Major determinants of hyperhomocysteinemia in peritoneal dialysis patients

Gut Microbiome, Functional Food, Atherosclerosis, and Vascular Calcifications-Is There a Missing Link?

The gut microbiome is represented by the genome of all microorganisms (symbiotic, potential pathogens, or pathogens) residing in the intestine. These ecological communities are involved in almost all metabolic diseases and cardiovascular diseases are not excluded. Atherosclerosis, with a continuously increasing incidence in recent years, is the leading cause of coronary heart disease and stroke by plaque rupture and intraplaque hemorrhage. Vascular calcification, a process very much alike with osteogenesis, is considered to be a marker of advanced atherosclerosis. New evidence, suggesting the role of dietary intake influence on the diversity of the gut microbiome in the development of vascular calcifications, is highly debated. Gut microbiota can metabolize choline, phosphatidylcholine, and L-carnitine and produce vasculotoxic metabolites, such as trimethylamine-N-oxide (TMAO), a proatherogenic metabolite. This review article aims to discuss the latest research about how probiotics and the correction of diet is impacting the gut microbiota and its metabolites in the atherosclerotic process and vascular calcification. Further studies could create the premises for interventions in the microbiome as future primary tools in the prevention of atherosclerotic plaque and vascular calcifications.

Decisive evidence corroborates a null relationship between MTHFR C677T and chronic kidney disease: A case-control study and a meta-analysis

BACKGROUND

Previous meta-analyses have explored the association between the C677T polymorphism of methyltetrahydrofolate reductase (MTHFR) and chronic kidney disease (CKD) but there were no studies with a decisive conclusion. Furthermore, the high heterogeneity among different populations is not yet interpreted.

OBJECTIVES

This study used trial sequential analysis (TSA) to evaluate whether the nowadays conclusion supported by current cumulative samples. We also applied case-weighted meta-regression to explore the potential gene-environment interactions.

METHODS

For the first stage of this study we conducted a case-control study involving 847 dialysis patients from 7 hemodialysis centers in Taipei during 2015 to 2018 and 755 normal controls from a health center in the Tri-Service General Hospital. The second stage combined the results from the first stage with previous studies. The previous studies were collected from PubMed, EMBASE, and Web of Science databases before January 2018.

RESULTS

From the case-control study, the T allele of MTHFR C677T appeared to have a protective effect on end-stage renal disease compared with the C allele [odds ratio (OR): 0.80, 95% CI (confidence interval) = 0.69-0.93]. However, the meta-analysis contradicted the results in Asian (OR = 1.12, 95% CI = 0.96-1.30). The same analysis was also applied in Caucasian and presented similar results from Asian (OR = 1.18, 95% CI = 0.98-1.42). The TSA showed our case-control study to be the decisive sample leading to a null association among Asian population. The high heterogeneity (I = 75%) could explain the contradictory results between the case-control study and the meta-analysis. However, further case-weighted meta-regression did not find any significant interaction between measured factors and MTHFR C677T on CKD.

CONCLUSIONS

High heterogeneities were found in both Caucasian and Asian, which caused the null relationship in meta-analysis while there were significant effects in individual studies. Future studies should further explore the high heterogeneity that might be hidden in unmeasured gene-environment interactions, to explain the diverse findings among different populations.

Peritoneal Homocysteine Clearance is Inefficient in Peritoneal Dialysis Patients

Objectives

To investigate the degree and the determinants of peritoneal homocysteine (Hcy) clearance and to compare measured Hcy clearance with the Hcy clearance predicted based on molecular weight (MW).

Design

Cross-sectional observational analysis.

Setting

Tertiary care institutional dialysis center.

Patients

Sixty-five stable peritoneal dialysis (PD) patients.

Outcome Measures

Fasting blood and 24-hour pooled dialysate effluents were collected for determination of peritoneal clearances of Hcy (CpHcy), urea (CpUr), and creatinine (CpCr). The dialysate-to-plasma creatinine ratio at 4 hours (D/P Cr 4 h) and levels of red cell folate, B12, ferritin, and C-reactive protein (CRP) were measured concurrently. Observed CpHcy was compared with predicted clearance, based on Hcy plasma protein binding and the relative molecular weights of Hcy, urea, and creatinine.

