Inherited Retinal Dystrophy Genetics

Overview

Well over 100 genes can cause an inherited retinal dystrophy (IRD), because the phototransduction cascade and the photoreceptor-RPE interface depend on many proteins. You don't need to memorize them as a list. For each disease, ask three questions: which cells fail first, does it get worse over time, and which one gene is the classic association?

Two terms explain why the genetics can feel confusing:

• Locus heterogeneity: many genes, one disease. Example: dozens of different genes can cause retinitis pigmentosa. Think "many Locations, one picture."
• Allelic heterogeneity: one gene, different diseases, depending on the mutation. Example: different RPGR mutations cause either X-linked RP or X-linked cone-rod dystrophy. Think "same gene, different Alleles, different picture."

Retinitis Pigmentosa

Example: a 25-year-old who struggles to find a seat in a dark cinema and keeps bumping into door frames, while reading vision is still fine.

In retinitis pigmentosa (RP) the rods die first, so nyctalopia (night blindness) comes first. A ring scotoma then forms and peripheral fields constrict toward the center. The fundus shows bone-spicule pigmentation in the mid-periphery, as pigment-laden cells migrate around vessels, with attenuated arterioles and waxy disc pallor.

Inheritance varies. Worldwide, autosomal recessive RP is most common (roughly 50 to 60%), then autosomal dominant (30 to 40%), then X-linked (5 to 15%). Autosomal dominant is the most common form in European populations, so a "most common" question depends on the population it asks about.

The Three Genes to Know

• RPGR, X-linked RP. The most severe form, with earlier onset and faster progression. It accounts for roughly 70 to 75% of X-linked RP and up to 10 to 20% of all RP. The protein sits in the photoreceptor connecting cilium, the narrow bridge proteins cross to reach the outer segment, so a defect disrupts that transport.
• RHO, autosomal dominant RP. The most common adRP gene in European populations, roughly 25 to 30% of cases, with over 150 known variants. Mutations act by disrupting disc assembly in a dominant-negative way, by misfolding with ER stress, or by constitutive light-independent activation.
• PRPF31, the disease that skips a generation. It encodes a pre-mRNA splicing factor and is the classic ophthalmic example of reduced penetrance. Disease occurs through haploinsufficiency, only when the normal allele cannot make enough protein. Carriers whose normal allele is expressed at a higher level (regulated by a modifier locus near CNOT3) stay unaffected. Example: an affected grandfather and grandchild, with a healthy parent in between who carries the same mutation. A negative family history doesn't rule it out.

Congenital Stationary Night Blindness

Example: a 9-year-old with lifelong, unchanging trouble seeing in dim light, whose vision and fundus look normal.

Congenital stationary night blindness (CSNB) is what the name says: night blindness from birth that stays the same. The photoreceptors don't die. Most forms are a signal failure at the synapse between photoreceptor and bipolar cell, which produces a negative ERG on dark-adapted bright-flash testing: a normal a-wave (photoreceptors work) with a reduced b-wave (the next cell doesn't respond). This is the Schubert-Bornschein pattern.

Complete and Incomplete CSNB

• Complete CSNB: only the ON pathway fails. Mutations in NYX (X-linked) or TRPM1, GPR179, or LRIT3 (autosomal recessive) disrupt the mGluR6 signaling cascade in ON-bipolar cells.
• Incomplete CSNB: both ON and OFF pathways fail. CACNA1F (X-linked) encodes the Cav1.4 calcium channel at the photoreceptor terminal, which drives glutamate release. Less glutamate reaches both pathways, so the ERG is abnormal under both light- and dark-adapted conditions.

Night Blindness With an Abnormal Fundus

• Oguchi disease (autosomal recessive; SAG or GRK1, the two proteins that switch rhodopsin off). It shows the Mizuo-Nakamura phenomenon: a golden-yellow fundus in the light that looks normal after prolonged dark adaptation, and turns gold again within minutes of light exposure. Stationary.
• Fundus albipunctatus (RDH5, retinoid recycling). Scattered yellow-white dots that spare the macula. The scotopic ERG looks markedly reduced at first but largely recovers after several hours in the dark, because the chromophore regenerates slowly rather than not at all. Stationary.

Two Look-Alikes That Are Not CSNB

• Retinitis punctata albescens looks almost identical to fundus albipunctatus, but it progresses, with ERG amplitudes and fields worsening over time. Follow-up, not the fundus photo, separates the two.
• Enhanced S-cone syndrome (NR2E3). Developing rod precursors become S-cones instead, so there is little real rod signal. Light- and dark-adapted ERGs look nearly identical, and a blue flash evokes an abnormally large response. It is slowly progressive. The more severe form, Goldmann-Favre syndrome, adds peripheral retinal degeneration and foveal schisis.

