Peter Vreken

Mutations in the 3β-hydroxysterol Δ24-reductase gene cause desmosterolosis, an autosomal recessive disorder of cholesterol biosynthesis

Desmosterolosis is a rare autosomal recessive disorder characterized by multiple congenital anomalies. Patients with desmosterolosis have elevated levels of the cholesterol precursor desmosterol, in plasma, tissue, and cultured cells; this abnormality suggests a deficiency of the enzyme 3β-hydroxysterol Δ24-reductase (DHCR24), which, in cholesterol biosynthesis, catalyzes the reduction of the Δ24 double bond of sterol intermediates. We identified the human DHCR24 cDNA, by the similarity between the encoded protein and a recently characterized plant enzyme—DWF1/DIM, from Arabidopsis thaliana—catalyzing a different but partially similar reaction in steroid/sterol biosynthesis in plants. Heterologous expression, in the yeast Saccharomyces cerevisiae, of the DHCR24 cDNA, followed by enzyme-activity measurements, confirmed that it encodes DHCR24. The encoded DHCR24 protein has a calculated molecular weight of 60.1 kD, contains a potential N-terminal secretory-signal sequence as well as at least one putative transmembrane helix, and is a member of a recently defined family of flavin adenine dinucleotide (FAD)–dependent oxidoreductases. Conversion of desmosterol to cholesterol by DHCR24 in vitro is strictly dependent on reduced nicotinamide adenine dinucleotide phosphate and is increased twofold by the addition of FAD to the assay. The corresponding gene, DHCR24, was identified by database searching, spans ∼46.4 kb, is localized to chromosome 1p31.1-p33, and comprises nine exons and eight introns. Sequence analysis of DHCR24 in two patients with desmosterolosis revealed four different missense mutations, which were shown, by functional expression, in yeast, of the patient alleles, to be disease causing. Our data demonstrate that desmosterolosis is a cholesterol-biosynthesis disorder caused by mutations in DHCR24.

Genotype and phenotype in patients with dihydropyrimidine dehydrogenase deficiency

Abstract Dihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal recessive disease characterised by thymine-uraciluria in homozygous deficient patients and has been associated with a variable clinical phenotype. In order to understand the genetic and phenotypic basis for DPD deficiency, we have reviewed 17 families presenting 22 patients with complete deficiency of DPD. In this group of patients, 7 different mutations have been identified, including 2 deletions [295–298delTCAT, 1897delC], 1 splice-site mutation [IVS14+1G>A)] and 4 missense mutations (85T>C, 703C>T, 2658G>A, 2983G>T). Analysis of the prevalence of the various mutations among DPD patients has shown that the G→A point mutation in the invariant splice donor site is by far the most common (52%), whereas the other six mutations are less frequently observed. A large phenotypic variability has been observed, with convulsive disorders, motor retardation and mental retardation being the most abundant manifestations. A clear correlation between the genotype and phenotype has not been established. An altered β-alanine, uracil and thymine homeostasis might underlie the various clinical abnormalities encountered in patients with DPD deficiency.

Disorders of mitochondrial fatty acyl-CoA β-oxidation

In recent years tremendous progress has been made with respect to the enzymology of the mitochondrial fatty acid β-oxidation machinery and defects therein. Firstly, a number of new mitochondrial β-oxidation enzymes have been identified, including very-long-chain acyl-CoA dehydrogenase (VLCAD) and mitochondrial trifunctional protein (MTP). Secondly, the introduction of tandem MS for the analysis of plasma acylcarnitines has greatly facilitated the identification of patients with a defect in fatty acid oxidation (FAO). These two developments explain why the number of defined FAO disorders has increased dramatically, making FAO disorders the most rapidly growing group of inborn errors of metabolism. In this review we describe the current state of knowledge of the enzymes involved in the mitochondrial oxidation of straight-chain, branched-chain and (poly)unsaturated fatty acyl-CoAs as well as disorders of fatty acid oxidation. The laboratory diagnosis of these disorders is described, with particular emphasis on the methods used to identify the underlying enzyme defect and the molecular mutations. In addition, a simple flowchart is presented as a guide to the identification of mitochondrial FAO-disorders. Finally, treatment strategies are discussed briefly.

Significantly reduced docosahexaenoic and docosapentaenoic acid concentrations in erythrocyte membranes from schizophrenic patients compared with a carefully matched control group

BACKGROUND Fatty acid research in schizophrenia has demonstrated an altered cell membrane phospholipid metabolism. Erythrocyte membrane phospholipid composition closest reflects that of neuronal membranes. METHODS (Poly)(un)saturated fatty acid concentrations were measured in the erythrocyte membranes of 19, consecutively admitted, medicated young schizophrenic patients and then compared with matched control subjects. Psychiatric symptomatology was rated with the Positive and Negative Symptom Scale and Montgomery-Asberg Depression Rating Scale. Because diet, hormones, and cannabis influence fatty acid metabolism, we included these factors in our study. RESULTS The most distinctive findings concerned the omega-3 series: C22:5 omega-3, C22:6 omega-3 (docosahexaenoic acid), and the sum of omega-3 fatty acids were significantly decreased. Interestingly, C20:4 omega-6 (arachidonic acid) was not lowered. In the omega-9 series, higher levels of C22:1 omega-9 and lower levels its elongation product, C24:1 omega-9 (nervonic acid), were found. Interestingly, the other arm of the desaturation-elongation sequence of C18:1 omega-9, C20:3 omega-9, was lower in patients. The total omega-9 fatty acid levels were also lower in patients. CONCLUSIONS Significant differences in erythrocyte fatty acid composition were found. The differences were not due to diet or hormonal status and could not be explained by the medication or cannabis use. No consistent pattern emerged from the different fatty acid abnormalities and the clinical symptom scores.

