By Jawad Braick

 Macular degeneration is progressive loss of vision to the point of legal blindness due to damage to the macula, the part of the eye responsible for detailed, sharp vision.  The macula contains photoreceptor cells, which take in light, and retinal pigmented epithelium (RPE) cells, which nourish the photoreceptors cells.  Macular degeneration begins to occur when RPE cells die and are unable to nourish the photoreceptors and in turn, they die and light isn’t taken in at that area of the eye.  Macular degeneration characterized by progressive loss of fine acuity vision in the central portion of the visual.

 Macular degeneration comes in two forms: age- related macular degeneration, AMD, and stargardt’s disease.  AMD is as a result of dyeing photoreceptor and RPE cells that occur because of the natural aging process.  Stargardt’s disease is caused by dyeing photoreceptor and RPE cells due to a genetic mutation. There are two forms of stargardt’s disease.

 The most common form of stargardt’s disease is the recessive form caused by a mutation in ABCA4 gene.  The ABCA4 gene is located on the short arm of chromosome 1 in position 22  More than 500 mutations in this gene have been known to cause macular degeneration, most of which change single amino acids

 The damage to photoreceptor cells is caused by the build up of toxic substances in the cell  When light enters the retina and is converted to electrical signals, phototransduction a substance called N-retinyl-phosphatidyletholamine (NR-PE) is produced as bi-product. A protein called flippase is supposed to bring NR-PE out of the cell but if this protein is malfunctioning NR-PE builds up in the cell  NR-PE then combines with another molecule to produce lipofuscin, a toxic substance which causes damage to the cells and they will eventually die.

 The rarer form of Stargardt’s Disease is the dominant form caused by a mutation in the EVOVL4 gene  The EVOVL4 gene is located on the long arm of chromosome 6 in position 14. At least 3 mutations in this gene have been known to cause macular degeneration.

 Death of the photoreceptor and RPE cells can also be caused by interference with cell functions.  The EVOVL4 gene codes for the protein elongase. Its job is to assist in the formation of very long- chain fatty acids by adding carbon molecules.  A mutated EVOVL4 gene sends premature signals in the instructions to make elongase. As a result, the protein is unable to stay in endoplasmic reticulum and clumps together (aggregates). These aggregates can not make very long-chain fatty acids can interfere with other cell functions and cause cell death.

 A stem cell is a self renewing cell capable of differentiating into a specialized cell  Pluripotent stem cells-stem cells that can differentiate into most cell types  Multipotent stem cells- stem cells that differentiate into a few cell types  Induced Pluripotent stem cells (iPS)-specialized cells that were reverted back to pluripotent stem cells  Embryonic stem cells-pluripotent stem cells from an embryo  Adult stem cells- multipotent stem cells from various areas of an adult body

 The eye is one of the few areas of the body that is immune privileged. This means that the immune system isn’t very active, in the eye.  Immune privilege is characterized by the ability to tolerate antigens without eliciting an inflammatory immune response.  Some mechanisms that maintain immune privilege in the eye are physical barriers, the inhibitory ocular environment, and the ability to regulate systemic immune responses.

 Immune privilege allows stem cells to work to their full potential.  Usually implanted stem cells are rejected by the body and are recognized as antigens by the immune system, whether they are from a donor or the patient themselves. As a result, immune system attacks and destroys the stem cells.

 Scientists hope to inject embryonic stem cells into the macula then the stem cells would differentiate into RPE  Also scientists, want to inject embryonic stem cell-derived RPE cells into the macula rather have them differentiate after being injected

 Two phase 1 trials were conducted in which two patients were injected were embryonic stem cell-derived RPE cells into one eye. One had AMD and the other had Stargardt’s Disease  Both patients tolerated the treatment well and said they had improved vision in the treated eye

 The inserted stem cells could proliferate and form a tumor  Even though the eye is immune privileged the immune system could still attack and destroy the stem cells  The cells may not integrate and function due to damage in tissue