title: Epigenetic allele silencing unveils recessive RYR1 mutations in core myopathies.
abstract: 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.
authors.affiliation: Dubowitz Neuromuscular Centre, Department of Paediatrics, Imperial College, Hammersmith Hospital London, United Kingdom.
authors.forename: Haiyan
authors.initials: H
authors.lastname: Zhou
authors.forename: Martin
authors.initials: M
authors.lastname: Brockington
authors.forename: Heinz
authors.initials: H
authors.lastname: Jungbluth
authors.forename: David
authors.initials: D
authors.lastname: Monk
authors.forename: Philip
authors.initials: P
authors.lastname: Stanier
authors.forename: Caroline A
authors.initials: CA
authors.lastname: Sewry
authors.forename: Gudrun E
authors.initials: GE
authors.lastname: Moore
authors.forename: Francesco
authors.initials: F
authors.lastname: Muntoni
mesh_terms: D000483:Alleles; D000818:Animals; D001374:Azacitidine / Q000031:analogs & derivatives / Q000494:pharmacology; D001483:Base Sequence; D016022:Case-Control Studies; D002478:Cells, Cultured; D018899:CpG Islands; D019175:DNA Methylation; D017931:DNA Primers / Q000235:genetics; D000077209:Decitabine; D044127:Epigenesis, Genetic; D005260:Female; D005333:Fetus / Q000378:metabolism; D020868:Gene Silencing; D005808:Genes, Recessive; D018392:Genomic Imprinting; D006801:Humans; D006877:Hydroxamic Acids / Q000494:pharmacology; D008297:Male; D051379:Mice; D008810:Mice, Inbred C57BL; D018482:Muscle, Skeletal / Q000378:metabolism; D032448:Myoblasts, Skeletal / Q000187:drug effects / Q000378:metabolism; D020512:Myopathy, Central Core / Q000235:genetics* / Q000378:metabolism; D010375:Pedigree; D017354:Point Mutation; D020641:Polymorphism, Single Nucleotide; D019837:Ryanodine Receptor Calcium Release Channel / Q000235:genetics* / Q000378:metabolism; D014018:Tissue Distribution
chemical_list: D017931:DNA Primers; D006877:Hydroxamic Acids; D019837:Ryanodine Receptor Calcium Release Channel; C012589:trichostatin A; D000077209:Decitabine; D001374:Azacitidine