Polymorphisms in porcine TRH and TRHR gene and associations with growth and fatness traits

Thyrotropin-releasing hormone (TRH), produced in the hypothalamus and stimulating secretion of thyrotropin from the pituitary, is central in regulating the hypothalamic–pituitary–thyroid (HPT) axis. The number of TRH receptors (TRHR) expressed on the surface of target cell is a crucial factor influencing the magnitude of TRH response. The objective of this study was to identify the DNA polymorphisms existing in pig TRH and TRHR gene and to evaluate their associations with porcine growth, carcass and meat quality traits. Six polymorphisms in TRH and seven in TRHR gene in total were identified by re-sequencing. Among the polymorphisms, TRH ss325994920 was a missense mutation at TRH precursor residue 173 causing Val>Met (GTG>ATG) and would generate mutant pFE22/pSE14 peptides which are non-TRH peptides derived from the TRH precursor and have roles in regulating hormone ACTH and PRL. Allele frequencies of the polymorphisms were evaluated in four Western and two Chinese pig breeds (N=228). Opposite alleles prevailed in Western pigs and Chinese pigs at loci ss325994933, ss325994918, ss325994923, ss325994924 and ss325994925, and existence of high linkage disequilibrium blocks in all six breeds were observed. A resource population (N=409) based on crosses of Pietrain and Jinhua pigs were used for the association tests. TRH gene was significantly (P<0.05) associated with intramuscular fat content, BFT at shoulder and ADG from 150 to 180days old, while TRHR gene was significantly associated with head weight and ADG from 90 to 120days in the Pietrain×Jinhua resource population. Thus, it was suggested that TRH and TRHR could be the candidate genes for growth, meat quality and fatness traits. As extended linkage disequilibrium existed in resource populations, the association results obtained in the present study need further verification. Though verifying test in Yorkshire (N=57), Landrace (N=27) and Duroc (N=30) showed no significant results, the sample number might not be big enough to be statistical powerful. Peripheral tissue expression profile of porcine TRH mRNA was also investigated in this study. Co-expression of TRH and its receptor TRHR in the same tissue, especially in fat tissue, indicated the possibility of the existence of local paracrine/autocrine regulations of TRH on respective peripheral tissue.