Mutations in GYPB exon 5 drive the S-s-U+(var) phenotype in persons of African descent: implications for transfusion.

BACKGROUND: The S-s-U- phenotype in African Americans is due to a GYPB deletion, however the molecular basis for the S-s-U+var phenotype is poorly understood. Variable reactivity of S-s-U+var RBCs with monoclonal anti-He or by anti-U has been demonstrated, however the underlying molecular bases for this phenotype remain to be established. STUDY DESIGN AND METHODS: Hemagglutination was performed on 104 S-s- blood samples using monoclonal anti-He and anti-U. GYPB was sequenced from selected samples. Allele and exon-specific PCR analysis was used to identify wild-type and mutant alleles.
RESULTS: The RBCs of 49-percent S-s- samples were identified as S-s-U+var by hemagglutination. Sequencing analysis of 41 samples revealed 1) a point mutation at +5 (g > t) of intron 5 that resulted in skipping of exon 5 in 34 samples; 2) two mutations (208G > T and 230C > T) caused partial skipping of exon 5 in four samples due to activation of a cryptic 3' splice site that resulted from a C > G transversion at nt251 present in all GYPB*S alleles and most GYPB*s alleles tested. Three samples were heterozygous for the mutated alleles. DISCUSSION: The S-s-U+var phenotype arises from changes in or around GYPB exon 5. The weak expression of U and in most examples, He, may be due to low levels of normal transcription of the variant gene or to posttranscriptional down regulation.