title: High level of functional polymorphism indicates a unique role of natural selection at human immune system loci.
abstract: Several studies have shown that immune system proteins have on average a higher rate of amino acid evolution between different species of mammals than do most other proteins. To test whether immune-system-expressed loci show a correspondingly elevated rate of within-species nonsynonymous (amino acid altering) polymorphism, we examined gene diversity (heterozygosity) at 4,911 single nucleotide polymorphism (SNP) sites at 481 protein-coding loci. At loci with nonimmune functions, gene diversity at nonsynonymous SNP sites was typically lower than that at silent SNP sites (those not altering the amino acid sequence) in the same gene, a pattern that is an evidence of purifying selection acting to eliminate slightly deleterious variants. However, this pattern was not seen at nonsynonymous SNPs causing conservative amino acid replacements in immune system proteins, indicating that the latter are subject to a reduced level of functional constraint. Similarly, immune system genes showed higher gene diversities in their 5' noncoding regions than did other proteins. These results identified certain immune system loci that are likely to be subject to balancing selection that acts to maintain polymorphism in either coding or regulatory regions.
authors.affiliation: Department of Biological Sciences, University of South Carolina, Coker Life Sciences Bldg., Columbia, 29208, USA. austin@biol.sc.edu
authors.forename: Austin L
authors.initials: AL
authors.lastname: Hughes
authors.forename: Bernice
authors.initials: B
authors.lastname: Packer
authors.forename: Robert
authors.initials: R
authors.lastname: Welch
authors.forename: Stephen J
authors.initials: SJ
authors.lastname: Chanock
authors.forename: Meredith
authors.initials: M
authors.lastname: Yeager
mesh_terms: D002877:Chromosomes, Human; D005787:Gene Frequency; D014644:Genetic Variation; D006801:Humans; D007109:Immunity / Q000235:genetics*; D015810:Linkage Disequilibrium; D020641:Polymorphism, Single Nucleotide; D022661:RNA, Untranslated; D012641:Selection, Genetic
chemical_list: D022661:RNA, Untranslated