Endocrine Abstracts

The somatostatin receptor 5 (sst5) and its truncated splicing variants (sst5TMD5, sst5TMD4) are considered putative biomarkers that can predict pharmacological response or aggressiveness in several endocrine-related pathologies, such as acromegaly. sst5 is encoded by a gene, SSTR5, that lacks introns within its coding sequence, and hence, the splicing variants identified are generated by non-canonical splicing events. However, the mechanisms underlying their genesis and regulation are still to be fully elucidated. Recent analyses of SSTR5 gene structure revealed the existence of a natural antisense transcript, named SSTR5-AS1, which overlaps with SSTR5 and encodes an intergenic long non-coding RNA that may be involved in the regulation of SSTR5 expression and processing. Likewise, recent studies indicate that DNA methylation within genic regions could influence exon inclusion and alternative splicing. Accordingly, in this study we have implemented pilot in silico analyses of the human SSTR5 and SSTR5-AS1 genes to explore the existence of CpG islands (high density CG regions susceptible to be methylated), located at their promoters, and, most interestingly, within their coding regions. Our ultimate goal was to ascertain if these processes could contribute to the regulation of the expression of SSTR5 and the generation of its variants in somatotropinomas. Firstly, we measured the mRNA levels of sst5, sst5TMD4 and sst5TMD5 variants and the SSTR5-AS1 by qPCR in a cohort of 11 normal pituitary (NPs) and 27 somatotropinoma samples. In addition, we studied the methylation status in four CpG areas of SSTR5 and SSTR5-AS1 genes. Our results revealed that somatotropinomas expressed significantly more sst5 than NPs, whereas no significant differences were found in the expression of the antisense transcript. In addition, expression of sst5TMD4 and sst5TMD5 variants was numerically, although not significantly increased in this pilot cohort of patients. Methylation analysis revealed that CpG sites were differentially methylated on SSTR5 and SSTR5-AS1 genes in acromegaly compared with NPs, which might explain the differential expression of sst5. Interestingly, mRNA levels of SSTR5 showed a direct correlation with SSTR5-AS1 expression, but not with those of the sst5 splicing variants, both in acromegaly and NP samples, which could suggest that the antisense may regulate the expression of the full-length sst5, without affecting its splicing variants. Summarizing, results from this work supports the notion that epigenetic and post-transcriptional events might contribute to regulate the expression of SSTR5 in acromegaly, while their precise involvement in SSTR5 splicing dysregulation remain to be determined.