Untargeted metabolomics and infrared ion spectroscopy identify biomarkers for pyridoxine-dependent epilepsy

BackgroundPyridoxine-dependent epilepsy (PDE-ALDH7A1) is an inborn error of lysine catabolism that presents with refractory epilepsy in newborns. Biallelic ALDH7A1 variants lead to deficiency of α-aminoadipic semialdehyde dehydrogenase/antiquitin, resulting in accumulation of piperideine-6-carboxylate (P6C), and secondary deficiency of the important cofactor pyridoxal-5'-phosphate (PLP, active vitamin B6) through its complexation with P6C. Vitamin B6 supplementation resolves epilepsy in patients, but intellectual disability may still develop. Early diagnosis and treatment, preferably based on newborn screening, could optimize long-term clinical outcome. However, no suitable PDE-ALDH7A1 newborn screening biomarkers are currently available.MethodsWe combined the innovative analytical methods untargeted metabolomics and infrared ion spectroscopy to discover and identify biomarkers in plasma that would allow for PDE-ALDH7A1 diagnosis in newborn screening.ResultsWe identified 2S,6S-/2S,6R-oxopropylpiperidine-2-carboxylic acid (2-OPP) as a PDE-ALDH7A1 biomarker, and confirmed 6-oxopiperidine-2-carboxylic acid (6-oxoPIP) as a biomarker. The suitability of 2-OPP as a potential PDE-ALDH7A1 newborn screening biomarker in dried bloodspots was shown. Additionally, we found that 2-OPP accumulates in brain tissue of patients and Aldh7a1-knockout mice, and induced epilepsy-like behavior in a zebrafish model system.ConclusionThis study has opened the way to newborn screening for PDE-ALDH7A1. We speculate that 2-OPP may contribute to ongoing neurotoxicity, also in treated PDE-ALDH7A1 patients. As 2-OPP formation appears to increase upon ketosis, we emphasize the importance of avoiding catabolism in PDE-ALDH7A1 patients.FundingSociety for Inborn Errors of Metabolism for Netherlands and Belgium (ESN), United for Metabolic Diseases (UMD), Stofwisselkracht, Radboud University, Canadian Institutes of Health Research, Dutch Research Council (NWO), and the European Research Council (ERC).

3-Methylglutaconic aciduria type I redefined: a syndrome with late-onset leukoencephalopathy

OBJECTIVE

3-Methylglutaconic aciduria type I is a rare inborn error of leucine catabolism. It is thought to present in childhood with nonspecific symptoms; it was even speculated to be a nondisease. The natural course of disease is unknown.

METHODS

This is a study on 10 patients with 3-methylglutaconic aciduria type I. We present the clinical, neuroradiologic, biochemical, and genetic details on 2 new adult-onset patients and follow-up data on 2 patients from the literature.

RESULTS

Two unrelated patients with the characteristic biochemical findings of 3- methylglutaconic aciduria type I presented in adulthood with progressive ataxia. One patient additionally had optic atrophy, the other spasticity and dementia. Three novel mutations were found in conserved regions of the AUH gene. In both patients, MRI revealed extensive white matter disease. Follow-up MRI in a 10-year-old boy, who presented earlier with isolated febrile seizures, showed mild abnormalities in deep white matter.

CONCLUSION

We define 3-methylglutaconic aciduria type I as an inborn error of metabolism with slowly progressive leukoencephalopathy clinically presenting in adulthood. In contrast to the nonspecific findings in pediatric cases, the clinical and neuroradiologic pattern in adult patients is highly characteristic. White matter abnormalities may already develop in the first decades of life. The variable features found in affected children may be coincidental. Long-term follow-up in children is essential to learn more about the natural course of this presumably slowly progressive disease. Dietary treatment with leucine restriction may be considered.

The Gln223Arg polymorphism in the leptin receptor is associated with familial combined hyperlipidemia

OBJECTIVE

Familial combined hyperlipidemia (FCH) is characterized by elevated levels of total cholesterol (TC), triglycerides (TG) and apolipoprotein B (apo B) and is associated with premature cardiovascular disease (CVD). Other features of FCH are obesity and insulin resistance. Serum leptin levels have also been associated with obesity, insulin resistance and atherosclerosis. Leptin exerts its effect through the leptin receptor (LEPR). The aim of this study is to determine whether the Gln223Arg polymorphism in the LEPR gene contributes to FCH and its associated phenotypes.

METHODS

The study population consists of 37 families, comprising 644 subjects, of whom 158 subjects were diagnosed as FCH. The FCH diagnosis was based on plasma TC and TG levels, adjusted for age and gender, and absolute apo B levels, according to our recently published nomogram. The Gln223Arg polymorphism was studied by restriction fragment length polymorphism-PCR.

