Functional validation of GPIHBP1 and identification of a functional mutation in GPIHBP1 for milk fat traits in dairy cattle

In a previous genome-wide association study (GWAS) on milk production traits in a Chinese Holstein population, we revealed that GPIHBP1 is a novel promising candidate gene for milk fat content traits. In this study, we performed over-expression and RNAi experiments on GPIHBP1 in bovine primary mammary epithelial cells. The results showed that the expression of several important milk fat-related genes (LPL, CD36, VLDLR, ACACA and FASN) increased or decreased when the expression of GPIHBP1 was up- or down-regulated. To identify the potential functional SNP involved, we explored the genetic variants of GPIHBP1 and found that a G/A mutation (chr14:2553998) in the promoter region of GPIHBP1 significantly reduced promoter activity and had an effect on transcription factor binding sites. This finding was consistent with the lower expression of GPIHBP1 observed in the mammary gland tissue of cows harboring the homozygous AA mutation compared with wild-type homozygous GG or heterozygous AG. Furthermore, association analysis showed that cows with the AA genotype outperformed those with the GG and AG genotypes in terms of the milk fat percentage. Our study demonstrates that GPIHBP1 could be a strong candidate gene for milk fat content traits and, in particular, the G to A mutation at chr14:2553998 within GPIHBP1 could be a functional mutation related to its effects.