N-methyl-N-nitrosourea-induced retinal degeneration in mice is independent of the p53 gene

Abstract

Purpose: A single systemic administration of N-methyl-N-nitrosourea (MNU) causes retinal degeneration involving photoreceptor cell loss within 7 days. MNU-induced photoreceptor cell loss is due to apoptosis and is a reliable animal model for human retinitis pigmentosa. The purpose of this study was to determine if p53 contributes to the development of MNU-induced retinal degeneration in mice.

Methods: Eight-week-old p53(-/-), p53(+/-), and p53(+/+) mice received an intraperitoneal injection of 60 mg/kg bodyweight of MNU. Age-matched p53(+/+) mice received the vehicle only (physiologic saline containing 0.05% acetic acid). Mice were sacrificed and necropsied 7 days after the treatment. Both eyes were examined histologically and morphometrically to determine retinal thickness, photoreceptor cell ratio, and retinal damage ratio.

Results: No mice died during the experiment, but the p53 null mice treated with MNU had a statistically significant weight loss compared to the other groups. Histologically, all MNU-treated mice, regardless of p53 gene status, experienced retinal degeneration characterized by photoreceptor cell loss (the disappearance of the outer nuclear layer and photoreceptor layer) in both the central and peripheral retina. All MNU-treated mice had significantly decreased retinal thickness and photoreceptor cell ratios at the central and peripheral retina and an increased retinal damage ratio compared to the vehicle-treated control. The retinal changes caused by MNU in p53(+/+), p53(+/-), and p53(-/-) mice were not significantly different and hence were related to a p53-independent apoptotic mechanism.

Conclusions: Because the absence of p53 did not prevent photoreceptor cell loss, we conclude that p53 is not essential for MNU-mediated photoreceptor cell degeneration.

Figure 1

Percent bodyweight change of initial bodyweight in N-methyl-N-nitrosourea (MNU) -treated p53^−/−, p53^+/−, and p53^+/+ mice and vehicle-treated p53^+/+ control mice 7 days after the treatment. The MNU-treated p53 null mice experienced a significant decrease in bodyweight as compared to vehicle-treated and MNU-treated p53^+/− and p53^+/+ mice. Asterisk is p<0.01 and compared with three other organs.

Figure 2

Retinal changes 7 days after a single systemic administration of 60 mg/kg N-methyl-N-nitrosourea (MNU) to p53^−/−, p53^+/−, and p53^+/+ mice. In MNU-treated p53^−/−, p53^+/−, and p53^+/+ mice, the outer nuclear layer and photoreceptor layer disappeared in both the peripheral and central retina. Equivalent levels of severe retinal degeneration were detected in MNU-treated p53^−/−, p53^+/−, and p53^+/+ mice. GCL, ganglion cell layer; IPL, inner plexiform layer; INL, inner nuclear layer; OPL, outer plexiform layer; ONL, outer nuclear layer; PRL, photoreceptor cell layer; and PEL, pigment epithelial cell layer. The sections were stained with hematoxylin and eosin. The scale bar represents 70 μm.

Figure 3

Photoreceptor cell ratio in the central and peripheral retina 7 days after a single systemic administration of 60 mg/kg N-methyl-N-nitrosourea (MNU) to p53^−/−, p53^+/−, and p53^+/+ mice. Photoreceptor cell ratio was calculated as [(outer retinal thickness)/(total retinal thickness)]×100. In mice treated with MNU, regardless of their p53 status, the total retinal thickness and the outer retinal thickness of the central and peripheral retina were significantly decreased, resulting in a statistically significant decrease in the photoreceptor ratio of MNU-treated mice compared to vehicle-treated controls. Asterisk is p<0.01 and compared with vehicle controls.

Figure 4

Retinal damage ratio in N-methyl-N-nitrosourea (MNU)-treated p53^−/−, p53^+/−, and p53^+/+ mice. The retinal damage ratio was calculated as the percentage of length of retina composed of less than four photoreceptor cells on the length of whole retina. Asterisk is p<0.01 and compared with vehicle controls.