Tufts University logoFor many years, researchers associated with human genetics have believed that expansions of GAA repeat resulting in this nucleotide triplet repeating hundreds or thousands of times. This was known to cause one of the most common hereditary neurological disorders known as Friedreich’s ataxia.

A team of researchers led by Sergei Mirkin now reveal to have created an experimental model that produces significant expansion of GAA repeats on a large scale during DNA replication. Experts working on the research were from the White Family Professor of Biology at Tufts’ School of Arts and Sciences.

Apparently, there is no treatment for Friedreich’s ataxia, which seems to damage the nervous system and could result in heart disease. The nativity of this disease was noted to have been confirmed through an investigation of genetic records of affected individuals and their families.

However, scientists have been unable to study the molecular progression that might cause the GAA sequence to greatly expand. Any significant development offering large scale expansions for experimental reasons could not be created due to the shortage of a model. Thus by creating this experimental model, Mirkin and his team were now able to examine GAA repeat expansions. Subsequently, they could identify cellular proteins that stopped normal replication and helped the elongated sequence.

“In essence we believe that the replication machinery occasionally gets tangled within a repetitive run, adding extra repeats while trying to escape. And the longer the repeat – the more likely the entanglement is. That is as if a car which entered a roundabout misses the right exit due the heavy traffic and has to make the whole extra circle before finally escaping,” says Mirkin.

Researchers began with common baker’s yeast because it permitted them to keep a check on the growth and genetic control of repeat expansions, which was nearly impossible in humans.

They inserted GAA repeats of different lengths resembling 50-to-150 triplet repeats into an intron of the specifically modified reporter gene. Later they found that extensive expansions of these repeats seemed to have taken place. These expansions blocked the union of RNA and, as a result, deactivated the gene.

By means of this approach, they observed substantial development of GAA repeats that ranged between 200 and 450 repeats. They discovered that the possibility of a repeat expansion increased as its growth extended, which appeared directly identical to that observed in the genetic record of humans with Friedreich’s ataxia.

Subsequently, Mirkin and his team conducted a genetic screen in order to identify yeast proteins which seem to influence repeat expansions. They established that the proteins inside the cell that help the smooth replication fork progression seemed to have reduced repeat expansions. However it appeared that the proteins responsible for the fork deviations, such as template switching and reversal raised repeat expansions.

This research has been published in the Molecular Cell.