Childhood Acute Myeloid Leukemia/Other Myeloid Malignancies Treatment (PDQ®): Treatment - Health Professional Information [NCI]

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This information is produced and provided by the National Cancer Institute (NCI). The information in this topic may have changed since it was written. For the most current information, contact the National Cancer Institute via the Internet web site at http://cancer.gov or call 1-800-4-CANCER.

General Information

Fortunately, cancer in children and adolescents is rare, although the overall incidence of childhood cancer has been slowly increasing since 1975.[1] Children and adolescents with cancer should be referred to medical centers that have a multidisciplinary team of cancer specialists with experience treating the cancers that occur during childhood and adolescence. This multidisciplinary team approach incorporates the skills of the primary care physician, pediatric surgical subspecialists, radiation oncologists, pediatric medical oncologists/hematologists, rehabilitation specialists, pediatric nurse specialists, social workers, and others to ensure that children receive treatment, supportive care, and rehabilitation that will achieve optimal survival and quality of life. (Refer to the PDQ Supportive and Palliative Care summaries for specific information about supportive care for children and adolescents with cancer.)

Guidelines for pediatric cancer centers and their role in the treatment of children with cancer have been outlined by the American Academy of Pediatrics.[2] At these pediatric cancer centers, clinical trials are available for most types of cancer that occur in children and adolescents, and the opportunity to participate in these trials is offered to most patients/families. Clinical trials for children and adolescents with cancer are generally designed to compare potentially better therapy with therapy that is currently accepted as standard. Most of the progress made in identifying curative therapies for childhood cancers has been achieved through clinical trials. Information about ongoing clinical trials is available from the NCI website.

Dramatic improvements in survival have been achieved for children and adolescents with cancer.[1] Between 1975 and 2010, childhood cancer mortality decreased by more than 50%. For acute myeloid leukemia, the 5-year survival rate increased over the same time from less than 20% to 68% for children younger than 15 years and from less than 20% to 57% for adolescents aged 15 to 19 years.[1] Childhood and adolescent cancer survivors require close follow-up because cancer therapy side effects may persist or develop months or years after treatment. (Refer to the PDQ summary on Late Effects of Treatment for Childhood Cancer for specific information about the incidence, type, and monitoring of late effects in childhood and adolescent cancer survivors.)

Myeloid Leukemias in Children

Approximately 20% of childhood leukemias are of myeloid origin and they represent a spectrum of hematopoietic malignancies.[3] The majority of myeloid leukemias are acute, and the remainder include chronic and/or subacute myeloproliferative disorders such as chronic myelogenous leukemia (CML) and juvenile myelomonocytic leukemia (JMML). Myelodysplastic syndromes (MDS) occur much less frequently in children than in adults and almost invariably represent clonal, preleukemic conditions.

Acute myeloid leukemia (AML) is defined as a clonal disorder caused by malignant transformation of a bone marrow-derived, self-renewing stem cell or progenitor, which demonstrates a decreased rate of self-destruction as well as aberrant, and usually limited, differentiation capacity. These events lead to increased accumulation in the bone marrow and other organs by these malignant myeloid cells. To be called acute, the bone marrow usually must include greater than 20% leukemic blasts, with some exceptions as noted in subsequent sections.

CML represents the most common of the chronic myeloproliferative disorders in childhood, although it accounts for only 10% to 15% of childhood myeloid leukemia.[3] Although CML has been diagnosed in very young children, most patients are aged 6 years and older. CML is a clonal panmyelopathy that involves all hematopoietic cell lineages. While the white blood cell (WBC) count can be extremely elevated, the bone marrow does not show increased numbers of leukemic blasts during the chronic phase of this disease. CML is nearly always characterized by the presence of the Philadelphia chromosome, a translocation between chromosomes 9 and 22 (i.e., t(9;22)) resulting in fusion of the BCR and ABL genes. Other chronic myeloproliferative syndromes, such as polycythemia vera and essential thrombocytosis, are extremely rare in children.

JMML represents the most common myeloproliferative syndrome observed in young children. JMML occurs at a median age of 1.8 years and characteristically presents with hepatosplenomegaly, lymphadenopathy, fever, and skin rash along with an elevated WBC count and increased circulating monocytes.[4] In addition, patients often have an elevated hemoglobin F, hypersensitivity of the leukemic cells to granulocyte-macrophage colony-stimulating factor (GM-CSF), monosomy 7, and leukemia cell mutations in a gene involved in RAS pathway signaling (e.g., NF1, KRAS/NRAS, PTPN11, or CBL).[4,5]

The transient myeloproliferative disorder (TMD) (also termed transient leukemia) observed in infants with Down syndrome represents a clonal expansion of myeloblasts that can be difficult to distinguish from AML. Most importantly, TMD spontaneously regresses in most cases within the first 3 months of life. TMD blasts most commonly have megakaryoblastic differentiation characteristics and distinctive mutations involving the GATA1 gene.[6,7] TMD may occur in phenotypically normal infants with genetic mosaicism in the bone marrow for trisomy 21. While TMD is generally not characterized by cytogenetic abnormalities other than trisomy 21, the presence of additional cytogenetic findings may predict an increased risk for developing subsequent AML.[8] Approximately 20% of infants with Down syndrome and TMD eventually develop AML, with most cases diagnosed within the first 3 years of life.[7,8] Early death from TMD-related complications occurs in 10% to 20% of affected children.[8,9] Infants with progressive organomegaly, visceral effusions, and laboratory evidence of progressive liver dysfunction are at a particularly high risk for early mortality.[8]

MDS in children represents a heterogeneous group of disorders characterized by ineffective hematopoiesis, impaired maturation of myeloid progenitors with dysplastic morphologic features, and cytopenias. Although the majority of patients have hypercellular bone marrows without increased numbers of leukemic blasts, some patients may present with very hypocellular bone marrow, making the distinction between severe aplastic anemia and low-blast count AML difficult.

There are genetic risks associated with the development of AML. There is a high concordance rate of AML in identical twins; however, this is not believed to be related to genetic risk, but rather to shared circulation and the inability of one twin to reject leukemic cells from the other twin during fetal development.[10,11,12] There is an estimated twofold- to fourfold-risk of fraternal twins each developing leukemia up to about age 6 years, after which the risk is not significantly greater than that of the general population.[13,14] The development of AML has also been associated with a variety of predisposition syndromes that result from chromosomal imbalances or instabilities, defects in DNA repair, altered cytokine receptor or signal transduction pathway activation, as well as altered protein synthesis.[15]

Inherited and Acquired Genetic Syndromes Associated with Myeloid Malignancies

  • Inherited syndromes
    • Chromosomal imbalances:
      • Down syndrome.
      • Familial monosomy 7.
    • Chromosomal instability syndromes:
      • Fanconi anemia.
      • Dyskeratosis congenita.
      • Bloom syndrome.
    • Syndromes of growth and cell survival signaling pathway defects:
      • Neurofibromatosis type 1 (particularly JMML development).
      • Noonan syndrome (particularly JMML development).
      • Severe congenital neutropenia (Kostmann syndrome).
      • Shwachman-Diamond syndrome.
      • Diamond-Blackfan anemia.
      • Congenital amegakaryocytic thrombocytopenia.
      • CBL germline syndrome (particularly in JMML).
  • Acquired syndromes
    • Severe aplastic anemia.
    • Paroxysmal nocturnal hemoglobinuria.
    • Amegakaryocytic thrombocytopenia.
    • Acquired monosomy 7.
  • Familial MDS and AML syndromes[16]
    • Familial platelet disorder with a propensity to develop AML (associated with germline RUNX1 mutations).
    • Familial MDS and AML syndromes with germline GATA2 mutations.
    • Familial MDS and AML syndromes with germline CEBPA mutations.[17]
    • Telomere biology disorders due to a mutation in TERC or TERT (i.e., occult dyskeratosis congenita).

