Genetics of HR AML
HR AML represents an extremely complex subgroup of adult AML, characterized by a variety of well-defined cytogenetic and/or genetic lesions, which contribute to the aggressive course of the disease and its intrinsic resistance to standard chemotherapeutic approaches. In this section, we provide an overview of the current biological knowledge for each specific genetic entity of HR-AML, according to the European LeukemiaNet classification [
1] (
Table 1).
t(6;9)(p23.3;q34.1)DEK::NUP214. NUP214 is a nucleoporin that binds to the cytoplasmic side of the nuclear pore complex (NPC), that is critical for nucleo-cytoplasmic transport of proteins and mRNA. Defective nuclear export derived from
DEK-NUP214 fusion induces the nuclear retention of transcription factors (TFs) that induce sustained
HOX gene upregulation.[
2] DEK is a chromatin-associated protein critical for the maintenance of chromatin stability.
t(v;11q23.3)KMT2A-rearranged. Acute leukemias carrying
KMT2A (MLL) translocations represent 5-10% of acute leukemia in all age, and up to 70% of infantile leukemia.[
8]
KMT2A fusion supports leukemogenesis by recruiting the superelongation complex (SEC), the histone H3K79 methyltransferase DOT1L and menin (MEN1), to induce the overexpression of AML TFs such as HOXA9, MEIS1 and MEF2C. [
9]
KMT2A-rearranged leukemias are featured by promiscuous expression of lineage markers and a propensity for lineage switching. [
10,
11]
t(9;22)(q34.1;q11.2)BCR::ABL1 (BCR-ABL+). This category comprises a subset of
de novo AML developed in patients without a history of chronic myeloid leukemia (CML) and lacking recurrent genetic aberrations affecting CEBPA or NPM1 genes, or cytogenetic alterations such as inv(16) or inv(3). Distinguishing BCR-ABL+ AML from a myeloid blast crisis of CML poses challenges. Unique to BCR-ABL+ AML are the loss of IKZF1 and CDKN2A, along with cryptic deletions in IGH and TRG genes, features not observed in myeloid blast crisis of CML. [
12] AML blasts in this category often aberrantly express CD19, CD7 and TdT. Although BCR-ABL+ AML generally falls under the adverse-risk category, it should be noted that cases associated with inv(16) or
NPM1 mutations may have favorable outcomes. [
13,
14,
15]
t(8;16)(p11.2;p13.3)KAT6A::CREBBP. It is a rare subset, representing 0.2 to 0.4% of all AML cases. CREBBP alterations in
de novo AML have been reported to be associated with poor prognosis. [
16] KAT6A, also known as MOZ or MYST3, encodes the monocytic leukemia zinc finger protein, a histone acetyltransferase of the MYST family that regulates gene transcription by activating RUNX1 transcription factor complex. CREBBP plays a critical role in transcription regulation. Similar to KAT6A, CREBBP has an intrinsic histone acetyltransferase activity.
EVI1-rearranged.GATA2, MECOM(EVI1) AML is characterized by the reposition of a distal GATA2 enhancer that activates
MECOM expression leading to GATA2 haploinsufficiency. About 20% of AML with inv(3)/t(3;3) harbor mutations in RUNX1, while around 25% exhibit mutations in IKZF1. Additionally, a subset of these AML cases presents with activating mutations in the RAS GTPase family member (NRAS or KRAS) or other signaling pathway proteins, such as PTPN11 and NF1, contributing to RAS signaling dysregulation and promoting AML cell proliferation. About 20% of patients have mutations in the polycomb protein ASXL1, and 30-60% has mutations in the spliceosomal machinery components, such as SF3B1 and U2AF1.
