Viruses and the brain
I am certain most of you would have heard that some of the therapies being developed for HD and other neurodegenerative indications involve the use of viruses to deliver a gene therapy agent. This is true in the case of HD, we are working with 3 companies that have efforts in this regard: Shire/Sangamo, Genzyme, and Uniqure. All of them are working in late preclinical studies to identify the best agent and the best virus type to deliver the therapy. All of them are working on a type of virus called adeno-associated virus (or AAV). This type of virus comes in many flavors (serotypes), and they differ in some significant respects from each other: their ability to infect various cell types (called tropism) with different efficiencies (neurons or glial cells, for instance), their ability to be recognized by the immune system (some of these viruses have co-existed with humans for a long time, and the body has generated neutralizing antobodies to them), and finally, their ability to be taken up by axons. This latter part is critical for HD, because ideally we want to ‘transduce’ (infect) as many brain cells as possible once we administer the viruses. The viruses are non-replicative (eg they dont have the capacity to divide once injected into the brain), and typically their distribution in the brain is limited. Multiple injections directly into the brain areas affected by HD is being explored, and only a subset of neurons in these areas will be targeted by the viruses. Will this be enough??
Some variants of AAV have recently been shown to be ‘taken up’ by axons or dendrites (the ‘branches’ of neurons), and the virus can then hitch a ride from there into the nucleus of the cell. In the nucleus, the virus will start expressing the gene therapy product. One injection -we hope- will be sufficient for those cells to express the gene therapy agent for life. In studies in primates and in humans, we know we can detect the gene therapy agents for at least 10 years after a single injection into the striatum. These studies were done in the context of expressing GDNF or Neurturin for the treatment of Parkinson’s. It is from these pioneering studies that we know that the AAV ’2′ serotype is safe in humans, and leads to long-term expression. This is a critical piece of the puzzle we are trying to solve.
One difference between AAV2 and other serotypes, however, is their ability to ‘travel’ up the axons…. why is this so important for HD?
Well, the striatum (the caudate and putamen areas of the brain) is a ‘relay station’ for the brain (think of the main train station in your town). Lots of axons arrive in the striatum from everywhere in the cortex and the thalamus. The striatum has the unenviable task of receiving many types of information, which it needs to bring together, and ‘decide’ what specific ‘response’ to trigger in response to that information, particularly in the areas of action selection and motor control. Well, the anatomy of the striatum makes it possible that, when injecting the viruses directly, they will be taken up by the many millions of axons, and infect cells far away in the cortex and thalamus. Since we still do not know where we need to suppress mutant HTT expression (although we are guessing the more cells without it the better for the patients!), the broader areas that we can target, the better. In this respect, not all serotypes of AAV are the same. There is a lot of effort now in selecting new variants that are better suited for broader distribution because of their ability to get transported by neurons.
OK, so this is where we are today. The therapies using AAVs (with some luck and good science) will reach patients, we expect, as early as next year. Exciting indeed.
BUT – as all of you know, HD is more than a disease of the striatum. Many other parts of the brain are affected, and cells also die in other regions of the brain. While we are excited about the current approaches (which are at the leading edge of scientific research today), we still think that if we could administer the viruses ‘systemically’ (eg take a pill or have an injection, so that the viruses go everywhere in your body), this would be the best way to approach treating HD. Remember we think that mutant HTT also causes damage to other cells of the body, outside of the brain. Recently, some labs have identified new AAV variants (some naturally occurring, some engineered in the lab), which when injected into the mouse bloodstream, they infect the brain. This is terribly exciting! Imagine if we could give an HD patient an injection once and be done with HD!!! We are FAR away from this scenario, but this is my personal goal, and that of many of my colleagues.
There are also other viruses which we are beginning to explore to achieve this ambitious objective. We are working with academic scientists (and will expand these efforts) to engineer viruses to do precisely that: cross the blood-brain barrier (BBB) and infect cells in the brain.
What are some of the hurdles in the way, briefly outlined?
- Ability to make large ‘titers’ (number of active virus particles/volume) so that we can give enough virus dose to infect the brain when delivered via the bloodstream
- Ability to bypass the immune system and ensure no negative immune reactions are seen
- Ability to cross the BBB by retargeting viruses to utilize some of the brain’s existing ‘entry gates’ (called ‘transcytosis mechanisms’) and allow the viruses to get access to brain cells. For these, viruses will be engineered to express proteins or antibodies which recognize proteins responsible for mediating the normal entry of other molecules into the brain.
- Ability to eliminate/decrease the ‘sinks’ that exist in the body, most notably the liver, lung and spleen. Most viruses, when injected into blood, accumulate very rapidly in these organs, so that in essence very little virus remains to cross into the brain. There are specific proteins, or glycoproteins, that might mediate these effects, so modifying viruses to eliminate their interaction with molecules in these organs might allow us to bypass them
OK- I think we have had enough information for today. This area -delivering gene therapy to the brain – is most critical for us. No one has yet succeeded in doing this for any brain disorder. We hope to do it. I am excited about this work…. are you?
have a great day