Glial cell-line derived neurotrophic factor (or GDNF) has been a topic of heated discussion in the Parkinson’s community for a long time. Most recently due to the announcement of the results of the Phase II Bristol GDNF clinical trial results, which did not meet the primary end points of the study (Click here to read more about that).
This week at the annual American Association of Neurological Surgeons conference in San Diego, the results of another GDNF clinical trial were presented.
This new study was a Phase I study assessing the safety and tolerability of a gene therapy approach for GDNF in people with Parkinson’s.
In today’s post, we will discuss what gene therapy is, what the new trial results indicate, and what the researchers may be planning to do next for this new clinical trial programme.
Every year members of the American Association of Neurological Surgeons gather together in one spot and compare data/research/clinical notes.
This year the 87th AANS Annual Scientific Meeting was held in spectacular San Diego.
San Diego. Source: AFP
From Saturday 13th April through till Wednesday 17th, clinicians and researchers attended lectures and discussed new data on every aspect of neurological surgery. While I did not (nor planned to) attend the meeting, I was very interested to learn more about one particular presentation.
It involved the announcement of the results of a clinical trial which was focused on a gene therapy approach for Parkinson’s.
The treatment involved GDNF (Click here to read the abstract).
What is GDNF?
GDNF stands for glial cell line-derived neurotrophic factor.
Glial cells are the support cells in the brain. While neurons are considered to be the ‘work horses’ of neurological function – passsing messages and storing memories – glial cells are in the background making sure that neurons are supported, protected and nurtured.
There are different types of glial cells, including astrocytes, oligodendrocytes and microglia. And each type has a specific function, for example microglia are the brain’s resident immune cells checking up on the health of the neurons while oligodendrocytes provide the neurons with a protective covering (called myelin sheath) which also helps to speed up the signalling of neurons.
Different types of cells in the brain. Source: Dreamstime
Astrocytes provide nutrients to neurons and make sure the environment surrounding the neurons is balanced and supportive. Glial cells are absolutely critical to the normal functioning of the brain.
The researchers that discovered GDNF found this protein in a cell culture of rat glial cells – hence the name: glial cell-line derived.
So that is the “glial cell-line” part of glial cell line-derived neurotrophic factor, now let’s focus on the latter part.
Neurotrophic factors (neurotrophic = Greek: neuron – nerve; trophikós – pertaining to food/to feed) are chemicals that nurture neurons and support growth. There are many types of neurotrophic factors, some having more beneficial effects on certain types of neurons and not other.
GDNF is one of these neurotrophic factors.
Of particular interest to us is that GDNF is very neuroprotective for dopamine neurons (Click here for a very good OPEN ACCESS review of GDNF biology). Dopamine neurons are one group of cells in the brain that are badly affected by Parkinson’s. Thus, any protein that protects them and stops them from dying is of great interest to the Parkinson’s research community.
Ok, and what is meant by gene therapy?
Gene therapy is an experimental treatment approach that involves treating medical conditions with DNA rather than drugs.
Gene therapy basically involves introducing a new piece of DNA or replacing a faulty piece of DNA within a population of cells. DNA, as you may remember from high school science class provides the instructions for making proteins in the a cell and these proteins are the bits that actually do stuff.
By introducing a new piece of DNA into a cell, the cell can start to produce a functioning protein that it may not normally produce. In some diseases, a cell may normally produce a particular protein, but because the genetic instructions in the DNA (a section of the DNA called a gene) for that protein have a small error (a genetic mutation), a non-functioning version of the protein is actually being produced. The introduction of the new correct (functioning) version of that piece of DNA (or gene) into a cell can start the production of a functional version of the protein.
Gene therapy. Source: yourgenome
Alternatively, a gene can be introduced into a cell which would cause the cell to produce a protein that that cell usually does not produce. This sort of approach is being used in gene therapy for cancer, where ‘suicide genes’ are being introduced into cancer cells. These cause the cancer cell to die, by initiate an auto-destruct sequence resulting in cell death (a process called apoptosis). Another approach the cancer field is using is introducing a gene into cancer cells that cause a protein to be produced on the surface of the cancer cell that attracts the attention of the immune system. This ‘marker gene’ causes the immune system to attack the cancer cell, resulting in the death of the cancer cell.
Taking this approach one step further, we can take sections of DNA that contain the genes involved with the production of a proteins that would be beneficial for the cell, such as GDNF. By then injecting a virus with the DNA for GDNF into the brain, we can produce GDNF in any infected cells (it’s slightly more complicated than that, but you get the basic idea).
Gene therapy for Parkinson’s disease. Source: Wiki.Epfl
I’m sorry, but did you say viruses?
Yes, if you remove the viral DNA from inside a virus and replace it with something useful, then a virus becomes a very useful biological delivery system. Far superior to anything we humans have devised thus far. Viruses are easy to produce and manipulate, and they can even be engineered to target specific cell types.
And these viruses have been engineered not to replicate. They deliver the proposed DNA and that is all.
I see. Has this gene therapy approach ever been tested in models of Parkinson’s?
Yes it has. Many times in fact.