RESULTS

Carriers of one or two Arg alleles had an increased risk of FCH, compared to subjects homozygous for the Gln allele (OR=1.6 [95% CI 1.0-2.4]). A difference in high-density lipoprotein cholesterol (HDL-c) levels was present between carriers and non-carriers of an Arg allele, 1.21 vs 1.28 mmol/l, respectively (P=0.04), but no differences in obesity, insulin resistance and other lipid parameters were found.

CONCLUSION

The Gln223Arg polymorphism in the LEPR gene is associated with FCH, which is supported by a significant association between HDL-c levels and the LEPR gene.

The effect of dantrolene sodium in Very Long Chain Acyl-CoA Dehydrogenase Deficiency

We present a patient, who experienced recurrent episodes of rhabdomyolysis. Her beneficial response to treatment with dantrolene sodium was previously reported. Adult onset Very Long Chain Acyl-CoA Dehydrogenase (VLCAD) deficiency has been diagnosed only recently. In adults, VLCAD deficiency results in recurrent fasting-, exercise-, or infection-induced muscle stiffness, muscle pain and myoglobinuria caused by rhabdomyolysis. This case illustrates for the first time the beneficial effect of dantrolene in VLCAD deficiency. We discuss the therapeutic mechanism of dantrolene sodium and its possible role as additional treatment modality for patients with VLCAD deficiency.

Phase I clinical and pharmacogenetic trial of irinotecan and raltitrexed administered every 21 days to patients with cancer

PURPOSE

Irinotecan and raltitrexed display schedule-dependent synergy in vitro, which supports the clinical investigation of the combination. Functional polymorphisms of the methylenetetrahydrofolate reductase (MTHFR) gene result in intracellular redistribution of folate derivatives, which may affect raltitrexed-associated cytotoxicity.

PATIENTS AND METHODS

Patients with a range of solid cancers and good performance status received irinotecan as a 90-minute infusion on day 1 and raltitrexed as a 15-minute infusion on day 2, repeated every 21 days. Samples were collected for MTHFR C677T genotyping and fasting plasma homocysteine during the first cycle.

RESULTS

Thirty-nine assessable patients received 127 cycles of therapy. Irinotecan doses ranged from 100 to 350 mg/m(2), and raltitrexed, 1.0 to 4.0 mg/m(2). Raltitrexed doses of more than 3.0 mg/m(2) were not tolerated and were associated with dose-limiting asthenia, diarrhea, and AST/ALT elevation. Irinotecan/raltitrexed doses of 350/3.0 mg/m(2) were well-tolerated; principal toxicities included neutropenia, diarrhea, and fatigue. Two partial responses were observed in patients with pretreated gastroesophageal cancers. Homozygotes with the MTHFR 677 TT polymorphism incurred significantly less raltitrexed-associated toxicity than those with either wild-type or heterozygous genotypes (P = .05). No significant differences were noted in plasma homocysteine values between the genotypic subtypes, and plasma homocysteine levels did not predict the risk of toxicity.

CONCLUSION

Irinotecan and raltitrexed doses of 350 and 3.0 mg/m(2) are recommended for further study on a day 1, 2 schedule every 21 days. Efficacy results suggest that trials in upper and lower gastrointestinal malignancies are warranted. MTHFR C677T genotypes may be predictive of clinical raltitrexed toxicity.

A second common variant in the methylenetetrahydrofolate reductase (MTHFR) gene and its relationship to MTHFR enzyme activity, homocysteine, and cardiovascular disease risk

Molecular defects in genes encoding enzymes involved in homocysteine metabolism may account for mild hyperhomocysteinemia, an independent and graded risk factor for cardiovascular disease (CVD). We examined the relationship of two polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene, the 677C-->T and 1298A-->C variants, to MTHFR activity, homocysteine concentrations, and risk of CVD in a population of 190 vascular disease patients and 601 apparently healthy controls. The mean specific and residual MTHFR activities were significantly lower in 677CT and 677TT individuals (both P<0.001). The 1298A-->C mutation alone showed no effect on MTHFR activities. However, when the 677C-->T genotype was taken into account, the 1298A-->C mutation also caused a significant decrease in MTHFR activities, which was observed in both the homozygous 1298CC (P<0.001) and the heterozygous 1298AC states (P=0.005). Both the 677TT as the 677CT genotypes were associated with significantly higher fasting and postload homocysteine levels than 677CC (P<0.001 and P=0.003, respectively). The 1298A-->C mutation had no effect on fasting or postload homocysteine levels. Since homocysteine itself is considered to be positively associated with the risk of CVD, these findings indicate that the 1298A-->C mutation cannot be considered a major risk factor for CVD.

