The retarded hair growth (rhg) mutation in mice is an allele of ornithine aminotransferase (Oat)

Intraretinal variation in disease severity in the Oatrhg mouse model of gyrate atrophy

Gyrate atrophy is an autosomal recessive retinal degeneration caused by pathogenic variants in the gene encoding ornithine aminotransferase (OAT), a mitochondrial enzyme required for ornithine degradation. Deficiency of OAT leads to hyperornithinemia and progressive chorioretinal atrophy that results in permanent vision loss. Strict dietary arginine restriction can slow the progression of the disease, but long-term adherence to the diet is challenging and not curative. Here, we characterize the retinal structure and function of the retarded hair growth (Oatrhg) mouse model of gyrate atrophy in order to identify appropriate outcome measures for future therapeutic approaches. Optical coherence tomography (OCT), histological sections, and retinal pigment epithelium (RPE) flat mounts of 12-month-old Oatrhg mice revealed a well-defined patch of atrophy in the superonasal and occasionally inferior retina, characterized by RPE cell mounding, migration, and hypertrophy. The remainder of the retina was indistinguishable from age-matched wild type controls, and full-field electroretinograms (ERGs) were not significantly different between Oatrhg and wild type mice. Therefore, unlike mice harboring the perinatal-lethal null mutation in OAT (OatΔ) which exhibit a loss of central photoreceptor cells and decreased ERG signal starting at 4 months, the Oatrhg mouse exhibits a milder phenotype with intraretinal variation in disease severity that is reminiscent of the regional predilection observed in patients. These structural abnormalities are not sufficient to negatively impact retina-wide function but are accessible to monitoring by multimodal retinal imaging for testing of novel treatments.

Liver-directed gene therapy for ornithine aminotransferase deficiency

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.

Preclinical Models of Retinitis Pigmentosa

Retinitis pigmentosa (RP) is the name for a group of phenotypically-related heritable retinal degenerative disorders. Many genes have been implicated as causing variants of RP, and while the clinical phenotypes are remarkably similar, they may differ in age of onset, progression, and severity. Common inheritance patterns for specific genes connected with the development of the disorder include autosomal dominant, autosomal recessive, and X-linked. Modeling the disease in animals and other preclinical systems offers a cost-conscious, ethical, and time-efficient method for studying the disease subtypes. The history of RP models is briefly examined, and both naturally occurring and transgenic preclinical models of RP in many different organisms are discussed. Syndromic forms of RP and models thereof are reviewed as well.

Modeling Rare Human Disorders in Mice: The Finnish Disease Heritage

The modification of genes in animal models has evidently and comprehensively improved our knowledge on proteins and signaling pathways in human physiology and pathology. In this review, we discuss almost 40 monogenic rare diseases that are enriched in the Finnish population and defined as the Finnish disease heritage (FDH). We will highlight how gene-modified mouse models have greatly facilitated the understanding of the pathological manifestations of these diseases and how some of the diseases still lack proper models. We urge the establishment of subsequent international consortiums to cooperatively plan and carry out future human disease modeling strategies. Detailed information on disease mechanisms brings along broader understanding of the molecular pathways they act along both parallel and transverse to the proteins affected in rare diseases, therefore also aiding understanding of common disease pathologies.

Ornithine-δ-Aminotransferase Inhibits Neurogenesis During Xenopus Embryonic Development

PURPOSE

In humans, deficiency of ornithine-δ-aminotransferase (OAT) results in progressive degeneration of the neural retina (gyrate atrophy) with blindness in the fourth decade. In this study, we used the Xenopus embryonic developmental model to study functions of the OAT gene on embryonic development.

METHODS

We cloned and sequenced full-length OAT cDNA from Xenopus oocytes (X-OAT) and determined X-OAT expression in various developmental stages of Xenopus embryos and in a variety of adult tissues. The phenotype, gene expression of neural developmental markers, and enzymatic activity were detected by gain-of-function and loss-of-function manipulations.

RESULTS

We showed that X-OAT is essential for Xenopus embryonic development, and overexpression of X-OAT produces a ventralized phenotype characterized by a small head, lack of axial structure, and defective expression of neural developmental markers. Using X-OAT mutants based on mutations identified in humans, we found that substitution of both Arg 180 and Leu 402 abrogated both X-OAT enzymatic activity and ability to modulate the developmental phenotype. Neurogenesis is inhibited by X-OAT during Xenopus embryonic development.

CONCLUSIONS

Neurogenesis is inhibited by X-OAT during Xenopus embryonic development, but it is essential for Xenopus embryonic development. The Arg 180 and Leu 402 are crucial for these effects of the OAT molecule in development.