Almost immediately after the discovery of GDNF was announced, researchers began trying to stick the DNA of GDNF into empty viruses with the goal of infecting cells in the brain and causing them to produce the protein. The first successful demonstration of this feat in a model of Parkinson’s was published in 1997:
Title: Dopaminergic neurons protected from degeneration by GDNF gene therapy.
Authors: Choi-Lundberg DL, Lin Q, Chang YN, Chiang YL, Hay CM, Mohajeri H, Davidson BL, Bohn MC.
Journal: Science. 1997 Feb 7;275(5301):838-41.
In this study, the researchers inserted the DNA for GDNF into an adenovirus, and injected it into the part of the brain where the dopamine neurons reside (the substantia nigra). This treatment resulted in a 3 fold reduction in the loss of dopamine neurons 6 weeks after a neurotoxin (6-OHDA) was delivered (compared with no gene therapy or an empty virus control treatment).
But the type of virus used in gene therapy is important. Adenoviruses are known to cause immune responses in mammals. And this has caused researchers to shift to AAV viruses.
What are AAV viruses?
Adeno-associated viruses (or AAV) are a kind of virus that are popular with researchers because A.) they readily infect human and primate cells, and B.) they produce little (if any) immune response, and C.) they are non-pathogenic (they don’t cause any known diseases).
Given these characteristics, AAVs have been used in most of the gene therapy clinical trials thus far:
AAV-based gene therapy clinical trials. Source: Wikipedia
They were originally discovered in the preparation of another type of virus, called an adenovirus (hence the name ‘Adeno-associated’). They were believed to simply be a contaminant of that preparation. Further research, however, revealed that AAVs belong to the Dependoparvo genus of viruses, which in turn belongs to the family Parvoviridae.
AAVs are single-stranded DNA viruses, and they are one of the smallest viruses (approximately 22 nm in diameter) with a non-enveloped capsid. The capsid is the shell surrounding the genetic material of the virus. Viruses are either enveloped or non-enveloped. “Enveloped” means that a second casing surrounds the capsid, providing further protection for the virus, while “non-enveloped” viruses have only the capsid.
Enveloped (left) vs Non-enveloped (right) viruses. Source: Differencebtwn
Given the reduced amount of casing, non-enveloped viruses are generally more virulent (more infectious) than enveloped viruses (a good example of a non-enveloped virus is the influenza virus). Non-enveloped viruses do not survive outside of an organism for long though.
The AAV capsid. Source: Wikipedia
And AAV viruses have been found to be very effective at delivering GDNF DNA into models of Parkinson’s, for example:
Title: Midbrain injection of recombinant adeno-associated virus encoding rat glial cell line-derived neurotrophic factor protects nigral neurons in a progressive 6-hydroxydopamine-induced degeneration model of Parkinson’s disease in rats.
Authors: Mandel RJ, Spratt SK, Snyder RO, Leff SE.
Journal: Proc Natl Acad Sci U S A. 1997 Dec 9;94(25):14083-8.
PMID: 9391156 (This report is OPEN ACCESS if you would like to read it)
In this study, the researchers injected of either an AAV GDNF virus or an AAV-control (empty) virus into the substantia nigra region of the rat brain 3 weeks prior to delivery of a neurotoxin (6-OHDA). When they anlysed the brains 4 weeks after the neurotoxin was administered, they found that the AAV GDNF virus protected the dopamine neurons significantly better than the AAV-control virus. The AAV GDNF virus treated animals had 94% of their dopamine neurons intact, compared to just 51% in the AAV-control virus treated animals.
All of these successful results of the GDNF-based gene therapy in models of Parkinson’s led researchers to test this approach in non-human primates (with the goal of ultimately testing it in humans). Despite the ethical issues surrounding the use of primates in research, health regulators still require new treatments to be tested in them before they will give the green light for clinical testing in humans.
The first demonstration of GDNF-based gene therapy in primates involved the delivery of a lentivirus containing GDNF DNA into a primate model of Parkinson’s. The results of that study were published in 2000:
Title: Neurodegeneration prevented by lentiviral vector delivery of GDNF in primate models of Parkinson’s disease.
Authors: Kordower JH, Emborg ME, Bloch J, Ma SY, Chu Y, Leventhal L, McBride J, Chen EY, Palfi S, Roitberg BZ, Brown WD, Holden JE, Pyzalski R, Taylor MD, Carvey P, Ling Z, Trono D, Hantraye P, Déglon N, Aebischer P.
Journal: Science. 2000 Oct 27;290(5492):767-73.
In this study, the researchers found that lentivirus-GDNF reversed the behavioural/motor deficits in a neurotoxin (MPTP)-based primate model of Parkinson’s and completely prevented dopamine neuron degeneration. And this lentivirus result has been replicated (Click here to read more about this).
But researchers have chosen to use AAV viruses in the clinical testing of GDNF-based gene therapy and this has also been tested in primates, with very positive results:
Title: Recombinant adeno-associated viral vector (rAAV) delivery of GDNF provides protection against 6-OHDA lesion in the common marmoset monkey
Authors: Eslamboli A, Cummings RM, Ridley RM, Baker HF, Muzyczka N, Burger C, Mandel RJ, Kirik D, Annett LE.