The methionine synthase reductase (MTRR) A66G polymorphism is a novel genetic determinant of plasma homocysteine concentrations

Epidemiological evidence has revealed that an elevated plasma homocysteine level (hyperhomocysteinemia) confers an increased risk of cardiovascular disease and neural tube defects. Hyperhomocysteinemia is caused by both nutritional (e.g. folate, vitamins B(6) and B(12)) and genetic factors, including functional polymorphisms of key enzymes involved in homocysteine metabolism. One such enzyme, methionine synthase reductase (MTRR), maintains adequate levels of methylcob(III)alamin, the activated cofactor for methionine synthase, which catalyzes the remethylation of homocysteine to methionine. A common MTRR polymorphism, i.e. a 66 A-->G substitution specifying an isoleucine to methionine substitution (I22M), was recently identified. To assess the influence of this polymorphism on total plasma homocysteine (tHcy), we undertook a genotype/phenotype analysis in a study population of 601 Northern-Irish men, aged 30--49, for which biochemical and genetic data relevant to folate/homocysteine metabolism had already been acquired. The 66AA genotype has a frequency of 29% in this population. We established that there was a significant influence of MTRR genotype on tHcy ranking (P=0.004) and that the 66AA genotype contributes to a moderate increase in tHcy levels across the distribution [OR 1.59 (95% CI: 1.10--2.25) for the 66AA genotype to be in the upper half of the tHcy distribution, P=0.03]. The homocysteine-elevating effect of the 66AA genotype is independent of serum folate, vitamin B(12) and vitamin B(6) levels. Based on published estimates of the enhanced cardiovascular disease risk conferred by defined increments of plasma tHcy, we estimate that 66AA homozygotes have, on average, an approximately 4% increase in cardiovascular disease risk compared to 66GG homozygotes. This study provides the first evidence that the MTRR A66G polymorphism significantly influences the circulating tHcy concentration.

A 31 bp VNTR in the cystathionine beta-synthase (CBS) gene is associated with reduced CBS activity and elevated post-load homocysteine levels

Molecular defects in genes encoding enzymes involved in homocysteine metabolism may account for mild hyperhomocysteinaemia, an independent and graded risk factor for cardiovascular disease (CVD). Although heterozygosity for cystathionine beta-synthase (CBS) deficiency has been excluded as a major genetic cause of mild hyperhomocysteinaemia in vascular disease, mutations in (non-)coding DNA sequences may lead to a mildly decreased CBS expression and, consequently, to elevated plasma homocysteine levels. We assessed the association between a 31 bp VNTR, that spans the exon 13-intron 13 boundary of the CBS gene, and fasting, post-methionine load and increase upon methionine load plasma homocysteine levels in 190 patients with arterial occlusive disease, and in 381 controls. The 31 bp VNTR consists of 16, 17, 18, 19 or 21 repeat units and shows a significant increase in plasma homocysteine concentrations with an increasing number of repeat elements, in particular after methionine loading. In 26 vascular disease patients the relationship between this 31 bp VNTR and CBS enzyme activity in cultured fibroblasts was studied. The CBS enzyme activity decreased with increasing number of repeat units of the 31 bp VNTR. RT-PCR experiments showed evidence of alternative splicing at the exon 13-intron 13 splice junction site. The 31 bp VNTR in the CBS gene is associated with post-methionine load hyperhomocysteinaemia that may predispose individuals to an increased risk of cardiovascular diseases.

Methylenetetrahydrofolate reductase genotypes and predisposition to atherothrombotic disease; evidence that all three MTHFR C677T genotypes confer different levels of risk

AIMS

Elevated plasma homocysteine is an independent risk factor for atherothrombotic disease. Individuals homozygous for the methylenetetrahydrofolate reductase (MTHFR) 677C allele exclusively accumulate 5methyltetrahydrofolate, the methyl donor for homocysteine remethylation, in their red blood cells; this contrasts with 677 TT homozygotes who also accumulate significant levels of non-methylated folate derivatives. Those with the MTHFR 677 TT, CT and CC genotypes may therefore differ qualitatively with respect to folate utilization and hence their capacity to remethylate homocysteine. This study was consequently designed to establish whether all three genotypes confer different levels of atherothrombotic risk.

METHODS AND RESULTS

The risk of atherothrombotic disease conferred by the MTHFR 677 CT and 677 CC genotypes was assessed using a 'restricted' meta-analysis approach applied to subjects from the first ten studies reporting a significantly increased risk conferred by the 677 TT genotype. The defined risk of the TT genotype in each of these ten studies was judged by us to denote 'genetic vulnerability' in the populations from which subjects were drawn. After proportional adjustment for the greater number of case TT homozygotes, the CT and CC frequencies observed in cases were compared with expectations based on the frequencies of these genotypes in controls. The observed CT frequency among cases was higher than expected in eight of the ten studies. In the meta-analysis, which included 1857 cases and 2942 controls, 847 (45.6%) cases, instead of the 777 (41.8%) expected, had the MTHFR CT genotype (P=0.010).

CONCLUSIONS

Our findings suggest that the three MTHFR C677T genotypes confer different levels of atherothrombotic risk in 'genetically vulnerable' populations: CT heterozygotes have an elevated risk over CC homozygotes. One explanation is that the CT genotype actively confers atherothrombotic risk. An alternative interpretation however, for which a biologically plausible mechanism is proposed, is that CC is a protective genotype.

