The human gene for γS-crystallin:: Alternative transcripts and expressed sequences from the first intron

Purpose: gamma S-crystallins are major components of adult vertebrate lenses. Here we examine the population of gamma S transcripts in adult human lens and the structure of the human CRYGS genes. Methods: Adult lens human transcripts were obtained from NEIBANK, an Expressed Sequence Tag (EST) analysis of human eye tissues. The human CRYGS gene was isolated as a PAC clone and sequenced by direct and PCR-based methods. Results: As judged by EST frequency, gamma S is one of the most abundant transcripts in the adult human lens, ranking just behind beta B2-, alpha B- and alpha A-crystallins. EST analysis reveals two transcript sizes resulting from alternative AATAAA and ATTAAA polyadenylation signals. In addition, one cDNA clone was found to contain a novel insert sequence that disrupted the open reading frame. Gene sequencing confirmed that this insert comes from intron 1 and is part of a sequence corresponding to a cluster of unidentified human transcripts in dbEST. Human and mouse gamma S gene proximal promoter sequences were compared and showed a high degree of evolutionary conservation, including consensus binding sites for transcription factors of the maf and SOX families. Conclusions: The human CRYGS gene can give rise to at least two transcripts through alternative polyadenylation. A minor transcript results from alternative splicing into sequences in intron 1. These sequences form part of a transcription unit (Mys) expressed in several non-lens tissues. The identity and function Mys of is not yet known, however, the cryptic splicing of CRYGS could produce a defective protein product, with potentially deleterious results for the adult human lens.