Clinical Features of Patients with ASXL1 and ASXL2 Mutations in Pediatric Acute Myeloid Leukemia

Background;

Acute myeloid leukemia (AML) is a molecularly and clinically heterogeneous disease. Currently, a number of gene mutations have been implicated in the pathogenesis of both adult and pediatric AML, including mutations of CEBPA,NPM1, DNMT3A, IDH1/2, TET2 and EZH2 in addition to RAS, KIT and FLT3, because the recent development of massively parallel sequencing technologies.

We have performed whole-exome sequencing of paired tumor-normal DNA from 19 patients, and identified 80 somatic mutations or 4.2 mutations per sample. Many of the recurrent mutations identified in this study involved previously reported targets in adult AML, such as FLT3, CEBPA, KIT, CBL, NRAS, WT1, BCORL1, EZH2, and major cohesin components including SMC3 and RAD21. In addition to these mutations, we also identified disease-associated candidate genes of ASXL2, PAX5 and others.

Recently, recurrent somatic mutations in ASXL1 have been reported to occur in patients with adult AML, and to be associated with adverse outcome. Another study suggested that ASXL1 or ASXL2 mutations were associated with a high incidence of relapse.

To reveal the significance of these mutations, we performed mutational analysis of ASXL1 and ASXL2 in 184 pediatric AML patients.

Methods;

Between 2006 and 2010, 485 de novo pediatric AML patients aged <18 years old participated in the Japanese AML-05 study conducted by the Japanese Pediatric Leukemia/Lymphoma Study Group (JPLSG). Among them, 369 samples were available. We analyzed the first half of these samples which were registered in the order (184 samples).

To estimate the frequency and prognostic impact of the ASXL1 and ASXL2 mutations in pediatric AML, we performed targeted sequencing of ASXL1 (exon 12) and ASXL2 (exon 12) genes using next-generation sequencer in 184 de novo AML patients including 51 patients with t(8;21). We validated the mutations in ASXL1 and ASXL2 by Sanger sequencing. Furthermore, we investigated the correlation among these mutations, other cytogenetic alterations and clinical characteristics.

Results and Discussion;

ASXL1 mutations were identified in 4 of 184 de novo pediatric AML patients (2.2 %) and all 4 ASXL1 mutation positive patients harbored t(8;21). Two of them relapsed, and one died after relapsed. On the other hand, ASXL2 mutations were identified in 10 of 184 de novo pediatric AML patients (5.4%) and 6 of them harbored t(8;21). Five of these 10 patients relapsed, and 2 of them died after relapsed. Especially, all of 4 patients without t(8;21) relapsed (2 in M5a and 2 with CBFA2T3-GLIS2 in M7), and 2 died after relapsed. Although only one ASXL2 patients with t(8;21) relapsed (1/6 or 17%), no ASXL2 positive patients with t(8;21) died. ASXL2 mutations were more observed in AML patients with t(8;21) (11.8%, 6/51), but not in 13 patients with inv(16). Overall survival of the patients with or without ASXL2 mutations were 80% and 66.7% (p=0.54), respectively. ASXL1 and ASXL2 mutations were mutually exclusive in this study.

Conclusion;

Ten of 184 patients (5.4%) had mutations of ASXL2 in pediatric AML, and the outcome of ASXL2 mutant patients with t(8;21) was favorable. Among the 51 pediatric AML patients with t(8;21), ASXL2 mutations were detected in 6 (11.8%) patients. All of them have been survived, suggesting that ASXL2 mutations in patients with t(8;21) may be associated with favorable prognosis in pediatric AML in contrast to adult AML. On the other hand, 4 (2.2%) of 184 patients had ASXL1 mutations, and all of them were t(8;21). In these 4 patients, 2 of them relapsed, and one died after relapsed. Although the number of patients is too small, ASXL1 mutations were not considered to be associated with favorable outcome. Both ASXL1 and ASXL2 mutations were detected at high frequency among pediatric AML patients with t(8;21) and mutual exclusive. As we consider that further study will be needed to clarify the significance of these mutations, we are now analyzing mutations in other exons of ASXL2, and would like to report these data in the annual meeting.