| Feature | Loss-of-Function (LCA1) | Gain-of-Function (CORD6) |
|---|---|---|
| Typical inheritance | Autosomal recessive; usually biallelic GUCY2D variants causing GUCY2D-LCA/LCA1 (pqac-00000002, pqac-00000003, pqac-00000010) | Autosomal dominant; usually heterozygous GUCY2D variants causing cone/cone-rod dystrophy, classically CORD6 (pqac-00000001, pqac-00000009, pqac-00000010) |
| Typical mutation classes | Null or severely damaging alleles predominate: nonsense, frameshift, splice-site, and severe missense variants that abolish or markedly reduce RetGC-1 activity (pqac-00000009, pqac-00000010, pqac-00000021) | Mostly missense variants, strongly enriched in exon 13/codon 838 hotspot, altering GCAP-mediated regulation rather than eliminating protein expression (pqac-00000009, pqac-00000019, pqac-00000024) |
| RetGC-1 enzyme function | RetGC-1/RETGC-1 (retinal guanylate cyclase-1) is absent or catalytically impaired, so cGMP cannot be adequately regenerated after phototransduction (pqac-00000002, pqac-00000011, pqac-00000013) | RetGC-1 is present but dysregulated, remaining excessively active or overly sensitive to GCAP control, causing excessive cGMP synthesis (pqac-00000009, pqac-00000010, pqac-00000011) |
| GCAP regulation | Normal Ca2+-dependent GCAP control is ineffective because enzyme activity is too low/absent to respond appropriately; recovery phase of phototransduction fails (pqac-00000011, pqac-00000013) | GCAP-dependent regulation is shifted abnormally; mutant RetGC-1 shows enhanced sensitivity or altered Ca2+ set-point, driving cyclase activity when it should be suppressed (pqac-00000010, pqac-00000011, pqac-00000012) |
| Primary biochemical defect | Failure of cGMP resynthesis in outer segments after light response (pqac-00000002, pqac-00000009, pqac-00000013) | Excess cGMP production and dysregulated cGMP signaling in darkness/recovery (pqac-00000009, pqac-00000010, pqac-00000012) |
| cGMP level consequence | cGMP becomes insufficient for normal reopening of cyclic nucleotide-gated channels; photoreceptors remain functionally silent/blind (pqac-00000009, pqac-00000011) | cGMP becomes abnormally elevated, promoting sustained channel opening and toxic signaling (pqac-00000009, pqac-00000010, pqac-00000012) |
| CNG channel status | CNG channels fail to reopen normally because cGMP is not adequately restored; phototransduction recovery is impaired (pqac-00000009, pqac-00000013) | CNG channels remain abnormally open or overactive due to high cGMP (pqac-00000009, pqac-00000010) |
| Calcium homeostasis | Abnormal signaling with impaired normal Ca2+-dependent recovery; severe dysfunction results from failure to restore the dark state (pqac-00000011, pqac-00000013) | Excess Na+ and Ca2+ influx through persistently open CNG channels causes Ca2+ overload and homeostatic failure (pqac-00000009, pqac-00000010, pqac-00000012) |
| Downstream cellular consequences | Severe photoreceptor dysfunction with markedly reduced or absent signaling despite relative anatomic preservation early in disease; blindness reflects failure of phototransduction rather than immediate cell loss (pqac-00000002, pqac-00000005, pqac-00000028) | Chronic cGMP/PKG activation, Ca2+ overload, calpain activation, ER stress, and downstream death pathways drive progressive cone-predominant degeneration (pqac-00000010, pqac-00000012) |
| Predominant photoreceptor effect | Congenital severe cone-rod or severe photoreceptor dysfunction; rods and cones both affected functionally, but central retinal structure may remain preserved for years (pqac-00000003, pqac-00000005, pqac-00000028) | Cone-dominant degeneration first, with later rod involvement; progressive cone dystrophy or cone-rod dystrophy phenotype (pqac-00000001, pqac-00000009, pqac-00000029) |
| Clinical phenotype | Leber congenital amaurosis 1 / early-onset severe retinal dystrophy with profound visual loss, nystagmus, photophobia, extinguished or severely reduced ERG (pqac-00000002, pqac-00000003, pqac-00000030) | Progressive cone dystrophy / cone-rod dystrophy with reduced visual acuity, dyschromatopsia, photophobia, macular atrophy, abnormal cone ERG and later rod involvement (pqac-00000001, pqac-00000007, pqac-00000029) |
| Age at onset | Congenital or infancy; usually first year or first 3 years of life (pqac-00000003, pqac-00000006, pqac-00000030) | Childhood to adolescence, often around the second decade, with later progressive decline (pqac-00000001, pqac-00000022, pqac-00000029) |
| Disease course | Severe but often relatively stable visual function over time in many cohorts, despite profound impairment from infancy (pqac-00000005, pqac-00000028, pqac-00000030) | Clearly progressive, with worsening BCVA over time and substantial risk of severe visual impairment by adulthood (pqac-00000029, pqac-00000034) |
| Structural preservation | Distinctive structure-function dissociation: OCT often shows preserved ellipsoid zone/outer nuclear layer centrally despite very poor vision and ERG (pqac-00000002, pqac-00000005, pqac-00000028) | Structural degeneration becomes more apparent over time, including reduced retinal thickness, loss of outer retinal bands, and macular atrophy (pqac-00000001, pqac-00000007, pqac-00000034) |
| Why gene augmentation is attractive | Strong rationale because disease is usually due to insufficient gene product/activity and many patients retain central photoreceptors structurally (pqac-00000002, pqac-00000014, pqac-00000016) | Gene augmentation alone is less straightforward because dominant toxicity/dysregulation must be suppressed or corrected, not just supplemented (pqac-00000009, pqac-00000010) |
| Suggested GO terms | GO:0007602 phototransduction; GO:0006171 cGMP biosynthetic process; GO:0050896 response to stimulus; GO:0006816 calcium ion transport; GO:0007601 visual perception (pqac-00000009, pqac-00000013) | GO:0007602 phototransduction; GO:0006171 cGMP biosynthetic process; GO:0051480 regulation of cytosolic calcium ion concentration; GO:0030168 platelet-derived growth factor receptor signaling?* not specific; GO:0097190 apoptotic signaling pathway; GO:0034976 response to endoplasmic reticulum stress (pqac-00000010, pqac-00000012) |
| Suggested pathway labels | Reactome/KEGG-relevant concepts: Phototransduction cascade; cyclic GMP signaling in photoreceptors; recovery phase of phototransduction (pqac-00000002, pqac-00000009, pqac-00000013) | Reactome/KEGG-relevant concepts: Phototransduction cascade; cGMP-PKG signaling; calcium-overload/stress-linked degeneration pathways downstream of abnormal phototransduction (pqac-00000010, pqac-00000012) |


*Table: This table compares the two major molecular mechanisms of GUCY2D-related retinopathy: recessive loss-of-function causing LCA1 and dominant gain-of-function causing CORD6. It is useful for linking genotype to biochemical dysfunction, retinal physiology, and clinical phenotype.*