Gene therapy has the aim of inserting a normal gene in place of a defective one. The next bold step would be to insert an “improved” gene in place of a normal one. This technology is in its infancy. Theoretically, there are two types of cells that gene therapy could effect. Somatic cells are most of the cells of the body. Then there are germline cells such as stem cells, sperm and eggs. The focus of all gene therapy at this time is with the cells of the body which are the somatic cells.
The highly controversial prospect of engineering germline cells such as stem cells, sperm and eggs has many ethical as well as scientific problems to surmount.
Genes can be modified outside the body which is called ex vivo. Here the cells are removed, modified and put back in the body. The method called in vivo works with the cells while in the body.
The new DNA needs a vehicle to get into the cells. The common vehicle of choice currently is viruses because viruses are programmed to get inside a cell and replicate themselves. The immune system is on the alert to detect unknown viruses and destroy them. This becomes another scientific problem that must be overcome.
If the new DNA gets successfully inserted in the cell without being detected by the immune system the genetic material in the virus becomes part of the genetic material of the host cell. The host cell now contains a new gene. The miracle happens when the host cell divides later. All of its descendants will will contain the new genes.
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