Results

Plasma concentrations of Hcy averaged 24.6 ± 1.1 μmol/L and were elevated above the upper limit of normal in 59 (91%) patients. The mean dialysate concentration of Hcy was 2.9 ± 0.3 μmol/L, equating to a daily peritoneal elimination of 34.6 ± 3.6 μmol. Observed CpHcy was closely approximated by predicted CpHcy (8.7 ± 0.6 L/week/1.73 m2vs 9.0 ± 0.3 L/week/1.73 m2respectively, p = 0.55). Patients maintained on automated PD ( n = 5) had a CpHcy similar to that of patients treated with continuous ambulatory peritoneal dialysis (8.9 ± 1.0 L/week/1.73 m2vs 8.7 ± 0.6 L/week/1.73 m2, p = 0.92). The CpHcy was significantly correlated with C-reactive protein (CRP), D/P creatinine, CpUr, CpCr, and peritoneal protein loss, but not with plasma Hcy, albumin, B12, ferritin, age, dialysis duration, peritonitis episodes, or daily dialysate effluent volume. By multivariate analysis, the only variables that remained significant independent predictors of CpHcy were CRP and D/P Cr 4 h. High and high-average transporters had a higher CpHcy than low and low-average transporters (9.7 ± 0.8 L/week/1.73 m2vs 7.0 ± 0.7 L/week/1.73 m2, p < 0.05), despite comparably elevated plasma Hcy concentrations [25.2 ± 1.5 μmol/L vs 23.4 ± 1.6 μmol/L, p = nonsignificant (NS)].

Conclusions

Elevated plasma concentrations of Hcy are not efficiently reduced by PD. The relatively low peritoneal clearance of Hcy is largely accounted for by a high degree of plasma protein binding and is significantly influenced by peritoneal membrane permeability.

Homocysteine, Vitamin B12, and Serum and Erythrocyte Folate in Peritoneal Dialysis and Hemodialysis Patients

Background

Plasma homocysteine (Hcy) is an independent risk factor for cardiovascular disease. High levels of plasma Hcy have been observed in end-stage renal disease patients. Few studies have compared peritoneal dialysis (PD) and hemodialysis (HD) patients and few data are available on erythrocyte folate (ery-F) levels in dialysis patients.

Objectives

To evaluate plasma Hcy concentrations, vitamin B12 (B12), and folate status in dialysis patients; to analyze the possible causes of high Hcy levels; to follow up changes in folate and B12 concentrations after 6 months.

Design

A cross-sectional observational study.

Setting

Nephrology division and laboratory of hematology in a university and clinical research hospital.

Patients

The study included 82 patients treated with PD for 37 ± 37 months and 70 patients treated with HD for 136 ± 95 months.

Laboratory Methods

Plasma Hcy was measured by the immunoenzymatic IMx Hcy FPIA method (Abbott Laboratories, Diagnostic Division, Abbott Park, IL, U.S.A.), serum folate (s-F) and ery-F by the Stratus folate fluorometric enzyme-linked assay, and B12 by the Stratus vitamin B12 fluorometric enzyme-linked assay (DADE-Behring, Newark, DE, U.S.A.).

Results

Ninety-six percent of PD and 97% of HD patients had Hcy levels above the cutoff (13.5 μmol/L). Homocysteine level was higher in HD than in PD patients, while the prevalence of hyperhomocysteinemia was similar with the two techniques. Erythrocyte folate was significantly higher in PD (1333 ± 519 pmol/L) than in HD (1049 ± 511 pmol/L, p < 0.01). Statistically significant correlations were observed between Hcy and B12, s-F, ery-F, and dialysis duration. Multivariate analysis showed a strong correlation between s-F and Hcy. After 6 months there were no differences in Hcy, B12, s-F, and ery-F levels.

Conclusions

Plasma Hcy levels were high in more than 95% of our dialysis patients, with no relation to the type of dialysis. Vitamin B12 and folate were normal in the majority of our patients. However, serum folate was the major determinant of Hcy levels. Such a relation between Hcy and folate suggests that levels of folate within the reference interval are inadequate for dialysis patients.

Homocysteine and IgA nephropathy: observational and Mendelian randomization analyses

BACKGROUND

High levels of plasma homocysteine occur almost uniformly in patients with end-stage renal disease (ESRD). IgA nephropathy (IgAN) is the most common form of primary glomerulonephritis and a common cause of ESRD in young adults. Here, we aimed to detect whether homocysteine was elevated and associated with clinical-pathologic manifestations of IgAN patients and tested its causal effects using a two-sample Mendelian randomization (MR) approach.

METHODS

For observational analysis, 108 IgAN patients, 30 lupus nephritis (LN) patients, 50 minimal change disease (MCD) patients, and 206 healthy controls were recruited from April 2014 to April 2015. Their plasma homocysteine was measured and clinical-pathologic manifestations were collected from medical records. For MR analysis, we further included 1686 IgAN patients. The missense variant methylenetetrahydrofolate reductase C677T (rs1801133) was selected as an instrument, which was genotyped by TaqMan allele discrimination assays.

