OAT — ornithine aminotransferase

The OAT gene provides the instructions for making an important enzyme called ornithine aminotransferase. This enzyme works inside the energy-producing centers of our cells, the mitochondria. Its main job is to help break down a substance called ornithine, which is a building block of proteins that our bodies produce during normal metabolism. By breaking down ornithine, the enzyme helps keep the levels of this substance balanced and also helps create other important molecules that the body needs to function properly. When there is a mutation or mistake in the OAT gene, the body cannot produce enough working ornithine aminotransferase enzyme. Because the enzyme isn't working right, ornithine cannot be broken down, and it builds up to very high, toxic levels in the blood and the eyes. At the same time, the body might not have enough of the other important molecules that the enzyme normally helps create. This imbalance is particularly harmful to the retina, the light-sensitive tissue at the back of the eye, and the choroid, the layer of blood vessels that nourishes it. This genetic condition is called gyrate atrophy of the choroid and retina. It is an inherited disease, meaning it is passed down from parents to their children. It follows an "autosomal recessive" pattern, which means a person must inherit two mutated copies of the gene (one from each parent) to develop the disease. People with gyrate atrophy typically start experiencing night blindness and nearsightedness in childhood. Over time, they slowly lose their side (peripheral) vision, and eventually, their central vision can also be affected. While there is currently no cure, treatments like a strict, specialized diet can help lower ornithine levels and slow down the vision loss.

Gene description: This gene encodes a mitochondrial enzyme that catalyzes the interconversion of ornithine and alpha-ketoglutarate to glutamate and glutamate semialdehyde.

Patient and family guide: The OAT gene provides the instructions for making an important enzyme called ornithine aminotransferase. This enzyme works inside the energy-producing centers of our cells, the mitochondria. Its main job is to help break down a substance called ornithine, which is a building block of proteins that our bodies produce during normal metabolism. By breaking down ornithine, the enzyme helps keep the levels of this substance balanced and also helps create other important molecules that the body needs to function properly. When there is a mutation or mistake in the OAT gene, the body cannot produce enough working ornithine aminotransferase enzyme. Because the enzyme isn't working right, ornithine cannot be broken down, and it builds up to very high, toxic levels in the blood and the eyes. At the same time, the body might not have enough of the other important molecules that the enzyme normally helps create. This imbalance is particularly harmful to the retina, the light-sensitive tissue at the back of the eye, and the choroid, the layer of blood vessels that nourishes it. This genetic condition is called gyrate atrophy of the choroid and retina. It is an inherited disease, meaning it is passed down from parents to their children. It follows an "autosomal recessive" pattern, which means a person must inherit two mutated copies of the gene (one from each parent) to develop the disease. People with gyrate atrophy typically start experiencing night blindness and nearsightedness in childhood. Over time, they slowly lose their side (peripheral) vision, and eventually, their central vision can also be affected. While there is currently no cure, treatments like a strict, specialized diet can help lower ornithine levels and slow down the vision loss.

Gene function: OAT is crucial for maintaining ornithine homeostasis in the retina. Its deficiency leads to gyrate atrophy of the choroid and retina, a progressive chorioretinal degeneration characterized by hyperornithinemia, causing photoreceptor and RPE dysfunction and loss, severely impacting vision.

Protein structure: The OAT gene encodes the ornithine aminotransferase protein, which is synthesized as a 439-amino acid precursor. This precursor contains an N-terminal mitochondrial targeting sequence of 34 amino acids that directs the protein to the mitochondrial matrix. Upon entry into the mitochondria, this leader sequence is cleaved off, resulting in the mature, functional protein of 405 amino acids with a molecular weight of approximately 45 kDa. Structurally, the mature OAT protein functions as a homotetramer, meaning it is composed of four identical protein subunits assembled together. Each subunit binds one molecule of pyridoxal 5'-phosphate (PLP), the active form of vitamin B6, which is an essential cofactor for its enzymatic activity. The PLP molecule is covalently linked to a specific lysine residue (Lys292) within the active site of the enzyme. The three-dimensional structure of OAT reveals a highly conserved fold typical of fold-type I pyridoxal-dependent enzymes, with a large domain that binds the PLP cofactor and a smaller domain that helps form the active site cleft where ornithine binds.

