New Horizons: Modeling Usher gene therapy treatments in animals
February 1, 2011
by Jennifer Phillips, Ph.D.
A study published in the journal Investigative Ophthalmology and Visual Science this month reports on a study that could lay the groundwork for a clinical trial at some point in the future. The laboratory of Jun Yang, at the Moran Eye Center in Salt Lake City, uses mouse models of Usher syndrome to study the molecular basis of the disease.
Last year, Yang and colleagues reported on a 'knock-out' mouse family in which the Ush2d gene has been deleted. These mice don't have the genetic information needed to make a functional copy of the Ush2d protein*, and thus have the mouse version of Usher syndrome type 2, with hearing loss from birth and vision loss later in life.
In this recent report, the Yang laboratory conducted a gene replacement experiment using this Ush2d knockout family of mice. They used a viral delivery system similar to what I've described here previously. Briefly, modified viruses containing the genetic code for Ush2d were delivered into the retinas of the Ush2d knockout mice. The basic mechanism is that, once the virus and its cargo is inside the retinal cells, the cellular machinery identifies the Ush2d genetic information associated with the virus and uses it as a template to make Ush2d protein.
So, what happens when you supply the blueprint for making Ush2d protein to an eye that can't make any on its own? The investigators limited this particular study to assessing the molecular changes resulting from the treatment. Using various methods of visualizing proteins within cells, they determined that:
1. Ush2d protein was made after viral injection.
Ush2d knockout mice don't make any Ush2d protein at all, so the presence of Ush2d protein in the retinas of treated mice show that the viral vector is successfully delivering its cargo so that it can be used to synthesize protein. Better still, the effect seems quite long lasting. The treatment was given in a single dose at about 3 weeks of age (that's young adulthood in mousie life cycles), and Ush2d protein was still being made at 6 months.
2. Ush2d protein is transported to the correct cellular location.
As discussed previously, photoreceptors have a lot of specialized parts that help them perform their sensory cell functions. Many Usher proteins are known to localize to the region of the connecting cilium and are thought to be important for cellular functions associated with the cilia. Determining that not only is Ush2d protein made in treated retinas, but the protein is also being delivered to the right place after it's made, is an important way to demonstrate that the replacement protein is behaving normally once it's produced.
3. Ush2d protein performs predicted molecular functions.
This was determined by looking for two other proteins that Ush2d is known to physically associate with. These happen to be the other two known Usher type 2 proteins, Ush2a and Ush2c. In the Ush2d knockout mice, Ush2a and Ush2c localization near the connecting cilium is lost-without the Ush2d protein to bind them, these other proteins just can't stay in the area. In Ush2d knockout mice treated with the virus, Ush2a and Ush2c proteins can again localize to the connecting cilium region-further evidence that the replacement protein should be capable of restoring normal Ush2d function.
The other part of this study dealt with the safety of the treatment. Viral delivery systems are pretty routine at this point, but it's still important to show that this particular virus with this particular cargo doesn't do any damage to the tissues of the eye or any other part of the body. These are, of course, important prerequisite experiments for any approval of a clinical trial.
Electroretinograms were measured from treated mice and other tests on the retinal tissue were performed to demonstrate that the intervention didn't lead to retinal damage or other problems with visual function early in life.
So, this is promising, but there's a fair amount we still don't know about this treatment that needs to be sorted out before the next step is taken.
Primarily, we still need data on whether or not this treatment actually helps the Ush2d mice see better. We know that the protein itself seems to be doing all the right things in there, but is that going to translate to better vision for the mouse-and, hopefully some day, human USH2D patients?
I hasten to point out that I'm not criticizing the paper in any way for not including this information. In general, if scientists waited until they had all the answers to every possible research question before they published, well, they'd never publish! For this study in particular, there's an extremely valid reason why the information is missing and that's because the loss of visual function in Ush2d knockout mice isn't detectable until after 2 years of age, which is really pretty old in mouse years. They can detect changes in the structure of the retina before then, but the ERGs are normal before then. Clearly any report of functional improvement with this treatment is going to have to include these late-stage analyses.
Once they have a sufficient number of mice raised to older ages, there are several smaller issues to address as well, such as the optimum time of treatment. From this recent IOVS report, we know that protein is still made ~5 months after treatment, but how much longer will it last? Will it have enough staying power to make a difference at that 2-year time point? Will supplying functional Ush2d protein earlier in life forestall the later effects regardless? Mouse and human lifespans are obviously very different, but these sorts of preliminary experiments could provide crucial information for planning any forthcoming clinical trials.
Dr. Yang's team has done wonderful work on this model thus far, and I look forward to what the next phase of this investigation will tell us about progress toward a potential treatment.
*Note: in the literature this protein is often referred to by the name 'whirlin'. For simplicity's sake I have substituted 'the Ush2d protein' in this article.
Yang, et al. (2010). Ablation of whirlin long isoform disrupts the USH2 protein complex and causes vision and hearing loss. PLoS Genetics 6 (5): e1000955
Zou, et al. (2011). Whirlin replacement restores the formation of the USH2 protein complex in whirlin knockout photoreceptors. IOVS 10-6141 (published ahead of print January 6, 2011).