Zhou Haiyan, Brockington Martin, Jungbluth Heinz, Monk David, Stanier Philip, Sewry Caroline A, Moore Gudrun E, Muntoni Francesco
Dubowitz Neuromuscular Centre, Department of Paediatrics, Imperial College, Hammersmith Hospital London, United Kingdom.
Am J Hum Genet. 2006 Nov;79(5):859-68. doi: 10.1086/508500. Epub 2006 Sep 21.
Epigenetic regulation of gene expression is a source of genetic variation, which can mimic recessive mutations by creating transcriptional haploinsufficiency. Germline epimutations and genomic imprinting are typical examples, although their existence can be difficult to reveal. Genomic imprinting can be tissue specific, with biallelic expression in some tissues and monoallelic expression in others or with polymorphic expression in the general population. Mutations in the skeletal-muscle ryanodine-receptor gene (RYR1) are associated with malignant hyperthermia susceptibility and the congenital myopathies central core disease and multiminicore disease. RYR1 has never been thought to be affected by epigenetic regulation. However, during the RYR1-mutation analysis of a cohort of patients with recessive core myopathies, we discovered that 6 (55%) of 11 patients had monoallelic RYR1 transcription in skeletal muscle, despite being heterozygous at the genomic level. In families for which parental DNA was available, segregation studies showed that the nonexpressed allele was maternally inherited. Transcription analysis in patients' fibroblasts and lymphoblastoid cell lines indicated biallelic expression, which suggests tissue-specific silencing. Transcription analysis of normal human fetal tissues showed that RYR1 was monoallelically expressed in skeletal and smooth muscles, brain, and eye in 10% of cases. In contrast, 25 normal adult human skeletal-muscle samples displayed only biallelic expression. Finally, the administration of the DNA methyltransferase inhibitor 5-aza-deoxycytidine to cultured patient skeletal-muscle myoblasts reactivated the transcription of the silenced allele, which suggests hypermethylation as a mechanism for RYR1 silencing. Our data indicate that RYR1 undergoes polymorphic, tissue-specific, and developmentally regulated allele silencing and that this unveils recessive mutations in patients with core myopathies. Furthermore, our data suggest that imprinting is a likely mechanism for this phenomenon and that similar mechanisms could play a role in human phenotypic heterogeneity.
基因表达的表观遗传调控是遗传变异的一个来源，它可以通过造成转录单倍剂量不足来模拟隐性突变。种系表观突变和基因组印记就是典型例子，尽管它们的存在可能难以揭示。基因组印记可能具有组织特异性，在某些组织中是双等位基因表达，在其他组织中是单等位基因表达，或者在普通人群中存在多态性表达。骨骼肌兰尼碱受体基因（RYR1）的突变与恶性高热易感性以及先天性肌病中央轴空病和多微小轴空病相关。此前从未认为RYR1会受表观遗传调控影响。然而，在对一组隐性核心肌病患者进行RYR1突变分析时，我们发现11名患者中有6名（55%）在骨骼肌中存在RYR1单等位基因转录，尽管其在基因组水平为杂合子。在可获取父母DNA的家庭中，分离研究表明未表达的等位基因是母系遗传的。对患者成纤维细胞和淋巴母细胞系的转录分析显示为双等位基因表达，这表明存在组织特异性沉默。对正常人类胎儿组织的转录分析表明，10%的病例中RYR1在骨骼肌、平滑肌、脑和眼中是单等位基因表达。相比之下，25份正常成人骨骼肌样本仅显示双等位基因表达。最后，对培养的患者骨骼肌成肌细胞施用DNA甲基转移酶抑制剂5-氮杂-脱氧胞苷可重新激活沉默等位基因的转录，这表明甲基化过度是RYR1沉默的一种机制。我们的数据表明，RYR1经历多态性、组织特异性和发育调控的等位基因沉默，并且这揭示了核心肌病患者中的隐性突变。此外，我们的数据表明印记可能是这一现象的一种机制，并且类似机制可能在人类表型异质性中起作用。
