Amino-acid metabolism and liver ultrastructure in hyperornithinemia with gyrate atrophy of the choroid and retina

Genome-wide association studies of metabolites in Finnish men identify disease-relevant loci

Few studies have explored the impact of rare variants (minor allele frequency < 1%) on highly heritable plasma metabolites identified in metabolomic screens. The Finnish population provides an ideal opportunity for such explorations, given the multiple bottlenecks and expansions that have shaped its history, and the enrichment for many otherwise rare alleles that has resulted. Here, we report genetic associations for 1391 plasma metabolites in 6136 men from the late-settlement region of Finland. We identify 303 novel association signals, more than one third at variants rare or enriched in Finns. Many of these signals identify genes not previously implicated in metabolite genome-wide association studies and suggest mechanisms for diseases and disease-related traits.

Characteristics of L-ornithine: 2-oxoacid aminotransferase and potential prenatal diagnosis of gyrate atrophy of the choroid and retina by first trimester chorionic villus sampling

A deficiency of the mitochondrial matrix enzyme L-ornithine: 2-oxoacid aminotransferase causes gyrate atrophy of the choroid and retina with hyperornithinemia (MIM 258870), a blinding degenerative disease, which is inherited as an autosomal recessive trait. We have developed a sensitive microradioisotopic method for enzyme assay by using 2-oxo-[5-14C] glutarate as the substrate and performing the separation of the product, [5-14C] glutamate from the substrate on a cation-exchange column. The enzyme activity was determined in human and rat tissues and in cultured cells. The enzyme activity in fibroblasts from a patient was deficient and that of the parents ranged between 25 and 60% of the control values. In addition we have found the enzyme expressed in native and cultured chorionic villi indicating a potential detection of the disease during the first trimester of pregnancy.

Ocular clinicopathologic study of gyrate atrophy

We performed a histopathologic study of whole globes obtained post mortem from a patient with well-documented, vitamin B6-responsive gyrate atrophy. The retina in the posterior pole had focal areas of photoreceptor atrophy with adjacent retinal pigment epithelial hyperplasia. An abrupt transition from the near normal retina to a zone of near total atrophy of the retina, retinal pigment epithelium, and choroid was present in the fundus midperiphery. Electron microscopic examination disclosed abnormalities of the mitochondria of the corneal endothelium and the non-pigmented ciliary epithelium. Similar, but less severe, mitochondrial abnormalities were present in the photoreceptors.

Pathology of iridectomy specimens in gyrate atrophy of the retina and choroid

Gyrate atrophy of the retina and choroid is an autosomal recessive disease characterized by progressive retinal degeneration and ornithine aminotransferase deficiency. We report here the new histological findings and ultrastructural changes in 3 iridectomy specimens from 2 Finnish patients with gyrate atrophy. The iridectomy specimens were removed during routine cataract extraction and studied with a transmission electron microscope. The dilator muscle showed atrophy, abnormal mitochondria, and tubular aggregate type structures similar to those found in skeletal muscle. Degenerative changes such as extracted cellular matrix, dropout of cellular organelles, and dilated intercellular spaces were observed in the pigmented posterior epithelium and the anterior iris epithelium.

Metabolic tapetoretinal degenerations

There are a number of metabolic diseases which cause tapetoretinal degeneration, suggesting that pure pigmentary retinopathy may also be metabolic in nature. On the other hand tapetoretinal degenerations may have various modes of inheritance, so we may conclude that the metabolic disorder at the basis of these diseases is not unique and that tapetoretinal degenerations are heterogenic. In this article, some 450 published reports on tapetoretinal degenerations are reviewed. Based on these reports, the clinical and ocular manifestations, laboratory and histopathological findings, inheritance patterns, and treatments of various syndromes characterized by tapetoretinal degenerations are described. It is hoped that the gathering together of this information in one source will acid in the future understanding of metabolically based eye disease.

