Book of Abstracts: Albany 2007
June 19-23 2007
Conformational Properties of Trinucleotide Repeats associated with Human Neurodegenerative Diseases
Unusual conformational properties of microsatellite DNA regions are the probable reason of their expansions in genomes, which lead to serious genetic diseases in some cases. Using CD and absorption spectroscopy and polyacrylamide gel electrophoresis, we studied conformational properties of (CCG)n/(CGG)n trinucleotide repeats associated with the fragile X chromosome, of (CAG)n connected with Huntington's chorea, and of its permuted analogue (GAC)n. The (GAC) repeats give rise to various human skeletal displasias the type of which is determined by the repeat number.
Our studies demonstrate (1) that the (CCG)n single strands can adopt hairpin and two types of quadruplexes: an intercalated quadruplex at very acidic pH values, and an unusual quadruplex consisting of two parallel-oriented hairpins, which is formed close to physiological conditions. The complementary (CGG)n strands are believed to form G-quadruplex that was suggested to be responsible for the sequence expansion. We have, however, shown (2) that the (CGG)n repeats form G-quadruplex very unwillingly, while the fragments longer than 8 repeats do not form the quadruplex at all. Interestingly, the (AGG) triplets, interrupting (CGG)n runs with healthy individuals, rather stabilize the G-quadruplex. The quadruplex, however, is not the substance preventing expansion of the repeat. Our not yet published results indicate that the AGG triplets represent joints, which fold the rigid (CGG)8-11 blocks into regularly packed multiple hairpins and prevent them from sliding, which is associated with expansion.
The study of (GAC)n showed (3) that the repeat could adopt a wide range of conformations including parallel and anti-parallel, right-handed as well as left-handed homoduplexes. The stability of particular conformers is influenced by the number of (GAC) repeats. In contrast, (CAG)n invariably adopted only a single conformation -- a stable hairpin under all solution conditions.
References and Footnotes
Laboratory of CD Spectroscopy of Nucleic Acids