The impact of genetic polymorphisms on the protein composition of ruminant milks

Abstract

-The purpose of this review is to give an overview of our current knowledge on the polymorphisms occurring in genes coding for milk proteins and responsible for quantitative variability in their expression, thus influencing the protein composition of livestock ruminant milk. The overall genomic organisation of the 6 main ruminant milk protein genes: a-lactalbumin, b-lactoglobulin and the four caseins (a s1 , a s2 , b and k), their chromosomal location and their expression pattern are first summarised before presenting general mechanisms controlling gene expression both at the transcriptional and the post-transcriptional levels. Polymorphisms found in cis-regulatory elements, mainly within the 5'-flanking region of the genes encoding b-lactoglobulin and a s1 -and a s2 -caseins, have been found, in cattle, to influence their transcription rate. In addition, polymorphisms found in the transcription unit, within intron as well as exon sequences, have been shown to be responsible for defects in the processing of primary transcripts and/or the export of messenger RNA to the cytoplasm. Mutations responsible for the occurrence of premature stop codons in a s1 -and b-casein mRNAs have been shown to be associated both with a decrease in the level of the relevant transcripts and the existence of multiple forms of messengers due to alternative splicing (exon skipping, usage of cryptic splice sites). Such a situation, well-exemplified by the gene encoding a s1 -casein in the goat, may have dramatic biological consequences (secretion pathway, casein micelle structure, fat content, etc.) by modifying the message and accordingly the primary structure of the protein as well as its expression. Since some of these polymorphisms dramatically affect technological properties of milk, including cheese yields and organoleptic characteristics, methods mainly based on the PCR technique have been designed and applied in selection and breeding programmes to improve milk protein quality.gene expression / milk protein / genetic polymorphism / ruminants Reprod. Nutr. Dev. 42 (2002) 433-459 433

Translational efficiency of casein transcripts in the mammary tissue of lactating ruminants

“…These extreme situations reflect a general phenomenon which can take more subtle appearances, in which relative proportions of caseins may vary significantly between species and even between individuals, within species. This is particularly exemplified, as far as α s1 -and β-caseins are concerned, in the goat species [8]. A minimum of 17 alleles have so been far characterized at the CSN1S1 locus [9], distributed among seven different classes of protein variants, associated with four levels of expression ranging between 0 (α s1 -CasO) and 3.5 g.L −1 (α s1 -CasA, B, C and M) per allele.…”

Milk quality – a future approach from a researcher’s point of view

“…So far, more than 18 alleles have been identified, and are distributed amongst seven different classes of protein variants which again are grouped in the so-called "strong", "medium", "weak" and "null" (Martin et al, 2002;Devold, 2004) indicating the level of synthesis of the α s1 -casein. The "null" variants, the 01 and 02 alleles, are associated with lack of synthesis of this protein (Leroux et al, 1990).…”

Short communication: Genetic variability in the predicted microRNA target sites of caprine casein genes

“…To date, 4 loci have been described in ruminants: α S1 -casein (CSN1S1), α S2 -casein (CSN1S2), β-casein (CSN2), and κ-casein (CSN3). Casein expression and secretion as micelles are highly regulated processes restricted to the lactation period and modulated by a complex network of environmental and genetic factors (Martin et al, 2002). Technological properties of milk are mainly determined by its casein content, highlighting the importance of understanding how these genes are regulated (Martin et al, 2002).…”

“…Casein expression and secretion as micelles are highly regulated processes restricted to the lactation period and modulated by a complex network of environmental and genetic factors (Martin et al, 2002). Technological properties of milk are mainly determined by its casein content, highlighting the importance of understanding how these genes are regulated (Martin et al, 2002). In a recent study, Ogorevc et al (2009) provided in silico evidence suggesting that several miRNA expressed in the bovine mammary gland might target casein genes, a feature that would affect their levels of expression.…”