For a number of nucleotide-repeat-expansion diseases, expansions occur within non-coding regions such as UTRs, introns, promoters, or within non-coding genes. The pathological effects of such non-coding expansions appear to be complex and varied. Frequently, non-coding expansions appear to result in repeat-containing transcripts with gain-of-function RNA toxicity. It at least one case, a profound impact on gene expression has been demonstrated (see FBhh0000123). In some cases, non-coding expansions appear to be translated into toxic peptides by repeat-associated non-AUG translation (RAN); some aspects of this phenomenon have been investigated in Drosophila human disease models (see FBhh0000137 and FBhh0000024).
This report includes model systems developed in Drosophila using UAS constructs to drive synthetic constructs of nucleotide repeats of variable composition and length. Synthetic alleles are designated with the prefix "Zzzz" in FlyBase (see the 'Alleles Reported to Model Human Disease' section, below); see also the allele report for CG9650CTG-240.4 (FBal0243220). See the 'Related Diseases' section, below, for a list of RNA repeat diseases characterized in flies.
Although RNA-mediated mechanisms of neurotoxicity may also play a role in diseases associated with polypeptide repeats, expanded CAG repeats within polyglutamine-containing proteins have been shown to be due (at least primarily) to the repeat-containing protein. Results in Drosophila addressing polyglutamine models are described in disease model reports 'polyglutamine diseases, polyQ models' (FBhh0000001), 'polyglutamine diseases' (FBhh0000218), and related disease models listed in these reports.
[updated Sep. 2022 by FlyBase; FBrf0222196]
A broad grouping of diseases caused by genes containing nucleotide repeat expansions (most commonly, but not exclusively, nucleotide triplet repeats); expansions may be located in exons, UTRs, or introns.
RNA-mediated mechanisms of neurotoxicity appear to play a significant role in the pathogenesis of repeat expansion diseases, probably in conjunction with the protein-mediated pathways. Multiple different mechanisms, such as the alteration of gene expression levels and splicing patterns, the generation of small RNAs, the induction of nucleolar stress, the promotion of bi-directional transcription and repeat-associated non-ATG translation of the disease locus, the activation of apoptotic signaling, and the sequestration of cellular components to RNA foci, have been shown for different repeat expansion diseases.
Non-coding expansion disorders typically involve large expansions (from ~100 to 1000 copies), which reside in the non-coding regions of genes, such as promoters, introns or untranslated regions (UTRs). Non-coding expansions typically result in repeat-containing transcripts with gain-of-function RNA toxicity. However, even non-coding expansions can still be translated into toxic peptides by repeat-associated non-ATG translation.