title: Epigenetic investigation of variably X chromosome inactivated genes in monozygotic female twins discordant for primary biliary cirrhosis.
abstract: Primary biliary cirrhosis (PBC) is an autoimmune chronic cholestatic liver disease with a strong genetic susceptibility due to the high concordance in monozygotic (MZ) twins and a striking female predominance. Women with PBC manifest an enhanced X monosomy rate in peripheral lymphocytes and we thus hypothesized an X chromosome epigenetic component to explain PBC female prevalence. While most genes on the female inactive X chromosome are silenced by promoter methylation following X chromosome inactivation (XCI), approximately 10% of X- linked genes exhibit variable escape from XCI in healthy females. This study was designed to test the hypothesis that susceptibility to PBC is modified by one or more X-linked gene with variable XCI status. Peripheral blood mRNA and DNA samples were obtained from a unique cohort of MZ twin sets discordant and concordant for PBC. Transcript levels of the 125 variable XCI status genes was determined by quantitative RT-PCR analysis and two genes (CLIC2 and PIN4) were identified as consistently downregulated in the affected twin of discordant pairs. Both CLIC2 and PIN4 demonstrated partial and variable methylation of CpG sites within 300 bp of the transcription start site that did not predict the XCI status. Promoter methylation of CLIC2 manifested no significant difference between samples and no significant correlation with transcript levels. PIN4 methylation showed a positive trend with transcription in all samples but no differential methylation was observed between discordant twins. A genetic polymorphism affecting the number of CpG sites in the PIN4 promoter did not impact methylation or transcript levels in a heterozygous twin pair and showed a similar frequency in independent series of unrelated PBC cases and controls. Our results suggest that epigenetic factors influencing PBC onset are more complex than methylation differences at X-linked promoters and variably 3 inactivated X-linked genes may be characterized by partial promoter methylation and biallelic transcription.
authors.affiliation: Medical Microbiology and Immunology, University of California at Davis, CA, USA.
authors.forename: Michelle M
authors.initials: MM
authors.lastname: Mitchell
authors.forename: Ana
authors.initials: A
authors.lastname: Lleo
authors.forename: Luca
authors.initials: L
authors.lastname: Zammataro
authors.forename: Marlyn J
authors.initials: MJ
authors.lastname: Mayo
authors.forename: Pietro
authors.initials: P
authors.lastname: Invernizzi
authors.forename: Nancy
authors.initials: N
authors.lastname: Bach
authors.forename: Shinji
authors.initials: S
authors.lastname: Shimoda
authors.forename: Stuart
authors.initials: S
authors.lastname: Gordon
authors.forename: Mauro
authors.initials: M
authors.lastname: Podda
authors.forename: M Eric
authors.initials: ME
authors.lastname: Gershwin
authors.forename: Carlo
authors.initials: C
authors.lastname: Selmi
authors.forename: Janine M
authors.initials: JM
authors.lastname: LaSalle
mesh_terms: D018118:Chloride Channels / Q000096:biosynthesis / Q000235:genetics; D041321:Chromosomes, Human, X / Q000235:genetics / Q000378:metabolism*; D015331:Cohort Studies; D018899:CpG Islands / Q000235:genetics; D019175:DNA Methylation; D005260:Female; D040181:Genetic Diseases, X-Linked / Q000235:genetics / Q000378:metabolism*; D006801:Humans; D008105:Liver Cirrhosis, Biliary / Q000235:genetics / Q000378:metabolism*; D008214:Lymphocytes / Q000378:metabolism; D000072340:NIMA-Interacting Peptidylprolyl Isomerase; D019696:Peptidylprolyl Isomerase / Q000096:biosynthesis / Q000235:genetics; D011110:Polymorphism, Genetic; D012333:RNA, Messenger / Q000096:biosynthesis / Q000235:genetics; D014158:Transcription, Genetic / Q000235:genetics; D014430:Twins, Monozygotic / Q000235:genetics / Q000378:metabolism*; D049951:X Chromosome Inactivation
chemical_list: C108828:CLIC2 protein, human; D018118:Chloride Channels; D000072340:NIMA-Interacting Peptidylprolyl Isomerase; D012333:RNA, Messenger; C000606846:PIN1 protein, human; C000606848:PIN4 protein, human; D019696:Peptidylprolyl Isomerase