PRPF3 — pre-mRNA processing factor 3

The PRPF3 gene provides instructions for making a protein called pre-mRNA processing factor 3. This protein is an essential part of a complex cellular machine called the spliceosome. The spliceosome acts like a genetic editor, cutting and pasting the raw genetic instructions (RNA) so they can be translated into functional proteins. While the PRPF3 protein is needed in cells throughout the entire body, it is particularly crucial for the health and survival of cells in the retina, the light-sensitive tissue at the back of the eye. When there is a mutation (a harmful change) in the PRPF3 gene, the spliceosome cannot assemble or function correctly. Because the cells in the retina have a very high demand for producing specific proteins to detect light and maintain vision, they are uniquely sensitive to these editing errors. Over time, the stress caused by inefficient genetic editing leads to the damage and death of the light-sensing cells (photoreceptors) and their supporting cells in the retina. Mutations in the PRPF3 gene cause a condition called retinitis pigmentosa 18 (RP18). This is an inherited eye disease that typically begins with night blindness in childhood or adolescence. As the disease progresses, patients gradually lose their peripheral (side) vision, creating a "tunnel vision" effect, and eventually, their central vision may also be affected. PRPF3 mutations are inherited in an autosomal dominant pattern, meaning that a person only needs one mutated copy of the gene (inherited from either parent) to develop the condition, and they have a 50% chance of passing it on to each of their children.

Gene description: This gene encodes a component of the U4/U6.U5 tri-snRNP, which is a key part of the spliceosome responsible for pre-mRNA splicing.

Patient and family guide: The PRPF3 gene provides instructions for making a protein called pre-mRNA processing factor 3. This protein is an essential part of a complex cellular machine called the spliceosome. The spliceosome acts like a genetic editor, cutting and pasting the raw genetic instructions (RNA) so they can be translated into functional proteins. While the PRPF3 protein is needed in cells throughout the entire body, it is particularly crucial for the health and survival of cells in the retina, the light-sensitive tissue at the back of the eye. When there is a mutation (a harmful change) in the PRPF3 gene, the spliceosome cannot assemble or function correctly. Because the cells in the retina have a very high demand for producing specific proteins to detect light and maintain vision, they are uniquely sensitive to these editing errors. Over time, the stress caused by inefficient genetic editing leads to the damage and death of the light-sensing cells (photoreceptors) and their supporting cells in the retina. Mutations in the PRPF3 gene cause a condition called retinitis pigmentosa 18 (RP18). This is an inherited eye disease that typically begins with night blindness in childhood or adolescence. As the disease progresses, patients gradually lose their peripheral (side) vision, creating a "tunnel vision" effect, and eventually, their central vision may also be affected. PRPF3 mutations are inherited in an autosomal dominant pattern, meaning that a person only needs one mutated copy of the gene (inherited from either parent) to develop the condition, and they have a 50% chance of passing it on to each of their children.

Gene function: PRPF3 plays a crucial role in the splicing of pre-mRNA, which is essential for the production of functional proteins. In the retina, proper splicing is vital for maintaining the integrity and function of photoreceptor cells and the retinal pigment epithelium. Mutations can disrupt this process, leading to the accumulation of abnormal proteins or reduced levels of essential ones, ultimately causing retinal degeneration and vision loss.

Protein structure: The PRPF3 gene encodes a protein of 683 amino acids with a molecular weight of approximately 77 kDa. The protein structure includes several distinct domains that are critical for its function in the spliceosome. It contains a PWI domain, a Prp3 domain, and a highly conserved C-terminal domain. The protein is characterized by multiple alpha-helices and beta-strands that facilitate its complex interactions with RNA and other proteins. The C-terminal domain of the PRPF3 protein is particularly important for its assembly into functional complexes. This region mediates the interaction with the U4/U6 snRNA duplex and is essential for binding to other spliceosomal proteins, including Prpf4, Prpf6, and PPIH, to form the U4/U6.U5 tri-snRNP complex. Notably, all known pathogenic mutations associated with retinitis pigmentosa are clustered in this highly conserved C-terminal region, underscoring its critical role in maintaining the stability and function of the spliceosome. The protein also undergoes post-translational modifications, including phosphorylation at specific residues (such as Thr494), which regulates its interactions and activity within the spliceosome.

