In a study to be published online and in the March 2005 issue of Investigative Ophthalmology and Visual Science, Dr Joseph Garcia, assistant professor of internal medicine at UT Southwestern, and colleagues, has identified a protein in mice called HIF-2alpha, which is important in retina formation. Controlling this protein, which is also found in humans, may help doctors treat or even prevent ROP before it happens.
Dr Garcia examined the eyes of mice genetically engineered to lack the HIF-2alpha gene. These mice could not make HIF-2alpha protein and had multiple visual defects associated with blindness. They were blind by one month of age.
"These mice initially behaved as if they were blind," Dr Garcia says. "And indeed, when we examined them, we found they had severe retinopathy."
Physiological studies using light, conducted by Dr David Birch, collaborating scientist from the Retina Foundation of the Southwest and adjunct professor of ophthalmology at UT Southwestern, determined how their eyes were failing. Photoreceptors were poorly developed throughout the periphery of the retina and the retina wasn't processing visual information. Additional investigation of the mice lacking HIF-2alpha revealed extensive malformation of the blood vessels in their eyes, leading to ischemia, or reduced blood flow.
HIF-2alpha belongs to a class of proteins called transcription factors, which turn specific genes in the cell nucleus on in response to signals from the environment. Genes activated by HIF-2alpha create proteins that help the cell respond to noxious stimuli. For HIF-2alpha, the stimulus is hypoxia, a condition in which oxygen levels are too low. While appropriate hypoxia is required for proper development in many cases, inappropriate or prolonged hypoxia can damage cells and tissues. HIF-2alpha acts on genes that help deal with the low oxygen environment by stimulating the production of antioxidant molecules and promoting blood and blood vessel formation to deliver much-needed oxygen.
Biochemical tests revealed that antioxidant genes turned on by HIF-2alpha were inefficiently activated when HIF-2alpha was missing. In addition, the gene for erythropoietin (Epo), essential for blood and blood vessel development, was also affected when HIF-2alpha was missing.
For premature babies, it's not the lack of HIF-2alpha but the hyperactivity of the protein that may influence the development of ROP, Dr Garcia says.
Preemies often have difficulty breathing due to under-developed lungs and are housed in special incubators that contain a higher percentage of oxygen than found in normal air. Exposure to this artificial environment tricks their bodies into accepting the high concentration of oxygen as normal.
When they are healthy enough, preemies can leave their incubators and start breathing normally. Their bodies, however, accustomed to higher oxygen, perceive normal oxygen as too low, and their bodies set off the protective hypoxic response. In the retina, HIF-2alpha may be triggered inappropriately, Dr Garcia suggests, and unnecessary blood vessels are created.
"Finding a way to shut down HIF-2alpha in ROP babies could prevent blindness when they come back down to regular oxygen," he says.
Dr Yu-Guang He, assistant professor of ophthalmology at UT Southwestern, sees ROP babies in his clinical practice. "Of preemies weighing less than 2.75 pounds, 50 percent will develop ROP, and of these, 10 percent develop the most severe type, which leads to blindness. Dr Garcia's work reveals an important mechanism in the development of ROP, with potential to prevent this devastating ailment. It provides new hope for these babies," he says.
Previous studies showed that antioxidant enzymes are regulated by HIF-2alpha. Antioxidant enzymes are vitally important for protecting against oxygen radicals, the levels of which are high in ischemic tissues such as in the eyes of mice lacking HIF-2alpha, Dr Garcia says.
"The ability of HIF-2alpha to control multiple essential genes may be required for proper eye development," he says. "Furthermore, there may be additional, and as of yet unidentified, HIF-2alpha controlled genes that also are important for eye development."