[Prevention of fragile X syndrome by prenatal genetic diagnosis: advantages and controversial aspects].

INTRODUCTION

Fragile X syndrome is the most common cause of hereditary mental retardation. Since the molecular mechanism causing it (anomalous expansion of the CGG triplet in the FMR1 gene and hypermethylation of its CpG island) was identified exactly ten years ago, it has been possible to give families in whom the syndrome is transmitted completely reliable prenatal genetic diagnosis of this.

OBJECTIVE

To report and discuss our experience in this field from 1994 to the present time.

PATIENTS AND

METHODS

During this period we performed 15 prenatal diagnoses: 14 in samples of chorionic villi from 13 pregnancies (one a twin pregnancy) and 1 using amniotic fluid. In all cases we used Southern blot molecular techniques with the StB12.3 probe, the PCR of CGG triplet and DXS548 in some cases.

RESULTS

Nine fetuses were normal. Of the other six foetuses, three had full mutation, one had deletion of the FMR1 gene, another was premutated and another had an allele in the grey zone (50 repetitions).

CONCLUSIONS

Molecular prenatal diagnosis of SXF is fast and 100% reliable, although from the technical point of view it is complicated and requires use of various molecular techniques. From the clinical point of view, the low rate of mutations found assures offspring, although molecular studies do not predict mental 'status' in either girls with complete mutation or children with permutation.