title: How many genes underlie the occurrence of common complex diseases in the population?
abstract: BACKGROUND
  Most common human diseases are due to complex interactions among multiple genetic variants and environmental risk factors. There is debate over whether variants of a relatively small number of genes, each with weak or modest individual effects, account for a large proportion of common diseases in the population, or whether a large number of rare variants with large effects underlie genetic susceptibility to these diseases. It is not clear how many genes are necessary to account for an appreciable population-attributable fraction of these diseases.
  
  
  METHODS
  In this analysis, we estimated the number of disease susceptibility genes needed to account for varying population attributable fractions of a common complex disease, taking into account the genotype prevalence, risk ratios for individual genes, and the model of gene-gene interactions (additive or multiplicative).
  
  
  RESULTS
  Very large numbers of rare genotypes (e.g. those with frequencies of 1 per 5000 or less) are needed to explain 50% of a common disease in the population, even if the individual risk ratios are large (RR = 10-20). On the other hand, only approximately 20 genes are usually needed to explain 50% of the burden of a disease in the population if the predisposing genotypes are common (> or = 25%), even if the individual risk ratios are relatively small (RR = 1.2-1.5).
  
  
  CONCLUSIONS
  Our results suggest that a limited number of disease susceptibility genes with common variants can explain a major proportion of common complex diseases in the population. Our findings should help focus the search for common genetic variants that provide the most important predispositions to complex human diseases.
authors.affiliation: National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention (CDC), Atlanta, GA 30333, USA. qay0@cdc.gov
authors.forename: Quanhe
authors.initials: Q
authors.lastname: Yang
authors.forename: Muin J
authors.initials: MJ
authors.lastname: Khoury
authors.forename: Jm
authors.initials: J
authors.lastname: Friedman
authors.forename: Julian
authors.initials: J
authors.lastname: Little
authors.forename: W Dana
authors.initials: WD
authors.lastname: Flanders
mesh_terms: D000465:Algorithms; D000483:Alleles; D015179:Colorectal Neoplasms / Q000235:genetics; D004781:Environmental Exposure / Q000009:adverse effects; D005787:Gene Frequency / Q000235:genetics; D005796:Genes / Q000235:genetics*; D020022:Genetic Predisposition to Disease / Q000235:genetics*; D014644:Genetic Variation / Q000235:genetics; D005838:Genotype; D006801:Humans; D008957:Models, Genetic; D016017:Odds Ratio; D010641:Phenotype; D015995:Prevalence; D012307:Risk Factors