Molecular function: The OAT gene encodes the enzyme ornithine aminotransferase (OAT), a pyridoxal phosphate (vitamin B6)-dependent mitochondrial matrix enzyme. OAT catalyzes the reversible transamination of the non-protein amino acid L-ornithine and alpha-ketoglutarate to form L-glutamate-gamma-semialdehyde (which spontaneously cyclizes to pyrroline-5-carboxylate, P5C) and L-glutamate. This reaction is a critical node connecting the urea cycle, the tricarboxylic acid (TCA) cycle, and the metabolism of several amino acids, including proline, glutamate, and arginine. In most tissues, the primary physiological direction of the OAT reaction is the catabolism of ornithine to P5C, which is subsequently converted to proline or glutamate. This pathway is essential for maintaining systemic ornithine homeostasis and preventing its toxic accumulation. In the retina and retinal pigment epithelium (RPE), the exact metabolic role of OAT is still being elucidated, but it is believed to be crucial for the local synthesis of proline, which is required for the high rate of protein synthesis in photoreceptors, or for the generation of glutamate, an important excitatory neurotransmitter in the retina. When OAT is deficient, ornithine cannot be properly broken down, leading to severe hyperornithinemia. The exact mechanism by which this metabolic defect causes chorioretinal degeneration is not fully understood. It is hypothesized to involve either direct toxicity from high ornithine levels, which may inhibit other critical enzymes like creatine synthesis, or a deficiency of downstream products like P5C and proline, which are vital for retinal cell survival and function. The resulting metabolic imbalance ultimately leads to the progressive death of RPE cells, photoreceptors, and the underlying choroid.

Expression pattern: The OAT gene is widely expressed across various tissues in the human body, reflecting its fundamental role in amino acid metabolism. High levels of OAT expression are found in the liver, kidney, and small intestine, which are the primary sites of the urea cycle and systemic ornithine metabolism. In these tissues, OAT plays a crucial role in regulating systemic ornithine levels and synthesizing proline and glutamate. In the eye, OAT is highly expressed in the retinal pigment epithelium (RPE) and the neurosensory retina. The robust expression in the RPE is particularly significant, as this cell layer is essential for the maintenance and survival of photoreceptors. The exact reason why OAT deficiency primarily manifests as a severe chorioretinal degeneration despite its ubiquitous expression remains an area of active research, but it is hypothesized that the retina and RPE are uniquely sensitive to either the toxic accumulation of ornithine or the local deficiency of its downstream metabolites, such as pyrroline-5-carboxylate (P5C) and proline.

Mutation spectrum: The mutation spectrum of the OAT gene is highly diverse, with over 60 pathogenic variants identified to date. These include missense, nonsense, frameshift, and splice-site mutations, as well as small deletions and insertions. Missense mutations are the most common type, often affecting highly conserved amino acid residues critical for the enzyme's structural stability, catalytic activity, or binding of the pyridoxal phosphate cofactor. While many mutations are private to individual families, some founder mutations have been identified in specific populations. The most notable is the p.Leu402Pro mutation, which is highly prevalent in the Finnish population due to a founder effect, accounting for the majority of gyrate atrophy cases in Finland. Another example is the p.Arg180Thr mutation, also found in Finnish patients. In other populations, the mutation spectrum is more heterogeneous, though certain variants may be more frequent in specific ethnic groups, such as the p.Met1Ile initiator mutation in Lebanese Maronites.

Pathogenic variants: 1. p.Leu402Pro (c.1205T>C): This is the most common pathogenic variant worldwide, primarily due to a strong founder effect in the Finnish population, where it accounts for the vast majority of gyrate atrophy cases. It results in a severe, pyridoxine-unresponsive phenotype. 2. p.Arg180Thr (c.539G>C): Another variant found with increased frequency in the Finnish population, also leading to a severe loss of enzyme activity and classic gyrate atrophy. 3. p.Met1Ile (c.3G>A): An initiator codon mutation that prevents the proper translation of the OAT protein. It is a well-characterized founder mutation in the Lebanese Maronite population. 4. p.Val332Met (c.994G>A): A notable missense mutation associated with the rare, pyridoxine-responsive form of gyrate atrophy. Patients with this variant may show a significant reduction in plasma ornithine levels when treated with high-dose vitamin B6. 5. p.Arg271Lys (c.812G>A): A well-documented missense mutation that affects a highly conserved residue, leading to loss of OAT function and typical gyrate atrophy presentation.

