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Aim The objective of this study was to characterize a DRB1 allele dropout in a blood sample from a 23 year-old female with precursor B-cell lymphoblastic leukemia, and to explore the mechanism that could underlie the observed allelic dropout. Methods Sequence Based Typing (SBT); Luminex-based reverse Sequence Specific Oligonucleotide Probes (rSSOP); Sequence Specific Primers (SSP). Results The patient was typed as: A∗23:01, 68:01; B∗44:03, 51:01; C∗04:01, 15:02; DRB1∗07:01; DQB1∗02:02, 04:02. To account for DRB1/DQB1 association, we repeated the SBT and rSSOP on the sample and obtained the same results. A verification sample was submitted six months later for ABDR typing. The sample showed the presence of a second DRB1 allele (DRB1∗08:02). We confirmed the second DRB1allele by re-extracting the sample and repeating rSSOP and SBT. A buccal swab sample was also run on both platforms and confirmed the DRB1 heterozygosity. The leftover extracted DNA from the initial testing was again run on both platforms. The sample demonstrated a homozygous DRB1∗07:01 with no evidence of second allele. To assess whether or not we could obtain the heterozygous result by a method that uses multiple primers with different binding sites, we ran the initial DNA sample on SSP tray. This test confirmed the presence of DRB1∗08:02 in the initial sample. Conclusions We hypothesize that blast cells may have undergone mutations in primer binding regions in SBT and SSO, leading to the apparent allelic imbalance seen in this case. If true, SSP was not affected by this phenomenon due to varying primers that target different binding sites in each well. Peripheral blood CBC performed when the initial sample was submitted indicated the presence of 97% blasts. No blasts were seen in peripheral blood of the verification sample, allowing us to achieve the correct HLA typing. This study confirms the importance of (1) verification typing and (2) knowledge of patient’s CBC result when a buccal swab may be deemed a better sample for typing. |
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