Mild elevations of plasma ornithine in homocystinuria

Hyperornithinemia was found on many occasions during monitoring of amino acids in the fasting plasma of homocystinuric patients. Ornithine concentrations were elevated 2- to 3-fold above the normal mean value. Hyperornithinemia was of much less marked degree than that seen in gyrate atrophy of the choroid and retina, and was not accompanied by overflow ornithinuria or hypolysinemia. Elevations in plasma ornithine concentrations in homocystinuric patients were generally proportional to elevations in the plasma concentration of homocystine, but not that of methionine.

Amino acids in retinitis pigmentosa

A survey of fasting whole blood amino acids in 65 patients with various subtypes of retinitis pigmentosa performed. Eight X-linked recessive patients showed decreased taurine and aspartate. Nineteen autosomal recessive patients, and to lesser extent 10 autosomal dominant patients, showed reduced levels of whole blood threonine and histidine. Branched-chain amino acids and arginine were present in increased amounts in 2 patients with Laurence-Moon-Bardet-Biedl syndrome. These findings in LMBB patients are probably related in part to their obesity and emphasise that appropriate controls are required, and other factors (including age) known to affect amino acid levels must be accounted for. The biochemical implications of our findings are not yet apparent.

Gyrate atrophy of the choroid and retina. Approaches to therapy

Gyrate atrophy of the choroid and retina is caused by deficient activity of ornithine ketoacid aminotransferase, a pyridoxal phosphate dependent enzyme. Besides the typical eye findings, abnormalities have been found on muscle biopsy, electro-encephalography, electromyography and electrocardiography, establishing this as a generalized disorder. Ornithine is markedly elevated in plasma and other body fluids. Plasma lysine, glutamate, glutamine and creatine are reduced. The possible contributions of these biochemical disturbances to the pathogenesis of gyrate atrophy are discussed. The disease is one of the few examples of an inherited chorioretinal dystrophy whose underlying biochemical defect is known. It therefore offers a unique opportunity to develop and test rational approaches to therapy. These include lowering of the abnormally high ornithine by dietary restriction of its precursor arginine, facilitation of ornithine excretion by administration of alpha-aminoisobutyric acid, replacement of deficient products such as lysine or creatine, or increasing residual enzyme activity by high levels of cofactor (vitamin B6). The results of several studies employing such approaches to therapy are presented as well as preliminary indications of possible benefit in a few patients.

Unusual clinical and ultrastructural features in a boy with biochemically typical mannosidosis

A 4 1/2-year-old boy with a history of recurring respiratory tract infections and seizures, and evidence of severe retardation of psychomotor development and growth, lacked the coarse facial features, skeletal changes and other clinical stigmata generally associated with mannosidosis, but the total alpha-mannosidase activity in his leukocytes, cultured fibroblasts and liver were no more than 10% of the control mean. Studies of the residual alpha-mannosidase enzyme suggest a specific deficiency of the thermostable isoenzyme with an acidic pH optimum. The alpha-mannosidase in the fibroblasts of our and another (control) patient with mannosidosis had a reduced affinity for the substrate 4-methylumbelliferyl-alpha-D-mannoside. Light microscopy of the liver biopsy showed an increase in connective tissue often distorting the hepatic architecture; numerous tiny vacuoles, small dense and lipid bodies in most hepatocytes, and similar but more extensive changes in sinusoidal cells; and sinusoidal pools of hepatocytic debris. Electron microscopy of hepatocytes revealed vacuoles similar but not identical to those described in reported mannosidosis patients, and in addition several forms of secondary lysosomes; prominent peroxisomes (microbodies); increased numbers of profiles of smooth endoplasmic reticulum; dilated rough endoplasmic reticulum containing traces of fine granulo-fibrillar material; increased numbers of rosettes of alpha particles of glycogen and reduced numbers of mitochondria with alterations in their distribution, size and configuration. It is believed that the usual clinical and hepatic ultrastructural features in our patient reflect another variant of mannosidosis.