Scientists at Tufts University have identified a molecular mechanism that could reverse the effects of Friedreich’s ataxia, a genetically induced neurodegenerative disease that leaves patients with difficulty walking, loss of limb sensation and impaired speech.
The researchers announced their impressive findings in the Proceedings of the National Academy of Sciences: that the genetic anomaly that causes the disease—a repetition error in the DNA sequence—could potentially be reversed.
Friedreich’s ataxia symptoms result from the destruction of nerve tissue in the spinal cord. This is caused by the presence of an expanded repetition of the genetic sequence ‘GAA’ in the FXNgene, which encodes for frataxin, a protein of the mitochondria. This is a rather rare genetic condition, with only one in 40,000 individuals being born with it.
Scientists believe that it may be possible to reverse this process by enhancing a natural process that contracts repetitive sequences in living tissue. Healthy people usually have 8 to 34 GAA repeats, carriers have 35 to 70 repeats, and individuals that exhibit disease symptoms have at least 70 though it is common for them to have hundreds of repeats in their genome.
An increased number of DNA repeats tends to correspond to an increase in a cells’ difficultly in “reading” or transcribing the FXN gene into the final protein required by the mitochondria, which in turn cease to function properly.
“The DNA repeats literally gum up the works,” said Sergei Mirkin, professor and chair of the Department of Biology at Tufts University. “They can also cause other mutations in the surrounding DNA, or make chromosomes extremely fragile, breaking into pieces, or rearranging themselves. If we can shrink the DNA repetition in tissues to levels found in healthy people, we might be able to stabilize the DNA and reduce the effects of disease.”
It is known that in patients’ tissues, the GAA repeats are unstable and continuously expand and contract. Understanding the mechanism of GAA repeat expansion and contraction — especially contraction — is important to developing a strategy for battling the currently incurable disease.
Numerous theories have been advanced as to how the DNA repeats contract, although the precise details of the mechanism remained largely unknown. The authors of this study developed an experimental model system in yeast (Saccharormyces cerevisiae) to quantitatively measure the effects of different interventions on contractions of DNA repeats.
They found that contractions usually occurred during the process of DNA replication referred to as “lagging strand synthesis.”
A normal DNA structure is comprised of a double stranded DNA helix, and it is separated into two individual strands for replication to occur. One strand called the “leading strand” (where the DNA replication enzymes follow the direction of the helicase), and the other strand is called the “lagging strand” (where DNA replication is done on the opposite strand in a piece-meal format). This lagging strand is so named because of its more complex synthesis process results in a limit on the rate at which the DNA can be copied.
The Tufts researchers found that the contraction of repeats depends on the ability of the DNA repeat to form an unusual triple-helical DNA structure along the lagging strand. A triple helix, in contrast to a double helix, consists of three DNA strands wrapped in a helical twist.
As the replication machinery moves across the lagging strand, it cannot easily bypass a triplex formed by the repeat. When the replication machinery jumps over this triple helix hurdle, the copied DNA strand ends up with fewer GAA repeats.
“While these results were uncovered in a yeast model, they do provide us with a clue into the mechanism of DNA repeat instability in Friedreich’s ataxia,” said Alexandra Khristich, graduate student in Mirkin’s lab and first author of the study. “I hope that our discovery would become a starting point for the potential development of therapeutic strategies that tip the balance toward DNA repeat contraction in patient tissues.”