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Gene Therapy II


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 problem of getting 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

Treating 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.

Treating Hemophilia

Hemophilia B

The 7 December 2017 issue of The New England Journal of Medicine carried a report of a study of 10 boys injected a single time with a vector containing

Eight of the ten boys went a year without needing any infusions of factor IX. Since then, further trials of AAV gene therapy for hemophilia B have shown such promising results that on 22 November 2022 the US Food and Drug Administration (FDA) approved its use (to be marketed as Hemgenix).

Hemophilia A

The 28 December 2017 issue of The New England Journal of Medicine carried a report of a Phase I/II study of seven men given a single intravenous injection of an AAV vector containing the DNA encoding Factor VIII (the clotting factor that results in hemophilia A if insufficient levels are present). Link.

After three years, the six men who received the highest dose of the vector havd sufficiently high levels of Factor VIII that they have been able to stop taking injections of Factor VIII and have been mostly free of any episodes of uncontrolled bleeding.

A phase 3 clinical trial involving 132 men for 2 years has demonstrated the effectiveness and safety of this gene therapy (2023).

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

Early trials in humans indicate that the use of gene therapy for single-gene disorders is promising. In addition to the results with hemophilia A and B described above,

Link to discussions of gene therapy using
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1 March 2023