The impact of genetic polymorphisms on the protein composition of ruminant milksPatrice Martina, Malgorzata Szymanowskab, Lech Zwierzchowskib and Christine Lerouxc
a Institut National de la Recherche Agronomique, Laboratoire de Génétique biochimique et de Cytogénétique, bâtiment Jacques Poly, Centre de Jouy, 78352 Jouy-en-Josas, France
b Institute of Genetics and Animal Breeding, Polish Academy of Sciences, Jastrzebiec, 05-552 Wólka Kosowska, Poland
c Institut National de la Recherche Agronomique, Unité de Recherches sur les Herbivores, équipe TALL, Centre de Theix, 63122 Saint-Genès-Champanelle, France
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: -lactalbumin, -lactoglobulin and the four caseins ( , , and ), 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 -lactoglobulin and - and -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 - and -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 -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.
Key words: gene expression / milk protein / genetic polymorphism / ruminants
Correspondence and reprints: Patrice Martin
© INRA, EDP Sciences 2002