Probing cataractogenesis associated with mevalonic aciduria

Retinitis pigmentosa and punctate cataracts in mevalonic aciduria

BACKGROUND

Mevalonic aciduria, caused by deficiency of mevalonate kinase, was the first recognized defect in the biosynthesis of cholesterol and isoprenoids. Ophthalmic features of this potentially blinding disorder include blue sclera, cataract, uveitis, optic atrophy, and, importantly, a retinitis pigmentosa-like retinopathy. To date, ∼30 cases of this rare autosomal recessive disorder have been reported, with no full characterization appearing in the ophthalmic literature.

METHODS

An 11-year-old white girl with mevalonic aciduria presented with decreasing peripheral vision and night blindness.

RESULTS

Examination revealed decreased central vision, punctate cataracts, and a retinitis pigmentosa-like retinopathy. Goldmann visual fields confirmed severe constriction in both eyes. A full-field electroretinogram was performed. A-waves, B-waves, and oscillatory potentials were all nonrecordable in both eyes, indicating severe bilateral retinopathy, affecting both cone- and rod-mediated responses. Dark adaptation testing showed severely impaired cone and rod function under dark-adapted (scotopic) conditions. Farnsworth-Munsell hue discrimination (FM-100 hue) testing was abnormal in both eyes.

CONCLUSION

The ocular findings in patients with mevalonic aciduria are heterogeneous and include blue sclerae, cataracts, uveitis, retinopathy, and optic atrophy. Visual prognosis is guarded; several patients surviving to adulthood have progressed to apparent legal blindness caused by cataracts and/or retinopathy.

Discordant expression of the sterol pathway in lens underlies simvastatin-induced cataracts in Chbb: Thom rats

Simvastatin rapidly induced cataracts in young Chbb:Thom (CT) but not Sprague Dawley (SD) or Hilltop Wistar (HW) rats. Oral treatment for 14 but not 7 days committed CT rat lenses to cataract formation. The cholesterol to phospholipid molar ratio in lenses of treated CT rats was unchanged. Differences between strains in serum and ocular humor levels of simvastatin acid poorly correlated with susceptibility to cataracts. No significant differences were found between rat strains in the capacity of simvastatin acid to inhibit lens-basal sterol synthesis. Prolonged treatment with simvastatin comparably elevated HMG-CoA reductase protein and enzyme activity in lenses of both cataract resistant and sensitive strains. However, in contrast to SD and HW rats, where sterol synthesis was markedly increased, sterol synthesis in CT rat lenses remained at baseline. Discordant expression of sterol synthesis in CT rats may be due to inadequate upregulation of lens HMG-CoA synthase. HMG-CoA synthase protein levels, and to a much lesser extent mRNA levels, increased in lens cortex of SD but not CT rats. Because upregulation of the sterol pathway may result in increased formation of isoprene-derived anti-inflammatory substances, failure to upregulate the pathway in CT rat lenses may reflect an attenuated compensatory response to injury that resulted in cataracts.