September 1, 2014
Update on HSV Research
We have recently extended our efforts to cure HSV infections by developing DNA editing enzymes as potential HSV treatments. I refer to these as “smart bombs” that can cleave the HSV-1 genome, and destroy the latent virus, if delivered to latently infected neurons using viral vectors. The only viral vectors that really make sense at this point are based on adeno-associated virus (AAV), which has been successfully used in gene therapy trials in humans. The big advantage of AAV is that you can get very high levels of virus—up to 10 billion infectious units per milliliter—and the Bloom lab has clearly shown, using an AAV that expresses green fluorescent protein (gfp), that he can infect essentially every single neuron in the trigeminal ganglia where HSV-1 establishes latency. The problem is that the AAV packaging size, that is the amount of DNA that it can fit into its viral capsid, is only ~4,600 bp.
Our initial efforts to use gene editing to destroy HSV-1, while very successful, used either transcription activator-like endonucleases (TALENs) or bacterial editing enzyme of the CRISPR/Cas9 family, derived from Streptococcus pyogenes (SPy), both of which work well but have size issues. The TALENs work as heterodimeric DNA binding and cleavage enzymes and, while AAV can package one TALEN, it cannot package two. The SPy Cas9 gene is so large that even on its own it is too big for AAV.
So, my long-term collaborator David Bloom and I have taken two approaches. On the one hand, we have generated AAVs that express one or the other TALEN and have generated viral stocks of each. We intend to mix these two stocks and then use the mixture to infect the trigeminal ganglia in mice at high levels of virus such that each neuron should be infected by both AAVs, allowing expression of the TALEN heterodimer and HSV-1 cleavage. These experiments are now in progress in latently HSV-1 infected mice in the lab of Dave Bloom at the University of Florida.
The second approach we have taken is to identify Cas9 proteins encoded by other bacteria that are highly active but small enough to fit into AAV. The one we are currently focused on is derived from Neisseria meningitidis (NMe). This Cas9 gene is only 3,200 bp in size, well below the 4,600 bp cutoff, and works well. Nevertheless, we have now cloned this into AAV, where it is well expressed, and sent it off to Dave Bloom to put into mice. While these are being tested, we are also testing Cas9 genes we have isolated from other bacteria and we are in the process of establishing a collaboration with a biotech company that focuses on the use of Cas9-derived DNA editing enzymes in the treatment of human disease. I have visited their headquarters and they have stated that they are enthusiastic about working with us on the goal of using Cas9 to cure HSV-1 and, especially, HSV-2. Hopefully, this will allow us to move this project along more rapidly, using new resources provided by this company.
So, some progress has been made but we haven't quite achieved full success. I’m very enthusiastic about the approach we are pursuing, because I really think this could be a way to actually destroy latent HSV genomes and lead to the cure we have all sought. I hope the next update will include the statement that we can at least cure mice! Once that is achieved, I think things will really start to move forward.
Frequently Asked Questions
Thank you for your interest in our research. We continue to receive a considerable number of inquiries on the report we published in 2008 in the scientific journal Nature. This report received extensive press coverage, some of it misleading. We will briefly answer some of the most common questions that we have been asked, and particularly how our work relates to a new treatment for cold sores, caused by Herpes Simplex Virus 1 (HSV-1), or genital ulcers, caused by the related but distinct HSV-2.
1) What have you accomplished?
Our work provides, for the first time, a molecular understanding of how HSV-1 establishes a life-long latent infection in the nerve cells of the face, and how it reactivates from latency to cause cold sores.
2) Have you developed a new treatment for cold sores?
The work we have performed provides a basis for the development of anti-HSV-1 and anti-HSV-2 drugs that might be able to permanently clear these viruses from patients.
3) Are these drugs being used on people?
No, but we hope to initiate trials in mice to study efficacy and toxicity in the near future. Due to limited funding, we are currently focused on providing more evidence in support of our hypothesis that viral reactivation can be regulated.
4) When might this drug reach the clinical trial stage?
We anticipate several years of animal experiments in mice followed by
approximately 1-2 years of toxicity studies in other animals, followed by
small studies in healthy volunteers. This timeline is dependent on
obtaining additional research funding, which we have not yet succeeded in
doing. After that, assuming things go well, this drug might proceed to
clinical trials in HSV-1 infected individuals. It is very possible that the
5) How does this relate to HSV-2, which causes genital herpes?
Most of our work so far has been on HSV-1, the cold sore virus, but HSV-2 is quite closely related. We are endeavoring to see if the lessons we have learned in HSV-1 also apply to HSV-2, and our recent results indicate that this is indeed the case (Umbach et al., Journal of Virology, 84, pp. 1189-1192, 2010). We hope to eventually start animal trials for an anti-HSV-2 drug, depending on the results with HSV-1 and our financial resources. Our initial data indicate that HSV-1 and HSV-2 may respond to distinct, but similar, drug therapies.
6) Would you be willing to accept a financial contribution to be used for your research focusing on HSV-1 and HSV-2?
If you are interested in making a contribution, you may do so in one of the
Cullen account 3990310"
Mail: Please make check payable to 'Duke University'In the memo line of the check, please write: Professor Bryan Cullen 3990310.
If have any questions, please contact:
Kristina Amidon Bowman
7) What can I use to treat chronic HSV-1-induced cold sores or
All treatment decisions should be made in consultation with your physician, and no general guidelines will apply to everybody. While there are no drugs that attack latent herpes viruses, three closely related prescription drugs (Zovirax/acyclovir; Famvir/famciclovir and Valtrex/valacyclovir) are potent inhibitors of active virus that work quite well and that you might discuss with your physician.
We hope this information answers your questions and is helpful. Please be assured that we are continuing to work on the problem of developing novel HSV-1 and HSV-2 treatment approaches.