Unbalanced expression of beta-casein variants A1 and A2 in Holstein-Friesian cows

Variant A1 of bovine beta-casein (CSN2) is known for producing beta-casomorphin-7 (BCM7), which is an opioid-like peptide released during gastrointestinal digestion. The aim of the study was to measure how much A1 and A2 protein variants occur in the milk of cows with different CSN2 genotypes. In a population of 113 A1A2 Holstein-Friesian cows, using the ELISA method, it was found that variant A2 was present at low content in milk (mean 6.31±3.09 ng/mL), but variant A1 reached almost seven times higher concentration (43.40 ng/mL±15.68 ng/mL). This unbalanced expression of the CSN2 alleles was not associated with the single nucleotide polymorphism within the 5’ flanking sequence known as beta-casein enhancer (BCE). Moreover, the origin of allele A1 (whether inherited from a sire or dam) did not explain its overexpression. Furthermore, using qRTPCR, it was shown that the mRNA levels of the CSN2 A1 and A2 alleles are at similar levels in the milk somatic cells of 16 A1A2 cows, suggesting that the unbalanced expression of CSN2 alleles could be determined by post-transcriptional events. Two SNPs were identified within the CSN2 3’ UTR in 109 cows with A1A2 genotype. The STarMiR software was used to predict microRNA targets and indicated that G/A was located within the canonical seed sites of bta-miR-145, potentially affecting miRNA-mRNA binding and translational repression of the CSN2 variant.