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Contributed by Akosua B Domfeh, MBchB, Nicole N. Esposito, MD, David N Finegold, MDDISCUSSION:
GYRATE ATROPHY (GA) of the choroid and retina (Figure 1) is a rare, autosomal recessive disease causing progressive chorioretinal degeneration resulting in blindness. It is caused by a deficiency of ornithine -aminotransferase (OAT) (1). It was first described as "atypical retinitis pigmentosa" by Jacobsohn in 1888.
Epidemiology:
There are more than 150 biochemically-documented cases known, with the highest rate in Finland (2).
Clinical Presentation:
Clinically, GA patients initially develop myopia and reduced night vision, followed by gradual reduction in peripheral vision with constriction of the visual fields, leading to complete loss of vision at around the fourth to fifth decade of life (3-5). Myopia and decreased night vision occur early, usually before the end of the first decade. The presence of numerous sharply demarcated circular areas with hyperpigmented margins in the anterior to mid peripheral retina is the earliest diagnostic funduscopic feature of GA (Figure 2). Reduced peripheral vision with constriction of the visual fields is obvious in the 2nd decade. Over time, these lesions increase in number and size, coalesce, and spread from the peripheral to the central fundus (Figure 3), forming lesions with a "gyrate" or garland-like border. Virtually all patients develop posterior subcapsular cataracts for unknown reasons late in the 2nd or early 3rd decade (3-4). Complete loss of vision is associated with involvement of the macula. Electroretinography (ERG) abnormalities are present at an early stage of the disease, with impaired rod and cone responses, ultimately progressing to a completely extinguished response (1, 6). In addition to the ocular findings, abnormalities in electroencephalograms, muscle and hair morphology and mitochondrial structure in the liver have been reported (7). Some patients present with mild mental retardation, delayed language development and speech defects (8). One study revealed degenerative lesions in the white matter of the brain in 50% of GA patients, and 70% of patients had premature atrophic changes, with a striking increase in the number of Virchow's spaces. Muscular atrophy associated with tubular aggregate deposition in type 2 muscle fibers is also seen in some patients (9-10).
Genetics:
OAT is encoded by the active structural OAT gene located on chromosome 10q26. More than 60 mutations have been identified, including missense mutations, nonsense mutations, microdeletions, microinsertions and substitutions altering splice consensus sequences. Misense mutations are the most common, and presumably result in premature termination of protein translation leading to an unstable enzyme (11-12).
Pathophysiology:
Ornithine is a non-essential amino acid that plays an important role in the metabolism of urea, creatinine and polyamines, and in the exchange of molecules between the urea and Kreb's cycles. Its major source in the diet is arginine. It is incorporated into protein as proline, glutamate or any of the alpha-ketoglutarate -derived nonessential aminoacids.
Hyperornithinemia associated with gyrate atrophy of the choroid and retina is a rare, autosomal recessive disorder resulting from a deficiency of the mitochondrial matrix enxyme, ornithine -aminotransferase (OAT)(1). OAT is a pyridoxal phosphate-dependent mitochondrial matrix enzyme expressed in most tissues, including kidney, small intestine, liver, and retina. This enzyme reversibly catalyses the transamination of ornithine and -ketoglutarate to D'-pyrroline 5-carboxylic acid and glutamic acid (Figure 4). High OAT activity has been documented in the retinal pigment epithelium (RPE), the cell layer external to the retinal photoreceptors. The metabolic function(s) of OAT in the RPE and elsewhere in the retina is poorly understood.
In gyrate atrophy, ornithine concentration is increased 10- to 15-fold above normal in all body fluids with concomitant overflow ornithinuria . Additionally, patients have modest but statistically significant reductions in plasma lysine, glutamate, glutamine and creatine.
The exact mechanism of chorioretinal degeneration remains unknown but it is believed to be due to a toxic effect of ornithine or one of its metabolites, such as urea. Alternatively, some have posed a deficiency of the reaction product, D'-pyrroline 5-decarboxylic acid or its metabolites, is the cause of chorioretinal degeneration.
Laboratory Investigations:
Ornithine levels are measured by high performance liquid chromatography (HPLC) with post column derivatization. Chromatography separates analytes based on their interactions with a mobile and stationary phase as they travel through a support medium, or "column". The compounds with a stronger affinity for the stationary phase are retained longer in the medium than those that favor the mobile phase. Retention time, or the time it takes a compound to elute off of the column, is thus characteristic of a compound. HPLC, rather than gas chromatography, is an ideal method for measurement of ornithine since it uses lower temperatures for separation, thereby achieving better separation of thermolabile organic compounds.