The human and mouse methylenetetrahydrofolate reductase (MTHFR) genes: genomic organization, mRNA structure and linkage to the CLCN6 gene

Methylenetetrahydrofolate reductase (MTHFR), a pivotal enzyme in folate metabolism, regulates the proportional distribution of one-carbon moieties between cellular methylation reactions and nucleic acid synthesis. The organization of the MTHFR gene and the structure of its mRNA were characterized in human and mouse. There are three mRNA transcripts of 2.8, 7.2 and 9.8 kb in human and two of 3.2 and 7.5 kb in mouse. Northern blot analysis revealed that human MTHFR MRNA is only present at low abundance in most tissues tested. Five kilobases of sequence flanking the 3' end of the human gene were isolated, and polyadenylation sites were defined by 3' RACE. The shorter 2.8 kb transcript and the two larger 7.2 and 9.8 kb transcripts utilize different polyadenylation signal sequences, 629 and 4937 bp downstream of the stop codon, respectively. The two mRNA species in mouse also result from differential polyadenylation. Approximately 7 and 3.5 kb upstream of the human and mouse genes, respectively, were isolated and sequenced. Transcription start sites in human MTHFR were mapped using 5' RACE. The 2.8 and 7.2 kb mRNAs originate from one of two transcription start sites that are 206 and 243 bp upstream of the ATG initiation codon, whereas transcription of the 9.8 kb mRNA is initiated at a start site located 2.8 kb upstream of the translation start codon. The putative MTHFR promoter does not have a TATA box but contains CpG islands and multiple potential Sp1 binding sites. The MTHFR gene was finely mapped to interval 16 of chromosome 1p36.3, a region deleted in many tumors, by establishing a close linkage to CLCN6, a putative chloride channel gene. A novel CA-repeat polymorphism identified within intron 2 of the CLCN6 gene may be useful in assessing loss of heterozygosity in such tumors. The multiple MTHFR mRNA species identified in this report may reflect an underlying complex set of gene regulatory mechanisms acting through an alternative transcription start site and/or polyadenylation signal sequence utilization.

Accurate and rapid "multiplex heteroduplexing" method for genotyping key enzymes involved in folate/homocysteine metabolism

BACKGROUND

Hyperhomocysteinemia, which is often associated with low folate status, is an independent risk factor for cardiovascular diseases and several other pathologies. The four most common functional polymorphisms in genes involved in folate/homocysteine metabolism are methylenetetrahydrofolate reductase (MTHFR) C677T and A1298C, methionine synthase (MS) A2756G, and cystathionine beta-synthase (CBS) 844ins68. The pathogenic impact of these variants is under active investigation in many laboratories. However, conventional genotyping methods, mostly using PCR followed by restriction enzyme digestion, often are compromised by partial fragment digestion. There is, therefore, a need to develop more reliable approaches to genotyping the above polymorphisms that may be applied in large-scale studies.

METHODS

Sequence-specific heteroduplex generators for each of the MTHFR and MS single nucleotide polymorphisms were generated by site-directed mutagenesis. These were subcloned into a single construct, pHcyHG-1, which could be multiplexed with a simple PCR amplification across the CBS 844ins68 polymorphic site to generate composite genotype-specific banding patterns from individual genomic DNA samples that could be electrophoretically resolved.

RESULTS

The "multiplex heteroduplexing" method yielded unambiguous MTHFR, MS, and CBS genotypes in a single-tube reaction that could be analyzed in a single gel run.

CONCLUSIONS

This method permits unambiguous genotyping of the four most common functional variants of enzymes involved in folate/homocysteine metabolism. It is rapid, reproducible, and inexpensive, and requires no special preparative or analytic facilities; consequently, it will facilitate large-scale studies of the genetic basis of hyperhomocysteinemia and the many pathologies that have been associated with this phenotype.

Cystathionine beta-synthase mutations in homocystinuria

The major cause of homocystinuria is mutation of the gene encoding the enzyme cystathionine beta-synthase (CBS). Deficiency of CBS activity results in elevated levels of homocysteine as well as methionine in plasma and urine and decreased levels of cystathionine and cysteine. Ninety-two different disease-associated mutations have been identified in the CBS gene in 310 examined homocystinuric alleles in more than a dozen laboratories around the world. Most of these mutations are missense, and the vast majority of these are private mutations. The two most frequently encountered of these mutations are the pyridoxine-responsive I278T and the pyridoxine-nonresponsive G307S. Mutations due to deaminations of methylcytosines represent 53% of all point substitutions in the coding region of the CBS gene.

