Two common polymorphisms in the APO A-IV coding gene: their evolution and linkage disequilibrium

Human apolipoprotein A-IV (APO A-IV) exhibits a common protein polymorphism detectable by isoelectric focusing (IEF) due to a single base substitution at codon 360 which replaces the frequently occurring glutamine residue (allele 1) with histidine (allele 2). Recently, sequence analysis of the APO A-IV coding region has revealed another common nucleotide substitution at codon 347 which converts the commonly present threonine residue (allele A) into serine (allele T). In order to investigate the extent of genetic variation at codon 347, we screened DNA samples from 192 unrelated individuals using a polymerase chain reaction based assay. The frequencies of the two alleles, A-IV*A and A-IV*T, were 0.81 and 0.19, respectively, with average heterozygosity 0.31. Genetic screening of the corresponding 192 plasma samples by IEF gave frequencies of 0.922 and 0.078 for the A-IV*1 and A-IV*2 alleles, respectively, at codon 360 with average heterozygosity 0.14. Genotype data at the two polymorphic sites were used to assign unequivocal haplotypes to all the 384 chromosomes. Of the expected four haplotypes (A1, T1, A2, and T2) only three were observed and their frequencies were 0.732 for A1, 0.190 for T1 and 0.078 for A2, with average heterozygosity 0.42. Although our data indicate significant linkage disequilibrium between the two sites (chi 21 = 7.65, P < 0.006, standardized disequilibrium constant phi = -0.14) the degree of nonrandom association varied between alleles at the two sites. Based upon allele frequency data and variable linkage disequilibrium between alleles, we propose that the A2 and T1 haplotypes may have evolved from the parental A1 haplotype by two independent mutations.