Abstract
The acetyl-CoA acyltransferase 2 (ACAA2) gene encodes an enzyme of the thiolase family that is involved in mitochondrial fatty acid elongation and degradation by catalyzing the last step of the respective β-oxidation pathway. The increased energy needs for gluconeogenesis and triglyceride synthesis during lactation are met primarily by increased fatty acid oxidation. Therefore, the ACAA2 enzyme plays an important role in the supply of energy and carbon substrates for lactation and may thus affect milk production traits. This study investigated the association of the ACAA2 gene with important sheep traits and the putative functional involvement of this gene in dairy traits. A single nucleotide substitution, a T to C transition located in the 3' untranslated region of the ACAA2 gene, was used in mixed model association analysis with milk yield, milk protein yield and percentage, milk fat yield and percentage, and litter size at birth. The single nucleotide polymorphism was significantly associated with total lactation production and milk protein percentage, with respective additive effects of 6.81 ± 2.95 kg and -0.05 ± 0.02%. Additionally, a significant dominance effect of 0.46 ± 0.21 kg was detected for milk fat yield. Homozygous TT and heterozygous CT animals exhibited higher milk yield compared with homozygous CC animals, whereas the latter exhibited increased milk protein percentage. Expression analysis from age-, lactation-, and parity-matched female sheep showed that mRNA expression of the ACAA2 gene from TT animals was 2.8- and 11.8-fold higher in liver and mammary gland, respectively. In addition, by developing an allelic expression imbalance assay, it was estimated that the T allele was expressed at an average of 18% more compared with the C allele in the udder of randomly selected ewes. We demonstrated for the first time that the variants in the 3' untranslated region of the ovine ACAA2 gene are differentially expressed in homozygous ewes of each allele and exhibit allelic expression imbalance within heterozygotes in a tissue-specific manner, supporting the existence of cis-regulatory DNA variation in the ovine ACAA2 gene. This is the first study reporting differential allelic imbalance expression of a candidate gene associated with milk production traits in dairy sheep.
Highlights
Milk yield represents more than two-thirds of the total income of the dairy sheep industry (Carta et al, 2009); the improvement of milk production is the most important breeding objective
We demonstrated for the first time that the variants in the 3′ untranslated region of the ovine acyltransferase 2 (ACAA2) gene are differentially expressed in homozygous ewes of each allele and exhibit allelic expression imbalance within heterozygotes in a tissue-specific manner, supporting the existence of cis-regulatory DNA variation in the ovine ACAA2 gene
To further confirm the initial results from the semiquantitative method consistent with allelic expression imbalance (AEI) in ACAA2 gene, we developed an AEI assay using TaqMan probes to estimate the T:C transcription ratio in cDNA samples corrected by the mean T:C ratio in genomic DNA (gDNA) samples and test for deviations from the expected ratio of 1 in the absence of AEI
Summary
Milk yield represents more than two-thirds of the total income of the dairy sheep industry (Carta et al, 2009); the improvement of milk production is the most important breeding objective. In Mediterranean countries, most of the ovine milk produced is used for the production of cheese commercialized as products of protected designation of origin (PDO) and other quality labels (Arranz and Gutiérrez-Gil, 2012). Marker-assisted selection could be of special interest for dairy sheep due to the great regional diversity of breeds, funding limitations, organizational difficulties (Arranz and Gutiérrez-Gil, 2012), and small population sizes (Garciá-Gámez et al, 2012).
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