The extent of STR chimerism in different biological samples following bone marrow transplantation: A case report

Biological samples recovered from recipients of allogeneic haematopoietic stem cell transplant (HSCT) contain genetic material from both donor and him/herself. This chimeric condition can greatly complicate analysis of a DNA evidence and undermine its power of discrimination, as the specimens could be mistakenly identified as a mixed sample when in fact it originated from a single person. Profiling reference samples could help clarify the profile anomalies, however, the degree of mixture between host’s and donor’s genetic materials has been reported to vary depending on the tissue from which DNA were obtained. As a result, knowledge to select appropriate sources of reference samples that would most likely reveal the person’s pre- and post-transplant alleles becomes necessary to minimise any possible misunderstanding during the subsequent DNA profile interpretation and comparison. This work investigated the extent of chimerism present in different types of biological samples collected from an individual who had undergone a bone marrow transplant as a child (post-transplant interval, PTI >28 years). DNA profiles from buccal cells, saliva, hair roots, and fingernail clippings were generated using AmpFℓSTR™ Identifiler™ Plus/Direct PCR Amplification Kit. The donor’s STR profile was used to identify donor- and host-specific alleles (DSA and HSA, respectively), and relative donor chimerism (%Ch) of each marker was calculated. Results showed that the saliva sample contained the highest level of chimerism, with complete profile of donor being detected and the mean %Ch was 34.0 ± 4.5. Four out of eight DSA dropped out from the buccal cell profile, and the mean %Ch was 24.2 ± 1.7. Hair roots and fingernail clippings yielded single source profiles and only host’s original alleles were present. Except for the fingernail results, these observations agreed with many of the previous studies, which further demonstrated the need for raising awareness among forensic genetic laboratories regarding the type of reference samples that should be collected in future cases.