SPATA7 — SPATA7, spermatogenesis associated protein 7

The SPATA7 gene provides instructions for making a protein that is essential for the health and function of the retina, the light-sensitive tissue at the back of the eye. Specifically, this protein acts like a bridge or scaffold in the "connecting cilium," a narrow passageway in the eye's light-detecting cells (photoreceptors). It helps transport other vital proteins from where they are made to where they are needed to detect light. When the SPATA7 gene is mutated, the protein it produces is either missing or doesn't work correctly. This disrupts the transport system within the photoreceptor cells, causing essential proteins to build up in the wrong places. As a result, the photoreceptor cells become damaged and eventually die, leading to vision loss. Depending on the specific mutation, this can cause Leber congenital amaurosis (LCA), which results in severe vision impairment from birth or early infancy, or juvenile retinitis pigmentosa (RP), which causes progressive vision loss starting in childhood. These conditions are inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the SPATA7 gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, typically do not show any symptoms of the condition. For families affected by SPATA7 mutations, genetic counseling can provide valuable information about the risks to future children and help them understand the disease and potential future treatments.

Gene description: Encodes a protein associated with spermatogenesis and essential for photoreceptor outer segment development and maintenance.

Patient and family guide: The SPATA7 gene provides instructions for making a protein that is essential for the health and function of the retina, the light-sensitive tissue at the back of the eye. Specifically, this protein acts like a bridge or scaffold in the "connecting cilium," a narrow passageway in the eye's light-detecting cells (photoreceptors). It helps transport other vital proteins from where they are made to where they are needed to detect light. When the SPATA7 gene is mutated, the protein it produces is either missing or doesn't work correctly. This disrupts the transport system within the photoreceptor cells, causing essential proteins to build up in the wrong places. As a result, the photoreceptor cells become damaged and eventually die, leading to vision loss. Depending on the specific mutation, this can cause Leber congenital amaurosis (LCA), which results in severe vision impairment from birth or early infancy, or juvenile retinitis pigmentosa (RP), which causes progressive vision loss starting in childhood. These conditions are inherited in an autosomal recessive pattern. This means that a person must inherit two mutated copies of the SPATA7 gene—one from each parent—to develop the disease. The parents, who each carry one mutated copy, typically do not show any symptoms of the condition. For families affected by SPATA7 mutations, genetic counseling can provide valuable information about the risks to future children and help them understand the disease and potential future treatments.

Gene function: SPATA7 is crucial for the structural integrity and function of photoreceptor outer segments in the retina. Mutations lead to their degeneration, impacting light detection and causing vision loss. It is involved in processes vital for maintaining healthy photoreceptor cells.

Protein structure: The SPATA7 gene encodes the spermatogenesis-associated protein 7, which in humans is a 599-amino acid protein. It contains several DNA-binding sites and three phosphorylation sites. The protein exists in at least two isoforms due to alternative splicing. SPATA7 functions as a putative scaffold protein. It localizes to the microtubule network and the ciliary axoneme of ciliated cells. In the retina, it is a critical component of the connecting cilium of photoreceptors, where it interacts directly with the coiled-coil domain of the RPGRIP1 protein, facilitating the stable assembly and localization of the ciliary RPGRIP1 protein complex.

Molecular function: The SPATA7 gene encodes a spermatogenesis-associated protein that functions as a ciliary protein. In the retina, it is essential for the proper assembly and localization of the ciliary RPGRIP1 protein complex at the connecting cilium of photoreceptor cells. SPATA7 directly interacts with and binds to the coiled-coil domain of RPGRIP1. This interaction is critical for protein trafficking across the connecting cilium from the inner segment to the outer segment of photoreceptors. Loss of SPATA7 function leads to a substantial reduction in RPGRIP1 levels at the connecting cilium and its mislocalization to the inner segment. Consequently, essential outer segment proteins, such as rhodopsin, accumulate in the inner segments and around the nucleus, triggering apoptotic degeneration of the photoreceptor cells. Thus, SPATA7 plays a vital role in photoreceptor cell maintenance and visual perception.