Molecular function: The PRPF3 gene encodes the pre-mRNA processing factor 3 (Prp3), a highly conserved 90 kDa protein that is an essential component of the spliceosome, the macromolecular complex responsible for removing introns from precursor messenger RNAs (pre-mRNAs). Specifically, Prp3 is a core component of the U4/U6 small nuclear ribonucleoprotein (snRNP) and the U4/U6.U5 tri-snRNP complex. Within the spliceosome, Prp3 plays a critical role in stabilizing the U4/U6 snRNA duplex and facilitating the assembly of the U4/U6.U5 tri-snRNP. It forms a stable trimeric complex with Prpf4 and PPIH, binding to the U4/U6 snRNA duplex primarily via interactions with U6 and U4/U6 RNA. Prp3 further stabilizes the tri-snRNP by interacting with other splicing factors, such as Prpf6 and Sart1. The C-terminus of Prp3 is also required for binding to RP9 (PAP-1), another protein implicated in autosomal dominant retinitis pigmentosa. Mutations in PRPF3 disrupt these critical interactions and impair spliceosome assembly. For example, the common p.Thr494Met mutation reduces Prp3 phosphorylation, impairs its association with Prpf4 and the U4/U6 snRNP, and causes the mutant protein to form aggregates in the nucleus. This leads to altered levels of snRNPs, delayed spliceosome assembly, and inefficient pre-mRNA splicing. The retina's unique vulnerability to these ubiquitous splicing defects is thought to arise from the exceptionally high demand for rapid and accurate splicing of retina-specific transcripts required for photoreceptor maintenance and function.

Expression pattern: The PRPF3 gene is ubiquitously expressed across a wide range of human tissues, consistent with its fundamental role in the core pre-mRNA splicing machinery. However, despite this ubiquitous expression, mutations in PRPF3 lead to a highly tissue-specific phenotype restricted to the retina. Within the eye, PRPF3 is highly expressed in both the neural retina and the retinal pigment epithelium (RPE). Studies in animal models have shown that PRPF3 expression is developmentally regulated and particularly high in retinal cells compared to other tissues. The high metabolic demand and rapid turnover of transcripts in photoreceptors and the RPE may make these cells uniquely vulnerable to subtle defects in spliceosome function or assembly caused by PRPF3 mutations.

Mutation spectrum: The mutation spectrum of the PRPF3 gene is remarkably narrow, with only a few pathogenic variants identified to date. Unlike many other genes associated with retinitis pigmentosa that harbor a wide variety of mutation types (e.g., nonsense, frameshift, splice-site), pathogenic variants in PRPF3 are almost exclusively missense mutations. These missense mutations are highly clustered in a specific hotspot region within exon 11, which encodes the highly conserved C-terminal domain of the protein. This domain is critical for protein-protein interactions within the spliceosome. The clustering of missense mutations and the absence of truncating mutations strongly suggest that PRPF3-associated retinitis pigmentosa is caused by a dominant-negative effect or a toxic gain of function, rather than haploinsufficiency.

Pathogenic variants: 1. p.Thr494Met (c.1481C>T) - The most common and extensively studied pathogenic variant in PRPF3. It causes classic autosomal dominant retinitis pigmentosa (RP18) and has been shown to impair protein phosphorylation, disrupt interactions with other spliceosome components, and cause nuclear aggregation of the mutant protein. 2. p.Pro493Ser (c.1477C>T) - A well-characterized missense mutation located adjacent to the Thr494 residue in the highly conserved C-terminal domain. It is also associated with autosomal dominant retinitis pigmentosa. 3. p.Ala489Asp (c.1466C>A) - A rarer missense mutation found in the same C-terminal hotspot region, identified in families with autosomal dominant retinitis pigmentosa.

Clinical significance: Mutations in the PRPF3 gene are a rare cause of autosomal dominant retinitis pigmentosa (adRP), specifically designated as retinitis pigmentosa 18 (RP18). Clinically, PRPF3-associated adRP manifests with classic features of the disease, including early-onset nyctalopia (night blindness), which typically begins in the first or second decade of life, though some patients may experience a later onset. This is followed by progressive loss of peripheral visual fields and eventual decline in central visual acuity. Ophthalmic examination of affected individuals reveals characteristic fundus changes, such as vessel attenuation, optic disc pallor, and mid-peripheral bone spicule pigmentation that progresses to retinal atrophy in later stages. Retinal pigment epithelium (RPE) mottling is also common, although the fundus may appear normal in the early stages of the disease. Electroretinography (ERG) typically shows severe rod-cone dystrophy with responses that are often extinguished by the fourth decade of life. The disease exhibits variable expressivity, meaning the severity and progression rate can vary significantly even among affected members of the same family. No systemic or extraocular anomalies have been reported in association with PRPF3 mutations.

Inheritance: Autosomal Dominant

Chromosomal location: 1q21.2

Genotype-phenotype correlations: Genotype-phenotype correlations in PRPF3-associated retinitis pigmentosa are challenging to establish definitively due to the rarity of the condition and the limited number of known pathogenic variants. The disease is characterized by variable expressivity, with significant differences in the age of onset, disease severity, and rate of progression observed even among individuals carrying the same mutation within a single family. The most common mutation, p.Thr494Met, is generally associated with classic, early-onset autosomal dominant retinitis pigmentosa. However, some patients with this mutation maintain good central vision into their later decades, while others experience severe visual impairment much earlier. The variable expressivity suggests that other genetic modifiers, environmental factors, or stochastic events may influence the clinical presentation and progression of the disease in individuals with PRPF3 mutations.

