Genetic variation at the ovine milk proteins in Italian ovine breeds: new knowledge.Original Title Variazione genetica delle lattoproteine in razze ovine Italiane: nuove conoscenze.

摘要

Genetic polymorphism is an important source of variation affecting both the qualitative and quantitative protein variation. An interesting and quite complex protein pattern was found also for ovine milk. A new impulse was given to this subject recently, with the description and characterization of further genetic variation. A total of 3 variants (A, B and C) of beta-lactoglobulin, 7 variants (A, C', C", D, E, H, I) of alpha_s1-casein (CSN1S1), and 2 variants (A and B) of alpha_s2-caseine (CSN1S2) have been clearly established until now. As for the more recently identified casein variants, some details need to be highlighted. The absence of exon 8 was found for CSN1S1*H, this aberration being caused by a genomic deletion across the exon-8 DNA. Sequencing of CSN1S1 cDNA and mature protein showed the absence of exon 7 from CSN1S1*I in comparison with the C" variant. This allelic aberration is correlated with a sequence difference in 5' splice donor sequence of intron 7, leading to upstream skipping of exon 7. The CSN1S2*B variant, first observed in the Italian Gentile di Puglia breed, differs from the most common A variant for two amino acid exchanges: Asp₇₅->Tyr₇₅ and Ile₁₀₅->Val₁₀₅. The first substitution, resulting in a loss of a negative charge, is responsible for the higher isoelectric point of the B protein variant, which allows its detection by isoelectric focusing (IEF). Since the Asp₇₅->Tyr₇₅ substitution modifies the protein electric charge, milk properties may result affected to some extent. Moreover, different single nucleotide polymorphisms (SNP) were identified at CSN1S1, beta-caseina (CSN2), and kappa-CN (CSN3), all resulting in amino acid exchanges. More recently, further polymorphisms were found at CSN2 with a silent mutation in the triplet coding for Gln₁₉₂, and a C to A transversion responsible for the amino acids exchange Leu₁₉₆->Ile₁₉₆. In addition, an undescribed CSN1S2 variant was characterized by two linked mutations: a C to G transversion, responsible for the aminoacid exchange Asn₂₀₀->Lys₂₀₀ and a T to A transversion at the 14th nucleotide of the 16th exon. The ovine milk proteins deserve a bigger attention that has to be directed to: (i) completely characterizing the newly identified polymorphisms in order to establish the genetic variants carrying the mutation/s; (ii) monitoring the genetic variation available in the different Italian breeds; (iii) understanding of the functional meaning of the SNPs, deletions, and genetic variants described.