The current study was designed to identity polymorphisms in the genes involved in milk lipid biosynthesis to provide animal breeders with tools that allow selection of animals producing milk with healthier fatty acid composition. High concentrations of saturated fatty acids (SFA) in human diets are known to increase plasma cholesterol concentrations and, as a result, increase the risk of developing cardiovascular diseases (CVD), the number one cause of death worldwide. Because bovine milk is one of the primary sources of SFA and individual atherogenic fatty acids such as palmitic (16:0) and myristic (14:0) in human diets the improvement of the healthfulness of milk through selection becomes one of the primary measures that has been taken with the intention of decreasing the incidence of CVD among humans.

The candidate gene approach was used to address the objectives of the study. Genes involved in milk triacylglycerol (TAG) biosynthesis, fatty acid uptake into mammary gland and fatty acid transport inside the mammary epithelial cells, and transcriptional regulation of some lipogenic genes were investigated. DNA sequencing was used to discover single nucleotide polymorphisms (SNPs) in the genes of interest. After genotyping animals on the study for the discovered SNPs, the intragenic haplotypes were reconstructed and tested for the association with milk fatty acid composition.

The glycerol-3-phosphate acyltransferases-1 and -4 (GPAT1 and GPAT4), 1-acylglycerol-3-phosphate acyltransferase-1 (AGPAT1), and phosphatidate phosphatase (LPIN1) genes from the TAG biosynthetic pathway were studied in the first set of experiments to test the associations of the polymorphisms in those genes with milk fatty acid composition. The polymorphisms in GPAT4 were associated with large differences in atherogenic index (AI), concentrations of SFA, unsaturated fatty acids (UFA), monounsaturated fatty acids (MUFA), polyunsaturated fatty acids (PUFA), SFA/UFA, concentrations of capric (10:0), lauric (12:0), palmitic (16:0), and oleic (18:1c9) acids, CLA (18:2c9, t11), C16 and C18 desaturation indices in milk. The size of the effects of GPAT4 polymorphisms for some of the traits was numerically at least the same or larger compared with the effect of DGAT1 A232K mutation, making the polymorphisms in GPAAT4 to be a very valuable tool for the improvement of the healthfulness of milk. Other polymorphisms significantly associated with the studied traits in the first set of experiments were in GPAT1 for milk fat percentage, concentrations of short- and medium-chain SFA, and myristoleic (14:1c9) acid concentration, and in AGPAT1 for the concentrations of linoleic (18:2c9, c12) acid and other UFA. The polymorphisms in GPAT1 can be used to select for animals producing milk with higher percentage of fat and desirable concentrations of short- and medium-chain SFA. The polymorphisms in AGPAT1 can be used to select for animals producing milk with higher concentration of UFA and linoleic (18:2c9, c12) acid, in particular.

In the second set of experiments, the polymorphisms in the solute carrier family 27 (SLC27A6), isoform A6 and fatty acid binding proteins-3 and -4 (FABP3 and FABP4) genes involved in fatty acid uptake into mammary gland and fatty acid transport inside the mammary epithelial cells were tested for the association with milk fatty acid composition. The haplotype effects of SLC27A6 were associated significantly with milk fat percentage, AI, the concentrations of SFA, UFA, MUFA, SFA/UFA, the concentrations of capric (10:0), lauric (12:0), myristic (14:0), and palmitic (16:0) acids. The size of the haplotype effects of SLC27A6 on the studied traits was large and numerically similar to the size of allelic effects of DGAT1 A232K mutation that makes the polymorphisms in SLC27A6 as valuable as the of DGAT1 A232K mutation to select for animals producing milk with higher fat percentage and healthier fatty acid composition. The haplotype effects of FABP4 were associated significantly with the concentrations of SFA, UFA, MUFA, PUFA, SFA/UFA, the concentrations of linoleic (18:2c9, c12) acid, CLA (18:2c9, t11), and C18 desaturation index.

The sterol regulatory element binding protein-1c (SREBP-1c) is involved in the transcriptional regulation of lipogenesis and its proteolytic activation is controlled by SREBP cleavage-activating protein (SCAP) and insulin-induced genes (Insig) that are all part of the SREBP pathway. In the third set of experiments, the significant association of the overall haplotype effect of SREBP1 with the concentrations of myristic (14:0), myristoleic (14:1c9) acids, and C14 desaturation index were detected. The overall haplotype effect of Insig1 was associated with the concentrations of PUFA and linoleic (18:2c9, c12) acid. There were no significant associations with milk fatty acid composition determined for SCAP.

In conclusion, we were able to identify polymorphisms in a number of genes that were associated significantly with milk fat percentage and fatty acid composition. The information about those polymorphisms can be used to select for animals producing healthier milk.