CGG Repeat-Associated Translation Mediates Neurodegeneration in Fragile X Tremor Ataxia Syndrome

Genetic decoding is not 'frozen' as was earlier thought, but dynamic. One facet of this is frameshifting that often results in synthesis of a C-terminal region encoded by a new frame. Ribosomal frameshifting is utilized for the synthesis of additional products, for regulatory purposes and for translational 'correction' of problem or 'savior' indels. Utilization for synthesis of additional products occurs prominently in the decoding of mobile chromosomal element and viral genomes. One class of regulatory frameshifting of stable chromosomal genes governs cellular polyamine levels from yeasts to humans. In many cases of productively utilized frameshifting, the proportion of ribosomes that frameshift at a shift-prone site is enhanced by specific nascent peptide or mRNA context features. Such mRNA signals, which can be 5' or 3' of the shift site or both, can act by pairing with ribosomal RNA or as stem loops or pseudoknots even with one component being 4 kb 3' from the shift site. Transcriptional realignment at slippage-prone sequences also generates productively utilized products encoded trans-frame with respect to the genomic sequence. This too can be enhanced by nucleic acid structure. Together with dynamic codon redefinition, frameshifting is one of the forms of recoding that enriches gene expression.

/pdf/ribosomal-frameshifting-and-transcriptional-slippage-from-2dxbxuyood.pdf

Ribosomal frameshifting and transcriptional slippage: From genetic steganography and cryptography to adventitious use.

https://www.cell.com/neuron/pdf/S0896-6273(15)00932-0.pdf

RAN Translation in Huntington Disease

https://dukespace.lib.duke.edu/dspace/bitstream/handle/10161/8406/Enns%20et%20al._Mutations%20in%20NGLY1%20Cause%20an%20Inherited%20Disorder.pdf;sequence=1

Mutations in NGLY1 cause an inherited disorder of the endoplasmic reticulum-associated degradation pathway

A nucleotide repeat expansion (NRE) within the chromosome 9 open reading frame 72 (C9orf72) gene was the first of this type of mutation to be linked to multiple neurological conditions, including amyotrophic lateral sclerosis and frontotemporal dementia. The pathogenic mechanisms through which the C9orf72 NRE contributes to these disorders include loss of C9orf72 function and gain-of-function mechanisms of C9orf72 driven by toxic RNA and protein species encoded by the NRE. These mechanisms have been linked to several cellular defects - including nucleocytoplasmic trafficking deficits and nuclear stress - that have been observed in both patients and animal models.

The expanding biology of the C9orf72 nucleotide repeat expansion in neurodegenerative disease

Spinocerebellar ataxia type 3 is caused by the expansion of the coding CAG repeat in the ATXN3 gene. Interestingly, a -1 bp frameshift occurring within an (exp)CAG repeat would henceforth lead to translation from a GCA frame, generating polyalanine stretches instead of polyglutamine. Our results show that transgenic expression of (exp)CAG ATXN3 led to -1 frameshifting events, which have deleterious effects in Drosophila and mammalian neurons. Conversely, transgenic expression of polyglutamine-encoding (exp)CAA ATXN3 was not toxic. Furthermore, (exp)CAG ATXN3 mRNA does not contribute per se to the toxicity observed in our models. Our observations indicate that expanded polyglutamine tracts in Drosophila and mouse neurons are insufficient for the development of a phenotype. Hence, we propose that -1 ribosomal frameshifting contributes to the toxicity associated with (exp)CAG repeats.

Expanded ATXN3 frameshifting events are toxic in Drosophila and mammalian neuron models

Autosomal dominant sensory ataxia is a rare genetic condition that results in a progressive ataxia that is caused by degeneration of the posterior columns of the spinal cord To date only two families have been clinically ascertained with this condition, both from Maritime Canada We previously mapped both families to chromosome 8p12-8q12 and have now screened the majority of annotated protein-coding genes in the shared haplotype region by direct DNA sequencing We have identified a putative pathogenic mutation in the gene encoding ring-finger protein RNF170, a potential ubiquitin ligase This mutation is a rare non-synonymous change in a well-conserved residue and is predicted to be pathogenic by SIFT, PolyPhen, PANTHER and Align-GVD Microinjection of wild-type or mutant orthologous messenger RNAs into zebrafish (Danio rerio) embryos confirmed that the mutation dominantly disrupts normal embryonic development Together these results suggest that the mutation in RNF170 is causal for the sensory ataxia in these families

A mutation in the RNF170 gene causes autosomal dominant sensory ataxia

Eighteen severe human diseases have so far been associated with trinucleotide repeat expansions coding for either polyalanine (encoded by a GCN repeat tract) or polyglutamine (encoded by a CAG repeat tract).  Among them, oculopharyngeal muscular dystrophy (OPMD), spinocerebellar ataxia type-3 (SCA3), and Huntington’s disease (HD) are late-onset autosomal-dominant disorders characterized by the presence of intranuclear inclusions (INIs).  We have previously identified the OPMD causative mutation as a small expansion (from 2 in normal to 7 in disease) of a GCG repeat tract in the PABPN1 gene.  In addition, -1 ribosomal frameshifting has been reported to occur in expanded CAG repeat tracts in the ATXN3 (SCA3) and HTT (HD) genes, resulting in the translation of a hybrid CAG/GCA repeat tract and the production of a polyalanine-containing peptide.  Previous studies on OPMD suggest that polyalanine-induced toxicity is very sensitive to the dosage and length of the alanine stretch.  Here we report the characterization of a polyclonal antibody that selectively recognizes pathological expansions of polyalanine in PABPN1.  Furthermore, our antibody also detects the presence of alanine proteins in INIs of SCA3 and HD patient samples.

Shawn J. Stochmanski

Papers

Expanded ATXN3 frameshifting events are toxic in Drosophila and mammalian neuron models

A mutation in the RNF170 gene causes autosomal dominant sensory ataxia

A polyalanine antibody for the diagnosis of oculopharyngeal muscular dystrophy and polyalanine-related diseases