Genetic Testing Unravels Unusual Patterns for Fragile-X Syndrome

Fragile-X syndrome is an X-linked disorder with a prevalence of about 1 in 3,600 to 4,000 in males and 1 in 4,000 to 6,000 in females and is the main cause of inherited intellectual disability (ID) and cognitive impairment.

Fragile-X syndrome (FXS) is a rare disease and ID is frequently associated with chromosomal imbalances that often involve the X chromosome, and most clinicians primarily request molecular diagnosis of FXS for patients affected by any type of ID or developmental delay.

Scientists at the University of Naples Federico II (Naples, Italy) and their colleagues studied three women affected by intellectual/psychomotor delay and primary or secondary amenorrhea, and six men affected by mental retardation or psychomotor delay. The enrollees underwent FXS molecular diagnosis and resulted to have normal CGG triplets in the Fragile X Mental Retardation 1 (FMR1) gene, but showed atypical X chromosome patterns. Further diagnostic investigations and, in two females, array-comparative genomic hybridization (array-CGH) revealed the possible genetic cause of their conditions.

The team extracted DNA from 10 mL of whole blood samples from each subject and the CGG repeat number of FMR1 alleles up to 160 CGG repeats was determined by capillary gel electrophoresis of fluorescent-labeled DNA amplicons using primers F (labeled with NED fluorochrome) and C. The triplet repeat primed polymerase chain reaction (TP-PCR) and the Southern blot analysis of genomic DNA digested with restriction enzymes were performed. The X-inactivation analysis was carried out with the PCR-based method for methylation-dependent amplification of the polymorphic triplet repeats at the human androgen receptor (HUMARA) gene. Amplification products were separated by capillary gel electrophoresis on the ABI Prism 3130 genetic analyzer.

The investigators reported that six men affected by ID and three women affected by ID and fragile X-associated premature ovarian failure/ primary ovarian insufficiency (POF/POI) underwent FXS molecular testing. They had normal FMR1 CGG repeats, but atypical X chromosome patterns. Further investigations revealed that the six males had Klinefelter syndrome (XXY), one female was a Turner mosaic (X0/XX) and two women had novel rearrangements involving X chromosome.

The authors concluded that diagnostic investigation of atypical patterns at FMR1 locus can address patients and/or their relatives to further verify the condition by performing karyotyping and/or array-CGH. They emphasize that, for all the presented patients, their diagnostic report concluded for the absence of pathological expansion in the FMR1 gene and the presence of analytical findings that were suggestive of X chromosome abnormality. The study was published online on November 21, 2017, in the journal Clinica Chimica Acta.

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University of Naples Federico II