The retina (Thin layer of neural cells covering the back two-thirds of the eyeball.) contains two types of photoreceptors: Rods and cones. Approximately 125 million rod receptors, located mostly at the periphery of the retina, more sensitive to light than cones, but not able to differentiate among colors, and not able to perceive shades of gray, black and white. Rod receptors function best in dim lightening. Approximately six million cone photoreceptors, located mainly in the macula (Small and highly sensitive part of the retina, responsible for detailed central vision, such as reading.) provide sharp vision, color and contrast discrimination center of the retina.
Achromatopsia (‘Day Blindness’) is a congenital (hereditary) eye defect, severe color blindness, poor detail vision, and photophobia, sensitivity to light. This hereditary condition, occurs one in about 30,000 people in the United States, and more likely to occur in men than women. Both parents must have the gene that causes the child to inherit both faulty genes. The disease does not affect the parents of the child, since they each only have one copy of the defective gene. There are two variations of achromatopsia, distinguished by severity of symptoms: Complete rod monochromatism (complete Achromatopsia), and incomplete rod monochromatism (Incomplete Achromatopsia) or blue cone monochromatism (type depends on which cones are affected). Complete rod monochromatism is rare, a total lack of cone function. Symptoms of this disorder include a sensitivity to light, even in normal lit rooms. Achromatopsia patients only capable perceive black, white and shades of gray. Also, symptoms include poor visual acuity (eye’s ability to distinguish shapes and objects at certain distances) and nystagmus, involuntary movement of the eyes. People with incomplete achromatopsia generally have a small amount of color vision with slightly improved vision compared to people with complete achromatopsia. Blue cone monochromatism disorder (Blue color blindness) in which the blue cones are normal, but red cones and green cones do not contribute to color vision, a rare disease, affects less 200,000 people in the United States population.
Diagnosis of Achromatopsia difficult because the back of the eye appear normal when examined. The symptoms and electroretinogram (May show abnormal photopic or daylight signal while maintaining normal night vision signal), provides the basis for diagnosis of this hereditary eye disorder. Also, ability to drive is impaired, patients have problems with traffic lights, brake lights, and reading certain colored signs. There are no medical or surgical treatments for achromatopsia. However, optical aids, sunglasses, and lowered illumination may provide some relief. Also, tinted contact lenses provide further relief see more clearly.
On May 22, 2007, paper published edition online of Nature Medicine, researchers at University of Florida and the Jackson Laboratory in Maine, restored sight in mice with achromatopsia, a form of hereditary blindness, diagnosis in humans. According to the research paper, researcher scientists Bo Chang describes the use of harmless virus (adeno-associated virus), deliver (injected into the subretinal space of the mouse eyes) corrective genes to mice for the treatment and cure of a genetic impairment, deprives them of vision. These mice found three years earlier with achromatopsia, while screening main strains of mice vision problems. The ability to deliver specific gene to cone cells, has implications for the treatment of all blinding diseases, not just inherited ones. Professor William Hauswirth of ophthalmic molecular genetics and member of the University of Florida Genetics Institute said: “Even in two very common types of blindness, age – related macular degeneration and diabetic retinopathy, if you can target cones you might be able to rescue that vision. The immediate impact could affect a few thousand patients, but there is the possibility of a longer — term impact that could affect millions of patients with other forms of blindness.” Within two months of the mice gene therapy injection, researchers measured the electrical activity in their retinas. They found that 19 of the 21 treated mice eyes responded positively to therapy: 17 of those 19 had electrical readings from their retinas on par with those taken in normal mice.
Retinitis pigmentosa (Inherited eye disease that affects the retina) affects about one in 3,000 Americans. As the disease progresses and more rod cells degenerate, patients lose their peripheral vision. Treating cones by gene therapy could provide a new treatment option.
The process by which to comprehend or measure gene therapy results, mice surrounded by four computer monitors simulated the appearance of being inside a moving drum with vertical stripes on the walls. Scientists observed mice could see the stripes because sighted animals naturally move their heads, in the same direction as the moving stripes. During the test, the stripes were made ever – narrower (similar to how the letters get smaller towards the bottom of an eye chart) — researchers assessed favorable improvement, after gene therapy, mice’s visual abilities. National Institutes of Health, the Foundation for Fighting Blindness, the Juvenile Diabetes Research Foundation, the Macula Vision Research Foundation, the Lions Central New York, Research to Prevent Blindness, Fight for Sight and NASA contributed grants for cone — target gene therapy research.
Professor Hauswirth and University of Florida have an interest in a biotechnology company, may pursue marketing the research gene technology.
In early May 2007, a trial study led by Robin Ali, a professor of human molecular genetics at University College London, developed after 15 years of test tube and live animal study, gene therapy treatment cure Leber’s Congenital Amaurocis, a major cause of congenital blindness. A man named Robert Johnson under went this procedure by which a needle was inserted in Mr. Johnson’s eye, into his retina, to replace the faulty gene (RPE65 gene) that had been blinding him for years (rare inherited eye disease), conducted at Moorfield Eye Hospital in London, England. United Kingdom doctors will have to wait months to determine if the procedure worked. Shortly after the procedure, a second patient received the same treatment and ten more scheduled to follow suit.
