Gene Therapy Sprouts New Neurons in Alzheimer’s Patients’ Brains


The journal JAMA Neurology has published a new study that describes an experimental gene therapy that reduces the rate at which nerve cells in the brains of Alzheimer’s patients degenerate and die. See Tuszynski, M. H., et al. (2015). Nerve Growth Factor Gene Therapy: Activation of Neuronal Responses in Alzheimer Disease. JAMA Neurology, published online August 24, 2015. DOI: 10.1001/jamaneurol.2015.1807.

In this study, targeted injections of a growth factor called “nerve growth factor” or NGF into the brain of patients rescued dying cells around the injection site, enhanced the growth of these cells and induced them to sprout new nerve fibers. Surprisingly, in some cases, these beneficial effects persisted for 10 years after the therapy was first delivered.

Alzheimer’s disease (AD) is the world’s leading form of dementia. It affects approximately 47 million people worldwide, and this number is expected to almost double every 20 years. Despite the huge amounts of time, effort, and money devoted to developing an effective cure, the vast majority of new drugs for AD have failed in clinical trials.

While these new results are preliminary findings, they come from the first human trials designed to test the potential benefits of NGF gene therapy for AD patients.

NGF was discovered in the 1940s by Rita Levi-Montalcini, who demonstrated, quite convincingly, that a small protein that she had isolated and purified promoted the survival of certain sub-types of sensory neurons during development of the nervous system. Since that time, others have shown that it also promotes the survival of neurons that produce acetylcholine in the basal forebrain; these cells die off at an alarming rate in AD patients.

In 2001, Mark Tuszynski and his coworkers at the University of California, San Diego School of Medicine initiated a clinical trial based on these laboratory findings. This trial was the first of its kind, and it was designed to investigate the ability of NGF gene therapy to slow or prevent the neuronal degeneration and cell death characteristic of AD.

In phase I of this trial, eight patients with mild AD received so-called “ex vivo” therapy to deliver the NGF gene directly into the brain. This trial extracted skin fibroblasts from the skin on the patient’s backs, and then genetically engineered those cells to express the NGF genes. These NGF-expressing cells were then implanted into the patients’ basal forebrain. Since NGF is too large to cross the blood-brain barrier, it had to be administered directly into the brain. Also, outside the brain, exogenous NGF can stimulate other nerve cells can cause unwanted side-effects such as pain and weight loss.

One of these patients died just 5 weeks after receiving the therapy. Tuszynski’s team secured permission to perform an autopsy of this patient, and in 2005 they reported that the treatment led to robust growth responses, and did not cause any adverse effects (Tuszynski, M. H., et al. (2005). A phase 1 clinical trial of nerve growth factor gene therapy for Alzheimer disease. Nature Medicine, 11: 551 – 555).

The latest results come from postmortem examination of these patients’ brains, all of whom had also been recruited in a safety trial between March 2001 and October 2012. Additionally, two other were included who had received in vivo therapy that included injecting a modified virus that carried the NGF gene into the basal forebrain.

Some of the participants died about one year after undergoing therapy, and others survived for 10 years after the treatment. These autopsies showed that all of them had responded to the treatment.

Essentially, all the brain tissue samples taken from around the implantation sites contained diseased neurons, as expected, but the cells were overgrown, and had sprouted axonal fibers that had grown towards the region into which NGF had been delivered. In contrast, samples taken from the untreated side of the brain exhibited no such response.

This trial was conducted to test the safety of the treatment and it did confirm that none of the patients experienced long-term adverse effects from the treatment, even after long periods of time. These results also suggest that NGF is successfully taken up by nerve cells following targeted delivery. Also the cells synthesize NGF protein so that its concentration dramatically increases in and around the delivery site. Probably the most exciting part of these findings is that the responses to NGF can persist for many years after the gene has been delivered into the brain.

Cholinergic Neuronal Hypertrophy and Sprouting Shown is labeling for p75, a neurotrophin receptor expressed on cholinergic neurons of the nucleus basalis of Meynert. Images were obtained 3 years after adeno-associated viral vectors (serotype 2)–nerve growth factor (AAV2-NGF) delivery (A-C) and 7 years after ex vivo gene transfer (D-F). A-C, Cholinergic neurons are labeled for p75 within the zone of transduction (A), 3 mm from the zone of transduction (B), and in a control Alzheimer disease (AD) brain of the same Braak stage (C). Cells near the NGF transduction region appear larger. The inset shows higher-magnification views of a typical neuron from each region. D, Shown is a graft of fibroblasts transduced to secrete NGF (yellow arrowhead) adjacent to the nucleus basalis of Meynert (red arrowheads). E, The graft (G) at higher magnification is densely penetrated by p75-labeled axons arising from the nucleus basalis of Meynert. These axons are sprouting toward the graft, a classic trophic response. F, Shown are p75-labeled axons from the nucleus basalis of Meynert sprouting toward the graft. Individual axons coursing toward the graft are evident (arrowheads). The bar represents 125 µm in A through C, 500 µm in D, and 100 µm in E and F.
Cholinergic Neuronal Hypertrophy and Sprouting
Shown is labeling for p75, a neurotrophin receptor expressed on cholinergic neurons of the nucleus basalis of Meynert. Images were obtained 3 years after adeno-associated viral vectors (serotype 2)–nerve growth factor (AAV2-NGF) delivery (A-C) and 7 years after ex vivo gene transfer (D-F). A-C, Cholinergic neurons are labeled for p75 within the zone of transduction (A), 3 mm from the zone of transduction (B), and in a control Alzheimer disease (AD) brain of the same Braak stage (C). Cells near the NGF transduction region appear larger. The inset shows higher-magnification views of a typical neuron from each region. D, Shown is a graft of fibroblasts transduced to secrete NGF (yellow arrowhead) adjacent to the nucleus basalis of Meynert (red arrowheads). E, The graft (G) at higher magnification is densely penetrated by p75-labeled axons arising from the nucleus basalis of Meynert. These axons are sprouting toward the graft, a classic trophic response. F, Shown are p75-labeled axons from the nucleus basalis of Meynert sprouting toward the graft. Individual axons coursing toward the graft are evident (arrowheads). The bar represents 125 µm in A through C, 500 µm in D, and 100 µm in E and F.

Now, does the observed cellular response to NGF alleviate disease symptoms? Although phase II trials testing the efficacy of the treatment are ongoing, preliminary findings from the initial study suggest that the therapy did indeed slow the rate at which mental function declined in one of the patients involved. These new results indicate that gene therapy is a viable strategy for treating Alzheimer’s and other neurodegenerative diseases, and warrants further research and development.

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Published by

mburatov

Professor of Biochemistry at Spring Arbor University (SAU) in Spring Arbor, MI. Have been at SAU since 1999. Author of The Stem Cell Epistles. Before that I was a postdoctoral research fellow at the University of Pennsylvania in Philadelphia, PA (1997-1999), and Sussex University, Falmer, UK (1994-1997). I studied Cell and Developmental Biology at UC Irvine (PhD 1994), and Microbiology at UC Davis (MA 1986, BS 1984).