Molecular basis of two novel high-prevalence antigens in the Kell blood group system, KALT and KTIM

The Kell blood group system consists of 25 antigens that result from single-nucleotide polymorphisms. Most polymorphic Kell antigens reside on the N-terminal domain of Kell before the zinc-binding catalytic motif, which is the major site for endothelin-3-converting enzyme activity. Kell antigens are important in transfusion medicine owing to their strong immunogenicity, and the corresponding antibodies are clinically significant. Two probands were studied whose serum samples contained antibodies to different high-prevalence Kell antigens.

Standard hemagglutination methods were used for serologic testing of Proband 1 and Proband 2. DNA was prepared from both probands and family members. The 19 exons and the intron-exon regions of KEL from both probands were amplified by polymerase chain reaction, and the sequences were compared with that of common KEL. The identified substitutions were located on a three-dimensional model of Kell generated based on the crystal structure of neutral endopeptidase, a homolog of Kell.

In Proband 1, a homozygous 1988G>A mutation (Arg623Lys) in Exon 17 was present. One sibling of Proband 1 was homozygous for 1988G>A. In Proband 2, a homozygous 1033G>A mutation (Asp305Asn) in Exon 8 was present. Three siblings of Proband 2 were heterozygous for 1033G>A.

The identified KEL mutations of the two probands are novel and inherited. The antigen absent from the red blood cells (RBCs) of Probands 1 and 2 are named KALT and KTIM, respectively. KALT is unique in that it is the only Kell antigen sensitive to treatment of RBCs by trypsin.