RESULTS

Majority of IgAN patients (93.52%, 101/108) showed elevated levels of plasma homocysteine (>10 μmol/L). Plasma homocysteine in IgAN patients was significantly higher than that in MCD patients (median: 18.32 vs. 11.15 μmol/L, Z = -5.29, P < 0.01) and in healthy controls (median: 18.32 vs. 10.00 μmol/L, Z = -8.76, P < 0.01), but comparable with those in LN patients (median: 18.32 L vs. 14.50 μmol/L, Z = -1.32, P = 0.19). Significant differences were observed in sub-groups of IgAN patients according to quartiles of plasma homocysteine for male ratio (22.22% vs. 51.85% vs. 70.37% vs. 70.37%, χ = 14.29, P < 0.01), serum creatinine (median: 77.00 vs. 100.00 vs. 129.00 vs. 150.00 μmol/L, χ = 34.06, P < 0.01), estimated glomerular filtration rate (median: 100.52 vs. 74.23 vs. 52.68 vs. 42.67 mL·min·1.73 m, χ = 21.75, P < 0.01), systolic blood pressure (median: 120.00 vs. 120.00 vs. 125.00 vs. 130.00 mmHg, χ = 2.97, P = 0.05), diastolic blood pressure (median 80.00 vs. 75.00 vs. 80.00 vs. 81.00 mmHg, χ = 11.47, P < 0.01), and pathologic tubular atrophy and interstitial fibrosis (T) (T0/T1/T2: 62.96%/33.33%/3.70% vs. 29.63%/40.74%/29.63% vs. 24.00%/48.00%/28.00% vs. 14.81%/37.04%/48.15%, χ = 17.66, P < 0.01). The coefficient of each rs1801133-T allele on homocysteine levels after controlling age and sex was 7.12 (P < 0.01). MR estimates showed causal positive effects of homocysteine on serum creatine (β = 0.76, P = 0.02), systolic blood pressure (β = 0.26, P = 0.02), diastolic blood pressure (β = 0.20, P = 0.01), and pathologic T lesion (β = 0.01, P = 0.01) in IgAN.

CONCLUSIONS

By observational and MR analyses, consistent results were observed for associations of plasma homocysteine with serum creatinine, blood pressures, and pathologic T lesion in IgAN patients.

Folic Acid and Vitamin B12 Administration in CKD, Why Not?

Patients affected by chronic kidney disease (CKD) or end-stage renal disease (ESRD) experience a huge cardiovascular risk and cardiovascular events represent the leading causes of death. Since traditional risk factors cannot fully explain such increased cardiovascular risk, interest in non-traditional risk factors, such as hyperhomocysteinemia and folic acid and vitamin B12 metabolism impairment, is growing. Although elevated homocysteine blood levels are often seen in patients with CKD and ESRD, whether hyperhomocysteinemia represents a reliable cardiovascular and mortality risk marker or a therapeutic target in this population is still unclear. In addition, folic acid and vitamin B12 could not only be mere cofactors in the homocysteine metabolism; they may have a direct action in determining tissue damage and cardiovascular risk. The purpose of this review was to highlight homocysteine, folic acid and vitamin B12 metabolism impairment in CKD and ESRD and to summarize available evidences on hyperhomocysteinemia, folic acid and vitamin B12 as cardiovascular risk markers, therapeutic target and risk factors for CKD progression.

Water soluble vitamins and peritoneal dialysis - State of the art

This review presents the results of a systematic literature search concerning water soluble vitamins and peritoneal dialysis modality. We provide an overview of the data available on vitamin requirements, dietary intake, dialysis related losses, metabolism and the benefits of supplementation. We also summarise the current recommendations concerning the supplementation of vitamins in peritoneal dialysis and discuss the safety of an administration of vitamins in pharmacological doses.

Effect of cholecalciferol on local arterial stiffness and endothelial dysfunction in children with chronic kidney disease

BACKGROUND

As cardiovascular factors are the leading cause of mortality in chronic kidney disease (CKD) and as vitamin D deficiency is prevalent in this population, we aimed to examine the effect of oral cholecalciferol on cardiac parameters and biomarkers for endothelial cell activation in children with CKD.

METHODS

Forty-one children with CKD and 24 healthy subjects free of any underlying cardiac or renal disease with low 25-hydroxyvitamin D3 (25OHD) levels were evaluated using echocardiography basally and following Stoss vitamin D supplementation. The local vascular stiffness and endothelial dysfunction markers were compared among the groups.

