Giving Genetic Disease the Finger

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Giving Genetic Disease the Finger

By Sam Jaffe

http://wired.com/news/print/0,1294,68019,00.html

02:00 AM Jul. 05, 2005 PT

Scientists are closing in on techniques that could let them safely

repair almost any defective gene in a patient, opening the door for

the first time to treatments for a range of genetic disorders that are

now considered incurable.

The breakthrough, announced in the journal Nature in June, relies on

so-called zinc fingers, named after wispy amino acid protuberances

that emanate from a single zinc ion. When inserted into human cells,

the fingers automatically bind to miscoded strands of DNA, spurring

the body's innate repair mechanism to recode the problem area with the

correct gene sequence.

A method for fixing miscoded DNA by injecting foreign genes into cells

won headlines three years ago when doctors in France and Britain

announced a handful of successful cures related to X-linked severe

combined immunodeficiency disease, or SCID, also known as " bubble boy "

disease. But that method was ultimately proven unsafe.

In a paper published earlier this month, scientists at California

biotechnology company Sangamo BioSciences showed that zinc fingers can

be used to erase targeted portions of DNA without risk of harmful side

effects.

" This doesn't just deliver a foreign gene into the cell, " said Nobel

Prize winner and CalTech President Baltimore, who with a Sangamo

paper co-author Mathew Porteus proposed this method to cure genetic

diseases. " It actually deletes the miscoded portion and fixes the

problem. "

At the heart of the breakthrough is the concept of " if it's broke,

break it some more. " Cells have a method of DNA repair called

homologous recombination, which fixes breaks in the double helix of

our chromosomes. But the process only repairs places where the DNA has

been cut, not where genes have been miscoded.

Using a package of synthesized zinc fingers, cells can be tricked into

doing nano-surgery on their own genes, Sangamo researchers found. The

zinc fingers home in like a guided missile on the exact spot in the

genome doctors are trying to target and then bind to it. DNA-devouring

enzymes then cut through the double helix of DNA at the exact

beginning and end of the targeted gene, and a template of donor DNA

helps rebuild the deleted strand.

While such a therapy has been theorized for years by Baltimore and

others, Sangamo scientists are the first to show test-tube results

with human cells. In a paper published June 2, Sangamo researchers

showed how they were able to correct the defective gene in 18 percent

of the T-cells extracted from the body of an X-linked SCID patient.

That should be enough to cure the disease, as it only takes one

corrected T-cell to repopulate a person's immune system with healthy

cells, according to Sangamo.

If successful in trials, Sangamo's technology would be the first

successful gene therapy, three decades after the concept of curing

diseases by tinkering with the genome was first proposed. Most gene

therapy trials have failed because the methods of inserting new genes

into cells (usually with modified viruses as vectors) haven't proved

to be effective enough.

One trial that did succeed, but then ended in tragedy, was a 2002

French X-linked SCID trial that used retroviruses to deliver a new

gene into the patients. The new gene cured the disease in 12 patients,

but went on to cause leukemia in three of them. It turned out the

foreign gene, in addition to producing the protein that vanquishes

X-linked SCID, had the unexpected side effect of sometimes turning on

a cancer-causing gene.

Sangamo's technology overcomes that problem. Whereas the French

viruses inserted the foreign gene randomly into the host cell's

genome, the zinc fingers are highly specific and can land only at the

targeted gene.

" They've certainly raised the bar for gene-therapy safety, " said

Wolfe, a zinc-finger researcher at the University of Massachusetts

Medical School in Worcester, Massachusetts. He points out that the

early proof-of-principle work was highly toxic to the cells. The zinc

fingers weren't specific enough and they created so many

double-stranded breaks in the DNA that a lot of the cells chose to

commit suicide rather than try to repair all the breaks. " They really

seem to have solved the toxicity problem altogether. "

Although X-linked SCID patients will probably be the first to try the

therapy, the technology is extremely versatile for a host of human

diseases. " Right now, its greatest weakness appears to be that it is

optimized for very small patches of gene repair, " said Baltimore. " If

it's a long sequence of DNA that has to be fixed, this might not be

the best way to do it. "

Nevertheless, there are a lot of ways to attack diseases without

replacing whole genes. Other potential targets for the therapy range

from many types of cancer to cystic fibrosis and even AIDS. " If they

can figure out how to optimize their zinc fingers for any spot on the

genome, this could target any gene you want it to, " said Wolfe.