Parity-dependent hairpin configurations of repetitive DNA sequence promote slippage associated with DNA expansion

Significance

We found that TGGAA DNA repeats, which are involved in the neurological disease spinocerebellar ataxia 31, are capable of assuming two different hairpin structures depending on repeat number parity. We determined the interconversion kinetics by single-molecule spectroscopy and probed the interconversion mechanism through elucidation of the TGGAA repeat stem structure. Our results suggest that the two hairpin structures interconvert through motion slippage, and the process can be explained by the overall stem stability and local destabilization of the kinked GGA motif. Divalent cations and stem length affected the equilibrium and kinetics of slippage. Our findings suggest a mechanism by which a binary dynamic property of DNA repeats may affect repeat expansion and may be applicable to other repetitive DNA systems.

Abstract

Repetitive DNA sequences are ubiquitous in life, and changes in the number of repeats often have various physiological and pathological implications. DNA repeats are capable of interchanging between different noncanonical and canonical conformations in a dynamic fashion, causing configurational slippage that often leads to repeat expansion associated with neurological diseases. In this report, we used single-molecule spectroscopy together with biophysical analyses to demonstrate the parity-dependent hairpin structural polymorphism of TGGAA repeat DNA. We found that the DNA adopted two configurations depending on the repeat number parity (even or odd). Transitions between these two configurations were also observed for longer repeats. In addition, the ability to modulate this transition was found to be enhanced by divalent ions. Based on the atomic structure, we propose a local seeding model where the kinked GGA motifs in the stem region of TGGAA repeat DNA act as hot spots to facilitate the transition between the two configurations, which may give rise to disease-associated repeat expansion.