TP53 mutations are found in approximately 25% of cases.[
17] Other mutations, albeit less frequently observed, occur in DNMT3, TET2 and IDH1/2 genes.[
18]
EVI1r AML often presents with monolobated megakaryocytes, multilineage dysplasia and normal/elevated blood platelet counts. [
19]
-5 or del(5q); -7; -17/abn(17p). These abnormalities are commonly observed in AML patients, previously treated with chemotherapy, including alkylating agents, platinum-based agents or antimetabolites. 5q deletion is typically large, involving ∼70 Mb of 5q14-q33 chromosome. This region includes haploinsufficent genes like
RPS14 (ribosomal protein S14) and
APC (adenomatous polyposis coli), microRNA genes (mir-145 and mir-146A) which are implicated in megakaryocytic dysplasia, as well as genes controlling hematopoietic stem cell expansion, such as
EGR1 and
CSNK1A1. [
20] Monosomy 7, the most common autosomal monosomy in AML, and frequently seen in therapy-related AML.[
20], can be also found in congenital diseases predisposing to myeloid neoplasms, such as those bearing germline GATA2 mutations, or affected by neurofibromatosis, and severe congenital neutropenia.[
21] The tumor suppressor genes located in chromosome 7 are believed to act in a haploinsufficient manner, and include
SAMD9/SAMD9L endosomal proteins,
EZH2 histone modifying enzyme and MLL3, that is associated with
Ras pathway mutations and
TP53 inactivation.[
21] 17p deletion or monosomy commonly involves the tumor suppressor gene p53 on band 17p13.1.
Complex karyotype (CK). CK is defined by the presence of ≥ 3 chromosomal abnormalities in the absence of specific recurring translocations or inversions included in the WHO classification, [
22] such as t(8;21), inv(16) or t(16;16), t(9;11), t(v;11)(v;q23.3), t(6;9), inv(3) or t(3,3). [
23] This subtype accounts for 10-12% of adult AML cases, with the most common chromosomal losses being 5q (80% of cases), 7q and 17p chromosomes. [
24] More recently, CK AML has been proposed to be further subclassified into
typical CK-defined by the presence of 5q, 7q abnormalities and/or 17p loss- and
atypical CK, which lacks these specific chromosomal abnormalities. Typical CK AML, often associated with
TP53 mutations (in 80% of cases), have very poor prognosis.[
24] In contrast, [
25]patients with
atypical CK AML, who are generally younger, frequently have mutations in
PHF6, FLT3-TKD, MED12 and NPM1, and tend to achieve a longer overall survival compared to those with typical CK AML. [
24]
Monosomal karyotype (MK). MK is defined by the presence of ≥2 distinct autosomal monosomies or a single autosomal monosomy accompanied by structural abnormalities (deletions of -X or -Y are not considered monosomies).[
26] MK AML occurs more frequently in therapy-related cases compared to
de novo AML, and is closely associated with alterations in the
TP53 gene, leading to significant chromosomal instability.[
27] The most common chromosomal alterations include monosomy 7 (∼35%), monosomy 5 (∼22%) -17 (∼11%). [
27]
Mutated RUNX1.
RUNX1 mutations typically affect the Rnt Homology Domain (RHD) or the Transactivation Domain (TAD) of the gene (located at 21q22), and encodes the alpha subunit of the Core Binding Factor (CBF). Given the association of RUNX1 mutations with autosomal dominant thrombocytopenia, it is advisable to screen for germline mutations among family members to rule out this hereditary condition.
RUNX1-mutated AML is predominantly observed in older male patients. It may be preceded by Fanconi anemia or congenital neutropenia. A prior history of myelodysplastic syndrome or prior exposure to radiation can be present. There is frequent association with
MLL-PTD or
ASXL1 mutations,[
28,
29] indicating a complex genetic landscape that influences disease progression and treatment response.
