P148 A case of a novel HLA allele generated by gene conversion detected by a next-generation sequencing assay

Abstract

Due to the expanded genomic coverage of many next-generation sequencing (NGS) assays for HLA typing, and their recent introduction into immunogenetics laboratories, there has been an increase in the identification of novel alleles. While the majority of novel alleles are simple, single nucleotide polymorphisms (SNP), others require an extensive knowledge of the NGS analysis software in order to be suitably identified. Herein, we describe a case of gene conversion, a primary way in which HLA diversity is maintained, in a novel DPB1 allele. An optimized NGSgo protocol (GenDx) was used for sequencing the DPB1 locus, with analysis by NGSengine (GenDx). Initially analyzed using default parameters, including both coding and non-coding regions of the amplicon, a typing assignment of DPB1∗02:01:19, and a novel allele, similar to DPB1∗512:01, with an SNP in exon 2 was obtained. Using a lab-developed process, the data was reanalyzed using exon 2 only, with a resulting assignment of DPB1∗40:01, without mismatches. Utilizing the full amplicon and manually aligned to DPB1∗40:01, the sample was reanalyzed, revealing that the novel allele matched DPB1∗40:01 in exons 1 and 2. However, multiple mismatches were present in the remaining sequence. This result indicated that, while the 5′ end of the novel allele likely derived from DPB1∗40:01, the 3′ end may have been derived from another allele. In order to determine which DPB1 allele may have served as the template for this gene conversion event, known DPB1 alleles were manually aligned to the sample, with the finding that the sequence for DPB1∗06:01:01 was identical to the sample sequence at exons 3–5. We concluded that the novel allele likely resulted from a gene conversion event between the DPB1∗40:01 and 06:01:01 alleles. With the increased adoption of NGS assays for HLA typing, the detection of novel alleles due to gene conversion will also likely increase. With the aid of a skilled analyst, and using the comprehensive process described here, the proper identification of such alleles may be simplified and reported appropriately to the clinician.