Transgene and host growth hormone gene expression in pituitary and nonpituitary tissues of normal and growth hormone transgenic salmon.

Growth hormone (GH) gene expression has been examined in control and transgenic coho salmon containing a transgene comprised of the sockeye salmon GH1 gene under the control of the MT-B promoter from the same species. This transgene dramatically enhances the growth of salmonids, and raises serum GH levels some forty-fold. Transcript levels from this transgene were detected by RT-PCR using construct-specific GH primers in all tissues examined (liver, kidney, skin, intestine, stomach, muscle, spleen, pyloric caeca), and ranged from 0.1 - 9.4 pg/50 microg total RNA in different tissues as estimated by dot blot analysis. Interestingly, GH gene expression was also observed in intestine of control coho salmon by RT-PCR capable of detecting host and transgene transcripts using general primers. Sequence analysis of the intestinal GH mRNA from controls indicated it was derived from the coho GH2 gene. GH mRNA abundance analyzed by northern analysis indicates lower levels are found in large (400-500 g) than small transgenic salmon (20-21 g). No molecular evidence for transgene expression was obtained in tissues from transgenic fry, despite an obvious increase in size relative to control siblings, suggesting very low levels of transgene expression early in development. GH mRNA levels (per microg RNA) were also examined in the pituitary gland, and were found to be significantly lower (P < 0.01) in transgenic coho compared to nontransgenic animals of the same size. Pituitary glands of transgenic animals were also smaller than control animals of the same size, and pituitary size, expressed as a proportion of body weight, decreased with body size in transgenic but not control animals. These results imply that pituitary GH expression is regulated by negative feed-back controls as occurs in other vertebrate systems. GH mRNA was examined in pituitary glands by whole-mount in situ hybridization, and, whereas overall levels appeared reduced in transgenic animals, the site of hybridization did not differ between transgenic and control glands.