The molecular basis of cystathionine beta-synthase deficiency in Dutch patients with homocystinuria: effect of CBS genotype on biochemical and clinical phenotype and on response to treatment

Homocystinuria due to cystathionine beta-synthase (CBS) deficiency, inherited as an autosomal recessive trait, is the most prevalent inborn error of methionine metabolism. Its diverse clinical expression may include ectopia lentis, skeletal abnormalities, mental retardation, and premature arteriosclerosis and thrombosis. This variability is likely caused by considerable genetic heterogeneity. We investigated the molecular basis of CBS deficiency in 29 Dutch patients from 21 unrelated pedigrees and studied the possibility of a genotype-phenotype relationship with regard to biochemical and clinical expression and response to homocysteine-lowering treatment. Clinical symptoms and biochemical parameters were recorded at diagnosis and during long-term follow-up. Of 10 different mutations detected in the CBS gene, 833T-->C (I278T) was predominant, present in 23 (55%) of 42 independent alleles. At diagnosis, homozygotes for this mutation (n=12) tended to have higher homocysteine levels than those seen in patients with other genotypes (n=17), but similar clinical manifestations. During follow-up, I278T homozygotes responded more efficiently to homocysteine-lowering treatment. After 378 patient-years of treatment, only 2 vascular events were recorded; without treatment, at least 30 would have been expected (P<.01). This intervention in Dutch patients significantly reduces the risk of cardiovascular disease and other sequelae of classical homocystinuria syndrome.

Detection of a novel deletion in the cystathionine beta-synthase (CBS) gene using an improved genomic DNA based method

We elucidated the intron-exon boundaries of the 15 coding exons of the human cystathionine beta-synthase (CBS) gene in order to establish an improved method based on PCR and direct sequencing for detection of CBS mutations. Using this method we identified the pathogenic mutations in two Danish siblings with CBS deficiency. Patients were compound heterozygotes: we detected the 833T-->C mutation and a novel 22 bp deletion of exon 4 (493-514del) that introduces a frameshift and a stop codon immediately after the deletion. The deletion resulted in no detectable mRNA from this allele, as assessed by sequencing of cDNA. The established method represents an improvement of the existing method based on sequencing of cDNA because it permits the detection of mutations within the entire coding region of the CBS gene from a peripheral blood sample, including splice mutations and mutations resulting in the lack or a reduced amount of transcript.

Identification of four novel mutations in severe methylenetetrahydrofolate reductase deficiency

Severe methylenetetrahydrofolate reductase (MTHFR) deficiency is an inborn error of folate metabolism, and is inherited as an autosomal recessive trait. MTHFR is a key enzyme in folate-dependent remethylation of homocysteine, and reduces 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate. Patients with this severe enzymatic deficiency are biochemically characterised by homocystinuria and hypomethioninaemia, and may suffer from neurological abnormalities, mental retardation and premature vascular disease. Here we report the molecular basis of severe MTHFR deficiency in four unrelated families from Turkish/Greek ancestry. By use of reverse-transcriptase (RT)-PCR, subsequently followed by direct sequencing analysis, we were able to identify four novel mutations in the MTHFR gene: two missense (983A-->G; 1027T-->G) and two nonsense (1084C-->T; 1711C-->T) mutations. Furthermore, a splice variant containing a premature termination codon, was observed in one patient, probably as a secondary effect of the 1027T-->G missense mutation. The ongoing identification and characterisation of mutations in the MTHFR gene will provide further insight into the heterogeneity of the clinical phenotype in severe MTHFR deficiency.

Homozygous cystathionine beta-synthase deficiency, combined with factor V Leiden or thermolabile methylenetetrahydrofolate reductase in the risk of venous thrombosis

Severe hyperhomocysteinemia in its most frequent form, is caused by a homozygous enzymatic deficiency of cystathionine beta-synthase (CBS). A major complication in CBS deficiency is deep venous thrombosis or pulmonary embolism. A recent report by Mandel et al (N Engl J Med 334:763, 1996) postulated factor V Leiden (FVL) to be an absolute prerequisite for the development of thromboembolism in patients with severe hyperhomocysteinemia. We studied 24 patients with homocystinuria caused by homozygous CBS deficiency from 18 unrelated kindreds for FVL and for the 677C-->T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene and investigated their possible interaction in the risk of venous thrombosis. Thrombotic complications were diagnosed in six patients, of whom only one was a carrier of FVL. On the contrary, thermolabile MTHFR caused by the 677C-->T mutation, was frequently observed among homocystinuria patients, especially among those with thromboembolic complications: three of six homocystinuria patients who had suffered from a thromboembolic event had thermolabile MTHFR. These data indicate that FVL is not an absolute prerequisite and probably not even a major determinant of venous thrombosis in homocystinuria, but, interestingly, thermolabile MTHFR may constitute a significant risk factor for thromboembolic complications in this inborn error of methionine metabolism.

