Leukemia, Acute Myeloid

A number sign (#) is used with this entry because of evidence that acute myeloid leukemia (AML) can be caused by heterozygous mutation in the CEBPA gene (116897) on chromosome 19p13. One such family has been reported.

Somatic mutations in several genes have been found in cases of AML, e.g., in the CEBPA, ETV6 (600618), JAK2 (147796), KRAS2 (190070), NRAS (164790), HIPK2 (606868), FLT3 (136351), TET2 (612839), ASXL1 (612990), IDH1 (147700), CBL (165360), DNMT3A (602769), NPM1 (164040), SF3B1 (605590), and KIT (164920) genes. Other causes of AML include fusion genes generated by chromosomal translocations; see, for example, 600358 and 159555.

Susceptibility to the development of acute myeloid leukemia may be caused by germline mutations in some genes, including GATA2 (137295), TERC (602322), and TERT (187270).

AML may also be part of the phenotypic spectrum of inherited disorders, including platelet disorder with associated myeloid malignancy (FPDMM; 601399), caused by mutation in the RUNX1 gene (151385), and telomere-related pulmonary fibrosis and/or bone marrow failure (PFBMFT1, 614742 and PFBMFT2, 614743), caused by mutation in the TERT or the TERC gene.

Clinical Features

Shields et al. (2003) published a case report on acute myeloid leukemia that presented as bilateral orbital myeloid sarcoma (or chloroma) in a previously healthy 25-month-old boy. Bone marrow biopsy revealed blasts and cells with maturing monocytic features. A final diagnosis of M5b AML was made. The authors reviewed the literature and concluded that leukemia may be the most likely diagnosis in a child with bilateral soft tissue orbital tumors.

Clinical Management

AML is often treated with allogeneic hematopoietic stem-cell transplantation (HSCT), and it is most sensitive to natural killer (NK)-cell reactivity. Venstrom et al. (2012) assessed clinical data, HLA genotyping results, and donor cell lines or genomic DNA for 1,277 patients with AML who had received HSCT from unrelated donors matched for HLA-A, -B, -C, -DR, and -DQ or with a single mismatch. They performed donor KIR genotyping and evaluated the clinical effect of donor KIR genotype and donor and recipient HLA genotypes. Patients with AML who received allografts from donors who were positive for KIR2DS1 (604952) had a lower rate of relapse than those with allografts from donors who were negative for KIR2DS1 (26.5% vs 32.5%; hazard ratio, 0.76; 95% confidence interval, 0.61 to 0.96; P = 0.02). Of allografts from donors with KIR2DS1, those from donors who were homozygous or heterozygous for HLA-C1 antigens could mediate this antileukemic effect, whereas those from donors who were homozygous for HLA-C2 did not provide any advantage. Recipients of KIR2DS1-positive allografts mismatched for a single HLA-C locus had a lower relapse rate than recipients of KIR2DS1-negative allografts with a mismatch at the same locus (17.1% vs 35.6%; hazard ratio, 0.40; 95% CI, 0.20 to 0.78; P = 0.007). KIR3DS1 (see 604946), in positive genetic linkage disequilibrium with KIR2DS1, had no effect on leukemia relapse but was associated with decreased mortality (60.1% vs 66.9% without KIR3DS1; hazard ratio, 0.83; 95% CI, 0.71 to 0.96; P = 0.01). Venstrom et al. (2012) concluded that activating KIR genes from donors were associated with distinct outcomes of allogeneic HSCT for AML. Donor KIR2DS1 appeared to provide protection against relapse in an HLA-C-dependent manner, and donor KIR3DS1 was associated with reduced mortality.

The transcription factor fusion CBFB (121360)-SMMHC (MYH11; 160745), expressed in AML with the chromosome inversion inv(16)(p13q22), outcompetes wildtype CBFB for binding to the transcription factor RUNX1, deregulates RUNX1 activity in hematopoiesis, and induces AML. Treatment of inv(16) AML with nonselective cytotoxic chemotherapy results in a good initial response but limited long-term survival. Illendula et al. (2015) reported the development of a protein-protein interaction inhibitor, AI-10-49, that selectively binds to CBFB-SMMHC and disrupts its binding to RUNX1. AI-10-49 restores RUNX1 transcriptional activity, displays favorable pharmacokinetics, and delays leukemia progression in mice. Treatment of primary inv(16) AML patient blasts with AI-10-49 triggers selective cell death. Illendula et al. (2015) concluded that direct inhibition of the oncogenic CBFB-SMMHC fusion protein may be an effective therapeutic approach for inv(16) AML.

