DISCUSSION

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DISCUSSION

McInnes: I think your model of ROM1 and RDS is in essence correct. Our initial antibody didn’t pick it up in cones, but Bob Molday has shown it is there. However, in the knockout mouse, the cones are ¢ne even though the rods are disappearing. If you are keeping the cells from dying in your type 2 patchy loss of function, why is this merely going to delay loss? If you correct the defect, then presumably they ought not to die.

Bird: In type 2 in which loss of function is due to cell death, gene therapy will slow down or delay visual loss, as opposed to type 1 in which the cells are dysfunctional but present you could induce recovery of function. This is

important to the detection of the therapeutic e¡ect. Obviously, what we would love to do is to cause recovery of function. This would be very easy to measure. If we are establishing priorities we want to choose the disorders where there are large populations of cells that aren’t working ¢rst. I am not saying that we shouldn’t do gene therapy in type 2, but the best we can expect there is delay of functional loss, and this will be much harder to measure and to be certain of.

Aguirre: In your population of autosomal dominant RP, I’d imagine that many of these patients have rhodopsin mutations. What is the percentage of patients that have type 2 versus type 1?

Bird: The work was done back in the 1980s. Rhodopsin mutations were found in several of those families, and we found that the mutations were in both categories. Most were type 1.

Daiger: I agree with the need for the patient registries that you described. Unfortunately, this would be di⁄cult to do in the USA, where we have human subjects constraints and a new law called HIPA, with draconian consequences for loss of con¢dentiality of patient material. Will this be easier in Europe?

Bird: We are extraordinarily fortunate in London in that Marcel Jay established a register in the 1970s, which now contains over 5000 families. Some of these are very big, with 100^200 a¡ected, traced back to the mid-19th century. We have close to 100% ascertainment of X-linked RP on our register. We have not seen an X-linked RP that we cannot connect to one of our families for about ¢ve years. We have about 60% of dominant disease on our register. There was a law proposed in Europe to make genetic registers illegal, so we were just going to call it a diagnostic index. Fortunately, this law was rejected. There is a data protection act, but we are allowed to store this information. We can be consulted just as a doctor consults another doctor, and we are allowed to release information that is medically relevant.

Daiger: It is an evolving issue in the USA, but it looks as if in order to have a name in such a registry with any identifying information, we need written consent from those individuals.

Bird: All of our subjects now must give written consent and can request at any time that their name be removed. In Europe the desire of insurance companies to have access to these lists will be resisted very strongly.

Hauswirth: Regarding your comment on the gene therapy application in type 2 only being able to slow down cell death, I agree this is the current way of thinking. But there are two scenarios in which therapy might be more applicable to type 2. The ¢rst one would be if it became possible to diagnose type 2 very early in life: then you might be able to deal with the toxic product and not have the disease at all. The second would involve gene correction therapy. This is where a mutant allele that is making a dominant negative product is turned into a normal gene. This should stop the disease in its tracks.

Bird: In type 2, the loss of rhodopsin is the only factor determining loss of sensitivity. Everything else appears to be normal. It would certainly be possible to stop the disease progressing. This work all dates from the early 1980s and has attracted little attention since.

Bok: Knowing your level of curiosity, I am surprised that you haven’t gone back and correlated the two types with the genotypes, which you must have in your ¢les.

Bird: The mutations known were limited to rhodopsin. It was a small sample of about 12 families, and it was a huge amount of work. We found rhodopsin mutations in both categories.

Dryja: Do di¡erent rhodopsin mutations cause type 1 and type 2?

Bird: Yes.

Dryja: And no one mutation can be in both groups? Bird: That’s correct. It is consistent within families. Bhattacharya: Did he ever study any of the 19q families?

Bird: No, that is precisely what we are doing now: we are recruiting the 19q families for study.

Zack: Looking to the future rather than to the past, it seems that there is a nice distinction between type 1 and type 2. Is there a technology that could be developed for physicians to be able to make this distinction more accurately and easily?

Bird: One diagnostic criterion is the temporal pro¢le of visual loss as a standard question. If someone says they had no problems by day until they were in their 30s but they have never really seen very well at night, this would be an indication that it is probably type 1. I recognize that patients are not always very good witnesses, and this is inde¢nite information. If you see someone who has had poor night vision for 30 years and there is not much pigmentation, it is probably type 1. What we need is some easy, rapid way of distinguishing one from another. Of the techniques available to us now, I would argue that auto£uorescence looks like the easiest way of doing this. OCT certainly is available, but auto£uorescence is quicker and easier.

Sieving: The di¡erence between type 1 and type 2 is interesting, but one of the features on your graph of type 2 is that sensitivity loss correlates with loss of quantum catch, and in type 1 it does not. However, type 1 cluster together at the top of the graph, in a region that would correspond with cone sensitivity. This indicates that there is no gradient of loss in type 1 cases and implies that any degree of constitutive activity leads to extensive rod desensitization. This would make it di⁄cult to restore rod function unless the de¢cit is totally reversed. By contrast, in RPE65 gene transfer the strategy is to provide something that is otherwise absent (i.e. RPE65 activity), and one would expect a gradient of restoration of function which would not require total reversal of the condition. I

think the restoration of vision in type 1 would be a di⁄cult target; half-restoration would not su⁄ce; you’d have to completely restore it.

Dryja: I had the impression that the measurements of rhodopsin in the retinas of patients tended to show that the rod photoreceptor cells are present in those cases. There is plenty of rhodopsin but the rods aren’t working. The hope is that we might be able to regain rod function.

Bird: If, for example, it was a noisy cell because rhodopsin was constitutively active, and you could suppress the expression of the noisy rhodopsin, this might work.

Sieving: Yes, but you would have to completely suppress the expression of noisy rhodopsin.

Dryja: In the case of a noisy cell it would probably be very di⁄cult. But if you provided functioning pigment to patients who have a non-functioning pigment, it would be theoretically bene¢cial.

Bird: Certainly, the rhodopsin is bleaching and regenerating since the re£ectometric assessment is dependent on this.

Aguirre: You mentioned that fundus auto£uorescence would be a good way of assessing patients with type 1 disease. Why? If you have rhodopsin, you would expect that the shedding of photoreceptor tips is ongoing even if it is not functioning, so there would be a normal level of fundus auto£uorescence.

Bird: Auto£uoresence can be used to measure physical loss of cells. Take Stargardt disease as an example. If you say you have a treatment for it, if an area of heightened auto£uorescence disappears without there being evidence of cell death, you have done something to that patient that would never occur without treatment. A possible measure of therapeutic e¡ect is disappearance of heightened auto£uorescence without accompanying cell death. This would be the same for age-related macular degeneration (AMD). Focal increases in auto£uoresence always leads to geographic atrophy in the natural history of disease.

Bhattacharya: Would you recommend that in all types of retinal disease that £uorescence imaging should be a standard test?

Bird: Yes. Any standard phenotyping needs to be quick and easy for patients. The initial measurements of rhodopsin levels done by Harris Ripps demanded a huge amount of the patient. A great deal of psychophysical techniques demands a great deal from the patient. It is a matter of choosing protocols that are acceptable to patients, and auto£uorescence imaging and optical coherence tomography (OCT) are both straightforward for the patient. I would argue that they should be part of the routine of phenotyping patients.

Bhattacharya: Is this a standard in the USA?

Sieving: Both auto£uorescence imaging and OCT appear to be quite useful approaches, but unfortunately they are not yet used frequently in the USA.

Farber: How much does the imaging equipment cost?