Mevalonic Aciduria — An Inborn Error of Cholesterol and Nonsterol Isoprene Biosynthesis

A two-year-old boy presented with severe failure to thrive, developmental delay, anemia, hepatosplenomegaly, central cataracts, and dysmorphic features. Quantitative analyses of urinary organic acids revealed massive excretion of mevalonic acid, a metabolic precursor of cholesterol and nonsterol isoprenes: 46,000 to 56,200 mmol per mole of creatinine, as compared with 0.2 to 0.3 mmol per mole in normal children. The mevalonic acid concentration in plasma was also greatly increased at 440 μmol per liter (normal, <0.05). The activity of mevalonate kinase, the enzyme that catalyzes the first step in mevalonate metabolism, was severely deficient in the patient's fibroblasts, lymphocytes, and lymphoblasts. In the subsequent pregnancy of the patient's mother, gas chromatography–mass spectrometry demonstrated a marked elevation of mevalonic acid in the mother's urine and a 3000-fold elevation, as compared with control levels in the amniotic fluid, suggesting that the fetus was affected. The diagnosis was confirmed by demonstration of the deficiency of mevalonate kinase in amniocytes and ultimately in liver from the abortus. Intermediate activities of the enzyme in both parents indicated an autosomal recessive mode of inheritance. These observations identify an inherited disorder of cholesterol and nonsterol isoprene biosynthesis in humans. (N Engl J Med 1986; 314: 1610–4.), MAMMALIAN cells have a great demand for cholesterol for the biosynthesis of membranes, bile acids, and steroid hormones. This requirement is met through two interacting pathways. The first is the uptake of external cholesterol, which is facilitated by the low-density-lipoprotein receptor. This receptor is abnormal in patients with familial hypercholesterolemia.¹ The second and quantitatively more important source of cholesterol² is de novo synthesis from acetate by means of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) (Fig. 1). This pathway also supplies the cells with at least four additional nonsterol isoprenes: ubiquinone, heme A, dolichol, and isopentenyl adenine.^{3 4 5} The synthesis of the first committed intermediate of.