Somatic Genomic Alterations in Haematological Tumours Can Interfere With Accurate <scp>HLA</scp> and Chimerism Diagnostics

Tumour cells, which are often found in the peripheral blood of patients with acute leukaemia, may harbour multiple somatic alterations throughout the genome, including changes in the HLA region and short tandem repeat (STR) regions. We investigated whether such somatic alterations interfere with HLA and chimerism diagnostics conducted in preparation for an allogeneic haematopoietic stem cell transplantation (allo-HSCT). This study describes 10 patient-based cases for which laboratory diagnostics were performed prior to a possible stem cell transplant in the Maastricht University Medical Center. In three acute leukaemia patients, somatic alterations were detected within the HLA region in peripheral blood samples: one case showed a complete loss of an HLA haplotype, while two cases exhibited somatic mutations affecting a single HLA class I gene. Additionally, seven patients with haematological malignancies revealed somatic variations within the STR regions, indicated by the presence of a third allele or the partial or complete loss of an allele in pre-transplant peripheral blood samples. In all patients, these somatic variations were confirmed by repeating the tests using buccal swab samples from patients or samples from family members. Furthermore, our study demonstrated that somatic alterations within STR regions used for chimerism testing occurred in 6% of the 176 patients who received an allo-HSCT between 2017 and 2022. This study underscores the clinical relevance of detecting somatic alterations prior to allo-HSCT, as they may interfere with HLA and STR analysis, potentially leading to HLA mistyping or incorrect chimerism detection. Additionally, it highlights the frequency with which genetic changes in tumour cells can affect chimerism diagnostics. The findings emphasise the vital importance of selecting the appropriate sample source for typing purposes and considering the patient's karyotype when choosing STRs, especially when tumour cells are present in the peripheral blood of patients with haematological malignancies.