Genetics and the eye: How the Human Genome Project is changing ophthalmology

With the recently completed genomic map to guide their efforts, researchers are years ahead of where they would be without it.

The virtual completion of the Human Genome Project in April has made work considerably easier for ophthalmic researchers.

The ophthalmic genetic research community has wasted no time digging in to the seemingly bottomless trove of new biological information made available by the 99% completed genome.

“The latest discoveries are extremely relevant to ophthalmology. There have already been big leaps forward, and we can expect many more,” said Irene Maumenee, MD, a specialist in genetic eye disease and professor at Johns Hopkins University’s Wilmer Eye Institute.

Scientists estimate that roughly 2,000 genes could be associated with genetic eye diseases, out of a total of 6,000 or so disease-related genes, according to Dr. Maumenee. In total, about 30,000 human genes have been identified through the Human Genome Project (HGP).

Researchers who have been laboring for years to determine the genetic basis of glaucoma, retinal disease, myopia and anterior segment conditions, among other disorders, now have a genomic map to guide their efforts.

Yet the challenges that lie ahead — isolating the ocular disease-related genes, understanding their encoded proteins and determining exactly how they express themselves in people — are daunting, and results will not happen overnight.

“Now that the genome has been fully sequenced, this still does not mean that the genes have been correlated with human eye diseases. There lies a major project ahead,” Dr. Maumenee said.

Wealth of information

While they are hardly complaining, most researchers will attest that they now have so much information at their fingertips that trying to digest it can be overwhelming. As information is unveiled, understanding it requires new approaches and new perspectives, including technological, chemical, physical and mathematical expertise.

“There has been a tremendous amount of information created by the Human Genome Project, but it has also created a psychological problem. The more doors that are open in research, the more questions that are raised,” said Mansoor Sarfarazi, PhD, professor of human genetics and director of the Molecular Ophthalmic Genetics Lab at the University of Connecticut Health Center.

Dr. Sarfarazi, whose team of investigators has played an instrumental role in uncovering what is known about the genetic basis of glaucoma, said his day-to-day work has been made immeasurably easier by the HGP.

Having the genome mapped has shaved years off the process of figuring out which genes might possibly be associated with glaucoma, he said.

“I have been in this business for over 20 years, and I remember that in the last 10 to 15 years we had to go and basically clone an entire (genomic) region by ourselves just to identify the normal genes sitting within our regions of interest,” Dr. Sarfarazi said. “But now this job has been done for us. (Because of the HGP), we can immediately see how many genes are within our regions of interest. It has saved us the years and years it would take to clone every single normal gene before it can even be tested as a candidate.

“Because of the Human Genome Project, you know exactly where these genes are, where they reside, and what are their exact genomic structure. So you can go and make a series of primers and amplify the axons of each gene and run it through a group of 50 or 100 glaucoma patients,” he said.

Evolution, not revolution

Despite the wealth of information, glaucoma researchers agree that the HGP is not going to by itself revolutionize their research, in that no major breakthroughs are going to happen quickly as a result of the completed genome.

Now that the new information has dramatically increased researchers’ knowledge about a particular region of the genome, they are engaged in “a lot of intelligent guessing as to which gene to go for first,” Dr. Sarfarazi said.

“Before, we knew that there were, say, 20 genes in a given region of interest. Now in addition to those 20 genes, there are another 20 or 30 that are predicted (to be of interest),” he said.

“So how do we go about it? Do we look at the predicted genes or the known genes first? There could be hundreds of genes in your region that you have to look at before you find your targeted gene,” Dr. Sarfarazi said.

Glaucoma newsbreak

One recent major newsbreak in glaucoma research occurred in 2002, before the completion of the HGP.

This was the identification of the optineurin gene, believed to cause open-angle glaucoma, by Dr. Sarfarazi and his team of researchers at the University of Connecticut Health Center. This was the third gene — out of an estimated 30 believed to be involved in the disorder — to be conclusively identified.

Dr. Sarfarazi’s group determined that the optineurin gene is expressed in the trabecular meshwork, nonpigmented ciliary epithelium, retina and brain. The gene, which has come to be known as optineurin, short for optic neuropathy-inducing protein, is believed to play a neuroprotective role.

This discovery was considered important to glaucoma research because previously there had been just two glaucoma-related genes identified. CYP1B1, encoding cytochrome P4501B1 enzyme, is mutated in primary congenital glaucoma, and MYOC, encoding myocilin, is mutated in juvenile onset primary open-angle glaucoma.

Retinovitreal advances

Another area of ophthalmic research that has much to gain from genetic advances is retinovitreal research.

Like glaucoma, the majority of macular and retinal diseases are not single-gene disorders, according to Anand Swaroop, PhD, a professor of ophthalmology and human genetics at the University of Michigan.

“Most are multifactorial disorders, such as macular degeneration and diabetes-related complications of the eye. For example, some people may never get retinopathy even if they have had diabetes for a long time and vice versa,” Dr. Swaroop said.

Now, with the sequencing of the genome project done, Dr. Swaroop said retinal researchers can look at the completed genome and have a much better idea of where to look for the more promising candidate genes and the mutations that might lead to disease.

“We really don’t have to have large families for the majority of diseases that affect large portions of the public. Now we look for DNA markers and expressed gene sequences that we can use for analysis — even if we don’t have large families — based on other criteria,” he said.

Age-related macular degeneration is a disease that gets its fair share of interest from the biotech industry because of its prevalence; it is viewed as the most potentially lucrative area of study. Yet researchers have been hard at work for years trying to elucidate the molecular basis for some of the lesser-known macular diseases.