Thermolabile methylenetetrahydrofolate reductase and factor V Leiden in the risk of deep-vein thrombosis

Mild hyperhomocysteinemia is an established risk factor for both arteriosclerosis and thrombosis, and may be caused by genetic and environmental factors. Methylenetetrahydrofolate reductase (MTHFR) catalyzes the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the cofactor for the methylation of homocysteine to methionine. Individuals with the thermolabile variant of MTHFR have decreased MTHFR activities, resulting in elevated plasma homocysteine concentrations. A homozygous 677C-->T transition in the MTHFR gene has recently been identified as the cause of reduced enzyme activity and thermolability of the protein. We studied the frequency of the homozygous mutant (+/+) genotype in 471 patients with deep-vein thrombosis and 474 healthy controls enrolled in The Leiden Thrombophilia Study (LETS), its interaction with factor V Leiden, and assessed the association between the MTHFR genotypes and plasma homocysteine concentration. Homozygosity for the 677C-->T polymorphism was observed in 47 (10%) patients, and in 47 (9.9%) controls (OR 1.01 [95% CI: 0.7-1.5]). No modified risk of the (+/+) genotype was observed in carriers of factor V Leiden. Our data suggest that, although the homozygous mutant genotype is associated with elevated plasma homocysteine concentrations, this homozygous mutation itself is not a genetic risk factor for deep-vein thrombosis, irrespective of factor V Leiden genotype.

Thermolabile methylenetetrahydrofolate reductase in coronary artery disease

BACKGROUND

Hyperhomocysteinemia, an independent and graded risk factor for coronary artery disease (CAD), may result from both environmental and hereditary factors. Methylenetetrahydrofolate reductase (MTHFR) catalyzes the conversion of methylenetetrahydrofolate to methyltetrahydrofolate, the methyl donor in the remethylation of homocysteine to methionine. A 677C-->T mutation in the MTHFR gene has been associated with elevated homocysteine concentrations in homozygous (+/+) individuals.

METHODS AND RESULTS

We assessed the frequency of this common mutation in 735 CAD patients from the Regression Growth Evaluation Statin Study (REGRESS), a lipid-lowering coronary-regression trial, and in 1250 population-based control subjects. Furthermore, the association between the mutation and serum homocysteine concentrations was studied. The frequency of the homozygous (+/+) mutation was 9.5% among patients versus 8.5% among control subjects, resulting in an odds ratio of 1.21 (95% confidence interval [CI], 0.87 to 1.68), relative to the (-/-) genotype. Homocysteine concentrations were significantly elevated in both (+/+) and (+/-) individuals compared with (-/-) individuals (median homocysteine levels, 15.4, 13.4, and 12.6 micromol/L, for (+/+), (+/-), and (-/-) individuals, respectively). For a summary estimation of the risk of the (+/+) genotype for CAD, we performed a meta-analysis on 8 different case-control studies on thermolabile MTHFR in CAD. In the meta-analysis, the homozygous (+/+) genotype was present in 299 of 2476 patients (12.1%) and in 257 (10.4%) of 2481 control subjects, resulting in a significant odds ratio of 1.22 (95% CI, 1.01 to 1.47) relative to the (-/-) genotype.

CONCLUSIONS

Both the homozygous (+/+) and heterozygous (+/-) genotype result in elevated homocysteine concentrations. From our meta-analysis, we conclude that the homozygous (+/+) genotype is a modest but significant risk factor for CAD.

A common 844INS68 insertion variant in the cystathionine beta-synthase gene

Mildly elevated plasma homocysteine has been shown to be associated with an elevated risk for cardiovascular disease. In this study, we analyzed the frequency of a common 844ins68 insertion variant in the cystathionine beta-synthase gene (CBS) in patients with arterial occlusive disease and in controls and assessed the association between the insertion variant and plasma homocysteine concentrations. The insertion variant was equally distributed between both study groups. Furthermore, the presence of this insertion variant, either in the heterozygous or the homozygous state, is not associated with hyperhomocysteinemia. We therefore conclude that this common 844ins68 variant is a neutral insertion variant.

Sequence analysis of the coding region of human methionine synthase: relevance to hyperhomocysteinaemia in neural-tube defects and vascular disease

Elevated homocysteine (Hcy) levels are observed in two apparently unrelated diseases: neural-tube defects (NTD) and premature vascular disease. Defective human methionine synthase (MS) could result in elevated Hcy levels. We sequenced the coding region of MS in 8 hyperhomocysteinaemic patients (4 NTD patients and 4 patients with pregnancies complicated by spiral arterial disease, SAD). We identified only one mutation resulting in an amino acid substitution: an A-->G transition at bp 2756, converting an aspartic acid (D919) into a glycine (G). We screened genomic DNA for the presence of this mutation in 56 NTD patients, 69 mothers of children with NTD, 108 SAD patients and 364 controls. There was no increased prevalence of the GG and AG genotypes in NTD patients, their mothers or SAD patients. The D919G mutation does not seem to be a risk factor for NTD or vascular disease. We then examined the mean Hcy levels for each MS genotype. There was no correlation between GG- or AG-genotype and Hcy levels. The D919G mutation is thus a fairly prevalent, and probably benign polymorphism. This study, though limited, provides no evidence for a major involvement of MS in the aetiology of homocysteine-related diseases such as NTD or vascular disease.

