This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "photoreceptor_degeneration#Rod Photoreceptor Apoptosis"). The module defines the expected pathophysiology structure; conforming nodes in disorder files should include the corresponding cell types, biological processes, and causal edges, specialized to the gene class involved. Key gene-class substitutions conforming nodes should make at the trigger node: phototransduction defects (e.g., RHO, PDE6B, CNGB1), photoreceptor ciliary/structural defects (e.g., RPGR, RP1, peripherin/PRPH2), RPE visual-cycle defects (e.g., RPE65, ABCA4), and pre-mRNA splicing-factor defects (e.g., PRPF31).
Photoreceptor Gene Defect
trigger
The initiating lesion is a mutation in a gene required for photoreceptor function or homeostasis. Across retinitis pigmentosa subtypes the gene class varies - phototransduction-cascade components, outer-segment and connecting-cilium structural proteins, RPE visual-cycle enzymes, or pre-mRNA splicing factors - but the mutation is expressed in or critically impairs rod photoreceptors. The defect disrupts phototransduction, outer-segment renewal, or retinol metabolism, destabilizing the photoreceptor before overt cell death.
Downstream
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Photoreceptor Metabolic and Oxidative Stress
Rod Photoreceptor Apoptosis
central effector
Accumulated metabolic and oxidative stress, dysregulated second messengers, and unfolded-protein-response signaling converge on programmed death of rod photoreceptors. Apoptosis is the central, conserved effector mechanism of rod loss across retinitis pigmentosa subtypes, although calpain-mediated and other non-apoptotic death pathways can also contribute. Because the disease mutation is expressed in rods, this is the primary, cell-autonomous death step.
Downstream
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Secondary Cone Degeneration and Outer Retinal Thinning
Secondary Cone Degeneration and Outer Retinal Thinning
effector
After rods are largely eliminated, cone photoreceptors degenerate in a characteristic non-cell-autonomous pattern even though the disease mutation is usually not expressed in cones. Proposed drivers include loss of rod-derived trophic support, nutrient and metabolic shortage, oxidative damage from the high post-rod-loss oxygen environment, and microglial inflammation. Progressive photoreceptor loss thins the outer retina. Cone death is the step that converts night blindness into disabling loss of daytime and central vision.
Downstream
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Progressive Visual Field Loss and Blindness
Progressive Visual Field Loss and Blindness
consequence
The clinical phenotype of retinitis pigmentosa / rod-cone dystrophy. Rod loss first produces night blindness (nyctalopia), then constriction of the peripheral visual field (tunnel vision), with central vision relatively preserved until cone degeneration advances. Eventually central vision is lost, progressing toward legal blindness. This is the conserved terminal consequence shared across all genetic subtypes.