(P-IG-28) JK Exon 4 Mutation Results in a Lack of Antigen Expression on Red Blood Cells

Background/Case Studies: The Kidd (JK) protein on RBCs functions as a urea transporter. It concentrates urea while maintaining red blood cell (RBC) osmotic stability. Confirmation of the JK antigen expression has been done through phenotyping RBCs with licensed antisera or genotyping using genomic DNA. Phenotype-genotype discordances are rare but can arise when variant alleles are present.

Study

Design/Methods: A 5 gallon repeat donor was genotyped using ID CORE XT. The donor was previously Jka phenotyped using licensed antisera in accordance with the manufacturers’ instructions on 6 different donations using 4 different antisera. Absorption/elution studies were performed using a pool of 5 patient-sourced anti-Jka. JK exons 3 to 10 were Sanger sequenced.

Results/Findings: The donor’s RBCs phenotyped Jk(a-) with all anti-Jka licensed reagents. Phenotyping confirmed the donor was Jk(b+). Adsorption/elution from a Jk(a+b+) control showed anti-Jka, and the donor’s eluate was nonreactive, indicating the absence of Jka on the donor RBCs. Sequencing of the JK exons revealed heterozygous single nucleotide variations at positions c.307T>C (p.Ser103Pro), c.588A>G (p.Pro196Pro) and c.838G>A (p.Asp280Asn), consistent with the ID CORE XT JK*01/JK*02 genotype but discordant with a Jk(a+b+) predicted phenotype. The c.307T>C variant is novel and has not been reported previously. A JK*01(307C)/JK*02 genotype is most consistent with the historical Jk(a-) phenotype.
During the past 10 years, blood from this donor has been successfully transfused to several patients with anti-Jka. No post-transfusion adverse events were reported. Molecular analysis shows that the small polar serine is replaced by the nonpolar proline at position 103. Serine at position 103 is located in the predicted second transmembrane domain and is conserved among mammals (29/29 species). Serine 103 appears engaged in a hydrogen bond with tyrosine at position 122, which may be lost when replaced by the nonpolar proline. Interestingly, another reported mutation with a serine to proline substitution at position 291 (c.807T>C, p.Ser291Pro) in the predicted seventh transmembrane domain is associated with a null phenotype.

Conclusions: Targeted antigen genotyping may not match the expressed phenotype. Combining past transfusion experience along with adsorption/elution studies, the JK*01(307C) allele may have a silencing effect on Jka expression and may be predicted as a null allele. It is best practice to deem the donor Jk(a-).

Importance of research: New JK*A variant on exon 04 (c.307T>C; AA change p.Ser103Pro) was discovered on a blood donor who had a JK*A/JK*B genotype but had been serologically tested as Jk(a-) on 6 separate donations. Current analysis demonstrates that this newly discovered variant creates a silencing effect of Jka expression on red blood cells, and therefore is likely a null allele.