Expression pattern: SPATA7 is broadly expressed in several tissues, with high levels found in the testis, brain, and retina. In the retina, it is expressed in multiple layers and is specifically localized to the connecting cilium between the inner and outer segments of both rod and cone photoreceptor cells. It is also found in the retinal pigment epithelium (RPE). During development, SPATA7 expression in the mouse retina progressively increases, coinciding with the development of photoreceptors. It is first clearly detected at postnatal day 4 and shows the strongest immunoreactivity in the photoreceptor cell layer by postnatal day 15. In the testis, it is localized to primary spermatocytes in the early prophase of meiosis I.

Mutation spectrum: The mutation spectrum of SPATA7 includes nonsense, frameshift (deletions and duplications), and missense mutations, as well as large deletions. The vast majority (over 90%) of mutant alleles are truncation mutations (nonsense and frameshift), which lead to a premature stop codon and likely result in a loss of protein function. Missense mutations and non-frameshift indels are much less common. Mutations are distributed across the gene, but specific locations may correlate with disease severity. For instance, mutations in the middle of the coding region are often linked to LCA, while those in the terminal exons are linked to juvenile RP. Several recurrent mutations have been identified in specific populations, such as the p.Arg108Ter mutation in Saudi Arabian and Dutch patients, and the p.Arg395Ter mutation in Portuguese patients.

Pathogenic variants: 1. p.Arg108Ter (c.322C>T) - A nonsense mutation found in exon 5, associated with Leber congenital amaurosis (LCA3) and juvenile retinitis pigmentosa. It has been identified in Saudi Arabian, Dutch, and Spanish patients. 2. p.Arg395Ter (c.1183C>T) - A nonsense mutation in exon 11, associated with juvenile retinitis pigmentosa. It has been identified in Portuguese patients. 3. c.961dupA - A 1-bp duplication in exon 8 causing a frameshift and premature termination, associated with LCA3. It has been found in patients of Middle Eastern and Pakistani origin. 4. c.1546delA - A 1-bp deletion in exon 12 causing a frameshift, associated with juvenile retinitis pigmentosa. It has been identified in French Canadian patients. 5. p.Gln123Ter (c.367C>T) - A nonsense mutation associated with LCA, identified in Chinese patients.

Clinical significance: Mutations in the SPATA7 gene are a rare cause of childhood retinal dystrophy, accounting for approximately 1.7% of cases in some cohorts. They primarily cause Leber congenital amaurosis type 3 (LCA3) and juvenile-onset retinitis pigmentosa (RP94). LCA3 is characterized by early-onset visual impairment, often presenting in infancy with poor visual pursuit, roving nystagmus, and amaurotic pupils. Visual acuity is typically worse than 6/60, and visual fields are severely constricted. Juvenile RP associated with SPATA7 mutations presents with a milder, early-onset severe retinal dystrophy phenotype. Patients may have minimal or no nystagmus initially due to relatively preserved cone function. Symptoms include nyctalopia (night blindness) during early childhood, progressive visual acuity and visual field loss, and eventual severe visual impairment. Fundus changes for both conditions include narrow arterioles, widespread retinal pigment epithelium (RPE) atrophy resulting in diffuse mottled hypopigmentation in the midperipheral retina, and relative parafoveal preservation. No extraocular systemic features have been reported.

Inheritance: Autosomal Recessive

Chromosomal location: 14q32.11

Genotype-phenotype correlations: There is a general genotype-phenotype correlation observed with SPATA7 mutations. Nonsense mutations located in the middle of the SPATA7 coding region tend to be associated with the more severe Leber congenital amaurosis (LCA) phenotype. In contrast, mutations located in the last two exons of the gene are often associated with the relatively milder juvenile retinitis pigmentosa (RP) phenotype. However, phenotypic variability exists even among individuals with the same mutations. For example, siblings homozygous for the same missense mutation have exhibited different clinical presentations, such as rod-cone versus cone-rod dystrophy. This suggests that other genetic modifiers or environmental factors may influence disease penetrance, expressivity, and progression rate.

