Gene therapy with AAV: too risky for a cure?

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Sandra Porter

Many medical conditions today are treated but never cured. Imagine, a child with a genetic disease like juvenile diabetes or hemophilia. This child will be taking expensive medications for their entire lives. In the case of some diseases the cost of the medications might be more than child or their parents can ever hope to earn in their lifetimes, much less spend on a life-saving drug.

This is one of the many reasons why people have placed such great hopes in gene therapy. If a disease results from a defective gene, and we could replace it or supplement it with a functional gene, perhaps we could have cures instead of treatments.

It seems simple.

But the many challenges are many. And we may have decide which risks are worth taking.

Obstacles to overcome
One challenge is the immune system. Our bodies are always ready to attack foreign proteins. Just like a child who was put up for adoption, isn't always recognized as family when they show up unexpected at the door, our bodies aren't likely to recognize a protein that we should have made but didn't. Even if it's missing because of genetic change, it will be seen as a foreign protein if it shows up unexpectedly, without proper warning.

Another challenge is cancer. One of the ways that cancer can begin is if a virus integrates into a chromosome and turns on genes that should be kept shut off. Human papilloma virus, the virus that causes cervical cancer, is one such case. HPV can insert itself into the genome and turn on genes for cell growth. If cells grow in the absence of their normal checks and balances, cancer can result.

How is this related to gene therapy?
A big part of gene therapy involves putting the correct gene into the genome to provide the protein that's missing from the gene that's defective. This requires getting the gene into the proper cells and somehow getting the gene into the chromosomal DNA.

How do you put a new gene into a chromosome? We look at how the experts do this in nature and see if we can't use the same techniques. In nature, the ones with the greatest expertise are viruses. AAV is one example.

AAV or adeno-associated viruses are viruses that can infect cells, and can integrate into chromosomal DNA through the work of enzymes that cut the chromosomal DNA and splice in the DNA from the virus.
Unfortunately, the genome is big and at this time, there's no way to control where the virus inserts itself. It's well known that viruses can integrate in innocuous positions or into parts of the chromosome are better left alone.

I'm not sure that we know much, right now, about why AAV picks one site over another.

In the July 27 issue of Science, researchers found some cases in mice, where the virus did indeed pick an unfortunate site to join a mouse chromosome (1). In their study, approximately one third of the mice that were treated with an AAV vector developed liver cancer. The researchers decided to investigate the sites where the virus joined the chromosomes.

They used PCR to create copies of the DNA at the junction sites, determined the DNA sequences from the PCR products, and used a sequence comparison program (probably BLAST although the article doesn't mention this) to determine where those DNA sequences were located in the mouse genome.

When the junction site DNA was examined more closely, they found that the integration sites from four tumor samples all mapped within the same 6, 000 base region in chromosome 12. Surprisingly (to me anyway), in the entire 3 billion base genome, two of the integration sites were only 12 bases apart, in the mir 341 micro RNA transcript.

I don't know if the researchers looked at all the sites where the virus integrated. To me, though, finding four integration sites so close together, makes me wonder if AAV integration isn't really that random.

It would be unfortunate if this location in chromosome 12 (or the human equivalent) is one that's preferred by AAV. Liver cancer isn't the sort of outcome that anyone would desire.

1. Donsante A, Miller DG, Li Y, Vogler C, Brunt EM, Russell DW, Sands MS. 2007. "AAV vector integration sites in mouse hepatocellular carcinoma." Science 27;317(5837):477.