Allele walking: a new and highly accurate approach to HLA-DRB1 typing. Application to DRB1*04 alleles

We have developed a typing method, which can be used even in small laboratories, to produce a highly accurate and reliable allele assignment in any homozygous or heterozygous situation. We have called the method allele walking (AW) and it consists of sequential rounds of PCR-RFLP. After digestion, electrophoresis separates alleles positive for the mutation from the negative alleles; the cleaved fragment is then recovered from the gel and analyzed for mutations at another codon. In this way, AW is able to positively ascertain which mutations are in the same chromosome (cis-linkage) and assigns alleles independently from each other. Artificial sites are created in the PCR step in order to positively detect substitutions not naturally recognized by any of the existing or convenient enzymes. We report the application of AW for typing the 22 DRB1*04 alleles. The first PCR-RFLP round groups DRB1*04 alleles. Subsequently, the mutations at codons 86, 74, 71, 57 and 37 can be analyzed for the unambiguous assignment of the majority of the alleles. Additional polymorphisms at different codons can be assayed to resolve any undetermined alleles. The viability of all the restriction sites used as well as the feasibility of AW were successfully tested.