Dispatches from ARVO, Day 2
May 8, 2012
by Jennifer Phillips, Ph.D.
Today was an 11-hour maelstrom* of really good science. Of all the great research stories I heard, there are several that will likely be of interest to our readers:
1. There were two posters there with updated information on the UshStat story. First, from the authors of the mouse studies I reported on previously, the continuation of the animal trials of this gene therapy: In this study, shaker1 mice treated with UshStat were subjected to all kinds of light conditions ranging from normal day/night illumination over several weeks to extremely bright light for a few hours. The retinas of these mice were compared with those of other shaker1 mice receiving a control injection, and in all cases, the UshStat protected the treated mice from the level of retinal degeneration experienced by controls. The researchers then applied the UshStat treatment to non-human primates in order to conduct safety studies. The determined that the treatment was tolerated well by the macaques and did many experiments to try and detect whether UshStat was detectable in body tissues other than the eye. An important safety consideration for gene therapy is that the DNA you are introducing won’t go ‘rogue’ and migrate to other tissues where it might have an effect you don’t intend. Encouragingly, no UshStat was detected in any of the numerous other body tissues or blood that were tested.
Another poster was presented by the researchers at the Casey Eye Institute who are conducting the clinical trials on UshStat and a similar gene therapy for Stargardt’s Macular Degeneration. No results of the treatment to report yet beyond the fact that the first patients have received their injections without incident. One of the images on the poster was a fundus photograph of an USH1B patient’s retina showing the site of injection that was administered two weeks ago. It’s very exciting to think that this trial is really happening, at last.
2. Another poster featured an alternate gene replacement therapy that’s being developed for USH1B patients, this one using a modified viral vector to deliver the genetic payload. The viral vector in question is called AAV (for adeno-associated virus) which, through natural talent augmented by human tinkering is extremely efficient at docking with and delivering its DNA content into human cells. The downside is that there’s a size limit to the DNA that can be packaged inside of it, so most of the Usher genes are just too big to fit. However, researchers from the Ophthalmology Department at the University of Florida were able to devise a way to deliver the whole Myo7a gene in two parts, which would be carried in separately by their respective vectors and then assemble into the complete gene once inside the cell. This has only been tested in cultured cells so far—no animal models yet—but it’s success thus far is promising indeed.
3. Following the trend of promising treatment directions for Usher syndrome, a fantastic story from one of my former collaborators, Jennifer Lentz, and her colleagues at Louisiana State University Health Sciences Center. They’re the ones who created the Ush1c mutant mouse containing a human ush1c mutation common to the Acadian population—a mutation that leads to improper splicing of the gene, thus producing a flawed protein that leads to Usher Syndrome Type 1C. Last year at ARVO, I reported some on some research that showed this improper splicing could be inhibited by the introduction of a small molecule designed to block the splicing machinery from reaching this mutated splice site. Over the past year, Lentz and colleagues have been testing out this molecule (an antisense oligonucleotide, if you’re trying to fill your science geek Bingo card) on the Ush1c ‘knock-in’ mice. They started by asking the very basic question, ‘will this molecule harm the mice?’ and as such didn’t go for great precision in their pilot experiments but instead injected a quantity of the molecule into the bellies of newborn mutant mice. They were then able to assess the development of the auditory and vestibular systems in these mice over the next few weeks. The first remarkable discovery was that the mice did not exhibit the strong circling behavior common to Usher 1 mice. They also performed much better on a variety of balance tests, indicating that the splice blocking treatment was actually working, and that normal Ush1c protein was being produced at significant levels over the mutant protein. Just as exciting, the young mice performed within normal limits on their hearing tests as well.
Tests on the retinal cell condition or function aren’t complete yet--recall that unlike many other Ush1 mice, these Ush1c ‘knock in’ mice do exhibit some retinal degeneration and loss of function, but not until later in life. However, this is an extremely encouraging beginning. In addition to the forthcoming vision tests, follow-up experiments in which the treatment is delivered into specific tissues (eyes and ears) are also planned.
4. Finally, an interesting treatment option that could forestall photoreceptor death was presented in one of the lectures I attended by an Ophthalmologist from UCSF. In this study, tiny capsules filled with cultured RPE cells were inserted into the retinas of several RP patients, one them an USH2A patient. As usual, in these types of trials, the other eye was left unimplanted to serve as a control. The RPE cells were engineered to excrete a molecule known to promote photoreceptor survival, and were encased in a special material that allowed small molecules to pass through—the photoreceptor survival factor could freely go out into the retina, while metabolic factors required for the continued happiness of the RPE cells could get into the capsule. This is a truly clever delivery system, as an adequate supply of the desired molecule (Ciliary Neurotrophic Factor, for you Bingo players) will continue to be produced for an extended period as long as the RPE cells inside the capsule remain viable. The end result? Over the course of a couple of years, the density of healthy cone photoreceptors was monitored in these patients. In the unoperated eyes, the number of cones decreased by 20-25%, but in the eyes implanted with the growth factor capsule, the numbers of healthy cones remained stable. While no reports of actual vision improvement were forthcoming, the fact that photoreceptors were apparently prevented from dying by this treatment is of clear benefit in the long term. Remember that the vast majority of the up and coming gene therapies will require for them to be some viable photoreceptors left in order to have an effect. Retinas that are too degenerated will likely have a very limited response to any kind of gene replacement if the cells in which those genes are required have already died. So preserving the cells, perhaps as a precursor to or in conjunction with a replacement therapy of some sort, could be a very promising option.
Oh, there’s so much more, but I’ll stop there with the hope that you will find these updates as encouraging as I do.
*I originally wrote ‘juggernaut’ here. Then I looked back at last year’s ARVO dispatches and discovered that I described ARVO Day 2 of 2011 as a juggernaut, so I came up with a new word. Basically, Day 2 is very intense.