Long-term functional correction of cystathionine ß-synthase deficiency in mice by adeno-associated viral gene therapy.

Cystathionine ß-synthase (CBS) deficiency is a recessive inborn error of sulfur metabolism characterized by elevated blood levels of total homocysteine (tHcy). Patients diagnosed with CBS deficiency are currently treated by a combination of vitamin supplementation and restriction of foods containing the homocysteine precursor methionine, but the effectiveness of this therapy is limited due to poor compliance. A mouse model for CBS deficiency (Tg-I278T Cbs-/- ) was used to evaluate a potential gene therapy approach to treat CBS deficiency utilizing an AAVrh.10-based vector containing the human CBS cDNA downstream of the constitutive, strong CAG promoter (AAVrh.10hCBS). Mice were administered a single dose of virus and followed for up to 1 year. The data demonstrated a dose-dependent increase in liver CBS activity and a dose-dependent decrease in serum tHcy. Liver CBS enzyme activity at 1 year was similar to Cbs+/- control mice. Mice given the highest dose (5.6 × 1011 genomes/mouse) had mean serum tHcy decrease of 97% 1 week after injection and an 81% reduction 1 year after injection. Treated mice had either full- or substantial correction of alopecia, bone loss, and fat mass phenotypes associated with Cbs deficiency in mice. Our findings show that AAVrh.10-based gene therapy is highly effective in treating CBS deficiency in mice and supports additional pre-clinical testing for eventual use human trials.

Liver transplant as a curative treatment in a pediatric patient with classic homocystinuria: A case report.

We report a patient with homocystinuria and hyperoxaluria who was cured of homocystinuria-related disease following liver transplant. The patient was diagnosed with homocystinuria as a newborn and was treated with dietary modifications and supplements. At 22 months, he passed a calcium oxalate stone and was found to have numerous bilateral kidney stones. Genetic testing confirmed primary hyperoxaluria, type 1. He underwent preemptive liver transplant at age four to treat primary hyperoxaluria. Following transplant, his serum methionine and homocysteine levels normalized, thus, demonstrating resolution of homocystinuria. Methionine and homocysteine levels remained normal 6 years later. Homocystinuria is associated with ophthalmologic, skeletal, neurologic, and thromboembolic complications. As cystathionine beta-synthase resides in the liver, transplant was hypothesized to be an effective treatment. Primary hyperoxaluria generally progresses to chronic kidney disease and is treated with combined kidney-liver transplant at the time of end stage kidney disease. Given this patient's dual diagnoses, we proceeded with preemptive liver transplantation. Three prior cases of patients with homocystinuria treated with liver transplantation have been reported. In all cases, transplant resolved metabolic effects. However, our case represents a pediatric patient without disease-related complications prior to transplant. This case supports liver-targeted gene therapies as an effective treatment for homocystinuria.

A cautionary tale of pyridoxine toxicity in cystathionine beta-synthase deficiency detected by two-tier newborn screening highlights the need for clear pyridoxine dosing guidelines.

Classic homocystinuria is due to deficiency of cystathionine beta-synthase (CBS), a pyridoxine-dependent enzyme that, depending on the molecular variants, may be co-factor responsive. Elevated methionine is often used as the primary analyte to detect CBS deficiency (CBSD) on newborn screening (NBS), but is limited by increased detection of other biochemical disorders with less clear clinical significance such as methionine aminotransferase (MAT) I/III heterozygotes. Our state has implemented a two-tier NBS algorithm for CBSD that successfully reduced the number of MATI/III heterozygotes, yet effectively detected a mild, co-factor responsive form of CBSD. After initial diagnosis, newborns with CBSD often undergo a pyridoxine challenge with high-dose pyridoxine to determine responsiveness. Here we describe our NBS-identified patient with a mild form of pyridoxine responsive CBSD who developed respiratory failure and rhabdomyolysis consistent with pyridoxine toxicity during a pyridoxine challenge. This case highlights the need for weight-based dosing and duration recommendations for pyridoxine challenge in neonates.

