Molecular basis of the Kell (K1) phenotype.

K1 (K, Kell) is a strong immunogen; its antibodies can cause severe reactions if incompatible blood is transfused and may cause hemolytic disease of the newborn in sensitized mothers. K1 is a member of the Kell blood group system, which is complex, containing over 20 different antigens. Some of the antigens are organized in allelic pairs of high and low prevalence whereas others are independently expressed. K1, which is present in 9% of the population, is antithetical to the high-prevalence K2 (k) antigen. We have determined the molecular basis of the K1/K2 polymorphism by sequencing the 19 exons of the Kell gene (KEL) of a K1/K1 person. Polymerase chain reaction was performed on genomic DNA isolated from peripheral blood and the amplified products were either directly sequenced or subcloned and sequenced. Comparisons of K1/K1 and K2/K2 DNA showed a C to T base substitution in exon 6 that predicts a threonine to methionine change at amino acid residue 193. This amino acid substitution occurs at a consensus N-glycosylation site (Asn. X. Thr) and probably prevents N-glycosylation, leading to a change in phenotype. The C to T substitution creates a Bsm I restriction enzyme site, which was tested in 42 different samples to confirm that this base change identifies the K1/K1 genotype. This test differentiates genotypes, K1/K1, K2/K2, and the K1/K2 heterozygote and should prove useful in the prenatal diagnosis of K1-related hemolytic disease of the newborn.