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Gene Therapy II

Problems

Reaching the goal of effective gene therapies for human diseases has been a difficult one.

Some of the problems that remain to be solved include:

Adeno-associated virus (AAV) — A possible solution?

Adeno-associated virus gets its name because it is often found in cells that are simultaneously infected with adenovirus. However, by itself it seems to be harmless.

Unlike adenovirus, AAV As for the last criterion — how to get the transgene to be expressed appropriately — that may be solved by using two AAV vectors simultaneously:

The strategy in action.

In the 1 January 1999 issue of Science, James M. Wilson and his colleagues reported the results of using this strategy in both mice and rhesus monkeys. They injected their experimental animals with two vectors.

Vector 1

This piece of DNA contained (among other things):

Vector 2

This piece of DNA contained (among other things):

The Experiment

The experimental animals were injected (in their skeletal muscles) with many copies of both vectors. Skeletal muscle was chosen because muscle fibers are multinucleate. Once across the plasma membrane, there are many nuclei which the vectors can enter and hence many opportunities to integrate into the DNA of the host.

Later the animals were injected with rapamycin. This small molecule is an immunosuppressant and is currently being tested in transplant recipients to help them avoid rejection of the transplant. It was used here because of its ability to simultaneously bind to the FRB and FKBP12 domains of the two gene products of vector 1. The resulting trimer is an active transcription factor for the erythropoietin gene.

The Results

In mice

In monkeys

The results were similar to those in mice, but the effect wore off after 4 months.

So here is a system where a gene introduced into an animal can then be

Curing Insulin-Dependent Diabetes Mellitus (IDDM) in mice and rats

Researchers in Seoul, Korea reported in the 23 November 2000 issue of Nature that they have used an AAV-type vector to cure Both groups of animals were injected (in their hepatic portal vein) with billions of copies of a complex vector containing:

The results:

Both groups of animals gained control over their blood sugar level and kept this control for over 8 months. When given glucose, they proceeded to synthesize the synthetic insulin which then brought their blood glucose back down to normal levels.

Curing hemophilia B in mice

Researchers at the Salk Institute reported (in the 30 March 1999 issue of the Proceedings of the National Academy of Sciences) work with mice These mice were injected (also in the hepatic portal vein) with DNA containing

The mice proceeded to make factor IX and were no longer susceptible to uncontrolled bleeding.

In later work, injection of embryonic stem cells with functioning factor IX genes into the liver of mice without the genes cured them.

Treating ALS

ALS (amyotrophic lateral sclerosis) is a human disease in which motor neurons degenerate. (It is often called "Lou Gehrig's disease" after the baseball player who died from it.)

A similar disease can be created in transgenic mice carrying mutant human genes (for superoxide dismutase) associated with ALS.

Researchers at the Salk Institute have slowed up the progression of the disease in these mice by injecting their skeletal muscles with an AAV vector containing the gene for insulin-like growth factor 1 (IGF-1). The vector

The results: destruction of motor neurons was reduced, and the mice lived longer than they otherwise would have.

The Outlook

It's a big jump from mice to humans, but these results indicate that the principle of gene therapy for single-gene disorders is valid.

And some early trials in humans look promising.

People with the rare disorder lipoprotein lipase deficiency are unable to process the globules (chylomicrons) of fat and protein that appear in the blood after a fat-containing meal because they lack functional copies of the gene encoding lipoprotein lipase. Intramuscular injection of an AAV vector containing the functional gene provides sufficient improvement, with apparent safety, that in October 2012, this agent (Glybera®) received approval for use in the European Union. It is the first gene therapy to receive such approval.


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30 November 2012