Human proteins are often used in the medical treatment of various human diseases. The most common way to produce proteins is through human cell culture, an expensive approach that rarely results in adequate quantities of the desired protein. Larger amounts of protein can be produced using bacteria or yeast. However, proteins produced in this way lack important post-translational modification steps necessary for protein maturation and proper functioning. Additionally, there are difficulties associated with the purification processes of proteins derived from bacteria and yeast. Scientists can obtain proteins purified from blood but there is always risk of contamination. For these reasons, new ways of obtaining low-cost, high-yield, purified proteins are in demand.
One solution is to use transgenic animals that are genetically engineered to express human proteins. Gene targeting using nuclear transfer is a process that involves removing nuclei from cultured adult cells engineered to have human genes and inserting the nuclei into egg cells void of its original nucleus.
Transgenic cows, sheep, and goats can produce human proteins in their milk and these proteins undergo the appropriate post-translational modification steps necessary for therapeutic efficacy. The desired protein can be produced up to 40 grams per liter of milk at a relatively low expense. Cattle and other animals are being used experimentally to express specific genes, a process known as "pharming". Using cloned transgenic animals facilitates the large-scale introduction of foreign genes into animals. Transgenic animals are cloned using nuclear gene transfer, which reduces the amount of experimental animals used as well as allows for specification of the sex of the progeny resulting in faster generation of breeding stocks.
Medical benefits from cloned transgenic animals expressing human proteins in their milk are numerous. For example, human serum albumin is a protein used to treat patients suffering from acute burns and over 600 tons are used each year. By removing the gene that expresses bovine serum albumin, cattle clones can be made to express human serum albumin. Another example is found at Genezyme transgenics, a biotech company that uses goats to produce human tissue plasminogen activator, a human protein involved in blood clotting cascades. Another biotech company, PPL Therapeutics, has a flock that produces alpha-1-antitrypsin, a drug currently in clinical trials for the use in treating patients with cystic fibrosis. Cows can also be genetically manipulated using nuclear gene transfer to produce milk that does not have lactose for lactose-intolerant people. here are also certain proteins in milk that cause immunological reactions in certain individuals that can be removed and replaced with other important proteins.
There is currently a significant shortage of organs for patients needing transplants. Long waiting lists lead to prolonged suffering and people often die before they find the necessary matches for transplantation. Transplantation technology in terms of hearts and kidneys is commonplace, but very expensive. Xenotransplantation, or the transplantation of organs from animals into humans, is being investigated, yet graft versus host rejection remains problematic. As an alternative to xenotransplantation, stem cells can be used therapeutically, such as in blood disorders where blood stem cells are used to deliver normal blood cell types. However, the availability of adequate amount of stem cells is a limiting factor for stem cell therapy.
One solution to supercede problems associated with transplantation or stem cell therapy is to use cloning technology along with factors that induce differentiation. The process is termed, "therapeutic cloning" and might be used routinely in the near future. It entails obtaining adult cells, reprogramming them to become stem cell-like using nuclear transfer, and inducing them to proliferate but not to differentiate. Then factors that induce these proliferated cells to differentiate will be used to produce specialized cell types. These now differentiated cell types or organs can then be transplanted into the same donor that supplied the original cells for nuclear transfer.
Although many applications of cloning technology remain in developmental stages, the therapeutic value has great potential. With technological advancements that allow scientists to broaden the applications of cloning becoming available almost daily, modern medicine stands to make rapid improvements in previously difficult areas.
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