Extensive Genomic Variability of Knops Blood Group Polymorphisms is Associated with Sickle Cell Disease in Africa

Abstract

Sickle cell disease (SCD) is a multisystem disorder characterized by chronic hemolytic anemia, vaso-occlusive crises, and marked variability in disease severity. Patients require transfusions to manage disease complications, with complements, directed by complement regulatory genes (CR1) and its polymorphisms, implicated in the development of alloantibodies. We hypothesize that CR1 polymorphisms affect complement regulation and function, leading to adverse outcome in SCD. To this end, we determined the genomic diversity of complement regulatory genes by examining single nucleotide polymorphisms associated with Knops blood group antigens. Genomic DNA samples from 130 SCD cases and 356 control Africans, 331 SCD cases and 497 control African Americans, and 254 Caucasians were obtained and analyzed, utilizing a PCR–-RFLP (polymerase chain reaction-restriction fragment length polymorphism) assay. Analyzing for ethnic diversity, we found significant differences in the genotypic and allelic frequencies of Sl1/Sl2 (rs17047661) and McCa/b (rs17047660) polymorphisms between Africans, African Americans, and Caucasians ( P < 0.05). The homozygote mutant variants had significantly higher frequencies in Africans and African Americans but were insignificant in Caucasians (80.2% and 59.6% vs 5.9% for Sl1/2; and 36% and 24% vs 1.8% for McCa/b). With SCD, we did not detect any difference among cases and controls either in Africa or in the United States. However, we found significant difference in genotypic ( P < 0.0001) and allelic frequencies ( P < 0.0001) of Sl1/Sl2 (rs17047661) and McCa/b (rs17047660) polymorphisms between SCD groups from Africa and the United States. There was no difference in haplotype frequencies of these polymorphisms among or between groups. The higher frequency of CR1 homozygote mutant variants in Africa but not United States indicates a potential pathogenic role, possibly associated with complicated disease pathophysiology in the former and potentially protective in the latter. The difference between sickle cell groups suggests potential genetic drift or founder effect imposed on the disease in the United States, but not in Africa, and a possible confirmation of the ancestral susceptibility hypothesis. The lower haplotype frequencies among sickle cell and control populations in the United States may be due to the admixture and the dilution of African genetic ancestry in the African American population.