FAM161A — family with sequence similarity 161 member A

The FAM161A gene provides instructions for making a protein that is essential for the health and survival of photoreceptors, the light-sensing cells in the retina at the back of the eye. This protein acts like a structural support or scaffolding within a specific part of the cell called the connecting cilium, which acts as a bridge for transporting vital materials between different parts of the photoreceptor. When the FAM161A protein is working correctly, it helps keep this bridge stable and organized. When a person inherits two mutated copies of the FAM161A gene (one from each parent), their body cannot produce functional FAM161A protein. Without this structural support, the connecting bridge in the photoreceptors becomes disorganized and collapses. As a result, the photoreceptor cells gradually deteriorate and die. This leads to a condition called retinitis pigmentosa (RP), specifically the type known as RP28. Because it requires two mutated copies to cause the disease, it is inherited in an autosomal recessive pattern. Parents who carry one mutated copy typically do not show any symptoms. For patients and their families, a diagnosis of FAM161A-related retinitis pigmentosa means they will experience a gradual loss of vision. The first symptom is usually night blindness, which often begins in childhood or early adulthood. Over time, patients lose their peripheral (side) vision, creating a "tunnel vision" effect. Fortunately, central vision and the ability to read or recognize faces are often preserved until the fourth or fifth decade of life. While there is currently no cure, research into gene therapies and other treatments is actively ongoing, offering hope for future interventions to slow or stop the vision loss.

Gene description: FAM161A is a ciliary protein involved in photoreceptor cilium maintenance and intracellular transport processes.

Patient and family guide: The FAM161A gene provides instructions for making a protein that is essential for the health and survival of photoreceptors, the light-sensing cells in the retina at the back of the eye. This protein acts like a structural support or scaffolding within a specific part of the cell called the connecting cilium, which acts as a bridge for transporting vital materials between different parts of the photoreceptor. When the FAM161A protein is working correctly, it helps keep this bridge stable and organized. When a person inherits two mutated copies of the FAM161A gene (one from each parent), their body cannot produce functional FAM161A protein. Without this structural support, the connecting bridge in the photoreceptors becomes disorganized and collapses. As a result, the photoreceptor cells gradually deteriorate and die. This leads to a condition called retinitis pigmentosa (RP), specifically the type known as RP28. Because it requires two mutated copies to cause the disease, it is inherited in an autosomal recessive pattern. Parents who carry one mutated copy typically do not show any symptoms. For patients and their families, a diagnosis of FAM161A-related retinitis pigmentosa means they will experience a gradual loss of vision. The first symptom is usually night blindness, which often begins in childhood or early adulthood. Over time, patients lose their peripheral (side) vision, creating a "tunnel vision" effect. Fortunately, central vision and the ability to read or recognize faces are often preserved until the fourth or fifth decade of life. While there is currently no cure, research into gene therapies and other treatments is actively ongoing, offering hope for future interventions to slow or stop the vision loss.

Gene function: FAM161A localizes to the connecting cilium of photoreceptors, a vital structure for transporting proteins and lipids from the inner to the outer segment. Its function is critical for maintaining the structural integrity and renewal of photoreceptor outer segments, which are responsible for light detection. Dysfunction leads to impaired ciliary transport, causing photoreceptor degeneration and retinal dystrophy.

Protein structure: The FAM161A gene encodes a protein of 660 amino acids with a calculated molecular mass of approximately 76 kDa. The protein contains a single, highly conserved domain known as UPF0564, which spans about 400 amino acid residues in the C-terminal region. This domain is crucial for the protein's function, as it mediates direct binding to microtubules and facilitates intermolecular interactions with other ciliary and centrosomal proteins. In the human retina, alternative splicing produces two main protein isoforms: a long isoform that includes exon 4 and a short isoform that lacks it. The protein does not have typical transmembrane domains but functions as a structural component of the cytoskeleton. It localizes to the basal body and connecting cilium of photoreceptors, where it assembles into the Golgi-centrosomal network and the cilia-basal body complex, acting as a scaffold to stabilize microtubules and organize the ciliary structure.

Molecular function: FAM161A encodes a ciliary-centrosomal protein that plays a crucial role in the structural maintenance and function of photoreceptor cells. It is a key component of the cilia-basal body complex and the microtubule-organizing center. FAM161A binds directly to microtubules, promoting the acetylation of α-tubulin, which is essential for the stabilization of the microtubule cytoskeleton within the connecting cilium. This structural integrity is vital for the transport of proteins and lipids between the inner and outer segments of photoreceptors. In addition to its ciliary role, FAM161A is a member of the Golgi-centrosomal interactome. It interacts with numerous other ciliary and centrosomal proteins, including LCA5 and CEP290, facilitating intracellular transport, Golgi maintenance, and centrosome organization. During the cell cycle, FAM161A follows the centrosome through all stages of mitosis. The loss of FAM161A function disrupts these intermolecular interactions and microtubule stability, leading to the disorganization of the connecting cilium, collapse of the outer segments, and eventual apoptotic death of photoreceptor cells.

