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Clearing Up Blurry Vision: Scientists gaze toward causes of myopia

Carrie Lock

Next time you can't make out a distant highway sign, blame your parents. Scientists in the United Kingdom have found that myopia, or nearsightedness, is predominantly hereditary, and they're beginning to unravel the genetic mechanism that causes the vision problem.

photo

ODD EYES. Elongated, myopic eyeball (top) focuses light in front of the retina, making distant images appear blurry. Normal eye (bottom) focuses light directly on the retina.

E. Roell

Roughly a third of people in the United States suffer from myopia—they clearly see close objects, such as words in a book, but things in the distance appear blurry. The anatomic root of the problem is an elongation of the eye as it grows, causing incoming light to focus in front of the retina, instead of squarely on it, explains Christopher J. Hammond of St. Thomas' Hospital in London.

Using a noninvasive technique, Hammond measured the sizes of the eyeballs of 280 sets of fraternal adult twins and 226 sets of identical twins. By mathematically modeling the differences in the eye sizes, Hammond found that genes accounted for 89 percent of nearsightedness, farsightedness, and other refractive vision problems, he reports in the August American Journal of Human Genetics.

To investigate what regions of DNA in the general population might have a connection to myopia, Hammond scanned the entire genome of the fraternal twins and found four sections linked to the eye problem. The most strongly linked segment contains 44 genes, including one specified as PAX6, which is already well-known to vision researchers. From fruit flies to humans, this gene is fundamental to eye growth in nearly all species that scientists have examined.

"We didn't think PAX6 would be involved in myopia," says Hammond. "Usually mutations [in PAX6] cause major anomalies in the eye, like aniridia," a condition in which the eye lacks the iris.

A closer genetic examination of the PAX6 portion didn't identify a specific variation that causes myopia. Hammond suggests two explanations. One possibility is that that an independent gene near PAX6, but outside the segment that the researchers examined most closely, plays a critical role in myopia. Another explanation could be that a source of myopia lies in a nearby gene that affects the activity of PAX6, speculates Hammond.

"[Myopia] could be affected by several genes and the environment as well," says J. Fielding Hejtmancik of the National Eye Institute in Bethesda, Md.

There's evidence that one major environmental contributor is close-up work. Researchers have observed that an increased literacy rate in a population is often followed by a dramatic rise in the rate of myopia, says Karla S. Zadnik of Ohio State University's College of Optometry in Columbus.

Do parents who became myopic because of heavy reading create an environment that encourages their kids to follow suit, she asks. Or do kids inherit a genetic propensity for myopia, and reading triggers it?

If scientists can determine the genetic mechanisms for myopia, they might develop targeted pharmaceutical agents that can halt or slow the excessive eye growth that causes it.

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References:

Hammond, C.J., et al. 2004. A susceptibility locus for myopia in the normal population is linked to the PAX6 gene region on chromosome 11: A genomewide scan of dizygotic twins. American Journal of Human Genetics 75(August):294–304. Abstract available at http://dx.doi.org/10.1086/423148.

Sources:

Christopher J. Hammond
Twin Research and Genetic Epidemiology Unit
St. Thomas' Hospital
London SE1 7EH
United Kingdom

J. Fielding Hejtmancik
National Eye Institute
31 Center Drive, MSC 2510
Bethesda, MD 20892

Karla S. Zadnik
College of Optometry
Ohio State University
338 West Tenth Avenue
Columbus, OH 43210


From Science News, Volume 166, No. 2, July 10, 2004, p. 19.