A Helicase Unwinds Hexanucleotide Repeat RNA G‑Quadruplexes and Facilitates Repeat-Associated Non-AUG Translation

The
expansion of a hexanucleotide repeat GGGGCC (G4C2) in the C9orf72 gene is the most common cause of amyotrophic lateral
sclerosis (ALS) and frontotemporal dementia (FTD). The G4C2 expansion
leads to repeat-associated non-AUG (RAN) translation and the production
of toxic dipeptide repeat (DPR) proteins, but the mechanisms of RAN
translation remain enigmatic. Here, we report that the RNA helicase
DHX36 is a robust positive regulator of C9orf72 RAN
translation. DHX36 has a high affinity for the G4C2 repeat RNA, preferentially
binds to the repeat RNA’s G-quadruplex conformation, and efficiently
unwinds the G4C2 G-quadruplex structures. Native DHX36 interacts with
the G4C2 repeat RNA and is essential for effective RAN translation
in the cell. In induced pluripotent stem cells and differentiated
motor neurons derived from C9orf72-linked ALS patients, reducing DHX36
significantly decreased the levels of endogenous DPR proteins. DHX36
is also aberrantly upregulated in tissues of C9orf72-linked ALS patients. These results indicate that DHX36 facilitates C9orf72 RAN translation by resolving repeat RNA G-quadruplex
structures and may be a potential target for therapeutic intervention.