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Retinitis pigmentosa slowly destroys the light-sensing cells of the retina, usually starting with night vision and side vision. More than eighty different genes can cause it, and a treatment aimed at one of them helps only the people who have that particular fault. The eye is the easiest organ to reach, so here the hard part is not getting the treatment in — it is that there are dozens of separate diseases wearing one name.
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Retinitis pigmentosa encompasses inherited rod-cone dystrophies caused by variants in more than 80 genes with autosomal dominant, recessive and X-linked inheritance. RHO, RPGR, USH2A and PDE6B are among the more common. Dominant-negative RHO variants are approached by allele-specific knockdown; loss-of-function forms by correction or gene addition. Mutation-agnostic strategies — optogenetic or neuroprotective — are pursued precisely because per-gene development is economically difficult across so many small populations.
The rare-disease economics problem, concentrated
Retinitis pigmentosa is the clearest illustration of a problem that runs through this whole field. The science for a given gene may be tractable; the population carrying that gene may be a few thousand people worldwide. The discontinuation of the CEP290 programme after a scientifically encouraging trial happened for exactly this reason. Regulators are now actively rethinking approval pathways for individualised therapies in response.
Sources
- MedlinePlus Genetics, U.S. National Library of Medicine
Retinitis pigmentosa ↗