Unusual presentation of fragile X syndrome

Detection of full fragile X mutation

In fragile X syndrome, the most common inherited cause of mental deficiency, the underlying mutation is a large increase in the number of CGG repeats in a gene on chromosome X. We have developed a polymerase chain reaction (PCR) method to amplify across the full mutation in affected individuals. In this report, a fragile X family including a positive prenatally diagnosed fetus was analysed by PCR, and the results are consistent with direct genomic Southern blot analysis. Genetic screening of at-risk populations for fragile X can now be achieved by PCR rapidly, inexpensively, and on small samples.

Molecular heterogeneity of the fragile X syndrome

The fragile X syndrome is an X-linked disorder which has been shown to be associated with the length variation of a DNA fragment containing a CGG trinucleotide repeat element at or close to the fragile site. Phenotypically normal carriers of the disorder generally have a smaller length variation than affected individuals. We have cloned the region in cosmids and defined the area containing the amplified sequence. We have used probes from the region to analyse the mutation in families. We show that the mutation evolves in different ways in different individuals of the same family. In addition we show that not all fragile X positive individuals show this amplification of DNA sequence even though they show expression of the fragile site at levels greater than 25%. One patient has alterations in the region adjacent to the CGG repeat elements. Three patients in fragile X families have the normal fragment with amplification in a small population of their cells. These observations indicate that there is molecular heterogeneity in the fragile X syndrome and that the DNA fragment length variation is not the only sequence responsible for the expression of the fragile site or the disease phenotype.