Nonsyndromic genetic susceptibility to AML is also being studied. For example, homozygosity for a specific IKZF1 polymorphism has been associated with an increased risk of infant AML.[18]

References:

  1. Smith MA, Altekruse SF, Adamson PC, et al.: Declining childhood and adolescent cancer mortality. Cancer 120 (16): 2497-506, 2014.
  2. Corrigan JJ, Feig SA; American Academy of Pediatrics: Guidelines for pediatric cancer centers. Pediatrics 113 (6): 1833-5, 2004.
  3. Smith MA, Ries LA, Gurney JG, et al.: Leukemia. In: Ries LA, Smith MA, Gurney JG, et al., eds.: Cancer incidence and survival among children and adolescents: United States SEER Program 1975-1995. Bethesda, Md: National Cancer Institute, SEER Program, 1999. NIH Pub.No. 99-4649, pp 17-34. Also available online. Last accessed August 02, 2017.
  4. Niemeyer CM, Arico M, Basso G, et al.: Chronic myelomonocytic leukemia in childhood: a retrospective analysis of 110 cases. European Working Group on Myelodysplastic Syndromes in Childhood (EWOG-MDS) Blood 89 (10): 3534-43, 1997.
  5. Loh ML: Recent advances in the pathogenesis and treatment of juvenile myelomonocytic leukaemia. Br J Haematol 152 (6): 677-87, 2011.
  6. Hitzler JK, Cheung J, Li Y, et al.: GATA1 mutations in transient leukemia and acute megakaryoblastic leukemia of Down syndrome. Blood 101 (11): 4301-4, 2003.
  7. Mundschau G, Gurbuxani S, Gamis AS, et al.: Mutagenesis of GATA1 is an initiating event in Down syndrome leukemogenesis. Blood 101 (11): 4298-300, 2003.
  8. Massey GV, Zipursky A, Chang MN, et al.: A prospective study of the natural history of transient leukemia (TL) in neonates with Down syndrome (DS): Children's Oncology Group (COG) study POG-9481. Blood 107 (12): 4606-13, 2006.
  9. Homans AC, Verissimo AM, Vlacha V: Transient abnormal myelopoiesis of infancy associated with trisomy 21. Am J Pediatr Hematol Oncol 15 (4): 392-9, 1993.
  10. Zuelzer WW, Cox DE: Genetic aspects of leukemia. Semin Hematol 6 (3): 228-49, 1969.
  11. Miller RW: Persons with exceptionally high risk of leukemia. Cancer Res 27 (12): 2420-3, 1967.
  12. Inskip PD, Harvey EB, Boice JD Jr, et al.: Incidence of childhood cancer in twins. Cancer Causes Control 2 (5): 315-24, 1991.
  13. Kurita S, Kamei Y, Ota K: Genetic studies on familial leukemia. Cancer 34 (4): 1098-101, 1974.
  14. Greaves M: Pre-natal origins of childhood leukemia. Rev Clin Exp Hematol 7 (3): 233-45, 2003.
  15. Puumala SE, Ross JA, Aplenc R, et al.: Epidemiology of childhood acute myeloid leukemia. Pediatr Blood Cancer 60 (5): 728-33, 2013.
  16. West AH, Godley LA, Churpek JE: Familial myelodysplastic syndrome/acute leukemia syndromes: a review and utility for translational investigations. Ann N Y Acad Sci 1310: 111-8, 2014.
  17. Tawana K, Wang J, Renneville A, et al.: Disease evolution and outcomes in familial AML with germline CEBPA mutations. Blood 126 (10): 1214-23, 2015.
  18. Ross JA, Linabery AM, Blommer CN, et al.: Genetic variants modify susceptibility to leukemia in infants: a Children's Oncology Group report. Pediatr Blood Cancer 60 (1): 31-4, 2013.

Classification of Pediatric Myeloid Malignancies

French-American-British (FAB) Classification for Childhood Acute Myeloid Leukemia

The first comprehensive morphologic-histochemical classification system for acute myeloid leukemia (AML) was developed by the FAB Cooperative Group.[1,2,3,4,5] This classification system, which has been replaced by the World Health Organization (WHO) system described below, categorized AML into the following major subtypes primarily based on morphology and immunohistochemical detection of lineage markers:

  • M0: Acute myeloblastic leukemia without differentiation.[6,7] M0 AML, also referred to as minimally differentiated AML, does not express myeloperoxidase (MPO) at the light microscopy level, but may show characteristic granules by electron microscopy. M0 AML can be defined by expression of cluster determinant (CD) markers such as CD13, CD33, and CD117 (c-KIT) in the absence of lymphoid differentiation.
  • M1: Acute myeloblastic leukemia with minimal differentiation but with the expression of MPO that is detected by immunohistochemistry or flow cytometry.
  • M2: Acute myeloblastic leukemia with differentiation.
  • M3: Acute promyelocytic leukemia (APL) hypergranular type. (Refer to the Acute Promyelocytic Leukemia section of this summary for more information on treatment options under clinical evaluation.)
  • M3v: APL, microgranular variant. Cytoplasm of promyelocytes demonstrates a fine granularity, and nuclei are often folded. Same clinical, cytogenetic, and therapeutic implications as FAB M3.
  • M4: Acute myelomonocytic leukemia (AMML).
  • M4Eo: AMML with eosinophilia (abnormal eosinophils with dysplastic basophilic granules).
  • M5: Acute monocytic leukemia (AMoL).
    • M5a: AMoL without differentiation (monoblastic).
    • M5b: AMoL with differentiation.
  • M6: Acute erythroid leukemia (AEL).
    • M6a: Erythroleukemia.
    • M6b: Pure erythroid leukemia (myeloblast component not apparent).
    • M6c: Presence of myeloblasts and proerythroblasts.
  • M7: Acute megakaryocytic leukemia (AMKL).

Other extremely rare subtypes of AML include acute eosinophilic leukemia and acute basophilic leukemia.

World Health Organization (WHO) Classification System

In 2001, the WHO proposed a new classification system that incorporated diagnostic cytogenetic information and more reliably correlated with outcome. In this classification, patients with t(8;21), inv(16), t(15;17), or MLL (KMT2A) translocations, which collectively constituted nearly half of the cases of childhood AML, were classified as AML with recurrent cytogenetic abnormalities. This classification system also decreased the bone marrow percentage of leukemic blast requirement for the diagnosis of AML from 30% to 20%; an additional clarification was made so that patients with recurrent cytogenetic abnormalities did not need to meet the minimum blast requirement to be considered AML.[8,9,10]

In 2008, the WHO expanded the number of cytogenetic abnormalities linked to AML classification, and for the first time included specific gene mutations (CEBPA and NPM mutations) in its classification system.[11] Such a genetically based classification system links AML class with outcome and provides significant biologic and prognostic information. With new emerging technologies aimed at genetic, epigenetic, proteomic, and immunophenotypic classification, AML classification will likely evolve and provide informative prognostic and biologic guidelines to clinicians and researchers.

WHO classification of AML

  • AML with recurrent genetic abnormalities:
    • AML with t(8;21)(q22;q22), RUNX1-RUNX1T1(CBFA2-AML1-ETO).
    • AML with inv(16)(p13.1;q22) or t(16;16)(p13.1;q22), CBFB-MYH11.
    • APL with t(15;17)(q24;q21), PML-RARA.
    • AML with t(9;11)(p22;q23), MLLT3(AF9)-MLL.
    • AML with t(6;9)(p23;q34), DEK-NUP214.
    • AML with inv(3)(q21;q26.2) or t(3;3)(q21;q26.2), RPN1-EVI1.
    • AML (megakaryoblastic) with t(1;22)(p13;q13), RBM15-MKL1.
    • AML with mutated NPM1.
    • AML with mutated CEBPA.
  • AML with myelodysplasia-related features.
  • Therapy-related myeloid neoplasms.
  • AML, not otherwise specified:
    • AML with minimal differentiation.
    • AML without maturation.
    • AML with maturation.
    • Acute myelomonocytic leukemia.
    • Acute monoblastic and monocytic leukemia.
    • Acute erythroid leukemia.
    • Acute megakaryoblastic leukemia.
    • Acute basophilic leukemia.
    • Acute panmyelosis with myelofibrosis.
  • Myeloid sarcoma.
  • Myeloid proliferations related to Down syndrome:
    • Transient abnormal myelopoiesis.
    • Myeloid leukemia associated with Down syndrome.
  • Blastic plasmacytoid dendritic cell neoplasm.

