n South African Medical Journal - Diagnostic, carrier and prenatal genetic testing for fragile X syndrome and other -related disorders in Johannesburg, South Africa : a 20-year review : Festschrift : Professor Trefor Jenkins - molecular genetics

Volume 103, Issue 12
  • ISSN : 0256-9574
  • E-ISSN: 2078-5135



Fragile X syndrome (FXS), the most common inherited cause of intellectual disability (ID) worldwide, is caused by the expansion of a CGG repeat in the fragile X mental retardation gene () gene.

To review, retrospectively, the genetic services for FXS and other -related disorders - including fragile X-associated tremor/ataxia syndrome (FXTAS) and -related primary ovarian insufficiency (POI) - at the Division of Human Genetics, Johannesburg, for diagnostic, carrier and prenatal genetic testing.
The records of 2 690 patients with ID and suspected FXS (ID/?FXS) who had genetic testing for between 1992 and 2012 were reviewed. Of these, 2 239 had diagnostic testing, 430 carrier or cascade testing and 17 prenatal testing for FXS. Four had FXTAS or POI testing. Polymerase chain reaction (PCR) and/or Southern blotting techniques were used to test the patientsâ?? samples for and FMR-2 expansions.
Of the 2 239 patients who had diagnostic testing, 128 (5.7%) had a full mutation, 12 (0.5%) had a premutation and 43 (1.9%) an intermediate allele. In 17 prenatal tests, eight foetuses tested positive for FXS. CGG repeat distribution analysis in 1 532 males negative for the expansion showed that 29 and 30 CGG repeats were the most common (61.1%), but distribution was significantly different in the black and white populations.
The findings support the presence of FXS, as the most common cause of ID, in all local populations. The CGG repeat distribution varied from that found in other studies. The number of family members tested was relatively low suggesting that many at-risk individuals are not being referred.

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