Instability of a 550-Base Pair DNA Segment and Abnormal Methylation in Fragile X Syndrome

Abstract

The fragile X syndrome, a common cause of inherited mental retardation, is characterized by an unusual mode of inheritance. Phenotypic expression has been linked to abnormal cytosine methylation of a single CpG island, at or very near the fragile site. Probes adjacent to this island detected very localized DNA rearrangements that constituted the fragile X mutations, and whose target was a 550-base pair GC-rich fragment. Normal transmitting males had a 150- to 400-base pair insertion that was inherited by their daughters either unchanged, or with small differences in size. Fragile X-positive individuals in the next generation had much larger fragments that differed among siblings and showed a generally heterogeneous pattern indicating somatic mutation. The mutated allele appeared unmethylated in normal transmitting males, methylated only on the inactive X chromosome in their daughters, and totally methylated in most fragile X males. However, some males had a mosaic pattern. Expression of the fragile X syndrome thus appears to result from a two-step mutation as well as a highly localized methylation. Carriers of the fragile X mutation can easily be detected regardless of sex or phenotypic expression, and rare apparent false negatives may result from genetic heterogeneity or misdiagnosis.

Génétique moléculaire humaine : des maladies monogéniques aux maladies complexes

“…Nous discuterons les différents aspects méthodologiques et pratiques de l'identification de ces gènes, ainsi que les défis qui attendent la médecine pour utiliser efficacement les nouveaux outils diagnostiques issus de ces travaux. < moléculaires ont été appliqués successivement à l'étude de la génétique des maladies monogé-niques classiques [1][2][3], à la cartographie systéma-tique du génome humain [4][5][6], puis à son séquencage [7,8] et, enfin, à l'étude des maladies fréquentes à forte composante génétique (ou maladies génétiques complexes) [9][10][11][12]. L'identification des mutations géniques, puis de la fonction du (des) produit(s) de ces gènes, permet de révéler la biologie normale et celle de la maladie.…”

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.…”

Epigenetics, fragile X syndrome and transcriptional therapy

“…In fact, the expanded and hypermethylated CGG sequence is replicating so late that (under particular culture conditions) it is not duplicated at mitosis and cannot condense at metaphase. All these fragile sites (including FRAXA, FRAXE, FRAXF, FRA10A, FRA11A, FRA11B, FRA12A, and FRA16A) are located in promoters and their expression associates with silencing of the respective genes [Oberlé et al, 1991;Knight et al, 1993;Ritchie et al, 1994;Nancarrow et al, 1995;Jones et al, 2000;Sarafidou et al, 2004;Debacker et al, 2007;Winnepenninckx et al, 2007]. …”