Mutated EZH2. Enhancer of Zeste Homolog 2 (EZH2) is a key component of the polycomb group (PcG) proteins, which are crucial for gene silencing via histone modifications. [
30] EZH2 composed the regulatory hub of PRC2, that functions as a histone H3 lysine 27 methyltransferase. [
30] Unlike its role in clonal haematopoiesis (CH), where EZH2 mutations are not typically implicated, these mutations are more commonly associated with the development of overt leukemia. [
31] EZH2 mutations could be initiating event or occur later on during leukemogenesis to drive clonal expansions. [
31] The prevalence of EZH2 mutations in
de novo AML ranges from 1-4% of patients. [
32,
33,
34] The
EZH2 gene is located at 7q36.1, a genomic region that is often deleted in AML (-7 or del7q), and associated with an adverse prognosis. In AML,
EZH2 frequently undergoes nonsense and frameshift mutations leading to its inactivation. Notably, mutations in the Serine and Arginine Rich Splicing Factor 2 (
SRSF2), which is a high-risk genomic entity,[
1] could affect EZH2 expression by modifying sequence-specific RNA binding activity of EZH2. This in turn alters the recognition of splicing enhancer motifs, leading to aberrant EZH2 splicing and nonsense mediated decay and decreased the expression of EZH2, thereby influencing H3K27me3 levels. Furthermore, mutations in ASXL1 gene, another polycomb-related protein mutated in HR-AML [
33] also decrease H3K27me3 levels by impairing PRC2 recruitment. This mechanisms contributes to the activation of
HOXA9-driven leukemogenesis.[
35] In myeloid neoplasms,
EZH2 mutations tend to be mutually exclusive with
SRSF2 and
U2AF1 mutations,[
36] while it is more frequently co-mutated with
ASXL1 and
TET2. [
36,
37]
Mutated ASXL1.
Additional sex combs-like 1 (
ASXL1) is a critical epigenetic modifier, whose mutations are commonly identified in CH. [
38,
39,
40] In murine models, ASXL1 knockdown leads to a myelodysplastic-like phenotype, primarily due to the loss of interaction with PRC2. [
35,
41,
42,
43] In myeloid neoplasms, the majority of ASXL1 mutations consist of frameshift or nonsense mutations at the exon 12. These mutations are mutually exclusive with
DNMT3A,
FLT3-ITD, and
NPM1 mutations, while ASXL1 mutations frequently co-occur with mutations in DNA methylation genes (such as
TET2, IDH1-2), spliceosomes (
U2AF1, SRSF2), transcription factors (
CEBPA, RUNX1, GATA2), signal transducers (
NRAS, JAK2, STAG2).[
44] In AML, the frequency of ASXL1 mutations is about 5-10%, [
33,
45] with a higher prevalence in older patients and those with secondary AML.
RUNX1 is the most frequent co-mutated gene and cooperates with mutant ASXL1 to support myeloid leukemogenesis
in vivo.[
46]
Mutated BCOR. The
BCL6 corepressor (
BCOR) is a tumor suppressor gene, that is dysfunctional in lymphoid and myeloid tumors. [
47] BCOR is a critical component of the noncanonical PRC1.1, that is recruited to specific chromatin regions in a context specific manner.[
47] Mutations of BCOR are detected in about 5% of adult
de novo AML and 4% of AML with myelodysplasia-related changes. [
33,
48] Frequency of
BCOR mutations is even higher in secondary AML. [
49] Most commonly, patients with BCOR-mutated AML carries a normal karyotype (NK). In AML with NK, about 45% of
BCOR-mutated AML have co-mutations with
DNMT3A and/or
RUNX1, while are mutually exclusive with
NPM1 and
FLT3 mutations. [
50,
51] Patients with
BCOR mutations usually have activated RAS signaling, due to high rate of
NRAS and
KRAS mutations. [
47]
In vivo,
BCOR leads to overt acute leukemia in the presence of co-mutations, such as
DNMT3A[
51] or
RAS mutations. [
52]
Spliceosome mutations (SRSF2, SF3B1, U2AF1, ZRSR2). The most commonly mutated genes in this category are splicing factor 3B subunit 1 (
SF3B1), serine and arginine rich splicing factor 2 (
SRSF2), U2 small nuclear RNA auxiliary factor 1 (
U2AF1) and zinc finger, CCCH type, RNA-binding motif and serine and arginine rich 2 (
ZRSR2),[
33] which are implicated in the early assembly of the spliceosome machinery. [
53] Mutations in splicing factors (SFmut) are predominantly early events in leukemogenesis. [