Fong et al. (2015) used primary mouse hematopoietic stem and progenitor cells immortalized with the fusion protein MLL-AF9 (see 159555) to generate several single-cell clones that demonstrated resistance, in vitro and in vivo, to the prototypical bromodomain and extra terminal protein (BET) inhibitor I-BET. Resistance to I-BET conferred cross-resistance to chemically distinct BET inhibitors such as JQ1, as well as resistance to genetic knockdown of BET proteins. Resistance was not mediated through increased drug efflux or metabolism, emerged from leukemia stem cells both ex vivo and in vivo. Chromatin-bound BRD4 (608749) was globally reduced in resistant cells, whereas the expression of key target genes such as Myc (190080) remained unaltered, highlighting the existence of alternative mechanisms to regulate transcription. Fong et al. (2015) demonstrated that resistance to BET inhibitors, in human and mouse leukemia cells, is in part a consequence of increased Wnt/beta-catenin (see 116806) signaling, and negative regulation of this pathway results in restoration of sensitivity to I-BET in vitro and in vivo. Fong et al. (2015) concluded that their findings provided insights into the biology of AML, highlighted potential therapeutic limitations of BET inhibitors, and identified strategies that may enhance the clinical utility of these unique targeted therapies.

Rathert et al. (2015) performed a chromatin-focused RNAi screen in a sensitive MLL-AF9;Nras(G12D)-driven AML mouse model to identify factors involved in primary and acquired BET resistance in leukemia. The screen showed that suppression of the Polycomb repressive complex-2 (PRC2; see 606245), contrary to effects in other contexts, promotes BET inhibitor resistance in AML. PRC2 suppression did not directly affect the regulation of Brd4-dependent transcripts, but facilitated the remodeling of regulatory pathways that restore the transcription of key targets such as Myc. Similarly, while BET inhibition triggered acute MYC repression in human leukemias regardless of their sensitivity, resistant leukemias were uniformly characterized by their ability to rapidly restore MYC transcription. This process involved the activation and recruitment of WNT (see 606359) signaling components, which compensated for the loss of BRD4 and drove resistance in various cancer models. Additional studies revealed that BET-resistant states are characterized by remodeled regulatory landscapes, involving the activation of a focal MYC enhancer that recruits WNT machinery in response to BET inhibition. Rathert et al. (2015) concluded that their results identified and validated WNT signaling as a driver and candidate biomarker of primary and acquired BET resistance in leukemia, and implicated the rewiring of transcriptional programs as an important mechanism promoting resistance to BET inhibitors and, potentially, other chromatin-targeted therapies.

Biochemical Features

Garzon et al. (2009) provided evidence supporting a tumor suppressor role for miR29A (610782) and miR29B (610783) in AML. Overexpression of both microRNAs reduced cell growth and induced apoptosis in AML cell lines. Injection of miR29B in a xenograft mouse model of AML resulted in tumor shrinkage. Northern blot analysis showed that the 2 microRNAs targeted genes involved in apoptosis, the cell cycle, and cell proliferation. Transfection of leukemic cells with miR29A and miR29B resulted in specific downregulation of CXXC6 (TET1; 607790), MCL1 (159552), and CDK6 (603368). Studies of 45 samples from patients with AML showed an inverse correlation between MCL1 and miR29B. Although 42% of the miR29A-correlated genes were also correlated with miR29B, there were some differences: genes related to protein metabolism were found overrepresented in miR29B-correlated genes, and genes related to immune function were overrepresented in miR29A-correlated genes. Finally, there was a downregulation of both miR29A and miR29B in primary AML samples with monosomy 7 (252270).

Pathogenesis

Kode et al. (2014) showed that an activating mutation of beta-catenin (116806) in mouse osteoblasts alters the differentiation potential of myeloid and lymphoid progenitors leading to development of AML with common chromosomal aberrations and cell-autonomous progression. Activated beta-catenin stimulates expression of the Notch (see NOTCH1, 190198) ligand Jag1 (601920) in osteoblasts. Subsequent activation of Notch signaling in hematopoietic stem cell progenitors induces the malignant changes. Genetic or pharmacologic inhibition of Notch signaling ameliorates AML and demonstrates the pathogenic role of the Notch pathway. In 38% of patients with myelodysplastic syndromes (see MDS, 614286) or AML, increased beta-catenin signaling and nuclear accumulation was identified in osteoblasts, and these patients showed increased Notch signaling in hematopoietic cells. Kode et al. (2014) concluded that their findings demonstrated that genetic alterations in osteoblasts can induce acute myeloid leukemia, identify molecular signals leading to this transformation, and suggested a potential novel pharmacotherapeutic approach to acute myeloid leukemia.

