Exploring the landscape of somatic <i>ASXL2</i> mutations in myeloid neoplasms: Frequency and clinical implications
Authors: Tareq Abuasab, Gautam Borthakur, Rashmi Kanagal‐Shamanna, Lucia Masarova, Keyur Patel, Koichi Takahashi, Prithviraj Bose, John Villarreal, Sherry Pierce, Tapan Kadia, Guillermo Garcia‐Manero, Nicholas J. Short, Courtney DiNardo, Naval Daver, Farhad Ravandi, Hagop Kantarjian, Srdan Verstovsek, Musa Yilmaz
Journal: American Journal of Hematology (2024)

Additional sex combs-like (ASXL) genes consist of three family members which are involved in epigenetic regulation and the three genes are: ASXL1, ASXL2, and ASXL3. Mutations in the family of ASXL have been identified in increased frequencies in myeloid neoplasms (MNs).1 While ASXL1 acts as a haploinsufficient tumor suppressor, mice models have shown that ASXL2 gene plays an essential role in normal hematopoiesis and its deletion in hematopoietic stem cell leads to myeloid malignancies. Compared with the incidence of ASXL1 mutations, ASXL2 mutations are less frequent in myeloid malignancies.2 Little is known about its clinical implications in patients with MNs. In this analysis, we identified the frequency, clinical characteristics, and mutational co-occurrences of ASXL2 across MNs, including acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and myeloproliferative neoplasms (MPN). Among 5746 patients with MNs (AML (N = 2473), MDS (N = 1477), MPN (N = 1299), MDS/MPN overlap neoplasms (N = 497)) presented to our institution between 2017 through 2022, we identified 50 (0.9%) with ASXL2 mutations (Figure S1, Consort Diagram; Table S1, Next generation sequencing 81-gene Panel, Table S2, ASXL2 mutations). Baseline characteristics of the study patients are shown in Table 1. Of the 50 patients, eight (6%), 20 (0.8%), 11 (0.7%), seven (0.5%), and four (0.8%) had CBF AML, non-CBF AML, MDS, MPN, and MDS/MPN, respectively. In the entire cohort, RAS (22%), SRSF2 (20%), TET2 (14%), and DNMT3A (14%), SF3B1 (12%) and RUNX1 mutation (12%) were the most commonly co-mutated genes (Figure S2, Bar plot of co-mutated genes). Six patients had coexisting ASXL1 mutations: three in AML, two in MPN, and one in MDS. Out of 2473 AML patients, 28 (1%) had ASXL2 mutation. The incidence of ASXL2 mutation was the most common in patients with CBF AML, 6% (8 of 142) versus 0.8% (20 of 2331) in non-CBF AML. In addition, among CBF AML patients, ASXL2 mutations were detected only in patients with t(8;21) RUNX1/RUNX1T1 (AML1/ETO), 15% (8/53). Consistent with previously published literature, none of the patients with inv(16)/t(16;16) CBFB/MYH11 had ASXL2 mutations (0 of 89). The most common co-mutations were SRSF2 (18%), IDH2 (18%), RUNX1 (14%), SF3B1 (14%), KIT (14%), and DNMT3A (14%). Of the 28 patients with AML, 16 were newly diagnosed (ND) and 12 had relapsed/refractory (R/R) disease. Among ND patients, all eight patients with CBF AML were treated with FLAG (fludarabine, cytarabine, and granulocyte colony-stimulating factor) based regimens and achieved complete remission with overall survival (OS) not reached. Because ASXL2 mutations were confined to the t(8;21) patients, we analyzed OS comparing t(8;21) with ASXL2 mutation versus t(8;21) with no ASXL2 mutations. The 3-year OS rates were not statistically different in t(8;21) patients with or without ASXL2 mutations, 87% versus 57%, respectively (p = .38) and median OS was not reached (Figure S3). Of the eight ND patients with non-CBF AML (two patients received intensive Rx, six patients received low-intensity Rx) had a median OS of 12.5 months. In order to compare the clinical impact of ASXL2 with ASXL1 mutations, we identified ASXL1 mutated patients from our institution database between 2017 and 2022. In ND patients with t(8;21), patients with ASXL2 or ASXL1 mutated AML had similarly favorable OS, (median not reached for both, p = .65) (Figure S4A). In non-CBF AML patients, OS rates were similarly poor with ASXL2 or ASXL1 (median 12.5 vs. 14 months, p = .85) (Figure S4B). Of the 12 R/R AML patients, all had non-CBF AML, the response rates after salvage chemotherapy and median OS rates were 67% and 6.2 months, respectively. There was no prior NGS data to evaluate whether ASXL2 emerged at relapse versus present at the time of first diagnosis. Eleven of 1477 (0.7%) MDS patients were identified to have ASXL2 mutation. Of the 11 patients, eight had ND disease and three had R/R disease. For the eight ND patients, most of the patients (63%) had lower risk disease based on the International Prognosis Scoring System (two low, five intermediate, one high). Most common co-mutations were SRSF2 (27%),TP53 (27%), one patient had co-occurring ASXL1 mutation. Of these eight patients, six received HMA based therapy (four had response −2 CR and 2 mCR- and two no response), one lenalidomide (achieved response). The OS was poor in patients with ND MDS with a median of 12.9 months only. There was no statistically significant OS difference between ASXL2 mutated ND MDS with historical ASXL1 mutated MDS (median OS 13 vs. 23 months, p = .29) (Figure S5). Out of seven patients with MPN and ASXL2 mutation, five patients had primary MF, one patient had post-essential thrombocythemia MF, and one patient had post-Polycythemia-Vera MF. Five patients (71%) harbored JAK2 V617F mutation, two (29%) MPL mutation, and three (43%) ASXL1 mutation. Six out of