Treatment of Cystathionine ß-Synthase Deficiency in Mice Using a Minicircle-Based Naked DNA Vector.

Cystathionine ß-synthase (CBS) deficiency is a recessive inborn error of metabolism characterized by extremely elevated total homocysteine (tHcy) in the blood. Patients diagnosed with CBS deficiency have a variety of clinical problems, including dislocated lenses, osteoporosis, cognitive and behavioral issues, and a significantly increased risk of thrombosis. Current treatment strategies involve a combination of vitamin supplementation and restriction of foods containing the homocysteine precursor methionine. Here, a mouse model for CBS deficiency (Tg-I278T Cbs-/-) was used to evaluate the potential of minicircle-based naked DNA gene therapy to treat CBS deficiency. A 2.3 kb DNA-minicircle containing the liver-specific P3 promoter driving the human CBS cDNA (MC.P3-hCBS) was delivered into Tg-I278T Cbs-/- mice via a single hydrodynamic tail vein injection. Mean serum tHcy decreased from 351 µM before injection to 176 µM 7 days after injection (p = 0.0005), and remained decreased for at least 42 days. Western blot analysis reveals significant minicircle-directed CBS expression in the liver tissue. Liver CBS activity increased 34-fold (12.8 vs. 432 units; p = 0.0004) in MC.P3-hCBS-injected animals. Injection of MC.P3-hCBS in young mice, subsequently followed for 202 days, showed that the vector can ameliorate the mouse homocystinuria alopecia phenotype. The present findings show that minicircle-based gene therapy can lower tHcy in a mouse model of CBS deficiency.

Biomarkers of oxidative stress, inflammation, and vascular dysfunction in inherited cystathionine ß-synthase deficient homocystinuria and the impact of taurine treatment in a phase 1/2 human clinical trial.

STUDY OBJECTIVE: A phase 1/2 clinical trial was performed in individuals with cystathionine ß synthase (CBS) deficient homocystinuria with aims to: (a) assess pharmacokinetics and safety of taurine therapy, (b) evaluate oxidative stress, inflammation, and vascular function in CBS deficiency, and (c) evaluate the impact of short-term taurine treatment. METHODS: Individuals with pyridoxine-nonresponsive CBS deficiency with homocysteine >50 µM, without inflammatory disorder or on antioxidant therapy were enrolled. Biomarkers of oxidative stress and inflammation, endothelial function (brachial artery flow-mediated dilation [FMD]), and disease-related metabolites obtained at baseline were compared to normal values. While maintaining current treatment, patients were treated with 75 mg/kg taurine twice daily, and treatment response assessed after 4 hours and 4 days. RESULTS: Fourteen patients (8-35 years; 8 males, 6 females) were enrolled with baseline homocysteine levels 161 ± 67 µM. The study found high-dose taurine to be safe when excluding preexisting hypertriglyceridemia. Taurine pharmacokinetics showed a rapid peak level returning to near normal levels at 12 hours, but had slow accumulation and elevated predosing levels after 4 days of treatment. Only a single parameter of oxidative stress, 2,3-dinor-8-isoprostaglandin-F2α, was elevated at baseline, with no elevated inflammatory parameters, and no change in FMD values overall. Taurine had no effect on any of these parameters. However, the effect of taurine was strongly related to pretreatment FMD values; and taurine significantly improved FMD in the subset of individuals with pretreatment FMD values <10% and in individuals with homocysteine levels >125 µM, pertinent to endothelial function. CONCLUSION: Taurine improves endothelial function in CBS-deficient homocystinuria in patients with preexisting reduced function.

Homocystinuria: Therapeutic approach.

