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)
Authors: Jahn Nikolaus , Agrawal Mridul , Bullinger Lars , Weber Daniela , Corbacioglu Andrea , Gaidzik Verena I. , Thol Felicitas , Heuser Michael , Krauter Juergen , Göhring Gudrun , Kuendgen Andrea , Fiedler Walter , Wattad Mohammad Amen , Held Gerhard , Kraemer Doris , Horst Heinz A. , Lübbert Michael , Ganser Arnold , Schlenk Richard F. , Dohner Hartmut , Dohner Konstanze , Paschka Peter 
Published: October 2019

Abstract
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 (ASXL2mut) 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 ASXL2mut 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 ASXL2mut 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 ASXL2mut 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 ASXL2mut and ASXL2 wildtype (ASXL2wt) with respect to sex, WBC, haemoglobin, platelets, LDH serum levels, and circulating or bone marrow blasts. Of note, ASXL2mut were not associated with increasing age, a finding which is commonly observed for ASXL1 mutations in AML. In terms of secondary chromosome aberrations ASXL2mut were frequently associated with del(9q) (P=.006), whereas they never co-occurred with trisomy 8. There was no significant association between ASXL2mut 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 ASXL2mut. There was no difference in complete remission (CR) rate after double induction between pts with ASXL2mut (88%) and those with ASXL2wt (92%); the same was true when comparing pts with ASXL1 or ASXL2 mutations (ASXL1/2mut) as one group (93%) versus those with ASXL1/2 wildtype. Neither ASXL2mut nor ASXL1/2mut 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. ASXL2mut did not impact achievement of CR or any survival endpoint. Nevertheless, the high incidence (16.2%) of ASXL2mut 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.

Disclosures
Horst: MSD: Research Funding; Pfizer: Research Funding; Amgen: Honoraria, Research Funding; Gilead: Honoraria, Research Funding; Boehringer Ingleheim: Research Funding. Schlenk:Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees; Novartis: Honoraria, Research Funding; Pfizer: Honoraria, Research Funding; Janssen: Membership on an entity's Board of Directors or advisory committees; Boehringer-Ingelheim: Honoraria; Arog: Honoraria, Research Funding; Teva: Honoraria, Research Funding.