seven patients had intermediate risk based on Dynamic International Prognostic Scoring System score. The 3-year OS rate was 100%. Out of 497 patients with MDS/MPN neoplasm, four (0.8%) had ASXL2 mutation. Two patients had ND chronic myelomonocytic leukemia, one patient with ND MDS/MPN unclassified, and one patient with R/R MDS/MPN with fibrosis. Our study reveals a distinct pattern regarding the occurrence of ASXL2 mutations in MNs, distinguishing them from the more prevalent ASXL1 mutations. ASXL1 mutations range from 5% to 40% in MNs, whereas ASXL2 mutations are notably less common in MNs, ranging from 0.5% to 1%.2 Specifically, our data confirms the rarity of ASXL2 mutations, with the exception of cases involving CBF AML. Notably, these mutations were exclusively observed in patients harboring the t(8;21)/RUNX1-RUNX1T1 (AML1-ETO) fusion, constituting approximately 15% of the analyzed cohort, while they were absent in patients with inv(16)/t(16;16) CBFB/MYH11. Importantly, our findings demonstrate that ASXL2 mutations did not exhibit an association with a poor prognosis in CBF AML. However, in the context of non-CBF AML and MDS, like ASXL1 mutations, ASXL2 mutations emerged as significant indicators of an unfavorable prognosis. Previous studies have reported a higher prevalence of ASXL2 mutations in CBF AML (RUNX1/RUNX1T1).3 Our study supports these findings and finds no evidence linking ASXL2 mutations to an increased risk of relapse or decreased survival in CBF AML. Notably, a study by Abdul-Wahab et al. delves further into the pathogenesis of ASXL2 mutations in CBF AML, unveiling ASXL2's crucial role in hematopoietic stem cell self-renewal and its contribution to RUNX1/RUNX1T1-induced leukemogenesis.1 Furthermore, the target genes of ASXL2 exhibit overlap with those of RUNX1 and RUNX1/RUNX1T1, underscoring ASXL2's pivotal role as a regulatory factor in hematopoiesis, mediating transcriptional effects that promote leukemogenesis driven by RUNX1/RUNXT1. Our study revealed a negative prognostic impact on non-CBF AML or MDS patients with ASXL2, similar to the poor prognosis associated with ASXL1 mutations. When comparing OS among patients with ASXL1 and ASXL2 mutated MDS or AML, ASXL2 mutations displayed a similarly unfavorable prognosis. The European Leukemia Network's AML risk criteria already designates ASXL1 mutations as adverse risk,4 and our research supports extending this classification to include ASXL2 mutations in MDS and non-CBF AML. However, further studies are necessary to provide additional evidence and validate these observations. Contrary to the prevailing literature indicating mutual exclusivity between ASXL1 and ASXL2 mutations,1, 5 our study revealed instances of co-occurrence between ASXL1 and ASXL2 mutations. Specifically, we identified six patients with coexisting ASXL1 mutations: three in AML, two in MPN, and one in MDS. However, it is important to note that the incidence of this co-occurrence was exceptionally rare within our large study cohort, comprising only six out of 5746 patients with myeloid malignancies. Within our cohort of patients with MPNs, we observed that all seven individuals with ASXL2 mutations were diagnosed with MF, while none had ET or PV. Surprisingly, the 3-year OS rate for patients harboring ASXL2 mutations was 100%. This suggests that ASXL2 mutations may not have a negative prognostic impact in MF. When comparing our findings to a study conducted by Wang et al., which focused on ASXL1-mutated myelofibrosis patients with a 3-year OS rate of 70%, it becomes evident that ASXL2 mutations might not have the same adverse effects as ASXL1 mutations in myelofibrosis patients.6 However, it is important to note that our study had a small sample size, and therefore, it is not possible to draw a definitive conclusion based on these results. In summary, our research findings indicate that ASXL2 mutations, although rare, can be detected across various myeloid malignancies, including AML, MDS, and MPNs. However, the frequency of ASXL2 mutations is generally low in myeloid malignancies, except in CBF AML, where they are limited to patients with the t(8;21)/RUNX1/RUNX1T1 (AML1-ETO) fusion, without significant prognostic implications. Conversely, in non-CBF AML or MDS, the presence of ASXL2 mutations was associated with an unfavorable prognosis. These results highlight the diverse impact of ASXL2 mutations in different subtypes of myeloid malignancies, emphasizing the need for further investigation into their underlying mechanisms and clinical implications. The authors have no competing financial interests nor other conflicts of interests to declare for this study. The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions. Figure S1. Consort diagram of the whole cohort. Figure S2. Bar plot of most common co-mutated genes. Figure S3. Kaplan–Meier analysis of overall survival (OS) in patients with newly diagnosed t(8;21) CBF-AML with and without ASXL2 mutation and Inv16 patients. Figure S4. Kaplan–Meier analysis of overall survival (OS) for newly diagnosed patients with core binding factor AML (A) and without core binding factor AML (B) with ASXL1 or ASXL2 mutation. Figure S5. Kaplan–Meier analysis of overall survival (OS) for MDS patients with ASXL1 and ASXL2 mutation. Table S1. Eighty-one gene panel. Table S2. ASXL2 mutations. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.