Somatic mutations and genomic alterations are frequent events in the clonal evolution of hematologic malignancies. Recent studies have reported copy neutral loss of heterozygosity (LOH) for the mismatched human leukocyte antigen (HLA) haplotype in patients relapsed after haploidentical hematopoietic cell transplantation (HCT) for a hematologic malignancy. Herein, we report 15 cases of somatic mutations in the HLA genes of patients with a variety of hematologic diseases, including acute myelogenous leukemia, acute lymphocytic leukemia, chronic lymphocytic leukemia, chronic myeloid leukemia, myelodysplastic syndrome, and non-Hodgkin's lymphoma, encountered at our institute over the past decade. While two of the cases were identified in patient relapse specimens collected post-HCT, 13 cases were found in peripheral blood specimens submitted for HLA typing prior to transplantation. Ten patients exhibited acquired LOH for all or part of one HLA haplotype. Five other cases involved somatic mutations in the nucleotide sequences of common HLA-A or HLA-B alleles. Since they are not systematically evaluated prior to HCT, acquired mutations in HLA genes are likely under reported. Beyond the implications for accurate HLA typing and donor selection, alternations that result in the loss of HLA expression may allow escape from immune surveillance and adversely impact transplant outcome.

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A project of the 15th International Histocompatibility Workshop examined the rarity of human leukocyte antigen (HLA) alleles. A section was constructed in the website, www.allelefrequencies.net to contain this data from different sources. A mechanism to search the data was implemented for use by any individual.

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