Karyotype and single nucleotide polymorphism of the prolactin gene in milking bucks

There is no genetic disease known currently to be linked to prolactin or its receptor in goats. Given the essential role of this hormonal system in the physiology of the mammary gland, we reasoned that genetic anomalies of Prl/PrlR genes could be related to the occurrence of mammary diseases. Karyotyping was performed, and the single nucleotide polymorphisms (SNPs) of the prolactin gene were investigated in goats with gynecomastia and galactorrhea (“milking bucks”). The results showed that the karyotype was identical between milking bucks and normal males (2n = 60, XY). Two heterozygous SNPs, G8895A and A8898C, were detected in exon 5 of the prolactin gene, and they corresponded to two amino acid substitutions, lysine (Lys) → glutamate (Glu)₁₄₅ and threonine (Thr) → proline (Pro)₁₄₆. The first mutation was found in only one milking buck; the second was found in both milking bucks and in four normal males. The alignment analysis for homology of the complete coding sequence (CDS) of the prolactin gene among cattle, sheep, goat, dog, rabbit and human indicated that the 145th amino acid, Glu, was highly conserved in the prolactin gene of all species that exhibited gynecomastia in males. However, the 146th amino acid was Pro for goat, sheep, cattle and dog, and isoleucine (Ile) for rabbit and human. These mutations occurred in the loop region, between the third and the fourth helices of prolactin, which would affect backbone chemical shifts indirectly by changing the average chemical environment of the backbone amides. It was presumed that the second mutation (Thr → Pro₁₄₆) may be associated with gynecomastia in bucks. Further studies are needed to confirm the assumption.