Histochemical Evaluation

The treatment for children with AML differs significantly from that for acute lymphoblastic leukemia (ALL). As a consequence, it is critical to distinguish AML from ALL. Special histochemical stains performed on bone marrow specimens of children with acute leukemia can be helpful to confirm their diagnosis, although such approaches have been mostly replaced by flow cytometric immunophenotyping. The stains most commonly used include myeloperoxidase, periodic acid-Schiff, Sudan Black B, and esterase. In most cases the staining pattern with these histochemical stains will distinguish AML from AMML and ALL (see below).

Table 1. Histochemical Staining Patternsa
M0 AML, APL (M1-M3) AMML (M4) AMoL (M5) AEL (M6) AMKL (M7) ALL
AEL = acute erythroid leukemia; ALL = acute lymphoblastic leukemia; AML = acute myeloid leukemia; AMKL = acute megakaryocytic leukemia; AMML = acute myelomonocytic leukemia; AMoL = acute monocytic leukemia; APL = acute promyelocytic leukemia; PAS = periodic acid-Schiff.
a Refer to the French-American-British (FAB) Classification for Childhood Acute Myeloid Leukemiasection of this summary for more information about the morphologic-histochemical classification system for AML.
b These reactions are inhibited by fluoride.
Myeloperoxidase - + + - - - -
Nonspecific esterases              
  Chloracetate - + + ± - - -
  Alpha-naphthol acetate - - +b +b - ±b -
Sudan Black B - + + - - - -
PAS - - ± ± + - +

Immunophenotypic Evaluation

The use of monoclonal antibodies to determine cell-surface antigens of AML cells is helpful to reinforce the histologic diagnosis. Various lineage-specific monoclonal antibodies that detect antigens on AML cells should be used at the time of initial diagnostic workup, along with a battery of lineage-specific T-lymphocyte and B-lymphocyte markers to help distinguish AML from ALL and bilineal (as defined below) or biphenotypic leukemias. The expression of various cluster determinant (CD) proteins that are relatively lineage-specific for AML include CD33, CD13, CD14, CDw41 (or platelet antiglycoprotein IIb/IIIa), CD15, CD11B, CD36, and antiglycophorin A. Lineage-associated B-lymphocytic antigens CD10, CD19, CD20, CD22, and CD24 may be present in 10% to 20% of AMLs, but monoclonal surface immunoglobulin and cytoplasmic immunoglobulin heavy chains are usually absent; similarly, CD2, CD3, CD5, and CD7 lineage-associated T-lymphocytic antigens are present in 20% to 40% of AMLs.[12,13,14] The aberrant expression of lymphoid-associated antigens by AML cells is relatively common but generally has no prognostic significance.[12,13]

Immunophenotyping can also be helpful in distinguishing some FAB subtypes of AML. Testing for the presence of HLA-DR can be helpful in identifying APL. Overall, HLA-DR is expressed on 75% to 80% of AMLs but rarely expressed on APL. In addition, APL cases with PML-RARA were noted to express CD34/CD15 and demonstrate a heterogeneous pattern of CD13 expression.[15] Testing for the presence of glycoprotein Ib, glycoprotein IIb/IIIa, or Factor VIII antigen expression is helpful in making the diagnosis of M7 (megakaryocytic leukemia). Glycophorin expression is helpful in making the diagnosis of M6 (erythroid leukemia).[16]

Less than 5% of cases of acute leukemia in children are of ambiguous lineage, expressing features of both myeloid and lymphoid lineage.[17,18,19] These cases are distinct from ALL with myeloid coexpression in that the predominant lineage cannot be determined by immunophenotypic and histochemical studies. The definition of leukemia of ambiguous lineage varies among studies, although most investigators now use criteria established by the European Group for the Immunological Characterization of Leukemias (EGIL) or the more stringent WHO criteria.[20,21,22] In the WHO classification, the presence of MPO is required to establish myeloid lineage. This is not the case for the EGIL classification.

The WHO classification system is summarized in Table 2.[22,23]

Table 2. Acute Leukemias of Ambiguous Lineage According to the WHO Classification of Tumors of Hematopoietic and Lymphoid Tissuesa
Condition Definition
NOS = not otherwise specified; WHO = World Health Organization.
a Béné MC: Biphenotypic, bilineal, ambiguous or mixed lineage: strange leukemias! Haematologica 94 (7): 891-3, 2009.[23]Obtained from Haematologica/the Hematology Journal websitehttp://www.haematologica.org.
Acute undifferentiated leukemia Acute leukemia that does not express any marker considered specific for either lymphoid or myeloid lineage
Mixed phenotype acute leukemia with t(9;22)(q34;q11.2);BCR-ABL1 Acute leukemia meeting the diagnostic criteria for mixed phenotype acute leukemia in which the blasts also have the (9;22) translocation or theBCR-ABL1rearrangement
Mixed phenotype acute leukemia with t(v;11q23);MLL(KMT2A) rearranged Acute leukemia meeting the diagnostic criteria for mixed phenotype acute leukemia in which the blasts also have a translocation involving theMLLgene
Mixed phenotype acute leukemia, B/myeloid, NOS Acute leukemia meeting the diagnostic criteria for assignment to both B and myeloid lineage, in which the blasts lack genetic abnormalities involvingBCR-ABL1orMLL
Mixed phenotype acute leukemia, T/myeloid, NOS Acute leukemia meeting the diagnostic criteria for assignment to both T and myeloid lineage, in which the blasts lack genetic abnormalities involvingBCR-ABL1orMLL
Mixed phenotype acute leukemia, B/myeloid, NOS-rare types Acute leukemia meeting the diagnostic criteria for assignment to both B- and T-lineage
Other ambiguous lineage leukemias Natural killer cell lymphoblastic leukemia/lymphoma

Leukemias of mixed phenotype comprise the following two groups of patients:

  1. Bilineal leukemias in which there are two distinct populations of cells, usually one lymphoid and one myeloid.
  2. Biphenotypic leukemias in which individual blast cells display features of both lymphoid and myeloid lineage.

Biphenotypic cases represent the majority of mixed phenotype leukemias.[17] B-myeloid biphenotypic leukemias lacking the TEL-AML1 fusion have a lower rate of complete remission (CR) and a significantly worse event-free survival (EFS) compared with patients with B-precursor ALL.[17] Some studies suggest that patients with biphenotypic leukemia may fare better with a lymphoid, as opposed to a myeloid, treatment regimen,[18,19,24] although the optimal treatment for patients remains unclear.

