Rethinking Unconventional Translation in Neurodegeneration

Abstract

Eukaryotic translation is tightly regulated to ensure that protein production occurs at the right time and place. Recent studies on abnormal repeat proteins, especially in age-dependent neurodegenerative diseases caused by nucleotide repeat expansion, have highlighted or identified two forms of unconventional translation initiation: usage of AUG-like sites (near cognates) or repeat-associated non-AUG (RAN) translation. We discuss how repeat proteins may differ due to not just unconventional initiation, but also ribosomal frameshifting and/or imperfect repeat DNA replication, expansion, and repair, and we highlight how research on translation of repeats may uncover insights into the biology of translation and its contribution to disease.

Keywords: ALS; C9ORF72; dipeptide repeat proteins; frontotemporal dementia; near-cognate start codon; repeat expansion; translation; upstream open reading frame.

Figure 1. Mechanisms of translation initiation

(A) During “conventional” translation, the ternary complex (TC) composed of eIF2, GTP, and Met-tRNA is joined by the 40S ribosomal subunit to form a 43S pre-initiation complex (PIC). The PIC interacts with eIF4F (composed of three factors: the cap binding protein eIF4E, the scaffold protein eIF4G, and the RNA helicase eIF4A) of the mRNA 5′ cap. The 40S then scans until if stalls at usually the first AUG, which often resides within a Kozak sequence. The 60S is then recruited and the full ribosome commences polypeptide synthesis. (B) One type of uORF translation is illustrated, in which a non-canonical CUG in the uORF is the initiating codon and translation of the uORF inhibits translation of the downstream ORF. (C) RAN translation proposes that the 40S and 60S ribosomal subunits directly bind the expanded repeats in mRNA through an unknown mechanism.

Figure 2. Different translation mechanisms that may operate in C9ORF72 ALS/FTD

(A) Genomic organization of the C9ORF72 gene (variant 3) containing expanded G₄C₂ repeats. (B) G₄C₂-contianing RNA that could be spliced intron or intron 1-retained mRNA. (C) C₄G₂-containing RNA whose 5′ and 3′ ends are unknown. (D) It is speculated that poly(GA) may be synthesized through CUG-initiated translation. (E) Different DPR proteins are thought to be produced through RAN translation. (F) The proposed mechanisms that may lead to the synthesis of chimeric DPR proteins from both sense and antisense repeat transcripts. (G) DPR proteins may be also synthesized through conventional translation from expanded C₄G₂ repeats-containing RNA. Three AUG start codons embedded in Kozak sequences are in-frame with poly(PG) coding sequence. All three DPR proteins, poly(PA), poly(PG) and poly(PR), may contain a N-terminus synthesized from nucleotide sequences 5′ to C₄G₂ repeats.