Homocystinuria is a disorder of sulfur metabolism pathway caused by deficiency of cystathionine ß-synthase (CBS). It is characterized by increased accumulation of homocysteine (Hcy) in the cells and plasma. Increased homocysteine results in various vascular and neurological complications. Present strategies to lower cellular and plasma homocysteine levels include vitamin B6 intake, dietary methionine restriction, betaine supplementation, folate and vitamin B12 administration. However, these strategies are inefficient for treatment of homocystinuria. In recent years, advances have been made towards developing new strategies to treat homocystinuria. These mainly include functional restoration to mutant CBS, enhanced clearance of Hcy from the body, prevention of N-homocysteinylation-induced toxicity and inhibition of homocysteine-induced oxidative stress. In this review, we have exclusively discussed the recent advances that have been achieved towards the treatment of homocystinuria. The review is an attempt to help clinicians in developing effective therapeutic strategies and designing novel drugs against homocystinuria.

The effect of dietary modulation of sulfur amino acids on cystathionine ß synthase-deficient mice.

Cystathionine ß synthase (CBS) is a key enzyme in the methionine and cysteine metabolic pathway, acting as a metabolic gatekeeper to regulate the flow of fixed sulfur from methionine to cysteine. Mutations in the CBS gene cause clinical CBS deficiency, a disease characterized by elevated plasma total homocysteine (tHcy) and methionine and decreased plasma cysteine. The treatment goal for CBS-deficient patients is to normalize the metabolic values of these three metabolites using a combination of vitamin therapy and dietary manipulation. To better understand the effectiveness of nutritional treatment strategies, we have performed a series of long-term dietary manipulation studies using our previously developed Tg-I278T Cbs(-/-) mouse model of CBS deficiency and sibling Tg-I278T Cbs(+/-) controls. Tg-I278T Cbs(-/-) mice have undetectable levels of CBS activity, extremely elevated plasma tHcy, modestly elevated plasma methionine, and low plasma cysteine. They exhibit several easily assayable phenotypes, including osteoporosis, loss of fat mass, reduced life span, and facial alopecia. The diets used in these studies differed in the amounts of sulfur amino acids or sulfur amino acid precursors. In this review, we will discuss our findings and their relevance to CBS deficiency and the concept of gene-diet interaction.

Lipid, Oxidative and Inflammatory Profile and Alterations in the Enzymes Paraoxonase and Butyrylcholinesterase in Plasma of Patients with Homocystinuria Due CBS Deficiency: The Vitamin B12 and Folic Acid Importance.

Cystathionine-ß-synthase (CBS) deficiency is the main cause of homocystinuria. Homocysteine (Hcy), methionine, and other metabolites of Hcy accumulate in the body of affected patients. Despite the fact that thromboembolism represents the major cause of morbidity in CBS-deficient patients, the mechanisms of cardiovascular alterations found in homocystinuria remain unclear. In this work, we evaluated the lipid and inflammatory profile, oxidative protein damage, and the activities of the enzymes paraoxonase (PON1) and butyrylcholinesterase (BuChE) in plasma of CBS-deficient patients at diagnosis and during the treatment (protein-restricted diet supplemented with pyridoxine, folic acid, betaine, and vitamin B12). We also investigated the effect of folic acid and vitamin B12 on these parameters. We found a significant decrease in HDL cholesterol and apolipoprotein A1 (ApoA-1) levels, as well as in PON1 activity in both untreated and treated CBS-deficient patients when compared to controls. BuChE activity and IL-6 levels were significantly increased in not treated patients. Furthermore, significant positive correlations between PON1 activity and sulphydryl groups and between IL-6 levels and carbonyl content were verified. Moreover, vitamin B12 was positively correlated with PON1 and ApoA-1 levels, while folic acid was inversely correlated with total Hcy concentration, demonstrating the importance of this treatment. Our results also demonstrated that CBS-deficient patients presented important alterations in biochemical parameters, possibly caused by the metabolites of Hcy, as well as by oxidative stress, and that the adequate adherence to the treatment is essential to revert or prevent these alterations.

Homocysteine contribution to DNA damage in cystathionine ß-synthase-deficient patients.

