Bidirectional expression of CUG and CAG expansion transcripts and intranuclear polyglutamine inclusions in spinocerebellar ataxia type 8

Abstract

We previously reported that a (CTG)n expansion causes spinocerebellar ataxia type 8 (SCA8), a slowly progressive ataxia with reduced penetrance. We now report a transgenic mouse model in which the full-length human SCA8 mutation is transcribed using its endogenous promoter. (CTG)116 expansion, but not (CTG)11 control lines, develop a progressive neurological phenotype with in vivo imaging showing reduced cerebellar-cortical inhibition. 1C2-positive intranuclear inclusions in cerebellar Purkinje and brainstem neurons in SCA8 expansion mice and human SCA8 autopsy tissue result from translation of a polyglutamine protein, encoded on a previously unidentified antiparallel transcript (ataxin 8, ATXN8) spanning the repeat in the CAG direction. The neurological phenotype in SCA8 BAC expansion but not BAC control lines demonstrates the pathogenicity of the (CTG-CAG)n expansion. Moreover, the expression of noncoding (CUG)n expansion transcripts (ataxin 8 opposite strand, ATXN8OS) and the discovery of intranuclear polyglutamine inclusions suggests SCA8 pathogenesis involves toxic gain-of-function mechanisms at both the protein and RNA levels.

Targeted degradation of sense and antisense C9orf72 RNA foci as therapy for ALS and frontotemporal degeneration

“…Although in most repeat expansion diseases, the sense strand transcript or its translated protein products are thought to have primacy in disease pathogenesis, discovery of antisense strand transcripts is leading to increasing appreciation of their potential significance in disease pathogenesis (45)(46)(47)(48)(49)(50)(51)56). In FXTAS, antisense strand transcripts, as well as sense strand transcripts, are up-regulated, polyadenylated at the 3′ end, and capped at the 5′ end (47), although only the sense strand is known to generate nuclear inclusions (57).…”

“…Recognizing that in several other examples of expanded nucleotide repeat diseases, bidirectional transcription of the repeat has been identified (45)(46)(47)(48)(49)(50)(51), and that a recent study reported antisense transcripts in C9orf72 patient nervous systems (20), we examined C9orf72 patient fibroblasts for accumulation of RNAs transcribed in the antisense direction. Using FISH with LNA probes to the antisense strand of the hexanucleotide repeat GGCCCC, RNA foci were identified in all six fibroblasts from C9orf72 expansion patients tested and were not observed in fibroblasts from three nonaffected individuals (Fig.…”

CAG repeat RNA as an auxiliary toxic agent in polyglutamine disorders

“…The expression of ATXN8 antisense CAG-containing transcript is correlated with its translation into a nearly pure polyglutamine protein, ataxin 8, which is found in intranuclear inclusions in Purkinje cells and in brainstem neurons of both SCA8 patients and BAC-SCA8 mice. 61 The concomitant expression of an untranslated sense CTG mutation in the 3'UTR of the ATXN8OS gene leads to the generation of CUG repeat transcript that becomes toxic via a gain-of-function mechanism, similarly to what is observed in DM1. In fact, in mice expressing a BAC transgene with the SCA8 mutation, the accumulation of SCA8 transcript with expanded CUG repeats leads to alternative splicing changes of GABA-A ©2 0 1 1 L a n d e s B i o s c i e n c e .…”

“…14,61 Alternative splicing changes in a number of CNS transcripts including the amyloid precursor protein (APP), microtubule-associated protein tau (MAPT), glutamate receptor NMDAR1 and MBNL1 are also found in human and mouse DM1 tissues. 29,62 Although the underlying mechanism for the CNS splicing changes in DM1 and SCA8 is unknown, the results suggest that the two CUG mutant transcripts cause brain defects via an RNA gain-of-function mechanism.…”

C9ORF72: grabbing a tiger by the tail

“…In the latter disorders, the major pathogenic function of repeat-expanded RNA arises from the formation of nuclear RNA foci [42,49,66]. As in these latter conditions, in C9ORF72 disease toxic RNA foci are formed from the expanded sequences through formation of hairpin structures, and these might sequester other RNA transcripts and binding partners [43] (see Gendron), thereby reducing key elements of transcription and protein production.…”