Expression pattern: FAM161A is highly expressed in the retina, specifically within photoreceptor cells. It localizes to the base of the connecting cilium, the basal body region, and the adjacent centriole. In addition to photoreceptors, FAM161A expression has been detected in the inner and outer plexiform layers (synaptic regions) and the ganglion cell layer of the retina. During embryonic development in mice, low levels of Fam161a transcripts are detected throughout the optic cup. After birth, expression becomes elevated and is predominantly confined to the photoreceptor layer. Beyond the retina, lower levels of FAM161A expression have been detected in other tissues, including the brain and testis, consistent with its role as a centrosomal and ciliary protein that may participate in broader cellular functions such as microtubule stabilization and Golgi-centrosomal network maintenance.

Mutation spectrum: The mutation spectrum of FAM161A is predominantly characterized by loss-of-function variants, including nonsense mutations, frameshifts (small deletions or insertions), and splice-site alterations. These mutations typically result in premature termination codons, leading to nonsense-mediated mRNA decay or the production of truncated, nonfunctional proteins. Over 13 distinct pathogenic mutations have been identified worldwide. A significant feature of the FAM161A mutation spectrum is the presence of strong founder mutations in specific populations. In the Israeli-Jewish population, two mutations—a frameshift (c.1355_6delCA) and a nonsense mutation (c.1567C>T)—account for nearly all cases of FAM161A-associated RP, making it the most common cause of arRP in this demographic (responsible for ~20% of cases). Another nonsense mutation, c.1309A>T (p.Arg437*), has been identified as a recurrent founder allele in Dutch and Belgian populations, as well as in British and Pakistani cohorts.

Pathogenic variants: 1. p.Thr452Serfs*3 (c.1355_6delCA) - A major founder frameshift mutation in the Israeli-Jewish population, particularly among those of North African descent, leading to a premature stop codon and loss of function. 2. p.Arg523* (c.1567C>T) - A common nonsense founder mutation in the Israeli-Jewish population, causing premature protein truncation and associated with a slightly more severe clinical phenotype. 3. p.Arg437* (c.1309A>T) - A recurrent nonsense founder mutation identified in Dutch, Belgian, British, and Pakistani populations, causing typical arRP with variable severity. 4. p.Arg229* (c.685C>T) - A nonsense mutation originally identified in the large Indian family used to map the RP28 locus, resulting in early protein truncation. 5. p.Arg596* (c.1786C>T) - A nonsense mutation identified in Palestinian families, leading to loss of function and typical arRP presentation.

Clinical significance: Mutations in the FAM161A gene are a major cause of autosomal recessive retinitis pigmentosa (arRP), designated as RP28. The clinical presentation typically begins with night blindness (nyctalopia) in the first or second decade of life, followed by progressive constriction of the visual fields. Best-corrected visual acuity is often preserved through the first three decades but severely deteriorates during the fourth and fifth decades, frequently leading to legal blindness by the sixth or seventh decade. Patients typically exhibit classic RP fundus changes, including waxy pallor of the optic discs and attenuation of retinal blood vessels. However, bone spicule-like pigmentary (BSP) changes often appear relatively late and may be mild or accompanied by nummular pigmentation. A characteristic feature observed on fundus autofluorescence is a hyper-autofluorescent ring around the fovea, which can be seen even at young ages. Optical coherence tomography (OCT) shows progressive thinning of the outer nuclear layer, though the foveal structure is relatively preserved until later stages. Moderate to high myopia is also a common finding in these patients.

Inheritance: Autosomal Recessive

Chromosomal location: 2p21

Genotype-phenotype correlations: The genotype-phenotype correlation for FAM161A mutations generally falls within the typical spectrum of autosomal recessive retinitis pigmentosa, but some specific trends have been noted. The vast majority of pathogenic variants are null mutations (nonsense or frameshift) that lead to a complete loss of functional protein. Patients homozygous for nonsense mutations (e.g., p.Arg523*) have been reported to manifest a somewhat more severe disease course compared to those with other variant types, though the overall progression remains relatively slow in the early decades. Despite the genetic homogeneity in populations with founder mutations, there can be considerable phenotypic variability even among individuals with the same genotype. For instance, the c.1309A>T (p.Arg437*) mutation has been associated with a diverse clinical phenotype ranging from mild disease with preserved central acuity to severe visual impairment. This variability suggests that genetic modifiers or environmental factors may influence the severity and progression rate of FAM161A-associated retinal degeneration.

