Post-mortem assessment of the short and long-term effects of the trophic factor neurturin in patients with α-synucleinopathies

• 4 autopsy cases of α‐synucleinopathies after AAV2-NRTN to putamen are reported
• All had motor deficits, clear nigrostriatal degeneration & alpha-synuclein aggregates
• They represent wide variations in time of transgene expression (1.5mo–4 yr) but nonetheless demonstrate persistent and stable NRTN expression
• Clear but modest neurotrophic response seen early; only modestly augmented at 4 yr
• Unique info into neurotrophic factor response in human degenerative brain provided

Substantial interest persists for developing neurotrophic factors to treat neurodegenerative diseases. At the same time, significant progress has been made in implementing gene therapy as a means to provide long-term expression of bioactive neurotrophic factors to targeted sites in the brain. Nonetheless, to date, no double-blind clinical trial has achieved positive results on its primary endpoint despite robust benefits achieved in animal models. A major issue with advancing the field is the paucity of information regarding the expression and effects of neurotrophic factors in human neurodegenerative brain, relative to the well-characterized responses in animal models. To help fill this information void, we examined post-mortem brain tissue from four patients with nigrostriatal degeneration who had participated in clinical trials testing gene delivery of neurturin to the putamen of patients. Each had died of unrelated causes ranging from 1.5-to-3-months (2 Parkinson's disease patients), to 4+-years (1 Parkinson's disease and 1 multiple-system atrophy-parkinsonian type patient) following gene therapy.Quantitative and immunohistochemical evaluation of neurturin, alpha-synuclein, tyrosine hydroxylase (TH) and an oligodendroglia marker (Olig 2) were performed in each brain. Comparable volumes-of-expression of neurturin were seen in the putamen in all cases (~ 15–22%; mean = 18.5%). TH-signal in the putamen was extremely sparse in the shorter-term cases. A 6-fold increase was seen in longer-term cases, but was far less than achieved in animal models of nigrostriatal degeneration with similar or even far less NRTN exposure. Less than 1% of substantia nigra (SN) neurons stained for neurturin in the shorter-term cases. A 15-fold increase was seen in the longer-term cases, but neurturin was still only detected in ~ 5% of nigral cells.These data provide unique insight into the functional status of advanced, chronic nigrostriatal degeneration in human brain and the response of these neurons to neurotrophic factor stimulation. They demonstrate mild but persistent expression of gene-mediated neurturin over 4-years, with an apparent, time-related amplification of its transport and biological effects, albeit quite weak, and provide unique information to help plan and design future trials.

This paper describes four autopsy cases of patients who had all been treated with the same AAV2 viral vector expressing the identical transgene (NRTN) using the same bioengineering and manufacturing process. At time of dosing, all showed similar motor deficits and responsiveness to l‐dopa. At autopsy, all showed clear evidence of significant degeneration in nigrostriatal neurons and similar accumulation of alpha‐synuclein aggregates in these specific neurons. Importantly, the cases reflect two widely different durations of transgene expression (1.5‐3 months vs 4‐years) following gene delivery. These cases therefore provide a very unique opportunity to gain important information regarding the effects of neurotrophic factors over time, under well‐controlled conditions. The data show persistence of stable NRTN expression from 1.5 months to 4 years. They also have a clear but very modest neurotrophic response within 1.5 to 3 months, that is further modestly augmented and persists at 4 years. However, even by this time point, the changes to nigrostriatal neurons are far less than those seen in animal studies, with the data pointing to deficits in retrograde transport in the degenerating neurons as responsible for these differences. This information provides unique insight into how neurotrophic factors respond in human neurodegenerative brain and points to certain factors must likely be accounted for in order to achieve therapeutic benefit from neurotrophic factors.Figure optionsDownload as PowerPoint slide