Cytogenetic Evaluation and Molecular Abnormalities

Genetic analyses of leukemia (using both conventional cytogenetic methods and molecular methods) should be performed on children with AML because both chromosomal and molecular abnormalities are important diagnostic and prognostic markers.[25,26,27,28,29,30,31] Clonal chromosomal abnormalities have been identified in the blasts of about 75% of children with AML and are useful in defining subtypes with particular characteristics (e.g., t(8;21), t(15;17), inv(16), 11q23 abnormalities, t(1;22)). Leukemias with the chromosomal abnormalities t(8;21) and inv(16) are called core-binding factor leukemias; core-binding factor (a transcription factor involved in hematopoietic stem cell differentiation) is disrupted by each of these abnormalities. Molecular abnormalities can help in risk stratification and treatment allocation. For example, mutations of NPM and CEBPA are associated with favorable outcome while certain mutations of FLT3 portend a high risk of relapse, and importantly, identifying these mutations may allow for targeted therapy.[32,33,34,35]

The 2016 revision to the WHO classification of myeloid neoplasms and acute leukemia emphasizes that recurrent chromosomal translocations in pediatric AML may be unique or of very different prevalence compared to adult AML.[36] In their supplemental materials, the authors denote the pediatric AML chromosomal translocations that are found by conventional chromosome analysis and those that are cryptic (identified only with fluorescent in situ hybridization or molecular techniques) and that occur at higher rates than in adults. These recurrent translocations are summarized in Table 3.[36] Also shown in Table 3 (bottom three rows) are additional relatively common recurrent translocations observed in children with AML.[28,29,37]

Table 3. Common Pediatric Acute Myeloid Leukemia (AML) Chromosomal Translocations
Gene Fusion Product Chromosomal Translocation Prevalence in Pediatric AML
a Cryptic chromosomal translocation.
KMT2A(MLL) translocated 11q23.3 25.0%
NUP98-NSD1a t(5;11)(q35.3;p15.5) 7.0%
CBFA2T3-GLIS2a inv(16)(p13.3;q24.3) 3.0%
NUP98-KDM5A4a t(11;12)(p15.5;p13.5) 3.0%
DEK-NUP214 t(6;9)(p23;q34.1) 1.7%
RBM15(OTT)-MKL1(MAL) t(1;22)(p13.3;q13.1) 0.8%
MNX1-ETV6 t(7;12)(q36.3;p13.2) 0.8%
KAT6A-CREBBP t(8;16)(p11.2;p13.3) 0.5%
RUNX1-RUNX1T1 t(8;21)(q22;q22) 13%-14%
CBFB-MYH11 inv(16)(p13.1;q22) or t(16;16)(p13.1;q22) 4%-9%
PML-RARA t(15;17)(q24;q21) 6%-11%

A unifying concept for the role of specific mutations in AML is that mutations that promote proliferation (Type I) and mutations that block normal myeloid development (Type II) are both required for full conversion of hematopoietic stem/precursor cells to malignancy.[38,39] Support for this conceptual construct comes from the observation that there is generally mutual exclusivity within each type of mutation, such that a single Type I and a single Type II mutation are present within each case. Further support comes from genetically engineered models of AML for which cooperative events rather than single mutations are required for leukemia development. Type I mutations are commonly in genes involved in growth factor signal transduction and include mutations in FLT3, KIT, NRAS, KRAS, and PTNP11.[40] Type II genomic alterations include the common translocations and mutations associated with favorable prognosis (t(8;21), inv(16), t(16;16), t(15;17), CEBPA, and NPM1). KMT2A rearrangements (translocations and partial tandem duplication) are also classified as Type II mutations.

Specific recurring cytogenetic and molecular abnormalities are briefly described below. The abnormalities are listed by those in clinical use that identify patients with favorable or unfavorable prognosis, followed by other abnormalities. The nomenclature of the 2016 revision of the WHO classification of myeloid neoplasms and acute leukemia is incorporated for disease entities where relevant.

Molecular abnormalities associated with favorable prognosis

Molecular abnormalities associated with favorable prognosis include the following:

  • Core-binding factor (CBF) AML includes cases with RUNX1-RUNX1T1 and CBFB-MYH11 fusion genes that disrupt the activity of core-binding factor, which contains RUNX1 and CBFB. These are specific entities in the 2016 revision of the WHO classification of myeloid neoplasms and acute leukemia.
    • AML with t(8;21)(q22;q22.1); RUNX1-RUNX1T1: In leukemias with t(8;21), the RUNX1 (AML1) gene on chromosome 21 is fused with the RUNX1T1 (ETO) gene on chromosome 8. The t(8;21) translocation is associated with the FAB M2 subtype and with granulocytic sarcomas.[41,42] Adults with t(8;21) have a more favorable prognosis than adults with other types of AML.[25,43] These children have a more favorable outcome compared with children with AML characterized by normal or complex karyotypes[25,44,45,46] with 5-year overall survival (OS) of 74% to 90%.[28,29,47] The t(8;21) translocation occurs in approximately 12% of children with AML.[28,29,47]
    • AML with inv(16)(p13.1;q22) or t(16;16)(p13.1;q22); CBFB-MYH11: In leukemias with inv(16), the CBF beta gene (CBFB) at chromosome band 16q22 is fused with the MYH11 gene at chromosome band 16p13. The inv(16) translocation is associated with the FAB M4Eo subtype.[48] Inv(16) confers a favorable prognosis for both adults and children with AML[25,44,45,46] with a 5-year OS of about 85%.[28,29] Inv(16) occurs in 7% to 9% of children with AML.[28,29,47] As noted above, cases with CBFB-MYH11 and cases with RUNX1-RUNX1T1 have distinctive secondary mutations; CBFB-MYH11 secondary mutations are primarily restricted to genes that activate receptor tyrosine kinase signaling (NRAS, FLT3, and KIT).[49,50]

      Both RUNX1-RUNX1T1 and DBFB-MYH11 subtypes commonly show mutations in genes that activate receptor tyrosine kinase signaling (e.g., NRAS, FLT3, and KIT); NRAS and KIT are the most commonly mutated genes for both subtypes. KIT mutations may indicate increased risk of treatment failure for patients with core-binding factor AML, although the prognostic significance of KIT mutations may be dependent on the mutant-allele ratio (high ratio unfavorable) and/or the specific type of mutation (exon 17 mutations unfavorable).[49,50] A study of children with RUNX1-RUNX1T1 AML observed KIT mutations in 24% of cases (79% being exon 17 mutations) and RAS mutations in 15%, but neither were significantly associated with outcome.[47]

      Although both RUNX1-RUNX1T1 and fusion genes disrupt the activity of core-binding factor, cases with these genomic alterations have distinctive secondary mutations.[49,50]

      • RUNX1-RUNX1T1 cases also have frequent mutations in genes regulating chromatin conformation (e.g., ASXL1 and ASXL2) and genes encoding members of the cohesin complex (approximately 40% and 20% of cases, respectively). Mutations in ASXL1 and ASXL2 and mutations in members of the cohesin complex are rare in CBFB-MYH11 leukemias.[49,50]
      • A study of 204 adults with RUNX1-RUNX1T1 AML found that ASXL2 mutations (present in 17% of cases) and ASXL1 or ASXL2 mutations (present in 25% of cases) lacked prognostic significance.[51] Similar results, albeit with smaller numbers, were reported for children with RUNX1-RUNX1T1 AML and ASXL1 and ASXL2 mutations.[52]
  • APL with PML-RARA: APL represents about 7% of children with AML.[29,53] AML with t(15;17) is invariably associated with APL, a distinct subtype of AML that is treated differently than other types of AML because of its marked sensitivity to arsenic trioxide and the differentiating effects of all-trans retinoic acid. The t(15;17) translocation or other more complex chromosomal rearrangements may lead to the production of a fusion protein involving the retinoid acid receptor alpha and PML.[54] The WHO 2016 revision does not include the t(15;17) cytogenetic designation to stress the significance of the PML-RARA fusion, which may be cryptic or result from complex karyotypic changes.[36]

    Utilization of quantitative reverse transcriptase-polymerase chain reaction (RQ-PCR) for PML-RARA transcripts has become standard practice.[55] RQ-PCR allows identification of the three common transcript variants and is used for monitoring response on treatment and early detection of molecular relapse.[56] Other much less common translocations involving the retinoic acid receptor alpha can also result in APL (e.g., t(11;17)(q23;q21) involving the PLZF gene).[57,58,59] Identification of cases with the t(11;17)(q23;q21) is important because of their decreased sensitivity to all-trans retinoic acid.[54,57]

