Technical note: Development of DNA quantitation and STR typing systems for Panthera tigris species determination and individual identification in forensic casework

The aim of this technical note is to provide an overview of methodical approaches used to develop molecular systems for species determination/DNA quantification called Ptig Qplex and individual identification called Ptig STRplex of Panthera tigris samples. Both systems will help to combat the illegal trade of endangered species and create a worldwide shared database of DNA profiles.

Investigation of Mosaicism Detected in STR Typing

Short tandem repeats (STRs) are the most popular markers for human identification in forensics. These markers can be easily analyzed through a multiplex polymerase chain reaction and electrophoresis and provide high discrimination power. However, in STR analysis, several atypical phenomena can be observed such as allelic dropouts, drop-ins, or imbalance, which may be due to DNA polymerase slippage or DNA degradation effects. The observed atypical STR profiles can also provide information for mixed DNA samples or chromosomal abnormalities. In this study, we report a case of mosaicism detected in routine casework of paternity testing. Hair samples from a phenotypically normal male were tested, and the result presented a typical STR profile of a female for the amelogenin gene (XX). Through chromosome analysis using peripheral blood, it was found that 45,X/46,XY mosaicism resulted in the discrepancy between the genotype and the phenotype. In addition, the amount of Y chromosome detected was particularly low in hair compared to that in blood. This study shows that mosaicism can make interpretation difficult during STR analysis and suggests that sample types and repeated analysis should be considered even for routine STR testing.

Sperm Cell Capture Based on ABH Antigen Differences to Separate Two Men in Mixed Seminal Stains

Personal identification of two individuals in mixed semen samples in forensic DNA testing in general usually involves analysis using autosomal and Y chromosome short tandem repeats (STRs). Results may exclude unrelated donors but cannot identify individuals. In this study, sperm cell capture based on ABH antigen differences was used to obtain the cells with the single ABO blood type. Immunohistochemical staining using labeled anti-A, anti-B, and anti-H antibodies and the laser microdissection system can be used to enrich sperm with different ABO types in mixed seminal stains from two individuals. Then, PCR amplification and capillary electrophoresis were performed to genotype the STR loci. To some extent, after sperm cell capture based on ABH antigen differences, autosomal STR typing using enriched single blood group cells can be utilized to partially identify different individuals in a mixed seminal stain sample from two individuals.

Development of a multiplex PCR short tandem repeat typing scheme for Candida krusei

Candida krusei is a human pathogenic yeast that can cause candidemia with the lowest 90-day survival rate in comparison to other Candida species. Infections occur frequently in immunocompromised patients and several C. krusei outbreaks in health care facilities have been described. Here, we developed a short tandem repeat (STR) typing scheme for C. krusei to allow for fast and cost-effective genotyping of an outbreak and compared identified relatedness of ten isolates to SNP calling from whole-genome sequencing (WGS). From a selection of 14 novel STR markers, six were used to develop two multiplex PCRs. Additionally, three previously reported markers were selected for a third multiplex PCR. In total, 119 C. krusei isolates were typed using these nine markers and 79 different genotypes were found. STR typing correlated well with WGS SNP typing, as isolates with the same STR genotype varied by 8 and 19 SNPs, while isolates that differed in all STR markers varied at least tens of thousands of SNPs. The STR typing assay was found to be specific for C. krusei , stable in 100 subcloned generations, and comparable to SNP calling by WGS. In summary, this newly developed C. krusei STR typing scheme is a fast, reliable, easy-to-interpret and cost-effective method compared to other typing methods. Moreover, the two newly developed multiplexes showed the same discriminatory power as all nine markers combined, indicating that multiplexes M3-1 and M9 are sufficient to type C. krusei .