Report on the Rare Diseases and Conditions Research Activities of the National Institutes of Health 1999
National Eye Institute (NEI)
Overview of Rare Diseases Research Activities
NEI was created on August 16, 1968, by Public Law 90-489 for the purpose of supporting and conducting research for improving the prevention, diagnosis, and treatment of diseases that affect the eye and vision. Eye diseases and blindness cost the Nation an estimated $38.4 billion per year. More than 12 million people in the United States suffer significant impairment of vision. Over the years, vision researchers supported by NEI have conducted many pioneering studies that have greatly advanced understanding of eye diseases, including those classified as rare, and provided eye care professionals with new tools and methods to prevent or cure many sight-threatening conditions. In June 1998, NEI published Vision Research-A National Plan: 1999-2003. This plan is the sixth in the series that dates back to the publication of Vision Research Program Planning in 1975. The development and publication of the plans address the visual health needs, including rare diseases of the eye and visual pathways, of the Nation.
Recent Scientific Advances in Rare Diseases Research
Retinitis pigmentosa (RP) is a group of blinding hereditary retinal degenerative diseases that are characterized by a progressive loss of vision due to degeneration of photoreceptor cells. The incidence of RP in the United States is about 1 in 3,500 births, and it affects more than 100,000 people. RP patients frequently report night blindness and loss of midperipheral vision during adolescence and are usually legally blind by the age of 40. Photoreceptor cells of the retina (the rods and cones) are responsible for the capture of light and the initiation of an electrical signal to the brain in the process of vision. The study of signaling in photoreceptor cells, termed the "visual phototransduction cascade," has provided a detailed molecular description of this pathway. One of the more promising areas to slow or stop the progression of this blinding eye disease and other inherited retinal degenerative diseases is gene therapy.
Dominantly inherited diseases, such as RP, are an especially challenging problem for development of gene-based therapies. Ideally, the defective gene has to be replaced to eliminate the protein product responsible for causing the photoreceptor degeneration and loss. For this to happen, the gene must be inactivated, repaired, or replaced. Recently, two different strategies have been developed by NEI-funded investigators to attack this problem. In the first case, triplex technology has been applied to the human rhodopsin gene that is involved in autosomal dominant RP (ADRP). The most common form of ADRP results from mutations in rhodopsin. Several so-called triplex-forming sites have been identified within the rhodopsin gene, and seven of these bind small DNA oligonucleotides. Such triplex-forming oligonucleotides (TFOs) bind to native DNA and offer a therapeutic potential by interrupting the expression of a disease-causing protein. In the second case, another group of investigators has developed a ribozyme-based approach to photoreceptor cell rescue. Ribozymes are small RNA molecules that cleave a complementary mRNA sequence, blocking production of its protein. Results are encouraging: two different and biologically active ribozymes slow the rate of retinal degeneration in a transgenic rat model of the disease.
Leber's Hereditary Optic Neuropathy
Leber's hereditary optic neuropathy (LHON) is a maternally inherited genetic disease that results in substantial loss of central vision in affected patients. Most genetic diseases are caused by mutations in chromosomal DNA inside the cell nucleus. LHON, however, is the first disease to be associated with mutations of the small amounts of DNA that reside inside the mitochondria (mtDNA). This DNA encodes for subunits of complex 1 of the respiratory chain, the key biochemical cascade that manufactures the cell's supply of the high-energy molecule ATP. The three most common mutations causing LHON have now been identified, providing a useful diagnostic test for LHON and new insight into the pathogenesis of the disease.
Retinopathy of prematurity (ROP) is a potentially blinding eye disorder of premature infants who typically weigh 2.75 pounds or less and have a gestational age of less than 31 weeks. ROP is associated with the growth of abnormal blood vessels that can lead to retinal scarring or detachment, resulting in vision loss. Each year, approximately 28,000 infants at risk for ROP are born in the United States, and about 14,000 to 16,000 are affected. In most cases, the disease regresses and leaves no permanent damage. However, 1,100 to 1,500 infants annually develop disease that is severe enough to require medical treatment.