RESULTS

Initial flow-mediated dilatation (FMD) measurements were lower and local arterial stiffness was significantly higher in patients. After vitamin D supplementation, these improved significantly in patients, while no significant change was observed for the healthy group. Homocysteine showed inverse correlation with baseline vitamin D level in CKD children and von Willebrand factor emerged as an independent risk factor for FMD impairment.

CONCLUSIONS

Our interventional study revealed the favorable effects of high-dose cholecalciferol on cardiovascular and endothelial parameters, implying the importance of vitamin D supplementation in children with CKD.

Homocysteine as a risk factor for cardiovascular disease in patients treated by dialysis: a meta-analysis

BACKGROUND

In the general population, increased homocysteine concentrations are a risk factor for cardiovascular disease and mortality. However, it is not known whether this also applies to patients with end-stage renal disease.

STUDY DESIGN

Meta-analysis of retrospective (11 studies including 1,506 individuals), prospective observational studies (12 studies including 1,975 individuals), and intervention trials (5 studies including 1,642 dialysis patients). Analyses were carried out separately, according to the study design.

SETTING & POPULATION

Studies of patients with end-stage renal disease treated by means of hemodialysis or peritoneal dialysis.

SELECTION CRITERIA FOR STUDIES

Studies investigating the association between total homocysteine level and cardiovascular disease or total mortality or the influence of vitamin supplementation on cardiovascular or mortality risk.

INTERVENTION

In intervention studies, vitamin preparations with folic acid alone or in combination with other vitamins, such as vitamin B(12) and B(6), were used.

OUTCOMES

In retrospective studies, cases are patients with cardiovascular diseases. Outcomes for prospective observational and intervention studies are cardiovascular events and total mortality.

RESULTS

In retrospective studies, there was no significant overall difference in homocysteine concentrations between cases and controls (weighted mean difference in homocysteine, 2.82 micromol/L; 95% confidence interval [CI], -2.22 to 7.86; P = 0.3). The pooled overall risk estimate for prospective observational studies suggests no association between homocysteine level (5-micromol/L increase) and total mortality (hazard ratio [HR], 1.02; 95% CI, 0.93 to 1.12; P = 0.7), but there was an association with cardiovascular events (HR, 1.09; 95% CI, 1.03 to 1.14; P = 0.001). In subgroup analysis of patients not receiving vitamins, an increase in homocysteine level was associated with increased mortality (HR, 1.07; 95% CI, 1.02 to 1.13; P = 0.01). For intervention trials with B vitamins, there was a significant risk reduction for cardiovascular disease (relative risk, 0.73; 95% CI, 0.56 to 0.94; P = 0.02), but no risk reduction for total mortality or the composite end point including total mortality (relative risk, 1.01; 95% CI, 0.88 to 1.15; P = 0.9).

LIMITATIONS

Many studies are small, which may lead to the observed heterogeneity. Some intervention trials are neither placebo controlled nor randomized. Separate analyses for specific end points and patients treated by means of hemodialysis or peritoneal dialysis were not possible.

CONCLUSION

Total homocysteine level may be a risk factor for cardiovascular events and total mortality in patients with end-stage renal disease not receiving vitamin supplementation or folic acid food fortification. There may be a potential for reducing cardiovascular disease in this population by folic acid supplementation.

Epigenetics in hyperhomocysteinemic states. A special focus on uremia

Aim of this article is to review the topic of epigenetic control of gene expression, especially regarding DNA methylation, in chronic kidney disease and uremia. Hyperhomocysteinemia is considered an independent cardiovascular risk factor, although the most recent intervention studies utilizing folic acid are negative. The accumulation of homocysteine in blood leads to an intracellular increase of S-adenosylhomocysteine (AdoHcy), a powerful competitive methyltransferase inhibitor, which is itself considered a predictor of cardiovascular events. The extent of methylation inhibition of each individual methyltransferase depends on the methyl donor S-adenosylmethionine (AdoMet) availability, on the [AdoMet]/[AdoHcy] ratio, and on the individual Km value for AdoMet and Ki for AdoHcy. DNA methyltransferases are among the principal targets of hyperhomocysteinemia, as studies in several cell culture and animal models, as well as in humans, almost unequivocally show. In vivo, DNA methylation may be also influenced by various factors in different tissues, for example by rate of cell growth, folate status, etc. and importantly inflammation. In chronic kidney disease and in uremia, hyperhomocysteinemia is commonly seen, and can be associated with global DNA hypomethylation, and with abnormal allelic expression of genes regulated through methylation. This alteration is susceptible of reversal upon homocysteine-lowering therapy obtained through folate administration. If this abnormality will translate itself in alterations of expression of genes relevant to the pathogenesis of this disease still remains to be established. In addition, these results establish a link between the epigenetic control of gene expression and xenobiotic influences, such as folate therapy.

