Abstract 1507: Multiple mechanisms of Myb gene activation revealed through transcriptome analysis of salivary gland adenoid cystic carcinoma

Abstract

Adenoid cystic carcinoma (ACC) of the salivary gland is a rare head and neck malignancy with poor long-term prognosis. Hallmark chromosomal translocations result in MYB to NFIB (t(6;9)) and MYBL1 to NFIB (t(8;9)) gene fusions. Gene rearrangement and aberrant expression of the MYB and MYBL1 genes, along with the encoded transcription factors, are the oncogenic driver in the majority of ACC tumors. Previous RNA-sequencing (RNA-seq) data collected from FFPE fixed ACC tumors illustrated MYB and MYBL1 elicit similar gene expression profiles in ACC, suggesting these transcription factors drive ACC oncogenes via similar mechanisms. Using these RNA-seq data we have identified EN1 and SOX4 as important Myb regulated genes and demonstrated activation of their promoters in reporter gene assays. We hope to utilize these reporters in high-throughput screens to identify new drug candidates for ACC. We have utilized RNA-seq data to identify an additional mechanism leading to MYB overexpression that appears unique to ACC tumors. In these tumors transcription of the MYB gene is aberrantly activated through use of an alternative promoter located within the first intron of the gene. Transcripts originating from this promoter have been confirmed in an ACC tumor via 5'RACE and the resulting Myb protein has a small N-terminal truncation eliminating the epitope recognized by a common Myb antibody. The alternative promoter is active and regulated in a cell type specific manner in reporter gene assays. Previous studies have shown chromosomal translocations juxtapose NFIB enhancers to activate MYB gene transcription. ChIP and 3C assays in fresh frozen ACC tumors will be utilized to test the hypothesis that NFIB enhancers are specifically recruited to the alternative MYB promoter, thereby activating gene transcription. Thus, multiple mechanisms lead to the aberrant over-expression and truncation of MYB and drives ACC oncogenesis. Citation Format: Candace Frerich, Kathryn Brayer, Brandon Painter, Scott Ness. Multiple mechanisms of Myb gene activation revealed through transcriptome analysis of salivary gland adenoid cystic carcinoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1507.