Shlush et al. (2014) found recurrent DNMT3A (602769) mutations at high allele frequency in highly purified hematopoietic stem cells (HSCs) as well as progenitor and mature cell fractions from the blood of AML patients, but these cells did not have the coincident NPM1 (164040) mutations present in AML blasts. DNMT3A mutation-bearing HSCs showed a multilineage repopulation advantage over nonmutated HSCs in xenografts, establishing their identity as preleukemic HSCs. Preleukemic HSCs were found in remission samples, indicating that they survive chemotherapy. Shlush et al. (2014) concluded that DNMT3A mutations arise early in AML evolution, probably in HSCs, leading to a clonally expanded pool of preleukemic HSCs from which AML evolves.

Santos et al. (2014) showed that the histone methyltransferase MLL4 (606834), a suppressor of B-cell lymphoma, is required for stem cell activity and an aggressive form of AML harboring the MLL-AF9 oncogene. Deletion of MLL4 enhances myelopoiesis and myeloid differentiation of leukemic blasts, which protects mice from death related to AML. MLL4 exerts its function by regulating transcriptional programs associated with the antioxidant response. Addition of reactive oxygen species scavengers or ectopic expression of FOXO3 (602681) protects MLL4-null MLL-AF9 cells from DNA damage and inhibits myeloid maturation. Similar to MLL4 deficiency, loss of ATM (607585) or BRCA1 (113705) sensitizes transformed cells to differentiation, suggesting that myeloid differentiation is promoted by loss of genome integrity. Santos et al. (2014) showed that restriction enzyme-induced double-strand breaks are sufficient to induce differentiation of MLL-AF9 blasts, which requires cyclin-dependent kinase inhibitor p21 (CDKN1A; 116899) activity. The authors concluded that they had uncovered an unexpected tumor-promoting role of genome guardians in enforcing the oncogene-induced differentiation blockade in AML.

By performing high-resolution proteomic analysis of human AML stem cell and non-stem cell populations, Raffel et al. (2017) found the branched-chain amino acid (BCAA) pathway enriched and BCAT1 (113520) protein and transcripts overexpressed in leukemia stem cells. Raffel et al. (2017) showed that BCAT1, which transfers alpha-amino groups from BCAAs to alpha-ketoglutarate, is a critical regulator of intracellular alpha-ketoglutarate homeostasis. Further to its role in the tricarboxylic acid cycle, alpha-ketoglutarate is an essential cofactor for alpha-ketoglutarate-dependent dioxygenases such as EGLN1 (606425) and the ten-eleven translocation (TET) family of DNA demethylases. Knockdown of BCAT1 in leukemia cells caused accumulation of alpha-ketoglutarate, leading to EGLN1-mediated HIF1-alpha (603348) protein degradation. This resulted in a growth and survival defect and abrogated leukemia-initiating potential. By contrast, overexpression of BCAT1 in leukemia cells decreased intracellular alpha-ketoglutarate levels and caused DNA hypermethylation through altered TET activity. AML with high levels of BCAT1 (BCAT1-high) displayed a DNA hypermethylation phenotype similar to cases carrying a mutant isocitrate dehydrogenase (see IDH1, 147700) (IDH-mut), in which TET2 (612839) is inhibited by the oncometabolite 2-hydroxyglutarate. High levels of BCAT1 strongly correlated with shorter overall survival in IDH-wildtype-TET2-wildtype, but not IDH-mut or TET2-mut, AML. BCAT1-high AML showed robust enrichment for leukemia stem cell signatures, and paired sample analysis showed a significant increase in BCAT1 levels upon disease relapse. In summary, by limiting intracellular alpha-ketoglutarate, BCAT1 links BCAA catabolism to HIF1-alpha stability and regulation of the epigenomic landscape, mimicking the effects of IDH mutations.

Abelson et al. (2018) used deep sequencing to analyze genes that are recurrently mutated in AML to distinguish between individuals who have a high risk of developing AML and those with benign age-related clonal hematopoiesis. They analyzed peripheral blood cells from 95 individuals that were obtained on average 6.3 years before AML diagnosis (pre-AML group), together with 414 unselected age- and gender-matched individuals (control group). Pre-AML cases were distinct from controls and had more mutations per sample, higher variant allele frequencies, indicating greater clonal expansion, and showed enrichment of mutations in specific genes. Genetic parameters were used to derive a model that accurately predicted AML-free survival; this model was validated in an independent cohort of 29 pre-AML cases and 262 controls. Abelson et al. (2018) developed an AML predictive model using a large electronic health record database that identified individuals at greater risk. The authors concluded that their findings provided proof of concept that it is possible to discriminate age-related clonal hematopoiesis from pre-AML many years before malignant transformation.

Cytogenetics

Loss of chromosome 5q is observed in 10 to 15% of patients with myelodysplastic syndrome (MDS) or acute myeloid leukemia and in 40% of patients with therapy-related MDS or AML. In addition, patients with 5q deletion syndrome (153550) show hematologic abnormalities, including refractory anemia and abnormal megakaryocytes. By cytogenetic analysis and hybridization techniques, Le Beau et al. (1993) identified a common 2.8-Mb critical region containing the EGR1 gene (128990) on chromosome 5q31 that was deleted in 135 patients with hematologic abnormalities and 5q deletions, including 85 patients with de novo MDS or AML, 33 with therapy-related MDS or AML, and 17 with MDS and the 5q deletion syndrome. Le Beau et al. (1993) postulated that EGR1 or another closely-linked gene may act as a tumor suppressor gene.

Baozhang et al. (1999) reported a family with 7 cases of related leukemias among 22 members in 3 consecutive generations consistent with autosomal dominant inheritance. One of the patients and her father were found to have rearrangement and a rearrangement/amplification, respectively, of the ERBB oncogene (131550).

Horwitz et al. (1996) reported evidence of anticipation in familial acute myelogenous leukemia. Horwitz et al. (1996) further studied those pedigrees and others from the literature. In 49 affected individuals from 9 families transmitting autosomal dominant AML, the mean age of onset was 57 years in the grandparental generation, 32 years in the parental generation, and 13 years in the youngest generation (p less than 0.001). Horwitz et al. (1996) also reported evidence of anticipation in autosomal dominant chronic lymphocytic leukemia (CLL; 151400) (p = 0.008). In 18 affected individuals from 7 pedigrees with autosomal dominant CLL, the mean age of onset in the parental generation was 66 years, versus 51 years in the younger generation. Based on this evidence of anticipation, Horwitz et al. (1996) suggested that dynamic mutations of unstable DNA sequence repeats could be a common mechanism of inherited hematopoietic malignancy. They proposed 3 possible candidate chromosomal regions for familial leukemia with anticipation: 21q22.1-22.2, 11q23.3 in the vicinity of the CBL2 gene (165360), and 16q22 in the vicinity of the CBFB gene (121360).

Mapping

Horwitz et al. (1997) presented evidence suggesting that there is a locus for acute myelogenous leukemia on chromosome 16q22. They studied a family with 11 relevant meioses transmitting autosomal dominant AML and myelodysplasia. They excluded linkage to 21q22.1-q22.2 and to 9p22-p21, and found a maximum 2-point lod score of 2.82 with the microsatellite marker D16S522 at recombination fraction theta = 0.0. Haplotype analysis showed a 23.5-cM region of 16q22 that was inherited in common by all affected family members and extended from D16S451 to D16S289. Nonparametric linkage analysis gave a p value of 0.00098 for the conditional probability of linkage. Mutation analysis excluded expansion of the AT-rich minisatellite repeat FRA16B fragile site and the CAG trinucleotide repeat in the E2F-4 transcription factor (600659). The 'repeat expansion detection' method, capable of detecting dynamic mutation associated with anticipation, more generally excluded large CAG repeat expansion as a cause of leukemia in this family.

Molecular Genetics

Mutations in CEBPA

In affected members of a family with acute myeloid leukemia, Smith et al. (2004) identified a germline 1-bp deletion (212delC; 116897.0007) in the CEBPA gene. Overt leukemia developed in the father at age 10 years, in the first-born son at age 30 years, and in the last-born daughter at age 18 years.

Mutations in GATA2

Hahn et al. (2011) analyzed 50 candidate genes in 5 families with a predisposition to myelodysplastic syndrome (614286) and acute myeloid leukemia, and in 3 of the families they identified a heritable heterozygous missense mutation in the GATA2 gene (T354M; 137295.0002) that segregated with disease and was not found in 695 nonleukemic ethnically matched controls.

Mutations in TERT

Calado et al. (2009) found a significantly increased number of germline mutations in the TERT gene in patients with sporadic acute myeloid leukemia compared to controls. One mutation in particular, A1062T (187270.0022), was 3-fold higher among 594 AML patients compared to 1,110 controls (p = 0.0009). In vitro studies showed that the mutations caused haploinsufficiency of telomerase activity. An abnormal karyotype was found in 18 of 21 patients with TERT mutations who were tested. Calado et al. (2009) suggested that telomere attrition may promote genomic instability and DNA damage, which may contribute to the development of leukemia.

Somatic Mutations in NPM1

NPM, a nucleocytoplasmic shuttling protein with prominent nucleolar localization, regulates the ARF (103180)/p53 (191170) tumor suppressor pathway. Chromosomal translocations involving the NPM gene cause cytoplasmic dislocation of the NPM protein. Falini et al. (2005) used immunohistochemical methods to study the subcellular localization of NPM in bone marrow biopsy specimens from 591 patients with primary AML. They then correlated the presence of cytoplasmic NPM with clinical and biologic features of the disease. Cytoplasmic NPM was detected in 35.2% of the 591 specimens from patients with primary AML but not in 135 secondary AML (sAML) specimens or in 980 hematopoietic or extrahematopoietic neoplasms other than AML. It was associated with a wide spectrum of morphologic subtypes of the disease, a normal karyotype, and responsiveness to induction chemotherapy, but not with recurrent genetic abnormalities. There was a high frequency of internal tandem duplications of FLT3 (136351) and absence of CD34 (142230) and CD133 (604365) in AML specimens with a normal karyotype and cytoplasmic dislocation of NPM, but not in those in which the protein was restricted to the nucleus. AML specimens with cytoplasmic NPM carried mutations in the NPM gene (see 164040.0001-164040.0004); this mutant gene caused cytoplasmic localization of NPM in transfected cells. All 6 NPM mutant proteins showed mutations in at least 1 of the tryptophan residues at positions 288 and 290 and shared the same last 5 amino acid residues (VSLRK). Thus, despite genetic heterogeneity, all NPM gene mutations resulted in a distinct sequence in the NPM protein C terminus. Falini et al. (2005) concluded that cytoplasmic NPM is a characteristic feature of a large subgroup of patients with AML who have a normal karyotype, NPM gene mutations, and responsiveness to induction chemotherapy. Grisendi and Pandolfi (2005) noted that NPM staining in cases of AML with aberrant cytoplasmic localization of the protein is mostly cytoplasmic, which suggests that the mutant NPM acts dominantly on the product of the remaining wildtype allele, causing its retention in the cytoplasm by heterodimerization.

By microRNA (miRNA) expression profiling, Garzon et al. (2008) identified 36 upregulated and 21 downregulated miRNAs in AML patients with NPM1 mutations compared with AML patients without NPM1 mutations. miR10A (MIRN10A; 610173) and miR10B (MIRN10B; 611576) showed the greatest upregulation, with increases of 20- and 16.67-fold, respectively. Mir22 (MIRN22; 612077) showed greatest downregulation, with a reduction of 0.31-fold. Garzon et al. (2008) concluded that AML with NPM1 mutations has a distinctive miRNA signature.

Ivey et al. (2016) used quantitative RT-PCR assays to detect minimal residual disease in 2,569 samples obtained from 346 patients with NPM1-mutated AML who had undergone intensive treatment in the National Cancer Research Institute AML17 trial. The authors used a custom 51-gene panel to perform targeted sequencing of 223 samples obtained at the time of diagnosis and 49 samples obtained at the time of relapse. Mutations associated with preleukemic clones were tracked by means of digital polymerase chain reaction. Molecular profiling highlighted the complexity of NPM1-mutated AML, with segregation of patients into more than 150 subgroups, thus precluding reliable outcome prediction. The determination of minimal residual disease status was more informative. Persistence of NPM1-mutated transcripts in blood was present in 15% of the patients after the second chemotherapy cycle and was associated with a greater risk of relapse after 3 years of follow-up than was an absence of such transcripts (82% vs 30%; hazard ratio 4.80; 95% CI 2.95-7.80; p less than 0.001) and a lower rate of survival (24% vs 75%; hazard ratio for death, 4.38; 95% CI 2.57-7.47; p less than 0.001). The presence of minimal residual disease was the only independent prognostic factor for death in multivariate analysis (hazard ratio, 4.84; 95% CI 2.57 to 9.15; p less than 0.001). These results were validated in an independent cohort. On sequential monitoring of minimal residual disease, relapse was reliably predicted by a rising level of NPM1-mutated transcripts. Although mutations associated with preleukemic clones remained detectable during ongoing remission after chemotherapy, NPM1 mutations were detected in 69 of 70 patients at the time of relapse and provided a better marker of disease status.

Other Somatic Mutations

In the bone marrow of a 4-year-old child with AML, Bollag et al. (1996) identified an insertion in the KRAS2 gene (190070.0008). Expression studies showed that the mutant KRAS2 protein caused cellular transformation and activated the RAS-mitogen-activated protein kinase signaling pathway.

Bone marrow minimal residual disease causes relapse after chemotherapy in patients with acute myelogenous leukemia. Matsunaga et al. (2003) postulated that the drug resistance is induced by the attachment of very late antigen-4 (VLA4; see 192975) on leukemic cells to fibronectin (135600) on bone marrow stromal cells. Matsunaga et al. (2003) found that VLA4-positive cells acquired resistance to anoikis (loss of anchorage) or drug-induced apoptosis through the phosphatidylinositol-3-kinase (see 601232)/AKT (164730)/Bcl2 (151430) signaling pathway, which is activated by the interaction of VLA4 and fibronectin. This resistance was negated by VLA4-specific antibodies. In a mouse model of minimal residual disease, Matsunaga et al. (2003) achieved a 100% survival rate by combining VLA4-specific antibodies and cytosine arabinoside, whereas cytosine arabinoside alone prolonged survival only slightly. In addition, overall survival at 5 years was 100% for 10 VLA4-negative patients and 44.4% for 15 VLA4-positive patients. Thus, Matsunaga et al. (2003) concluded that the interaction between VLA4 on leukemic cells and fibronectin on stromal cells may be crucial in bone marrow minimal residual disease and AML prognosis.

Barjesteh van Waalwijk van Doorn-Khosrovani et al. (2005) analyzed 300 patients newly diagnosed with AML for mutations in the coding region of the ETV6 gene and identified 5 somatic heterozygous mutations (e.g., 600618.0001 and 600618.0002). These ETV6 mutant proteins were unable to repress transcription and showed dominant-negative effects. The authors also examined ETV6 protein expression in 77 patients with AML and found that 24 (31%) lacked the wildtype 57- and 50-kD proteins; there was no correlation between ETV6 mRNA transcript levels and the loss of ETV6 protein, suggesting posttranscriptional regulation of ETV6.

Lee et al. (2006) identified heterozygosity for mutations in the JAK2 gene (147796.0001 and 147796.0002) in bone marrow aspirates from 3 (2.7%) of 113 unrelated patients with AML.

Delhommeau et al. (2009) analyzed the TET2 gene (612839) in bone marrow cells from 320 patients with myeloid cancers and identified TET2 defects in 2 patients with primary AML and 5 patients with secondary AML.

Mardis et al. (2009) used massively parallel DNA sequencing to obtain a very high level of coverage of a primary, cytogenetically normal, de novo genome for AML with minimal maturation (AML-M1) and a matched normal skin genome. Mardis et al. (2009) identified 12 somatic mutations within the coding sequences of genes and 52 somatic point mutations in conserved or regulatory portions of the genome. All mutations appeared to be heterozygous and present in nearly all cells in the tumor sample. Four of the 64 mutations occurred in at least 1 additional AML sample in 188 samples that were tested. Mutations in NRAS (164790) and NPM1 (164040) had been previously identified in patients with AML, but 2 other mutations had not been identified. One of these mutations, in the IDH1 (147700) gene, was present in 15 of 187 additional AML genomes tested and was strongly associated with normal cytogenetic status; it was present in 13 of 80 cytogenetically normal samples (16%). The other was a nongenic mutation in a genomic region with regulatory potential and conservation in higher mammals; it is at position 108,115,590 of chromosome 10. The AML genome that was sequenced contained approximately 750 point mutations, of which only a small fraction are likely to be relevant to pathogenesis.

Gelsi-Boyer et al. (2009) presented evidence that the ASXL1 gene (612990) may act as a tumor suppressor in myeloid malignancies. They identified heterozygous somatic mutations in the ASXL1 gene in 5 (16%) of 38 myelodysplastic syndrome/acute myeloid leukemia samples. Somatic ASXL1 mutations were also found in 19 (43%) of 44 chronic myelomonocytic leukemia (CMML; see 607785) samples. All the mutations were in exon 12 and resulted in truncation of the C-terminal PHD finger of the protein. The findings suggested that regulators of gene expression via DNA methylation, histone modification, and chromatin remodeling could be altered in myelodysplastic syndromes and some leukemias. The same group (Carbuccia et al., 2009) identified heterozygous somatic truncating ASXL1 mutations in 5 (7.8%) of 64 myeloproliferative neoplasms, including 1 essential thrombocythemia (187950), 3 primary myelofibrosis (254450), and 1 AML.

Harutyunyan et al. (2011) analyzed biopsy specimens of myeloproliferative neoplastic tissue from 330 patients for chromosomal aberrations associated with leukemic transformation. Three hundred and eight of the patients had chronic-phase myeloproliferative neoplasms and 22 had postmyeloproliferative-phase neoplasm secondary acute myeloid leukemia. Among those 22 patients, 1 carried the MPL W515L mutation and all others carried the JAK2 V617F mutation. Six of the 22 patients carried somatic mutations of TP53 (191170). Three of the patients had independent mutations on both TP53 alleles, and 2 had homozygous mutations because of an acquired uniparental disomy of chromosome 17p. None of the patients with TP53 mutations had amplification of chromosome 1q involving the MDM4 gene (604704). Harutyunyan et al. (2011) concluded that TP53 mutations are strongly associated with transformation to AML in patients with myeloproliferative neoplasms (p = 0.003). Harutyunyan et al. (2011) also found amplification of a region of chromosome 1q harboring the MDM4 gene in 18.18% of patients with secondary AML (p less than 0.001).

