Correction of ornithine accumulation prevents retinal degeneration in a mouse model of gyrate atrophy of the choroid and retina

Abstract

Deficiency of ornithine-delta-aminotransferase (OAT) in humans results in gyrate atrophy of the choroid and retina (GA), an autosomal recessive disorder characterized by ornithine accumulation and a progressive chorioretinal degeneration of unknown pathogenesis. To determine whether chronic, systemic reduction of ornithine can prevent this form of retinal degeneration, we used an arginine-restricted diet to maintain long term reduction of ornithine in a mouse model of OAT-deficiency (Oat(-/-)) produced by gene targeting. We evaluated the mice over a 12-month period by measurement of plasma amino acids, electroretinograms, and retinal histologic and ultrastructural studies. We found that an arginine-restricted diet substantially reduces plasma ornithine levels and completely prevents retinal degeneration in Oat(-/-). This result indicates that ornithine accumulation is a necessary factor in the pathophysiology of the retinal degeneration in GA and that restoration of OAT activity in retina is not required for effective treatment of GA.

Figures

Figure 1

Ornithine metabolic pathways. The vertical bar indicates the site of the block caused by OAT deficiency. ASA, argininosuccinic acid; CAP, carbamyl phosphate.

Figure 2

Selected plasma amino acid levels of Oat−/− mice on an arginine-restricted diet over 12 months (a) or Oat−/− mice on a standard diet (b). Ranges of plasma ornithine and lysine in control animals are indicated with dark or light gray rectangles, respectively. In a, the Oat−/− mice were placed on an arginine-restricted diet at the age of 6 weeks, as indicated by the arrow. Each point represents the mean plasma concentration for the indicated amino acid from 5–8 Oat−/− mice. In b, each point represents the mean of measurements for the indicated amino acid in 5–8 adult Oat−/− animals on a standard diet.

Figure 3

Growth of Oat−/− and control mice on an arginine-restricted diet over 12 months. The animals were placed on an arginine-restricted diet at the age of 6 weeks, as indicated by an arrow. Each point and vertical line indicates the mean and range of weight of the 5–8 animals in each group, respectively.

Figure 4

ERG amplitudes of Oat−/− and control mice on either a standard (a) or an arginine-restricted (b) diet over 12 months of life. Oat−/− and control mice were divided into groups of 5–8 animals. In a, a group of control and a group of Oat−/− mice were placed on standard diet for 12 months whereas in b, groups of control and Oat−/− mice were placed on an arginine-restricted diet at age 6 weeks. The symbols represent the mean, and the vertical bars the range, of ERG amplitudes recorded from control mice (open triangles) or Oat−/− mice (filled triangles) at indicated ages. The ERG amplitude was calculated as the difference between the peak of the A wave and the peak of the B wave (33).

Figure 5

Retinal histopathology of Oat−/− mice on an arginine-restricted diet (a) or a standard diet (b) at age 12 months. Note the reduced number of ONL cells and loss of IS and OS in the right panel. The arrows indicate abnormal RPE cells. GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; ONL, outer nuclear layer; IS and OS, inner and outer segments, respectively, of photoreceptors.

Figure 6

Electron micrographs of retinas from Oat−/− mice on an arginine-restricted diet (a) or a standard diet (b) at age 12 months. The arrow heads indicate normal appearing photoreceptor outer segments in a and a near complete loss of the photoreceptor outer segments in b. The asterisks in b indicate photoreceptor nuclei.

Figure 7

Electron micrograph of the outer retinas of the Oat−/− mice on an arginine-restricted diet (a) or a standard diet (b) at age 12 months. The large arrow heads indicate the basal infoldings of retinal pigment epithelium, healthy-appearing in a and virtually gone in b. The small arrows indicate the apical processes of the retinal pigment epithelium, long and delicate in a and short and disorganized in b. CC, choriocapillaris; N, nuclei of the retinal pigment epithelium; M, melanosome; OS, photoreceptor outer segment; L, lipid droplets; G, glycogen accumulation.