  • AML with mutated NPM1: NPM1 is a protein that has been linked to ribosomal protein assembly and transport as well as being a molecular chaperone involved in preventing protein aggregation in the nucleolus. Immunohistochemical methods can be used to accurately identify patients with NPM1 mutations by the demonstration of cytoplasmic localization of NPM.[60] Mutations in the NPM1 protein that diminish its nuclear localization are primarily associated with a subset of AML with a normal karyotype, absence of CD34 expression,[61] and an improved prognosis in the absence of FLT3-internal tandem duplication (ITD) mutations in adults and younger adults.[61,62,63,64,65,66]

    Studies of children with AML suggest a lower rate of occurrence of NPM1 mutations in children compared with adults with normal cytogenetics. NPM1 mutations occur in approximately 8% of pediatric patients with AML and are uncommon in children younger than 2 years.[32,33,39,67]NPM1 mutations are associated with a favorable prognosis in patients with AML characterized by a normal karyotype.[32,33,39] For the pediatric population, conflicting reports have been published regarding the prognostic significance of an NPM1 mutation when a FLT3-ITD mutation is also present, with one study reporting that an NPM1 mutation did not completely abrogate the poor prognosis associated with having a FLT3-ITD mutation,[32,68] but with other studies showing no impact of a FLT3-ITD mutation on the favorable prognosis associated with an NPM1 mutation.[33,39]

  • AML with biallelic mutations of CEBPA: Mutations in the CCAAT/Enhancer Binding Protein Alpha gene (CEBPA) occur in a subset of children and adults with cytogenetically normal AML.[69] In adults younger than 60 years, approximately 15% of cytogenetically normal AML cases have mutations in CEBPA.[65] Outcome for adults with AML with CEBPA mutations appears to be relatively favorable and similar to that of patients with core-binding factor leukemias.[65,70] Studies in adults with AML have demonstrated that CEBPA double-mutant, but not single-allele mutant, AML is independently associated with a favorable prognosis,[71,72,73,74] leading to the WHO 2016 revision requiring biallelic mutations for the disease definition.[36]

    CEBPA mutations occur in 5% to 8% of children with AML and have been preferentially found in the cytogenetically normal subtype of AML with FAB M1 or M2; 70% to 80% of pediatric patients have double-mutant alleles, which is predictive of a significantly improved survival and similar to the effect observed in adult studies.[34,75] Although both double- and single-mutant alleles of CEBPA were associated with a favorable prognosis in children with AML in one large study,[34] a second study observed inferior outcome for patients with single CEBPA mutations.[75] However, very low numbers of children with single-allele mutants were included in these two studies (only 13 in toto), making a conclusion regarding the prognostic significance of single-allele CEBPA mutations in children premature.[34] In newly diagnosed patients with double-mutant CEBPA AML, in addition to usual family history queries, germline screening should be considered because 5% to 10% of these patients are reported to have a germline CEBPA mutation.[69]

  • Myeloid leukemia associated with Down syndrome (GATA1 mutations): GATA1 mutations are present in most, if not all, Down syndrome children with either TAM or AMKL.[76,77,78,79]GATA1 mutations were also observed in 9% of non-Down syndrome children and 4% of adults with AMKL (with co-existence in 9 of 10 cases of amplification of the Down syndrome Critical Region on chromosome 21).[80]GATA1 is a transcription factor that is required for normal development of erythroid cells, megakaryocytes, eosinophils, and mast cells.[81]GATA1 mutations confer increased sensitivity to cytarabine by down-regulating cytidine deaminase expression, possibly providing an explanation for the superior outcome of children with Down syndrome and M7 AML when treated with cytarabine-containing regimens.[82]

Molecular abnormalities associated with an unfavorable prognosis

Molecular abnormalities associated with an unfavorable prognosis include the following:

  • Chromosomes 5 and 7: Chromosomal abnormalities associated with poor prognosis in adults with AML include those involving chromosome 5 (monosomy 5 and del(5q)) and chromosome 7 (monosomy 7).[25,43,83] These cytogenetic subgroups represent approximately 2% and 4% of pediatric AML cases, respectively, and are also associated with poor prognosis in children.[28,43,83,84,85,86,87]

    In the past, patients with del(7q) were also considered to be at high risk of treatment failure and data from adults with AML support a poor prognosis for both del(7q) and monosomy 7.[30] However, outcome for children with del(7q), but not monosomy 7, appears to be comparable to that of other children with AML.[29,86] The presence of del(7q) does not abrogate the prognostic significance of favorable cytogenetic characteristics (e.g., inv(16) and t(8;21)).[25,86,88]

    Chromosome 5 and 7 abnormalities appear to lack prognostic significance in AML patients with Down syndrome who are 4 years of age and younger.[89]

  • AML with inv(3)(q21.3;q26.2) or t(3;3)(q21.3;q26.2); GATA2, MECOM: MECOM at chromosome 3q26 codes for two proteins, EVI1 and MDS1-EVI1, both of which are transcription regulators. The inv(3) and t(3;3) abnormalities lead to overexpression of EVI1 and to reduced expression of GATA2.[90,91] These abnormalities are associated with poor prognosis in adults with AML,[25,43,92] but are very uncommon in children (<1% of pediatric AML cases).[28,45,93] Abnormalities involving MECOM can be detected in some AML cases with other 3q abnormalities and are also associated with poor prognosis.
  • FLT3 mutations: Presence of a FLT3-ITD mutation appears to be associated with poor prognosis in adults with AML,[94] particularly when both alleles are mutated or there is a high ratio of the mutant allele to the normal allele.[95,96]FLT3-ITD mutations also convey a poor prognosis in children with AML.[35,68,97,98,99,100] The frequency of FLT3-ITD mutations in children is lower than that observed in adults, especially for children younger than 10 years, for whom 5% to 10% of cases have the mutation (compared with approximately 30% for adults).[99,100,101] The prevalence of FLT3-ITD is increased in certain genomic subtypes of pediatric AML, including those with the NUP98-NSD1 fusion gene, of which 80% to 90% have FLT3-ITD.[102,103] Approximately 15% of patients with FLT3-ITD have NUP98-NSD1, and patients with both FLT3-ITD and NUP98-NSD1 have a poorer prognosis than do patients with FLT3-ITD and without NUP98-NSD1.[103]

    For APL, FLT3-ITD and point mutations occur in 30% to 40% of children and adults.[95,98,99,104,105,106,107,108] Presence of the FLT3-ITD mutation is strongly associated with the microgranular variant (M3v) of APL and with hyperleukocytosis.[98,106,109,110] It remains unclear whether FLT3 mutations are associated with poorer prognosis in patients with APL who are treated with modern therapy that includes all-trans retinoic acid and arsenic trioxide.[104,105,108,109,111,112,113,114]

    Activating point mutations of FLT3 have also been identified in both adults and children with AML, though the clinical significance of these mutations is not clearly defined.

Other molecular abnormalities observed in pediatric AML

Other molecular abnormalities observed in pediatric AML include the following:

  • KMT2A (MLL) gene rearrangements: KMT2A gene rearrangement occurs in approximately 20% of children with AML.[28,29] These cases, including most AMLs secondary to epipodophyllotoxin,[115] are generally associated with monocytic differentiation (FAB M4 and M5). KMT2A rearrangements are also reported in approximately 10% of FAB M7 (AMKL) patients (see below).[80,116]

    The most common translocation, representing approximately 50% of KMT2A-rearranged cases in the pediatric AML population, is t(9;11)(p22;q23) in which the KMT2A gene is fused with MLLT3(AF9) gene.[117] The WHO 2016 revision defined AML with t(9;11)(p21.3;q23.3); MLLT3-KMT2A as a distinctive disease entity. However, more than 50 different fusion partners have been identified for the KMT2A gene in patients with AML. The median age for 11q23/KMT2A-rearranged cases in children is approximately 2 years, and most translocation subgroups have a median age at presentation of younger than 5 years.[117] However, pediatric cases with t(6;11)(q27;q23) and t(11;17)(q23;q21) have significantly older median ages at presentation (12 years and 9 years, respectively).[117]

    Outcome for patients with de novo AML and KMT2A gene rearrangement is generally reported as being similar to that for other patients with AML.[25,28,117,118] However, as the KMT2A gene can participate in translocations with many different fusion partners, the specific fusion partner appears to influence prognosis, as demonstrated by a large international retrospective study evaluating outcome for 756 children with 11q23- or KMT2A-rearranged AML.[117] For example, cases with t(1;11)(q21;q23), representing 3% of all 11q23/KMT2A-rearranged AML, showed a highly favorable outcome with 5-year event-free survival (EFS) of 92%.

