Identification of a novel hybrid glycophorin gene encoding GP.Hop

BACKGROUND: The GP.Hop (Mi.IV) phenotype expresses the MNS low-incidence an tigens Mur, Hop, TSEN, MINY, and MUT. Because serologically similar MNS phe notypes expressing some or ail of these antigens were shown to be carried b y hybrid GP(B-A-B) proteins, it was proposed that a similar protein would b e found for GP.Hop. The identification of a second GP.Hop propositus (ES) i nitiated a study to determine the molecular basis of this phenotype. STUDY DESIGN AND METHODS: Serologic tests and immunoblotting analysis with glycophorin-specific antibodies were performed. GYPB, the gene encoding the GPB protein, was cloned and sequenced after reverse transcription PCR ampl ification of total RNA isolated from ES. GYPB-specific primers encompassing GYPB pseudoexon 3, intron 3, and exon 4 were also used to clone and sequen ce genomic DNA from ES and MH, the original GP.Hop proband. RESULTS: Serologic and immunochemical data confirmed that ES's RBCs carried antigens associated with the GP.Hop phenotype. Sequencing of ES's cDNA dem onstrated the presence of genes predicted to encode s-specific GPB and an S -specific GP(B-A-B) hybrid in which the 3' end of GYPB pseudoexon 3 had bee n replaced by a short nucleotide sequence from exon 3 of the GPA gene (GYPA ). The hybrid nucleotide sequence contained sequence motifs previously show n to be required for the expression of the Mur, Hop, TSEN, MINY, and MUT, w hich is consistent with their presence as detected serologically. Genomic D NA analysis found that the crossover point in GYPB pseudoexon 3 was identic al in ES and MH. CONCLUSIONS: The GP.Hop phenotype is produced by a hybrid GP(B-A-B) protein caused by a DNA insertion of GYPA into GYPB. The composition of the hybrid protein is GPB(1-26)-GP psi B(27-50)-GPA(51-58)-GPB(S)(59-103).