Genetic Stability of Short Tandem Repeat (STR) Loci in Human Term Placentas

Short tandem repeat (STR) genotyping is widely used in human identity testing and is the gold standard for diagnosing and classifying hydatidiform moles and gestational trophoblastic neoplasms. Although STR loci are known to be highly stable, recent whole-genome sequencing studies have revealed confined mosaicism and extensive genetic alterations in term placentas, raising concerns about the reliability of STR-based diagnostics in placental tissues. To assess the genetic stability of STR loci in human term placentas, we performed STR genotyping using a commercial 16-locus kit (PowerPlex 16) on 5 anatomically distinct regions from each of 10 term placentas. Chorionic villi and maternal decidua were microdissected and separately analyzed. All samples yielded adequate PCR amplification, with precise allele assignments at all loci. No evidence of allelic gain/loss, aneuploidy, or off-ladder alleles was observed. The STR profiles of chorionic villi were consistent across all sampled quadrants within each placenta and matched one maternal allele at each locus, confirming biparental inheritance and high allelic stability. Despite previously reported placental genomic mosaicism, our findings demonstrate that STR genotyping remains robust and reliable in term placentas. These data support continued use of STR analysis in the diagnosis of molar pregnancies and trophoblastic neoplasia without concern for placental-specific genetic instability.