Researchers from Vanderbilt University have found that genetic variation in mitochondrial DNA can affect a person's risk of developing age-related macular degeneration (AMD). The results of this first study that analyzes the relationship between AMD and changes in the mitochondrial genome are published in the open-access journal PLoS ONE.

AMD is the chief cause of blindness in Caucasians who are over the age of 50 and affects as many as 10 million people in the United States. It is a condition characterized by thinning and sometimes bleeding of the center of the inner lining of the eye (the macula area of the retina). This results in severe vision loss that makes activities such as reading, driving, watching television, and identifying faces extremely difficult or impossible.

Mitochondria - basic components of many types of cells - are known as the cell's "power plants" and generate most of the chemical energy used in the cell. Though most of the DNA in a cell is contained in the nucleus, the mitochondria also have a DNA. Lead author of the study Jeff Canter, M.D. M.P.H. (Center for Human Genetics Research) says that people are usually not aware that humans have two genomes. "We have the nuclear genome - the 'human genome' - that makes the cover of all the magazines, and then we also have this tiny genome in mitochondria in every cell."

Canter and colleagues set out to investigate a possible association between AMD and a specific variation in the mitochondrial genome that occurs in about 10% of Caucasians called mitochondrial haplogroup T. "We suspect that this variant will be one of a small group of important genetic variations that underlie AMD," said Canter. "By knowing this, we have a better chance of predicting accurately who will get the disease."

Canter remarked that the genetics of AMD has become a popular research area. Co-author Jonathan Haines, Ph.D. (Center for Human Genetics Research), an expert in AMD genetics and treatment, recently led a team that found a variation in the Complement Factor H (CFH) gene that accounted for up to 43% of AMD. In addition, researchers have linked AMD to variations in ApoE2 and a gene called LOC387715 on chromosome 10. Haines and colleagues reported that smoking and variations on the chromosome 10 gene raised the risk of AMD.

This current study not only analyzed the mitochondrial genome but also investigated variation in these nuclear genes in 280 AMD cases and 280 age-matched controls. The researchers were able to demonstrate that the variation in mitochondrial genome was independent of the known nuclear factors.

Haines said that, "We're at the stage where we can use genetic information to predict who is likely to develop AMD well before they actually develop it. Now we can conduct trials of preventive treatments something's that never been possible before."

Canter added that these findings could lead to personalized medicine. "I can see a day when physicians order genotyping on patients at a certain age to determine risk for AMD and put things in place - dietary changes, antioxidants, increased screening - that could prevent the disease." Canter emphasized that variation in the mitochondrial genome has been linked to a wide variety of diseases including.

Regarding the fact that mitochondrial variation has been associated with neurodegenerative diseases like Parkinson's and Alzheimer's as well as breast cancer and trauma survival, Canter concludes: "It's important to realize that there's another genome in the mitochondria, and even though there are not many genes there, they're important."

Mitochondrial DNA Polymorphism A4917G Is Independently Associated with Age-Related Macular Degeneration
Canter JA, Olson LM, Spencer K, Schnetz-Boutaud N, Anderson B, et al.
PLoS ONE (2008). 3(5): e2091.
doi:10.1371/journal.pone.0002091
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: Peter M Crosta