    While reports from single clinical trial groups have variably described more favorable prognosis for cases with AML with t(9;11)(p21.3;q23.3); MLLT3-KMT2A, the international retrospective study did not confirm the favorable prognosis for this subgroup.[25,28,117,119,120,121] An international collaboration evaluating pediatric AMKL observed that the presence of t(9;11), which was seen in approximately 5% of AMKL cases, was associated with an inferior outcome compared with other AMKL cases.[116]

    Several additional KMT2A-rearranged AML subgroups appear to be associated with poor outcome. For example, cases with the t(10;11) translocation are a group at high risk of relapse in bone marrow and the CNS.[25,29,122] Some cases with the t(10;11) translocation have fusion of the KMT2A gene with the AF10-MLLT10 at 10p12, while others have fusion of KMT2A with ABI1 at 10p11.2.[123,124] The international retrospective study found that these cases, which present at a median age of approximately 1 year, have a 5-year EFS in the 20% to 30% range.[117] Patients with t(6;11)(q27;q23) and with t(4;11)(q21;q23) also have a poor outcome, with a 5-year EFS of 11% and 29%, respectively, in the international retrospective study.[117] A follow-up study by the international collaborative group demonstrated that additional cytogenetic abnormalities further influenced outcome of children with KMT2A translocations, with complex karyotypes and trisomy 19 predicting poor outcome and trisomy 8 predicting a more favorable outcome.[125]

  • AML with t(6;9)(p23;q34.1); DEK-NUP214: t(6;9) leads to the formation of a leukemia-associated fusion protein DEK-NUP214.[126,127] This subgroup of AML has been associated with a poor prognosis in adults with AML,[126,128,129] and occurs infrequently in children (less than 1% of AML cases). The median age of children with DEK-NUP214 AML is 10 to 11 years, and approximately 40% of pediatric patients have FLT3-ITD.[130,131] t(6;9) AML appears to be associated with a high risk of treatment failure in children, particularly for those not proceeding to allogeneic stem cell transplantation.[28,127,130,131]
  • Molecular subgroups of non-Down syndrome acute megakaryoblastic leukemia (AMKL): AMKL accounts for approximately 10% of pediatric AML and includes substantial heterogeneity at the molecular level. Molecular subtypes of AMKL are listed below.
    • CBFA2T3-GLIS2: CBFA2T3-GLIS2 is a fusion resulting from a cryptic chromosome 16 inversion (inv(16)(p13.3q24.3)).[132,132,133,134,135,136] It occurs almost exclusively in non-Down syndrome AMKL, representing 16% to 27% of pediatric AMKL and presenting with a median age of 1 year.[80,134,137,138] It appears to be associated with unfavorable outcome,[80,132,136,137,138] with EFS at 2 years less than 20% in two reports that included 28 patients.[80,136,138]
    • KMT2A-rearranged: Cases with KMT2A translocations represent 10% to 17% of pediatric AMKL with MLLT3 (AF9) being the most common KMT2A transfusion partner.[80,116,137]KMT2A-rearranged cases appear to be associated with inferior outcome among children with AMKL with OS rates at 4 to 5 years of approximately 30%.[80,116,137] An international collaboration evaluating pediatric AMKL observed that the presence of t(9;11);MLLT3-KMT2A, which was seen in approximately 5% of AMKL cases (n = 21), was associated with an inferior outcome (5-year OS approximately 20%) compared with other AMKL cases and other KMT2A-rearrangements (n = 17), each with 5-year OS of 50% to 55%.[116] Inferior outcome was not observed for patients (n = 17) with other KMT2A-rearrangements.
    • NUP98-KDM5A4: NUP98-KDM5A4 is observed in approximately 10% of pediatric AMKL, [80,137] and are observed at much lower rates in non-AMKL cases.[138]NUP98-KDM5A4 cases showed a trend towards inferior prognosis, though the small number of cases studied limits confidence in this assessment.[80,137]
    • RBM15-MKL1: The t(1;22)(p13;q13) translocation that produces RBM15-MKL1 is uncommon (<1% of pediatric AML) and is restricted to acute megakaryocytic leukemia (AMKL).[28,138,139,140,141,142,143] Studies have found that t(1;22)(p13;q13) is observed in 10% to 18% of children with AMKL and evaluable cytogenetics or molecular genetics.[80,116,137] Most AMKL cases with t(1;22) occur in infants, with the median age at presentation (4-7 months) being younger than that for other children with AMKL.[116,134,144] Cases with detectable RBM15-MKL1 fusion transcripts in the absence of t(1;22) have also been reported because these young patients usually have hypoplastic bone marrow.[141]

      An international collaborative retrospective study with 51 t(1;22) cases reported that patients with this abnormality had a 5-year EFS of 54.5% and an OS of 58.2%, similar to the rates for other children with AMKL.[116] In another international retrospective analysis of 153 cases with non-Down syndrome AMKL with samples available for molecular analysis, the 4-year EFS for patients with t(1;22) was 59% and OS was 70%, significantly better than AMKL patients with other specific genetic abnormalities (CBFA2T3/GUS2, NUP98/KDM5A4, KMT2A rearrangements, monosomy7).[137]

    • HOX-rearranged: Cases with a gene fusion involving a HOX cluster gene represented 15% of pediatric AMKL in one report.[80] This report observed that these patients appear to have a relatively favorable prognosis, though the small number of cases studied limits confidence in this assessment.
    • GATA1 mutated: GATA1-truncating mutations in non-Down syndrome AMKL arise in young children (median age 1 to 2 years) and are associated with amplification of the Down syndrome critical region on chromosome 21.[80] These patients represented approximately 10% of non-Down syndrome AMKL and appeared to have a favorable outcome if there were no prognostically unfavorable fusion genes also present, although the number of patients studied reported was small (n = 8).[80]
  • t(8;16) (MYST3-CREBBP): The t(8;16) translocation fuses the MYST3 gene on chromosome 8p11 to CREBBP on chromosome 16p13. t(8;16) AML occurs rarely in children, and in an international BFM AML study of 62 children, presence of this translocation was associated with younger age at diagnosis (median, 1.2 years), FAB M4/M5 phenotype, erythrophagocytosis, leukemia cutis, and disseminated intravascular coagulation.[145] Outcome for children with t(8;16) AML appears similar to other types of AML. A substantial proportion of infants diagnosed with t(8;16) AML in the first month of life show spontaneous remission, although AML recurrence may occur months to years later.[145,146,147,148,149,150,151] These observations suggest that a watch and wait policy could be considered in cases of t(8;16) AML diagnosed in the neonatal period if close long-term monitoring can be ensured.[145]
  • t(7;12)(q36;p13): The t(7;12)(q36;p13) translocation involves ETV6 on chromosome 12p13 and variable breakpoints on chromosome 7q36 in the region of MNX1 (HLXB9).[152] The translocation may be cryptic by conventional karyotyping and in some cases may be confirmed only by FISH.[153,154,155] This alteration occurs virtually exclusively in children younger than 2 years, is mutually exclusive with KMT2A (MLL) rearrangement, and is associated with a high risk of treatment failure.[28,29,39,153,154,156]
  • NUP98 gene fusions: NUP98 has been reported to form leukemogenic gene fusions with more than 20 different partners.[157] In the pediatric AML setting, the two most common fusion genes are NUP98-NSD1 and NUP98-KDM5A4 (JARID1A), with the former observed in one report in approximately 15% of cytogenetically normal pediatric AML and the latter observed in approximately 10% of pediatric AMKL (see above).[80,102,134] AML cases with either NUP98 fusion gene show high expression of HOXA and HOXB genes, indicative of a stem cell phenotype.[127,134]

