A simple screening assay for the most common JK*0 alleles revealed compound heterozygosity in Jk(a-b-) probands from Guam

The Jk(a-b-) phenotype results from alterations in the JK gene and is characterized by absence of the RBC urea transporter in the cell membrane. The frequency of Jk(a-b-) varies among populations, but this phenotype is most commonly found in people of Polynesian and Finnish descent. Although rare, Jk(a-b-) individuals present a clinical challenge because anti-Jk3 is produced readily in response to transfusion and pregnancy, and Jk(a-b-) blood is not routinely available. Identification of Jk(a-b-) patients and donors is most often performed serologically. However, ten JK*o alleles have been identified, and this information can be used in DNA-based typing. We selected five JK*o alleles that had been encountered by our reference laboratory in two or more samples from unrelated individuals and designed an allele-specific primer PCR assay for use as an initial screening tool. After in-house validation, we tested genomic DNA from a family: a mother and her two sons referred to us for genetic investigation of their Jk(a-b-) phenotypes. Two different nucleotide substitutions, -1g>a in intron 5 (IVS5) and 956C>T in exon 10, originally associated with Polynesian and Indian/African populations respectively, were identified in the family. The mother and one son were compound heterozygotes, and the second son was homozygous for IVS5-1g>a. We conclude that the effort to design and validate such a screening assay was cost-efficient when compared with DNA sequencing costs. Furthermore, selection of the more common JK*o mutations was a practical approach that resulted in rapid identification of the genetic bases behind the Jk(a-b-) phenotypes in this unusual family. Although an obvious target for eventual inclusion into high-throughput genotyping platforms for clinical diagnostic services, current systems are very limited. Our approach provides a simple and inexpensive method for the identification of these rare alleles.