Individual assignment of body fluids in mixed stains by synonymous SNP analysis

Genotyping of transcribed single nucleotide polymorphisms (SNPs) in cDNA molecules derived from tissue-specific mRNAs has a potential for linking cell types to donors in mixed stains. To test this hypothesis, a single base extension assay targeting the synonymous SNP marker rs857870 in the myeloid cell nuclear differentiation antigen (MNDA) gene was designed. In blood/semen mixtures from donors with informative MNDA genotype (opposite homozygotes), only the MNDA allele of the blood contributor was observed in cDNA, even at blood/semen ratios of 1/20, corresponding to the limit of detection for mixtures by STR profiling. In blood/saliva mixtures, the MNDA allele of the saliva donor was sporadically seen in cDNA, probably due to trace amounts of blood in the donor's saliva, suggesting that the informativeness of the assay may be limited in presence of cell types with partially overlapping mRNA profiles.