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Thursday, November 27th 2008

Don't Run And Get A Bone Marrow Transplant Just Yet

An American living in Berlin has apparently been “cured” of his HIV infection after undergoing a bone marrow transplant as a treatment for leukemia.

While interesting no one currently living with HIV should run to their physician expecting for this to be a viable option. As a British ‘expert’ commenting on the case put it,

“The problem is most people with HIV live in sub-Saharan Africa and this is hugely expensive, you have to find a matched donor, and it’s a pretty severe and painful operation.

“So it’s going to be an option for very few people.”

Finding a donor is the key hurdle. When someone has leukemia one of the options is to kill off all of their hematopoietic stem cells which are making the leukemic white blood cells. Then you replace them with the progenitor cells from someone else. Such is what a bone marrow transplant is.

To make it clear even though HIV infects T Cells, a type of white blood cell; you are not clearing the viral infection by doing a bone marrow transplant.

Instead, in this particular case, the patient got a bone marrow transplant from a donor whose white blood cells are resistant to infection from the HIV. For years people have been aware about mutations in the genes that code for specific proteins on the surface of T Cells and these mutations appear to confer resistance to HIV infection. These proteins on the T Cells appear to be used by the HIV to gain entrance to the cells. Think of them as door handles, as one piece I was reading described it.

The most prominent of these proteins is CCR5. Theoretically, and as it apparently worked in this particular patient’s case, if you could replace the T Cells of a patient with HIV with cells who had the “misformed” CCR5 protein then the infected patient would now have resistance to HIV infection and the virus in his body (depending on the serotype) would no longer be able to enter and infect his white blood cells.

A novel idea which appears to have worked in this case. This patient needed a bone marrow transplant for his leukemia and so they found a donor who also had the CCR5 gene mutation and, apparently, “cured” the patient of both of his diseases.

Unfortunately finding a bone marrow donor for a patient is already difficult. Like in all transplants specific antigens have to match up. But to make it tougher only about 10% of the world’s population has the CCR5 gene mutation. You have cut the available pool to search for a bone marrow donor by 90%. Already bad odds have just gotten terrible.

Beyond the costs and the novelty, finding donors for huge numbers of HIV infected patients is simply not feasible. So, bone marrow transplant will likely never be a realistic “cure” for the vast majority of those suffering from HIV. It does however, as the story says, raise the profile of studies on gene therapy to grant those already suffering with HIV a mutated CCR5 gene.

I truly believe that a “cure” for HIV is within our grasp. Bone marrow transplant simply isn’t going to be it however.

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