Spinocerebellar Ataxia Type 31 Is Associated with “Inserted” Penta-Nucleotide Repeats Containing (TGGAA)n

Abstract

Spinocerebellar ataxia type 31 (SCA31) is an adult-onset autosomal-dominant neurodegenerative disorder showing progressive cerebellar ataxia mainly affecting Purkinje cells. The SCA31 critical region was tracked down to a 900 kb interval in chromosome 16q22.1, where the disease shows a strong founder effect. By performing comprehensive Southern blot analysis and BAC- and fosmid-based sequencing, we isolated two genetic changes segregating with SCA31. One was a single-nucleotide change in an intron of the thymidine kinase 2 gene (TK2). However, this did not appear to affect splicing or expression patterns. The other was an insertion, from 2.5-3.8 kb long, consisting of complex penta-nucleotide repeats including a long (TGGAA)n stretch. In controls, shorter (1.5-2.0 kb) insertions lacking (TGGAA)n were found only rarely. The SCA31 repeat insertion's length inversely correlated with patient age of onset, and an expansion was documented in a single family showing anticipation. The repeat insertion was located in introns of TK2 and BEAN (brain expressed, associated with Nedd4) expressed in the brain and formed RNA foci in the nuclei of patients' Purkinje cells. An electrophoretic mobility-shift assay showed that essential splicing factors, serine/arginine-rich splicing factors SFRS1 and SFRS9, bind to (UGGAA)n in vitro. Because (TGGAA)n is a characteristic sequence of paracentromeric heterochromatin, we speculate that the insertion might have originated from heterochromatin. SCA31 is important because it exemplifies human diseases associated with "inserted" microsatellite repeats that can expand through transmission. Our finding suggests that the ectopic microsatellite repeat, when transcribed, might cause a disease involving the essential splicing factors.

CAG repeat RNA as an auxiliary toxic agent in polyglutamine disorders

“…11,[15][16][17] In HDL2, the CTG repeat occurs in variably spliced exons and is either in an untranslated region or encodes a polyleucine or polyalanine protein. 15 RNA-dependent pathogenesis of HDL2 includes nuclear retention of the JPH3 transcript with expanded CUG repeats in neurons of the frontal cortex and other brain regions, and MBNL1 co-localization with the CUG RNA foci, which results in cell toxicity.…”

“…The RNA gain-of-function mechanism initially described in DM1, 18,20,35 wherein repeat expansions are found to be toxic only at the RNA level, now also explains some aspects of pathogenesis in other non-coding expansion disorders, including FXTAS, SCA8, SCA31, HDL2 as well as SCA10 and DM2. 11,[13][14][15][16][17] SCA8 is a unique among these diseases because…”

“…The involvement of other repeat-harboring transcripts in neurodegeneration via an RNA gain-of-function mechanism has been described not only for trinucleotide repeat disorders, such as FXTAS, SCA8 and HDL2, [13][14][15] but also for DM2, SCA10 and SCA31. 9,11,16,17 In the RNA gain-of-function mechanism, the expanded repeat of a flawed RNA exerts its toxic function by sequestration and subsequent loss-of-function of RNA binding proteins, such as the muscleblind-like (MBNL) family. Resultant ribonucleoprotein complexes become trapped in the nucleus where they form microscopic bodies and become toxic.…”

“…Although unstable trinucleotide repeats are the most common repeats that cause neurological disorders, other repeats, such as tetra-and pentanucleotides, may also expand, resulting in DM2 as well as SCA10 and SCA31. [9][10][11] Intragenically located expanded trinucleotide repeats exert their pathogenic effect on transcript and/or protein levels. For repeat mutations present in non-protein coding sequences, an RNA gain-of-function mechanism, which explains how transcripts of expanded alleles exert toxicity, has been proposed (reviewed in refs.…”

Expansions, contractions, and fragility of the spinocerebellar ataxia type 10 pentanucleotide repeat in yeast

“…Because a DUE is involved in replication initiation in vivo, it was proposed that (ATTCT) n repeats expansions could result from multiple reinitiation of DNA replication within this sequence (9,10). Another example of an expandable repeat that is not expected to form alternative DNA structures is a sequence, (TGGAA) n , responsible for SCA31 (11). We were interested, therefore, in whether the same mechanisms govern expansions of the structureprone and structure-proof repeats.…”