Blinding Retinal Diseases

Meet Pearl, the new DNA-sequencer in PRRL (Pediatric Retinal Research Laboratory). She can sequence 1.2 Gigabases (1.2 billion) of DNA sequence by simultaneously sequencing 4.5-million pieces of DNA, each about 250 base-pairs long, in just several hours.

What we study

In the ERI laboratories directed by Dr. Mitton, the Control of Gene Expression Lab and the Pediatric Retinal Research Lab, we carry out research, and some therapeutic development, involving diseases that are responsible for most new cases of blindness in the United States each year and also some very rare inherited blinding genetic diseases. What most all of this research currently has in common is the retinal vasculature, the critical and essential blood supply of the neural retina.

Retinal Vascular Diseases cause most new blindness

The neural retina is a part of our central nervous system, but one outside the skull in our eyes. Photoreceptors that detect light must send their signals to other neurons to get the visual information to our brain for visual perception of the world around us. Some of these retinal neurons include bipolar cells, that are the go-between connection from photoreceptor cells to ganglion cells. Ganglion cells are neurons that have very long axons that extend from your eye through the optic nerve to make connections to neurons in the brain. Over a million such axons come together in a bundle and exit the back of the eye and are together what we call the optic nerve. Other retinal neurons called horizontal cells and amacrine cells, add horizontal connections to provide substantial processing of the visual information even before it exits the eye for the brain. All of these neurons, required for vision, can stop functioning and even die if the blood supply in the neural retina is compromised.

The disease that causes most new cases of blindness each year in developed nations is Diabetic Retinopathy. Over 11 million Americans have this condition, which has increased in incidence over 85%, (yes 85%) since 2002! A combination of our aging population and increasing numbers of people who are overweight and eating processed food diets have combined to cause this epidemic rise in this vision robbing condition. The small retinal blood vessels lose their ability to regulate and control a strong blood-retinal-barrier and some microvessels close off and degrade. This leaves regions of the retina without enough oxygen and glial cells (non-neural cells) of the retina produce higher amount of Vascular Endothelial Growth Factor (VEGF) than normal which causes more blood-retinal-barrier loss and also triggers growth of new but leaky blood vessels. Neovessels. This process is called neovascularization. Unlike the vessels formed during normal development, these neovessels are not physically robust. Leaking of blood, clotting, fibrosis and contraction can also cause retinal detachments and tearing. Currently, anti-VEGF drugs and steroids are injected into the eye to help control this disease process. Laser ablation is also used to remove parts of the retina that have lost their blood supply, to reduce the demand for oxygen. None of these treatments cure the condition or prevent the disease from continuing.

We currently carry out NEI/NIH funded research to determine how different forms of VEGF-A, which change in Diabetic Retinopathy and ROP (Retinopathy of Prematurity), activate and regulate the blood-retinal-barrier. Most of this work uses Primary Human Retinal Microvascular Endothelial Cells from organ donor eyes. These cells line the inside of blood vessels in the retina and they are the key keepers of the blood-retinal-barrier.

ROP – Retinopathy of Prematurity

ROP affects pre-term infants that are very immature at their time of early birth. Similar to diabetic retinopathy, ROP affected retinas experience a lack of oxygen and a rise of VEGF too. In the case of ROP, early birth halts normal development of the retinal blood vessels. Normally these grow out from the optic disc, where the optic nerve and retinal vessels enter the eye, towards the outer edge of the retina. In very early pre-term babies this process becomes halted leaving a blood supply only in the central retina. The peripheral retina becomes oxygen starved as the photoreceptor cells and other neurons mature and attempt to do their functions. Photoreceptor cells, bipolar cells and ganglion cells have very high oxygen demands. This causes loss of these essential cells over time and also can trigger neovascular growth, leaking, retinal tearing and blindness. Because of this, our research on the effects of VEGF-A changes on retinal endothelial cells also helps us to understand ROP.

Rare Blinding Inherited Retinal Diseases: FEVR, ND, RS

While some genetic conditions are rare disease, they still affect hundreds to a few thousand children around the United States and the world. In Norrie Disease (ND) and Familial Exudative VitreoRetinopathy, normal term babies have retinas that look like ROP. There is a deficiency in the developmental growth of blood vessels from the optic disc to the peripheral edge of the neural retina. These are genetic mutations, changes to the DNA sequence of key genes involved in regulating retinal vascular development. Genes with names like NDP (Norrin, the Norrie Disease Protein), FZD4 (Frizzled-4), TSPAN12 and LRP5 . In endothelial cells that line the interior of blood vessels, The proteins FZD4/TSPAN12/LRP5 form a receptor complex that binds Norrin as a Norrin receptor. Another non-vascular disease we investigate is Retinoschisis (RS). A non-functional RS1 gene product causes a physically weak neural retina because the RS protein is like a protein linking glue that helps retinal neurons and layers bind strongly to each other.

Using the Illumina Sequencer, named Pearl, we have developed a targeting sequencing strategy to sequence at least 8 genes in about 40 different persons at the same time. In doing so we have also reduced the cost of sequencing just two of these genes $4,500 to cost about $350 to sequence 8 genes. This is important to help research and discover mutations in Families affected by these pediatric retinal disease because American health insurance does not cover the cost of this kind of sequencing. The much smaller cost and higher speed of testing will make it possible to get sequencing research completed on more patients.