First-of-its-kind test on a microchip array will help diagnose retinitis pigmentosa

Sep 19 2005

Rapid genetic testing for eye disease is becoming a reality, thanks to a technology developed at the University of Michigan Kellogg Eye Center.

Scientists have created a first-of-its-kind test on a microchip array that will help physicians hone their diagnoses for patients with the blinding disease known as retinitis pigmentosa (RP). The screening technique has proven to be reliable and cost-effective.

In the September issue of Investigative Ophthalmology & Visual Science (IOVS), scientists at the U-M Department of Ophthalmology and Visual Sciences report on the arRP-I sequencing array, the first technology to screen simultaneously for mutations in multiple genes on a single platform.

This is a novel tool for scientists and physicians alike, says lead author and Kellogg scientist Radha Ayyagari, Ph.D. "For diseases that are associated with multiple genes, like RP, we now have a new and faster method for identifying the underlying genetic basis. This is also useful in analyzing complex patterns of inheritance and for understanding how causative genes might interact with each other."

RP is a group of diseases, affecting one in every 3,500 individuals, in which retinal degeneration leads to blindness or severe vision loss.

Among the outward signs and symptoms are loss of peripheral vision, night blindness, and abnormal results from an electroretinogram (ERG), a test that measures the electrical activity and function of the retina. A patient with the autosomal recessive form of the disease (arRP) has inherited one gene from each parent, neither of whom is affected by RP.

It is nearly impossible to identify which form of the disease a patient has through a clinical examination alone, notes John R. Heckenlively, M.D., a specialist in inherited eye disease who also participated in the study.

"Identifying the precise genetic mutation responsible for an individual's disease will allow us to provide a precise diagnosis, and this knowledge will also allow us to apply genetic therapies as they are developed," he says.

Some clues to treatments are beginning to emerge in animal models, and scientists expect future therapies to be very specific to the type of RP.

"Perhaps one patient will benefit from dramatically limiting exposure to sun or artificial light, and another will use certain vitamins or supplements to stop progression of the disease," says Heckenlively. "Obtaining a molecular diagnosis is going to be very important in helping to guide gene-based treatments for patients in the coming years," he concludes.

Ayyagari's study involved 70 individuals with a clinical diagnosis of arRP. Thirty-five had not been previously screened, and 35 others with known genetic mutations were screened to validate the results.

The arRP-I chip contained sequences, or genetic codes, of 11 genes that carry approximately 180 mutations associated with early-onset retinal degenerations. To date more than 30 genes have been identified for various forms of RP. Ayyagari notes that while the size of the chip currently limits the ability to array all known RP genes, larger platforms are likely to be available soon.

The arRP-I chips designed by the Kellogg research team produced 97.6 percent of the sequence analyzed with greater than 99 percent accuracy and reproducibility. The material cost of the arRP-I chip was 23 percent less that of current sequencing methods. That figure does not take into account the substantial savings in time and labor realized by analyzing multiple genes at once. These chips can detect both previously known and novel mutations.

Kellogg scientists and physicians expect that genetic technologies will grow dramatically in the next five years, particularly as additional space becomes available in the recently approved expansion to the Eye Center.

A proposed expansion of the U-M's eye disease genetic testing and counseling center will allow Ayyagari and Heckenlively to screen large numbers of interested patients, provide counseling and education on the implications of genetic testing, and advance the pace of research toward targeted genetic therapies for RP and other inherited eye diseases.