The pathogenesis of ROP is not well understood. Lighting levels in hospital nurseries have been suggested as a factor in the development of ROP, but previous research was conflicting and inconclusive. To clarify this issue, NEI sponsored a multicenter clinical trial, The Effects of Light Reduction on Retinopathy of Prematurity (Light-ROP). In this study, a control group of infants was exposed to normal nursery lighting, and another group wore specialized goggles that dramatically reduced light exposure to the equivalent of wearing very dark sunglasses. Researchers found that the difference in the incidence of confirmed ROP between the two groups was not statistically significant-54% for the infants wearing the goggles and 58% for the babies who did not wear goggles. Thus, reducing nursery lighting by having infants wear the type of goggles used in this study would probably not affect their risk of developing ROP.
Another clinical trial supported by NEI, the Supplemental Therapeutic Oxygen for Prethreshold Retinopathy of Prematurity (STOP-ROP) multicenter trial, was undertaken to test the efficacy, safety, and costs of providing supplemental oxygen in moderately severe ROP (prethreshold ROP). Eligible infants were randomized to oxygen administration with continuous saturation monitoring at conventional levels (target pulse oximetry, 89-94% saturation) versus supplemental levels (target pulse oximetry, 96-99% saturation). A total of 649 infants were enrolled from 30 centers over 5 years. Of those, 325 received conventional oxygen supplementation and 324 received supplemental oxygen. The results of the STOP-ROP study show that use of supplemental oxygen did not cause additional progression of prethreshold ROP but also did not significantly reduce the number of infants requiring peripheral retinal ablative surgery. The study also showed that supplemental oxygen increased the risk of adverse pulmonary events, including pneumonia or exacerbations of chronic lung disease, and the need for oxygen, diuretics, and hospitalization at 3 months of corrected age. Although the relative risk/benefit of supplemental oxygen for each infant must be individually considered, clinicians need no longer be concerned that supplemental oxygen, as used in this study, will worsen active prethreshold ROP.
Early eye development of the vertebrate lens is controlled by specific genes that operate in a hierarchy of expression. The first of these genes to be identified was Pax-6. Mutations in Pax-6 are responsible for causing aniridia, a congenital malformation of the eye in which the iris is not completely formed and that results in cataract formation and congenital glaucoma. Subsequent to this discovery, Pax-6 expression was found in other embryonic tissue, including the tissues destined to form the nose, suggesting its more general involvement in craniofacial development. The significance of Pax-6 as a key developmental regulator has been substantiated in several experimental systems, most notably mouse and Drosophila, and its protein product is now characterized at the structural and functional levels. Researchers view Pax-6 as a "master gene" controlling the expression of downstream genes during development. Recent studies have described several of genes downstream of Pax-6 that may play a significant role in eye formation. As these genes and their products are characterized, the developmental hierarchy controlling ocular and more generally craniofacial development will be pieced together to form a picture of the developmental process and enhance understanding of the molecular basis of early eye developmental defects such as aniridia.
X-Linked Juvenile Retinoschisis
Juvenile retinoschisis is a degenerative X-linked recessive retinal disorder that gradually robs the patient of useful vision. The disease is characterized by the formation of cystlike structures on the retina. Although not usually diagnosed until children start school, visual acuity is often reduced to 20/200 by the time the child reaches puberty. The disease symptoms are similar to those of macular degeneration, and, in fact, most cases of juvenile macular degeneration are caused by retinoschisis. Peripheral vision is also affected by retinoschisis. Other complications associated with juvenile retinoschisis are strabismus, nystagmus, retinal detachment, and massive vitreous hemorrhages.
Researchers funded by NEI have localized the molecular defect of retinoschisis to the XLRS1 gene. These scientists have reported finding 23 different XLRS1 gene mutations in 28 patients affected by the disorder. Additionally, these researchers have demonstrated that molecular screening for retinoschisis is an effective diagnostic tool in at-risk males.
The hypothesis that lacrimal gland secretory cells actively provoke Sjögren's syndrome autoimmune responses has gained support from analyses of the intracellular traffic of histocompatibility molecules and autoantigens. The hypothesis is gaining further support from new experiments based on autologous mixed cell reactions that may recreate autoimmune responses under defined cell culture conditions. A new autoantigen (the cytoskeletal component -fodrin) implicated in Sjögren's syndrome autoimmunity has been identified. The autoantigen appears to have considerable specificity, because antibodies to it were found in the serum of 95% of patients with Sjögren's syndrome. No antibodies were found in healthy individuals or in patients with other autoimmune disorders such as SLE and rheumatoid arthritis. Thus, the autoantigen may have considerable diagnostic potential. Moreover, neonatal vaccination with -fodrin prevented development of the disease in mice, opening the possibility of new therapeutic approaches for Sjögren's syndrome.
