| Abstract: |
In July 2000, the IMGT/HLA Database described 18 HLA-A and B alleles detected by DNA typing but not detected by serologic reagents. These known HLA expression variants, commonly referred to as "null" or "low expression" alleles, are the result of insertions, deletions and/or nucleotide substitutions in exons 1, 2, 3 and 4 and introns 1 and 2 of HLA-A and B alleles. During routine HLA typing of patients and family, we identified 10 haplotypes with HLA-A, B or C alleles not detected by serology but identified by DNA typing (not including HLA-C alleles known to be undetectable by serology). DNA sequencing identified A*2409N (n=1) in linkage with B*4001, DRB1*0404 and A*2402102L (n=4) on 4 unique haplotypes: 1) B*0801, DRB1*0301; 2) B*3502, DRB1*1104; 3) B*3701, DRB1*0407; 4) B*4403, DRB1*0801. Also, single examples of 5 previously unknown HLA class I expression variant alleles (indicated by bold italics in the table) were characterized. A Cw B DRB1 Mutation location *23XX *04 *4403 *07 Exon 3, TGC>TG4, stop codon *24XX 3 *4002 *1101 Exon 3, GAC>TAC *0201 *04XX *4403 *07 Exon 7, single base deletion *0201 nt *39XX *1101 Exon 3, CG deletion, stop codon *0201 *05 *44XX *0301 Exon 3, CAG>rAG, stop codon Immunoprecipitation studies suggest that the A*24XX allele may be a low expression variant due to the aspartic acid to tyrosine change at residue 119, a position predicted to hydrogen bond with beta-2-microglobulin. The 2 nucleotide deletion of B*39XX, the C to T change in B*44XX, and the C to A change in A*23XX generate stop codon sequences. Sequencing of Cw*04XX found a single base deletion in exon 7 and Cw4 protein was not detected in IEF gels. Our study of over 10,000 patient and family haplotypes suggests a frequency for known or novel HLA-class I expression variant alleles of approximately 0.1%. In allogeneic transplantation it is important to know if HLA antigens are expressed or not. Our detection of single examples of 5 novel "null" alleles suggests not only that exquisitely sensitive HLA-DNA typing methods are essential to provide optimal unrelated donor matching in hematopoietic stem cell transplantation, but also that serologic testing may be necessary to assure detection of novel HLA class I expression variant alleles. © 2001 Blackwell Science Ltd. |