Ding et al. (2012) determined the mutational spectrum associated with relapse of AML by sequencing the primary tumor and relapse genomes from 8 AML patients, and validated hundreds of somatic mutations using deep sequencing. This method allowed them to define clonality and clonal evolution patterns precisely at relapse. In addition to discovering novel, recurrently mutated genes (e.g., WAC; SMC3, 606062; DIS3, 607533; DDX41, 608170; and DAXX, 603186) in AML, Ding et al. (2012) identified 2 major clonal evolution patterns during AML relapse: (1) the founding clone in the primary tumor gained mutations and evolved into the relapse clone, or (2) a subclone of the founding clone survived initial therapy, gained additional mutations, and expanded at relapse. In all cases, chemotherapy failed to eradicate the founding clone. The comparison of relapse-specific versus primary tumor mutations in all 8 cases revealed an increase in transversions, probably due to DNA damage caused by cytotoxic chemotherapy. Ding et al. (2012) concluded that AML relapse is associated with the addition of new mutations and clonal evolution, which is shaped, in part, by the chemotherapy that the patients receive to establish and maintain remissions.

The Cancer Genome Atlas Research Network (2013) analyzed the genomes of 200 clinically annotated adult cases of de novo AML, using either whole-genome sequencing (50 cases) or whole-exome sequencing (150 cases), along with RNA and microRNA sequencing and DNA methylation analysis. A total of 23 genes were significantly mutated, and another 237 were mutated in 2 or more samples. Nearly all samples had at least 1 nonsynonymous mutation in 1 of 9 categories of genes that were deemed relevant for pathogenesis. The authors identified recurrent mutations in the NPM1 gene in 54/200 (27%) samples, in the FLT3 gene (136351) in 56/200 (28%) samples, in the DNMT3A gene (602769) in 51/200 (26%) samples, and in the IDH1 or IDH2 (147650) genes in 39/200 (20%) samples.

Brewin et al. (2013) noted that the study of the Cancer Genome Atlas Research Network (2013) did not reveal which mutations occurred in the founding clone, as would be expected for an initiator of disease, and which occurred in minor clones, which subsequently drive disease. Miller et al. (2013) responded that genes mutated almost exclusively in founding clones in their study included RUNX1 (151385) (9 of 9 mutations in founding clones), NPM1 (164040) (3 of 3 clones), U2AF1 (191317) (5 of 5 clones), DNMT3A (38 of 40 clones), IDH2 (13 of 14), IDH1 (147700) (15 of 17 clones), and KIT (164920) (5 of 6). In contrast, mutations in NRAS, TET2 (612839), CEBPA, WT1 (607102), PTPN11 (176876), and FLT3 were often found in subclones, suggesting that they were often cooperating mutations.

Therapy-Related Acute Myeloid Leukemia

Wong et al. (2015) sequenced the genomes of 22 patients with therapy-related AML (t-AML) and showed that the total number of somatic single-nucleotide variants and the percentage of chemotherapy-related transversions are similar in t-AML and de novo AML, indicating that previous chemotherapy does not induce genomewide DNA damage. Wong et al. (2015) identified 4 cases of t-AML/t-MDS in which the exact TP53 mutation found at diagnosis was also present at low frequencies (0.003-0.7%) in mobilized blood leukocytes or bone marrow 3 to 6 years before the development of t-AML/t-MDS, including 2 cases in which the relevant TP53 mutation was detected before any chemotherapy. Moreover, functional TP53 mutations were identified in small populations of peripheral blood cells of healthy chemotherapy-naive elderly individuals. Finally, in mouse bone marrow chimeras containing both wildtype and Tp53 +/- hematopoietic stem/progenitor cells (HSPCs), the Tp53 +/- HSPCs preferentially expanded after exposure to chemotherapy. Wong et al. (2015) concluded that these data suggested that cytotoxic therapy does not directly induce TP53 mutations. Rather, they supported a model in which rare HSPCs carrying age-related TP53 mutations are resistant to chemotherapy and expand preferentially after treatment. The early acquisition of TP53 mutations in the founding HSPC clone probably contributes to the frequent cytogenetic abnormalities and poor responses to chemotherapy that are typical of patients with t-AML/t-MDS.