[Cardiovascular morbidity and mortality risk factors in peritoneal dialysis patients]

Cardiovascular (CVS) morbidity and mortality in the end-stage renal disease (ESRD) patients on peritoneal dialysis therapy is 10-30 folds higher than in general population. The prevalence of well known traditional risk factors such as age, sex, race, arterial hypertension, hyperlipidaemia, diabetes, smoking, physical inactivity is higher in the uraemic patients. Besides these, there are specific, nontraditional risk factors for dialysis patients. Mild inflammation present in peritoneal dialysis (PD) patients which can be confirmed by specific inflammatory markers is the cause of CVS morbidity and mortality in these patients. Hypoalbuminaemia, hyperhomocysteinaemia and a higher level of leptin are important predictors of vascular complications as well as CVS events in the PD patients. Plasma norepinephrine, an indicator of sympathetic activity, is high in the ESRD patients and higher in the PD patients than in the patients on haemodialysis (HD). Therefore, norepinephrine may be a stronger risk factor in the PD patients. The same applies to asymmetric dimethylargine (ADMA), an endogenous inhibitor of nitric oxide synthase, which is an important risk factor of CVS morbidity and mortality 15 % higher in the PD than the HD patients. Hyperphosphataemia, secondary hyperparathyroidism and high calcium x phosphate product have been associated with the progression of the coronary artery calcification and valvular calcifications and predict all-cause CVS mortality in the PD patients. Residual renal function (RRF) declines with time on dialysis but is slower in the PD than the HD patients. RRF decline is associated with the rise of proinflammatory cytokines and the onset of hypervolaemia and hypertension which increase the risk of CVS diseases, mortality in general and CVS mortality. In conclusion, it is very important to establish all CVS risk factors in the PD patients to prevent CVS diseases and CVS mortality in this population.

Epidemiology of dialysis patients and heart failure patients

The epidemiology of maintenance dialysis patients and heart failure patients has striking similarities. Both groups have a high prevalence of comorbid conditions, a high hospitalization rate, a low self-reported quality of life, and an excessively high mortality risk, mostly because of cardiovascular causes. Observational studies in both dialysis and heart failure patients have indicated the lack of a significant association between the traditional cardiovascular risk factors and mortality, or the existence of a paradoxic or reverse association, in that obesity, hypercholesterolemia, and hypertension appear to confer survival advantages. The time discrepancy between the 2 sets of risk factors, that is, overnutrition (long-term killer) versus undernutrition (short-term killer) may explain the overwhelming role of malnutrition, inflammation, and cachexia in causing the reverse epidemiology, which may exist in more than 20 million Americans. We have reviewed the opposing views about the concept of reverse epidemiology in dialysis and heart failure patients, the recent Die Deutsche Diabetes Dialyze study findings, and the possible role of racial disparities. Contradictory findings on hyperhomocysteinemia in dialysis patients are reviewed in greater details as a possible example of publication bias. Additional findings related to intravenous iron and serum ferritin, calcium, and leptin levels in dialysis patients may enhance our understanding of the new paradigm. The association between obesity and increased death risk in kidney transplanted patients is reviewed as an example of the reversal of reverse epidemiology. Studying the epidemiology of dialysis patients as the archetypical population with such paradoxic associations may lead to the development of population-specific guidelines and treatment strategies beyond the current Framingham cardiovascular risk factor paradigm.

Clinical significance of homocysteine in elderly hospitalized patients

Serum homocysteine levels, which increase with age, are now recognized as a vascular risk factor and are related to the development of heart failure and dementia in the elderly. However, relatively low serum homocysteine levels have also been reported to be an adverse prognostic factor in dialysis patients. The objective of the study was to analyze the prevalence, clinical significance, and prognostic value of serum homocysteine levels in patients older than 65 years, admitted to a general internal medicine hospitalization unit. We studied 337 hospitalized patients, 184 males and 153 females, aged 77.2+/-0.4 years, whose admission was not determined by an acute vascular event. We recorded past vascular events and vascular risk factors. We determined the body mass index (weight in kilograms divided by the square of height in meters), and cholesterol, triglyceride, folate, vitamin B12, and homocysteine levels. We also studied 36 control subjects (18 males and 18 females) of similar age. After discharge, we assessed the survival status of 301 patients by telephone recall. Survival curves were plotted by the method of Kaplan and Meier. Median survival was 1186 days. The 15th (9.6 micromol/L) and 50th (14.4 micromol/L) percentiles, as the lowest and highest cut-off points, were empirically defined as those related to a shorter survival. Serum homocysteine concentration was significantly positively correlated with age and serum creatinine and albumin concentrations, and negatively correlated with serum cobalamin and folate concentrations. The average serum homocysteine concentration for the patients group, as a whole, was 16.5+/-0.5 micromol/L, not significantly different from the control group, but with a much greater dispersion, as patients with congestive heart failure or cognitive impairment had higher serum homocysteine concentrations, and patients with sepsis, leukocytosis, and hypoalbuminemia had lower concentrations. Malnutrition was associated both with abnormally high and low homocysteine concentrations, and abnormally low and abnormally high homocysteine concentrations were both associated with higher mortality. In conclusion, low homocysteine levels in elderly non-vitamin-supplemented hospitalized patients should not be interpreted as a protective factor in some individuals. Instead, it may be considered as an effect of an inflammatory-malnutrition process associated with a poor prognosis.

