Clinical Utility and Performance of Methylation-Specific Triplet-Primed PCR for Fragile X Syndrome Diagnosis

Fragile X syndrome (FXS) is the most common cause of intellectual disability. It is usually caused by the expansion of the CGG trinucleotide repeat (>200 repeats) in FMR1, resulting in DNA hypermethylation and gene silencing. Conventional FXS diagnosis is based on a combination of PCR and Southern blot (SB) analysis, which is technically challenging and labor-intensive. Methylation-specific triplet-primed PCR (msTP-PCR) has been proposed to overcome these limitations in detecting FMR1 expansion and methylation status. However, its performance in FXS diagnosis in clinical laboratory settings has not been extensively studied. An msTP-PCR assay was validated, implemented, and compared versus the conventional diagnostic protocol in clinical samples. A total of 420 clinical samples (339 male subjects and 81 female subjects) previously genotyped for FMR1 CGG repeat expansion using reference methods (PCR-based methods and/or SB analysis) were investigated using msTP-PCR. The results showed high concordance with the reference results for allele categorization, repeat number correlation, and methylation status. However, discordant results were observed in rare cases of female patients with complex mosaic normal, premutation, and full mutation alleles, which need further confirmation via SB analysis. Nonetheless, these results confirm that the msTP-PCR method is a useful alternative technique for FXS diagnosis and prenatal testing, as it is rapid, reliable, cost-effective, and can potentially reduce the requirement for SB analysis.