Detection of Cryptic Fragile X Full Mutation Alleles by Southern Blot in a Female and Her Foetal DNA via Chorionic Villus Sampling, Complicated by Mosaicism for 45,X0/46,XX/47,XXX

• Main Authors: Alison Pandelache, David Francis, Ralph Oertel, Rebecca Dickson, Rani Sachdev, Ling Ling, Dinusha Gamage, David E. Godler
• Format: Article
• Language: English
• Published: MDPI AG 2021-05-01
• Series: Genes
• Subjects: fragile; X syndrome; FMR1; mosaicism; CGG; cultured chorionic villus
• Online Access: https://www.mdpi.com/2073-4425/12/6/798

We describe a female with a 72 CGG <i>FMR1</i> premutation (PM) (CGG 55–199) and family history of fragile X syndrome (FXS), referred for prenatal testing. The proband had a high risk of having an affected pregnancy with a full mutation allele (FM) (CGG > 200), that causes FXS through hypermethylation of the <i>FMR1</i> promoter. The CGG sizing analysis in this study used AmplideX triplet repeat primed polymerase chain reaction (TP-PCR) and long-range methylation sensitive PCR (mPCR). These methods detected a 73 CGG PM allele in the proband’s blood, and a 164 CGG PM allele in her male cultured chorionic villus sample (CVS). In contrast, the Southern blot analysis showed mosaicism for: (i) a PM (71 CGG) and an FM (285–768 CGG) in the proband’s blood, and (ii) a PM (165 CGG) and an FM (408–625 CGG) in the male CVS. The <i>FMR1</i> methylation analysis, using an EpiTYPER system in the proband, showed levels in the range observed for mosaic Turner syndrome. This was confirmed by molecular and cytogenetic karyotyping, identifying 45,X0/46,XX/47,XXX lines. In conclusion, this case highlights the importance of Southern blot in pre- and postnatal testing for presence of an FM, which was not detected using AmplideX TP-PCR or mPCR in the proband and her CVS.