Identification and sequence analysis of discordant phenotypes between AmpFlSTR SGM Plus and PowerPlex 16

Concordance testing between Powerplex ESI 16 Fast System and VeriFiler Express

DNA profiles generated by different STR kits may show different alleles for identical amplified loci. This well-known phenomenon affects the smooth transition of data generated by new STR kits to a database or casework laboratory or cross-laboratory comparison of STR profiles. As in other DNA databases throughout the world, it has become clear that the number of the analyzed loci should be expanded for a variety of reasons, such as partial profiles resulting from low copy-number DNA template or degraded samples, working with mixtures or when prevalence of familial inbreeding. In the course of introducing a new STR kit, VeriFiler™ Express (Applied Biosystems, Foster City, CA, USA), we compared genotyping data of 1568 samples amplified by the VeriFiler™ Express with the data generated on the same samples by the Powerplex™ ESI FAST (Promega, Madison WI, USA) kit. Discordance was noted in 20 samples (1.27%), 14 (0.89%) of them showing allele dropout mismatch and six (0.38%) showing an additional fixed-size third allele. These rates are well above the reported rates of 0.4% for this kit. Since correct genotyping and accurate consistent allele assignment is of paramount importance, it seems timely to recommend for DNA laboratories and genetic-match search systems to take these possible inconsistencies into account.

Population genetics data for 22 autosomal STR loci in European, South Asian and African populations using SureID® 23comp Human DNA Identification Kit

Allele frequency data for 22 short tandem repeat loci; D18S1364, D1S1656, D13S325, D5S2800, D9S1122, D4S2366, D3S1744, D12S391, D11S2368, D21S2055, D20S482, D8S1132, D7S3048, D2S441, D19S253, D10S1248, D17S1301, D22-GATA198B05, D16S539, D6S474, D14S1434 and D15S659 from the SureID^® 23comp Human DNA Identification Kit have been determined for unrelated individuals in European, South Asian and African populations. Deviations from Hardy-Weinberg equilibrium were observed in loci D1S1656 and D19S253 in European; D18S1364, D6S474 and D14S1434 in South Asian; and D9S1122 and D8S1132 in African populations (p-value <0.05). However, after Bonferroni correction no significant deviations were observed (p-value <0.002). The most discriminating loci were D1S1656 and D12S391 for European (PD=0.977), D21S2055 for South Asian (PD=0.980), and D21S2055 and D7S3048 for African (PD=0.972) populations. The match probabilities were 1 in 6.7×10²⁵ for European, 1 in 1.4×10²⁶ for South Asian and 1 in 1.6×10²⁶ for African populations. These findings established the high discriminatory capacity and robustness of the tested STR loci for forensic identification and kinship testing.

Kit-dependent discrepancy in D16S539 and general considerations for database matches

Throughout the last decade more companies have been offering multiplex PCR kits for forensic STR typing. As a consequence, it has been demonstrated, that an observed genotype may unexpectedly vary at a single locus when different STR kits have been used. Analysing STR profiles which have to be entered in a national database, unknown or undetected primer binding site mutations, insertions or deletions within the flanking region of STR loci may hinder matches and therefore have far-reaching consequences. The current study is a further example indicating that sequence variations in flanking regions are a common problem within STR typing which should not be underestimated. A deletion of 16 nucleotides close to the primer binding site downstream of the repeat sequence resulted in deviant genotypes at the D16S539 locus according to different STR kits used.

Null alleles and sequence variations at primer binding sites of STR loci within multiplex typing systems

Rare variants are widely observed in human genome and sequence variations at primer binding sites might impair the process of PCR amplification resulting in dropouts of alleles, named as null alleles. In this study, 5 cases from routine paternity testing using PowerPlex^®21 System for STR genotyping were considered to harbor null alleles at TH01, FGA, D5S818, D8S1179, and D16S539, respectively. The dropout of alleles was confirmed by using alternative commercial kits AGCU Expressmarker 22 PCR amplification kit and AmpFℓSTR^®. Identifiler^® Plus Kit, and sequencing results revealed a single base variation at the primer binding site of each STR locus. Results from the collection of previous reports show that null alleles at D5S818 were frequently observed in population detected by two PowerPlex^® typing systems and null alleles at D19S433 were mostly observed in Japanese population detected by two AmpFℓSTR™ typing systems. Furthermore, the most popular mutation type appeared the transition from C to T with G to A, which might have a potential relationship with DNA methylation. Altogether, these results can provide helpful information in forensic practice to the elimination of genotyping discrepancy and the development of primer sets.

A silent allele in the locus D5S818 contained within the PowerPlex®21 PCR Amplification Kit

Three paternity tests cases were found with a single locus mismatch at the locus D5S818 with PowerPlex®21 PCR Amplification Kit (Promega). Forward and reverse primers were redesigned to type the samples again and to evaluate if there were alleles dropped out. The results showed the existence of a silent allele 12 in all the three families, due to a point mutation that changed cytosine to adenine at 90 nucleotides upstream from the 5' end of the AGAT repeat sequences in all the six individuals. A single locus mismatch due to a silent allele may occur in any locus using any kit. Therefore, we recommend using multiple kits to confirm the results in paternity testing cases with mismatches, especially when there is a single locus mismatch with homozygote involved.

