ASXL1 mutations identify a high-risk subgroup of older patients with primary cytogenetically normal AML within the ELN Favorable genetic category

Abstract

The associations of mutations in the enhancer of trithorax and polycomb family gene ASXL1 with pretreatment patient characteristics, outcomes, and gene-/microRNA-expression profiles in primary cytogenetically normal acute myeloid leukemia (CN-AML) are unknown. We analyzed 423 adult patients for ASXL1 mutations, other prognostic gene mutations, and gene-/microRNA-expression profiles. ASXL1 mutations were 5 times more common in older (≥ 60 years) patients (16.2%) than those younger than 60 years (3.2%; P < .001). Among older patients, ASXL1 mutations associated with wild-type NPM1 (P < .001), absence of FLT3-internal tandem duplications (P = .002), mutated CEBPA (P = .01), and with inferior complete remission (CR) rate (P = .04), disease-free survival (DFS; P = .03), overall survival (OS; P = .006), and event-free survival (EFS; P = .002). Within the European LeukemiaNet (ELN) genetic categories of older CN-AML, ASXL1 mutations associated with inferior CR rate (P = .02), OS (P < .001), and EFS (P < .001) among ELN Favorable, but not among ELN Intermediate-I patients. Multivariable analyses confirmed associations of ASXL1 mutations with unfavorable CR rate (P = .03), DFS (P < .001), OS (P < .001), and EFS (P < .001) among ELN Favorable patients. We identified an ASXL1 mutation-associated gene-expression signature, but no microRNA-expression signature. This first study of ASXL1 mutations in primary CN-AML demonstrates that ASXL1-mutated older patients, particularly within the ELN Favorable group, have unfavorable outcomes and may be candidates for experimental treatment approaches.

Figure 1

ASXL1 exon 12 mutations in primary CN-AML patients 60 years of age or older. (A) Localization of sequence variations within ASXL1 exon 12 found among 234 older CN-AML patients. Each arrow represents one of the nonsynonymous changes, except for known SNPs, which are not displayed. Top part of the panel: nonsense and frame shift mutations leading to truncation of the protein-coding sequence (indicated by red arrows). All these alterations were considered pathogenic mutations for the analysis of clinical outcomes. One patient had both a nonsense and a frame shift mutation. Bottom part: missense variations that alter single amino acids (indicated by black arrows). Patients who only had such missense variations were excluded from analyses of clinical correlations and outcomes. Known functional domains in ASXL1 exon 12 are designated by the shaded areas. NR indicates the RAR-binding nuclear receptor domain; and PHD, the plant homeodomain. (B) Relationship between ASXL1 mutations and other common gene mutations in 38 older CN-AML patients. Each column represents 1 patient. The topmost row indicates whether a patient had c.1934dupG (red) or another ASXL1 mutation (blue). The following rows represent 10 different types of mutations in 8 different genes that were found together with mutated ASXL1: purple represents the presence of the mutation; white, absence of the mutation; and gray fields, missing data. For CEBPA-mut patients, “s” indicates single and “d” double (biallelic) CEBPA mutations. The bottom row displays the ELN genetic category for each patient: green represents ELN Favorable; and yellow, ELN Intermediate-I.

Figure 2

Survival of CN-AML patients 60 years of age or older, according to ASXL1 mutation status. (A) DFS. (B) OS. (C) EFS.

Figure 3

Survival of older CN-AML patients in the ELN Favorable and ELN Intermediate-I genetic groups, according to ASXL1 mutation status. (A) OS of ELN Favorable group patients. (B) OS of ELN Intermediate-I group patients. (C) EFS of ELN Favorable group patients. (D) EFS of ELN Intermediate-I group patients. ELN Fav. indicates ELN Favorable category; and ELN Int.-I, ELN Intermediate-I category.

Figure 4

Heat map of the gene-expression signature associated with ASXL1 mutations in older patients with primary CN-AML. Differential gene expression was studied within the subset of NPM1-wt/FLT3-ITD-negative patients, to avoid confounding by those gene mutations. In this subset, a comparison of ASXL1-mut (n = 26) and ASXL1-wt (n = 39) patients identified 92 differentially expressed probe sets. The heat map shows expression levels of these 92 probe sets in all 185 older CN-AML patients with available microarray data. The NPM1-wt/FLT3-ITD-negative subset where the signature was derived is indicated by the gray bar at the bottom of the heat map. Rows represent probe sets; and columns, individual patients. Patients are grouped by ASXL1 mutation status, and genes are ordered by hierarchical cluster analysis. Expression values of the probe sets are represented by color: blue represents expression less than the median value for the given probe set; and red, expression greater than the median value for the given probe set. Arrows identify genes that are discussed in the text, with the vertical bar indicating multiple probe sets representing the same gene (CYP1B1).