High blood levels of homocysteine (Hcy) are found in patients affected by homocystinuria, a genetic disorder caused by deficiency of cystathionine ß-synthase (CBS) activity, as well as in nutritional deficiencies (vitamin B12 or folate) and in abnormal renal function. We previously demonstrated that lipid and protein oxidative damage is increased and the antioxidant defenses diminished in plasma of CBS-deficient patients, indicating that oxidative stress is involved in the pathophysiology of this disease. In the present work, we extended these investigations by evaluating DNA damage through the comet assay in peripheral leukocytes from CBS-deficient patients, as well as by analyzing of the in vitro effect of Hcy on DNA damage in white blood cells. We verified that DNA damage was significantly higher in the CBS-deficient patients under treatment based on a protein-restricted diet and pyridoxine, folic acid, betaine and vitamin B12 supplementation, when compared to controls. Furthermore, the in vitro study showed a concentration-dependent effect of Hcy inducing DNA damage. Taken together, the present data indicate that DNA damage occurs in treated CBS-deficient patients, possibly due to high Hcy levels.

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 ß-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.

Cystathionine beta-synthase deficiency causes fat loss in mice.

Cystathionine beta synthase (CBS) is the rate-limiting enzyme responsible for the de novo synthesis of cysteine. Patients with CBS deficiency have greatly elevated plasma total homocysteine (tHcy), decreased levels of plasma total cysteine (tCys), and often a marfanoid appearance characterized by thinness and low body-mass index (BMI). Here, we characterize the growth and body mass characteristics of CBS deficient TgI278T Cbs(-/-) mice and show that these animals have significantly decreased fat mass and tCys compared to heterozygous sibling mice. The decrease in fat mass is accompanied by a 34% decrease in liver glutathione (GSH) along with a significant decrease in liver mRNA and protein for the critical fat biosynthesizing enzyme Stearoyl CoA desaturase-1 (Scd-1). Because plasma tCys has been positively associated with fat mass in humans, we tested the hypothesis that decreased tCys in TgI278T Cbs(-/-) mice was the cause of the lean phenotype by placing the animals on water supplemented with N-acetyl cysteine (NAC) from birth to 240 days of age. Although NAC treatment in TgI278T Cbs(-/-) mice caused significant increase in serum tCys and liver GSH, there was no increase in body fat content or in liver Scd-1 levels. Our results show that lack of CBS activity causes loss of fat mass, and that this effect appears to be independent of low serum tCys.

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.

Differential effects of low and high dose folic acid on endothelial dysfunction in a murine model of mild hyperhomocysteinaemia.

The exact mechanism(s) by which hyperhomocysteinaemia promotes vascular disease remains unclear. Moreover, recent evidence suggests that the beneficial effect of folic acid on endothelial function is independent of homocysteine-lowering. In the present study the effect of a low (400 microg/70 kg/day) and high (5 mg/70 kg/day) dose folic acid supplement on endothelium-dependent relaxation in the isolated perfused mesenteric bed of heterozygous cystathionine beta-synthase deficient mice was investigated. Elevated total plasma homocysteine and impaired relaxation responses to methacholine were observed in heterozygous mice. In the presence of N(G)-nitro-L-arginine methyl ester relaxation responses in wild-type tissues were reduced, but in heterozygous tissues were abolished. Clotrimazole and 18alpha-glycyrrhetinic acid, both inhibitors of non-nitric oxide/non-prostanoid-induced endothelium-dependent relaxation, reduced responses to methacholine in wild-type but not heterozygous tissues. The combination of N(G)-nitro-L-arginine methyl ester and either clotrimazole or 18alpha-glycyrrhetinic acid completely inhibited relaxation responses in wild-type tissues. Both low and high dose folic acid increased plasma folate, reduced total plasma homocysteine and reversed endothelial dysfunction in heterozygous mice. A greater increase in plasma folate in the high dose group was accompanied by a more significant effect on endothelial function. In the presence of N(G)-nitro-L-arginine methyl ester, a significant residual relaxation response was evident in tissues from low and high dose folic acid treated heterozygous mice. These data suggest that the impaired mesenteric relaxation in heterozygous mice is largely due to loss of the non-nitric oxide/non-prostanoid component. While low dose folic acid may restore this response in a homocysteine-dependent manner, the higher dose has an additional effect on nitric oxide-mediated relaxation that would appear to be independent of homocysteine lowering.

Effects of betaine in a murine model of mild cystathionine-beta-synthase deficiency.

Cystathionine-beta-synthase (CBS) is required for transsulfuration of homocysteine, an amino acid implicated in vascular disease. We studied homocysteine metabolism in mice with mild hyperhomocysteinemia due to a heterozygous disruption of the Cbs gene. Mice were fed diets supplemented with betaine or dimethylsulfonioacetate (DMSA); betaine and DMSA provide methyl groups for an alternate pathway of homocysteine metabolism, remethylation by betaine:homocysteine methyltransferase (BHMT). On control diets, heterozygous mice had 50% higher plasma homocysteine than did wild-type mice. Betaine and DMSA had similar effects in both genotype groups: liver betaine increased dramatically, while plasma homocysteine decreased by 40% to 50%. With increasing betaine supplementation, homocysteine decreased by 75%. Plasma homocysteine and BHMT activity both showed a strong negative correlation with liver betaine. Homocysteinemia in mice is sensitive to a disruption of Cbs and to methyl donor intake. Because betaine leads to a greater flux through BHMT and lowers homocysteine, betaine supplementation may be beneficial in mild hyperhomocysteinemia.

Homocysteine, endothelial dysfunction, and coronary artery disease: emerging strategy for secondary prevention.

Atherosclerosis is an important medical problem of the 21st century, but traditional risk factors could only account for 50% of the problem. Hyperhomocysteinemia is emerging as an independent atherosclerosis risk factor, associated with folate deficiency, renal failure, and relative deficiency of MTHFR (C677T polymorphism) or other enzymes depending on gender, age, and smoking status. Hyperhomocysteinemia has been reported to occur in 11-22% of western people, in 3-5% of normal asymptomatic Chinese subjects aged 18-70 years in Hong Kong, Macau, Sydney, and San Francisco, 23-36% of Chinese in Hong Kong with premature coronary artery disease, and 29% of a nonselective series of coronary subjects in Hong Kong. Evidence is accumulating that documents its associations with atherosclerosis disease in both case-control observations and prospective cohort studies, in vitro experiments, and in vivo experimental models in both animals and human subjects, as well as the successful improvement by homocysteine-lowering of endothelial function as surrogate atherosclerosis endpoints in asymptomatic human and coronary patients (secondary prevention). A number of large scale homocysteine-lowering trials are currently underway for stroke and heart attacks prevention. Collectively these trials will include more than 65,000 patients at high-risk for cardiovascular and stroke events, and should provide a reliable evidence-base for prevention.

Oxidative stress and platelet activation in homozygous homocystinuria.

BACKGROUND: Severe hyperhomocysteinemia due to cystathionine beta-synthase deficiency (CbetaSD) is associated with early atherothrombotic vascular disease. Homocysteine may exert its effects by promoting oxidative damage. In the present study, we investigated whether in vivo formation of 8-iso-prostaglandin (PG) F(2alpha), a platelet-active product of arachidonic acid peroxidation, is enhanced in CbetaSD and whether it correlates with in vivo platelet activation, as reflected by thromboxane (TX) metabolite excretion. METHODS AND RESULTS: Urine and blood samples were obtained from patients with homozygous CbetaSD (n=13) and age-matched healthy subjects. Urinary 8-iso-PGF(2alpha) excretion was significantly higher in CbetaSD patients than in control subjects (640+/-384 versus 213+/-43 pg/mg creatinine; P=0.0015) and correlated with plasma homocysteine (rho=0.398, P=0.0076). Similarly, urinary 11-dehydro-TXB(2) excretion was enhanced in CbetaSD (1166+/-415 versus 324+/-72 pg/mg creatinine; P=0.0015) and correlated with urinary 8-iso-PGF(2alpha) (rho=0.362, P=0.0153). Vitamin E supplementation (600 mg/d for 2 weeks) was associated with a statistically significant increase in its plasma levels (from 16.6+/-4.6 to 40.4+/-8.7 micromol/L, P=0.0002) and with reductions in 8-iso-PGF(2alpha) (from 790+/-159 to 559+/-111 pg/mg creatinine, P=0.018) and 11-dehydro-TXB(2) (from 1273+/-383 to 913+/-336 pg/mg creatinine, P=0.028). A statistically significant inverse correlation was found between urinary 8-iso-PGF(2alpha) and plasma vitamin E levels (rho=-0.745, P=0.0135). CONCLUSIONS: The results of the present study suggest that enhanced peroxidation of arachidonic acid to form bioactive F(2)-isoprostanes may represent an important mechanism linking hyperhomocysteinemia and platelet activation in CbetaSD patients. Moreover, they provide a rationale for dose-finding studies of vitamin E supplementation in this setting.

Endothelial dysfunction and elevation of S-adenosylhomocysteine in cystathionine beta-synthase-deficient mice.

Hyperhomocysteinemia is associated with increased risk for cardiovascular events, but it is not certain whether it is a mediator of vascular dysfunction or a marker for another risk factor. Homocysteine levels are regulated by folate bioavailability and also by the methyl donor S-adenosylmethionine (SAM) and its metabolite S-adenosylhomocysteine (SAH). We tested the hypotheses that endothelial dysfunction occurs in hyperhomocysteinemic mice in the absence of folate deficiency and that levels of SAM and SAH are altered in mice with dysfunction. Heterozygous cystathionine beta-synthase-deficient (CBS(+/-)) and wild-type (CBS(+/+)) mice were fed a folate-replete, methionine-enriched diet. Plasma levels of total homocysteine were elevated in CBS(+/-) mice compared with CBS(+/+) mice after 7 weeks (27.1+/-5.2 versus 8.8+/-1.1 micromol/L; P<0.001) and 15 weeks (23.9+/-3.0 versus 13.0+/-2.3 micromol/L; P<0.01). After 15 weeks, but not 7 weeks, relaxation of aortic rings to acetylcholine was selectively impaired by 35% (P<0.05) and thrombomodulin anticoagulant activity was decreased by 20% (P<0.05) in CBS(+/-) mice. Plasma levels of folate did not differ between groups. Levels of SAH were elevated approximately 2-fold in liver and brain of CBS(+/-) mice, and correlations were observed between plasma total homocysteine and SAH in liver (r=0.54; P<0.001) and brain (r=0.67; P<0.001). These results indicate that endothelial dysfunction occurs in hyperhomocysteinemic mice even in the absence of folate deficiency. Endothelial dysfunction in CBS(+/-) mice was associated with increased tissue levels of SAH, which suggests that altered SAM-dependent methylation may contribute to vascular dysfunction in hyperhomocysteinemia.

Homocystinuria due to cystathionine beta-synthase deficiency in Ireland: 25 years' experience of a newborn screened and treated population with reference to clinical outcome and biochemical control.