Peritoneal clearance of homocysteine with icodextrin or standard glucose solution exchange

BACKGROUND

The aim of the study was to assess plasma homocysteine concentration in peritoneal dialysis patients, and to compare the effect of different peritoneal solutions (glucose-based and icodextrin-based) on peritoneal clearance of homocysteine.

METHODS

The study group comprised 10 chronic peritoneal dialysis patients; the control group comprised 15 healthy, age-matched non-obese subjects with normal renal function. Patients with vitamin B(12) or folate deficiency were excluded. In all subjects, plasma homocysteine and dialysis adequacy parameters were assessed at baseline. The clearance study was carried out with 2.27% glucose and 7.5% icodextrin solutions (12-h dwell time).

RESULTS

Mean dialysate concentration of homocysteine was similar for both glucose and icodextrin solutions (8.3 +/- 3.2 and 8.4 +/- 1.9 micromol/L, respectively), but homocysteine clearance was significantly higher for icodextrin than glucose solution (1.82 +/- 0.57 vs 1.39 +/- 0.53 mL/min per 1.73 m(2)P = 0.01). Net ultrafiltration after icodextrin solution was also higher than after glucose solution (599 +/- 136 mL vs 134 +/- 337 mL, P < 0.01). A correlation between total plasma level of homocysteine and its peritoneal clearance was found (r = 0.69; P = 0.03).

CONCLUSION

It appears that peritoneal elimination of homocysteine depends primarily on its plasma concentration. Icodextrin-based solution for peritoneal dialysis seems to be more efficient in homocysteine elimination than a standard glucose-based solution.

Acute effect of amino acid peritoneal dialysis solution on vascular function

BACKGROUND

Oral ingestion of proteins or amino acids is associated with endothelial dysfunction. The effect of commercial amino acid peritoneal dialysis solutions on vascular function is unknown.

OBJECTIVE

We compared the acute effect of intraperitoneal amino acid administration with that of intraperitoneal glucose administration on vascular function in peritoneal dialysis patients.

DESIGN

In an open-label randomized, controlled, crossover and observer-blinded trial, we examined the acute effect of an intraperitoneal application of 2 L commercial 1.1% amino acid solution compared with that of a 2.27% glucose solution in 13 peritoneal dialysis patients. The primary endpoint was the change in forearm reactive hyperemia 6 h after instillation of either dialysis solution.

RESULTS

After 6 h of dwell time, reactive hyperemia was substantially impaired after administration of the amino acid solution compared with the glucose solution (median difference: 202%; 95% CI: 57%, 368%; P = 0.007). In a comparison of differences between values at 6 h and those before treatment, reactive hyperemia significantly decreased during the dwell with the amino acid dialysis solution compared with that with the glucose dialysis solution (median difference: 242%; 95% CI: 53%, -457%; P = 0.013). In an analysis of smoking and nonsmoking patients separately, the difference in forearm blood flow between the 2 treatments was still statistically significant.

CONCLUSIONS

One 6-h dwell with a commercial amino acid dialysis solution acutely impairs forearm reactive hyperemia in smoking and nonsmoking peritoneal dialysis patients. Because endothelial dysfunction is associated with increased morbidity and mortality, the long-term use of these solutions may increase the risk of cardiovascular disease.

