RNA-Mediated Neurodegeneration Caused by the Fragile X Premutation rCGG Repeats in Drosophila

Abstract

Fragile X syndrome carriers have FMR1 alleles, called premutations, with an intermediate number of 5' untranslated CGG repeats between patients (>200 repeats) and normal individuals (<60 repeats). A novel neurodegenerative disease has recently been appreciated in some premutation carriers. As no neurodegeneration is seen in fragile X patients, who do not express FMR1, we hypothesize that lengthened rCGG repeats of the premutation transcript may lead to neurodegeneration. Here, using Drosophila melanogaster, we show that 90 rCGG repeats alone are sufficient to cause neurodegeneration. This phenotype is neuron specific and rCGG repeat dosage sensitive. Although devoid of mutant protein, this neurodegeneration exhibits neuronal inclusion bodies that are Hsp70 and ubiquitin positive. Overexpression of Hsp70 could suppress the neurodegeneration. These results demonstrate that neurodegenerative phenotype associated with fragile X premutation is indeed caused by the lengthened rCGG repeats and provide the first in vivo experimental demonstration of RNA-mediated neurodegeneration.

Imperfect CAG Repeats Form Diverse Structures in SCA1 Transcripts

“…1B). These results are in accord with the observations made for shorter model transcripts (CNG) 16 and (CNG) 17 containing the artificial G-C clamp at the base of their hairpin stem (20). For comparison the model transcripts having not-clamped repeats were shown to have freedom of alignment and formed several alternative "slipped" structures each containing 4-nucleotide terminal loop (20).…”

“…This clamp forces the repeats to a single alignment, and the 7-nucleotide loop is formed in the transcript containing an even number of repeats whereas the 4-nucleotide loop is present in those having an odd repeat number. This pattern of the stem structure and loop sizes follows those observed for shorter (CAG) 16 and (CAG) 17 repeat tracts containing an artificial G-C clamp (20). Thus, it may be concluded from the above that both the uninterrupted normal repeats as well as expanded repeats present in mutation carriers form the same type of regular hairpin structure (Fig.…”

“…2C). Only at the temperature of 70°C and higher the structure formed by the helices H4 and 14 12-1-1-1-14 ⌬ ϭ Ϫ2 29 (CAG) 16 CAUCAGCAU (CAG) 10 16-1-1-1-10 ⌬ ϭ ϩ6 (CAG) 12 CAU (CAG) 16 12-1-16 ⌬ ϭ Ϫ4 30 (CAG) 12 CAUCAGCAU (CAG) 15 12-1-1-1-15 ⌬ ϭ Ϫ3 (CAG) 13 CAUCAGCAU (CAG) 14 13-1-1-1-14 ⌬ ϭ Ϫ1 31 (CAG) 11 CAUCAGCAUCAGCAU (CAG) 15 11-1-1-1-1-1-15 ⌬ ϭ Ϫ4 32 (CAG) 14 CAUCAGCAU (CAG) 15 14-1-1-1-15 ⌬ ϭ Ϫ1 (CAG) 12 CAUCAGCAU (CAG) 17 12- H3 was melted and all internucleotide bonds of that region showed similar reactivity. The structures formed by the repeats containing asymmetrically located interruptions are somewhat different (Fig.…”

Repetitive sequences that shape the human transcriptome

“…In support of this hypothesis and the unified model for RNA pathogenesis in TREDs, there are recent results showing RNAmediated neurodegeneration caused by the FMR1 permutation [55]. The hairpin structure formed by the CGG repeats in the FMR1 transcript [38,56] together with its 5-10-fold increased level [57] likely to activate RNA pathogenesis in permutation carriers [55].…”

“…The hairpin structure formed by the CGG repeats in the FMR1 transcript [38,56] together with its 5-10-fold increased level [57] likely to activate RNA pathogenesis in permutation carriers [55]. The resulting syndromes, tremor-ataxia and premature ovarian failure, differ significantly in their clinical features from the fragile X syndrome developed in full mutation carriers [57].…”