Long CCG Triplet Repeat Blocks Exclude Nucleosomes: A Possible Mechanism for the Nature of Fragile Sites in Chromosomes

Comparative analysis of DNA methylation in transgenic mice with unstable CGG repeats from FMR1 gene

“…8 Although in most cases FMR1 full mutation alleles are hypermethylated, unmethylated alleles in the full mutation range have also been described and are generally associated with a less severe phenotype. [8][9][10][11][12][13] However methylation in absence of repeat expansion leading to fragile X syndrome is not reported so far.…”

Chromatin Remodeling by SWI/SNF Results in Nucleosome Mobilization to Preferential Positions in the Rat Osteocalcin Gene Promoter

“…However, it appears that these sequences may exhibit certain parameters, such as the presence of AA duplets or AAA triplets, or CTG triplets every 10 or 11 bp (6,7). Interestingly, there have also been reports suggesting the existence of "nucleosome-excluding" sequences, which include short repeats of adenine (A 16 ) (8), long CCG triplet repeats (9), TGGA repeats (10), or the [(G/C) 3 NN] n motif (11). These nucleosome-excluding sequences may lead to nucleosome-free promoter domains that could allow preferential access for transcription factors to their cognate elements.…”

Fragile X spectrum disorders

“…One suggests that the observed increase of acetylated histones at the FMR1 promoter (118) could increase the FMR1 gene transcription. Second, the long tracts of CGGrepeats have been shown to exclude nucleosomes in vitro (119) and if this occurs in vivo it may increase the accessibility of transcription factors to the promoter. Third, the R-loops formed by the CGG-repeats (120,121) may lead to chromatin decondensation (122).…”