Abstract 3649: Allelic imbalance in <i>KMT2A</i>-rearranged infant acute lymphoblastic leukemia

Abstract Background Infant acute lymphoblastic leukemia (ALL) is a malignant disorder with poor clinical outcome. It is well-known that infant ALL cases with rearrangement of the KMT2A gene (KMT2A-r) have an even poorer prognosis than non-KMT2A-r cases. Interestingly, KMT2A-r infant ALL cases have remarkably few other genetic alterations. We hypothesized that non-coding events in cancer genomes (e.g. loss of expression or methylation) may play a role in this disease. Such events can be captured using genomic analyses in haploid genomes using analytical approaches for allelic imbalance quantification. Here, we examine whether allelic imbalance is a feature of infant ALL. Methods We performed whole genome sequencing (WGS) and RNA sequencing on peripheral blood or bone marrow specimens from 29 KMT2A-r cases and 14 non-KMT2A-r cases at diagnosis (DX), remission (MD), and relapse (RL) as applicable. WGS data from MD samples was phased using a 1000 Genomes reference panel. Biallelic expression was measured on the phased genome for transcripts with at least 2 SNPs and at least 15 aligned reads. Lesser allele fraction (LAF; allele fraction for the less prevalent allele) for DX/RL versus MD samples was compared for transcripts that had LAFs available in at least 3 cases using t-test. Transcript-level p-values were aggregated at the gene level using the Sidak method. Results Allelic imbalance in expression (LAF <= 0.2) was observed in an average of about 600 genes per sample in infant ALL regardless of timepoint. Disease-specific allelic imbalance (skewed in DX samples but not in paired MD samples) was detected in 431 genes for the KMT2A-r cohort and 77 genes for the non-KMT2A-r cohort. A total of 38 genes with allelic imbalance were shared between the two cohorts. Notably, KMT2A was observed to be imbalanced in KMT2A-r samples. Genes of known significance that were found to be skewed included HOXA9 and PARP8. Discussion Our study suggests that allelic imbalance quantification may help uncover novel molecular mechanisms in infant ALL, especially KMT2A-r cases. However, similar to gene expression, the patterns of allelic imbalance in KMT2A-r cases at DX do not allow efficient prediction of which patients go on to relapse. Citation Format: Byunggil Yoo, Midhat S. Farooqi, Rumen Kostadinov, Warren Cheung, Emily Farrow, Shannon Kelley, Neil Miller, Bing Ge, Margaret Gibson, Patrick Brown, Erin M. Guest, Tomi Pastinen. Allelic imbalance in KMT2A-rearranged infant acute lymphoblastic leukemia [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 3649.

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