Genotype/Phenotype Correlations

Schlenk et al. (2008) studied 872 patients younger than 60 years of age with cytogenetically normal AML and compared mutation status of the NPM1 (164040), FLT3 (136351), CEBPA (116897), MLL (159555), and NRAS (164790) genes in leukemia cells with clinical outcome. There was an overall complete remission rate of 77%. The genotype of mutant NPM1 without FLT3 internal tandem duplications (FLT3-ITD), the mutant CEBPA genotype, and younger age were each significantly associated with complete remission. The authors also found that the benefit of postremission hematopoietic stem cell transplant was limited to the subgroup of patients with the prognostically adverse genotype FLT3-ITD or the genotype consisting of wildtype NPM1 and CEBPA without FLT3-ITD.

Gale et al. (2008) found that 354 (26%) of 1,425 patients with AML had the FLT3 internal duplication. The median total mutant level for all patients was 35% of total FLT3, but there was wide variation with levels ranging from 1 to 96%. There was a significant correlation between worse overall survival, relapse risk, and increased white blood cell count with increased mutant level, but the size of the duplication and the number of mutations had no significant impact on outcome. Those patients with the FLT3 duplication had a worse risk of relapse than patients without the FLT3 duplication. Among a subset of 1,217 patients, 503 (41%) had a mutation in the NPM1 gene (164040), and 208 (17%) had mutations in both genes. The presence of an NPM1 mutation had a beneficial effect on the remission rate, most likely due to a lower rate of resistant disease, both in patients with and without FLT3 duplications. Gale et al. (2008) identified 3 prognostic groups among AML patients: good in those with only a NPM1 mutation; intermediate in those with either no FLT3 or NPM1 mutations or mutations in both genes; and poor in those with only FLT3 mutations.

Boissel et al. (2011) reviewed the work of several others and performed their own analysis of 205 patients with cytogenetically normal AML, and found that patients with IDH2(R172) mutations had a worse prognosis from those with IDH2(R140) mutations (e.g., 147650.0001). That patients with IDH2(R172) mutations had an unfavorable prognosis by comparison had been noted by Marcucci et al. (2010). The frequency of IDH2(R172) mutations was lower than that of IDH2(R140) mutations among cytogenetically normal AML patients. Boissel et al. (2011) cautioned that patients should be separated by mutation status for prognostic analysis.

Activating internal tandem duplication (ITD) mutations in FLT3 (FLT3-ITD) are detected in approximately 20% of acute myeloid leukemia patients and are associated with a poor prognosis. Abundant laboratory and clinical evidence, including the lack of convincing clinical activity of early FLT3 inhibitors, suggested that FLT3-ITD probably represents a passenger lesion. Smith et al. (2012) reported point mutations at 3 residues within the kinase domain of FLT3-ITD that confer substantial in vitro resistance to AC220 (quizartinib), an active investigational inhibitor of FLT3, KIT (164920), PDGFRA (173490), PDGFRB (173410), and RET (164761); evolution of AC220-resistant substitutions at 2 of these amino acids was observed in 8 of 8 FLT3-ITD-positive AML patients with acquired resistance to AC220. Smith et al. (2012) concluded that their findings demonstrated that FLT3-ITD can represent a driver lesion and valid therapeutic target in human AML.

Animal Model

Jin et al. (2006) found that treatment with activating monoclonal antibodies to CD44 (107269) markedly reduced leukemic repopulation in nonobese diabetic (NOD)/severe combined immunodeficiency (SCID) mice challenged with human AML cells. Absence of leukemia following serial tumor transplantation experiments in mice demonstrated direct targeting of AML leukemic stem cells (LSCs). Treatment of engrafted mice with anti-CD44 reduced the number of Cd34 (142230)-positive/Cd38 (107270)-negative primitive stem cells and increased the number of Cd14 (158120)-positive monocytic cells. Anti-CD44 treatment also diminished the homing capacity of SCID leukemia-initiating cells to bone marrow and spleen. Jin et al. (2006) concluded that CD44 is a key regulator of AML LSCs, which require a niche to maintain their stem cell properties. They suggested that CD44 targeting may help eliminate quiescent AML LSCs.

Mullican et al. (2007) generated Nr4a1 (139139)/Nr4a3 (600542) double-null mice and observed the development of rapidly lethal acute myeloid leukemia involving abnormal expansion of hematopoietic stem cells and myeloid progenitors, decreased expression of JunB (165161) and c-Jun (165160), and defective extrinsic apoptotic signaling (FASL, 134638; TRAIL, 603598). Leukemic blast cells from 46 AML patients with a variety of cytogenetic abnormalities all showed downregulation of NR4A1 and NR4A3 compared to CD34+ cells from normal controls, suggesting that epigenetic silencing of these receptors may be an obligate event in human AML development.