Replication and Expansion of Trinucleotide DNA Repeats
Trinucleotide repeats attracted wide attention, since their expansion was found to cause a growing number of human neurological disorders. The mechanisms of repeats? expansion remain unknown, but there are several indications that it might be caused by anomalous replication of those repeats. Using two-dimensional electrophoretic analysis of replication intermediates, we showed that trinucleotide repeats indeed stall the replication fork progression in both bacterial and yeast systems, and the strength of replication inhibition increases with repeats? length. We also detected an underreplication of the lagging strand at the vicinity of replication stall sites, indicating that an unusual conformation of the repeat-containing lagging strand template might prevent the lagging strand synthesis and consequently attenuate progression of the whole replication fork. In accord with these expectations, mutations in the components of the replication fork involved in the coordination of the leading and lagging strand syntheses alleviate repeat-caused replication blockage. The same mutations lead to an increase in the frequency of trinucleotide repeats expansion in yeast. These data point to a direct connection between replication and expansion of trinucleotide repeats. We suggest that tightly coordinated syntheses of the leading and lagging DNA strands during ordinary replication prevent trinucleotide repeats from expanding. At the same time, a slight miscoordination of these processes due to either nonessential mutations in the replication apparatus or speedy replication during embryonic development might lead to an expansion.
Sergei M. Mirkin
Department of Molecular Genetics,