P171 Description of a novel HLA-DPB1 allele with polymorphic sites in exon 1

Abstract

We describe a new HLA-DPB1 allele, which was first identified in an ASHI PT sample (HT-202) from an African American donor, and recently identified again in a 22-year-old African-American woman undergoing HLA typing as a potential bone marrow donor for her father. High-resolution HLA genotyping was performed using sequence based typing and analyzed using SBTengine software (GenDX, Netherlands). Typing of the PT sample showed allele pair ambiguity of DPB1∗03:01, 105:01 vs DPB1∗124:01, 463:01. Consensus results for the PT sample were DPB1∗03:01, 105:01, however, exon 1 sequence revealed heterozygous peaks at positions 24 (R) and 47 (Y). Given that DPB1∗03:01 and 105:01 are identical in exon 1 and that DPB1∗124:01 does not have a known exon 1 sequence, our lab typed the sample as DPB1∗124:01, 463:01. However it is possible the sample is HLA-DPB1∗03:01 and the new allele which is identical to DPB1∗105:01 except for exon 1. Long range sequencing encompassing exon 2 and 3 would be needed to rule out these two possible allele pairs. At this time, this would only be possible using the PacBio system or a method that allows for chromosome separation and sequencing of DPB1 alleles separately. We encountered the same exon 1 sequence producing heterozygous peaks at positions 24 and 47 while performing HLA typing for a potential bone marrow donor. Exclusion of exon 1 yielded DPB1∗01:01, 105:01, but HLA-DPB1 typing did not match any known allele in the IMGT/HLA database when exon 1 was included in analysis. Family analysis confirms that one of the donor’s alleles is DPB1∗01:01, since this is also present in the donor’s father. Her second allele has identical sequence to DPB1∗105:01 in exons 2, 3, and 4. The new allele contains single nucleotide substitutions at positions 24 and 47 of exon 1. Position 24 contains a G > A substitution; both codons code for alanine, making this a silent mutation. Position 47 contains a C > T substitution, resulting in an amino acid substitution of methionine for threonine. These findings represent a new HLA-DPB1 allele. This polymorphic site has previously been described in HLA-DPB1∗107:01.