PolyQ-independent toxicity associated with novel translational products from CAG repeat expansions.

Paige Rudich,Todd Lamitina,Simon Watkins,David R Borchelt

doi:10.1371/journal.pone.0227464

Paige Rudich, Todd Lamitina + Show 2 more

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https://doi.org/10.1371/journal.pone.0227464

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Abstract

Expanded CAG nucleotide repeats are the underlying genetic cause of at least 14 incurable diseases, including Huntington’s disease (HD). The toxicity associated with many CAG repeat expansions is thought to be due to the translation of the CAG repeat to create a polyQ protein, which forms toxic oligomers and aggregates. However, recent studies show that HD CAG repeats undergo a non-canonical form of translation called Repeat-associated non-AUG dependent (RAN) translation. RAN translation of the CAG sense and CUG anti-sense RNAs produces six distinct repeat peptides: polyalanine (polyAla, from both CAG and CUG repeats), polyserine (polySer), polyleucine (polyLeu), polycysteine (polyCys), and polyglutamine (polyGln). The toxic potential of individual CAG-derived RAN polypeptides is not well understood. We developed pure C. elegans protein models for each CAG RAN polypeptide using codon-varied expression constructs that preserve RAN protein sequence but eliminate repetitive CAG/CUG RNA. While all RAN polypeptides formed aggregates, only polyLeu was consistently toxic across multiple cell types. In GABAergic neurons, which exhibit significant neurodegeneration in HD patients, codon-varied (Leu)38, but not (Gln)38, caused substantial neurodegeneration and motility defects. Our studies provide the first in vivo evaluation of CAG-derived RAN polypeptides in a multicellular model organism and suggest that polyQ-independent mechanisms, such as RAN-translated polyLeu peptides, may have a significant pathological role in CAG repeat expansion disorders.

Highlights

DNA repeat expansions are the genetic cause of >30 different diseases, most of which affect the nervous system and are currently incurable [1]
Diseases caused by exonic CAG repeat expansions are often referred to as ‘polyQ’ diseases, because the expanded CAG repeat is inframe with the encoded protein and leads to the translation of a series of glutamine (Q)
We investigated the cell biological and pathophysiological properties of the newly discovered CAG-derived Repeat-associated non-AUG dependent (RAN) polypeptides

Summary

Introduction

DNA repeat expansions are the genetic cause of >30 different diseases, most of which affect the nervous system and are currently incurable [1]. Despite the dogma that expanded polyQ proteins are the molecular cause of toxicity in exonic CAG repeat expansions, significant data suggests that polyQ oligomers or aggregates may not always be associated with toxicity. Patient genetic data shows that uninterrupted CAG repeat length determines the age of disease onset in HD through a mechanism separate from its glutamine-coding property [11, 12].

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