GENOTYPING AND PROTEIN PROFILING OF MILK β-CASEIN VARIANTS (A1 and A2) IN INDIAN BREEDS OF MILCH CATTLE
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Date
2018-03
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SRI VENKATESWARA VETERINARY UNIVERSITY TIRUPATI - 517 502. (A.P.) INDIA
Abstract
ABSTRACT:
The exotic and cross bred cattle milk contain A1 β-casein variant, whereas
indigenous cattle milk contains A2 β-casein variant. They have structural diversity at
67th amino acid residue proline in A2 which is replaced by histidine in A1 during
course of evolution. The histidine at 67th amino acid residue generates a proteolytic
cleavage site in A1 leading to the production of betacasomorphin-7 (BCM-7) that is
known to be a risk factor for several human diseases. The serious health concern
associated with A1 milk consumption endures an urgent need to select cattle breeds
with higher genotypic and allelic frequencies of A2 and conserve indigenous cattle
breeds through breeding programmes. In this context, the present study was
undertaken to screen seventy animals belonging to five indigenous cattle breeds of
India viz., Deoni, Sahiwal, Ongole, Punganur, Malnad Gidda and Holstein Friesan
cross (twelve from each breed and ten from Deoni) to characterize β-casein variants
(A1 and A2) geotypically by PCR-RFLP with TaqI restriction enzyme and also to
study milk β-casein variants (A1 and A2) by IEF. PCR-RFLP with Taq1 restriction enzyme yielded two genotypes as A1A2 (251 and 213 bp) and A2A2 (251 bp) in all indigenous cattle breeds studied, whereas only heterozygous (A1A2) genotype was found in the HF crossbreds. The genotypic frequency of A1A2 heterozygous variant of exon 7 of CSN2 gene was the highest for HF cross (1.0) followed by Sahiwal (0.75), Deoni (0.58), Malnad Gidda (0.40),
Punganur (0.17) and Ongole (0.083). Whereas, for A2A2 homozygous variant was the
highest for Ongole (0.92) followed by Punganur (0.83), Malnad Gidda (0.60), Deoni
(0.42) and Sahiwal (0.25). The A1 allelic frequency of exon7 of CSN2 gene was the
highest in HF cross (0.50) followed by Sahiwal (0.38), Deoni (0.29), Malnad Gidda
(0.20), Punganur (0.083) and Ongole (0.042), whereas the highest A2 allelic
frequency was observed in Ongole (0.96) followed by Punganur (0.92), Malnad Gidda
(0.80), Deoni (0.71), Sahiwal (0.63) and HF cross (0.50).
The IEF gel electrophoresis pattern of milk β-casein protein showed two
distinct bands corresponding to A1 and A2 β-casein protein variants at pI values 5.22
and 5.14 respectively and this polymorphism correlated positively with the results of
PCR-RFLP with TaqI restriction enzyme. The present study unveils the existing fact
that there is incorporation of exotic germplasm in native cattle breeds to varying
degrees in different breeds and emphasizes an immediate need for genotypic
screening of bulls for β-casein gene variant A2 for documentation and further
measures to improve the indigenous breeds with appropriate breeding strategies.
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