Research and therapeutic approaches: Currently, there are no FDA-approved therapies specifically for SPATA7-associated inherited retinal diseases. Management is primarily supportive, focusing on maximizing remaining vision through low-vision aids, orientation and mobility training, and regular monitoring for complications like cataracts. However, gene therapy is an active area of research for SPATA7 mutations. Preclinical studies have shown promising results using adeno-associated virus (AAV) vectors to deliver a functional copy of the Spata7 gene. Specifically, AAV8(Y733F)-mediated gene therapy in a Spata7 knockout mouse model demonstrated long-term improvement of photoreceptor function and structure, suggesting its potential as a treatment for human patients. While these approaches are still in the preclinical pipeline and have not yet reached human clinical trials, they represent a significant step toward a targeted treatment for SPATA7-related blindness.

Diagnostic testing: Mutations in SPATA7 are typically detected through next-generation sequencing approaches, including targeted retinal gene panels, whole exome sequencing, or whole genome sequencing. Clinical evaluation often involves electroretinography (ERG), which shows absent responses or severe rod-cone dystrophy, and optical coherence tomography (OCT), which may reveal preserved foveal ellipsoid zones but outer retinal loss parafoveally. Genetic counseling is essential for affected individuals and their families. Since SPATA7-related conditions are inherited in an autosomal recessive manner, parents of an affected child are obligate carriers, and each subsequent child has a 25% chance of inheriting the condition. Carrier screening and prenatal testing can be offered to at-risk family members. Genetic counselors can help families understand the inheritance pattern, prognosis, and available research or clinical trials.

Animal models: Spata7 knockout mice have been instrumental in understanding the gene's function. These mice exhibit severe early-onset retinal defects, with progressive thinning of the outer nuclear layer, indicating photoreceptor degeneration. Cone degeneration proceeds at a lower rate compared to rods. The mice show shortened outer segments and disorganized disc membranes. Rod function declines significantly by postnatal day 15 and is almost undetectable by 12 months, mimicking the human phenotype of Leber congenital amaurosis and retinitis pigmentosa. Conditional knockout models have further refined this understanding. Photoreceptor-specific deletion of Spata7 causes both rod and cone dysfunction and degeneration, while RPE-specific deletion does not impair retinal function or cell survival, confirming that Spata7 is primarily required within photoreceptors for their survival.

Population genetics: The carrier frequency for SPATA7 mutations varies by population. In a study analyzing carrier frequencies for recessive retinal diseases, the overall carrier frequency per subpopulation for various genes ranged from 1 in 2.4 to 1 in 9.3 individuals, though specific rates for SPATA7 alone are generally much lower in the general population. Certain mutations have been found more frequently in specific ethnic groups, such as the p.Arg108Ter mutation in Saudi Arabian populations and the c.961dupA mutation in Pakistani populations, suggesting possible founder effects in these consanguineous communities.

Selected references: 1. Wang H, et al. Mutations in SPATA7 cause Leber congenital amaurosis and juvenile retinitis pigmentosa. Am J Hum Genet, 2009. PMID: 19268277 2. Eblimit A, et al. Spata7 is a retinal ciliopathy gene critical for correct RPGRIP1 localization and protein trafficking in the retina. Hum Mol Genet, 2015. PMID: 25398945 3. Mackay DS, et al. Screening of SPATA7 in patients with Leber congenital amaurosis and severe childhood-onset retinal dystrophy reveals disease-causing mutations. Invest Ophthalmol Vis Sci, 2011. PMID: 21310915 4. Xiao X, et al. Spectrum, frequency, and genotype-phenotype of mutations in SPATA7. Mol Vis, 2019. PMID: 31908400 5. Zhong H, et al. AAV8(Y733F)-mediated gene therapy in a Spata7 knockout mouse model of Leber congenital amaurosis and retinitis pigmentosa. Gene Ther, 2015. PMID: 25965394 6. Eblimit A, et al. Conditional loss of Spata7 in photoreceptors causes progressive retinal degeneration in mice. Exp Eye Res, 2018. PMID: 29198616