Mevalonic aciduria: pathobiochemical effects of mevalonate kinase deficiency on cholesterol metabolism in intact fibroblasts

Severe phenotypic spectrum of mevalonate kinase deficiency with minimal mevalonic aciduria

Mevalonate kinase deficiency is a rare autosomal recessively inherited organic aciduria with a complex multi-systemic phenotype. We describe two deceased patients with clinically severe mevalonate kinase (MK) deficiency confirmed by MK mutation analysis. The phenotype in our patients ranged from neonatal hydrops in the first patient to severe failure to thrive, hepatosplenomegaly, recurrent febrile episodes and lymphadenopathy in the second. Both infants excreted relatively low amounts of mevalonic acid intermittently.

Diagnostics and therapeutic insights in a severe case of mevalonate kinase deficiency

Mevalonate kinase deficiency is a rare inborn disorder of isoprenoid and sterol biosynthesis characterized by a recurrent autoinflammatory syndrome and, in most severe cases, psychomotor delay. Clinical manifestations can be very complex and, in some cases, mimic a chronic inflammatory disease. Diagnosis is also complex and often requires immunologic, genetic, and biochemical investigations. There is no standardized therapy, but biological agents could help to control inflammatory complaints in some cases. A severe case of mevalonate kinase deficiency that was associated with nephritis and successfully treated with anakinra (interleukin 1 receptor antagonist) is reported here, and new insights into diagnosis and therapy of this complex disorder are discussed.

Isoprenoids: Remarkable diversity of form and function

The isoprenoid biosynthetic pathway is the source of a wide array of products. The pathway has been highly conserved throughout evolution, and isoprenoids are some of the most ancient biomolecules ever identified, playing key roles in many life forms. In this review we focus on C-10 mono-, C-15 sesqui-, and C-20 diterpenes. Evidence for interconversion between the pathway intermediates farnesyl pyrophosphate and geranylgeranyl pyrophosphate and their respective metabolites is examined. The diverse functions of these molecules are discussed in detail, including their ability to regulate expression of the beta-HMG-CoA reductase and Ras-related proteins. Additional topics include the mechanisms underlying the apoptotic effects of select isoprenoids, antiulcer activities, and the disposition and degradation of isoprenoids in the environment. Finally, the significance of pharmacological manipulation of the isoprenoid pathway and clinical correlations are discussed.

Regulation of isoprenoid/cholesterol biosynthesis in cells from mevalonate kinase-deficient patients

Mevalonic aciduria (MA) and hyper-IgD and periodic fever syndrome (HIDS) are two inherited disorders both caused by depressed mevalonate kinase (MK) activity. MK is the first enzyme to follow the highly regulated 3-hydroxy-3-methylglutaryl (HMG)-CoA reductase (HMGR), which catalyzes the rate-limiting step in the isoprenoid/cholesterol biosynthesis pathway. In fibroblasts of MA patients, but not of HIDS patients, HMGR activity is elevated under normal growth conditions. This activity is down-regulated when cells are supplemented with the isoprenoid precursors geraniol, farnesol, and geranylgeraniol, and a mixture of 25-hydroxycholesterol and cholesterol. This indicates that the regulation of the pathway in these cells is not disturbed. The elevated HMGR activity is probably due to a shortage of non-sterol isoprenoid end products, as indicated by normal HMGR mRNA levels in MA fibroblasts. Furthermore, the HMGR activity in MA cells was more sensitive to geranylgeraniol suppression and less sensitive to sterol suppression than the HMGR activity in low density lipoprotein receptor-deficient cells. HMGR activity in MA cells was down-regulated also by addition of its product mevalonate to the culture medium. Thus, it appears that the elevation of mevalonate levels, which are high in MA patients and moderate in HIDS patients, allows the cells to compensate for the depressed MK activity. Indeed, the isoprenylation of Ras and RhoA protein appeared normal in HIDS and MA fibroblasts under normal conditions but showed increased sensitivity toward inhibition of HMGR by simvastatin. Our results indicate that MK-deficient cells maintain the flux through the isoprenoid/cholesterol biosynthesis pathway by elevating intracellular mevalonate levels.

