RAN translation and frameshifting as translational challenges at simple repeats of human neurodegenerative disorders

Wojciechowska, Marzena; Olejniczak, Marta; Galka-Marciniak, Paulina; Jazurek, Magdalena; Krzyzosiak, Wlodzimierz J.

The RNA structures formed by the repeats are recognized as potential triggers when trying to decode the mechanism of RAN translation initiation. Their formation and stability affect the abundance of RAN-translated products. For longer repeats that have a more stable RNA structure, increased RAN-translated protein levels were detected (14, 51) . It is known that repeats that undergo RAN translation can form hairpin or G-quadruplex structures, which have different molecular architecture and stability ( Figure 1 and Supplementary Figure S2 ). Our previous in vitro analysis of RNA structures formed by different triplet motifs revealed the following order of hairpin stability: CGG > CAG > CUG > CCG (52) . The crystallographic structures of CNG repeat duplexes (N = A,U,G,C) showed distortions of their double-stranded helices caused by noncanonical N-N base pairs, implying both similarities and differences in protein binding by repeat hairpins (53) (54) (55) . In addition, we have demonstrated that in certain repeat containing transcripts, e.g. HTT/HD, AR/SBMA and CACNA1/SCA6, the repeat flanking sequences may contribute to hairpin stability (56) . Additionally, the presence of interruptions in repeat tracts strongly influences the stability of the hairpin structure they adopt (57) . Could such properties determine RAN translation process intensity in different disease models? The available data show differences in RAN translation efficiency between the C9ORF72 antisense and sense transcripts, in which C 4 G 2 repeats adopt hairpin structures, whereas G 4 C 2 repeats may also fold into more stable G-quadruplex structures (47) . The intramolecular G-quadruplexes are adopted by RNA motifs having the following consensuses sequence G 3+ N 1−7 G 3+ N 1−7 G 3+ N 1−7 G 3+ (N represent any nucleotide, 3+ means 3 and more and 1−7 means number ranging from 1 to 7). The stability of these structures depends on several factors including the presence of potassium ions, number of consecutive guanines and separating nucleotides between G-tracts (58, 59) . Thus RAN translation from the latter transcript adopting G-quadruplex structure seems to be more robust (15, 17, 22) . For this same reason, normal CGG repeats of sense FMR1 transcripts might undergo the noncanonical translation that typically occurs on expanded repeats. Interestingly, the presence of a G-quadruplex in the 5 UTRs was demonstrated to impair cap-dependent translation, promoting cap-independent translation, as G-quadruplexes were revealed to be structural components of some human IRES sequences (60) .