Delivering a gene therapy directly to the substantia nigra, an area of the brain affected by Parkinson’s disease, led to sustained protein production for up to eight years, a post-mortem of analysis of two patients revealed.
The therapy’s use failed to show significant benefits in its clinical trials, but these evaluations — covering the longest number of years for such an analysis in gene therapy trial participants — are likely to help researchers working to improve the effectiveness of this approach.
The study, “Long-term post-mortem studies following neurturin gene therapy in patients with advanced Parkinson’s disease,” was published in the journal Brain.
Parkinson’s is caused by the dysfunction or death of nerve cells that produce dopamine (dopaminergic neurons). A neurotransmitter, dopamine is produced in the substantia nigra, a part of the brain the controls balance and movement.
CERE-120, an investigative gene therapy first developed Ceregene, now part of Sangamo Therapeutics, is designed to protect these neurons. It works to deliver a gene called NRTN directly to the substantia nigra and surrounding area known as the putamen. This gene carries the instructions for a protein known as neurturin (NRTN) — a neurotrophic factor — which supports the growth and survival of neurons.
In animal models, the human NRTN gene carried on a harmless virus known as adeno-associated virus serotype 2 (the CERE-120 therapy), and surgically implanted, was seen to protect dopaminergic neurons and led to an increase in dopamine production.
An early open-label Phase 1 clinical trial (NCT00252850) in Parkinson’s patients showed CERE-120 was safe and well-tolerated, with some patients reporting benefits. However, a double-blind Phase 2 trial (NCT00400634), in which CERE-120 was surgically delivered to the putamen, failed to show significant improvement compared to those given a sham surgery.
Post-mortem studies of those treated showed persistent but limited NRTN protein production in the putamen. These levels were not sufficient to provide significant benefits.
A second trial (NCT00985517) was designed to enhance NRTN production by delivering a higher dose to a larger area of the putamen as well as directly to the substantia nigra. Again, CERE-120 failed to show improvement beyond sham surgery in this new group of Parkinson’s patients.
To better understand the reasons for this failure, researchers at the Rush University Medical Center in Chicago conducted post-mortem assessments on two Parkinson’s patients who participated in the CERE-120 gene therapy clinical trials.
One was from the first Phase 2 trial (putamen only) and survived for 10 years after surgery, while the other was enrolled in the second Phase 2 trial (putamen plus substantia nigra) and lived for another eight years.
As a comparison, the team also evaluated the brains of two age-matched Parkinson’s patients who were not given gene therapy, and those of two age-matched people who neither had Parkinson’s nor other psychiatric or neurological illnesses at the time of their death.
In both treated patients, there was a persistent but limited production of NRTN in the putamen, and an associated increase in levels of an enzyme known as tyrosine hydroxylase (TH) — a key enzyme in the production of dopamine. TH levels were substantially higher in the case with combined putamen plus substantia nigra delivery compared to putamen delivery alone.
The NRTN protein was found in up to 19% of remaining dopaminergic neurons in the substantia nigra of the patient who received CERE-120 delivered to the putamen only. In the patient with CERE-120 delivered to both the putamen and substantia nigra, NRTN was detected in up to 39% of the remaining neurons.
This protein was not detected in samples from patients not treated with gene therapy, or people without Parkinson’s.
NRTN signaling works through a multi-component system, including a receptor known as RET. Consistently, RET expression levels were higher in the patient with combined CERE-120 delivery than in the patient with putaminal CERE120 delivery only.
In Parkinson’s patients, dopaminergic neurons are damaged by the buildup of the alpha-synuclein protein, which forms clumps called Lewy bodies. Studies have suggested that alpha-synuclein can reduce the expression of the RET receptor, limiting NRTN production.
Alpha-synuclein clumps were found in neurons that also showed NRTN, RET, and TH. No differences were seen in the numbers of Lewy bodies between the two treated and untreated Parkinson’s patients.
Finally, low levels of the viral AAV vector were detected in both the putamen and the substantia nigra of patients compared to animal models, but its presence did not cause inflammation.
“In summary we demonstrate that gene delivery of NRTN can induce long-standing transgene [artificially introduced gene] expression in Parkinson’s disease subjects lasting for at least 8–10 years with prominent upregulation of TH in focal areas of the putamen and substantia nigra that express NRTN,” the researchers wrote.
“If gene therapy is ever to be considered as a treatment for Parkinson’s disease, we will have to find a way to meaningfully increase TH-positive terminals in the striatum [brain area that includes the putamen], as motor benefits are primarily dependent on TH expression in the putamen,” they added.
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