Several recent reports on ongoing clinical trials for gene therapies indicate that even preliminary studies with only a handful of patients can yield results with the potential to alter the course of the entire field. So after each description below, I offer a DNA Science “lesson learned” assessment: why the study is important.
INTO THE BRAIN – PARKINSON’S DISEASE
Gene therapy typically delivers a functioning version of a gene to cells needing it. Investigators Stéphane Palfi MD of AP-HP, Groupe Henri-Mondor Albert-Chenevier in Créteil, France and Roger Barker, PhD, at Addenbrooke’s Hospital in Cambridge, UK, have expanded the reach of gene therapy by delivering the trio of genes whose encoded proteins enable cells to make dopamine, the neurotransmitter that’s depleted as Parkinson’s disease (PD) progresses. Preliminary results on the gene therapy appear in The Lancet. Oxford Biomedica, a company developing “gene-based medicines,” is funding the trial of the triplo-gene therapy for Parkinson’s, called ProSavin.
In a healthy brain, neurons in the substantia nigra make dopamine. Their axons project to the striatum, where they release the neurotransmitter so neurons there can sop it up. Three enzymes control dopamine synthesis: two convert the amino acid tyrosine to levodopa, and a third converts the levodopa to dopamine.
Treating PD is an ever-changing question of balance. Oral levodopa can offset the dopamine deficit, but after a few years, motor symptoms develop. These include uncontrollable movements (tardive dyskinesia) and “on-off phenomena,” which are periods of improved mobility interspersed with periods of impairment, sometimes severe.
Where there are missing enzymes, gene therapy
is an option, and several have been tried for Parkinson’s disease. The safest gene therapy vector (a disabled virus that delivers the gene), adeno-associate virus (AAV), can’t carry a very large payload, only one smallish gene at a time. So the researchers turned to a larger vehicle to deliver the trio of genes, the lentivirus that causes swamp fever in horses, equine infectious anemia (EIA) virus. Many gene therapy experiments use a more familiar lentivirus – HIV.
Instead of delivering the genes to the substantia nigra neurons that normally make dopamine, the gene therapy infuses gene-loaded viral vectors into both sides of the brain, right into the striatum. The resident neurons then, presumably, pump out what’s needed, even though they don’t normally do so. The goal: to “convert striatal cells into so-called ‘dopamine factories,” the researchers write.
The horse virus seems to offer the optimal combination of features. It doesn’t have the image problem of HIV, nor does it insert into oncogenes, causing cancer, as other retroviruses can do and have done in gene therapy trials. EIA also targets neurons, which don’t divide.
In the trial, 15 patients with advanced PD received low, medium, or high doses, while continuing to take levodopa. And so far it’s all good, up to 4 years later. ProSavin appears to be safe, and all of the patients reported improvements by 6 months. Eleven of the 15 needed to decrease the levodopa, with those in the highest gene therapy dose group needing the most reduction – suggesting that the little dopamine factories work. But the researchers caution that the results of the uncontrolled trial could be due to a placebo effect, something that’s been seen before in Parkinson’s research.
Lesson learned: Gene therapy can deliver components of a pathway – not just a single gene.
WHEN SKIN COMES UNDONE – EPIDERMOLYSIS BULLOSA
The skin is much more than a surface to smear make-up on. In addition to holding our insides in, it regulates body temperature, lets wastes out, keeps water in, and activates vitamin D. A square inch of skin houses 650 sweat glands, 20 blood vessels, 1000 or so nerve endings, 60,000 pigment cells, and a bunch of hair follicles.
About two-thirds of the way down a hair follicle lies a region called the bulge that houses stem cells that have the ability to divide to give rise to either hair or skin. These stem cells were discovered when physicians who treat severe burns noted that new skin forms around hair follicles.
In a group of inherited disorders called epidermolysis bullosa (EB), the layers of the skin separate. The skin is very fragile and blisters easily. Mutations in any of several genes cause EB, and subtypes are classified by the extent to which skin layers pull away from the basement membrane that normally separates the epidermis from the dermis beneath.
About 70% of affected individuals have the “simplex” form of EB that usually peels skin from the hands and feet. It’s manageable, and often several family members have it. Another 25% have the dystrophic form, with more widespread blistering that is replaced with scars that gradually tighten the body. Only about 5% of people with EB have the junctional form, in which the coming apart of skin layers is everywhere, even inside the throat. It can be deadly. Treatment for EB relieves symptoms, and bone marrow transplants have helped some children with the dystrophic form.
Seven years ago, Michele De Luca, MD and his colleagues at the Center for Regenerative Medicine at the University of Modena and Reggio Emilia, in Italy, sampled stem cells from the palm epidermis of a 37-year-old man named Claudio who has junctional EB. They used retroviruses to give the stem cells working copies of the gene encoding laminin 332-Β3, a linchpin-like protein that fastens skin layers. The doctored cells were grafted to the man’s thighs.
