One sample, three genotypes: A flanking region deletion at the D19S433 locus causes genotyping discrepancies between...

Short tandem repeats (STRs) are widely used in forensic genetics for individual identification. While traditional STR analysis relies on capillary electrophoresis (CE), next-generation sequencing (NGS) offers advantages such as full allelic sequence resolution, improving sensitivity and discrimination power. However, genetic variations in flanking regions can lead to discordant genotyping results between CE and NGS approaches, as well as among different analysis software. During the GEDNAP Proficiency Test 65, a genotyping discrepancy was observed at the D19S433 locus. The sample was analyzed using the ForenSeq® DNA Signature Prep Kit on the MiSeq FGx® Sequencing System, yielding a genotype 11.2,16 when analyzed with the ForenSeq™ Universal Analysis Software. This result differed from the 11.1,16 genotype reported by GEDNAP's CE-based results. Sequencing data from ForenSeq was further reanalyzed with STRait Razor Online and STRNaming, resulting in a genotype 11.1,16. Additional testing with three different CE kits (AmpFLSTR™ Identifiler™ Plus, NGM SElect™, and GlobalFiler™) produced a 16,16 genotype, leading to three different genotype assignments for the same sample. A 3-bp TCT deletion in the 5' flanking region of D19S433, located within the International Society for Forensic Genetics (ISFG) minimum reporting range was identified as the cause of these genotyping inconsistencies. Long-read sequencing with PacBio Sequel II technology confirmed that no additional variants were present in the primer binding regions, demonstrating that the TCT deletion alone was responsible for the discrepancies. This study highlights the impact of flanking region mutations on allele calling across different STR typing technologies and the lack of consensus in sequence analysis among bioinformatics pipelines, emphasizing the need to incorporate the ISFG minimum range in the regions sequenced and reported by NGS kits to ensure inter-laboratory and inter-kit consistency, ultimately minimizing discrepancies in forensic STR typing.