Pretransplant HLA mistyping in diagnostic sample of a T-ALL patient due to loss of heterozygosity in the major histocompatibility complex

The degree of HLA compatibility between donor and recipient in hematopoietic stem cell transplantation is critical. In this report, we describe an acute lymphoblastic leukemia case with loss of heterozygosity (LOH) encompassing the entire HLA. HLA molecular typing was performed on peripheral blood (PB) and buccal swabs (BS). Chromosomal microarray analysis (CMA) was performed using a whole genome platform. Typing results on PB sample collected during blast crisis demonstrated homozygosity at the-B,-C,-DR, and -DP loci. A BS sample demonstrated heterozygosity at the above loci. A subsequent PB sample drawn after count recovery confirmed heterozygosity. The CMA performed on PB samples collected during blast crisis revealed a large terminal region of copy-neutral LOH involving chromosome region 6p25.3p21.31, spanning approximately 33.32 Mb. The results of the CMA assay on sample collected after count recovery did not demonstrate LOH. LOH at the HLA gene locus may significantly influence the donor search resulting in mistakenly choosing homozygous donors. We recommend confirming the HLA typing of recipients with hematological malignancies when homozygosity is detected at any locus by using BS samples, or alternatively from PB when remission is achieved. •Loss of heterozygosity at HLA gene locus may influence the donor search resulting in mistakenly choosing homozygous donors.•We described an ALL case with LOH encompassing the entire HLA, which was the reason of HLA homozygous loci.•Homozygosity in HLA typing of hematological malignancies recipients should be confirmed by BS samples or post remission PB.