Delving into DNA Repeat Expansion
Sergei Mirkin, PhD, joined the Department of Biology in 2007. His research group explores the complex structure and function of DNA. Current research projects include replication and expansion of simple DNA repeats, transcription–replication interplay and its effect on the organization and stability of the genome, and unusual DNA structures, including DNA triplexes. Mirkin earned his MS in genetics from Moscow State University and his PhD in molecular biology from the Russian Academy of Sciences. He held faculty positions at the University of Illinois at Chicago for 16 years before coming to Tufts, where he holds the White Family Chair in Biology.
“My lab works on the mechanisms of a very unusual group of human hereditary disorders such as Huntington’s disease, myotonic dystrophy and Friedreich’s ataxia,” says Mirkin. “It’s an unusual group in that the pattern of inheritance is characterized by a phenomenon called ‘anticipation,’ which means if a person in a family has a mild form of the disease, there’s a fairly high likelihood that his or her children will have a more severe form of the disease, and their children in turn will have a very serious congenital form of the disease.” These diseases are known as repeat expansion diseases because they are associated with long, repetitive sequences of DNA. The length of the repetition, and the severity of the disease, tends to increase with each generation. For example, a person with 40 repetitions would be minimally affected, one with a few hundred would be moderately affected, and one with several thousand repeats would be severely affected.
“When my lab came to this problem in the mid-1990s, we decided to look at what was going on when the replication machinery goes through those repeats,” says Mirkin, “and it appeared that it gets stuck.” In order to study repeat expansions in a controllable manner, the research group then developed an experimental model using baker's yeast, a simple unicellular organism that has DNA very similar to that of humans. They designed a system that would easily show when a particular DNA repeat expanded to a critical length. They could then select for those yeast cultures and look at the effect of specific genes on DNA repeats. “What we found so far is that most of the genes affecting this process are genes involved in DNA replication,” says Mirkin. “That allowed us to propose a model of how these repeats could expand during DNA replication.” The model describes how the DNA replication machine gets tangled in repetitive DNA strands replicating them over and over again (similar to a sewing machine jam) until it eventually escapes and moves on down the DNA molecule. The model also points to specific proteins that either promote or inhibit repeat expansions.
More than 30 repeat expansion diseases are known. “The most common is fragile X syndrome, occurring in 1 in 1,000 newborns and resulting in fairly severe mental retardation when a particular DNA repeat exceeds 200 copies,” Mirkin explains. “When the repeat is fewer than 200 but greater than, say, 50 copies, this doesn’t result in mental retardation but in another syndrome called fragile X–associated tremor and ataxia syndrome which occurs at the age of 60 or so. This is very common in elderly people, and until the early 2000s, it was commonly misdiagnosed as Parkinson’s disease.” The other known repeat expansion diseases are more rare than fragile X syndrome; however, carriers of these diseases who do not yet show symptoms are more common.
“We have two long-term goals,” says Mirkin. “One is we want to do the same kind of system but now in cultured mammalian cells. We would be interested to establish contacts with people who actually have cells obtained from patients with these diseases. We know reasonably well the genetic control of this process in yeast, which isn’t necessarily the same in humans, but that we can check using mammalian and human cells.” Mirkin plans to test the candidate genes discovered from yeast screens in experiments with human cells using RNA interference technology.
“The second direction is a bit more translational,” says Mirkin. “You can’t help but wonder if you can somehow use this knowledge to treat these diseases.” Mirkin’s second long-term goal is to develop a high-throughput drug screening system to use with his experimental expansion systems. He would like to find a collaborator interested in working on drug screens. “So you see we have very ambitious plans,” says Mirkin. “It’s really an exciting time for the development of the biology department at Tufts.”
For more information, please go to http://ase.tufts.edu/biology/labs/mirkin.