Research and therapeutic approaches: Currently, there are no FDA-approved therapies specifically targeting PRPF3-associated retinitis pigmentosa. Management is primarily supportive, focusing on maximizing remaining vision through low-vision aids, regular monitoring for complications like cataracts or macular edema, and genetic counseling. Unlike RPE65-associated retinal dystrophy, which has an approved gene therapy (Luxturna), PRPF3 mutations present unique challenges for therapeutic development because they cause disease through a dominant-negative or toxic gain-of-function mechanism rather than simple loss of function. Research into therapeutic approaches for PRPF3-associated disease is ongoing and primarily in the preclinical stages. Because adding a healthy copy of the gene (traditional gene augmentation) may not be sufficient to overcome the toxic effects of the mutant protein, researchers are exploring alternative strategies. These include allele-specific knockdown approaches using RNA interference (RNAi) or antisense oligonucleotides (ASOs) to selectively silence the mutant PRPF3 allele while preserving the expression of the wild-type allele. Additionally, CRISPR/Cas9 gene editing is being investigated as a potential method to directly correct the mutation at the genomic level. While these approaches show promise in cellular and animal models, they have not yet advanced to clinical trials for PRPF3 patients.

Diagnostic testing: Diagnostic testing for PRPF3-associated retinitis pigmentosa typically involves next-generation sequencing (NGS) approaches. Targeted retinal dystrophy gene panels that include PRPF3 are the most common and cost-effective initial test for patients presenting with clinical signs of retinitis pigmentosa. If panel testing is inconclusive, whole exome sequencing (WES) or whole genome sequencing (WGS) may be employed to identify pathogenic variants. Genetic counseling is an essential component of the diagnostic process. Since PRPF3 mutations cause autosomal dominant retinitis pigmentosa, affected individuals have a 50% chance of passing the pathogenic variant to each of their offspring. Genetic counselors play a crucial role in explaining the inheritance pattern, discussing the variable expressivity and potential age of onset, and facilitating cascade testing for at-risk family members. Prenatal testing and preimplantation genetic testing may also be discussed as options for family planning.

Animal models: Animal models have been instrumental in understanding the pathogenesis of PRPF3-associated retinitis pigmentosa. Initial studies using Prpf3 knockout mice and zebrafish models demonstrated that decreased levels of the RNA splicing factor Prpf3 do not cause photoreceptor degeneration, suggesting that the disease mechanism is not haploinsufficiency but rather a toxic gain of function or dominant-negative effect caused by missense mutations. Homozygous Prpf3 gene trap mice are embryonic lethal before embryonic day 14, indicating that Prpf3 is essential for early development. To better mimic the human disease, researchers developed knock-in mouse models carrying the most common disease-causing mutation, Prpf3-T494M. These knock-in mice exhibit late-onset pathological changes primarily in the retinal pigment epithelium (RPE), including loss of basal infoldings, vacuolization, and accumulation of deposits, which are more severe in homozygous animals. These changes are associated with decreased rod function and defective RPE phagocytosis, highlighting the critical role of the RPE in the pathogenesis of PRPF3-associated retinal degeneration.

Population genetics: Pathogenic variants in the PRPF3 gene are a very rare cause of retinitis pigmentosa, accounting for approximately 1% of all autosomal dominant retinitis pigmentosa (adRP) cases worldwide. Because the condition is autosomal dominant and highly penetrant, the concept of a "carrier frequency" (which typically applies to recessive conditions where individuals can carry a mutation without showing symptoms) is less relevant; individuals with a pathogenic PRPF3 variant will generally develop the disease. The prevalence of PRPF3 mutations varies slightly among different populations, but no major founder effects have been widely reported. The p.Thr494Met mutation is the most frequently observed variant across various ethnic groups, suggesting it may represent a mutational hotspot rather than a single ancestral founder mutation.

Selected references: 1. Růžičková Š, Staněk D. Mutations in spliceosomal proteins and retina degeneration. RNA Biol. 2017;14(5):544-552. PMID: 27982730 2. Graziotto JJ, et al. Three gene-targeted mouse models of RNA splicing factor RP show late-onset RPE phagocytosis defects and abnormal visual function. Invest Ophthalmol Vis Sci. 2011;52(1):190-198. PMID: 20811066 3. Comitato A, et al. Mutations in splicing factor PRPF3, causing retinal degeneration, form detrimental aggregates in photoreceptor cells. Hum Mol Genet. 2007;16(14):1699-1707. PMID: 17517693 4. Graziotto JJ, et al. Decreased levels of the RNA splicing factor Prpf3 in mice and zebrafish do not cause photoreceptor degeneration. Invest Ophthalmol Vis Sci. 2008;49(9):3830-3838. PMID: 18552388 5. Vaclavik V, et al. Variable phenotypic expressivity in a Swiss family with autosomal dominant retinitis pigmentosa due to a T494M mutation in the PRPF3 gene. Mol Vis. 2010;16:467-475. PMID: 20300566 6. Martínez-Gimeno M, et al. Mutations in the pre-mRNA splicing-factor genes PRPF3, PRPF8, and PRPF31 in Spanish families with autosomal dominant retinitis pigmentosa. Invest Ophthalmol Vis Sci. 2003;44(5):2171-2177. PMID: 12714658