Increased urinary leukotriene E(4) during febrile attacks in the hyperimmunoglobulinaemia D and periodic fever syndrome

BACKGROUND

The hyperimmunoglobulinaemia D and periodic fever syndrome is a hereditary periodic fever, caused by deficiency of the enzyme mevalonate kinase. It is unclear how this defect leads to recurrent fever episodes.

AIM

To assess the involvement of cysteinyl leukotrienes in the pathogenesis of fever attacks as reflected by urinary leukotriene E(4) (LTE(4)) excretion.

METHODS

Urinary LTE(4) was measured in seven patients while febrile and afebrile.

RESULTS

LTE(4) was raised during fever in all subjects (46-199 nmol/mol creatinine, mean 92; normal <40). Urinary LTE(4) was normal between attacks, as well as in normal children with fever as a result of miscellaneous causes.

CONCLUSION

Our results suggest that cysteinyl leukotrienes play a role in the pathophysiology of this disorder. As no effective treatment is yet available, leukotriene receptor antagonists might offer a new therapeutic approach for patients with the hyperimmunoglobulinaemia D and periodic fever syndrome.

Normal cholesterol synthesis in human cells requires functional peroxisomes

To evaluate the importance of peroxisomes in cholesterol metabolism we measured the rate of cholesterol synthesis in cultured skin fibroblasts from 16 patients in whom deficiency of peroxisomes had been established. Seven complementation groups were studied, consisting of one six member group, one three member group, three groups comprising single cases and two groups with two cases each. On the average, cholesterol synthesis was below control values in all the 16 peroxisome-deficient fibroblast cell cultures. The range of cholesterol synthesis in these cells was 2% to 84% of normal values. These data strongly suggest that peroxisomes are essential for normal cholesterol synthesis in human fibroblasts.

Phenotypic heterogeneity in the syndromes of 3-methylglutaconic aciduria

Combined 3-methylglutaconic and 3-methylglutaric aciduria, one of the more common urinary organic acid abnormalities, has been observed in at least three clinical syndromes. We studied an additional seven patients with 3-methylglutaconic aciduria, four of whom were best categorized as having the type II syndrome, two as having an "unspecified" syndrome, and one who may have had a primary urea cycle defect. In cultured cells and autopsy tissues derived from patients with the type II and unspecified syndromes, we were unsuccessful in identifying a defect in the leucine degradative pathway distal to 3-methylcrotonyl-coenzyme A carboxylase and in the cholesterol biosynthetic pathway between 3-hydroxy-3-methylglutaryl-coenzyme A reductase and diphosphomevalonate decarboxylase. Further assessment of the cholesterol biosynthetic pathway in several patients with one of the two types of disease also provided no defined abnormality. The primary metabolic defects in the type II and unspecified syndromes remain undefined.

Facts and artefacts in mevalonic aciduria: development of a stable isotope dilution GCMS assay for mevalonic acid and its application to physiological fluids, tissue samples, prenatal diagnosis and carrier detection

A stable isotope dilution assay using D3-mevalonic acid was developed and applied to the study of mevalonic aciduria. The method also appears to be suitable for the evaluation of different therapeutic regimens in patients with hypercholesterolemia. Mevalonic acid was isolated by liquid partition chromatography and quantified as the underivatized lactone by means of ammonia chemical ionization selected ion monitoring capillary gas chromatography-mass spectrometry. In heterozygotes there was significantly greater urinary excretion of mevalonic acid, while the range of enzymatic activity of mevalonate kinase showed an overlap with that of controls. The analysis of amniotic fluids of two pregnancies at risk for mevalonic aciduria showed a 3277-fold elevation as compared to controls in the first case, diagnostic of an affected fetus, and a normal value in the second one. Mevalonic acid concentration was much increased in tissues of the affected and aborted fetus. Concentrations ranged from 840 to 1120 mumol/kg in various tissues and were as high as 1810 mumol/kg in brain. Concentrations in control fetal tissues were approximately 1 mumol/kg.