Clinical significance: Mutations in the OAT gene cause gyrate atrophy of the choroid and retina (GACR), a rare, autosomal recessive inherited retinal dystrophy. The disease is characterized by a classic triad of early-onset high myopia and astigmatism, night blindness (nyctalopia) beginning in the first decade of life, and progressive loss of peripheral vision. The hallmark clinical sign is the appearance of sharply demarcated, circular patches of chorioretinal atrophy in the mid-periphery of the fundus. Over time, these atrophic patches enlarge and coalesce, spreading towards the posterior pole and the macula, eventually leading to severe visual impairment or legal blindness, typically by the fourth to fifth decade of life. In addition to the retinal manifestations, patients frequently develop early-onset posterior subcapsular cataracts, usually in their late teens or twenties, which further compromise vision. Systemically, the defining biochemical feature of GACR is hyperornithinemia, with plasma ornithine levels typically 10 to 20 times higher than normal. While the disease is primarily ocular, some patients may exhibit mild systemic features, such as sparse, fine hair, and occasionally, mild proximal muscle weakness or electromyographic abnormalities, though these are generally subclinical. The severity and rate of progression can vary among individuals, but the visual prognosis is generally poor without intervention.

Inheritance: Autosomal Recessive

Chromosomal location: 10q26.13

Genotype-phenotype correlations: Genotype-phenotype correlations in gyrate atrophy are complex and not entirely straightforward, partly due to the rarity of the disease and the large number of private mutations. However, some general patterns have emerged. The majority of patients with classic, severe GACR have mutations that result in a complete or near-complete loss of OAT enzyme activity, such as nonsense mutations, frameshifts, or missense mutations affecting critical active site residues. A notable exception is a subset of patients who exhibit a milder phenotype and a slower rate of disease progression. These individuals often possess specific missense mutations that leave residual enzyme activity, which can sometimes be enhanced by supplementation with pyridoxine (vitamin B6), the essential cofactor for OAT. For example, certain mutations (e.g., p.Val332Met) have been associated with this pyridoxine-responsive form of the disease. In these patients, high-dose vitamin B6 supplementation can significantly lower plasma ornithine levels and alter the clinical course. However, the majority of OAT mutations are pyridoxine-unresponsive, and the severity of the retinal degeneration in these cases is primarily dictated by the degree of hyperornithinemia and the age at which dietary intervention is initiated.

Research and therapeutic approaches: The current standard of care for gyrate atrophy is primarily dietary management aimed at reducing plasma ornithine levels. This involves a highly restrictive, low-protein diet that is specifically limited in arginine, the metabolic precursor to ornithine. To ensure adequate nutrition and protein synthesis, patients are supplemented with essential amino acids (excluding arginine). When strictly adhered to, this diet can significantly lower ornithine levels and has been shown to slow or halt the progression of chorioretinal degeneration. However, the diet is extremely difficult to maintain long-term. In a small subset of patients with specific mutations (e.g., p.Val332Met), high-dose supplementation with pyridoxine (vitamin B6) can enhance residual enzyme activity and lower ornithine levels, offering a less restrictive treatment option. Given the challenges of dietary management, there is significant interest in developing gene therapy for gyrate atrophy. The goal is to deliver a functional copy of the OAT gene to restore enzyme activity. Preclinical studies in animal models have shown great promise. For instance, liver-directed gene therapy using adeno-associated virus (AAV) vectors has successfully corrected systemic hyperornithinemia and prevented retinal degeneration in Oat-deficient mice. Other approaches are exploring combined intraocular (subretinal or intravitreal) and intravenous AAV delivery to target both the systemic metabolic defect and the local retinal pathology. While no gene therapies are currently FDA-approved for gyrate atrophy (unlike Luxturna for RPE65-associated IRDs), natural history studies like the Gyrate Atrophy Ocular and Systemic Study (GYROS) are actively underway to establish clinical endpoints and prepare for future human clinical trials.

