Fragile X Genotype Characterized by an Unstable Region of DNA

Abstract

DNA sequences have been located at the fragile X site by in situ hybridization and by the mapping of breakpoints in two somatic cell hybrids that were constructed to break at the fragile site. These hybrids were found to have breakpoints in a common 5-kilobase Eco RI restriction fragment. When this fragment was used as a probe on the chromosomal DNA of normal and fragile X genotype individuals, alterations in the mobility of the sequences detected by the probe were found only in fragile X genotype DNA. These sequences were of an increased size in all fragile X individuals and varied within families, indicating that the region was unstable. This probe provides a means with which to analyze fragile X pedigrees and is a diagnostic reagent for the fragile X genotype.

Rapid preparation of diagnostic probes for the fragile X syndrome by direct PCR amplification of human chromosomal DNA

“…FX311 was a fragile X-negative male, and showed normal size of the restriction fragment. These results were consistent with the ones reported by other groups (Verkerk et al, 1991 ;Yu et al, 1991 ;Kremer et aL, 1991), and any size variations in the restriction fragments were not observed among normal DNA samples in our assay conditions.…”

“…The somatic variations observed in the affected patients might be the consequence of unequal sister chromatid exchange after DNA replication of the mutated X chromosome. In addition we observed in our preliminary experiments on fragile X families that the size of PstI-fragments were increased when the mutant chromosomes were transmitted by females, but not were increased when transmitted by males as previously reported by Yu et aL (1991).…”

Unstable expansion of triplet repeats as a new disease mechanism for neurodegenerative diseases

“…Fragile X syndrome has been found to be caused by expansion of a CGG trinucleotide repeat in the 5'-UTR Oberle et al, 1991;Yu et al, 1991;Verkerk et al, 1991), while expansion of a CTG trinucleotide repeat in the T-UTR has been identified in myotonic dystrophy (Mahadevan et al, 1992;Fu et al, 1992;Brook et al, 1992). Expansion of an intronic GAA trinucleotide repeat has recently been identified in Friedreich's ataxia (Campuzano et al, 1996).…”

“…Fragile X syndrome is an X-linked recessive disease and the leading cause for mental retardation in male Oberle et al, 1991;Yu et al, 1991;Verkerk et al, 1991). Friedreich's ataxia is an autosomal recessive neurodegenerative disorder characterized by ata• decreased or absent tendon reflexes and impairment of deep sensation (Campuzano et al, 1996).…”

“…The gene product of FMR1 gene, the causative gene for fragile X syndrome, has been suggested to have a function as an RNA-binding protein Oberle et al, 1991;Yu et al, 1991;Verkerk et al, 1991), and that of the myotonin kinase gene to be a protein kinase based on the nucleotide sequence homology with other protein kinases (Mahadevan et al, 1992;Fu et al, 1992;Brook et al, 1992;Campuzano et al, 1996). The function of frataxin, the gene product of the gene for Friedreich's ataxia, remains to be elucidated.…”