Quantitative plasma acylcarnitine analysis using electrospray tandem mass spectrometry for the diagnosis of organic acidaemias and fatty acid oxidation defects

Electrospray tandem mass spectrometry (ESI-MS/MS) of acylcarnitines has been successfully applied in newborn screening for defects in fatty acid oxidation and organic acidaemias (Millington et al 1990; Rashed et al 1995a, 1997). In addition, acylcarnitine analysis has also been applied for the selective screening for these disorders (Chace et al 1997; Van Hove et al 1993, 1995), for post-mortem diagnosis using bile fluid (Rashed et al 1995b) and for prenatal diagnosis of organic acidaemias and defects of fatty acid oxidation (Nada et al 1996; Shigematsu et al 1996). Since for selective screening for inborn errors of metabolism, apart from urine samples, mostly serum or plasma samples are sent in, we developed a quantitative ESI-MS/MS acylcarnitine analysis in plasma and included the analysis of free carnitine in the same assay. The results show that this analysis is highly sensitive and reproducible and therefore suitable for selective screening of fatty acid oxidation defects, organic acidaemias and secondary carnitine deficiency.

Inborn errors of pyrimidine degradation: clinical, biochemical and molecular aspects

The pyrimidines, uracil and thymine, are degraded in four steps. The first three steps of pyrimidine catabolism, controlled by enzymes shared by both pathways, result in the production of the neurotransmitter amino acid β-alanine from uracil and the nonfunctional (R)-(-)-β-aminoisobutyrate from thymine. The fourth step is controlled by several aminotransferases, which have different affinities for β-alanine, β-aminoisobutyrate and GABA. Defects concerning the first three steps all lead to a reduced production of β-alanine; defects of the transaminases involving the metabolism of β-alanine and GABA lead to accumulation of these neurotransmitter substances. In addition, other metabolites will accumulate or be reduced depending on the specific enzyme defect. Analysis of the abnormal concentrations of these metabolites in the body fluids is essential for the detection of patients with pyrimidine degradation defects. Clinically these disorders are often overlooked because symptomatology is highly aspecific. The growth in our knowledge concerning inborn errors of pyrimidine degradation has emphasized the importance of the clinical awareness of these defects as a possible cause of neurological disease and a contraindication for treatment of cancer patients with certain pyrimidine analogues. The various defects are discussed and attention is paid to clinical, genetic and diagnostic aspects.

A point mutation in an invariant splice donor site leads to exon skipping in two unrelated Dutch patients with dihydropyrimidine dehydrogenase deficiency

SummaryDihydropyrimidine dehydrogenase (DPD) deficiency is an autosomal recessive disease characterized by thymine-uraciluria and associated with a variable clinical phenotype. In order to identify the molecular defect underlying complete DPD deficiency in a Dutch patient previously shown to have a 165 base pair deletion in the mature DPD mRNA, we cloned the genomic region encompassing the skipped exon and its flanking intron sequences. Sequence analysis revealed that the patient was homozygous for a single G→A point mutation in the invariant GT dinucleotide splice donor site downstream of the skipped exon. The same mutation was identified in another, unrelated, Dutch patient. Because this mutation destroys a uniqueMaeII restriction site, rapid screening using restriction enzyme cleavage of the amplified genomic region encompassing this mutation is possible. Analysis of 50 controls revealed no individuals heterozygous for this mutation

Nomenclature for human DPYD alleles.

To standardize DPYD allele nomenclature and to conform with international human gene nomenclature guidelines, an alternative to the current arbitrary system is described. Based on recommendations for human genome nomenclature, we propose that each distinct allele be designed by DPYD followed by an asterisk and an Arabic numeral. The number specifies the key mutation and, where appropriate, a letter following the number indicates an additional mutation on the mutant allele. Criteria for classification as a distinct allele are also presented.

Rapid stable isotope dilution analysis of very-long-chain fatty acids, pristanic acid and phytanic acid using gas chromatography-electron impact mass spectrometry.

A common feature of most peroxisomal disorders is the accumulation of very-long-chain fatty acids (VLCFAs) and/or pristanic and phytanic acid in plasma. Previously described methods utilizing either gas chromatography alone or gas chromatography-mass spectrometry are, in general, time-consuming and unable to analyze VLCFAs, pristanic and phytanic acid within a single analysis. We describe a simple, reproducible and rapid method using gas chromatography/mass spectrometry with deuterated internal standards. The method was evaluated by analysing 30 control samples and samples from 35 patients with defined peroxisomal disorders and showed good discrimination between controls and patients. This method is suitable for routine screening for peroxisomal disorders.

Rapid analysis of conjugated bile acids in plasma using electrospray tandem mass spectrometry: application for selective screening of peroxisomal disorders

The final step in bile acid biosynthesis takes place in peroxisomes and involves oxidative cleavage of the side-chain of C 27 -β-cholestanoic acids, leading to the formation of the primary bile acids cholic and chenodeoxycholic acid. Therefore, in a number of peroxisomal disorders, including both peroxisomal biogenesis defects and isolated defects of peroxisomal β-oxidation, the accumulation of serum C 27 -5β-cholestanoic acids can be observed (Bjorkhem 1994; Clayton et al 1987; Libert et al 1991; Wanders et al 1995). Since routine gas chromatography mass-spectrometry (GC-MS) analysis of plasma bile acids requires laborious sample preparation including enzymatic and/or chemical deconjugation of plasma bile acids prior to derivatization and GC-MS analysis, we have developed a rapid HPLC electrospray tandem mass spectrometry (ESI-MS/MS) method for analysing taurine- and glycine-conjugated bile acids.