Friedreich ataxia (FA) (GAA)n repeats may attenuate DNA replication fork progression in vivo, according to an article published in the journal Molecular and Cellular Biology.
The researchers cloned FA (GAA)n repeats of various lengths into a Saccharomyces cerevisiae plasmid and analyzed their effects on DNA replication through 2-dimensional electrophoresis of replication intermediates.
According to the results, premutation- and disease-size repeats hindered the DNA replication fork progression in vivo, whereas normal-size repeats did not affect the process of replication.
The observed threshold repeat length for replication stalling in yeast was approximately 40 repeats, which was comparable with the threshold length for repeat expansion in humans.
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Replication stalling was highly dependent on the orientation of the (GAA)n repeats. Delayed DNA replication was pronounced only when the repeat’s homopurine strand was positioned as the lagging strand template.
Moreover, the length polymorphism of the (GAA)n · (TTC)n repeat in expansions and contractions dramatically increased in the repeat’s orientation, which is responsible for the replication stalling.
“We believe that the mechanisms of replication blockage by the (GAA)n · (TTC)n repeat could be grounded in its structural features. This repeat can adopt a triple-helical H-DNA conformation in vitro, preventing efficient DNA polymerization through it,” Krasilnikova and Mirkin wrote.
“The apparent link between the replication stalling and length polymorphism of the repeat points to a new model for the repeat expansion.”
FA is caused by an expansion of the (GAA)n repeat within the first intron of the frataxin (X25) gene. The results obtained in this study represent the first direct proof of the effects of (GAA)n repeats on DNA replication in vivo.
Krasilnikova MM, Mirkin SM. Replication stalling at Friedreich’s ataxia (GAA)n repeats in vivo. Mol Cell Biol. Published online March 27, 2023. doi:10.1128/MCB.24.6.2286-2295.2004