Tight translation regulation of the canonical C9ORF72 through a multi-component system
C9ORF72 is involved in multiple neuronal functions and a major factor in Amyotrophic Lateral Sclerosis, a fatal neurodegenerative disease. Pathogenicity is implemented through an intronic repeat expansion in the genes mRNA leader that produces long, repetitive RNA and dipeptides when translated through a non-canonical mechanism. In contrast, despite the presence of ribosome-occupied, regulatory elements in the mRNA leader, nothing is known about C9ORF72 translation regulation under normal conditions. Surprisingly, when analyzing a series of mutants of the C9ORF72 mRNA leader, we found that translation of C9ORF72 is tightly regulated through a multi-layer system. First, non-canonical, multi-initiation upstream open reading frames (nc-uORFs) in all three frames, a start-stop element in frame 2, and a canonical uORF in frame 1work together to keep baseline C9ORF72 translation very low. Second, a strong secondary structure enhances these repressive elements, mainly the two nc-uORFs in frames 0 and 2. Finally, as initiation at the nc-uORFs reduces initiation at the downstream start-stop, the nc-uORFs effectively dampen the start-stops repressive function, forming a feedforward loop. We hypothesize that this buffered repressor system has likely evolved to ensure reliable, noise-insensitive expression of this critical regulator of neuronal function.