Using FastID to analyze complex SNP mixtures from indoor dust.

Forensically relevant single nucleotide polymorphisms (SNPs) can provide valuable supplemental information to short tandem repeats (STRs) for investigative leads, and genotyping can now be streamlined using massively parallel sequencing (MPS). Dust is an attractive evidence source, as it accumulates on undisturbed surfaces, often is overlooked by perpetrators, and contains sufficient human DNA for analysis. To assess whether SNPs genotyped from indoor dust using MPS could be used to detect known household occupants, 13 households were recruited and provided buccal samples from each occupant and dust from five predefined indoor locations. Thermo Fisher Scientific Precision ID Identity and Ancestry Panels were utilized for SNP genotyping, and sequencing was completed using Illumina® chemistry. FastID, a software developed to permit mixture analysis and identity searching, was used to assess whether known occupants could be detected from associated household dust samples. A modified "subtraction" method was also used in FastID to estimate the percentage of alleles in each dust sample contributed by known and unknown occupants. On average, 72% of autosomal SNPs were recovered from dust samples. When using FastID, (a) 93% of known occupants were detected in at least one indoor dust sample and could not be excluded as contributors to the mixture, and (b) non-contributor alleles were detected in 54% of dust samples (29 ± 11 alleles per dust sample). Overall, this study highlights the potential of analyzing human DNA present in indoor dust to detect known household occupants, which could be valuable for investigative leads.

An alternate workflow for preparing Precision ID Ancestry and Identity Panel libraries for Illumina sequencing.

Single nucleotide polymorphisms (SNPs) are well-established for forensic applications. Although they are not compatible with existing criminal databases, they offer some advantages over short tandem repeat (STR) markers including smaller amplicons, no stutter artifacts, and biogeographic ancestry and phenotype predictions. The Precision ID NGS System, a commercial workflow by Thermo Fisher Scientific, offers a streamlined solution for genotyping forensically relevant SNPs using next-generation sequencing. The Precision ID Ancestry and Identity Panels combined target 289 SNPs, and their sensitivity, reproducibility, and accuracy have been evaluated by the forensic community. The aim of this study was to develop an alternative workflow to genotype these SNP panels using Illumina chemistry. Commercial genomic DNAs (gDNAs) (n, 3) were amplified using three uracil-tolerant polymerase master mixes. Resulting amplicons were prepared into libraries using the KAPA Hyper Prep Kit (KAPA Biosystems) and sequenced via Illumina's MiniSeq. Reads were analyzed using a published analysis pipeline to compile final genotypes with read depth information. Phusion U Multiplex PCR Master Mix (Thermo Fisher Scientific) statistically outperformed the other master mixes tested (P <0.0001), with respect to the number of SNPs genotyped. To ensure a workflow using Phusion U would be compatible across diverse samples, we optimized PCR cycle number using the same commercial gDNAs (n, 3), reference buccal swabs (n, 3), and environmental (household dust) samples (n, 6). Using the developed workflow, 93.9% of all SNPs were successfully genotyped across sample types. Implementation of the developed workflow should be straightforward for forensic laboratories and suitable for processing reference and casework samples.

Single Fragment or Bulk Soil DNA Metabarcoding: Which is Better for Characterizing Biological Taxa Found in Surface Soils for Sample Separation?

In forensic geology casework, sample size typically limits routine characterization of material using bulk approaches. To address this, DNA-based characterization of biological taxa has received attention, as the taxa present can be useful for sample-to-sample comparisons and source attribution. In our initial work, low biodiversity was captured when DNA barcodes were Sanger-sequenced from plant and insect fragments isolated from 10 forensic-type surface soils. Considering some forensic laboratories now have access to massively parallel sequencing platforms, we assessed whether biological taxa present in the same surface soils could be better characterized using DNA metabarcoding. To achieve this, plant and animal barcodes were amplified and sequenced on an Illumina MiniSeq for three different DNA sample types (n = 50): individual fragments used in our initial study, and 250 and 100 mg of bulk soil (from the 10 sites used in the initial study). A total of 572 unique target barcode sequences passed quality filtering and were used in downstream statistical analyses: 54, 321, and 285 for individual fragments, 100 mg, and 250 mg bulk soil samples, respectively. Plant barcodes permitted some spatial separation of sample sites in non-metric multidimensional scaling plots; better separation was obtained for samples prepared from bulk soil. This study confirmed that bulk soil DNA metabarcoding is a better approach for characterizing biological taxa present in surface soils, which could supplement traditional geologic examinations.