Persisting fetal microchimerism does not interfere with forensic Y-chromosome typing

Forensic Y-chromosome typing applies Y-chromosomal polymorphisms to the analysis of male/female mixed stains such as vaginal swabs in rape cases. The sensitivity of this approach exceeds that of cytological techniques combined with autosomal DNA typing. Y-chromosome typing is based on the assumption that Y-chromosomal DNA found in tissue or secretions of women must originate from a male individual, usually the perpetrator. Nevertheless, it was shown recently that fetal cells can migrate into the female body during pregnancy and can persist for decades ("persisting fetal microchimerism"). The body of a woman after a pregnancy with a male embryo can thus display a small fraction of fetal cells with Y-chromosomes. Using high sensitivity PCR protocols (reamplification with nested primers and up to 60 PCR cycles) fetal cells were previously identified in a number of maternal tissues including skin, blood, muscle and solid organs. It is, however, not clear at present, whether these cells can occur in vaginal secretions, and whether they are capable of producing false positive results in forensic Y-chromosome typing. To evaluate these questions, 66 blood samples of women with at least one son and nine vaginal swabs of women without sexual intercourse in the last 2 weeks were amplified for a stretch of the SRY gene. Eight thyroid gland tissues with already established male fetal microchimerism were used as positive control samples. Blood samples of 10 young girls without history of pregnancy were used as negative controls. Using a PCR with 10 ng of extracted DNA and 30 PCR cycles ("routine sensitivity assay") none of the samples yielded positive results. However, in a PCR with 200 ng of extracted DNA and 45 PCR cycles ("high sensibility assay"), 14% of the blood samples of mothers and 33% of the vaginal swabs amplified for SRY. Our results thus show that increasing the sensitivity of the PCR method and the amount of template DNA produce positive results while protocols used for routine Y-chromosomal typing with small amounts of DNA (approximately 10 ng of DNA) and with a limited number of PCR cycles (approximately 30) can clearly eliminate this peril.