Human SP-A Locus: Allele Frequencies and Linkage Disequilibrium between the Two Surfactant Protein A Genes

Two surfactant protein A (SP-A) genes and several alleles for each SP-A locus have been previously described. In this report we investigate the potential usefulness of the SP-A loci as markers for genetic studies. We establish conditions that allow the identification of alleles with very similar sequences; We also determine the degree of polymorphism for each SP-A locus: The heterozygosity and polymorphism information content (PIC) values for the SP-A1 locus are 0.63 and 0.55, respectively, and for the SP-A2 locus are 0.50 and 0.56. In the course of these studies, we identify one new SP-A2 allele and show that the SP-A1 and SP-A2 loci are in linkage disequilibrium (P < 0.000001). We also identify 19 of the 20 possible haplotypes in a population of n = 239. Nine of the observed haplotypes reach statistical significance (P < 0.01) in this population, and the segregation of two haplotypes (6A²/1A⁰ and 6A⁴/1A) without recombination is verified in a family pedigree. These data together indicate that both SP-A loci are sufficiently polymorphic to be good markers for use in genetic studies. Furthermore, the finding of one novel allele suggests that additional unknown SP-A alleles are yet to be found.