Published online 27 October 2005
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Depletion of TDP 43 overrides the need for exonic and intronic splicing enhancers in the human apoA-II gene
1International Centre for Genetic Engineering and Biotechnology Padriciano 99, I-34012 Trieste, Italy 2Department of Physiology and Pathology, University of Trieste Via A. Fleming 22, 34127 Trieste, Italy
*To whom correspondence should be addressed. Tel: +39 040 3757337; Fax: +39 040 3757361; Email: baralle{at}icgeb.org
Received August 2, 2005. Revised September 19, 2005. Accepted September 27, 2005.
Exon 3 of the human apolipoprotein A-II (apoA-II) gene is efficiently included in the mRNA although its acceptor site is significantly weak because of a peculiar (GU)16 tract instead of a canonical polypyrimidine tract within the intron 2/exon 3 junction. Our previous studies demonstrated that the SR proteins ASF/SF2 and SC35 bind specifically an exonic splicing enhancer (ESE) within exon 3 and promote exon 3 splicing. In the present study, we show that the ESE is necessary only in the proper context. In addition, we have characterized two novel sequences in the flanking introns that modulate apoA-II exon 3 splicing. There is a G-rich element in intron 2 that interacts with hnRNPH1 and inhibits exon 3 splicing. The second is a purine rich region in intron 3 that binds SRp40 and SRp55 and promotes exon 3 inclusion in mRNA. We have also found that the (GU) repeats in the apoA-II context bind the splicing factor TDP-43 and interfere with exon 3 definition. Significantly, blocking of TDP-43 expression by small interfering RNA overrides the need for all the other cis-acting elements making exon 3 inclusion constitutive even in the presence of disrupted exonic and intronic enhancers. Altogether, our results suggest that exonic and intronic enhancers have evolved to balance the negative effects of the two silencers located in intron 2 and hence rescue the constitutive exon 3 inclusion in apoA-II mRNA.
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