American Red Cross Pitman, New Jersey, United States
Background/Case Studies: The Kx antigen (XK:1) is a multipass transmembrane protein that is covalently linked by a disulfide bond to the Kell glycoprotein. The Kx antigen is encoded by the XK gene located on the X chromosome at Xp21. Loss of expression of Kx is associated with the McLeod phenotype which is characterized by acanthocytosis and depressed expression of KEL system antigens. There are 48 reported XK alleles associated with the null kx- phenotype. The null alleles include six deletions involving an exon or the entire gene, 17 insertion/deletions, 11 nonsense single nucleotide variants (SNV) codons, seven splicing defects and seven non-synonymous single nucleotide variants. Here we describe molecular characterization of a Caucasian male blood donor with the McLeod phenotype. The donor was identified in 2012 during random screening with anti-Kpb. The donor RBCs initially typed negative for K, Jsb, Kpa, Kpb but weak microscopic agglutinates were noted; his RBCs typed 1+ with anti-k. Acanthocytes were noted in the microscopic readings, along with the weak reactivity, which alerted staff to the possible McLeod phenotype status.
Study
Design/Methods: The blood donor RBCs were typed using standard agglutination methods. Sanger sequencing of the three exons and splice sites of the XK gene was performed. Resulting sequences were aligned to the reference sequence using Sequencher software (GeneCodes). ISBT 019 KX Allele table v5.0 30-JUN-2021 (https://www.isbtweb.org/resource/019kx.html) was reviewed.
Results/Findings: Donor XK exons 1 and 2 had no sequences that differed from the consensus. Exon 3 was hemizygous for a single nucleotide variant (SNV) at c.664. The c.664C>T variant changes an asparagine to a premature stop codon at amino acid residue 222. Since the initial discovery of this donor’s rare phenotype, they have donated red cells 38 times. Conclusions: According to NCBI, the XK c.664C>T is part of a tri-allelic SNV (rs2146834483). The third allele c.664G encodes glycine at p.222, is associated with a null phenotype and has been assigned XK*N.27 on the ISBT XK allele table. The XK c.664T is not listed in the XK allele table. The allele has been given provisional assignment of XK*N.49. This donor is currently the only active blood donor in the American Rare Donor Program with the McLeod phenotype and only one of very few donors known globally. Some individuals with the McLeod phenotype have clinical conditions including chronic granulomatous disease (CGD) and Duchenne Muscular Dystrophy due to large deletions that encompass not only all or part of the XK gene but neighboring genes including CYBB and DMD, the genes impacted in these disorders, respectively.
Importance of research: The McLeod phenotype is rare and can be associated with genetic disorders. Requests for McLeod red cell units are difficult to fulfill and finding and characterizing new donors with the McLeod phenotype are important to fulfilling the need. in this case, a novel allele was identified with a non-synonymous SNV. Cataloging these variants can aid in our understanding of the molecular basis of the phenotype.
