Incidence and Prognostic Relevance of ASXL2 Mutations in Adult CBF-AML with t(8;21)(q22;q22): A Study of the German-Austrian AML Study Group (AMLSG)

Background: The ASXL2 (Additional Sex comb-like 2) gene on chromosome 2p23.3 encodes an epigenetic regulator that is thought to act through histone modification and thereby regulating gene transcription in a context-dependent manner. Recently, ASXL2 mutations (ASXL2^mut) were found with a high incidence (~23%) in a cohort of 110 pediatric or adult AML patients (pts) with t(8;21)(q22;q22) (Micol et al., Blood 2014).

Aim: We assessed the frequency and prognostic impact of ASXL2^mut in the context of other clinical and genetic factors in a large clinically well-defined cohort of intensively treated adults with t(8;21) AML. In addition, the stability of ASXL2 mutation status was analysed in a subset of pts at the time of relapse.

Methods: Diagnostic samples from 204 adults with t(8;21);RUNX1/RUNX1T1-positive AML [median age: 49 years, range: 18-82] were analysed for ASXL2^mut in exon 11 and 12 using a combination of PCR-based amplification and subsequent direct DNA-sequencing. Paired samples at diagnosis and relapse were evaluated for the ASXL2 mutation status in a subset of 22 pts. Additional mutation analyses were performed for KIT, FLT3 (ITD and TKD), NRAS, and ASXL1 genes. All pts received intensive treatment either within AMLSG trials (n=155) or according to standard chemotherapy regimens.

Results: Thirty four ASXL2^mut were identified in 33 (16.2%) of the 204 pts. All mutations (frame-shifts, n=32; non-sense, n=2) created stop codons leading to premature protein truncation with loss of the terminal PHD domain; 27% of the mutations affected codon T740 in exon 12. At diagnosis, there was no significant difference between pts with ASXL2^mut and ASXL2 wildtype (ASXL2^wt) with respect to sex, WBC, haemoglobin, platelets, LDH serum levels , and circulating or bone marrow blasts. Of note, ASXL2^mut were not associated with increasing age, a finding which is commonly observed for ASXL1 mutations in AML. In terms of secondary chromosome aberrations ASXL2^mut were frequently associated with del(9q) (P=.006), whereas they never co-occurred with trisomy 8. There was no significant association between ASXL2^mut and all other gene mutations analysed. Analysis for ASXL2 mutation status in 22 paired samples obtained at diagnosis and relapse showed a high stability since the mutation was still present in two pts at relapse whereas none of the remaining 20 ASXL2 wildtype cases acquired ASXL2^mut. There was no difference in complete remission (CR) rate after double induction between pts with ASXL2^mut (88%) and those with ASXL2^wt (92%); the same was true when comparing pts with ASXL1 or ASXL2 mutations (ASXL1/2 ^mut) as one group (93%) versus those with ASXL1/2 wildtype. Neither ASXL2^mut nor ASXL1/2^mut did impact the endpoints event-free-survival, cumulative incidence of relapse, relapse-free and overall survival.   

Conclusions: Beside KIT and NRAS, ASXL2 is among the most frequently mutated genes in t(8;21) AML. ASXL2^mut did not impact achievement of CR or any survival endpoint. Nevertheless, the high incidence (16.2%) of ASXL2^mut in t(8;21) AML and the exclusivity to this subgroup of core-binding factor AML implies a peculiar role of ASXL2 in the leukemogenesis of t(8;21) AML providing a basis for further studies.