The 677C-->T mutation in the methylenetetrahydrofolate reductase gene: associations with plasma total homocysteine levels and risk of coronary atherosclerotic disease

Homozygosity for a 677C-->T mutation at the locus that codes for 5,10-methylenetetrahydrofolate reductase (MTHFR), a folate-dependent crucial enzyme in homocysteine metabolism, may render the enzyme thermolabile and less active and has been associated with increased levels of plasma total homocysteine (tHcy). We assessed whether this mutation was associated with increased risk of coronary atherosclerosis and plasma levels of tHcy and furthermore studied whether folate status would modify the associations. Data were collected from subjects with substantial coronary atherosclerosis (> or = 90% occlusion in one and > or = 40% occlusion in a second coronary artery, referred to as cases, n = 131) or virtually no coronary narrowing (referred to as coronary controls, n = 87) and from a population-based control group (n = 100), all residing in the Rotterdam area, The Netherlands. Both males and females, aged 25-65 years were studied. The frequency of homozygosity for the mutation (+/+) in cases (10.0%) did not significantly differ statistically from that observed in coronary controls (11.5%, P = 0.71), population-based controls (7.0%, P = 0.43), or combined control groups (9.1%, P = 0.80). In the overall group (as well as in the three subgroups), plasma tHcy levels, fasting and to a lesser extent after a methionine-loading test, were higher in +/+ subjects than in homozygous normal subjects (-/-), whereas heterozygous subjects (+/-) had intermediate levels (Ptrend = 0.001). The +/+ subjects with erythrocyte folate levels < 790 nmol/l (population median) had a 77%, (95% CI, 27-144%) higher geometric mean fasting tHcy (21.4, micromol/l) than those with higher erythrocyte folate (12.1 micromol/l). The odds ratio (OR) of coronary atherosclerosis for +/+ subjects, with +/- and -/- subjects as the reference group, in analyses with combined control groups, was 1.1 (95% CI, 0.5-2.4). The ORs were 2.2 (95% CI, 0.7-6.8) and 0.6 (95% CI, 0.2-1.7) among subjects with low and high folate levels, respectively. Our study indicates that homozygosity for the 677C-->T MTHFR mutation, especially in combination with low folate status, predisposes to high plasma levels of fasting tHcy. However, homozygosity for this mutation, whether or not in combination with low folate status, was not associated with increased risk of coronary artery disease.

C677T mutation of methylenetetrahydrofolate reductase gene determined in blood or plasma by multiple-injection capillary electrophoresis and laser-induced fluorescence detection

We constructed an assay to detect the common C677T mutation in the methylenetetrahydrofolate reductase gene. The mutation creates a Hinfl recognition site detected by restriction cleavage of a 198-bp fragment amplified in the polymerase chain reaction (PCR). Digested samples were subjected to capillary electrophoresis with laser-induced fluorescence detection (CE-LIF), with hydroxypropylmethylcellulose as the sieving matrix and SYBR Green I as the fluorescent dye. After amplification but before digestion, we added to the PCR mixture a fragment with the HinfI recognition site and a 15-bp truncation at the 3' end. Using this procedure, we could (a) verify completeness of digestion and monitor injection, (b) assign genotypes on the basis of pattern recognition, and (c) develop a multiple-injection mode with simultaneous separation of as many as eight samples. A seminested PCR protocol in combination with CE-LIF allowed genotyping of plasma/serum samples 20 years old.

Defective cystathionine beta-synthase regulation by S-adenosylmethionine in a partially pyridoxine responsive homocystinuria patient

We determined the molecular basis of cystathionine beta-synthase (CBS) deficiency in a partially pyridoxine-responsive homocystinuria patient. Direct sequencing of the entire CBS cDNA revealed the presence of a homozygous G1330A transition. This mutation causes an amino acid change from aspartic acid to asparagine (D444N) in the regulatory domain of the protein and abolishes a TaqI restriction site at DNA level. Despite the homozygous mutation, CBS activities in extracts of cultured fibroblasts of this patient were not in the homozygous but in the heterozygous range. Furthermore, we observed no stimulation of CBS activity by S-adenosylmethionine, contrary to a threefold stimulation in control fibroblast extract. The mutation was introduced in an E. coli expression system and CBS activities were measured after addition of different S-adenosylmethionine concentrations (0-200 microM). Again, we observed a defective stimulation of CBS activity by S-adenosylmethionine in the mutated construct, whereas the normal construct showed a threefold stimulation in activity. These data suggest that this D444N mutation interferes in S-adenosylmethionine regulation of CBS. Furthermore, it indicates the importance of S-adenosylmethionine regulation of the transsulfuration pathway in homocysteine homeostasis in humans.