Journal: Exp Neurol. 2003 Nov;184(1):536-48.
And all of these results collectively gave the research community confidence in taking GDNF gene therapy to clinical testing for Parkinson’s.
Hang on a second, these reports were published in 2000 and 2003. Why has the clinical trial taken so long?
Because at the time that these reports were being published, GDNF was being clinically tested in Parkinson’s using a direct administration of GDNF protein approach (tubes were inserted into the brains of participants and GDNF was periodically infused), which…. well, let’s just say it has been a bit of a roller coaster saga (Click here to read more about that).
I see. So what did the new clinical study involve?
This Phase I clinical trial, involved 13 people with advanced Parkinson’s having a one-time injection of AAV2 virus containing GDNF-DNA injected into a region of the brain called the putamen on both sides of the head. The goal of the study was to investigate the safety, tolerability, and potential clinical effects of this treatment. There were three different doses of GDNF AAV virus used in this study (a low dose (9 x 1010vg in 6 participants), a medium dose (3 x 1011vg in 6 participants); and a high dose (9 x 1011vg in 1 participant)) (Click here to read more about the details of this trial – though please note that some of the trial details have changed).
One quick question: what is the putamen?
As I mentioned above, dopamine neurons in the brain reside in an area called the substantia nigra, near the base of the brain, but they project their branches (or axons) to the several other areas, including the putamen, and this is where they release most of their dopamine.
The projections of the substantia nigra dopamine neurons & location of the putamen. Source: MyBrainNotes
Appreciating that the putamen is where many of the axons of the dopamine neurons can be found, the researchers hoped that by delivering GDNF to that region they would encourage the dopamine neurons to not only survive, but also to grow more branches. An example of regenerative medicine.
Previous research in models of Parkinson’s suggested that delivering GDNF to the substantia nigra protected the dopamine neurons, but not their branches. By delivering GDNF to the putamen, the researchers may be able to protect both the cell bodies and the branches.
Let’s continue: Both pre-operatively and at 6-12 month intervals post-operatively, the participants in the study were assessed using the Unified Parkinson’s Disease Rating Scale (UPDRS) to determine any benefits in their motor function. They were also given brain imaging (PET), which was conducted to measure any change in dopamine activity.
And what did the results suggest?
So all I know is what has been provided in the abstract, but it sounds like the participants tolerated the AAV2-GDNF treatment very well.
In addition, the UPDRS clinical assessment scores remained stable across the time frame of the study, and in 12 of the 13 participants there was a 54% increase in dopamine activity at 18 months after AAV2-GDNF treatment (this effect ranged between 8-130% across the paritipants).
Based on these satisfactory results, the researchers are now planning a follow-up clinical trial (Click here to read the abstract).
Do we know anything about the follow up study?
The Phase I study was coordinated by Prof Krystof Bankiewicz of University of California, San Francisco (UCSF).
Prof Bankiewicz. Source: Swiatlekarza
And this summer (2019), he will be setting up a Phase II clinical trial for AAV-GDNF with the help of a private-public entity, called Brain Neu Bio. The sites for the larger Phase II will include those that were in place for Phase I study at UCSF and new sites at Ohio State University and in Europe (specifically Warsaw, Poland).
The new phase II trial will differ from the Phase I study in that they will be:
- Enrolling people with moderate Parkinson’s instead of moderate-to-severe PD
- Delivering the virus from the back of the skull, rather than the top of the skull
- Increasing the vector volume (the volume is expected to triple in an effort to increase the coverage of the putamen to at least 50% (it was only 26% coverage in the Phase I study)
By increasing the coverage of the putamen (that is, infecting more cells in the putamen with the AAV-GDNF virus treatment), the researchers are hoping to produce more GDNF. Ideally, more GDNF will produce a better outcome.
There is currently no details regarding the size of the study or format, but we will keep an eye out for them and mention them here when they become available (Click here to read more about what is known regarding this proposed Phase II study).
So what does it all mean?
GDNF is a contenscious topic for many folks in the Parkinson’s community – it represents hope to some, and gets the blood boiling in others. And there is certainly going to be more discussion about it as the Bristol GDNF clinical trial results get dissected, digested, and absorbed (Click here for a great over view of the current discussions that are being had on this topic). As such, I was almost reluctant to write this post for fear of the consequences (raised expectations, etc). But better sense won the day.
As we wait for the results of the Herantis Pharma clinical trial of CDNF (another neurotrophic protein) in Parkinson’s, it is encouraging for the neurotrophic field that this new GDNF clinical trial is being reported. Neurotrophic proteins represent one method of regenerative medicine for Parkinson’s. It is important to remember, though, that this current study is very small (only 13 individuals), it is ‘open-label’ (which means that everyone involved knows what is happening and what treatment is being administered), and it was not designed to test the efficacy of the GDNF AAV treatment. Thus, we must be very careful with our interpretation of any results.
I will be looking out for the publication of the full results of this study and for any news of a larger follow-up study.
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