Effect of Incremental Doses of Folate on Homocysteine and Metabolically Related Vitamin Concentrations in Nondiabetic Patients on Peritoneal Dialysis

The role of folate supplementation in reducing hyperhomocystinemia in patients on dialysis has been reported, but the optimal dose of folate is still unknown. The aim of the present study was to investigate whether greater than 5 mg/day folate supplementation provides any additional effect on plasma homocysteine (HCY) levels. The study was prospective, open, and had no control group. Of the 64 eligible nondiabetic patients on peritoneal dialysis with hyperhomocystinemia (>20 micromol/L), 56 were given oral folate (5 mg/day) for 3 months. When Hcy did not fall below 20 micromol/L, folate doses were increased by 5 mg every 3 months to up to 15 mg/day. With 5 mg/day supplementation, serum folate concentrations increased above the upper confidence limit in 23 patients and erythrocyte folate concentrations in 27 patients. Hcy levels decreased to less than 15 micromol/L in 6 cases and by more than 50% in 12 cases. Nineteen of the remaining patients were given 10 mg/day folate. After increasing the dose, serum and erythrocyte folate levels rose above the upper detection limit. In one patient, plasma Hcy concentrations decreased to less than 15 micromol/L. Ten patients were given 15 mg/day oral folate for an additional 3 months with no effect on homocystinemia. This study confirms that oral folate supplementation may improve hyperhomocystinemia even in patients on dialysis with normal serum or erythrocyte folate concentrations. In fact, serum and erythrocyte levels cannot predict the effect of supplementation on plasma Hcy levels. However, 5 mg/day folate supplementation normalized Hcy in 10% of cases and reduced Hcy levels in another 21%. Increasing the folate dose to greater than 5 mg/day had a minimal (10 mg/day) or no (15 mg/day) additional effect on Hcy concentrations. Despite the minimal effect of increasing folate doses, given the low cost, the absence of side effects, and the high cardiovascular risk for patients on peritoneal dialysis, a careful attempt to increase the dose of oral folate up to 10 mg/day might be suggested.

Effect of TCN2 776C>G on vitamin B12 cellular availability in end-stage renal disease patients

BACKGROUND

Transcobalamin II is a serum protein that transports vitamin B12 from the intestine to the tissues. This complex, holo-transcobalamin II, may reflect vitamin B12 availability in the body. Conflicting data exist with regard to the effect of a polymorphism in the gene coding for transcobalamin II, TCN2 776C>G, on transcobalamin II levels in the general population, which in turn may affect holo-transcobalamin II, vitamin B12, as well as total homocysteine (tHcy) plasma levels. The effect of TCN2 776C>G on vitamin B12 cellular availability in dialysis patients is unknown.

METHODS

We examined the effect of TCN2 776C>G on holo-transcobalamin II, vitamin B12, and tHcy plasma concentrations in 120 dialysis patients.

RESULTS

Holo-transcobalamin II levels were normal or supranormal in all patients and showed a linear association with albumin (r = 0.205, P = 0.025) and with vitamin B12 (r = 0.778, P = 0.001), but not with age, creatinine, body mass index, tHcy, ln-tHcy, vitamin B6, plasma folate, and red blood cell folate concentration. TCN2 776C>G showed no effect on holo-transcobalamin II, vitamin B12, and tHcy concentration [one-way analysis of variance (ANOVA), post-hoc Scheffe test]. Multiple linear regression analyses showed that albumin and B12 are independently associated with holo-transcobalamin II, whereas TCN2 776C>G and MTHFR 677C>T had no effect. Independent predictors of ln-tHcy included creatinine, red blood cell folate, and the MTHFR 677TT genotype. There was also an effect of the TCN2 776CC genotype on ln-tHcy levels in this multivariate analysis, however, that deserves cautious interpretation because there was no effect of TCN2 genotypes by ANOVA and Scheffe test [median ln-tHcy concentrations according to TCN2 genotypes (micromol/L): CC, 3.22; CG, 3.30; GG, 3.23].

CONCLUSION

TCN2 776C>G does not influence holo-transcobalamin II or vitamin B12 levels, and has no major effect on tHcy concentrations of end-stage renal disease patients.

Effect of glutamate carboxypeptidase II and reduced folate carrier polymorphisms on folate and total homocysteine concentrations in dialysis patients

This study was designed to examine the effect of two single nucleotide polymorphisms in the reduced folate carrier 1 (RFC1 80G>A) and the glutamate carboxypeptidase 2 (GCP2 1561C>T) gene on total homocysteine (tHcy) plasma level and folate status in 120 chronic dialysis patients. Red blood cell folate concentration was higher in patients with the GCP2 CT or TT genotype (ANOVA, P = 0.04). Among patient groups with different RFC1 genotypes, red blood cell folate level was not significantly different. A multivariate analysis confirmed that the GCP2 1561C>T genotype (P = 0.011) had a significant influence on the red blood cell folate concentration. Overall, serum folate, creatinine, and the GCP2 polymorphism explained nearly 50% of the variance of red blood cell folate. A linear multivariate regression analysis showed that red blood cell folate (P < 0.001), creatinine (P < 0.001), and the 5,10-methylenetetrahydrofolate reductase (MTHFR) 677T allele (P = 0.013) are independent predictors of tHcy plasma level explaining 49% of the variance of tHcy plasma concentration. GCP2 1561C>T and RFC1 80G>A showed no effect on tHcy and folate plasma level. In conclusion, GCP2 1561C>T, but not RFC1 80G>A, is a predictor of red blood cell folate level in chronic dialysis patients. Both polymorphisms have no major effect on tHcy plasma concentration in end-stage renal disease patients.

