Identification of a novel epigenetic marker for typical and mosaic presentations of Fragile X syndrome.

Background: Fragile X syndrome (FXS) is primarily due to CGG repeat expansions in the gene. alleles are classified as normal (N), intermediate (I), premutation (PM), and full mutation (FM). FXS patients often carry an FM, but size mosaicism can occur. Additionally, loss of methylation boundary upstream of repeats results in methylation spreading to promoter in FXS patients.

Research Design And Methods: This pilot study investigated the methylation boundary and adjacent regions in 66 males with typical and atypical FXS aged 1 to 30 years (10.86 ± 6.48 years). AmplideX mPCR kit was used to discriminate allele profiles and methylation levels. CpG sites were assessed by pyrosequencing.

Results: 40 out of 66 FXS patients (60.6%) showed an exclusive FM (n = 40), whereas the remaining (n = 26) exhibited size mosaicism [10 PM_FM (15.15%); 10 N_FM (15.15%); 2 N_PM_FM (3%)]. Four patients (6.1%) had deletions near repeats. Noteworthy, a CpG within intron 2 displayed hypomethylation in FXS patients and hypermethylation in controls, demonstrating remarkable specificity, sensitivity, and accuracy when a methylation threshold of 69.5% was applied.

Conclusions: Since intragenic methylation is pivotal in gene regulation, the intronic CpG might be a novel epigenetic biomarker for FXS diagnosis.