title: High frequency of skewed X-chromosome inactivation in females with autoimmune thyroid disease: a possible explanation for the female predisposition to thyroid autoimmunity.
abstract: BACKGROUND
  Autoimmune thyroid diseases (AITD) comprise Graves' disease (GD) and Hashimoto's thyroiditis (HT). They are characterized by loss of immunological self-tolerance and female preponderance. Theoretically, X chromosome inactivation (XCI) and resultant tissue chimerism could offer an explanation for the female predisposition to AITD.
  
  
  OBJECTIVE
  Our aim was to examine whether skewed XCI is associated with AITD.
  
  
  METHODS
  We first conducted a classical case-control study of twin individuals with and without AITD, and then a case-control study of twin pairs discordant for AITD.
  
  
  METHODS
  Participants included 32 female twins with AITD and a control group of 96 healthy female twin individuals.
  
  
  METHODS
  XCI analysis was performed by enzymatic predigestion of DNA with a methylation-sensitive enzyme followed by PCR of the polymorphic CAG repeat of the androgen receptor gene. The XCI pattern was classified as skewed when 80% or more of the cells preferentially inactivated the same X chromosome.
  
  
  METHODS
  We assessed the prevalence of skewed XCI.
  
  
  RESULTS
  The frequency of skewed XCI in female twins with AITD, GD, and HT was 34, 37, and 31%, respectively, which was higher than the prevalence in the corresponding control populations, 11% (P = 0.003), 14% (P = 0.045), and 8% (P = 0.057), respectively. Similar results were found in twin pairs discordant for AITD. Overall, skewed XCI was associated with an increased risk of developing AITD, with an odds ratio of 9.0 (95% confidence interval, 1.64-49.4) (P = 0.022).
  
  
  CONCLUSIONS
  These observations suggest a possible role of XCI in the etiology of AITD and may in part explain the female preponderance of AITD.
authors.affiliation: Department of Endocrinology, Odense University Hospital, Sönder Boulevard 29, 5000 Odense C, Denmark. thomas.brix@ouh.fyns-amt.dk
authors.forename: Thomas Heiberg
authors.initials: TH
authors.lastname: Brix
authors.forename: Gun Peggy S
authors.initials: GP
authors.lastname: Knudsen
authors.forename: Marianne
authors.initials: M
authors.lastname: Kristiansen
authors.forename: Kirsten Ohm
authors.initials: KO
authors.lastname: Kyvik
authors.forename: Karen Helene
authors.initials: KH
authors.lastname: Orstavik
authors.forename: Laszlo
authors.initials: L
authors.lastname: Hegedüs
mesh_terms: D000328:Adult; D016022:Case-Control Studies; D005260:Female; D020022:Genetic Predisposition to Disease; D006111:Graves Disease / Q000209:etiology / Q000235:genetics*; D006801:Humans; D013967:Thyroiditis, Autoimmune / Q000209:etiology / Q000235:genetics*; D049951:X Chromosome Inactivation / Q000235:genetics*