Three novel ABCC5 splice variants in human retina and their role as regulators of ABCC5 gene expression

Background: The ABCC5 gene encodes an organic anion pump of the ATP-binding cassette (ABC) transporter family, subclass C. The exact physiological function of ABCC5 however is not known. Here, we have isolated three novel ABCC5 splice variants and characterized their role in the regulation of ABCC5 gene expression. Results: Two additional exons within intron 5 of the ABCC5 gene were identified; one of the exons exhibits alternative donor splice sites. Differential usage of these exons generates three short ABCC5 transcripts named ABCC5_SV1, ABCC5_SV2 and ABCC5_ SV3. The variants share the first five exons with the ABCC5 gene but differ in their 3' sequences. ABCC5 and its novel isoforms are abundantly expressed in the human retina. Splice variant ABCC5_SV1 and ABCC5_SV2 contain premature stop codons. While inhibition of nonsense- mediated mRNA decay selectively stabilized ABCC5_SV1 but not ABCC5_SV2, the amount of full length ABCC5 mRNA was simultaneously reduced. A negative regulatory effect on full length ABCC5 expression was also observed when the ABCC5 isoforms were silenced with siRNA duplexes. Finally, we show that the evolutionarily conserved ABCC5_SV2 transcript is translated into a protein abundantly present in endothelial cells of inner retinal blood vessels and along RPE membranes. Conclusion: Our data suggest that alternative splicing of the ABCC5 gene has functional consequences by modulating ABCC5 gene expression. In addition, at least one ABCC5 splice variant is protein-coding and produces a truncated ABCC5 protein isoform with thus far unknown functional properties in the retina.