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Differential Alternative Splicing of a Novel FGFR3 Variant Involved in African American Prostate Cancer Disparities Open Access

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Prostate cancer (PCa) is the most diagnosed cancer in men and the second leading cause of male cancer-related deaths annually in the United States (Siegel, Miller, and Jemal 2019). Despite increased screening and decreasing mortality rates of PCa, African American (AA) men have significantly higher rates of PCa incidence, high-risk cancer, and mortality compared to European American (EA) men (Cooperberg 2013). AA men are 1.7 times more likely to be diagnosed with PCa (DeSantis et al. 2016; Kelly et al. 2017). Additionally, the mortality ratio of AA compared to EA PCa patients (2.4 to 1) is the largest of any other malignancy in the United States (Rebbeck 2017a). Even after adjusting for clinical and epidemiological factors, AA men still have significantly higher occurrence and mortality rates, suggesting differences in biology and genetics may be playing a role in this disparate disease burden (Evans et al. 2008; Tyson and Castle 2014). Alternative splicing (AS) is the major mechanism for post-transcriptional regulation of gene expression, mRNA diversity, and protein modification. While AS has been shown to play a role in PCa development and progression, a link between AS and PCa health disparities has been largely unexplored. Recently, a novel splice variant of the fibroblast growth factor receptor 3 (FGFR3), a known proto-oncogene, was identified as a possible candidate for differential AS (dAS) in AA PCa (Wang et al. 2017). Here, we aim to clone and functionally characterize this heretofore unreported exon 14-skipped splice variant of FGFR3 and characterize the molecular ramifications of this variant on PCa oncogenesis. Additionally, we propose that differential gene expression of RNA binding proteins, such as splicing factors, may be an upstream regulator of FGFR3 dAS.In this study, we successfully cloned and sequenced a previously unreported splice variant of FGFR3. This novel variant lacks exon 14, generating a short variant (FGFR3-S) that is predominantly expressed in AA PCa compared to the long variant (FGFR3-L) containing exon 14. We show that FGFR3-S promotes increased proliferation, migration, invasion, downstream oncogenic signaling and decreased caspase activity compared to FGFR3-L. We also demonstrate that FGFR3-S has increased resistance to the pan-FGFR small molecule inhibitor dovitinib both in vitro and when examining tumor growth and metastasis in vivo. We have identified seven splicing factors that are differentially expressed in AA versus EA PCa and shown that two, SRSF2 and hnRNPF, are directly involved in exon 14 skipping of FGFR3. Knockdown of SRSF2 and HNRNPF leads to splice switching (i.e. preferential exon 14 inclusion over exon 14 skipping), a decrease in invasion and proliferation, and increase in caspase activity. Meanwhile, overexpression of FGFR3-S rescues this decreased oncogenic phenotype. This work identifies a novel FGFR3 splice variant and supports the hypothesis that dAS plays a crucial role in PCa health disparities.

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