RPGRIP1 — X-linked retinitis pigmentosa GTPase regulator-interacting protein 1

The RPGRIP1 gene provides instructions for making a protein that is essential for the health and function of photoreceptors, the light-sensing cells in the retina. This protein acts like an anchor or scaffold at a critical junction in the cell called the connecting cilium, helping to organize other important proteins and ensuring that the photoreceptor cells develop and function properly. When there are harmful mutations in both copies of the RPGRIP1 gene, the protein cannot perform its anchoring job. As a result, the light-sensing parts of the photoreceptor cells fail to form correctly and eventually degenerate. This leads to severe vision problems from a very early age, such as Leber congenital amaurosis, which causes profound vision loss, involuntary eye movements, and light sensitivity starting at birth or early childhood. In some cases, it causes cone-rod dystrophy, which also leads to progressive vision loss.

Gene description: RPGRIP1 encodes a scaffolding protein localized to the connecting cilium of photoreceptors. It is essential for normal photoreceptor development and maintenance, and mutations in this gene cause severe inherited retinal dystrophies such as Leber congenital amaurosis and cone-rod dystrophy.

Patient and family guide: The RPGRIP1 gene provides instructions for making a protein that is essential for the health and function of photoreceptors, the light-sensing cells in the retina. This protein acts like an anchor or scaffold at a critical junction in the cell called the connecting cilium, helping to organize other important proteins and ensuring that the photoreceptor cells develop and function properly. When there are harmful mutations in both copies of the RPGRIP1 gene, the protein cannot perform its anchoring job. As a result, the light-sensing parts of the photoreceptor cells fail to form correctly and eventually degenerate. This leads to severe vision problems from a very early age, such as Leber congenital amaurosis, which causes profound vision loss, involuntary eye movements, and light sensitivity starting at birth or early childhood. In some cases, it causes cone-rod dystrophy, which also leads to progressive vision loss.

Gene function: RPGRIP1 functions as a scaffolding protein at the connecting cilium of photoreceptor cells. It is required for anchoring the retinitis pigmentosa GTPase regulator (RPGR) protein to the cilium and plays a crucial role in photoreceptor outer segment disk morphogenesis and ciliary protein trafficking.

Protein structure: RPGRIP1 is a multi-domain protein containing an N-terminal coiled-coil domain, two central C2 domains that may bind calcium, and a C-terminal RPGR-interacting domain (RID) that folds into a C2 domain architecture.

Molecular function: RPGRIP1 acts as a structural component and scaffolding protein at the ciliary transition zone (connecting cilium) of photoreceptors. It interacts with RPGR via its C-terminal RPGR-interacting domain (RID), tethering RPGR to the cilium. It also interacts with other ciliary proteins like NPHP4. This scaffolding function is critical for the nucleocytoplasmic shuttling, cytoskeletal organization, and vesicular trafficking required for the daily renewal and morphogenesis of photoreceptor outer segments.

Mutation spectrum: The mutation spectrum includes nonsense, frameshift deletions, missense, and splice-site alterations. Nonsense and frameshift mutations typically result in premature termination codons and null alleles, causing severe Leber congenital amaurosis. Missense mutations, which may retain partial protein function, are more commonly associated with the less severe cone-rod dystrophy phenotype.

Clinical significance: Mutations in RPGRIP1 lead to a loss of RPGR at the connecting cilium of photoreceptors, disrupting normal photoreceptor outer segment disk morphogenesis and organization. This causes rapid degeneration of both rod and cone photoreceptors, leading to severe visual deficiency, nystagmus, and early-onset blindness in Leber congenital amaurosis, or progressive vision loss in cone-rod dystrophy.

Inheritance: Autosomal Recessive

Chromosomal location: 14q11.2

Research and therapeutic approaches: Gene replacement therapy is currently under development for RPGRIP1-associated retinal dystrophies. Preclinical studies using adeno-associated virus (AAV) vectors, such as the Anc80 vector, to deliver a functional copy of the human RPGRIP1 gene have shown efficacy in preserving photoreceptors and slowing disease progression in animal models, with clinical trials being prepared.