Research and therapeutic approaches: Currently, there are no FDA-approved therapies specifically for FAM161A-associated retinitis pigmentosa, and clinical management focuses on supportive care, such as low-vision aids and monitoring for complications like cataracts or macular edema. However, several investigational therapeutic strategies are in the preclinical pipeline. The most prominent approach is gene augmentation therapy using Adeno-Associated Virus (AAV) vectors to deliver a functional copy of the FAM161A gene to photoreceptors. Recent preclinical studies in Fam161a knockout mice have demonstrated that successful gene therapy requires precise regulation; specifically, using a weak promoter (FCBR1-F0.4) and co-administering vectors encoding both the long and short human isoforms of FAM161A is necessary to properly restore the connecting cilium structure and improve retinal function without causing toxicity from overexpression. Another promising pipeline strategy involves the use of translational read-through inducing drugs (TRIDs) for patients with nonsense mutations, which constitute a large portion of FAM161A cases. Preclinical in vitro studies using patient-derived fibroblasts have shown that TRIDs can induce the read-through of premature stop codons, restoring the production of full-length FAM161A protein and improving ciliogenesis and microtubule localization. These approaches are currently advancing through preclinical animal model testing to evaluate their safety and efficacy for future human clinical trials.

Diagnostic testing: Diagnosis of FAM161A-associated retinitis pigmentosa is typically achieved through comprehensive genetic testing, such as targeted inherited retinal disease (IRD) gene panels or whole exome sequencing (WES). These tests identify biallelic pathogenic variants in the FAM161A gene. Given the high prevalence of specific founder mutations in certain populations (e.g., the Israeli-Jewish population), targeted screening for known variants like c.1355_6delCA and c.1567C>T may be a first-line approach in these demographics. Genetic counseling is highly recommended for affected individuals and their families. Since the disease follows an autosomal recessive inheritance pattern, parents of an affected individual are obligate carriers, and siblings have a 25% chance of being affected. Carrier screening and prenatal testing can be offered to at-risk family members, particularly in populations with a high carrier frequency. Counselors should discuss the progressive nature of the disease, the relatively preserved central vision in early adulthood, and the potential for future therapeutic interventions.

Animal models: Two main mouse models have been developed to study FAM161A-associated retinal degeneration. The first is a GeneTrap knockout model that showed progressive retinal degeneration but was considered "leaky" as it weakly expressed a truncated form of the protein. A more recent and accurate knock-in (KI) mouse model (Fam161a tm1b/tm1b) was generated using CRISPR/Cas9 to introduce a frameshift or nonsense mutation (such as p.Arg512*, equivalent to human p.Arg523*). These models exhibit shortened and disorganized photoreceptor connecting cilia, with outer segment disks oriented perpendicularly and a wider, shorter outer segment base. The models demonstrate that loss of Fam161a leads to progressive photoreceptor degeneration, thinning of the outer nuclear layer, and gradual decline in visual acuity and electroretinographic (ERG) responses starting around 1 month of age. Molecular markers of degeneration, such as microglial activation and CALPAIN-2 expression, appear early. These models confirm that FAM161A is essential for the structural integrity of the connecting cilium and the survival of photoreceptors, providing a valuable platform for testing gene therapies and translational read-through inducing drugs (TRIDs).

Population genetics: FAM161A mutations exhibit striking population-specific prevalence due to founder effects. In the general global population, FAM161A mutations are a rare cause of autosomal recessive retinitis pigmentosa (arRP), accounting for roughly 1-2% of cases in North America and Europe. However, in the Israeli-Jewish population, FAM161A is the single most common cause of arRP, responsible for approximately 18-20% of cases. This high prevalence is driven by two major founder mutations: c.1355_6delCA (estimated carrier frequency of 1:32 in Jews of North African descent) and c.1567C>T. Another founder mutation, c.1309A>T, accounts for about 2% of arRP cases in the Dutch and Belgian populations. These founder effects highlight the importance of targeted genetic screening in specific ethnic groups.

Selected references: 1. Bandah-Rozenfeld D, et al. Homozygosity mapping reveals null mutations in FAM161A as a cause of autosomal-recessive retinitis pigmentosa. Am J Hum Genet, 2010. PMID: 20705279 2. Langmann T, et al. Nonsense mutations in FAM161A cause RP28-associated recessive retinitis pigmentosa. Am J Hum Genet, 2010. PMID: 20705278 3. Di Gioia SA, et al. FAM161A, associated with retinitis pigmentosa, is a component of the cilia-basal body complex and interacts with proteins involved in ciliopathies. Hum Mol Genet, 2012. PMID: 22940612 4. Beryozkin A, et al. Unique combination of clinical features in a large cohort of 100 patients with retinitis pigmentosa caused by FAM161A mutations. Sci Rep, 2020. PMID: 32938965 5. Arsenijevic Y, et al. Fine-tuning FAM161A gene augmentation therapy to restore retinal function. EMBO Mol Med, 2024. PMID: 38504136 6. Beryozkin A, et al. A new mouse model for retinal degeneration due to Fam161a deficiency. Sci Rep, 2021. PMID: 33441865 7. Zach F, et al. The retinitis pigmentosa 28 protein FAM161A is a novel ciliary protein involved in intermolecular protein interaction and microtubule association. Hum Mol Genet, 2012. PMID: 22791751