A2-MILK – A NEW DIRECTION IN ANIMAL GENETICS

     There are a lot of people in many countries of the world who suffer from intolerance to cow's milk, that cause digestive disorders and some specific diseases. It has long been thought that this phenomenon is due to a lack of activity of the specific enzyme (lactase), which decompose milk sugar into a digestible form. But it was recently been found that milk intolerance is also due to specific type of milk protein, beta-casein, which content compose about 40% of the whole casein fraction in milk. There are two most common and genetically determined varieties of beta-casein molecule – A1 and A2 form. One type of beta-casein, A1, is harmful to the human body, and another type, A2, is not, and these forms are differed only by one amino acid residue in the molecule. In the case of beta-casein type A1 there is histidine at position 67 of the protein molecule, and for A2 – is proline at the same position. When milk, containing A1 form of beta-casein, enters the gastrointestinal tract, β-casomorphin is formed in the organism, which cause different digestive disorders and, if taken for a long time, can lead to diabetes and other serious diseases. This does not happen when using milk that contains A2 form of beta-casein. It was found that the cause for the replacement of the amino acid in the protein molecule is a point mutation in the beta-casein gene, which appeared many years ago in populations of European cattle breeds (for instance – in the ancestor populations of Holstein cattle breed). These cattle are superior to other breeds by milk productivity parameters, due to which it has become practically "transcontinental" and is used for intensive "Holsteinization" of local livestock in many countries. In the local populations of native cattle breeds in African and Asian countries, which have not yet undergone “Holsteinization”, this mutation has not been identified at all. All cattle in these countries, as well as goats and buffaloes, have only the A2 allele of the beta-casein gene. The same applies to breast milk. For recent years, in populations of Holstein cows the part of individuals that producing A1 milk reaches 75%, and A2 – does not exceed 25%, respectively.

     Currently, several methods are known for detecting different allelic variants of the beta-casein gene in the experimental cattle populations of different breeds. To identify the individuals with different allelic forms of beta-casein gene (A1 and A2), study not milk, but its producers directly, the animals – by the analysis of the blood samples, hair follicles or other biological material that containing DNA. Since the genotype of the animal does not change during the life cycle, individuals of the desired type it can be detected regardless of their age and sex.

    This year in molecular genetic laboratory of the Animal Biology Department (Faculty of Livestock Raising and Water Bioresources) started scientific work by the theme “To develop the technology of molecular-genetic support of the selection process for the creation of herds of cows that produce A2 milk”. The general aim of this study is to develop the technology of molecular genetic support of the selection process for the creation of herds of cows producing A2 milk. Currently, the effectiveness of different methods (ACRS-PCR, AS-PCR) of typing cattle by beta-casein (CSN2) gene has been studied and the best of them has been improved. In particular, the selection of optimal concentrations of reagents, amplification and restriction reactions parameters, electrophoresis conditions are carried out. This will allow starting next months the genotyping of cattle populations in several farms located in Kyiv and Cherkasy regions to determine their genetic structure and identify the different genotypes by beta-casein locus to create experimental populations of cows that producing A2 milk. Next, it is planned to form a specific “selection nucleus” of these breeds, select individuals with the desired genotype to obtain young animals that are carriers of A2 type beta-casein for using in the farther selection work. According to the implementation this work and developing technology, that mentioned above, and recommendations for its practical application in livestock farms and complexes, as well as a set of other organizational measures, by the end of 2023 will create the conditions for industrial production of A2 milk intended for infant and dietary nutrition, for consumption by people of all ages without threats to their health.

Roman Kulibaba,
Professor, Department of Animal Biology,
Faculty of Livestock Raising and Water Bioresources