The G4C2 hexanucleotide repeat expansion, located in the first intron of the C9ORF72 gene, represents a major genetic hallmark of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. Several hypotheses have been proposed on how the transcribed repeat RNA leads to the development of amyotrophic lateral sclerosis and frontotemporal lobar degeneration. However, despite their importance, factors affecting the transcription of expanded-repeat RNA are not well known. As transcription is dependent on the DNA containing the expanded repeats, it is crucial to understand its structure. G-quadruplexes are known to affect expression on the level of DNA, therefore whether they form on the expanded-repeat DNA constitutes an important biological question. Using nuclear magnetic resonance and circular dichroism spectroscopy we show that DNA G4C2 with varying number of repeats d(G4C2)n form planar guanine quartets characteristic of G-quadruplexes. Additionally, we show DNA G-quadruplexes can form inter- and intra-molecularly in either parallel or anti-parallel orientation, based on d(G4C2) sequence length. This potential structural heterogeneity of longer disease-relevant repeats should therefore be taken into account when studying their role in disease pathogenesis.
COBISS.SI-ID: 27972647
Amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) are devastating neurodegenerative diseases that form two ends of a complex disease spectrum. Aggregation of RNA binding proteins is one of the hallmark pathologic features of ALS and FTDL and suggests perturbance of the RNA metabolism in their etiology. Recent identification of the disease-associated expansions of the intronic hexanucleotide repeat GGGGCC in the C9ORF72 gene further substantiates the case for RNA involvement. The expanded repeat, which has turned out to be the single most common genetic cause of ALS and FTLD, may enable the formation of complex DNA and RNA structures, changes in RNA transcription, and processing and formation of toxic RNA foci, which may sequester and inactivate RNA binding proteins. Additionally, the transcribed expanded repeat can undergo repeat-associated non-ATG-initiated translation resulting in accumulation of a series of dipeptide repeat proteins. Understanding the basis of the proposed mechanisms and shared pathways, as well as interactions with known key proteins such as TAR DNA-binding protein (TDP-43) are needed to clarify the pathology of ALS and/or FTLD, and make possible steps toward therapy development.
COBISS.SI-ID: 27699239