Genetic variability in the ACE gene region surrounding the Alu I/D polymorphism is maintained by balancing selection in human populations

OBJECTIVE: Angiotensin-converting enzyme plays a critical role in the maintenance of cardiovascular homeostasis. Extensive research has aimed at identifying ACE genetic variants responsible for variation in enzyme plasma concentrations and associated with human diseases. These efforts have been hampered by the extensive linkage disequilibrium across the gene and the identity or location of the functional polymorphism(s) is at presently unknown. The aim of our study was to verify whether the Alu insertion/deletion (Alu I/D) polymorphism or any linked variant has been maintained by natural selection in human populations. METHODS: We resequenced a gene region surrounding the Alu I/D polymorphism in four human populations; we applied population neutrality tests and performed haplotype analysis for this region. RESULTS: We observed high levels of nucleotide diversity, an excess of intermediate frequency alleles and, at least in African populations, a higher level of within-species diversity compared with interspecific divergence. Analysis of haplotype genealogy indicated the presence of two major clades separated by deep branches with a coalescence time older than 1.5 million years. All these features strongly suggest the action of balancing selection and we verified that the selection signature is restricted to the gene region surrounding the Alu I/D. CONCLUSION: Our data imply the presence of a functional polymorphism in the Alu I/D region and illustrate the contribution of evolutionary models to classic single nucleotide polymorphism-phenotype association approaches by providing information about the localization of candidate functional variants.