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Original Investigation |

Nerve Growth Factor Gene Therapy Activation of Neuronal Responses in Alzheimer Disease

Mark H. Tuszynski, MD, PhD1,2; Jennifer H. Yang, BS1; David Barba, MD3; Hoi-Sang U, MD3; Roy A. E. Bakay, MD4; Mary M. Pay, NP1; Eliezer Masliah, MD1; James M. Conner, PhD1; Peter Kobalka, MD1,5; Subhojit Roy, MD, PhD1,5; Alan H. Nagahara, PhD1
[+] Author Affiliations
1Department of Neuroscience, University of California, San Diego, La Jolla
2Veterans Affairs Medical Center, San Diego, California
3Department of Neurosurgery, University of California, San Diego, La Jolla
4Department of Neurosurgery, Rush University Medical Center, Chicago, Illinois
5Department of Pathology, University of California, San Diego, La Jolla
JAMA Neurol. 2015;72(10):1139-1147. doi:10.1001/jamaneurol.2015.1807.
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Importance  Alzheimer disease (AD) is the most common neurodegenerative disorder and lacks effective disease-modifying therapies. In 2001, we initiated a clinical trial of nerve growth factor (NGF) gene therapy in AD, the first effort at gene delivery in an adult neurodegenerative disorder. This program aimed to determine whether a nervous system growth factor prevents or reduces cholinergic neuronal degeneration in patients with AD. We present postmortem findings in 10 patients with survival times ranging from 1 to 10 years after treatment.

Objective  To determine whether degenerating neurons in AD retain an ability to respond to a nervous system growth factor delivered after disease onset.

Design, Setting, and Participants  Patients in this anatomicopathological study were enrolled in clinical trials from March 2001 to October 2012 at the University of California, San Diego, Medical Center in La Jolla. Ten patients with early AD underwent NGF gene therapy using ex vivo or in vivo gene transfer. The brains of all 8 patients in the first phase 1 ex vivo trial and of 2 patients in a subsequent phase 1 in vivo trial were examined.

Main Outcomes and Measures  Brains were immunolabeled to evaluate in vivo gene expression, cholinergic neuronal responses to NGF, and activation of NGF-related cell signaling. In 2 patients, NGF protein levels were measured by enzyme-linked immunosorbent assay.

Results  Among 10 patients, degenerating neurons in the AD brain responded to NGF. All patients exhibited a trophic response to NGF in the form of axonal sprouting toward the NGF source. Comparing treated and nontreated sides of the brain in 3 patients who underwent unilateral gene transfer, cholinergic neuronal hypertrophy occurred on the NGF-treated side (P < .05). Activation of cellular signaling and functional markers was present in 2 patients who underwent adeno-associated viral vectors (serotype 2)–mediated NGF gene transfer. Neurons exhibiting tau pathology and neurons free of tau expressed NGF, indicating that degenerating cells can be infected with therapeutic genes, with resultant activation of cell signaling. No adverse pathological effects related to NGF were observed.

Conclusions and Relevance  These findings indicate that neurons of the degenerating brain retain the ability to respond to growth factors with axonal sprouting, cell hypertrophy, and activation of functional markers. Sprouting induced by NGF persists for 10 years after gene transfer. Growth factor therapy appears safe over extended periods and merits continued testing as a means of treating neurodegenerative disorders.

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Figure 1.
Alzheimer Disease Pathology and Site of Gene Delivery in a Patient 7 Years After Gene Transfer

All participants in this study exhibited Alzheimer pathology. A, The hippocampus shows numerous plaques. The inset shows diffuse and dense-core plaques. B, Tangles and dense neurofibrillary tau pathology are present (also evident in the inset), indicated by tau phosphorylation (PHF-1). C, Shown is an overview of the left half of a normal human brain, with the boxed region indicating a region containing neurons of the nucleus basalis of Meynert that are targeted by nerve growth factor gene delivery. Myelin staining is reproduced with permission from the Atlas of the Human Brain.19 D, Shown is Nissl staining of a graft of autologous fibroblasts that were genetically modified to secrete human nerve growth factor and injected into the nucleus basalis of Meynert. Cells had been injected 7 years previously, and the graft is located between the parallel lines. Cells survived and exhibited typical fibroblast morphology (inset). E, In the adjacent section of D, p75 neurotrophin receptor immunolabeling shows basal forebrain cholinergic axons penetrating into the graft (between the parallel lines) in a linear fashion, and this concentration of p75 fibers is only present at graft sites (confirmed by Nissl staining). The bar represents 300 µm in A (inset 50 µm), 300 µm in B (inset 50 µm), 6 mm in C, 30 µm in D, and 25 µm in E.

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Figure 2.
Adeno-Associated Viral Vectors (Serotype 2)–Nerve Growth Factor Gene Expression

A, Nerve growth factor labeling shows the site of nerve growth factor gene delivery in the nucleus basalis of Meynert (arrowhead) under the anterior commissure (ac). The patient had received the injection 3 years previously. B, Another site from same brain shows nerve growth factor uptake in neurons in the region of the nucleus basalis of Meynert (arrowhead). The inset shows a single neuron with granular intraneuronal labeling. C and D, Higher magnification of nerve growth factor–expressing neurons (C, from the region of the arrowhead in B) compared with less intense labeling in the nucleus basalis of Meynert neurons located 3 mm from the injection site (D). The bar represents 325 µm in A, 250 µm in B, and 100 µm in C and D.

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Figure 3.
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.

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Figure 4.
Cell Activation After Nerve Growth Factor (NGF) Gene Delivery

A and B, In regions of NGF expression (CREB-near), there is induction of phosphorylated CREB expression, a canonical mediator of downstream neurotrophin signaling and cell activation23 compared with regions of the nucleus basalis of Meynert 3 mm from the site of NGF injection (CREB-far). Arrowheads indicate examples of individual nuclei labeled for CREB. The patient had received adeno-associated viral vectors (serotype 2)–NGF (AAV2-NGF) 3 years earlier. A similar response was evident in a patient who had received AAV2-NGF and survived 1 year. C-F, An individual neuron expressing NGF (green) also expressed c-fos, another canonical marker of neurotrophin-mediated activation of cell signaling.23 As expected, c-fos labeling is nuclear (arrowheads). This image is from the brain of the patient who had undergone AAV2-NGF gene transfer 3 years earlier. F, Staining with DAPI for cell nuclei. G-I, Shown is NGF expression in degenerating neurons expressing tau pathology. G, Neurons in the nucleus basalis of Meynert exhibit characteristic tau pathology, indicated by tau phosphorylation (AT180 antibody). H, Two neurons from G also express NGF (red arrowheads). A third neuron in the field (yellow arrowhead) expressed tau but not NGF. I, The overlay shows that actively degenerating neurons can be infected by AAV2-NGF to express a potentially therapeutic gene product. DAPI indicates 4′,6-diamidino-2-phenylindole. The bar represents 50 µm in A and B, 35 µm in C through F, and 25 µm in G through I.

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