The first report of genetic polymorphisms of the equine growth hormone gene and its association with conformation and muscle-related traits in thoroughbred-Turkmen crossbred horses

The growth hormone (GH) gene is a significant for growth in livestock. The equine growth hormone gene is located on chromosome 11, comprising 5 exons and 4 introns. Numerous studies have explored genetic diversity among various horse breeds, yet limited research exists on Thoroughbred-Turkmen crossbred horses. The aim of the present study was to investigate the polymorphisms of exon 4 and 5 of the growth hormone gene and their associations with conformation and muscle-related traits in crossbred horses.

Blood samples from 96 horses underwent DNA extraction via a modified salting-out method. PCR-SSCP analysis indicated polymorphisms in exons 4 and 5 of the growth hormone gene. The band patterns A, B, and C frequencies for exon 4 were 36.45 %, 75 43.79 %, and 19.79 %, respectively. For exon 5, the band patterns A and B frequencies were 10.41 % and 89.58 %, respectively. The results of the sequencing analysis identified a novel single nucleotide polymorphism (SNP) (OP765334.1: 249(C/T)) located within exon 4, in addition to four other SNPs in exon 5, specifically OP765336.1: 118 T > C, OP765336.1: 142 G > A, OP765336.1: 175 T > C, and OP765336.1: 238 C > G, which collectively result in three missense SNPs leading to amino acid substitutions at positions 118, 142, and 175. These substitutions correspond to alterations in the amino acid sequences, notably aspartic acid (D), leucine (L), and alanine (A), respectively. The association analysis demonstrated a significant association between gender and various phenotypic measures, including neck midpoint circumference (NMC), forearm midpoint circumference (FMC), eye to eye width (EEW), muzzle circumference (MC), jaw width (JW), neck circumference at throat latch (NCTL), neck circumference at base (NCB) and fore cannon midpoint circumference (FCMC) with a statistical significance of P < 0.05. Therefore, our findings indicate that body size is predominantly shaped by demographic characteristics, with age and gender exerting considerable influence on these phenotypic traits.

The investigation of genetic variants in the GH gene across various livestock species has revealed significant associations with growth traits, underscoring the need for further research with larger sample sizes. This is particularly important as the current studies indicate varying degrees of significance in the association between GH gene polymorphisms and growth metrics, suggesting that more comprehensive data could clarify these associations.