“A couple of industry people have begun to take an interest in our work on macular degeneration. Nobody, unfortunately, seems to care about relatively rare diseases like retinitis pigmentosa or Leber’s (congenital amaurosis); these are of no interest to the drug companies at this point,” Dr. Swaroop said. “Now that many genes for such diseases have been identified, we are hoping to design better strategies that can be used to treat people with different retinal diseases. Such approaches should attract the attention of the pharmaceutical industry.”

Juan Verdaguer T., MD, of Santiago, Chile, described some of the lesser-known diseases in which the responsible genes have been identified.

For example, a Chilean family with complete achromatopsia, an autosomal recessive disease, was presented in an isolated, rural area of central Chile. This area has a high inbreeding coefficient, according to Dr. Verdaguer.

Researchers from the University of Chile’s INTA determined that the disorder was caused by a novel mutation in the CNGB 3 gene, located on chromosome 8, consisting of two nucleotide changes.

Dr. Verdaguer said that other diseases have been linked to specific genes, including X-linked juvenile retinoschisis, familial exudative vitreoretinopathy, Best macular dystrophy, Stargardt macular dystrophy and fundus flavimaculatus.

Happening everywhere

These efforts are going on all over the world and have been since well before the HGP was completed. Now the efforts should be accelerated, according to Dr. Maumenee.

“It’s all happening now — there are efforts everywhere. In villages, communities, tribes in the Middle East, you can identify a specific mutation, then identify the carriers. What decisions people make based on this information is a different issue entirely,” Dr. Maumenee said.

Recessive diseases are considerably less prevalent in the United States than in small villages and islands in other parts of the world where there is more inbreeding, she noted.

“There is a tremendous deficit of identified recessive genes and mutations in people as compared to mice. Mice have been used in labs since the 1920s. We are dealing with inbred strains, and they suffer overwhelmingly from recessive diseases,” Dr. Maumenee said. “But understanding recessive genes is so critical because their biochemical pathways are very significant to our understanding how one gene interacts with another.”

For some diseases, such as retinoblastoma, the gene and mutation have been known for years, making it much easier to identify the children who are at risk for the disease.

“It has made a tremendous difference in management of retinoblastoma. It will happen for other diseases as well,” Dr. Maumenee said.

Like glaucoma, Leber’s congenital amaurosis, though rare, is an area that should prove valuable if the underlying genetic factors can be clarified.

“It only affects roughly 1 in every 50,000 people, but there are many major genes — maybe 20 or 30 — at the basis of this disease. These are genes that if you understand them, you will understand the significant steps in development of the retina and retinal function,” Dr. Maumenee said.

Genes and myopia

Genetic research is also making headway in the study of myopia, the most common eye disease. Terri L. Young, MD, associate professor of ophthalmology and pediatrics with the Children’s Hospital of Philadelphia and the University of Pennsylvania, has been involved in a long-term effort to define the genes involved in myopia and identify the mutations that cause this potentially blinding condition.

She said her laboratory has mapped three autosomal dominant high myopia loci and has refined the X-linked high myopia locus.

Currently, her research team is working to identify gene mutations in families that map to existing myopia loci, and to identify additional families with heritable patterns of high myopia. The researchers hope to gain a better understanding of the myopia gene mutations in order to start to work toward developing pharmacological interventions to prevent severe near-sightedness.

A global effort

Paul A. Sieving, MD, PhD, director of the National Eye Institute (NEI), is adamant that genetic ophthalmic research needs must incorporate an element of international collaboration.

“The genetic basis of eye disease and vision disorders spreads across the entirety of the world’s community,” Dr. Sieving said. “Unlocking the keys to genetic eye disease provides wonderful opportunities for international research collaborations, and ultimately these discoveries will help people in every country who have limited vision.”

There were 462 active grants for genetics and gene-based research issued by the NEI during fiscal year 2002, according to Dr. Sieving.

That represents $160 million in funding, compared to about $70 million in 1998.

At this rate, it could take less than 10 years for the new genetic knowledge to trickle down to the mainstream of patient care, Dr. Maumenee said.

“There is so much interest, and people have come to recognize the power of this. Everyone is getting on the bandwagon now,” she said. “It is also a genuine relief to be able to tell people that hoping for treatment is realistic and may be developed for many blinding diseases within the next 10 to 20 years.”

For Your Information:

• Irene Maumenee, MD, can be reached at the Maumenee Building, Suite 517, 600 North Wolfe St., Baltimore, MD 21287-9237; (410) 955-5214; fax: (410) 614-4363; e-mail: JHCHED@jhmi.edu.
• Mansoor Sarfarazi, PhD, can be reached at the Molecular Ophthalmic Genetics Laboratory, Surgical Research Center, Department of Surgery, University of Connecticut Health Center, 263 Farmington Ave., Farmington, CT 06030; (860) 679-3629; fax: (860) 679-7524; e-mail: mansoor@neuron.uchc.edu.
• Anand Swaroop, PhD, can be reached at 537, KEC, University of Michigan, 1000 Wall St., Ann Arbor, MI 48105; (743) 615-2246; fax: 734-647-0228; e-mail: swaroop@umich.edu.
• Juan Verdaguer T., MD, can be reached at Fundación Oftalmológica Los Andes, Las Hualtatas 5951, Santiago, Chile; (562) 370-46-24; fax: (562) 219-9116; e-mail: verdague@vtr.net.
• Terri L. Young, MD, can be reached at The Children’s Hospital of Philadelphia, Division of Ophthalmology, 34th Street and Civic Center Boulevard, Philadelphia, PA 19104-4399; (215) 590-7199; fax: (215) 590-3850; e-mail: youngt@email.chop.edu.
• Paul A. Sieving, MD, PhD, can be reached at the National Eye Institute, 31 Center Drive, Building 31, Room 6A03, Bethesda, MD 20892-3655; (301) 496-2234; fax: (301) 496-9970; Web site: www.nei.nih.gov.