A method to determine the 5' end of the binding site of primers included in a commercially available forensic human identification kit

Analysis for short tandem repeat (STR) loci is widely performed in forensic laboratories for human identification that utilizes commercially available kits that employ fluorescently labeled primers and capillary electrophoresis. With only a few exceptions, the sequences of the primers included in a kit are not disclosed by the kit manufacturers. Therefore, we developed a simple method to determine the 5' end of the binding site of the primers included in commercial kits. Our method requires only custom primers and human genome sequence data and routinely used equipment and consumables. One or two custom primers are added to the PCR reaction mixture containing kit primers and input human DNA prior to amplification, and PCR products are separated by capillary electrophoresis after amplification. With this method we can determine which primer of the pair is fluorescently labeled and the 5' end of the binding site of primers based on the changes in an electropherogram that are caused by the addition of the custom primer(s), and the human genome sequence data. This method is also useful for the determination of the shortest possible lengths of labeled kit primers.

Identification of new primer binding site mutations at TH01 and D13S317 loci and determination of their corresponding STR alleles by allele-specific PCR

Several commercial multiplex PCR kits for the amplification of short tandem repeat (STR) loci have been extensively applied in forensic genetics. Consequently, large numbers of samples have been genotyped, and the number of discordant genotypes observed has also increased. We observed allele dropout with two novel alleles at the STR loci TH01 and D13S317 during paternity testing using the AmpFℓSTR Identifiler PCR Amplification Kit. The lost alleles reappeared when alternative PCR primer pairs were used. A sequence analysis revealed a G-to-A substitution 82 bases downstream of the last TCAT motif of the repeat region at the TH01 locus (GenBank accession: D00269) and a G-to-T substitution 90 bases upstream of the first TATC motif of the repeat region at the D13S317 locus (GenBank accession: G09017). The frequencies of these two point mutations were subsequently investigated in the Chinese population using sequence-specific primer PCR (SSP-PCR), but neither of these mutations was detected in any of the samples tested. In addition, the DNA samples in which the mutations were identified were amplified to type the point mutations by SSP-PCR to determine the corresponding STR alleles at the two loci. Subsequently, the amplified PCR products with different point mutations and STR repeat numbers were directly sequenced because this strategy overcomes the appearance overlapping peaks generated by different STR alleles and accurately characterizes genotypes. Thus, our findings not only provide useful information for DNA databases and forensic identification but also establish an effective strategy for typing STR alleles with primer binding site mutations.

Development and validation for identity testing of I-DNADuo, a combination of I-DNA1 and a new multiplex system, I-DNA2

The I-DNADuo multiplex system combination is composed of previously validated I-DNA1 and a new short tandem repeat (STR) multiplex named I-DNA2 that analyses 11 STR loci plus amelogenin. I-DNADuo, with amplicon sizes ranging from 57 to 298 bp, is specifically designed to analyse amelogenin and 15 STR loci (ten of them plus amelogenin in duplicate), including all the STR loci of the CODIS, ISSL and ECL databases, and seven of the eight in GCL. The validation of I-DNADuo shows that it is a highly sensitive, robust multiplex system for obtaining individual genetic profiles and for detecting and preventing allelic dropouts.

A silent allele in the locus D19S433 contained within the AmpFlSTR Identifiler PCR Amplification Kit

We present two cases where a single locus mismatch was found in the locus D19S433 using the AmpFlSTR Identifiler PCR Amplification Kit (Applied Biosystems) (Identifiler Kit) during paternity and maternity tests. This mismatch differed from the mismatch pattern where there is usually a one repeat difference. We designed forward and reverse primers so that they were positioned further away from the primer set contained in the Identifiler Kit. The results showed the existence of a silent allele 13 in both families, due to a point mutation that changed guanine to adenine at 32 nucleotides downstream from the 3' end of the AAGG repeat sequences in all four members. A single locus mismatch due to a silent allele may occur in any locus using any kit. Accordingly, we should pay attention to this silent allele when carrying out human identification and parentage analysis.

A D19S433 primer binding site mutation and the frequency in Japanese of the silent allele it causes

Short tandem repeat studies are powerful tools for parentage analysis and for identification of missing persons, victims of murder, and victims of mass fatalities when reference samples are unavailable. The primer in the Identifiler kit failed to amplify an allele at the D19S433 locus, producing a silent ("null") allele. The causal mutation is a base change (G>A) 32 nucleotides downstream from the 3' end of the AAGG repeats. The silent alleles are problematical in parentage analysis because when transmitted, they can cause a parent-child inconsistency that is unrelated to Mendelian genetics. The inconsistency is sometimes termed an "apparent opposite homozygosity" and it produces false evidence of nonparentage. Alternative primers were designed to amplify the D19S433 locus alleles and they detect the silent allele. Frequencies of the (no longer) silent allele were determined to be 0.0114 in 176 people from Shizuoka (Honshu) and 0.0128 in 156 people from Okinawa.