|MadSci Network: Medicine|
Hi Mike, I can see you know a bit about the subject but I will keep the answer fairly easy for any newcomers to the topic who come across this. DNA from transplanted material ( blood, kidney etc ) keeps on working as normal until the cell dies. There are two very obvious and very unfortunate signs of this. Consider the following scenario - Jane Doe donates an organ to Joe Public. Jane's organ is made of cells which carry markers on the surface. These markers identify the cells as JANE. Once the organ is transplanted it carries on living inside Joe's body - the organ does its job and the cells that make up the organ continue to live, divide and die just as though the organ was still inside Jane. The problem is that the DNA inside the cells instructs the cells to produce the cell marker JANE. Joe's immune system can recognise this marker as a foreign object and sets about killing the cells in the transplanted organ and eventually destroying the whole organ. This is the first sign - organ rejection. The other sign of transplanted DNA working as normal occurs when the transplanted material contains cells from the donor's immune system - this occurs if the organ still has blood inside it or if the donated material is blood or bone marrow. In this case cells from Jane's immune system find themselves surrounded by cells all carrying the cell marker JOE. The transplanted cells then function as normal and multiply, produce antibodies to the JOE cell marker and try to destroy those cells carrying the marker. This is called graft versus host disease. These problems can be avoided in several ways - 1) Testing the cell markers present on the donor and recipient cells. JANE and JOE are quite different markers but LESLEY and LESLIE are quite similar and so the chances or organ rejection are lower. 2) Immunosuppressent drugs can stop Joe's immune system attacking the JANE cell marker. 3) Removing any residual immune cells in the transplant material - with blood this is done by exposing the bag of blood to gamma radiation which kills the immune cells but doesn't affect the red cells that transport oxygen around the body. Transplant organs can have most but not quite all of the blood flushed out of them with saline. Unfortunately option 3 doesn't work with bone marrow as the marrow produces many of the cells that form the immune system and a bone marrow transplant is given to patients with irreparably damaged bone marrow who would otherwise die of anaemia or infection. When transplanted cells die ( either normally by apoptosis or by immune or infectious attack ) the recipient body treats them like its own - the DNA is broken down into its constituent sub-units which can be recycled or removed from the body. There seems to be no evidence that DNA from transplants can act like a virus and enter other cells in the recipient's body and result in unusual gene expression. If such a process was possible there would be at least 2 sources of evidence - 1) The large number of transplant recipients over the last 30 years. 2) The human body itself - every day millions of cells die and are replaced, if salvaged DNA was capable of entering other cells and being expressed then we would see tissues and organs growing in appropriate places. It would take a number of unusual and complicated circumstances to provide transplanted material with the opportunity to insert DNA into the recipient's genome. Hope this helps. NIGE
Try the links in the MadSci Library for more information on Medicine.