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New Approaches to cure deafness: gene therapy and stem cell therapy
Hearing loss is the most common congenital disability of newborns. Over 1 in 500 babies are born with sensorineural hearing loss (SNHL or nerve deafness.) We estimate that 6 million Americans suffer from the most advanced stages of SNHL: severe to profound deafness. Although there is no cure for SNHL hearing aids assist many people and for severe to profound loss, cochlear implants have proven to be highly effective, safe and reliable.
At the University of Miami Ear Institute, our 12 doctors of audiology are able to identify two sub-types of sensorineural hearing loss; sensory and neural. Sensory loss is due to absent or damaged hair cells in the inner ear. Neural loss is due to damaged nerve cells that conduct signals from hair cells to the brain. The vast majority of deafness is caused by sensory hearing loss (loss of hair cells). Hair cell loss can be caused by many factors including noise, head injury, medications, aging or genetic defects. Aging of the ear may begin at puberty but usually only progresses to become a problem for seniors. Hearing loss progresses as more and more of the approximately 150,000 hair cells in each ear degenerate. When hair cells die they cannot regenerate and the loss is permanent. . Nonetheless, recent advances in hearing science give hope for the future. These include gene therapy and stem cell therapy.
Genes are carried on chromosomes of every cell in the body. They work to control the function of organs including the cochlear organ and also to build organs. When a gene does not work properly, malfunctions occur in specific areas of the body.
Gene therapy works in two ways: 1) a properly functioning gene is put into a human cell to replace an abnormal gene and; 2) a developmental gene, usually only active in embryos, builds new cells in the diseased organ. Over 4,000 people have been treated with gene therapy since 1990.
The Miami Institute for Human Genomics (MIHG) at the University of Miami, Miller School of Medicine has been responsible for decoding of many genes within the human genome. The University of Miami Ear Institute is working closely with the MIHG to address and eventually cure deafness in some populations. Malfunction of the Connexin 26 gene prevents hair cells from functioning properly and is the most common cause of congenital deafness. Replacing a malfunctioning Connexin 26 gene with a normal one may provide restoration of hearing. It is possible that replacing the Connexin 26 gene may only be effective in very young patients and will not be widely available for many years.
Another approach is to inject genes into the inner ear that are responsible for growing new hair cells. The mouse Math1 gene and its human equivalent, the Hath1 gene have the potential to restore hair cells. However, the only way to deliver these genes has been by attaching them to a virus which, by its nature, inserts itself into the nucleus of a cell and turns that cell into a hair cell. It is easy to imagine the safety issues that must be addressed to prevent such genes from ending up in the wrong place. For example, a rapidly growing nest of hair cells in the brain could turn into a benign tumor. Nonetheless, gene therapy has much to offer to the eventual cure of some forms of deafness.
Stem Cell Therapy
Stem cells share the promise of treating deafness by restoring the function of the cochlea. Both stem cells taken from an embryo (embryonic stem cells) and those taken from certain organs such as the skin, bone marrow, umbilical cord blood and even the inner ear, have the potential to generate new cochlear hair cells. Since most severe to profound sensorineural hearing loss (SNHL) is caused by loss of hair cells, regenerating those cells is our current goal.
Embryonic stem cells have two main characteristics. First, these cells can replicate themselves or develop into other types of cells throughout a lifetime. The second characteristic of stem cells is that they can develop into any type of cell in the body, such as hair cells of the inner ear.
Embryonic stem cells exist in the human embryo and are ideally removed about two weeks after conception. Cells removed from the embryo at that time can create more stem cells or daughter cells that can develop any cell type. An entire rodent can be grown from a single embryonic stem cell and they can also be used replace hair cells in the inner ear.
More recently it has been found, possibly as a result of political pressure not to violate living embryos, that adult skin cells can be treated with several genes or gene-like chemicals to turn them into stem cells. That process is much simpler than obtaining embryonic stem cells and has great potential for restoring hair cells.
A third type of stem cell is called the adult stem cell. While adult stem cells donít have the ability to recreate every cell type in the body, they can regenerate cells of the organ from which they are removed. Because adult stem cells are present in the inner ear, they may be used to regenerate hair cells. This has already occurred in animal models. In addition, stem cells from the umbilical cord or bone marrow may have the ability to repair the inner ear. As with gene therapy, safety issues are critical to work out prior out prior to any form of human testing.
The goal of using stem cells to create effective treatment for deafness is still many years away. If you would like to support research on gene therapy or stem cell therapy, please contact Michael Foden, Executive Director of Development at (305) 243-6256.
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