LAUSR

Cleavage and polyadenylation is a fundamental process in the control of gene expression, yet how cancer cells deregulate this process to generate cancer-causing alterations is only beginning to be appreciated. We previously showed that in human breast cancer, premature polyadenylation of the tumor suppressor gene MAGI3 causes the expression of a truncated protein that dominantly promotes malignant growth via the Hippo pathway. This established a precedent for premature polyadenylation as an aberrant process causing transformation in vitro and in vivo. However, little is known about the molecular mechanism that causes premature polyadenylation, or what types of genes it affects in cancer. Despite the absence of cis-acting mutations, premature polyadenylation of MAGI3 occurs specifically following the gene’s large internal exon. We therefore asked whether an epigenetic mechanism might underlie the phenomenon of premature polyadenylation in cancer. Here, we report that genes with large internal exons are susceptible to truncation by premature polyadenylation. In addition to MAGI3, premature polyadenylation truncates the gene products of two tumor suppressor genes with large internal exons, BRCA1 and LATS1. These truncated proteins are upregulated in breast cancer cells compared to non-transformed mammary epithelial cells and enable downstream oncogenic events instead of providing tumor suppressive functions. Large internal exons are infrequently found in the genome as the mechanics of efficient splice site recognition favor small exons. Interestingly in transcripts, large internal exons are conspicuously enriched in N 6 -methyladenosine (m 6 A), though the functional significance of these epitranscriptomic marks remains unknown. Molecularly, we find that methylated levels of N 6 -adenosine in the large internal exons of BRCA1, LATS1 and MAGI3 transcripts are significantly reduced in breast cancer cells that upregulate the truncated gene products compared to non-transformed mammary epithelial cells. These results raise the intriguing possibility that m 6 A marks in large internal exons are mechanistically involved in repressing premature polyadenylation. We are currently investigating this hypothesis by perturbing N 6 -adenosine methylation in cells and examining the functional consequences. In summary, our study suggests that premature polyadenylation may be a more pervasive alteration mechanism than previously appreciated in cancer and begins to illuminate a gene region-specific mechanism responsible for repressing premature polyadenylation. Citation Format: Thomas K. Ni, Charlotte Kuperwasser. Premature polyadenylation causes oncogenic truncations of the tumor suppressor genes BRCA1 , LATS1 and MAGI3 in breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4995. doi:10.1158/1538-7445.AM2017-4995