A complex case of paternity testing

The report describes a complex case of paternity identification managed at Forensic Genetic laboratory of CEINGE Advanced Biotecnology in Naples (Italy). A woman requested a paternity testing to know whether a man was the father of her child. Buccal swabs were used to obtain DNA specimens. The child's autosomal profile presented a mixed profile. After exclusion of possible sources of contamination, this scenario was compatible with a mother/son mixture. Actually, the child, affected by acute lymphoblastic leukemia, underwent to an allogeneic hematopoietic stem cell transplantation with his mother as donor a few years before. The PowerQuant® System highlighted a concentration autosomal/Y ratio of almost 2. In the absence of the recipient's autosomal profile prior the transplant, the known maternal profile and the mixed child profile were compared locus by locus. The pairs of mother-son alleles for each locus were identified applying the deconvolution rules of the two contributors in the mixture, where one of them is known. Hence, we chose the most probable pair, considering the profile mixture ratio woman/man of 1.8. According to the International Society for Forensic Genetics guidelines, the biostatistical interpretation was based on a likelihood ratio (LR) approach. Furthermore, LRmix and Familias 3 software were used to interpret the mixture and the kinship. This case highlights that we should bear in mind the possible occurrence of chimerism in reference material derived from buccal swabs, if a subject had an allogeneic hematopoietic stem cell transplantation. In similar case, other specimens, like hair roots, should be used, provided that they are available.