Trisomy 21 disclosure using STR and SNP markers typed by MiSeq FGx™ Forensic Genomics System

The presence of a tri-allelic pattern at a single locus in a multiplex short tandem repeat (STR) profile is a rarely observable event. Generally, based on peak height measured by the capillary electrophoresis (CE) method and combination of alleles, the tri-allelic pattern is distinguishable into two predominant types: type 1 and 2, which are caused, respectively, by somatic mutations and chromosomal rearrangements. When tri-allelic patterns at more than one STR located on the same chromosome are detected, there is a reasonable suspicion of a trisomy due to an extra copy of a chromosome. Therefore, information on the type of three-band pattern is usually limited to STRs localized on the same chromosome included in the forensic kit in use and sometimes in insufficient numbers to classify this event correctly. The opportunity to extend this evaluation to additional markers, such as SNPs detectable using NGS, has not yet been explored. In this study, using the ForenSeq™ DNA Signature Prep kit, two cases of autosomal aneuploidy were revealed on chromosome 21, relying not only on STRs assessment but also extending the analysis to the five identity-informative single nucleotide polymorphisms (iiSNPs) localized on chromosome 21.