Abstract
Previously, we have shown that alleles of the BM1500 microsatellite, located 3.6 kb downstream of the leptin gene in cattle, were associated with carcass fat measures in a population of 154 unrelated beef bulls. Subsequently, a cytosine (C) to thymine (T) transition that encoded an amino acid change of an arginine to a cysteine was identified in exon 2 of the leptin gene. A PCR-RFLP was designed and allele frequencies in four beef breeds were correlated with levels of carcass fat. The T allele was associated with fatter carcasses and the C allele with leaner carcasses. The frequencies of the SNP alleles among breeds indicated that British breeds have a higher frequency of the T allele whereas the continental breeds have a higher occurrence of the C allele. A ribonuclease protection assay was developed to quantify leptin mRNA in a separate group of animals selected by genotype. Animals homozygous for thymine expressed higher levels of leptin mRNA. This may suggest that the T allele, which adds an extra cysteine to the protein, imparts a partial loss of biological function and hence could be the causative mutation.
Highlights
Leptin is the hormone product of the obese gene that acts on central and peripheral tissues to modulate appetite and energy metabolism [16]
Among the six bulls sequenced, one SNP was found in leptin exon 2 and five SNPs were found in exon 3
All three bulls selected for sequencing based on high fat phenotype were homozygous for thymine and a cysteine amino acid in the leptin molecule at this position
Summary
Leptin is the hormone product of the obese gene that acts on central and peripheral tissues to modulate appetite and energy metabolism [16]. Plasma leptin levels in cattle and sheep increase linearly with increased body fat mass and with increased energy balance [2,3,6]. In vitro studies suggest that leptin can directly modulate energy metabolism in peripheral tissues and may antagonize insulin activities in adipose [21] and muscle [22]. These physiological properties support leptin as a strong candidate gene for evaluation of genetic polymorphisms that could affect carcass fat content in cattle
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