Molecular defects underlying the Kell null phenotype

Expression of the Kell blood group system is dependent on two proteins; Kel l and XK, that are linked by a single disulfide bond. Kell, a type II membr ane glycoprotein, is a zinc endopeptidase, while XX, which has 10 transmemb rane domains, is a putative membrane transporter. A rare phenotype termed K ell null (Ko) is characterized by the absence of Kell protein and Kell anti gens from the red cell membrane and diminished amounts of XK protein. We de termined the molecular basis of eight unrelated persons with Ko phenotypes by sequencing the coding and the intron-exon splice regions of KEL and, in some cases, analysis of mRNA transcripts and expression of mutants on the c ell surface of transfected cells. Six subjects were homozygous: four with p remature stop codons, one with a 5' splice site mutation, G to A, in intron 3, and one with an amino acid substitution (S676N) in exon 18. Two Ko pers ons with premature stop codons had identical mutations in exon 4 (R128Stop) , another had a different mutation in exon 4 (C83Stop), and the fourth had a stop codon in exon 9 (Q348Stop). Two Ko persons were heterozygous for two mutations. One had a 5' splice site mutation (G to A) in intron 3 of one a llele that caused aberrant splicing and exon skipping, and the other allele had an amino acid substitution in exon 10 (S363N). The other heterozygote had the same amino acid substitution in exon 10 (S363N) in one allele and a premature stop codon in exon 6 (R192Stop) in the other allele. The S363N a nd S676N mutants, expressed in 293T cells, were retained in a pre-Golgi com partment and were not transported to the cell surface, indicating that thes e mutations inhibit trafficking. We conclude that several different molecul ar defects cause the Kell null phenotype.