Influence of template DNA degradation on the genotyping of SNPs and STR polymorphisms from forensic materials by PCR

Detection of single nucleotide polymorphisms (SNPs) and short tandem repeat (STR) polymorphisms by PCR is widely used to analyze degraded DNAs in forensic science. The success of DNA analysis from human remains largely depends on the quality of the template DNA. We examined two SNPs (HLA-DQA1 and ABO) and two STR polymorphisms (VWA and CD4) by SSCP gel or denaturing gel electrophoresis, using two kinds of degraded DNA samples (165 teeth and blood stains contaminated with saliva) derived from the same person and investigated the influence of template DNA degradation on genotyping. As the degradation of DNA proceeds, unbalanced amplification of alleles occurred in the analysis of both SNPs and STRs, followed by allele drop, and further by loss of amplification. Non-target allelic products of STRs were amplified from highly degraded DNA samples; however, false allelic products of SNPs were not amplified from them. Amplification efficiency increased in proportion to the decrease of PCR target size, but reduction of the PCR target sizes also increased the chances of amplifying contaminating DNA, especially in highly degraded DNA specimens. The present results will help investigators to evaluate the genotyping of highly degraded DNA samples in forensic casework.