Leber Congenital Amaurosis

Example: a 3-month-old with poor fixation, roving eye movements, and constant eye rubbing, whose fundus looks close to normal.

Leber congenital amaurosis (LCA) is the most severe end of the IRD spectrum, with severe visual impairment from birth. Look for poor fixation and roving nystagmus in the first months, the oculodigital (Franceschetti) sign of persistent eye poking and rubbing, and sluggish or nearly absent pupillary reflexes. The fundus can look near normal early on, so full-field ERG settles the diagnosis: flat or severely reduced in both eyes.

LCA2 is caused by RPE65 mutations, a gene in the visual cycle that recycles retinal in the RPE. Subretinal AAV2 gene therapy was proven first in Briard dogs and then in humans, leading to voretigene neparvovec (Luxturna), FDA approved in 2017 as the first approved in vivo gene therapy for a genetic disease of any kind.

Macular Dystrophies

Stargardt Disease: ABCA4

Example: a teenager with falling central vision whose fundus shows yellow flecks.

Stargardt disease is the most common inherited macular dystrophy of childhood, caused by autosomal recessive ABCA4 mutations (formerly ABCR). ABCA4 is a transporter in rod outer segment disc membranes that exports N-retinylidene-phosphatidylethanolamine. Without it, toxic bisretinoid lipofuscin (A2E) builds up, the RPE atrophies, and photoreceptors are lost. The fundus shows a "beaten-bronze" fovea surrounded by pisciform (fish-shaped) yellow-white flecks.

Best Disease: BEST1

Example: a child with an egg-yolk lesion at the fovea and a parent with the same finding.

Best vitelliform macular dystrophy is caused by autosomal dominant BEST1 mutations. Bestrophin-1 is a calcium-activated chloride channel in the RPE's basolateral membrane, and its failure impairs fluid transport across the RPE. The result is a vitelliform ("egg-yolk") subfoveal lesion and a markedly subnormal Arden ratio on electrooculogram (EOG), detectable even in carriers before any lesion appears.

Cone-Rod vs. Rod-Cone Dystrophy

The first word of the name tells you which cell dies first, and that gives you the symptoms.

ABCA4, GUCY2D, CRX, and RPGR can all cause cone-rod dystrophy.

Syndromic Retinal Dystrophies

Usher Syndrome

Usher syndrome is RP with hearing loss, and the most common cause of combined deaf-blindness worldwide.

Bardet-Biedl Syndrome: A Ciliopathy

Bardet-Biedl syndrome (BBS) is a rod-cone dystrophy with systemic features. BBS genes encode the BBSome, an octameric protein complex that controls intraflagellar transport and membrane protein trafficking in the photoreceptor connecting cilium. This is the same structure affected in RPGR disease, which is why both are called ciliopathies.

Choroideremia and Its Carrier Phenotype

Choroideremia is an X-linked progressive chorioretinal degeneration caused by CHM mutations. CHM encodes Rab escort protein 1 (REP-1), needed for prenylation and trafficking of Rab GTPases in the RPE and choroid.

Example: a man with progressive field loss whose mother has good central vision but a patchy fundus. Random X-inactivation (the Lyon hypothesis) leaves female carriers with a mosaic of patchy chorioretinal atrophy, because some cells use the mutant X and others the normal one. Most keep normal central vision, though some develop symptomatic peripheral field loss over time.

Albinism

All forms share reduced melanin in the developing RPE, which disrupts two things: the fovea and the wiring at the chiasm.

• Foveal hypoplasia: an underdeveloped or absent foveal pit, which lowers acuity and is commonly associated with photophobia and congenital nystagmus.
• Chiasmal misrouting: too many temporal retinal fibers cross at the chiasm. This gives an asymmetric cortical response on visual evoked potential (VEP) testing, the Apkarian VEP pattern, a measurable sign of the misrouting itself.

The Three Forms

• Oculocutaneous albinism type 1 (TYR): mutations in tyrosinase, the rate-limiting enzyme in melanin synthesis. OCA1A has no enzyme activity: white hair, white skin, translucent irides. OCA1B has reduced activity, so some pigment builds up with age and the phenotype is milder.
• Ocular albinism type 1, Nettleship-Falls (GPR143/OA1): X-linked, with foveal hypoplasia, nystagmus, and reduced acuity, while skin and hair look relatively normal. Carrier females show a mosaic fundus pigment pattern.
• Hermansky-Pudlak syndrome: a rare form of oculocutaneous albinism caused by defects in BLOC complexes and the AP-3 adaptor complex, the machinery that builds lysosome-related organelles. These include melanosomes (albinism), platelet dense granules (bleeding from storage pool deficiency), and pulmonary lamellar bodies (pulmonary fibrosis). Pulmonary fibrosis is the most lethal complication, so patients need monitoring beyond the eye.

High-Yield Pearls

Related Topics

Continue exploring Basic Sciences: