Plasma phospholipid dysregulation in patients with cystathionine-β synthase deficiency

BACKGROUND & AIMS

Patients with cystathionine β-synthase deficiency (CBSD) exhibit high circulating levels of homocysteine and enhanced lipid peroxidation. We have characterized the plasma lipidome in CBSD patients and related lipid abnormalities with reactions underlying enhanced homocysteine levels.

METHODS AND RESULTS

Using an ultra-high-performance liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry method, plasma lipids were determined with an untargeted lipidomics approach in 11 CBSD patients and 11 matched healthy subjects (CTRL). Compared to CTRL, CBSD patients had a higher medium and long-chain polyunsaturated fatty acids (PUFA) content in phosphatidylethanolamine (PE) and lysophosphatidylethanolamine (LPE) species (p < 0.02), and depletion of phosphatidylcholine (PC; p = 0.02) and of lysophosphatidylcholine (LPC; p = 0.003) species containing docosahexaenoic acid (DHA), suggesting impaired phosphatidylethanolamine-N-methyltransferase (PEMT) activity. PEMT converts PE into PC using methyl group by S-adenosylmethionine (SAM) thus converted in S-adenosylhomocysteine (SAH). Whole blood SAM and SAH concentrations by liquid chromatography tandem mass spectrometry were 1.4-fold (p = 0.015) and 5.3-fold (p = 0.003) higher in CBSD patients than in CTRL. A positive correlation between SAM/SAH and PC/PE ratios (r = 0.520; p = 0.019) was found.

CONCLUSIONS

A novel biochemical abnormality in CBSD patients consisting in depletion of PC and LPC species containing DHA and accumulation of PUFA in PE and LPE species is revealed by this lipidomic approach. Changes in plasma SAM and SAH concentrations are associated with such phospholipid dysregulation. Given the key role of DHA in thrombosis prevention, depletion of PC species containing DHA in CBSD patients provides a new direction to understand the poor cardiovascular outcome of patients with homocystinuria.

Metabolic Serendipities of Expanded Newborn Screening

Incidental findings on newborn screening (NBS) are results that are not the target of screening within a given NBS program, but rather are found as a result of the screening and resulting diagnostic workup for that target. These findings may not have an immediate clinical impact on the newborn, but are sometimes an additional benefit of NBS programs and may be considered secondary targets of NBS programs. This work describes four case reports that had incidental findings on the NBS, which eventually led to the diagnosis of another metabolic disease instead of the one that was initially suspected. The first case was a new defect in the cationic amino acid transporter-2 (CAT-2), which was oriented as an arginase-1 deficiency in the newborn. The second case was a maternal glutaric aciduria type 1 (GA-1) that mimicked a carnitine transporter deficiency in the newborn. The third report was a case of lysinuric protein intolerance (LPI), which appeared as high levels of citrulline on the NBS. The fourth case was a mother with homocystinuria that was diagnosed during the biochemical study of vitamin B₁₂ status. All cases provide new or interesting data that will help guide differential diagnosis in the future.

Effects of medical food leucine content in the management of methylmalonic and propionic acidemias

PURPOSE OF REVIEW

The current review highlights the varied effects of medical foods high in leucine (Leu) and devoid of valine (Val) and isoleucine (Ile) in the management of methylmalonic acidemia (MMA) and propionic acidemia and cobalamin C (cblC) deficiency, aiming to advance dietary practices.

RECENT FINDINGS

Leu is a key metabolic regulator with a multitude of effects on different organ systems. Recent observational studies have demonstrated that these effects can have unintended consequences in patients with MMA as a result of liberal use of medical foods. The combination of protein restriction and medical food use in MMA and propionic acidemia results in an imbalanced branched-chain amino acid (BCAA) dietary content with a high Leu-to-Val and/or Ile ratio. This leads to decreased plasma levels of Val and Ile and predicts impaired brain uptake of multiple essential amino acids. Decreased transport of methionine (Met) across the blood-brain barrier due to high circulating Leu levels is of particular concern in cblC deficiency in which endogenous Met synthesis is impaired.

SUMMARY

Investigations into the optimal composition of medical foods for MMA and propionic acidemia, and potential scenarios in which Leu supplementation may be beneficial are needed. Until then, MMA/propionic acidemia medical foods should be used judiciously in the dietary management of these patients and avoided altogether in cblC deficiency.

The proteome of cblC defect: in vivo elucidation of altered cellular pathways in humans

Methylmalonic acidemia with homocystinuria, cobalamin deficiency type C (cblC) (MMACHC) is the most common inborn error of cobalamin metabolism. Despite a multidrug treatment, the long-term follow-up of early-onset patients is often unsatisfactory, with progression of neurological and ocular impairment. Here, the in-vivo proteome of control and MMACHC lymphocytes (obtained from patients under standard treatment with OHCbl, betaine, folate and L-carnitine) was quantitatively examined by two dimensional differential in-gel electrophoresis (2D-DIGE) and mass spectrometry. Twenty three proteins were found up-regulated and 38 proteins were down-regulated. Consistent with in vivo studies showing disturbance of glutathione metabolism, a deregulation in proteins involved in cellular detoxification, especially in glutathione metabolism was found. In addition, relevant changes were observed in the expression levels of proteins involved in intracellular trafficking and protein folding, energy metabolism, cytoskeleton organization and assembly. This study demonstrates relevant changes in the proteome profile of circulating lymphocytes isolated from treated cblC patients. Some results confirm previous observations in vivo on fibroblast, thus concluding that some dysregulation is ubiquitous. On the other hand, new findings could be tissue-specific. These observations expand our current understanding of the cblC disease and may ignite new research and therapeutic strategies to treat this disorder.

Simultaneous determination of cystathionine, total homocysteine, and methionine in dried blood spots by liquid chromatography/tandem mass spectrometry and its utility for the management of patients with homocystinuria

BACKGROUND

Disorders of homocysteine and B-vitamin metabolism represent a significant problem in clinical practice. Establishing the diagnosis requires specialized tests with demanding preanalytical requirements. To advance the detection of patients with these disorders, we developed a method for the simultaneous determination of cystathionine (Cysta), methionine (Met) and total homocysteine (tHcy) in dried blood spots (DBSs).

METHODS

A punch from a DBS sample was mixed with a solution of isotopically labeled internal standards, and analytes were extracted using methanol/0.1% formic acid/0.5mol/L dithiothreitol. The extract was injected into an LC-MS/MS system operating in MRM mode.

RESULTS

The analytical performance of the method employing DBS is adequate for its purpose and the type of sample. Compared with Cysta, tHcy and Met plasma levels, our method exhibited a negative bias between -3.8% and -42.2% due to the lower concentrations of these analytes in erythrocytes. The tHcy level and the Met/Cysta ratio in DBS enabled the clear detection of 12 patients with disorders of transsulfuration and with genetic and nutritional remethylation defects.

CONCLUSIONS

The ease of collecting and transporting DBS samples may advance diagnostic procedures in patients with neuropsychiatric disorders and thromboembolism. Consequently, this approach may facilitate detection and simplify the monitoring of patients with homocystinuria.

Glutathione metabolism in cobalamin deficiency type C (cblC)

BACKGROUND

Methylmalonic aciduria with homocystinuria, cblC defect, is the most frequent disorder of vitamin B12 metabolism. CblC patients are commonly treated with a multidrug therapy to reduce metabolite accumulation and to increase deficient substrates. However the long-term outcome is often unsatisfactory especially in patients with early onset, with frequent progression of neurological and ocular impairment. Recent studies, have shown perturbation of cellular redox status in cblC. To evaluate the potential contribution of oxidative stress into the patophysiology of cblC defect, we have analyzed the in vivo glutathione metabolism in a large series of cblC deficient individuals.

METHODS

Levels of different forms of glutathione were measured in lymphocytes obtained from 18 cblC patients and compared with age-matched controls. Furthermore, we also analyzed plasma cysteine and total homocysteine.

RESULTS

We found an imbalance of glutathione metabolism in cblC patients with a significant decrease of total and reduced glutathione, along with a significant increase of different oxidized glutathione forms.

CONCLUSIONS

These findings show a relevant in vivo disturbance of glutathione metabolism underlining the contribution of glutathione pool depletion to the redox imbalance in treated cblC patients. Our study may be helpful in addressing future research to better understanding the pathogenetic mechanism of the disease and in developing new therapeutic approaches, including the use of novel vitamin B₁₂ derivatives.

Dietary practices in pyridoxine non-responsive homocystinuria: a European survey

BACKGROUND

Within Europe, the management of pyridoxine (B6) non-responsive homocystinuria (HCU) may vary but there is limited knowledge about treatment practice.

AIM

A comparison of dietetic management practices of patients with B6 non-responsive HCU in European centres.

METHODS

A cross-sectional audit by questionnaire was completed by 29 inherited metabolic disorder (IMD) centres: (14 UK, 5 Germany, 3 Netherlands, 2 Switzerland, 2 Portugal, 1 France, 1 Norway, 1 Belgium).

RESULTS

181 patients (73% >16 years of age) with HCU were identified. The majority (66%; n=119) were on dietary treatment (1-10 years, 90%; 11-16 years, 82%; and >16 years, 58%) with or without betaine and 34% (n=62) were on betaine alone. The median natural protein intake (g/day) on diet only was, by age: 1-10 years, 12 g; 11-16 years, 11 g; and >16 years, 45 g. With diet and betaine, median natural protein intake (g/day) by age was: 1-10 years, 13 g; 11-16 years, 20 g; and >16 years, 38 g. Fifty-two percent (n=15) of centres allocated natural protein by calculating methionine rather than a protein exchange system. A methionine-free l-amino acid supplement was prescribed for 86% of diet treated patients. Fifty-two percent of centres recommended cystine supplements for low plasma concentrations. Target treatment concentrations for homocystine/homocysteine (free/total) and frequency of biochemical monitoring varied.

CONCLUSION

In B6 non-responsive HCU the prescription of dietary restriction by IMD centres declined with age, potentially associated with poor adherence in older patients. Inconsistencies in biochemical monitoring and treatment indicate the need for international consensus guidelines.

Neonatal aortic thrombosis as a result of congenital homocystinuria

BACKGROUND

Arterial thrombosis, that too in aorta is rare in neonates.

CASE CHARACTERISTICS

A 4-day-old presented with non-recordable BP in lower limbs. Doppler ultrasonography of abdomen revealed aortic thrombus.

OBSERVATION

Serum homocysteine level was elevated (25.5 umol/L).

OUTCOME

Thrombus resolved with subcutaneous LMW heparin therapy for 2 weeks.

MESSAGE

Congenital classic homocystinuria can rarely cause aortic thrombosis in neonatal period.

[Outcomes of patients with combined methylmalonic acidemia and homocystinuria after treatment]

OBJECTIVE

Combined methylmalonic acidemia with homocystinuria is a common form of methylmalonic acidemia in China. Patients with this disease can progress to death without timely and effective treatment. This study aimed to analyze the treatment outcomes of patients with combined methylmalonic acidemia and homocystinuria.

METHOD

From September 2004 to April 2012, 58 patients with combined methylmalonic acidemia and homocystinuria (34 males and 24 females) were diagnosed and treated in our hospital. Fifty cases were from clinical patients including 42 early-onset cases and 8 late-onset cases. Their age when they were diagnosed ranged from 18 days to 30.8 years. The other 8 cases were from newborn screening. All the patients were treated with vitamin B12, betaine, folic acid, vitamin B6, and L-carnitine. The physical and neuropsychological development, general laboratory tests, the levels of amino acids, acylcarnitines, and homocysteine in blood, and organic acids in urine were followed up.

RESULT

The follow-up period ranged from 1 month to 7.1 years. Three cases died (all were early-onset cases). In the other patients after treatment, the symptoms such as recurrent vomiting, seizures, lethargy, and poor feeding disappeared, muscle strength and muscle tension were improved, and general biochemical abnormalities such as anemia and metabolic acidosis were corrected. Among the surviving 55 cases, 49 had neurological impairments such as developmental delay and mental retardation. The median levels of blood propionylcarnitine and its ratio with acetylcarnitine, serum homocysteine, and urine methylmalonic acid were significantly decreased (P < 0.01), from 7.73 µmol/L (ranged from 1.5 to 18.61 µmol/L), 0.74 (ranged from 0.29 to 2.06), 97.3 µmol/L (ranged from 25.1 to 250 µmol/L) and 168.55 (ranged from 3.66 to 1032.82) before treatment to 2.74 µmol/L (ranged from 0.47 to 12.09 µmol/L), 0.16 (ranged from 0.03 to 0.62), 43.8 µmol/L (ranged from 17 to 97.8 µmol/L) and 6.81 (ranged from 0 to 95.43) after treatment, respectively.

CONCLUSION

Patients with combined methylmalonic acidemia and homocystinuria respond to a combined treatment consisting of supplementation of hydroxycobalamin, betaine, folic acid, vitamin B6 and L-carnitine with clinical and biochemical improvement. But the long-term outcomes are unsatisfactory, with neurological sequelae in most patients.

[Clinical management of homocystinuria: case report and review of the literature]

Homocystinuria is a congenital disorder of methyonine metabolism that leads to increased plasmatic, urinary and tissue deposits of methyonine and its main metabolite: homocysteine. Homocysteine deposits are toxic for the skeletal system (osteoporosis), the eyes (lens dislocation), central nervous system (seizures, psychiatric disorders) and also induce vascular damage (stroke and other cardiovascular events). This article reports two patients with homocystinuria in two siblings, followed by a concise review on the therapeutic strategies available for this disorder.

Long-term betaine therapy in a murine model of cystathionine beta-synthase deficient homocystinuria: decreased efficacy over time reveals a significant threshold effect between elevated homocysteine and thrombotic risk

Classical homocystinuria (HCU) is caused by deficiency of cystathionine β-synthase and is characterized by connective tissue disturbances, mental retardation and cardiovascular disease. Treatment for pyridoxine non-responsive HCU typically involves lowering homocysteine levels with a methionine-restricted diet and dietary supplementation with betaine. Compliance with the methionine-restricted diet is difficult and often poor. Investigating optimization of the efficacy of long-term betaine treatment in isolation from a methionine-restricted diet is precluded by ethical considerations regarding patient risk. The HO mouse model of HCU developed in our laboratory, exhibits constitutive expression of multiple pro-inflammatory cytokines and a hypercoagulative phenotype both of which respond to short-term betaine treatment. Investigation of the effects of long-term betaine treatment in the absence of methionine-restriction in HO HCU mice revealed that the ability of betaine treatment to lower homocysteine diminished significantly over time. Plasma metabolite analysis indicated that this effect was due at least in part, to reduced betaine-homocysteine S-methyltransferase (BHMT) mediated remethylation of homocysteine. Western blotting analysis revealed that BHMT protein levels are significantly repressed in untreated HCU mice but are significantly induced in the presence of betaine treatment. The observed increase in plasma homocysteine during prolonged betaine treatment was accompanied by a significant increase in the plasma levels of TNF-alpha and IL-1beta and reversion to a hypercoagulative phenotype. Our findings are consistent with a relatively sharp threshold effect between severely elevated plasma homocysteine and thrombotic risk in HCU and indicate that the HO mouse model can serve as a useful tool for both testing novel treatment strategies and examining the optimal timing and dosing of betaine treatment with a view toward optimizing clinical outcome.

Experimental evidence of oxidative stress in plasma of homocystinuric patients: a possible role for homocysteine

Homocystinuria is an inherited disorder biochemically characterized by high urinary excretion of homocystine and increased levels of homocysteine (Hcy) and methionine in biological fluids. Affected patients usually have a variety of clinical and pathologic manifestations. Previous experimental data have shown a relationship between Hcy and oxidative stress, although very little was reported on this process in patients with homocystinuria. Therefore, in the present study we evaluated parameters of oxidative stress, namely carbonyl formation, malondialdehyde (MDA) levels, sulfhydryl content and total antioxidant status (TAS) in patients with homocystinuria at diagnosis and under treatment with a protein restricted diet supplemented by pyridoxine, folate, betaine, and vitamin B(12). We also correlated plasma Hcy and methionine concentrations with the oxidative stress parameters examined. We found a significant increase of MDA levels and carbonyl formation, as well as a reduction of sulfhydryl groups and TAS in plasma of homocystinuric patients at diagnosis relatively to healthy individuals (controls). We also verified that Hcy levels were negatively correlated with sulfhydryl content and positively with MDA levels. Furthermore, patients under treatment presented a significant reduction of the content of MDA, Hcy and methionine concentrations relatively to patients at diagnosis. Taken together, the present data indicate that lipid and protein oxidative damages are increased and the antioxidant defenses diminished in plasma of homocystinuric patients, probably due to increased reactive species elicited by Hcy. It is therefore presumed that oxidative stress participates at least in part in the pathogenesis of homocystinuria.

Determination of cystathionine beta-synthase activity in human plasma by LC-MS/MS: potential use in diagnosis of CBS deficiency

Cystathionine β-synthase (CBS) deficiency is usually confirmed by assaying the enzyme activity in cultured skin fibroblasts. We investigated whether CBS is present in human plasma and whether determination of its activity in plasma could be used for diagnostic purposes. We developed an assay to measure CBS activity in 20 μL of plasma using a stable isotope substrate - 2,3,3-(2)H serine. The activity was determined by measurement of the product of enzyme reaction, 3,3-(2)H-cystathionine, using LC-MS/MS. The median enzyme activity in control plasma samples was 404 nmol/h/L (range 66-1,066; n = 57). In pyridoxine nonresponsive CBS deficient patients, the median plasma activity was 0 nmol/ho/L (range 0-9; n = 26), while in pyridoxine responsive patients the median activity was 16 nmol/hour/L (range 0-358; n = 28); this overlapped with the enzyme activity from control subject. The presence of CBS in human plasma was confirmed by an in silico search of the proteome database, and was further evidenced by the activation of CBS by S-adenosyl-L-methionine and pyridoxal 5'-phosphate, and by configuration of the detected reaction product, 3,3-(2)H-cystathionine, which was in agreement with the previously observed CBS reaction mechanism. We hypothesize that the CBS enzyme in plasma originates from liver cells, as the plasma CBS activities in patients with elevated liver aminotransferase activities were more than 30-fold increased. In this study, we have demonstrated that CBS is present in human plasma and that its catalytic activity is detectable by LC-MS/MS. CBS assay in human plasma brings new possibilities in the diagnosis of pyridoxine nonresponsive CBS deficiency.

Complete deficiency of methylenetetrahydrofolate reductase in mice is associated with impaired retinal function and variable mortality, hematological profiles, and reproductive outcomes

Severe deficiency of methylenetetrahydrofolate reductase (MTHFR) with homocystinuria can result in early demise or later-onset neurological impairment, including developmental delay, motor dysfunction, and seizures. We previously characterized BALB/c Mthfr (-/-)mice as a model for this disorder and have recently backcrossed the disrupted allele onto the C57Bl/6 background to examine the variable phenotypes in MTHFR deficiency. Compared with BALB/c Mthfr (-/-)mice, C57Bl/6 Mthfr (-/-)mice have enhanced survival rates (81% vs 26.5%). Four-day-old BALB/c mutant pups had lower body, brain, and spleen weights relative to their wild-type counterparts compared with C57Bl/6 mutants. Pregnant BALB/c Mthfr (+/-)mice had increased resorptions and embryonic delays compared with wild-type littermates, whereas these outcomes in C57Bl/6 c Mthfr (+/-)mice were similar to those of wild-type C57Bl/6 mice. BALB/c-mutant pups had altered hematological profiles (higher hematocrit, hemoglobin, and white blood cell counts, with lower platelet counts) compared with C57Bl/6 mutants. Mutants of both strains had similar degrees of hepatic steatosis, hepatic activity of betaine:homocysteine methyltransferase, and altered cerebellar histology. Electroretinograms (ERG) in C57Bl/6 Mthfr (-/-)mice revealed decreased amplitude of scotopic and photopic waves in 6-week-old mice, with normalized ERGs at 13 weeks. Plasma homocysteine was modestly higher in C57Bl/6 compared with BALB/c mice. Our results emphasize the variable presentation of MTHFR deficiency in different genetic backgrounds and suggest that plasma homocysteine is not a predictor of severity. In addition, our novel findings of decreased spleen weights, thrombocytopenia, and impaired retinal function warrant investigation in patients with severe MTHFR deficiency or other forms of homocystinuria.

The effect of homocysteine on the clinical outcomes of ventilated patients with severe sepsis

BACKGROUND

There is considerable evidence that elevated plasma homocysteine levels are associated with a prothrombotic milieu, whereas activation of the coagulation cascade is an important component of the pathogenesis of sepsis. The protein C pathway has been reported to play a central role both in the propagation of sepsis and a hyperhomocysteinemia-induced hypercoagulable state. Our primary aim was to measure plasma homocysteine levels in mechanically ventilated patients with severe sepsis/septic shock and to assess the association of these levels with relevant clinical outcomes.

METHODS

The study cohort included 102 mechanically ventilated patients with severe sepsis or septic shock. Demographics, comorbidities, clinical data and severity scores were recorded. Plasma homocysteine, vitamin B12, folate, creatinine, and protein C levels were measured in all study subjects upon enrollment, and genotyping for the C677T and A1298C polymorphisisms of the methylenetetrahydrofolate reductase (MTHFR) gene and for factor V Leiden (FVL) mutations was performed as well. The primary outcomes were mortality at 28 and 90 days; secondary outcomes included the number of days without renal or cardiovascular failure and the ventilator-free days during the study period.

RESULTS

Homocysteine levels were not significantly associated with any primary or secondary outcomes in the multivariable analysis. In addition, a synergistic effect of homocysteine with protein C levels was not detected.

CONCLUSION

Our data suggest that plasma homocysteine levels may not inform the prognosis of mechanically ventilated patients with severe sepsis/septic shock.

Are heterocygotes for classical homocystinuria at risk of vitamin B12 and folic acid deficiency?

OBJECTIVES/DESIGN

Comparative cross-sectional study to assess homocysteine and vitamin status in carriers of CBS gene mutations.

METHOD

Subjects included 34 parents (13 males, 21 females, age 27-59 years) of 30 patients with classical homocystinuria due to homozygous cystathionine beta-synthase deficiency. Control subjects were matched for gender and age (13 males, 21 females, age 25-59 years). All subjects were of Qatari origin, had normal liver and renal function tests and had not taken drugs or vitamin supplements prior to the study. The concentrations of homocysteine, folic acid and vitamins B6 and B12 in blood were determined after an overnight fast.

RESULTS

Heterozygous carriers had significantly increased fasting levels of homocysteine compared to controls (9.1 vs. 8.1 micromol/l, P=0.012). Both folic acid (328 vs. 478 pmol/l, P=0.002) and vitamin B12 concentrations (232 vs. 287 pmol/l, P=0.013) were reduced whilst there was no significant difference in vitamin B6 levels between the two groups (5.8 vs. 6.44 microg/l).

CONCLUSIONS

Increased homocysteine concentrations in CBS gene mutation carriers are associated with reduced concentrations of folic acid and vitamin B12 in blood. In view of the adverse effects of mild hyperhomocysteinemia, routine testing of vitamin status in parents of homocystinuria patients may be warranted. The causal relationship and pathophysiological consequences are uncertain; it is likely that CBS gene mutation carriers need higher doses of dietary vitamins.

Reduction of the false-positive rate in newborn screening by implementation of MS/MS-based second-tier tests: the Mayo Clinic experience (2004-2007)

The continued expansion of newborn screening programmes to include additional conditions increases the responsibility of newborn screening laboratories to provide testing with the highest sensitivity and specificity to allow for identification of affected patients while minimizing the false-positive rate. Some assays and analytes are particularly problematic. Over recent years, our laboratory tried to improve this situation by developing second-tier tests to reduce false-positive results in the screening for congenital adrenal hyperplasia (CAH), tyrosinaemia type I, methylmalonic acidaemias, homocystinuria, and maple syrup urine disease (MSUD). Beginning in 2004, this approach was applied to Mayo's newborn screening programme and resulted in a false-positive rate of 0.09%, a positive predictive value of 41%, and a positive detection rate of 1 affected case in 1672 babies screened.

Implementation of extended neonatal screening and a metabolic unit in the State of Qatar: developing and optimizing strategies in cooperation with the Neonatal Screening Center in Heidelberg

Qatar is a country in the Gulf area and member of the Gulf Cooperation Council states. The country is populated by original Qatari tribes that amount to about 200,000 people and about 600,000 expatriates mainly from Arabic and Asian countries. Inbreeding over centuries and high rates of consanguinity in the Qatari population and in some groups of expatriates, in addition to large family sizes and rapid population growth, have contributed to a high frequency of autosomal recessive disorders. In December 2003 Hamad Medical Corporation in Doha and the University Children's Hospital of Heidelberg, Germany, started an extended state-wide neonatal screening programme for metabolic and endocrine disorders, with the laboratory situated in Heidelberg, Germany. All aspects of the screening process had to be adapted to the unique situation of the laboratory being 6000 km from the birthplace of the neonates. Within 32 months, samples of 25,214 neonates were screened. In 28 cases an endocrine or metabolic diagnosis was identified (incidence 1:901, in Germany 1:1728). In particular, a variety of monogenic metabolic diseases were prevalent, with 19 patients detected giving an incidence of metabolic diseases of 1:1327 (Germany 1:2517). Each euro spent on the screening programme saved more than 25 euros in health and social costs. The programme revealed a high incidence of treatable inborn metabolic diseases in the population of Qatar. A reliable screening for classical homocystinuria showing a unique incidence of >1:3000 and for sickle cell disease has now been added.

Class selection of amino acid metabolites in body fluids using chemical derivatization and their enhanced 13C NMR

We report a chemical derivatization method that selects a class of metabolites from a complex mixture and enhances their detection by 13C NMR. Acetylation of amines directly in aqueous medium with 1,1'-13C(2) acetic anhydride is a simple method that creates a high sensitivity and quantitative label in complex biofluids with minimal sample pretreatment. Detection using either 1D or 2D 13C NMR experiments produces highly resolved spectra with improved sensitivity. Experiments to identify and compare amino acids and related metabolites in normal human urine and serum samples as well as in urine from patients with the inborn errors of metabolism tyrosinemia type II, argininosuccinic aciduria, homocystinuria, and phenylketonuria demonstrate the method. The use of metabolite derivatization and 13C NMR spectroscopy produces data suitable for metabolite profiling analysis of biofluids on a time scale that allows routine use. Extension of this approach to enhance the NMR detection of other classes of metabolites has also been accomplished. The improved detection of low-concentration metabolites shown here creates opportunities to improve the understanding of the biological processes and develop improved disease detection methodologies.

Challenges in the diagnosis of Marfan syndrome

Marfan syndrome (MFS) is a multisystem disorder of connective tissue that is inherited in an autosomal dominant fashion, and results from mutations in the FBN1 gene on chromosome 15. Diagnosis is challenging as it requires definition of diverse clinical features and input from a variety of specialists. Genetic testing of FBN1 is time consuming, expensive and complex, and may not solve the diagnostic dilemma. Failure to make a diagnosis or making an inappropriate diagnosis of MFS has social, lifestyle and medical consequences for the individual as well as the family.

cblE type of homocystinuria due to methionine synthase reductase deficiency: functional correction by minigene expression

The cblE type of homocystinuria is a rare autosomal recessive disorder caused by impaired reductive activation of methionine synthase. Although earlier biochemical studies proposed that the methionine synthase enzyme might be activated by two different reducing systems, mutations were reported in only the methionine synthase reductase gene (MTRR) in cblE patients. The pathogenicity of MTRR mutations, however, has not yet been tested functionally. We report on nine patients of European origin affected by the cblE type of homocystinuria. They presented between 2 weeks and 3 years of age (median age 4 weeks) with anemia, which was macrocytic in only three patients, and with neurological involvement in all but two cases. Bone marrow examination performed in seven patients showed megaloblastic changes in all but one of them. All patients exhibited moderate to severe hyperhomocysteinemia (median plasma total homocysteine [Hcy] 92 mumol/L, range 44-169), while clearly reduced methionine was observed only in four cases. Pathogenic mutations were identified in both parental alleles of the MTRR gene in all patients. Five known (c.903+469T>C, c.1361C>T, c.1459G>A, c.1557-4_1557+3del7, and c.1622_1623dupTA) and three novel mutations (c.7A>T, c.1573C>T, and c.1953-6_1953-2del5) were detected. Importantly, transfection of fibroblasts of cblE patients with a wild-type MTRR minigene expression construct resulted in a significant approximately four-fold increase of methionine synthesis, indicating correction of the enzyme defect. Our study shows a link between a milder predominantly hematological presentation and homozygosity for the c.1361C>T mutation, but no other obvious genotype-phenotype correlation. The identification of mutations in the MTRR gene, together with restoration of methionine synthesis following MTRR minigene expression in cblE cells confirms that this disease is caused by defects in the MTRR gene.

Liver failure and neurologic disease in a patient with homocystinuria

A homocystinuric patient with the typical clinical and metabolic features of the disorder started treatment at 22 years of age. A diet with controlled methionine intake resulted in good metabolic control for five years, but control could not be attained for the following 1 1/2 years. This was accompanied by the development of paraparesis and increasing liver failure. A liver transplantation was successful in achieving metabolic control without the need for any dietary restrictions.

B-Vitamin Treatment Trialists' Collaboration. Homocysteine-lowering trials for prevention of cardiovascular events: a review of the design and power of the large randomized trials

BACKGROUND

Dietary supplementation with folic acid and vitamin B12 lowers blood homocysteine concentrations by about 25% to 30% in populations without routine folic acid fortification of food and by about 10% to 15% in populations with such fortification. In observational studies, 25% lower homocysteine has been associated with about 10% less coronary heart disease (CHD) and about 20% less stroke.

METHODS

We reviewed the design and statistical power of 12 randomized trials assessing the effects of lowering homocysteine with B-vitamin supplements on risk of cardiovascular disease.

RESULTS

Seven of these trials are being conducted in populations without fortification (5 involving participants with prior CHD and 2 with prior stroke) and 5 in populations with fortification (2 with prior CHD, 2 with renal disease, and 1 with prior stroke). These trials may not involve sufficient number of vascular events or last long enough to have a good chance on their own to detect reliably plausible effects of homocysteine lowering on cardiovascular risk. But, taken together, these 12 trials involve about 52,000 participants: 32,000 with prior vascular disease in unfortified populations and 14,000 with vascular disease and 6000 with renal disease in fortified populations. Hence, a combined analysis of these trials should have adequate power to determine whether lowering homocysteine reduces the risk of cardiovascular events within just a few years.

CONCLUSION

The strength of association of homocysteine with risk of cardiovascular disease may be weaker than had previously been believed. Extending the duration of treatment in these trials would allow any effects associated with prolonged differences in homocysteine concentrations to emerge. Establishing a prospective meta-analysis of the ongoing trials of homocysteine lowering should ensure that reliable information emerges about the effects of such interventions on cardiovascular disease outcomes.

Molecular analysis of homocystinuria in Brazilian patients

BACKGROUND

Cystathionine beta-synthase (CBS) deficiency is the most common cause of homocystinuria. However, no data are available concerning the molecular basis of this disease in Brazilian populations.

METHODS

We studied 14 Brazilian patients from 11 unrelated families using a combined screening approach, involving restriction analysis, single-strand conformational polymorphism (SSCP) scanning, and sequencing.

RESULTS

All patients presented homocysteine levels higher than 200 mumol/l before the beginning of treatment. The most common CBS gene mutations, p.G307S (c.919G > A) and p.I278T (c.833T > C), were evaluated and the allele c.919A was not found. One allele with the c.844 ins68 (4.5%) in the CBS gene was found. Three families (6 patients) presented the allele c.833 C (13.6%), without the insertion in the heterozygous state. SSCP scanning and sequencing showed 3 alleles p.T191M (13.64%) in 2 families. One allele with a novel mutation was found in exon 4 (c.168T > A) of the CBS gene (4.5%). We also analyzed c.677C > T and c.1298A > C polymorphisms in the methylenetetrahydrofolate reductase (MTHFR) gene and the 2756A > G polymorphism in the methionine synthase (MTR) gene. The frequencies of mutated alleles were: 50% c.677T and 18.2% c.1298C for MTHFR, and 27.3% c.2756G for MTR.

CONCLUSION

In spite of the high level of racial mixing in the country, Brazilian homocystinuric patients did not present a high prevalence of the most common mutations described in the literature.

Homocysteine and endothelial function in human studies

The endothelium plays a key role in the pathophysiology of vascular disease. Impaired flow-mediated dilatation (FMD) is a measure of endothelial dysfunction resulting from reduced bioavailability of nitric oxide (NO). Patients with homocystinuria manifest with impaired FMD, but in mild hyperhomocysteinemia, the evidence is conflicting. Oral loading with methionine or homocysteine impairs FMD, but it remains unproven that this effect is mediated directly by homocysteine. In addition, there is no clear consensus as to a mechanisms by which homocysteine would induce endothelial dysfunction. Folate administration lowers plasma homocysteine and enhances FMD. However, the effect of folate only appears to occur at high doses and with a time course that would indicate that it is acting by a mechanism independent of homocysteine lowering. It is possible that folate, in pharmacological doses, may enhance the NO activity by influencing NO-tetrahydrobiopterin interactions. These studies provide some insights and raise intriguing questions concerning the relationship between homocysteine, folate, and endothelial function. However, changes in FMD may not translate into vascular endpoints, and the outcomes of clinical intervention trials with different doses of folic acid are awaited with interest.

Reduction of butyrylcholinesterase activity in rat serum subjected to hyperhomocysteinemia

In the present study we investigate the effect of homocysteine (Hcy) administration, the main metabolite accumulating in homocystinuria, on butyrylcholinesterase (BuChE) activity in serum of rats. For the acute treatment, 29-day-old Wistar rats received one subcutaneous injection of Hcy (0.6 micromol/g) or saline (control) and were killed 1 h later. For the chronic treatment, Hcy was administered subcutaneously to rats from the 6th to the 28th day of life. Control rats received saline. The rats were killed 12 h after the last injection. In another set of experiments, rats were pretreated for one week with vitamins E and C or saline and 12 h after the last injection received one single injection of Hcy or saline, being killed 1 h later. Serum was used to determine BuChE activity. Our results showed that acute and chronic administration of Hcy significantly decreased BuChE activity. Furthermore, vitamins E and C per se did not alter BuChE activity, but prevented the reduction of this enzyme activity caused by acute administration of Hcy. The data suggest that the inhibitory effect of Hcy on BuChE activity is probably mediated by free radicals, since vitamins E and C administration prevented such effect.

Homocysteine and Venous Thrombosis

Elevated plasma total homocysteine concentration is a risk factor for venous thrombosis. The association is well established in patients with homocystinuria irrespective of the genetic etiology and metabolic background. Homocystinuria is a human model of chronic exposure to very high concentrations of plasma homocysteine and reflects an abnormal amino acid metabolism. Elevated homocysteine levels in patients with venous thrombosis have attracted considerable interest because homocysteine is a potentially reversible thrombophilic marker for venous thrombosis. In contrast to homocystinuria, hyperhomocysteinemia is mild and reflects environmental and constitutional factors such as age, intake of B-vitamins, derangements of metabolism, and renal impairment. This review examines the evidence for the relationship of homocysteine with risk of venous thrombosis in homocystinuria and in the general population.

Plasma albumin cysteinylation is regulated by cystathionine beta-synthase

High homocysteine (Hcy) levels are a well-known independent risk factor for endothelial damage in atherosclerosis. We examined whether a rat intestinal model of ischemia-reperfusion was associated with high Hcy and with the modification of plasma albumin into cysteinylated species (CysAlb). The three treatment groups were as follows: midline abdominal incision (group A, n=10), followed by ligation of the superior mesenteric artery for a period of 2h (group B, n=3), and followed by reperfusion for 1h (group C, n=10). Hcy levels were 2.5-fold higher in group C than group A (p<0.05). 100% and 73.44+/-0.04% of Alb were modified into Cys species in groups C and B, respectively, compared to 51.2% in group A. A cystathionine beta-synthase (CBS) deficient mouse model, known to have high plasma Hcy levels, was also used to determine the extent of CysAlb. Hcy levels, %CysAlb, and %HcyAlb were 180.1+/-45.7 microM, 0%, and 23.4+/-4.4% in CBS deficient mice, while in control mice, those values were 5.7+/-1.8 microM, 24.2+/-4.1%, and 0%, respectively (p<0.05). High CysAlb and Hcy levels were observed in a rat model of bowel ischemia/reperfusion while high HcyAlb and Hcy levels with no CysAlb were observed in the CBS deficient mice. CysAlb may serve as a biomarker for the severity of gut ischemia, and high Hcy may explain endothelial damage associated with this model. Additionally, active CBS is essential for the formation of CysAlb.

Homocysteine – a newly recognised risk factor for osteoporosis

Osteoporosis is a widespread problem, which frequently has devastating health consequences through its association with fragility fractures. The total number of fractures, and hence the cost to society, will increase dramatically over the next 50years as a result of demographic changes in the number of elderly people. Thus, prevention of osteoporosis by identifying risk factors or risk indicators, as well as the development of new treatment strategies, are major issues. Recent data suggest that homocysteine (Hcy), folate, vitamin B

Determination of total homocysteine in dried blood spots using high performance liquid chromatography for homocystinuria newborn screening

BACKGROUND

The most widely used method for newborn screening for homocystinuria (HCU) is a semi-quantitative bacterial inhibition assay for measuring methionine concentration in dried blood spots (DBS). Because this method has resulted in a number of missed cases due to many factors, we developed a high performance liquid chromatography (HPLC) method with fluorescence detection to measure total homocysteine (tHcy) in DBS which might be useful for newborn screening for HCU.

METHODS

One disk of DBS 3 mm in diameter was sonicated in 10 min. The extract was reduced with dithioerythritol and was derivatized with 4-aminosulfonyl-7fluoro-2,1,3-benzoxadiazole before injection into HPLC.

RESULTS

This method showed good linearity (r = 0.996), precision (coefficient of variation range 2.7-5%), and excellent correlation coefficient between DBS and serum tHcy, both in control (r = 0.932) and patient samples (r = 0.952). By this method, the mean tHcy concentration in DBS of preterm newborns, full-term newborns, and adults was 1.4 +/- 1.0, 2.5 +/- 1.6, and 4.9 +/- 1.5 micro mol/L, respectively. The mean tHcy DBS concentration in two cases of cystathionine-beta-synthase deficiency and one case of 5,10-methylentetrahydrofolate reductase deficiency was 22.7 +/- 2.88, 29.3 +/- 1.90, and 41.3 micro mol/L, respectively.

CONCLUSIONS

The present method, which is rapid, user friendly and reliable, seems applicable to newborn screening of HCU in place of methionine measurement.

Limited effectiveness of betaine therapy for cystathionine beta synthase deficiency

BACKGROUND

Orally administered betaine (Bet) is regarded as an effective and safe therapy for homocystin-uria. However, even when patients' serum Bet concentrations are increased by supplementation, serum homocysteine (Hcy) concentrations are often not lowered to the normal range. The present study tested the hypothesis that with relatively high serum methionine (Met), serum Hcy does not adequately decrease, even when serum Bet concentrations are potentially therapeutic.

METHODS

The present study examines the relationship between these amino acids by high-performance liquid chromatography (HPLC) in a total of 63 samples obtained over 2 years from two patients with cystathionine beta synthase (CBS) deficiency.

RESULTS

When serum Met was less than 80 micro mol/L (1.2 mg/dL), the treatment reduced serum Hcy to within the normal range. When serum Met exceeded 80 micro mol/L, serum Hcy showed only a limited decrease, despite sufficient doses of Bet (serum concentration, over 250 micro mol/L). The findings of the present study suggest that it is necessary to follow a low methionine diet that keeps serum Met within the normal range when treating patients with homocystinuria due to CBS deficiency when Bet is administered. Homocystinuria is a rare congenital metabolic disease and the data presented in the present paper, although it relates to only two patients, is worth reporting.

Homocysteine and cysteine - albumin binding in homocystinuria: assessment of cysteine status and implications for glutathione synthesis?

Measurement of plasma total cysteine rather than free dimeric cystine gives a better indication of cysteine status in homocystinuric patients. This is the result of displacement of cysteine from albumin by homocysteine and is related to the plasma homocysteine concentration. In control subjects the free/bound cyst(e)ine ratio was independent of albumin and total cysteine concentrations. In homocystinuric (HCU) patients both free and total cyst(e)ine values differed significantly from control values (P < 0.001) but whilst free cystine considerably overlapped control values the total cysteine concentrations were almost invariably lower. The possible consequences of this on glutathione synthesis was explored by assay of plasma total glutathione but no evidence for glutathione deficiency was found. Measurement of total cysteine, rather than free cystine, provides a better indication of cysteine status in HCU.

Newborn screening of homocystinuria: quantitative analysis of total homocyst(e)ine on dried blood spot by liquid chromatography with fluorimetric detection

Identification of homocystinuric newborns is hindered by the pitfalls of neonatal screening programs. We propose a fluorimetric HPLC method with a rapid pre-analytical step for homocysteine determination from neonatal dried blood spot cards. Homocysteine in blood spots sampled among 2000 healthy newborns on living day 4, averaged 2.92+/-2.07 microM (range 0.4-7.5). In eight homocystinuric control children, mean values were 61.71+/-52.84 microM (range 18.9-145.7). The method showed a good linearity (r=0.999), precision (RSD<7%) and recovery (95%). The correlation between blood spots and plasma samples was r=0.90. This method has all the essential features for a homocystinuria screening program: an easy and rapid pre-analytical step combined with method linearity and precision.

Pharmacokinetics of oral betaine in healthy subjects and patients with homocystinuria

AIMS

Large oral doses of betaine have proved effective in lowering plasma homocysteine in severe hyperhomocysteinaemia. The pharmacokinetic characteristics and metabolism of betaine in humans have not been assessed and drug monitoring for betaine therapy is not available. We studied the pharmacokinetics of betaine and its metabolite dimethylglycine (DMG) in healthy subjects and in three patients with homocystinuria.

METHODS

Twelve male volunteers underwent an open-label study. After one single administration of 50 mg betaine kg-1 body weight and during continuous intake of twice daily 50 mg kg-1 body weight, serial blood samples and 24 h urines were collected to determine betaine and DMG plasma concentrations and urinary excretion, respectively. Patients were evaluated after one single dose of betaine.

RESULTS

We found rapid absorption (t(1/2),abs 00.28 h, s.d. 0.17) and distribution (t(1/2), lambda1 00.59 h, s.d. 0.22) of betaine. A Cmax of 0.94 mmol l-1 (s.d. 0.19) was reached after tmax 00.90 h (s.d. 0.33). The elimination half life t(1/2), z was 14.38 h (s.d. 7.17). After repeated dosage, t(1/2), lambda1 (01.77 h, s.d. 0.75) and t(1/2), z (41.17 h, s.d. 13.50) increased significantly (95% CI 0.73, 01.64 h and 19.90, 33.70 h, respectively), whereas absorption remained unchanged. DMG concentrations increased significantly after betaine administration and accumulation occurred to the same extent as with betaine. Renal clearance was low and urinary excretion of betaine was equivalent to 4% of the ingested dose. Distribution and elimination kinetics in homocystinuric patients appeared to be accelerated.

CONCLUSIONS

Betaine plasma concentrations change rapidly after ingestion. Elimination half-life increased during continuous dosing over 5 days. Betaine is mainly eliminated by metabolism. More pharmacokinetic and pharmacodynamic studies in hyperhomocysteinaemic patients are needed to refine the current treatment with betaine.

Molecular and clinical characterisation of homocystinuria in two Austrian families with cystathionine beta-synthase deficiency

The influence of the genotype on the phenotypic expression of homocystinuria due to cystathionine beta-synthase (CBS) deficiency is frequently unclear. We therefore investigated the genotype and the phenotype of CBS deficiency in two Austrian families also considering genetic polymorphisms with a putative association with vascular disease (MTHFR 677C-->T, MTHFR 1298A-->C, F5 1691G-->A, F2 20210G-->A) and response to therapy. We identified the CBS 833T-->C/1058C-->T and CBS 828ins104/1358del134 compound heterozygous genotype in our index patients. Both patients showed mental retardation and ectopia lentis. CBS 833T-->C/1058C-->T was associated with severe vascular complications, which was not the case for CBS 828ins104/1358del134. The patient with CBS 828ins104/1358del134 was negative for F5 1691G-->A, F2 20210G-->A, MTHFR 677C-->T, and MTHFR 1298A-->C, while the patient with CBS 833T-->C/1058C-->T was heterozygous for MTHFR 1298A-->C. A combination therapy including pyridoxine, folic acid, hydroxycobalamin, and betaine failed to lower total homocysteine plasma levels below 50 mumol/L in both patients. In summary, our study demonstrates that the CBS 833C/1058T-MTHFR 1298AC genotype can be related to severe vascular disease, while the CBS 828ins104/1358del134-MTHFR 1298AA genotype presents with a somewhat milder clinical phenotype. Both genotypes do not allow for normalisation of total homocysteine plasma levels following vitamin therapy.

A pilot study of homocyst(e)ine levels in essential hypertension: relationship to von Willebrand factor, an index of endothelial damage

An interaction between homocyst(e)ine and the endothelium in hypertensive patients may promote thrombogenesis and atherogenesis, leading to adverse cardiovascular events. We hypothesized that homocyst(e)ine levels are abnormal in patients with essential hypertension, and that this may be related to an adverse effect on the vascular endothelium. Accordingly, we compared plasma levels of homocyst(e)ine and von Willebrand factor (marking endothelial damage) in 83 patients (43 men; mean age 54 +/- standard deviation 15.9 years) with essential hypertension (> 160/90 mm Hg), with levels in 25 healthy normotensive controls (13 men; mean age 56+/-11.8 years). Baseline levels of the markers and other clinical indices were then related to adverse cardiovascular events at follow-up. Plasma homocyst(e)ine (P = .0001) and von Willebrand factor (P = .031) levels were significantly higher in hypertensives compared to controls. After a mean follow-up of 76 patients for 45 months (range, 1 to 66 months), 17 subjects experienced an end point of either cardiovascular death (n = 10) or adverse cardiovascular event (n = 7). Comparing these 17 with the 59 free of an end point, the former were older (P = .0002) and had a longer duration of known hypertension (P = .018). There was a nonsignificant trend toward higher median plasma homocyst(e)ine levels in the patients sustaining a vascular end point (P = .07). In this pilot study, we suggest that essential hypertension may be associated with increased plasma homocyst(e)ine levels, but that this amino acid is unrelated to endothelial damage (von Willebrand factor), clinical indices, or prognosis.

Disposition of homocysteine in subjects heterozygous for homocystinuria due to cystathionine beta-synthase deficiency: relationship between genotype and phenotype

We have investigated 31 subjects from five unrelated families with one or more members with cystathionine beta-synthase (CBS) deficiency. On the basis of their CBS genotype, the subjects were grouped as normal (n = 11) or heterozygotes (n = 20). Based on pyridoxine effect in the probands, the heterozygotes were further classified as pyridoxine-responsive (n = 9) or non-responsive (n = 11). Heterozygous subjects had normal fasting total plasma homocysteine (tHcy), but median urinary tHcy excretion rate was significantly elevated compared to healthy controls (0.39 micromol/h vs 0.24 micromol/h, P < 0.05). An abnormal tHcy response after methionine loading identified 73% of the pyridoxine non-responsive heterozygotes, but only 33% of the pyridoxine responsive participants. The increase in cystathionine or the change in tHcy relative to cystathionine did not improve diagnostic accuracy of the methionine loading test. After Hcy loading, the maximal increase in tHcy was significantly elevated, whereas t(1/2) was normal in heterozygotes. In conclusion, a single biochemical test cannot discriminate CBS heterozygotes from controls. Abnormal tHcy response after methionine loading was the most sensitive test. Our data suggest that the urinary tHcy excretion rate is a simple, non-invasive approach for studying mild disturbances in Hcy metabolism.

Reducing cardiovascular risk in diabetes. Which factors to modify first?

Up to 80% of diabetic patients die of macrovascular complications, including CAD, stroke, and peripheral vascular disease. Because of the growing numbers of diabetic patients and the increased mortality after their first cardiovascular event, it is critical to identify and treat risk factors early and aggressively in these patients. Numerous studies in patients with type 2 diabetes have shown the benefits of aggressive treatment of blood pressure and lipids to levels that 10 years ago would have seemed abnormally low. The downward changes in "normal" limits can be frustrating to primary care physicians, but advances in treatment are redefining "normal" levels required to avoid complications in this high-risk population.

Vascular complications of severe hyperhomocysteinemia in patients with homocystinuria due to cystathionine beta-synthase deficiency: effects of homocysteine-lowering therapy

Homocystinuria (HCU) due to cystathionine beta-synthase (CBS) deficiency leads to severe hyperhomocysteinemia (HHcy). Vascular events (VE) remain the major cause of morbidity and mortality in the untreated patients with HCU. The study on the natural history of untreated HCU disclosed that, at the time of maximal risk, in other words beyond 10 years old, there was one event per 25 years. Recent studies from Australia (n = 32), The Netherlands (n = 28), and Ireland (n = 24) have documented the effects of long-term treatment on the vascular outcome of a total of 84 patients with 1314 patient-years of treatment for HCU. The mean (range) age was 27.8 (2.5 to 70) years. Five VE were recorded during treatment; one pulmonary embolism, two myocardial infarctions, and two abdominal aneurysms. All five VE occurred in B6-responsive patients at a mean (range) age of 48.8 (30 to 60) years. In 1314 patient-years of treatment, 53 VE would have been expected if they remained untreated; instead only 5 were documented, relative risk = 0.091 (95% confidence interval [CI] 0.043 to 0.190; p < 0.001). Appropriate homocysteine-lowering therapy for severe HHcy significantly reduced the vascular risk in patients with HCU. VE were rare with treatment despite the fact that the post-treatment homocysteine levels were several times higher than the cutoff point for homocysteine in the normal population. The present findings may have relevance to the current concept of "mild HHcy" as a risk factor for vascular disease, with elevated plasma homocysteine levels considerably lower than that of the post-treatment levels in this group of reported patients.

Hyperhomocysteinemia, vascular pathology, and endothelial dysfunction

Hyperhomocysteinemia has been associated with premature atherothrombotic vascular disease. It is not known whether hyperhomocysteinemia induces a distinct type of vascular disease. Its interaction, if any, with traditional risk factors also remains unclear. The pathophysiological mechanisms linking hyperhomocysteinemia to vascular disease have been extensively studied in vitro and in animals. From these studies, it has been suggested that homocysteine limits the bioavailability of nitric oxide (NO), increases oxidative stress, stimulates smooth cell proliferation, and alters elastic wall properties. The relevance of these proposed mechanisms in vivo is unclear, because clinical studies have yielded controversial results with regard to the relation between plasma homocysteine levels and indices of endothelial function, such as brachial artery flow-mediated vasodilatation and plasma levels of endothelium-derived marker proteins. Up till now, there have been no controlled data on the effects of homocysteine-lowering treatment on vascular function or clinical end points. The precise mechanisms (if any) by which homocysteine mediates its adverse vascular effects are in fact unknown but may relate to impaired endothelial and smooth muscle cell function.

Homocysteine, coagulation, platelet function, and thrombosis

Over the last 30 years, a growing body of evidence has documented the role of hyperhomocysteinemia (HHcy) as an independent vascular risk factor. However, the mechanisms through which elevated circulating levels of homocysteine (Hcy) cause vascular injury and promote thrombosis remain elusive. Most findings have been achieved in in vitro studies employing exceedingly high concentrations of Hcy, whereas only a few studies have been carried out in vivo in humans. In homocystinuric patients, homozygotes for mutations of the gene coding for the cystathionine beta-synthase enzyme, abnormalities of coagulation variables reflecting a hypercoagulable state, have been reported. In vitro studies provide a biochemical background for such a state. In homocystinuric patients, an in vivo platelet activation has also been reported. The latter abnormality is not corrected by the bolus infusion of concentrations of hirudin, which determines a long-lasting impairment of the conversion of fibrinogen to fibrin by thrombin; in contrast, it appears at least in part lowered by the administration of the antioxidant drug probucol. During the autooxidation of Hcy in plasma, reactive oxygen species are generated. The latter initiate lipid peroxidation in cell membranes (potentially responsible for endothelial dysfunction) and in circulating lipoproteins. Oxidized low-density lipoproteins (LDL) may trigger platelet activation as well as some of the hemostatic abnormalities reported in such patients. Thus the oxidative stress induced by Hcy may be a key process in the pathogenesis of thrombosis in HHcy. Accumulation of adenosylhomocysteine in cells (a consequence of high circulating levels of homocysteine) inhibits methyltransferase enzymes, in turn preventing repair of aged or damaged cells. This mechanism has been recently documented in patients with renal failure and HHcy and provides an additional direction to be followed to understand the tendency to thrombosis in moderate HHcy.

Genetic modulation of homocysteinemia

With the identification of hyperhomocysteinemia as a risk factor for cardiovascular disease, an understanding of the genetic determinants of plasma homocysteine is important for prevention and treatment. It has been known for some time that homocystinuria, a rare inborn error of metabolism, can be due to genetic mutations that severely disrupt homocysteine metabolism. A more recent development is the finding that milder, but more common, genetic mutations in the same enzymes might also contribute to an elevation in plasma homocysteine. The best example of this concept is a missense mutation (alanine to valine) at base pair (bp) 677 of methylenetetrahydrofolate reductase (MTHFR), the enzyme that provides the folate derivative for conversion of homocysteine to methionine. This mutation results in mild hyperhomocysteinemia, primarily when folate levels are low, providing a rationale (folate supplementation) for overcoming the genetic deficiency. Additional genetic variants in MTHFR and in other enzymes of homocysteine metabolism are being identified as the cDNAs/genes become isolated. These variants include a glutamate to alanine mutation (bp 1298) in MTHFR, an aspartate to glycine mutation (bp 2756) in methionine synthase, and an isoleucine to methionine mutation (bp 66) in methionine synthase reductase. These variants have been identified relatively recently; therefore additional investigations are required to determine their clinical significance with respect to mild hyperhomocysteinemia and vascular disease.

Optic atrophy in association with cobalamin C (cblC) disease

PURPOSE

To report the association of optic atrophy with cobalamin C (cblC) disease.

METHODS

Descriptive case reports on three patients, two of whom were siblings.

RESULTS

All three patients with cblC disease exhibited bilateral optic atrophy with decreased visual acuity. Of the two siblings, the younger sister had received cobalamin supplements from birth and the mother had been given cobalamin supplements prenatally.

CONCLUSION

These three cases confirm the association of optic atrophy with cblC disease. Early treatment with cobalamin supplements does not appear to prevent the development of optic atrophy.

Methionine transamination in patients with homocystinuria due to cystathionine beta-synthase deficiency

To assess the ability of patients with homocystinuria due to cystathionine beta-synthase (CBS) deficiency to perform the reactions of the methionine transamination pathway, the concentrations of the products of this pathway were measured in plasma and urine. The results clearly demonstrate that CBS-deficient patients develop elevations of these metabolites once a threshold near 350 micromol/L for the concurrent plasma methionine concentration is exceeded. The absence of elevated methionine transamination products previously reported among 16 CBS-deficient B6-responsive patients may now be attributed to the fact that in those patients the plasma methionine concentrations were below this threshold. The observed elevations of transamination products were similar to those observed among patients with isolated hypermethioninemia. Plasma homocyst(e)ine did not exert a consistent effect on transamination metabolites, and betaine appeared to effect transamination chiefly by its tendency to elevate methionine. Even during betaine administration, the transamination pathway does not appear to be a quantitatively major route for the disposal of methionine.

Tissue factor pathway inhibitor levels in patients with homocystinuria

Thrombotic events are a well-recognized complication of homocystinuria. However, the mechanisms involved in the atherogenic and thrombotic effects of homocyst(e)ine remain incompletely understood. The objective of this study was to determine the role of endothelial cell activation/damage as indicated by levels of thrombomodulin, tissue factor and tissue factor pathway inhibitor, and factor VII activity in patients with homocystinuria. Six patients with homocystinuria, nonresponsive to pyridoxine, treated only with trimethylglycine (betaine) were injected with a bolus of 20 IU/kg body weight of unfractionated commercial heparin to induce the release of tissue factor pathway inhibitor from the vascular endothelium. Tissue factor, thrombomodulin, and factor VII activity were measured by enzyme-linked immunosorbent assay and clotting assay before heparin administration. Tissue factor pathway inhibitor antigen and activity were measured before and 5 minutes after the bolus of heparin. Levels of homocyst(e)ine were elevated (patients: 144.2+/-19.2 micromol/L; controls: 10.2+/-0.9 micromol/L); however, levels of thrombomodulin, tissue factor, and tissue factor pathway inhibitor antigen were not statistically different from the control group. In contrast, tissue factor pathway inhibitor activity showed a significantly increased level (patients: 2.09+/-0.34 U/L; controls: 1.14+/-0.20 U/L; p<0.05) that was correlated with homocyst(e)ine. Factor VII activity was significantly decreased (patients: 64.7+/-5.1%; controls: 91.4+/-4.7%; p<0.05) and inversely correlated with homocyst(e)ine. After heparin the patients released higher amounts of tissue factor pathway inhibitor antigen and activity compared with the control group; however, the difference was not statistically significant. Although not treated with antithrombotic drugs, none of the patients had any thromboembolic complications after starting betaine. In addition to betaine treatment, the enhanced factor pathway inhibitor antigen activity observed in this small series of patients suggests that factor pathway inhibitor antigen may play an additional, as yet unexplained, role in this genetic disorder.