Hoping to prevent blindness in premature babies, UT Southwestern Medical Center researchers have identified a protein that responds to oxygen levels in cells and tissues and also affects the developing eye.
Premature birth carries the risk of multiple disabilities, including retinopathy of prematurity, or ROP. ROP affects the retina, the part of the eye that detects light before it is transformed in the brain to an image. ROP babies have poor vision, and in many cases, go blind.
In a study to be published online and in the March issue of Investigative Ophthalmology and Visual Science, Dr. Joseph Garcia, assistant professor of internal medicine at UT Southwestern, and colleagues, have identified a protein in mice called HIF-2a, 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-2a gene. These mice could not make HIF-2a protein and had multiple visual defects associated with blindness. They were blind by 1 month of age.
"These mice initially behaved as if they were blind," Dr. Garcia said. "And indeed, when we examined them, we found they had severe retinopathy."
Physiological studes 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. Additional investigation of the mice lacking HIF-2a revealed extensive malformation of the blood vessels in their eyes, leading to ischemia, or reduced blood flow.
HIF-2a 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-2a create proteins that help the cell respond to noxious stimuli.
For HIF-2a, 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-2a 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-2a were inefficiently activated when HIF-2a was missing. In addition, the gene for erythropoietin (Epo), essential for blood and blood vessel development, was also affected when HIF-2a was missing.
For premature babies, it's not the lack of HIF-2a but the hyperactivity of the protein that may influence the development of ROP, Dr. Garcia said.
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-2a may be triggered inappropriately, Dr. Garcia suggested, and unnecessary blood vessels are created.
"Finding a way to shut down HIF-2a in ROP babies could prevent blindness when they come back down to regular oxygen," he said.
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 said.
Previous studies showed that antioxidant enzymes are regulated by HIF-2a. 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-2a, Dr. Garcia said.
"The ability of HIF-2a to control multiple essential genes may be required for proper eye development," he said. "Furthermore, there may be additional, and as of yet unidentified, HIF-2a controlled genes that also are important for eye development."
Other contributors to this research were Dr. Kan Ding, postdoctoral fellow in internal medicine at UT Southwestern, Dr. Marzia Scortegagna, former postdoctoral fellow in internal medicine, and Robyn Seaman, research assistant at the Retina Foundation of the Southwest.