MmuEX0028099 @ mm9

C-terminal region, partly encoded by exon 7, mediates homotypic interactions which may stabilize intra- and/or inter-ring interactions

The exon 7 region enhances MBNL1-MBNL1 dimerization properties

The presence of alternative ex.54nt always significantly elevated MBNL1 and MBNL2 protein levels (_2.7 and _4.0-times, respectively). There were marginal changes of the GFP-MBNL level for constructs carrying ex.36nt (_1.3-times) and ex.95nt (_0.8-times). There were no significant differences between the activities of the analyzed proteins for the majority (67%) of AS events. However, these exons might have either a positive or negative effect on some specific AS events (Figure 5A and C). Interestingly, exOFF events predominated in AS events with a negative effect of ex.36nt (95%; P = 0.002) and a positive effect of ex.95nt (90%; P = 0.010). All together, these data indicate that the presence of sequences encoded by these alternative exons can significantly modulate MBNL splicing activity, but this regulation depends strongly on the targeted RNA.

Inclusion/exclusion of the exon affects splicing activity with sequential binding motifs, as shown by competition assays between isoforms.

Exon 7 is the most differentially included exon in cancer, both in cell lines and in patients' samples. It confirms exon 7 is fundamental for MBNL1 protein homodimerization. Use of splice-switching antisense oligonucleotides (AONs) or siRNAs to compare the effect of MBNL1 splicing isoform switching with knockdown shows that whereas the absence of MBNL1 is tolerated in cancer cells, the expression of isoforms lacking ex7 (MBNL1 deltaex7) induces DNA damage and inhibits cell viability and migration, acting as dominant negative proteins.