Functional and Clinical Relevance of Splicing Factor SRSF1 in Multiple Myeloma (MM)

The mechanisms modifying expression and function of genes, such as alternative pre-mRNA splicing or microRNA (miRNA) activity, need to be considered in order to provide a more accurate genomic framework for clinical correlation, as well as for high value therapeutic target discovery. Aberrant splicing of numerous genes has been reported in other malignancies, including a small number of genes reported in MM. The emerging focus therefore, has been to understand the molecular mechanisms driving alternate splicing. Several studies provide evidence that an abnormally expressed splicing factor can have oncogenic properties by impacting alternative splicing of cancer-associated genes.

Based on our previously described significant role of E2F1 and its heterodimerization partner Dp1 in myeloma, we evaluated their chromatin binding targets related with splicing factors. Using genome wide chromatin and transcription landscape mapping techniques, we observed both transcription factors bound to the promoter of several splicing factors including Serine Arginine-Rich Splicing Factor 1 (SRSF1). Further evaluation in our gene expression profile from 170 newly-diagnosed myeloma patients, identified high expression of SRSF1 in MM compared to normal plasma cells, with a significant difference in the non-hyperdiploid subtype. Importantly, we have observed significant coexpression of Dp1 and SRSF1 in this dataset suggesting a mechanism for SRSF1 upregulation in tumors with elevated Dp1. SRSF1 has been shown to be overexpressed in human tumors with elevated Myc. No correlation between Myc with SRSF1 was observed in myeloma suggesting that SRSF1 expression is mediated by Myc-independent mechanism.

We next explored the functional role of SRSF1 in MM. In gain-of-function experiments, enforced expression of SRSF1-eGFP in MM1S significantly increased proliferation. Conversely, downregulation of SRSF1 with specific shRNAs in MM cell lines significantly inhibited MM cell proliferation and cell survival. These data underscore the oncogenic potential of SRSF1 in MM. A significant reduction in SRSF1 at mRNA and protein levels was observed after E2F1 and/or Dp1 gene silencing. Moreover, peptide-based strategy to abrogate interaction between Dp1-E2F1 led to decreased SRSF1 expression levels.

Importantly, the increased expression of SRSF1 was linked with overall survival in 2 independent MM datasets, highlighting for the first time the clinical relevance of splicing related factors in myeloma.

In conclusion our results indicate a functional role and clinical significance of a gene involved in regulation of gene splicing. This study highlights the need to further understand the splicing pattern in myeloma and also supports the emerging concept that splicing programs, together with transcriptional programs participate in the altered cellular function during tumor initiation and progression.