P466: AMPLIFIED EPOR/JAK2 GENES DEFINE A UNIQUE SUBTYPE OF ACUTE ERYTHROID LEUKEMIA

Abstract

Background: Acute erythroid leukemia (AEL) is a rare subtype of acute myeloid leukemia (AML) characterized by erythroid predominant proliferation and classified into two subtypes with pure erythroid (PEL) and erythroid/myeloid (EML) phenotypes based on the degree of erythroid hyperplasia. Despite an intensive mutational analysis, the mechanism of erythroid hyperplasia in AEL is still poorly understood and so are feasible therapeutic targets. Aims: To understand the mechanism of erythroid dominant phenotype of AEL and identify potential therapeutic targets for AEL. Methods: We analyzed a total of 124 adult AEL cases, where whole genome/exome sequencing of 35 cases were followed by targeted-capture sequencing in all cases. RNA sequencing was also performed in 23 cases. The mutational profile of AEL cases was compared to that of 409 cases with non-erythroid AML (non-AEL). Patient-derived xenograft (PDX) mouse models of AEL with JAK2 and/or EPOR amplification were established from 6 AEL patients with these abnormalities. These models were tested for their response to JAK1/2 inhibitor. Results: In accordance with a recent report, AEL cases were classified into 4 genomic groups (A-D), which are characterized by biallelic TP53 mutations and complex karyotype (Group-A), mutated NPM1 (Group-B) and STAG2 (Group-C), and other mutations in histone modifiers and transcription factors (Group-D). In particular, all but one PEL cases belonged to Group-A. Also found in non-AEL cases, these group-defining lesions in AEL were uniquely associated with focal gains/amplifications of EPOR, JAK2, and/or ERG/ETS2 loci (Group-A), PTPN11 mutations (Group-B), and KMT2A-PTD (Group-C), which might be responsible for the AEL phenotype. Highly enriched in PEL cases (7/13), EPOR/JAK2 focal gains/amplifications were implicated in their extreme erythroid hyperplasia and associated with particularly poor prognosis, even compared to other Group-A cases, who had shorter survival than those in other groups. As expected, JAK2/EPOR-amplified cases showed upregulated STAT5 signaling compared to non-AEL cases, which however, was also observed in other AEL cases, suggesting that upregulated STAT5 signaling is a hallmark of AEL. Based on these findings, we tested the effect of JAK2 inhibition on cell growth of EPOR/JAK2-amplified AEL cells in in vitro culture and xenograft model. Of interest, ruxolitinib-mediated JAK2 inhibition resulted in significantly suppressed in vitro cell growth of EPOR/JAK2-amplified AEL cells and prolonged overall survival in 4 PDX models with phospho-STAT5 downregulation, although other 2 models were resistant to JAK2 inhibition with persistent STAT5 activation. Summary/Conclusion: AEL is a heterogenous subgroups of AML characterized in common by upregulated JAK/STAT5 signaling. Gains/amplifications of JAK2/EPOR are frequent in TP53-mutated cases, particularly those with the PEL phenotype, and could be exploited as potential therapeutic targets using JAK2 inhibitors.