Summary: Copies for CAG and CTG triplets repeat themselves numerous times and disrupt normal gene sequences in some neurodegenerative diseases, a new study reports.
Source: Tufts University.
Tufts University researchers have discovered a possible explanation for the occurrence of a genetic error that causes over a dozen neuromuscular and neurodegenerative disorders, including Huntington’s disease, myotonic dystrophy and forms of spinocerebellar ataxia.
The error occurs as copies of three-letter sequences of DNA–known as CAG and CTG triplets–expand and repeat themselves hundreds or even thousands of times, disrupting normal gene sequences.
Genetic analyses in baker’s yeast now reveal that these large-scale expansions are controlled by genes that have been implicated in a process for repairing DNA breaks, leading the researchers to surmise that the expansions occur while breaks are being healed. Baker’s yeast, known as Saccharomyces cerevisiae, is frequently used in scientific research.
The findings are published in Nature Structural & Molecular Biology.
“We think these large-scale repeat expansions could occur in a single step,” said Sergei Mirkin, Ph.D., White Family Chair in Biology at Tufts School of Arts and Sciences and corresponding author on the paper. “The DNA replication machinery stalls within those repeats, which ultimately results in the formation of DNA breaks. To heal those breaks, cells seem to involve a special path of repair, called break-induced replication, which sacrifices the fidelity of DNA synthesis for the sake of a quick fix. As a result, large numbers of extra repeats can be added while the break is healed.”
Previous studies have identified certain mutations in DNA replication machinery that account for small-scale repeat expansions in which only a few extra repeats are added, but the cause of rapid accumulation of hundreds of repeats has remained unclear until now.
To detect and analyze large-scale expansions of CAG and CTG repeats, Mirkin and his colleagues developed an experimental system in yeast that observed simultaneous additions of more than 100 triplets, a first for such systems. This observation is similar to what happens in human disease.
Jane Kim, Ph.D., who was a research assistant professor in Mirkin’s lab when the research was conducted and who is the paper’s first author, said the model suggests answers to two questions that have puzzled researchers.
“Not only does this model explain how additions of hundreds of repeats can occur in human diseases, but it can also account for a bias towards expansions observed in human pedigrees,” said Kim, now an assistant professor at California State University San Marcos.
The authors note that the timing of expansion in human cases is still unclear and presents an important area of future investigation.
About this genetics research article
The study was performed in Mirkin’s lab by Kim with the help of two former undergraduate students, Samantha Harris, currently a student at Harvard Medical School, Teresa Dinter, currently a technical research assistant at Brigham & Women’s Hospital and graduate student Kartik Shah, currently a scientist at Amgen.
Funding: Research in Mirkin’s lab is supported by the National Institute of General Medical Sciences (NIGMS) of the National Institutes of Health (awards R01GM60987 and P01GM105473) and by a generous contribution from the White family. Kim was supported by the NIH Training in Education and Critical Research Skills postdoctoral program (K12GM074869) and by Tufts University. Harris received support from REU award NSF DBI 1263030. Dinter received support from Tufts Summer Scholars.
Source: Patrick Collins – Tufts University Image Source: NeuroscienceNews.com image is in the public domain. Original Research:Abstract for “The role of break-induced replication in large-scale expansions of (CAG)n/(CTG)n repeats” by Jane C Kim, Samantha T Harris, Teresa Dinter, Kartik A Shah and Sergei M Mirkin in Nature Structural & Molecular Biology. Published online December 5 2016 doi:10.1038/nsmb.3334
Cite This NeuroscienceNews.com Article
[cbtabs][cbtab title=”MLA”]Tufts University. “Researchers Uncover Possible Source of Genetic Errors Causing Multiple Diseases.” NeuroscienceNews. NeuroscienceNews, 5 December 2016. <https://neurosciencenews.com/genetics-diseases-errors-5685/>.[/cbtab][cbtab title=”APA”]Tufts University. (2016, December 5). Researchers Uncover Possible Source of Genetic Errors Causing Multiple Diseases. NeuroscienceNews. Retrieved December 5, 2016 from https://neurosciencenews.com/genetics-diseases-errors-5685/[/cbtab][cbtab title=”Chicago”]Tufts University. “Researchers Uncover Possible Source of Genetic Errors Causing Multiple Diseases.” https://neurosciencenews.com/genetics-diseases-errors-5685/ (accessed December 5, 2016).[/cbtab][/cbtabs]
The role of break-induced replication in large-scale expansions of (CAG)n/(CTG)n repeats
Expansions of (CAG)n/(CTG)n trinucleotide repeats are responsible for over a dozen neuromuscular and neurodegenerative disorders. Large-scale expansions are commonly observed in human pedigrees and may be explained by iterative small-scale events such as strand slippage during replication or repair DNA synthesis. Alternatively, a distinct mechanism may lead to a large-scale repeat expansion as a single step. To distinguish between these possibilities, we developed a novel experimental system specifically tuned to analyze large-scale expansions of (CAG)n/(CTG)n repeats in Saccharomyces cerevisiae. The median size of repeat expansions was ~60 triplets, although we also observed additions of more than 150 triplets. Genetic analysis revealed that Rad51, Rad52, Mre11, Pol32, Pif1, and Mus81 and/or Yen1 proteins are required for large-scale expansions, whereas proteins previously implicated in small-scale expansions are not involved. From these results, we propose a new model for large-scale expansions, which is based on the recovery of replication forks broken at (CAG)n/(CTG)n repeats via break-induced replication.
“The role of break-induced replication in large-scale expansions of (CAG)n/(CTG)n repeats” by Jane C Kim, Samantha T Harris, Teresa Dinter, Kartik A Shah and Sergei M Mirkin in Nature Structural & Molecular Biology. Publishe online December 5 2016 doi:10.1038/nsmb.3334