RESUMEN
Spinocerebellar ataxia type 8 (SCA8) is caused by a bidirectionally transcribed CTG·CAG expansion that results in the in vivo accumulation of CUG RNA foci, an ATG-initiated polyGln and a polyAla protein expressed by repeat-associated non-ATG (RAN) translation. Although RAN proteins have been reported in a growing number of diseases, the mechanisms and role of RAN translation in disease are poorly understood. We report a novel toxic SCA8 polySer protein which accumulates in white matter (WM) regions as aggregates that increase with age and disease severity. WM regions with polySer aggregates show demyelination and axonal degeneration in SCA8 human and mouse brains. Additionally, knockdown of the eukaryotic translation initiation factor eIF3F in cells reduces steady-state levels of SCA8 polySer and other RAN proteins. Taken together, these data show polySer and WM abnormalities contribute to SCA8 and identify eIF3F as a novel modulator of RAN protein accumulation.
Asunto(s)
Envejecimiento/metabolismo , Factor 3 de Iniciación Eucariótica/metabolismo , Proteínas del Tejido Nervioso/metabolismo , Degeneraciones Espinocerebelosas/metabolismo , Sustancia Blanca/metabolismo , Envejecimiento/genética , Envejecimiento/patología , Animales , Factor 3 de Iniciación Eucariótica/genética , Células HeLa , Humanos , Ratones , Ratones Transgénicos , Proteínas del Tejido Nervioso/genética , Degeneraciones Espinocerebelosas/genética , Degeneraciones Espinocerebelosas/patología , Sustancia Blanca/patologíaRESUMEN
Spinocerebellar ataxia type 8 (SCA8), a dominantly inherited neurodegenerative disorder caused by a CTGâ¢CAG expansion, is unusual because most individuals that carry the mutation do not develop ataxia. To understand the variable penetrance of SCA8, we studied the molecular differences between highly penetrant families and more common sporadic cases (82%) using a large cohort of SCA8 families (n = 77). We show that repeat expansion mutations from individuals with multiple affected family members have CCGâ¢CGG interruptions at a higher frequency than sporadic SCA8 cases and that the number of CCGâ¢CGG interruptions correlates with age at onset. At the molecular level, CCGâ¢CGG interruptions increase RNA hairpin stability, and in cell culture experiments, increase p-eIF2α and polyAla and polySer RAN protein levels. Additionally, CCGâ¢CGG interruptions, which encode arginine interruptions in the polyGln frame, increase toxicity of the resulting proteins. In summary, SCA8 CCGâ¢CGG interruptions increase polyAla and polySer RAN protein levels, polyGln protein toxicity, and disease penetrance and provide novel insight into the molecular differences between SCA8 families with high vs. low disease penetrance.
Asunto(s)
Degeneraciones Espinocerebelosas , Expansión de Repetición de Trinucleótido , Ataxia , Humanos , Proteínas del Tejido Nervioso/genética , Penetrancia , Proteínas , ARN Largo no Codificante/genética , Degeneraciones Espinocerebelosas/genéticaRESUMEN
Since the discovery of repeat-associated non-ATG (RAN) translation, and more recently its association with amyotrophic lateral sclerosis/frontotemporal dementia, there has been an intense focus to understand how this process works and the downstream effects of these novel proteins. RAN translation across several different types of repeat expansions mutations (CAG, CTG, CCG, GGGGCC, GGCCCC) results in the production of proteins in all three reading frames without an ATG initiation codon. The combination of bidirectional transcription and RAN translation has been shown to result in the accumulation of up to six mutant expansion proteins in a growing number of diseases. This process is complex mechanistically and also complex from the perspective of the downstream consequences in disease. Here we review recent developments in RAN translation and their implications on our basic understanding of neurodegenerative disease and gene expression.