INTERRUPTIONS IN THE TRIPLET REPEATS OF SCA1 AND FRAXA REDUCE THE PROPENSITY AND COMPLEXITY OF SLIPPED STRAND DNA (S-DNA) FORMATION

Models for the disease-associated expansion of trinucleotide repeats i nvolve the participation of alternative DNA structures during replicat ion, repair, or recombination. CAT or AGG interruptions within the (CA G)(n) or (CGG)(n) repeats of SCA1 or FRAXA, respectively, confer incre ased genetic stability to the repeats. In this study, we report the fo rmation of slipped strand structures (S-DNA) using genomic sequences c ontaining pure and interrupted SCA1 and FRAXA repeats having lengths a bove and below the genetic stability thresholds. S-DNA forms within th e repeats during annealing of complementary strands containing equal l engths of repeats. Increased lengths of pure repeats led to an increas ed propensity for S-DNA formation. CAT or AGG interruptions have both quantitative and qualitative effects upon S-DNA formation: they decrea se the total amount of slipped structures as well as limit the specifi c isomers formed. This demonstrates a unifying inhibitory effect of in terruptions in both (GAG),, and (CGG),, tracts. We also present transm ission stability data for SCA1 and FRAXA alleles spanning the threshol ds and compare these with the ability to form slipped structures. The effect of both the length and purity of the repeat tract on the propen sity of slipped structure formation correlates with their effect on ge netic instability and disease, suggesting that S-DNA structures may be models for mutagenic intermediates in instability.