Alternative transcription of the mouse Gh gene identifies an immune-associated transcript with species-specific structural divergence.

Growth hormone (GH) in mice is primarily expressed in the anterior pituitary, although Gh expression has been reported in extrapituitary tissues, including immune organs. However, the structure of immune-associated Gh transcripts remains poorly characterized. To determine whether splenic Gh transcripts differ from pituitary Gh mRNA, 5'- and 3'-rapid amplification of cDNA ends (RACE) analyses were performed. While 3' RACE showed a shared polyadenylation site, 5' RACE identified a novel exon located approximately 2 kb upstream of the conventional exon 1, generating a transcript (spl-Gh mRNA) with a distinct first exon but shared downstream exons with pituitary Gh mRNA (pit-Gh mRNA). RT-PCR analysis revealed that spl-Gh mRNA is predominantly expressed in immune tissues such as spleen and bone marrow, and its distribution did not correlate with Pit-1 mRNA expression. Quantitative RT-PCR further demonstrated that spl-Gh mRNA was expressed at levels comparable to those of pit-Gh mRNA in the mouse spleen, indicating that spl-Gh is one of the major Gh transcript forms in this tissue. Sequence analysis indicated that spl-Gh mRNA is predicted to retain coding potential for a GH protein. Comparative genomic analyses further demonstrated that genomic features associated with the spl-Gh transcriptional unit are conserved only in a subset of closely related Mus species. In contrast, although a spl-Gh-related transcript was detected in rat spleen, no properly spliced mouse-like transcript was identified under the present experimental conditions. The detected transcript exhibited intron retention and an in-frame stop codon, suggesting that it is unlikely to produce a functional GH protein. These findings identify a distinct immune-associated Gh transcript generated through alternative transcription of the mouse Gh gene and suggest that immune-associated Gh transcriptional mechanisms have undergone species-specific divergence among rodents. Together, these findings reveal previously unrecognized complexity in Gh gene regulation and highlight species-specific differences in immune-associated Gh transcripts.