    The NUP98-NSD1 fusion gene, which is often cytogenetically cryptic, results from the fusion of NUP98 (chromosome 11p15) with NSD1 (chromosome 5q35).[102,103,127,158,159,160,161] This alteration occurs in approximately 4% to 7% of pediatric AML cases.[36,37,102,127,160] The highest frequency in the pediatric population is in the 5- to 9-year age group (approximately 8%), with lower frequency in younger children (approximately 2% in children younger than 2 years). NUP98-NSD1 cases present with high WBC count (median, 147 × 109 /L in one study).[102,103] Most NUP98-NSD1 AML cases do not show cytogenetic aberrations.[102,127,158] A high percentage of NUP98-NSD1 cases (74% to 90%) have FLT3-ITD.[37,102,103] A study that included 12 children with NUP98-NSD1 AML reported that although all patients achieved CR, presence of NUP98-NSD1 independently predicted for poor prognosis, and children with NUP98-NSD1 AML had a high risk of relapse, with a resulting 4-year EFS of approximately 10%.[102] In another study that included children (n = 38) and adults (n = 7) with NUP98-NSD1 AML, presence of both NUP98-NSD1 and FLT3-ITD independently predicted for poor prognosis; patients with both lesions had a low CR rate (approximately 30%) and a low 3-year EFS rate (approximately 15%).[103]

  • RAS mutations: Although mutations in RAS have been identified in 20% to 25% of patients with AML, the prognostic significance of these mutations has not been clearly shown.[39,162,163,164] Mutations in NRAS are observed more commonly than KRAS mutations in pediatric AML cases.[39,40]RAS mutations occur with similar frequency for all Type II alteration subtypes with the exception of APL, for which RAS mutations are seldom observed.[39]
  • KIT mutations: Mutations in KIT occur in approximately 5% of AML, but in 10% to 40% of AML with core-binding factor abnormalities.[39,40,165,166] The presence of activating KIT mutations in adults with this AML subtype appears to be associated with a poorer prognosis compared with core-binding factor AML without KIT mutation.[165,167,168] The prognostic significance of KIT mutations occurring in pediatric core-binding factor AML remains unclear,[169,170,171,172] although the largest pediatric study reported to date observed no prognostic significance for KIT mutations.[173]
  • WT1 mutations: WT1, a zinc-finger protein regulating gene transcription, is mutated in approximately 10% of cytogenetically normal cases of AML in adults.[174,175,176,177] The WT1 mutation has been shown in some,[174,175,177] but not all,[176] studies to be an independent predictor of worse disease-free, event-free, and OS of adults. In children with AML, WT1 mutations are observed in approximately 10% of cases.[178,179] Cases with WT1 mutations are enriched among children with normal cytogenetics and FLT3-ITD, but are less common among children younger than 3 years.[178,179] AML cases with NUP98-NSD1 are enriched for both FLT3-ITD and WT1 mutations.[102] In univariate analyses, WT1 mutations are predictive of poorer outcome in pediatric patients, but the independent prognostic significance of WT1 mutation status is unclear because of its strong association with FLT3-ITD and its association with NUP98-NSD1.[102,178,179] The largest study of WT1 mutations in children with AML observed that children with WT1 mutations in the absence of FLT3-ITD had outcomes similar to that of children without WT1 mutations, while children with both WT1 mutation and FLT3-ITD had survival rates less than 20%.[178]
  • DNMT3A mutations: Mutations of the DNA cytosine methyltransferase gene (DNMT3A) have been identified in approximately 20% of adult AML patients, being uncommon in patients with favorable cytogenetics but occurring in one-third of adult patients with intermediate-risk cytogenetics.[180] Mutations in this gene are independently associated with poor outcome.[180,181,182]DNMT3A mutations are virtually absent in children.[183]
  • IDH1 and IDH2 mutations: Mutations in IDH1 and IDH2, which code for isocitrate dehydrogenase, occur in approximately 20% of adults with AML,[184,185,186,187,188] and they are enriched in patients with NPM1 mutations.[185,186,189] The specific mutations that occur in IDH1 and IDH2 create a novel enzymatic activity that promotes conversion of alpha-ketoglutarate to 2-hydroxyglutarate.[190,191] This novel activity appears to induce a DNA hypermethylation phenotype similar to that observed in AML cases with loss of function mutations in TET2.[189] Mutations in IDH1 and IDH2 are rare in pediatric AML, occurring in 0% to 4% of cases.[183,192,193,194,195,196] There is no indication of a negative prognostic effect for IDH1 and IDH2 mutations in children with AML.[192]
  • CSF3R mutations: CSF3R is the gene encoding the granulocyte colony-stimulating factor (G-CSF) receptor, and activating mutations in CSF3R are observed in 2% to 3% of pediatric AML cases.[197] These mutations lead to enhanced signaling through the G-CSF receptor, and they are primarily observed in AML with either CEBPA mutations or with core-binding factor abnormalities (RUNX1-RUNX1T1 and CBFB-MYH11).[197] The clinical characteristics of and prognosis for patients with CSF3R mutations do not seem to be significantly different from those of patients without CSF3R mutations.

    Activating mutations in CSF3R are also observed in patients with severe congenital neutropenia. These mutations are not the cause of severe congenital neutropenia, but rather arise as somatic mutations and can represent an early step in the pathway to AML.[198] In one study of patients with severe congenital neutropenia, 34% of patients who had not developed a myeloid malignancy had CSF3R mutations detectable in peripheral blood neutrophils and mononuclear cells, while 78% of patients who had developed a myeloid malignancy showed CSF3R mutations.[198] A study of 31 patients with severe congenital neutropenia who developed AML or MDS observed CSF3R mutations in approximately 80%, and also observed a high frequency of RUNX1 mutations (approximately 60%), suggesting cooperation between CSF3R and RUNX1 mutations for leukemia development within the context of severe congenital neutropenia.[199]

Classification of Myelodysplastic Syndromes in Children

The FAB classification of myelodysplastic syndromes (MDS) was not completely applicable to children.[200,201] Traditionally, MDS classification systems have been divided into several distinct categories based on the presence of the following:[201,202,203,204]

  • Myelodysplasia.
  • Types of cytopenia.
  • Specific chromosomal abnormalities.
  • Percentage of myeloblasts.

A modified classification schema for MDS and myeloproliferative disorders (MPDs) was published by the WHO in 2008 and included subsections that focused on pediatric MDS and MPD.[205] The bone marrow and peripheral blood findings for the myelodysplastic syndromes according to the 2008 WHO classification schema [205] are summarized in Tables 4 and 5.

A pediatric approach to the WHO classification of myelodysplastic and myeloproliferative diseases was published in 2003.[10] A retrospective comparison of the WHO classification to the Category, Cytology, and Cytogenetics system (CCC) and to a Pediatric WHO adaptation for MDS/MPD, has shown that the latter two systems appear to effectively classify childhood MDS better than the more general WHO system.[206] For instance, while refractory anemia with ring sideroblasts is rare in children, refractory anemia and refractory anemia with excess blasts are more common. When such refractory cytopenias with excess blasts (5%-20%) are associated with recurrent cytogenetic abnormalities usually associated with AML, a diagnosis of the latter should be made and treated accordingly.

