title: Recurrence risk modelling of the genetic susceptibility to ankylosing spondylitis.
abstract: OBJECTIVE
  It has long been suspected that susceptibility to ankylosing spondylitis (AS) is influenced by genes lying distant to the major histocompatibility complex. This study compares genetic models of AS to assess the most likely mode of inheritance, using recurrence risk ratios in relatives of affected subjects.
  
  
  METHODS
  Recurrence risk ratios in different degrees of relatives were determined using published data from studies specifically designed to address the question. The methods of Risch were used to determine the expected recurrence risk ratios in different degrees of relatives, assuming equal first degree relative recurrence risk between models. Goodness of fit was determined by chi(2) comparison of the expected number of affected subjects with the observed number, given equal numbers of each type of relative studied.
  
  
  RESULTS
  The recurrence risks in different degrees of relatives were: monozygotic (MZ) twins 63% (17/27), first degree relatives 8.2% (441/5390), second degree relatives 1.0% (8/834), and third degree relatives 0. 7% (7/997). Parent-child recurrence risk (7.9%, 37/466) was not significantly different from the sibling recurrence risk (8.2%, 404/4924), excluding a significant dominance genetic component to susceptibility. Poor fitting models included single gene, genetic heterogeneity, additive, two locus multiplicative, and one locus and residual polygenes (chi(2) >32 (two degrees of freedom), p<10(-6) for all models). The best fitting model studied was a five locus model with multiplicative interaction between loci (chi(2)=1.4 (two degrees of freedom), p=0.5). Oligogenic multiplicative models were the best fitting over a range of population prevalences and first degree recurrence risk rates.
  
  
  CONCLUSIONS
  This study suggests that of the genetic models tested, the most likely model operating in AS is an oligogenic model with predominantly multiplicative interaction between loci.
authors.affiliation: Spondyloarthritis Research Group, Wellcome Trust Centre for Human Genetics, Roosevelt Drive, Headington, OX3 7BN, UK. mbrown@well.ox.ac.uk
authors.forename: M A
authors.initials: MA
authors.lastname: Brown
authors.forename: S H
authors.initials: SH
authors.lastname: Laval
authors.forename: S
authors.initials: S
authors.lastname: Brophy
authors.forename: A
authors.initials: A
authors.lastname: Calin
mesh_terms: D005260:Female; D020022:Genetic Predisposition to Disease; D006801:Humans; D008285:Major Histocompatibility Complex / Q000235:genetics; D008297:Male; D008957:Models, Genetic; D016017:Odds Ratio; D018570:Risk Assessment / Q000379:methods; D012307:Risk Factors; D013167:Spondylitis, Ankylosing / Q000235:genetics*