Homocystinuria (HCU) due to cystathionine beta-synthase deficiency (Mudd et al 1964) was independently described by Gerritsen and colleagues (USA) and Carson and colleagues (Northern Ireland) in 1962. The worldwide frequency of HCU has been reported as 1 in 344,000, while that in Ireland is much higher at 1 in 65,000, based on newborn screening and cases detected clinically. The national newborn screening programme for HCU in Ireland was started in 1971 using the bacterial inhibition assay. A total of 1.58 million newborn infants have been screened over a 25-year period up to 1996. Twenty-five HCU cases were diagnosed, 21 of whom were identified on screening. The remaining four HCU cases were missed and presented clinically; three of these were breast-fed and one was pyridoxine responsive. Twenty-four HCU cases were pyridoxine nonresponsive. Once the status of pyridoxine responsiveness was identified, all pyridoxine nonresponsive cases, but one, were started on a low methionine, cystine-enhanced diet supplemented with pyridoxine, vitamin B12 and folate. Dietary treatment commenced within 6 weeks of birth (range 8-42 days) for those cases detected by screening, while for the late-detected cases treatment was started upon presentation and diagnosis. Biochemical control was monitored measuring deproteinized plasma methionine, free homocystine and cystine at least once a month. Review of the clinical outcome of the 25 HCU cases with 365.7 patient-years of treatment revealed no HCU-related complications in 18 screened, dietary-treated cases. Fifteen of these had lifetime medians of free homocystine < or = 11 mumol/L (range 4-11). The remaining three cases with higher lifetime medians of free homocystine (18, 18 and 48 mumol/L) have developed increasing myopia recently. Among the three screened non-dietary-compliant cases, two have ectopia lentis, one has osteoporosis and two have mental handicap. Of the four cases missed on screening, three presented with ectopia lentis after the age of 2 years. There were no thromboembolic events in any of the 25 HCU cases. The lifetime medians for methionine ranged from 47 to 134 mumol/L. The Irish HCU clinical outcome data suggest that newborn screening, early commencement of dietary treatment and a lifetime median of free homocystine of < or = 11 mumol/L had significantly reduced the probability of developing complications when it was compared to the untreated HCU data (Mudd et al 1985).

Strategies for the treatment of cystathionine beta-synthase deficiency: the experience of the Willink Biochemical Genetics Unit over the past 30 years.

Strategies for the treatment of cystathionine beta-synthase (CBS) deficiency include (1) increasing residual enzyme activity by giving pyridoxine in those patients with vitamin responsive variants, (2) reducing the load on the affected pathway with a low methionine diet and supplementing the diet with cysteine; and (3) giving betaine in order to utilise alternative pathways to remove homocyst(e)ine. In our experience of over 30 years in the diagnosis and management of patients with CBS deficiency, a normal outcome can only be achieved in patients diagnosed and treated from infancy. Pyridoxine combined with folic acid prevents further deterioration in pyridoxine responsive patients. Dietary treatment of patients with non-pyridoxine responsive CBS deficiency becomes more difficult outside childhood but since late complications are not uncommon must be continued for life. Betaine can be effective in this group but compliance is often poor.

Thiamine (vitamin B1) supplementation does not reduce fasting blood homocysteine concentration in most homozygotes for homocystinuria.

Homozygotes for homocystinuria due to cystathionine synthase (CS) deficiency accumulate homocysteine and methionine in their blood and tissues. High-dose pyridoxin, folic acid, vitamin B12, or betaine are therapeutical options to lower the elevated homocysteine concentration. These compounds stimulate the transsulfuration or remethylation of homocysteine. Despite such treatment, elevated blood homocysteine concentrations may persist in many homocystinurics. Therefore, it is warranted to study alternative regimen to reduce the blood homocysteine concentration in homocystinurics. Apart from entering the transsulfuration pathway, methionine can be catabolized via the transamination pathway, by conversion into 4-methylthio-2-oxobutyrate (MTOB), followed by oxidative decarboxylation of MTOB to 3-methylthiopropionate. Thiamine pyrophosphate, the active form of thiamine, is a cofactor of the supposed rate-limiting oxidative decarboxylation in the transamination of methionine. The effect of thiamine administered in 2 or 3 daily doses of 25 mg orally, was studied in nine homozygote CS deficient patients. Methionine levels decreased in 6 out of 9 patients. In 8 out of 9 patients, however, the levels of plasma homocysteine remained virtually unchanged, as did the serum transamination metabolites in all patients. We conclude that vitamin B1 cannot be used as an additional homocysteine-lowering treatment in most homozygotes for homocystinuria.