Molecular genetic analysis in mild hyperhomocysteinemia: a common mutation in the methylenetetrahydrofolate reductase gene is a genetic risk factor for cardiovascular disease

Mild hyperhomocysteinemia is an established risk factor for cardiovascular disease. Genetic aberrations in the cystathionine beta-synthase (CBS) and methylenetetrahydrofolate reductase (MTHFR) genes may account for reduced enzyme activities and elevated plasma homocysteine levels. In 15 unrelated Dutch patients with homozygous CBS deficiency, we observed the 833T-->C (I278T) mutation in 50% of the alleles. Very recently, we identified a common mutation (677C-->T; A-->V) in the MTHFR gene, which, in homozygous state, is responsible for the thermolabile phenotype and which is associated with decreased specific MTHRF activity and elevated homocysteine levels. We screened 60 cardiovascular patients and 111 controls for these two mutations, to determine whether these mutations are risk factors for premature cardiovascular disease. Heterozygosity for the 833T-->C mutation in the CBS gene was observed in one individual of the control group but was absent in patients with premature cardiovascular disease. Homozygosity for the 677C-->T mutation in the MTHFR gene was found in (15%) of 60 cardiovascular patients and in only 6 (approximately 5%) of 111 control individuals (odds ratio 3.1 [95% confidence interval 1.0-9.2]). Because of both the high prevalence of the 833T-->C mutation among homozygotes for CBS deficiency and its absence in 60 cardiovascular patients, we may conclude that heterozygosity for CBS deficiency does not appear to be involved in premature cardiovascular disease. However, a frequent homozygous mutation in the MTHFR gene is associated with a threefold increase in risk for premature cardiovascular disease.

Two novel missense mutations in the cystathionine beta-synthase gene in homocystinuric patients

Direct sequencing of the coding region of the cystathionine beta-synthase (CBS) gene in two homocystinuric patients revealed the presence of two novel missense mutations. The first mutation, a 1111G-->A transition, resulted in the substitution of the evolutionary conserved valine-371 by a methionine residue (V371M) and created a new NlaIII restriction site. The second mutation, a G-->A transition at base-pair 494, resulted in an amino acid change from cysteine to tyrosine (C165Y) and abolished a BsoFI restriction site. Both mutations were found in a compound heterozygous state with the previously described 833T-->C transition.

A candidate genetic risk factor for vascular disease: a common mutation in methylenetetrahydrofolate reductase

Hyperhomocysteinaemia has been identified as a risk factor for cerebrovascular, peripheral vascular and coronary heart disease. Elevated levels of plasma homocysteine can result from genetic or nutrient-related disturbances in the trans-sulphuration or re-methylation pathways for homocysteine metabolism. 5, 10-Methylenetetrahydrofolate reductase (MTHFR) catalyzes the reduction of 5, 10-methylenetetrahydrofolate to 5-methyltetrahydrofolate, the predominant circulatory form of folate and carbon donor for the re-methylation of homocysteine to methionine. Reduced MTHFR activity with a thermolabile enzyme has been reported in patients with coronary and peripheral artery disease. We have identified a common mutation in MTHFR which alters a highly-conserved amino acid; the substitution occurs at a frequency of approximately 38% of unselected chromosomes. The mutation in the heterozygous or homozygous state correlates with reduced enzyme activity and increased thermolability in lymphocyte extracts; in vitro expression of a mutagenized cDNA containing the mutation confirms its effect on thermolability of MTHFR. Finally, individuals homozygous for the mutation have significantly elevated plasma homocysteine levels. This mutation in MTHFR may represent an important genetic risk factor in vascular disease.

Aberrant splicing of the COL4A5 gene in patients with Alport syndrome

A variety of mutations have been identified in the X-linked type IV collagen alpha 5 chain (COL4A5) gene in patients with Alport syndrome. A substantial number of these mutations were predicted to have an effect on RNA splicing. For 4 such mutations in our group of patients the effect of the DNA mutation on the COL4A5 mRNA structure and stability was analysed. An alteration of the invariant splice acceptor site of intron 41 resulted in a shift of the actual splicing to either a cryptic splice site within exon 42 or the normal splice site in the next intron. A single base substitution of the final nucleotide of exon 48 resulted in the removal of the entire exon. Two frameshift mutations, a 10 basepair duplication in exon 49 and a single base deletion in exon 41, were incorporated in the mRNA as such and resulted in a stretch of missense codons terminated by a premature stop codon. Exon skipping was occasionally observed in these samples, but not reproducibly in every experiment. In healthy controls exon skipping was never detected. Analysis of female carriers revealed that in only one case was the stability of the mutated mRNA reduced in comparison with the normal transcript. The extent to which the non-collagenous domain was predicted to be deleted correlated with the severeness of the disease.