Aberrant splicing of intron 1 creates a novel null HLA-B*1501 allele

A comparison of serological and DNA HLA class I typing data identified a se rological "blank" HLA-B15 antigen in a volunteer donor on the bone marrow r egistry Isoelectric focusing and Western blot analysis of a cell line estab lished from this individual confirmed that the HLA-B15 antigen is not expre ssed at the cell surface. Nucleotide sequence analysis of the HLA-B*15 null allele revealed a 10-bp deletion near the 3' end of intron 1, when compare d to the normal HLA-B*1501 sequence. All of the HLA-B*15 specific cDNA clon es examined retained the intron 1 sequence, Reverse transcription-polymeras e chain reaction (RT-PCR) and Southern blot analysis demonstrated that the HLA-B*15 mRNA molecule contained the intron 1 sequence, indicating an inabi lity to efficiently splice out intron 1 from the mRNA transcript. The reten tion of the mutated intron 1 sequence in the mRNA causes a frameshift and p remature termination of translation at the start of exon 2, explaining the HLA-B*1501 null phenotype, Our data predicts that the HLB-B*1501 null allel e would express a small truncated protein containing the signal sequence fu sed to an ORF within intron 1 and terminating (out of frame) just within ex on 2.