Journal of Nucleic Acids 3 Figure 1: Alternative splicing of VEGF-A pre-mRNA. The pre-mRNA of VEGF-A undergoes alternative splicing leading to pro-angiogenic isoforms notated with the number of amino acids and containing as last exon, the exon 8a stemming from the Proximal Splicing Site (PSS) located at the beginning of exon 8. The more recent subfamily of VEGF isoforms containing ﬁve members so far, are anti-angiogenic and contain as last exon, the exon 8b resulting of the splicing at the Distal Splicing Site (DSS) located after the exon 8a. Eﬀectively, VEGFxxxb homodimers bind to the VEGF receptors and block the downstream signalling pathway [44]. On the other hand, VEGFxxxb can heterodimerize with VEGF preventing VEGF-mediated activation of VEGF receptors [42]. Consequently, the presence of VEGFxxxb may have a double anti-angiogenic action through (i) direct downregulation of VEGF receptors signalling pathways and (ii) by inhibiting VEGF activation of the pathway. This ﬁnding could explain why patients treated with BVZ do not have as good results as expected if VEGFxxxb isoforms are still present. Furthermore, treatment of breast, lung, colon, and kid- ney cancers has combined BVZ to conventional chemother- apies. A phase III trial of metastatic breast cancers, at primary diagnosis treated with BVZ plus Paclitaxel, showed a better beneﬁt on progression-free survival than Paclitaxel alone [54]. Similarly, combination of BVZ plus Paclitaxel- Carboplatin in a randomized study of non-small-cell lung cancer showed a better beneﬁt than Paclitaxel-Carboplatin alone [55]. Also, the addition of BVZ to Irinotecan, Flu- orouracil and Leucovorin for treatment of metastatic col- orectal cancer improved survival [56]. Finally, a randomized phase III trial of metastatic RCC showed the improvement on progression-free survival of the addition of BVZ to IFN as ﬁrst-line treatment [12]. However, the relapse to therapy or an acquired resistance may be explained by either the redundancy of angiogenic factors or the selection of aggressive cells, showing the limit of these treatments and the necessity to switch to RTKI to bypass resistance to anti-VEGF therapy. We can hypothesize that the chemotherapeutic agents used in combination with BVZ may normalize the VEGF/VEGFxxxb ratio in favour of VEGFxxxb. Hence, targeting the “good and bad” VEGF isoforms may lead to selection of more aggressive cells rendering the therapeutic combination totally ineﬃcient and even deleterious. Thus, in order to get the expected beneﬁt, two diﬀerent strategies could be investigated: either targeting speciﬁcally the pro- angiogenic VEGF isoform or treating patients with BVZ only in cases where the VEGFxxxb isoforms are absent, although it represents a third of patients as described above. 6. Regulation of VEGF Splicing Leading to Targeting Splicing for Therapy The study of VEGFxxxb isoforms regulation may be key for targeting the angiogenic balance in cancers and other pathologies. Most genes, like VEGF, can produce diﬀerent transcripts, which result in the production of diﬀerent protein isoforms. This phenomenon, known as alternative splicing, is mainly regulated by SR proteins. One of these proteins, ASF/SF2, has been described as a protooncogene with it being suﬃcient to induce cell transformation [57]. These observations provide a link between angiogenesis, cancer, and splicing. The study of VEGF splicing has illustrated in particular the need of SR proteins such