"Our findings show a possible reason that cells with a certain type of mutation (expansion of repetitive DNA) die prematurely," said Catherine Freudenreich, assistant professor of biology at the School of Arts and Sciences at Tufts. "We may be able to use this information to stop or slow the development of some of these degenerative diseases that affect thousands of people every year."
She and her colleagues – post-doctoral fellow Mayurika Lahiri and former Tufts undergraduate researchers Tanya Gustafson and Elizabeth Majors – published their findings, "Expanded CAG repeats activate the DNA damage checkpoint pathway" in the July 23 issue of the journal Molecular Cell.
Freudenreich, a molecular biologist, studies the unstable elements in the human genome, particularly the type of unstable element called "trinucleotide repeat sequences," whose expansion causes numerous human genetic diseases such as Huntington's disease (a degenerative neurological disease) and myotonic dystrophy (a type of muscular dystrophy). There are more than 15 repeat expansion diseases, all of which are of special interest because they are caused by a highly unusual DNA mutation, one in which a repetitive DNA sequence expands from a small number of copies to a larger number. For example, 20 copies of a DNA sequence (such as CAG) could expand to 70 or 100 copies to cause disease.
With a grant for more than $1 million from the National Institutes of Health, Freudenreich's team investigated whether the presence of expanded repeats in a cell is recognized by the cell as damage and, if so, whether the cell activates the surveillance system that facilitates repair (called the "DNA damage checkpoint pathway"). They found that the proteins that signal that DNA damage is present are activated when a cell contains an expanded CAG repeat sequence.
"We know this because when we used cells that were defective in the checkpoint proteins, there was a large increase in chromosome breakage at the expanded repeat. This means that cells with expanded repeats are particularly vulnerable and if their surveillance mechanism fails for any reason they will probably die. This is important because cell death is a hallmark of most of the CAG expansion diseases, leading to neurodegeneration in Huntington's disease and muscle degeneration in myotonic dystrophy."
Although some of the reasons for the degeneration are understood, activation of the checkpoint pathway is a possible contributor that wasn't recognized before.
In the study, the researchers also found that when some checkpoint proteins were absent, the CAG repeat became very instable, contracting at an increased frequency. This means the checkpoint status of a cell can influence repeat stability.
"This is interesting because it still isn't understood why the repeat doesn't change size in some cell types, but is very unstable in others," Freudenreich said.
For example, in Huntington's disease the repeat is prone to expansion during sperm development, which leads to inheritance of even longer repeats in the resulting children and a worsening of the disease in the next generation. The CAG repeat also expands further in the affected brain cells of patients, which could explain why those brain cells die first. So identification of factors that limit expansion, such as these checkpoint proteins, could be very useful in controlling the inheritance and severity of the repeat expansion diseases.
"Freudenreich's study is an example of the innovative genetic research being done at Tufts and it will have long-term impact on future understanding of the mechanisms responsible for genomic instability and their relationship with certain inherited diseases," said Susan Ernst, dean of the School of Arts and Sciences at Tufts and a professor of biology.
EDITORS NOTE: Here is a link to Freudenreich's web page at Tufts: http://ase.tufts.edu/BIOLOGY/faculty/bios/freudenreich/freudenreich.html
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Journal
Molecular Cell