The Biophysical Characterization of the Higher-order Structures of the C9orf72 repeat

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are diseases that form a spectrum of neurodegenerative disorders. Back in 2011, it was shown that expansion of (GGGGCC)n•(GGCCCC)n in the C9orf72 gene is the main cause of ALS/FTD. Being G-rich on one strand, and C-rich on the other strand, the C9orf72 repeats would appear to have a large propensity to form secondary structures. It has been shown that different repeats of the G-rich DNA and RNA, can form G-quadruplexes- also known as four-stranded DNA or RNA- in-vitro. To date, the exact mechanism leading to pathogenesis is still not known. However, the two major theories regarding pathogenesis are loss-of-function or gain-of-function mechanisms. Supporting the gain-of-function mechanism for pathogenesis, multiple proteins have been shown to bind the C9orf72 repeat. Interestingly, some of these proteins were shown to have structure-specific binding to the higher-order structures mentioned above. Structure-specific protein binding has previously been reported for other repeat expansion diseases as well. Our studies have focused on the biophysical characterization of the structure of the G-rich and C-rich strands of the C9orf72 repeat and their binding with biologically relevant proteins and small molecules by using CD spectroscopy, UV spectroscopy and gel electrophoresis. Non-canonical structures may be important intermediates in mutagenesis; therefore, it is important to assess the relative stability of these structures and their binding to biologically relevant molecules.

Keywords:

C9orf72 , G-quadruplexes , i-Motifs , Structure-specific protein binding