The WHO classification schema has a subgroup that includes JMML (formerly juvenile chronic myeloid leukemia), CMML, and Ph chromosome-negative CML. This group can show mixed myeloproliferative and sometimes myelodysplastic features. JMML shares some characteristics with adult CMML [207,208,209] but is a distinct syndrome (see below). A subgroup of children younger than 4 years at diagnosis with JMML associated with monosomy 7, are considered to have a subtype of JMML characterized by lower WBC, higher percentage of circulating monocytes, higher mean cell volume for red blood cells, a lower bone marrow myeloid to erythroid ratio and often, normal to moderately increased fetal hemoglobin.

The International Prognostic Scoring System is used to determine the risk of progression to AML and the outcome in adult patients with MDS. When this system was applied to children with MDS or JMML, only a blast count of less than 5% and a platelet count of more than 100 x 109 /L were associated with a better survival in MDS, and a platelet count of more than 40 x 109 /L predicted a better outcome in JMML.[210] These results suggest that MDS and JMML in children may be significantly different disorders than adult-type MDS.

MDS in children with monosomy 7 and high-grade MDS behaves more like MDS in adults and are best classified as adult MDS, as well as treated with allogeneic hematopoietic stem cell transplantation.[211,212] The risk group or grade of MDS is defined according to International Prognostic Scoring System guidelines.[213]

Table 4. World Health Organization Classification of Bone Marrow and Peripheral Blood Findings for Myelodysplastic Syndromes (MDS)a
Type of MDS Bone Marrow Peripheral Blood
a Adapted from Arber et al.[11]
b Note that cases with pancytopenia would be classified as MDS-U.
c When the marrow has <5% myeloblasts, but the peripheral blood has 2%-5% myeloblasts, RAEB-1 should be diagnosed.
d If Auer rods are present and there are <5% myeloblasts in the peripheral blood and the marrow has <10% myeloblasts, the diagnosis should be RAEB-2.
e Recurring chromosomal abnormalities in MDS: Unbalanced: +8, -7 or del(7q), -5 or del(5q), del(20q), -Y, i(17q) or t(17p), -13 or del(13q), del(11q), del(12p) or t(12p), de(9q), idic(X)(q13); Balanced: t(11;16)(q23;p13.3), t(3;21)(q26.2;q22.1), t(1;3)(p36.3;q21.2), t(2;11)(p21;q23), inv(3)(q21q26.2), t(6;9)(p23;q34). The WHO classification notes that the presence of these chromosomal abnormalities in presence of persistent cytopenias of undetermined origin should be considered to support a presumptive diagnosis of MDS when morphological characteristics are not observed.
Refractory cytopenias with unilineage dysplasia (RCUD) Unilineage dysplasia: Unicytopenia or bicytopeniab
-Refractory anemia (RA) - =10% in one myeloid lineage Blasts (none or <1%)c
-Refractory neutropenia (RN) - <5% blasts  
-Refractory thrombocytopenia (RT) - <15% ring sideroblasts  
 
Refractory anemia with ring sideroblasts (RARS) Erythroid dysplasia only Anemia
<5% blasts No blasts
=15% ring sideroblasts  
 
Refractory cytopenia with multilineage dysplasia (RCMD) Dysplasia in =10% of cells in =2 myeloid lineages Cytopenia(s)
<5% blasts Blasts (none or <1%)c
±15% ring sideroblasts No Auer rods
No Auer rods <1×109 monocytes/L
 
Refractory anemia with excess blasts-1 (RAEB-1) Unilineage or multilineage dysplasia Cytopenia(s)
5%-9% blastsc <5% blastsc
No Auer rods No Auer rods
  <1×109 monocytes/L
 
Refractory anemia with excess blasts-2 (RAEB-2) Unilineage or multilineage dysplasia Cytopenia(s)
10%-19% blasts <5%-19% blasts
Auer rods ±d Auer rods ±d
  <1×109 monocytes/L
 
MDS associated with isolated del(5q) Normal to increased megakaryocytes (hypolobulated nuclei) Anemia
<5% blasts Normal to increased platelet count
No Auer rods Blasts (none or <1%)
Isolated del(5q)  
 
Myelodysplastic syndrome-unclassified (MDS-U) Dysplasia in <10% of cells in =1 myeloid cell lineage Cytopenias
Cytogenetic abnormality associated with diagnosis of MDSe =1% blastsc
<5% blasts  
 
Childhood myelodysplastic syndrome Refer to Table 5for more information.
-Provisional entity: Refractory cytopenia of childhood (RCC)f

The diagnostic criteria for childhood myelodysplastic syndrome (refractory cytopenia of childhood [RCC]-provisional entry) include the following:

  • Persistent cytopenia with <5% bone marrow blasts and <2% peripheral blood blasts.
  • Dysplastic changes should be present.
Table 5. Definitions for Minimal Diagnostic Criteria for Childhood Myelodysplastic Syndrome (MDS) (Provisional Entity: Refractory Cytopenia of Childhood [RCC])a
Erythroid Lineage Myeloid Lineage Megakaryocyte Lineage
a Adapted from Baumann et al.[214]
b Bone marrow trephine/biopsy may be required as bone marrow in childhood RCC is often hypocellular.
c Characteristics include abnormal nuclear lobulation, multinuclear cells, presence of nuclear bridges.
d Presence of pseudo-Pelger-Huet cells, hypo- or agranular cytoplasm, giantband forms.
e Megakaryocytes have variable size and often round or separated nuclei; the absence of megakaryocytes does not exclude the diagnosis of RCC.
Bone Marrow Aspirateb Dysplasia and/or megablastoid changes in =10% of erythroid precursorsc Dysplasia in =10% of granulocytic precursors and neutrophils Micromegakaryocytes plus other dysplastic featurese
  <5% blastsd  
 
Bone Marrow Biopsy Presence of erythroid precursors No additional criteria Micromegakaryocytes plus other dysplastic featurese
Increased proerythroblasts   Immunohistochemistry positive for CD61 and CD41
Increased number of mitoses    
 
Peripheral Blood   Dysplasia in =10% of neutrophils  
  <2% blasts  

Stage Information

There is presently no therapeutically or prognostically meaningful staging system for these myeloid malignancies. Leukemia is considered to be disseminated in the hematopoietic system at diagnosis, even in children with AML who present with isolated chloromas (also called granulocytic sarcomas). If these children do not receive systemic chemotherapy, they invariably develop AML in months or years. AML may invade nonhematopoietic tissue such as meninges, brain parenchyma, testes or ovaries, or skin (leukemia cutis). Extramedullary leukemia is more common in infants than in older children with AML.[215]

Newly diagnosed

Childhood AML is diagnosed when bone marrow has greater than 20% blasts. The blasts have the morphologic and histochemical characteristics of one of the FAB subtypes of AML. It can also be diagnosed by biopsy of a chloroma. For treatment purposes, patients with clonal cytogenetic abnormalities typically associated with AML, such as t(8:21) (RUNX1-RUNX1T1), inv(16)(CBFB-MYH11), t(9;11)(MLL-MLLT3(AF9)) or t(15;17)(PML-RARA) and who have less than 20% bone marrow blasts, are considered to have AML rather than myelodysplastic syndrome.[216]

In remission

Remission is defined in the United States as peripheral blood counts (WBC count, differential, and platelet count) rising toward normal, a mildly hypocellular to normal cellular marrow with fewer than 5% blasts, and no clinical signs or symptoms of the disease, including in the CNS or at other extramedullary sites. Achieving a hypoplastic bone marrow is usually the first step in obtaining remission in AML with the exception of the M3 (acute promyelocytic leukemia [APL]); a hypoplastic marrow phase is often not necessary before the achievement of remission in APL. Additionally, early recovery marrows in any of the subtypes of AML may be difficult to distinguish from persistent leukemia, although the application of flow cytometric immunophenotyping and cytogenetic/molecular testing have made this less problematic. Correlation with blood cell counts and clinical status is imperative in passing final judgment on the results of early bone marrow findings in AML.[217] If the findings are in doubt, the bone marrow aspirate should be repeated in 1 to 2 weeks.[215]

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