Corneal dystrophies are a heterogeneous group of conditions that involve abnormal corneal development and result in defects in structure or clarity. Although relatively rare in the United States, the most common corneal dystrophy is keratoconus, which is characterized by a progressive thinning process of the cornea that may be accompanied by scarring. Keratoconus leads to progressive nearsightedness, astigmatism, and a cone-shaped cornea. Clinical care for keratoconus is time consuming for patients and doctors because of its chronic progression and the difficulty of achieving a stable contact lens fit for visual rehabilitation.
Recent biochemical investigation into the pathology of this disorder suggests that the loss of corneal stroma could result from either increased levels of proteases and other catabolic enzymes or decreased levels of proteinase inhibitors. Studies of corneal 1-proteinase inhibitor and 2-macroglobulin support the hypothesis that degrading processes may be aberrant in keratoconus. Both inhibitors are markedly diminished in the epithelium of keratoconus corneas.
Retinoblastoma (RB) is mainly a disease of childhood and is now one of the best understood of all solid tumors. Ninety percent of individuals who inherit specific mutations in the RB gene will develop the tumor. Each year, 300 to 400 new cases of RB are diagnosed in the United States. Unfortunately, the most prevalent treatment for RB at this time is surgical removal of the affected eye. Scientists supported by NEI have recently published results of experiments that were undertaken to determine the toxicity and dose-response of vitamin D3 for the treatment of retinoblastoma. An analogue of vitamin D3 has been shown to inhibit growth of RB tumors in transgenic mice. The results of this research showed that, in transgenic mice, it was possible to achieve an effective dose of the vitamin D3 analog that had no systemic toxicity.
Motor Neural-Ophthalmic Disorders
Blepharospasm, a motor neuro-ophthalmic disorder, is characterized by a forcible involuntary closure of the eyelids that may last as long as several minutes and usually occurs in people at midlife. Although the etiology of this condition is not well understood, researchers believe that the condition may be a form of PD in some people. NEI-supported researchers have recently reported the development of a rat model to study blepharospasm. The researchers depleted a small amount of striatal DA and slightly weakened the eyelid closure muscle (orbicularis oculi) in these animals. The results of this research showed that neither of these procedures produced the characteristic uncontrollable blinking spasm of blepharospasm by itself, but the procedures combined produced forceful blinking and spastic lid closure.
NEI-supported scientists have recently reported that injecting doxorubicin (DXN), an anticancer drug, directly into the eyelids may offer permanent relief from blepharospasm. DXN acts by reducing the number of muscle fibers in the obicularis oculi muscle by as much as 70%, offering alleviation from the spasms that are characteristic of this disease. However, to fully alleviate the spasms, patients often must undergo a series of painful DNX injections. Researchers supported by NEI are trying to find a way to increase the myotoxic effects of the DNX injection so that the patient can have permanent relief of spasms with a minimum of DXN treatments. Researchers injected bupivacaine and hyaluronidase 18 hours apart into the eyelids of rabbits. Two days later, the researchers injected the injured eyelids with DXN. The results of these experiments showed that the myotoxic effects of DXN were increased when the drug was injected 2 days after the animals' eyelids were injured with bupivacaine. The skin irritation that normally occurs as a result of DXN injection was also no worse in the dual-drug regimen than when DXN was administered alone. The dual-drug administration may help to reduce the number of DXN injections required to permanently stop the disabling eyelid spasms while minimizing the painful side effects that patients experience from the DXN injection.
Rare Diseases Research Initiatives
NEI will continue to fund high-quality investigator-initiated research on the prevention, etiology, pathology, and clinical intervention of rare diseases that cause visual impairment and disability.
Rare Diseases-Related Program Activities
The National Advisory Eye Council and NEI have established the following goals for rare diseases research in Vision Research: A National Plan 1999-2003:
- Identify novel causes of inherited retinal degenerations: further examine the cellular and molecular mechanisms whereby identified gene defects cause retinal degenerations.
- Further develop and critically evaluate therapies involving gene delivery, growth factors, and transplantation for the treatment of retinal disease.
- Improve understanding of ocular surface physiology.