Diagnostic testing: The diagnosis of gyrate atrophy is typically suspected based on the characteristic clinical findings of high myopia, night blindness, and the pathognomonic scalloped areas of chorioretinal atrophy. The biochemical hallmark and primary diagnostic test is the measurement of plasma amino acids, which reveals markedly elevated levels of ornithine (hyperornithinemia), often 10-20 times the normal range. Confirmatory diagnosis is achieved through molecular genetic testing of the OAT gene. This can be performed using targeted gene panels for inherited retinal diseases, whole exome sequencing (WES), or whole genome sequencing (WGS) to identify biallelic pathogenic variants. Genetic counseling is essential for affected individuals and their families, as GACR follows an autosomal recessive inheritance pattern. Carrier testing for at-risk relatives and prenatal diagnosis for pregnancies at increased risk are possible if the pathogenic variants in the family are known. Early diagnosis is crucial, as early initiation of dietary interventions can significantly alter the disease course.

Animal models: Animal models have been crucial for understanding the pathophysiology of gyrate atrophy and testing potential therapies. The primary model is the Oat knockout (Oat-/-) mouse, which exhibits hyperornithinemia and develops progressive chorioretinal degeneration that closely mimics the human disease. However, Oat-/- mice typically die within 48 hours of birth due to metabolic crisis unless rescued with arginine supplementation until weaning. Another important model is the spontaneous recessive mutation 'retarded hair growth' (rhg) mouse, which has a missense mutation (p.Gly353Ala) in the Oat gene. These mice show delayed growth, defective hair development, and premature death, along with elevated plasma ornithine and reduced liver OAT activity. Compound heterozygous rhg/Oat-null and homozygous rhg/rhg mice also develop chorioretinal degeneration similar to human gyrate atrophy. These models have revealed that the retinal degeneration is likely driven by the accumulation of ornithine or the deficiency of its downstream products, such as pyrroline-5-carboxylate (P5C) and proline. Furthermore, these animal models are currently being used to test the efficacy of gene therapy approaches. Recent studies using adeno-associated virus (AAV) vectors for liver-directed or combined intraocular and intravenous gene delivery in these mice have shown promising results in correcting hyperornithinemia and preserving retinal structure and function, paving the way for future human clinical trials.

Population genetics: Gyrate atrophy is an extremely rare disorder globally, with an estimated prevalence of less than 1 in 100,000 individuals. However, the carrier frequency and disease prevalence are significantly higher in certain populations due to founder effects. The most striking example is in Finland, where the disease prevalence is estimated to be around 1 in 50,000, and the carrier frequency for the predominant p.Leu402Pro mutation is approximately 1 in 100. Another notable population with a higher prevalence is the Lebanese Maronite community, where the p.Met1Ile mutation is common. In most other parts of the world, the disease is exceedingly rare, and patients are often compound heterozygotes for different, private mutations.

Selected references: 1. Mitchell GA, et al. An initiator codon mutation in ornithine-delta-aminotransferase causing gyrate atrophy of the choroid and retina. J Clin Invest. 1988;81(2):630-633. PMID: 3422055 2. Brody LC, et al. Ornithine delta-aminotransferase mutations in gyrate atrophy. Allelic heterogeneity and functional consequences. J Biol Chem. 1992;267(5):3302-3307. PMID: 1737784 3. Peltola KE, et al. Ophthalmologic heterogeneity in subjects with gyrate atrophy of choroid and retina harboring the L402P mutation of ornithine aminotransferase. Ophthalmology. 2001;108(4):721-729. PMID: 11297489 4. Montioli R, et al. Deficit of human ornithine aminotransferase in gyrate atrophy: Molecular, cellular, and clinical aspects. Biochim Biophys Acta Proteins Proteom. 2021;1869(1):140555. PMID: 33152522 5. Kaiser-Kupfer MI, et al. A specific enzyme defect in gyrate atrophy. Am J Ophthalmol. 1978;85(2):200-204. PMID: 623190 6. Dell'Aquila F, et al. Combined intraocular and intravenous gene delivery for therapy of gyrate atrophy of the choroid and retina. Mol Ther. 2024;32(7):2150-2163. PMID: 38702930