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*0 alleles have been identified, and this information can be used in DNA-based typing. We selected five JK*0 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*0 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. Immunohematology 2009;25:165–169.