Complex alternative splicing partially inactivates the human chorionic somatomammotropin-like (hCS-L) gene

The human growth hormone/human chorionic somatomammotropin (hGH/hCS) gene cluster contains five genes: hGH-N, hGH-V, hCS-A, hCS-B, and hCS-L. The expression of the first four genes has been well documented. In contrast, the hCS-L gene has been considered a pseudogene inactivated by loss of the normal intron 2 splice donor site. Previously our laboratory has shown that hCS-L transcripts are present in human placenta and that their levels are induced during the second trimester. More detailed studies of hCS-L transcript processing and mRNA structure are hindered by overwhelming levels of the structurally similar hCS-A and hCS-B transcripts in the placenta. To circumvent this problem, we have established stably transfected cell lines selectively expressing the hCS-L gene. Analysis of hCS-L mRNA from these cell lines demonstrates at least five major alternative splicing pathways, four of which could be confirmed qualitatively by parallel analysis of placental RNA. This analysis reveals an unexpectedly high frequency of exon 2 skipping (73%) as well as utilization of three competing exon 3 splice acceptor sites. Since exon 2 encodes the signal peptide, the majority of hCS-L transcripts are unable to express a secreted protein. Three of the defined hCS-L mRNAs contain an extended open reading frame similar to that present in the functional GH and CS genes. All three of these hCS-L transcripts are of minor abundance and only two, hCS-L(L) and hCS-L(L'), contain exon 2. In vitro translation and signal peptide processing of hCS-L(L) mRNA yields a 20-kDa hCS-L isoform in vitro. These data confirm the placental expression of the hCS-L gene, demonstrate surprising complexity in the splicing of its transcripts, predict that the majority of processed hCS-L mRNAs are nonfunctional, and identify specific, low abundance mRNAs that may encode novel gestational Hormones.