Variation in bovine leptin gene affects milk fatty acid composition in New Zealand Holstein Friesian × Jersey dairy cows.

Leptin is a protein hormone secreted from white adipose tissue. It regulates food/feed intake, body weight, immune function and reproduction. In our investigation, the polymerase chain reaction (PCR) amplification coupled with single-strand conformational polymorphism (SSCP) analysis was used to reveal variation in bovine leptin gene (LEP) in New Zealand (NZ) Holstein Friesian  $\times$  Jersey (HF  $\times$  J) dairy cows. Subsequent sequence analysis of a 430 bp amplicon spanning the entirety of exon 3 and part of the intron 2 region revealed three variant sequences ( $A_3$ , $B_3$ and $C_3$ ) containing a total of five nucleotide substitutions, all of which have been reported previously. Using general linear mixed-effect model analyses, the presence of variant $A_3$ (the most common variant) was associated with a decreased level of C15:1, C18:1 trans-11, C18:1 all trans, C18:2 trans-9, cis-12, C22:0 and C24:0 levels but increased levels of C12:1 and C13:0 iso ( $p<0.05$ ). Variant $B_3$ was associated with reduced levels of C6:0, C8:0, C11:0, C13:0 and C20:0 but increased C17:0 iso and C24:0 levels ( $p<0.05$ ). Variant $C_3$ was associated with decreased C17:0 iso levels but increased C20:0 ( $p<0.05$ ) levels. In a genotype model, the $A_3{B}_3$ genotype was associated with increased levels of C22:0 and C24:0 but decreased C8:0, C10:0, C11:0, C13:0, C15:0 and grouped medium-chain fatty acid (MCFA) levels ( $p<0.05$ ). Genotype $A_3{C}_3$ was found to be associated with decreased levels of C10:0, C11:0, C13:0 and grouped MCFA ( $p<0.05$ ). This is the first report of findings of this kind in NZ HF  $\times$  J cows, and they suggest that variation in exon 3 of bovine leptin gene could be explored as a means of decreasing the concentration of saturated fatty acids in milk.