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.

Translational frameshifting within the ATXN3 and HTT transcripts occurs both in vitro and in vivo. Frameshifted species containing polyAla tracts have been detected in the brain and lymphoblastoid cells of SCA3 patients as well as in transgenic Drosophila models and in various mammalian cells that have been transfected either with full-length or truncated ATXN3 sequence with expanded CAG repeats (Supplementary Table S3 and Figure S3 ) (26) (27) (28) . In the case of HD, frameshift products that contain polyAla or polySer tracts were observed in postmortem brains of HD patients and in transgenic HD mice (HD-N171-N82Q) (29) . Only polyAla-containing proteins were observed in other studies where two different cell lines, neuroblastoma N2a and epithelial HeLa cells, were transfected with the CAG repeat of Htt exon 1 (Supplementary Table S3 ) (30) . Immunodetection with specific C-terminal antibodies accompanied by Western blotting was mostly used to detect abnormal SCA3 proteins. In the case of HD, valuable information concerning the mechanism of frameshifting was also obtained with the use of MALDI-TOF mass spectrometry (MS) (30) . Frameshifting occurred with an equal stochastic probability at any CAG codon within the expanded CAG repeat, giving rise to proteins with various hybrid polyGln/polyAla stretches (30) . As shown by MALDI-TOF-MS analysis, frameshifted proteins formed in N2a cells expressing Htt51Q contained Gln:Ala ratios ranging from Gln1:Ala49 to Gln48:Ala2 in their repeat tract. This finding indicates that a (−1) frameshift can occur at any CAG codon within the CAG stretch, and its frequency is modulated by the number of CAG codons to be translated (30) .