Bidwell Christopher A, Kramer Lauren N, Perkins Allison C, Hadfield Tracy S, Moody Diane E, Cockett Noelle E
Department of Animal Sciences, Purdue University, 125 South Russell Street, West Lafayette, IN 47907-2042 USA.
BMC Biol. 2004 Aug 6;2:17. doi: 10.1186/1741-7007-2-17.
The callipyge mutation is located within an imprinted gene cluster on ovine chromosome 18. The callipyge trait exhibits polar overdominant inheritance due to the fact that only heterozygotes inheriting a mutant paternal allele (paternal heterozygotes) have a phenotype of muscle hypertrophy, reduced fat and a more compact skeleton. The mutation is a single A to G transition in an intergenic region that results in the increased expression of several genes within the imprinted cluster without changing their parent-of-origin allele-specific expression.
There was a significant effect of genotype (p < 0.0001) on the transcript abundance of DLK1, PEG11, and MEG8 in the muscles of lambs with the callipyge allele. DLK1 and PEG11 transcript levels were elevated in the hypertrophied muscles of paternal heterozygous animals relative to animals of the other three genotypes. The PEG11 locus produces a single 6.5 kb transcript and two smaller antisense strand transcripts, referred to as PEG11AS, in skeletal muscle. PEG11AS transcripts were detectable over a 5.5 kb region beginning 1.2 kb upstream of the PEG11 start codon and spanning the entire open reading frame. Analysis of PEG11 expression by quantitative PCR shows a 200-fold induction in the hypertrophied muscles of paternal heterozygous animals and a 13-fold induction in homozygous callipyge animals. PEG11 transcripts were 14-fold more abundant than PEG11AS transcripts in the gluteus medius of paternal heterozygous animals. PEG11AS transcripts were expressed at higher levels than PEG11 transcripts in the gluteus medius of animals of the other three genotypes.
The effect of the callipyge mutation has been to alter the expression of DLK1, GTL2, PEG11 and MEG8 in the hypertrophied skeletal muscles. Transcript abundance of DLK1 and PEG11 was highest in paternal heterozygous animals and exhibited polar overdominant gene expression patterns; therefore, both genes are candidates for causing skeletal muscle hypertrophy. There was unique relationship of PEG11 and PEG11AS transcript abundance in the paternal heterozygous animals that suggests a RNA interference mechanism may have a role in PEG11 gene regulation and polar overdominance in callipyge sheep.
臀肌肥大突变位于绵羊18号染色体上的一个印记基因簇内。臀肌肥大性状表现出极性超显性遗传，因为只有继承突变父本等位基因的杂合子（父本杂合子）具有肌肉肥大、脂肪减少和骨骼更紧凑的表型。该突变是基因间区域中单个A到G的转换，导致印记簇内几个基因的表达增加，而不改变其亲本来源的等位基因特异性表达。
基因型对携带臀肌肥大等位基因的羔羊肌肉中DLK1、PEG11和MEG8的转录本丰度有显著影响（p < 0.0001）。相对于其他三种基因型的动物，父本杂合动物肥大肌肉中DLK1和PEG11的转录水平升高。PEG11基因座在骨骼肌中产生一个6.5 kb的单一转录本和两个较小的反义链转录本，称为PEG11AS。PEG11AS转录本在PEG11起始密码子上游1.2 kb处开始的5.5 kb区域内可检测到，并跨越整个开放阅读框。通过定量PCR分析PEG11的表达表明，父本杂合动物肥大肌肉中有200倍的诱导，纯合臀肌肥大动物中有13倍的诱导。在父本杂合动物的臀中肌中，PEG11转录本比PEG11AS转录本丰富14倍。在其他三种基因型动物的臀中肌中，PEG11AS转录本的表达水平高于PEG11转录本。
臀肌肥大突变的作用是改变肥大骨骼肌中DLK1、GTL2（此处原文有误，应为PEG11）、PEG11和MEG8的表达。DLK1和PEG11的转录本丰度在父本杂合动物中最高，并表现出极性超显性基因表达模式；因此，这两个基因都是导致骨骼肌肥大的候选基因。父本杂合动物中PEG11和PEG11AS转录本丰度存在独特关系，这表明RNA干扰机制可能在臀肌肥大绵羊的PEG11基因调控和极性超显性中起作用。
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