Deficit of human ornithine aminotransferase (hOAT), a mitochondrial tetrameric pyridoxal-5'-phosphate (PLP) enzyme, leads to gyrate atrophy of the choroid and retina (GA). Although 70 pathogenic mutations have been identified, only few enzymatic phenotypes are known. Here, we report biochemical and bioinformatic analyses of the G51D, G121D, R154L, Y158S, T181M, and P199Q pathogenic variants involving residues located at the monomer-monomer interface. All mutations cause a shift toward a dimeric structure, and changes in tertiary structure, thermal stability, and PLP microenvironment. The impact on these features is less pronounced for the mutations of Gly51 and Gly121 mapping to the N-terminal segment of the enzyme than those of Arg154, Tyr158, Thr181, and Pro199 belonging to the large domain. These data, together with the predicted ΔΔG values of monomer-monomer binding for the variants, suggest that the proper monomer-monomer interactions seem to be correlated with the thermal stability, the PLP binding site and the tetrameric structure of hOAT. The different impact of these mutations on the catalytic activity was also reported and discussed on the basis of the computational information. Together, these results allow the identification of the molecular defects of these variants, thus extending the knowledge of enzymatic phenotypes of GA patients.

Gyrate atrophy of choroid and retina (GACR) is a chorioretinal degeneration caused by pathogenic variants in the gene encoding ornithine aminotransferase (OAT), an enzyme mainly expressed in liver. Affected patients have increased ornithine concentrations in blood and other body fluids and develop progressive constriction of vision fields leading to blindness. Current therapies are unsatisfactory and better treatments are highly needed. In two mouse models of OAT deficiency that recapitulates biochemical and retinal changes of GACR, we investigated the efficacy of an intravenously injected serotype 8 adeno-associated (AAV8) vector expressing OAT under the control of a hepatocyte-specific promoter. Following injections, OAT-deficient mice showed reductions of ornithine concentrations in blood and eye cups compared with control mice injected with a vector expressing green fluorescent protein. AAV-injected mice showed improved electroretinogram response and partial restoration of retinal structure up to one-year post-injection. In summary, hepatic OAT expression by AAV8 vector was effective at correction of hyperornithinemia and improved function and structure of the retina. In conclusion, this study provides proof-of-concept of efficacy of liver-directed AAV-mediated gene therapy of GACR.

Gyrate atrophy is a rare autosomal recessive inherited genetic disease. Progressive deterioration of peripheral night vision and blindness are the foremost clinical manifestations of the disease caused by mutations of ornithine aminotransferase gene. The presented case was an 18-year-old male referred for a progressive reduction of visual acuity, which started when the subject was 7 years old, blurred vision, and hypotonic muscles. The findings by liquid chromatography with tandem mass spectrometry and high-performance liquid chromatography methods exhibited a high level of ornithine: 248 μmol/L (reference range: 44-206 μmol/L) and 818 μmol/L (reference: 25-123 μmol/L), respectively. After genetic counseling and conducting further investigation, a novel mutation (c.425-1G>A) in ornithine aminotransferase gene was recognized through whole exome sequencing and the mutation was verified using Sanger sequencing method, which is associated with gyrate atrophy phenotype. The exact mechanism of chorioretinal atrophy in hyperornithinemia is not known but the increased ornithine level is the clinical manifestation of gyrate atrophy of choroid and retina, muscle weakness, moderate mental retardation, and low cerebral creatine. Pathogenic variant in the ornithine aminotransferase gene associated with gyrate atrophy, may be beneficial as a biomarker to initial diagnosis and treatment of gyrate atrophy disease.

Gyrate atrophy of choroid and retina is an autosomal recessive condition characterized by peripheral multiple sharp areas of chorioretinal atrophy which become confluent with age. Macula and central vision is typically involved late in the disease. Macular involvements such as cystoid macular edema, epimacular membrane, and choroidal neovascularization have been reported in gyrate atrophy. In this report, we present a family with diminished central vision presenting within 8 years of age. All of three siblings had typical peripheral chorioretinal atrophic lesions of gyrate atrophy and hyperornithinemia. On spectral domain optical coherence tomography, two of elder siblings showed macular edema. Hyporeflective spaces appeared to extend from outer nuclear layer to the inner nuclear layer level separated by multiple linear bridging elements in both eyes. Ultrawide field fluorescein angiogram (UWFI) even in late phase did not show any leak at macula suggesting foveoschisis. Foveoschisis in gyrate atrophy has not been reported before.