A year later, the grafted areas on the man’s legs looked pretty good – no blisters, infection, itching or inflammation, plus normal color and sensations. The healed skin had normal laminin adhering the layers, while surrounding skin was still ulcerated. Three years later, when the man hurt himself, his cut leg skin healed as if it had always been there.
The researchers waited 6 ½ years, to allow the grafted stem cells to go through about 80 division cycles, to see what would happen. The areas still look terrific, but the analysis, published December 26 in Stem Cell Reports, held a surprise.
It wasn’t terribly surprising that it took only a few gene-boosted epidermal stem cells to heal the legs – just 5 to 10 stem cells per 10 square millimeters, about the size of a large pea. The resident keratinocytes made the laminin, indicating that the stem cells had done what stem cells do: divide, differentiate, and replace, while maintaining the small population of stem cells to keep things going. (Many media reports that define stem cells as “turning into any cell type” ignore the more important function of self-renewal. If a stem cell doesn’t self-renew, it isn’t a stem cell.)
The surprise was that the stem cells taking up residence in the man’s legs bore a biological memory of where they’d come from – the palms. Not only was the new skin thick like palm skin, but it produced keratin 9, found normally only in keratinocytes in the soles and palms. “This finding suggests that adult stem cells primarily regenerate the tissue in which they normally reside, with little plasticity to regenerate other tissues,” De Luca said.
Lesson Learned: Stem cells aren’t a blank slate; if they are moved, they can retain echoes of their origins.
CORRECTING ANOTHER EYE DISEASE — CHOROIDEREMIA
Gene therapy has been making headlines in ophthalmology since 2007, when the first young people began to see the world for the first time after receiving working RPE65 genes to treat Leber congenital amaurosis type 2 (LCA2). Nearly 300 people have had that gene therapy, in several clinical trials. Check out this DNA Science post from November: Another Blind Boy Sees the Light Thanks to Gene Therapy.
Last week Robert MacLaren, MD, PhD, professor of ophthalmology at the Nuffield Laboratory of Ophthalmology, University of Oxford and colleagues published early results that gene therapy works for a different form of inherited blindness, choroideremia. That report is also in The Lancet.
The mutation behind choroideremia is in a gene called CHM, which is on the X chromosome. In the 1 in 50,000 people who have the condition, degeneration extends through several layers of the retina, in a patchy pattern. Matt During, MD, PhD, a professor of neuroscience at Ohio State University Medical Center and designer of the viral vector used in the clinical trial, described choroideremia when he told me about the exciting results last week. “A teenage boy will start losing night vision. Later he loses the peripheral visual field and then central vision, until he’s legally blind in his 50s.” I wrote about the technical details at Medscape Medical News.
The small gene, isolated affected body part, and gradual clinical course make choroideremia a perfect candidate for gene therapy. And the astounding success of the LCA2 trials indicated that even patients with just an “island” of photoreceptors left can improve.
Like the Parkinson’s trial, the blindness trial was open label with escalating doses, delivering the gene aboard the small-capacity adeno-associated virus 2 (AAV2). Billions of vectors were slipped beneath the most sensitive part of the retina in 6 men, their untreated eyes serving as controls. But their retinas had to be locally detached to deliver the genes. One reason for the phase 1/2 clinical trial was to assess recovery from the detachment. Not only did the retinas quickly slip back into place, but the men reported improved visual acuity and light sensitivity in the treated eyes.
The results were better than expected. “When we started, our hypothesis was not to get recovery, but just to arrest progression, and it might take one to two years to see that. We weren’t expecting such early and dramatic improved function,” Dr. During said. That’s why they published so soon.
If the promising results persist, the gene therapy will be done on younger patients, who would likely do even better because they have more “islands” of preserved photoreceptors than older patients. And in the future, genetic testing could identify boys who will be affected and perhaps gene therapy deployed to prevent visual loss.
(For updates on gene therapy clinical trials for eyes, see the terrific tables from Irv Arons, a former consultant to the ophthalmic industry. They include Leber hereditary optic neuropathy, wet age-related macular degeneration, Stargardt’s macular dystrophy, achromatopsia, and forms of retinitis pigmentosa.)
Lesson Learned: The retina can be detached to deliver gene therapy, and recover. Fast.
I’ll describe other recent gene therapy successes in the March issue of print Scientific American. And maybe we’ll hear from next week’s Phacilitate Cell and Gene Therapy Forum in Washington, DC. Also check out my gene therapy book, which tells the story of one of the first patients to become able to see thanks to gene therapy – 8-year-old Corey, now poster boy for the Foundation Fighting Blindness.
Gene Therapy News: Brain, Skin, Eye by PLOS Blogs Network, unless otherwise expressly stated, is licensed under a Creative Commons Attribution 4.0 International License.