Role and Mechanism of Action of Exonic Splicing Regulatory Sequences

Goina, Elisa (2008). Role and Mechanism of Action of Exonic Splicing Regulatory Sequences. PhD thesis The Open University.



Exonic mutations can result in altered protein function by affecting exon recognition during splicing. To understand this mechanism I have extensively evaluated the exonic regulatory elements affected by the disease G to T mutation at position +6 of BRCA1 exon 18. This substitution induces the exclusion of the exon and it has been suggested that it disrupts an ASF/SF2-dependent enhancer. Using a pulldown assay with an internal standard, I show that WT and T6 sequences bind ASF/SF2 with similar efficiency, which is significantly lower compared to the binding to a typical enhancer derived from the fibronectin EDA exon. Consistent with the absence of an ASF/SF2 enhancing effect, siRNA depletion of ASF/SF2 did not induce exon WT exclusion indicating that ASF/SF2 is not essential for BRCA1 exon 18 splicing. However, depletion or overexpression of ASF/SF2 resulted in a decrease or increase of T6 exon 18 inclusion, respectively. By a series of exonic deletions in the WT and T6 BRCA1 minigenes, an enhancer sequence was identified in position 23-32 which mediates the observed T6-specific, ASF/SF2-dependent splicing activation. Furthermore, extensive mutations analysis indicated that the T6 mutant creates a sequence with a predominantly inhibitory function. Double site-directed mutations showed that the point mutations affecting the “TAG” sequence, between +6 and +8 positions, completely restore normal splicing. Indeed, RNA protein interaction and siRNA experiments showed that the skipping of T6 BRCA1 exon 18 is due to the creation of a silencer element. This sequence specifically binds to the proteins hnRNP A1/A2 and to DAZAP1. By siRNA experiments, DAZAP1 and hnRNP A1/A2 are shown to be involved redundantly in the regulation of the defective BRCA1 exon 18. The present results support a gain-of-function model for the BRCA1 T6 exon 18 and indicate that the binding of the hnRNP A1/A2 and DAZAP1 is the primary determinant of exon skipping.

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