circSMARCA5-SRSF1 interaction is functionally involved in GBM pathogenesis

Glioblastoma multiforme (GBM) is the most aggressive human brain tumor with a median survival of 15 months. The standard treatments of GBM and the total medical resection are unable to contrast this mortal cancer. For these reasons new diagnostic approaches and treatment strategies are needed: the identification of molecular features of this cancer may allow to create a personalized therapy. Circular RNAs (circRNAs) are a new class of non-coding RNAs (ncRNAs) highly enriched in brain, stable within the cells and detectable in body fluids. Even though a lot of studies have proposed their potential roles, the biological importance of circRNAs is still object of debate. This thesis investigated the putative involvement of circRNAs in GBM pathogenesis. Our group has shown that circSMARCA5 is significantly downregulated in GBM biopsies and its expression is associated to the glioma grade malignancy. Functional analysis showed that circSMARCA5 negatively regulated migration of U87MG cells overexpressing circSMARCA5. Serine/arginine-rich splicing factor 1 (SRSF1) is one of the predicted interactors of circSMARCA5. Is has been proposed that circSMARCA5 may regulate the alternative splicing of serine and arginine rich splicing factor 3 (SRSF3), a known SRSF1 splicing target. Interestingly, SRSF3 is known to act together with other splicing factors, polypyrimidine tract binding protein 1 (PTBP1) and polypyrimidine tract binding protein 2 (PTBP2), that positively regulate glioma cells migration. Successively, we have demonstrated the physical interaction between SRFS1 and circSMARCA5. One of the most interesting splicing targets of SRSF1 in GBM is the vascular endothelial growth factor A (VEGFA). Expression analysis of the total VEGFA (VEGFAtot) and its splicing variants (Iso8a to Iso8b) transcripts showed that circSMARCA5 regulated the alternative splicing of VEGFA mRNA by binding to SRSF1. In addition, blood vascular microvessel density evaluated in GBM negatively correlated with the expression of circSMARCA5, while positively correlated with that of SRSF1 and Iso8a/Iso8b ratio. Kaplan-Meier survival analysis showed that GBM patients with low circSMARCA5 expression had lower overall and progression free survival rates. Collectively these data convincingly suggest that circSMARCA5 could be considered a promising druggable tumor suppressor in GBM. Moreover, the interaction with the splicing factor SRSF1 makes circSMARCA5 an upstream regulator of pro- to anti-angiogenic VEGFA isoforms ratio within GBM cells and a highly promising GBM prospective anti-angiogenic molecule.