title: Antagonistic pleiotropy as a widespread mechanism for the maintenance of polymorphic disease alleles.
abstract: BACKGROUND
  Many serious diseases have a genetic basis which, from an evolutionary point of view, should have been selected against, resulting in very low frequencies. The remarkable sustained prevalence of a number of disease-associated alleles is therefore surprising. We believe that antagonistic pleiotropy, when multiple effects of a gene have opposing effects on fitness (e.g., sickle cell disease), may be more widespread than typically considered. We hypothesize that, rather than being an exception to the rule of genetic disorders, antagonistic pleiotropy may be common.
  
  
  METHODS
  We surveyed the medical literature in order to determine whether sufficient evidence exists to reassess the nature of antagonistic pleiotropy; from being considered an unusual scenario to one that is anticipated. We also used a simple population genetic model to examine the feasibility of antagonistic pleiotropy to act as a mechanism to maintain polymorphism for serious genetic disorders even if the benefits are subtle.
  
  
  RESULTS
  We identified a number of examples of antagonistic pleiotropy where the deleterious effect, the beneficial effect, and the exact molecular cause have been demonstrated. We also identified putative cases in which there is circumstantial evidence or a strong reason to expect antagonistic pleiotropy in a genetic disorder. The population genetic model demonstrates that alleles with severe deleterious health effects can be maintained at medically relevant frequencies with only minor beneficial pleiotropic effects.
  
  
  CONCLUSIONS
  We believe that our identification of several cases of antagonistic pleiotropy, despite the lack of research on this question and the varied natures of the types of these disorders, speaks to both the underappreciated nature of this phenomenon and its potentially fundamental importance. If antagonistic pleiotropy is as common as our research suggests, this may explain why so many serious diseases, even apparently environmentally caused ones, have a genetic component. Furthermore, acceptance of a genome full of antagonistically pleiotropic genetic interactions poses important implications for clinical treatment and disease prevention research, especially genetically based therapies.
authors.affiliation: Biology Department, California State University Long Beach, 1250 Bellflower Blvd, Long Beach, CA 90840, USA. ashley.carter@csulb.edu
authors.forename: Ashley J R
authors.initials: AJ
authors.lastname: Carter
authors.forename: Andrew Q
authors.initials: AQ
authors.lastname: Nguyen
mesh_terms: D000483:Alleles; D005260:Female; D005298:Fertility / Q000235:genetics; D058685:Genetic Pleiotropy; D020022:Genetic Predisposition to Disease / Q000235:genetics*; D006801:Humans; D006816:Huntington Disease / Q000235:genetics; D008297:Male; D008957:Models, Genetic; D009369:Neoplasms / Q000235:genetics; D011110:Polymorphism, Genetic