BioEd Online

Researchers have halted a fatal brain disease by delivering a therapeutic gene to the stem cells that mature into blood cells.

The gene was transferred using a virus derived from HIV, a technique that researchers have pursued for more than a decade but has not been successful in humans until now.

Together with his colleagues, paediatric neurologist Patrick Aubourg at INSERM — France's main biomedical research agency — and at the Saint-Vincent de Paul Hospital in Paris, developed the system to treat X-linked adrenoleukodystrophy (ALD), a neurodegenerative disease that affects young males.

ALD results in severe degeneration of the myelin sheath, a structure that is crucial for brain-cell function. The disease is caused by mutations in a gene encoding the ALD protein, which has an important role in cells that make up the myelin sheath and in microglia, the immune cells that reside in the brain. The mutations lead to a deficiency of ALD protein and a subsequent loss of myelin, which begins affecting cognitive function in boys aged 6 to 8 years; most die before reaching adolescence.

Aubourg and his colleagues removed haematopoietic stem cells from two young male patients with ALD and infected the cells with an HIV-derived virus carrying the normal, non-mutated gene for the ALD protein. The patients underwent chemotherapy to eradicate their bone marrow to stop them producing further stem cells, and then received an infusion of their own fixed stem cells carrying the normal gene.

Brain scans and cognitive tests demonstrated that the disease stopped progressing after 14–16 months and was still stabilized after a further 12 months. Neither patient showed any sign of cancer, which can be a tragic consequence of virus-delivered gene therapy. The authors report their results in Science¹.

"It's the first time that a really severe disease of the brain has been treated with success by gene therapy," says Aubourg. "My hope is it gives a boost to the gene-therapy field. A lot of trials using [other viral techniques] were stopped because there was no efficacy, but now others can use this kind of virus to treat other diseases."

Match failure

Currently, the only effective way of treating ALD is bone-marrow transplantation, which replaces a patient's haematopoietic stem cells with those from a healthy donor. These cells give rise to all blood cells, including immune cells such as microglia, which are altered in ALD. But transplants are only possible when there is a compatible donor — and even if a donor is found, there is still a high mortality rate in patients with ALD who have received a transplant.

Aubourg and his team wanted to improve the treatment by using gene therapy to correct a patient's own haematopoietic stem cells. Viruses have long been the tools of choice for delivering genetic material to cells because it's what they do naturally. But most viruses can only get their genes into a host cell's genome if that cell is actively dividing — a problem for cells that divide very slowly, such as stem cells.

Gene-therapy vectors based on lentiviruses, such as HIV, are an exception: they can integrate genes into a target cell regardless of whether the cell is dividing or not.

However, a major concern with any virus used for gene therapy is safety. Viruses can integrate their genes in unpredictable locations in the host cell's genome — including locations that may activate cancer-causing genes. To overcome this problem, Aubourg and his colleagues used a modified lentivirus that would not activate nearby genes.

Vector victory

When Aubourg and his team tested the patients' blood for cells that come from the same line as microglia, they found that about 15% of the cells had received the non-mutated gene.

They analysed the safety of the gene's insertion by searching for its positions in the genomes of targeted cells. Cancerous cells divide rapidly so a cancer-causing insertion would quickly become more common than any others. But the team found that the insertion locations were diverse and remained so, indicating that no insertion was activating cancer-causing genes.

"Leukaemia is the worst-case scenario [in gene therapy]," says Aubourg. "We're now on three years and did not observe any biological effects. We need a longer follow-up, but we are quite reassured now."

"It's a huge advance," says Mark Kay, director of the Program in Human Gene Therapy at Stanford University School of Medicine in California. "If you look in general at the vectors we use for gene therapy, we've really come a long way. This is the first successful use of lentiviral vectors, and it gives me a lot more cautious optimism moving forward."