Gene Therapy Protects Against Diabetic Nerve Damage

MANCHESTER, England, May 31 ? An investigational gene therapy offers promise for diabetic neuropathy, investigators here reported. It would prod the body into ramping up growth factor production.

"Our approach to gene therapy is quite different to previous attempts at treatment," said David Tomlinson, Ph.D., of the University of Manchester here. "We use a DNA-binding protein called zinc finger protein transcription factor (ZFP-TF) to poke life into the patient's own genes and produce a growth factor that has a role in nerve protection and regeneration."

In preclinical studies, a single injection of the DNA-binding protein protected nerve function, stimulated nerve growth, and prevented tissue neuropathy related tissue damage, Dr. Tomlinson and colleagues reported in the June issue of Diabetes.

Sangamo BioSciences of Richmond, Calif., reported that the compound has been tested in 12 diabetic patients, with mild injection-site reactions in four. The clinical data were not discussed in the Diabetes paper.

"We look forward to the next stage of development of this novel therapeutic when phase-two clinical trials start later this year," said Edward Lanphier, the company's president and CEO.

The therapeutic compound is designed to stimulate or up-regulate the expression of the gene encoding for vascular endothelial growth factor (VEGF-A), which has been shown to have direct neurotrophic and neuroprotective properties.

"ZFP-TF-driven activation of the endogenous gene results in expression of all of the VEGF-A isoforms, a fact that may be of significance for recapitulation of the proper biological responses stimulated by this potent neuroprotective growth factor," Dr. Tomlinson and colleagues wrote.

They demonstrated the potential therapeutic effects of the compound in both in vitro studies and in animal models of diabetic neuropathy.

In both cultured human and rat cell lines, the engineered ZFP-TF activated VEGF-A, and the secreted growth factor protein then protected neuroblastoma cell lines from growth arrest when they were exposed to a serum starvation environment.

They then looked at the ability of the DNA-binding protein to protect both sensory and motor nerve function in rats with streptozotocin-induced diabetes.

Following a dose-ranging study, the animals were injected with plasmid DNA encoding the ZFP-TF in the left hind limb, and motor and sensory nerve-conduction velocities were measured. Untreated limbs in the same animals served as controls, as did diabetic rats injected with empty plasmid vaccine, and non-diabetic animals.

The investigators observed that after a single injection, diabetic rats treated with the three highest doses of ZFP-TF had a dose-dependent, progressively increasing difference between the treated and untreated limbs in both sensory- and motor-nerve conduction velocities.

"In all experiments at these higher doses, the conductance velocity of the nerve on the treated side was always higher than the untreated side, and the differences were all significant (P

The investigators also showed that repeated injections of the compound at two, four, and six weeks also produced a robust neuroprotective effect, suggesting that if the treatment is found to be safe and effective in humans, it might be useful for the treatment of chronic diabetic neuropathy, the authors suggested.

"This striking result in terms of the potency and duration of effect supports earlier work with a plasmid encoding the VEGF-A165 cDNA, demonstrating sustained improvement in nerve conduction velocities up to 10 weeks after gene transfer," the investigators wrote. "While angiogenesis was proposed as a possible mechanism responsible for the therapeutic effect after delivery of the VEGF-A cDNA, the data presented here strongly suggest a direct neuroprotective effect of VEGF-A."