There are five commonly used separation techniques in liquid chromatography (adsoprtion, partition, ion-exchange, affinity, and size-exclusion). Ornithine is measured via the lithium ion-exchange method, in which the column is coated with fully sulfonated, cross-linked, spherical polystyrene molecules. The mobile phase consists of an acid buffer, a neutral buffer, and column regenerant. This results in a pH gradient. Thus, as the sample is injected into the HPLC, amino acids within the sample are mobilized in the order of their isoelectric points. For example, acidic amino acids, such as glutamate, will elute early and basic amino acids, such as lysine, will elute late. After separation, the "post column derivatization" or "reaction" occurs in a heated reactor, where it is mixed with a reagent (ninhydrin) to form a purple compound (Ruhemann's Purple). This allows for fluorescent detection of individual amino acids as they elute off of the column.
Differential Diagnosis:
It is very important to distinguish gyrate atrophy from retinitis pigmentosa (RP) since treament of these two diseases differ. RP is group of inherited disorders in which abnormalities of the photoreceptors (rods and cones) of the retina lead to progressive visual loss. Symptoms include night blindness, loss of peripheral vision and eventual blindness. The main distinguishing factor beween RP and GA are the fundal findings and the hyperornithenemia seen in GA. In RP there is RPE hyperpigmentation in the form of "bone spicules" that alternate with atrophic regions, attenuation of the arterioles, and waxy pallor of the optic nerve head. This is in contrast to GA where there are circumscribed, discrete round patches of choroidal and retinal atrophy occuring in the midperiphery, progressing to coalescing sharply defined, scalloped defects of the pigment epithelium and choroid.
In the absence of clinical information for correlation, the only other condition in which elevated serum ornithine levels are detected is in hyperornithinemia-hyperammonemia-homocitrullinemia (HHH) syndrome. HHH syndrome, even rarer than GA with only about 50 reported cases, is a rare inborn error of metabolism associated with growth and developmental delays, learning disabilities, and, in most cases, ataxia. It is caused by a mutation in the gene encoding an inner mitochondrial membrane transport protein, resulting in the inability to transport ornithine into the mitochondrial matrix. Ornithine is thus unable to be "recycled" back into the urea cycle via condensation into citrulline by ornithine transcarbamylase (OTC) (refer to Figure 4), resulting in elevated serum ornithine. In contrast to GA, the urea cycle is impeded in HHH syndrome since ornithine cannot be transported inside the mitochondria, resulting in elevated blood ammonia levels. Additionally, OTC converts lysine to homocitrulline in the absence of ornithine, causing high levels of homocitrulline. Patients with HHH syndrome present at virtually any age with some type of neurologic dysfunction. Interestingly, chorioretinal atrophy is a rare finding (17).
Treatment:
The goal of treatment in GA is to maintain serum ornithine levels as close to normal as possible. Development of effective therapy for GA has been difficult, in part because of the limited understanding of the pathogenesis of chorioretinal degeneration. Furthermore, the long time course and extensive allelic heterogeneity make evaluation of any therapeutic intervention difficult. A minority of patients (<5%) displays reduction in plasma ornithine in response to pharmacologic doses of pyridoxine hydrochloride (vitamin B6). These individuals have OAT alleles with substitution (not missense) mutations affecting the vitamin B6 binding site of the enzyme (12). Since some patients may respond to supplementation with pyridoxine (vitamin B6), all patients are initially given a trial of this vitamin to determine to what extent, if any, it will lower plasma ornithine levels. Both pyridoxine-responsive and -nonresponsive patients are then placed on a low-protein, low-arginine diet.
In the management of children, expertise is required to be certain that growth and development remain normal while ornithine levels are lowered with a low-protein diet. An arginine-free essential amino acid mixture, depending on the patient's age, is used to provide sufficient nitrogen and meet essential amino acid requirements. In adults, a low-protein diet is also likely to result in amino acid deficiency. Thus, adults are also placed on an arginine-free essential amino acid mixture. Lysine supplementation may be necessary, depending on plasma levels. As a precaution, all patients are placed on a multivitamin preparation with minerals. In addition to receiving regular ocular examinations, all patients have their amino acid and protein levels monitored periodically (12-15). Despite arginine restricted diets, the vast majority of patients eventually develop blindness, though most studies report a delay in symptomatology. Most investigators thus believe arginine-restricted diets are non-curative but do serve to delay the progression of chorioretinal degeneration.
Creatine and proline supplementation has been instituted for therapy by some workers because of the hypothesis that gyrate atrophy may be due to a deficiency of creatine or proline, respectively. However, these studies failed to show significant improvement in the chorioretinal degeneration.
Genetic counseling of family members and evaluation of their blood ornithine levels also forms an important part of management, since ornithine is elevated even in the presymptomatic stage. With the insurgence of gene therapy, some authors are hypothesizing that development of strategies for somatic gene therapy to either replace the abnormal gene or genetically engineer normal OAT enzyme levels will go a long way in the treatment of GA patients (16). Currently, however, the value of early and accurate diagnosis of GA cannot be overemphasized since early correction of ornithine accumulation is critical for delaying the onset of irreversible retinal degeneration.
REFERENCES:
Contributed by Akosua B Domfeh, MBchB, Nicole N. Esposito, MD, David N Finegold, MD
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