Structural organization and expression pattern of the canine RPGRIP1 isoforms in retinal tissue

PURPOSE. To examine the structure and expression of RPGRIP1 in dog retina. METHODS. Determination of the structural analysis and expression pattern of canine RPGRIP1 (cRPGRIP1) was based on cDNA amplification. Absolute quantification of the expression level of cRPGRIP1 splice variants was determined by qRT-PCR. Regulatory structures were examined by computational analysis of comparative genomics. RESULTS. cRPGRIP1 encompasses 25 exons that harbor a 3627-bp open reading frame (ORF) encoding a 1209-amino-acid (aa)-predicted protein. In addition to the main transcript, five full-length and several partial cRPGRIP1 isoforms were identified revealing four alternative 3'-terminal exons-24, 19a, 19c, and 19d-three of which could potentially produce C-terminally truncated proteins that lack the RPGR-interacting domain. A complex organization of the 5'-UTR for the cRPGRIP1 splice products have been described, with a common promoter driving multiple isoforms, including four full-length transcripts using the 3'-terminal exon 24. In addition, a potential alternative internal promoter was revealed to initiate a t least two cRPGRIP1 splice variants sharing the same 3'-terminal exon 19c. Transcription initiation sites were ighly supported by conserved arrangements of cis-elements predicted in a bioinformatic analysis of orthologous RPGRIP1 promoter regions. CONCLUSIONS. The use of alternative transcription start and termination sites results in substantial heterogeneity of cRPGRIP1 transcripts, many of which are likely to have tissue-specific expression. The identified exon-intron structure of cRPGRIP1 isoforms provides a basis for evaluating the gene defects underlying inherited retinal disorders in dogs.