54] Mutations in splicing factors accounts for about 18% of adult AML, [
33] are more frequent in older age, and commonly associate with multilineage dysplasia. [
55] While mutations of
SF3B1, SRSF2 and
U2AF1 are gain-of-function, determining a change of amino acid residues,[
56] mutations of
ZRSR2 are inactivating nonsense or frameshift. [
56] Mutations in SF are always heterozygous and mutually exclusive between each other. [
56]
However, pattern of co-mutations between
STAG2, RUNX1, SRSF2 and
ASXL1 (SRSA genes) [
57] or between
SRSF2 and
IDH2 [
56] have been described in human AML. In mice,
SF3B1, U2AF1 and
SRSF2 mutations cause aberrant hematopoiesis and the acquisition of myelodysplastic-like phenotypes. [
58,
59,
60,
61] Mechanisms of splicing factors dysregulation in myeloid leukemogenesis have been extensively reviewed. [
62] Briefly, several studies have analyzed the impact of mutations of specific splicing gene and implication for leukemogenesis: i) Mutations in SRSF2 and U2AF1 yield alternative exon usage; ii) ZRSR2 mutant induces the retention of minor introns (U12-type); [
63] and iii) SF3B1 mutant instigates the usage of alternative branch points to cause an alternative 3′ splice site. [
64,
65]SF mutations induce mis-splicing of hematopoietic regulators, such as
EZH2 in
SRSF2-mutated MDS. [
58]
Mutated STAG2 (cohesin complex). Mutations in the cohesin subunit SA-2 (STAG2) define AML with myelodysplasia-related gene mutations irrespective of prior MDS [
1] and are considered a marker of poor prognosis. STAG2, together with double-strand-break repair rad21 homologue (RAD21), and structural maintenance of chromosomes (SMC1A and SMC3) form the core of the cohesion complex, that surrounds sister chromatids during replication, and support the transition from metaphase to anaphase. [
66] The roles of cohesin mutations in leukemogenesis are multiple, as they can induce aneuploidy through mis-segregation of sister chromatids, or remodel 3D chromosome topology and chromatin interactions. [
66]
In vivo, mutated cohesion subunits induce the acquisition of a pre-leukemic phenotype, with altered erythroid and myeloid lineages differentiation. Mutations in the cohesion genes ranges between 6-13% in AML [
67,
68] are mutually exclusive, and can be accompanied by NK or CK. Most
STAG2 mutations are nonsense or frameshift, leading to protein truncation or loss-of-function. [
63]
STAG2 mutations are often, if not always, associated with
RUNX1, SRSF2 and
ASXL1 mutations.[
63] Although
STAG2 mutations classify within the adverse-risk category, their prognostic significance appears to be linked to the presence of other co-mutations. When multivariate analysis are adjusted for mutation in
BCOR, ASXL1 and
RUNX1 - which are more commonly found in
STAG2-mutated AML compared to other subsets- STAG2 mutations lose their independent prognostic impact. Intriguingly, mutated STAG2 significantly increases the sensitivity of AML cells to poly ADP-ribose polymerase (PARP), such as talazoparib. [
69,
70] This suggests that the presence of STAG2 mutations could potentially be exploited to tailor more effective therapeutic strategies in this setting.
Mutated TP53. The majority of
TP53 mutations are missense, with hotspots in arginine residues, though other mutational events have been reported, including insertions, deletions and frameshift mutations. More frequently, the mutation occurs in the DNA binding domain, with loss of function of p53 tumor suppressor, despite some mutations can lead to gain-of-function through the binding of mutant p53 to other tumor suppressors such as p63 and p73. [
71] The frequency of
TP53 mutations in
de novo AML ranges from 5-10% increasing to approximately 30% in cases of therapy-related AML and AML with complex cytogenetics.
TP53 mutations are particularly prevalent in AML cases that exhibit CK, chromotripsis or a monosomal karyotype. [
72] Interstingly
, TP53 mutations are less commonly found with mutations in
DNMT3A, TET2 and
IDH1-2. [
72] Moreover, the variant allele frequency of
TP53 appears to be directly correlated with the level of cytogenetic complexity and inversely correlated with overall survival in AML patients. [
73]