Genetic determinants of the homocysteine level

Elevated total homocysteine (tHcy) plasma concentrations indicate folate and/or vitamin B12 deficiency and are associated with cardiovascular disease and neural tube defects. Evidence has accumulated that folate-, vitamin B12-, and Hcy-metabolism are under genetic control. Because Hcy metabolism is impaired in renal failure, MTHFR 677 C>T, GCP2 1561C>T, RFC1 80G>A, and TCN2 776G>C may further aggravate hyperhomocysteinemia in these patients. The most consistent effect on tHcy plasma concentrations is observed for 677C>T of MTHFR, whereas GCP2, RFC1, and TCN2 polymorphisms show no major effect on tHcy concentrations. Much is yet to be learned about the impact of genetic variants on tHcy levels, human diseases, the genetic-nutrient interactions, as well as the pharmacogenetic consequences in Hcy and vitamin metabolism.

[Atherosclerosis and uremia: signifance of non-traditional risk factors]

Arteriosclerosis, atherosclerosis and vascular calcification are causally related to the high morbidity and mortality of patients with chronic renal failure. Oxidative stress and carbonyl stress of uremia, dialysis procedure and/or intravenous iron therapy result in AGE (advanced glycation end-product), ALE (advanced lipoxidation end-product) and AOPP (advanced oxidation protein product) formation, favouring together with elevated CRP (C-reactive protein) levels the development of cardiovascular and cerebrovascular complications. Enhanced plasma levels of homocysteine and ADMA (asymmetric dimethylarginine) contribute to this process. In addition, in chronic renal insufficiency hyperphosphatemia and an enhanced calcium x phosphorus ion product are associated with the morbidity and mortality of the patients, particularly in the presence of fetuin deficiency. Phosphorus, AGEs and AOPPs, beside other factors, catalyze the conversion of vascular smooth muscle cells to osteoblast--like cells (particularly in the presence of monocytes/macrophages), resulting in bone matrix protein formation. Other risk factors, such as age, male sex, smoking, hypertension, diabetes, chronic inflammation, insulin resistance or dyslipidemia (enhanced non-HDL-cholesterol) also contribute to the atherosclerotic risk profile of the patient with chronic renal insufficiency. While there is growing understanding of the mechanisms involved in arteriosclerosis, atherosclerosis and vascular calcification in uremia, we are still missing effective therapeutic maneuvers for reduction of excess mortality in uremic patients.

Hyperhomocysteinemia and its relationship to cardiovascular disease in ESRD: influence of hypoalbuminemia, malnutrition, inflammation, and diabetes mellitus

BACKGROUND

In the general population, a mildly elevated plasma total homocysteine (tHcy) level is an independent and graded risk factor for cardiovascular disease (CVD). In patients with end-stage renal disease (ESRD), CVD is highly prevalent and a major cause of premature mortality, and plasma tHcy levels are as much as three to four times greater than in the general population. Several other risk factors, such as diabetes mellitus (DM), inflammation, and malnutrition, also are prevalent and contribute to CVD in patients with ESRD, and there are strong associations between inflammation, malnutrition, and hypoalbuminemia in these patients. Several investigations in patients with ESRD have shown the important role of vitamin status for plasma tHcy, but little attention has been given to the influence of nutritional status. However, it is obvious that hypoalbuminemia is of interest because a substantial fraction of tHcy (>70%) is protein bound, mainly to albumin.

RESULTS

In studies of patients with ESRD in whom the prevalence of hyperhomocysteinemia was very high (>90%), tHcy level was strongly related to serum albumin level, and patients with malnutrition had lower levels of both tHcy and serum albumin than those with normal nutritional status. Furthermore, inflammation, DM, and CVD are associated with hypoalbuminemia and therefore a lower degree of hyperhomocysteinemia. In our studies, in different groups of patients with ESRD, we showed that greater tHcy levels were associated with lower CVD mortality. However, this apparently paradoxical association between lower CVD mortality and lower plasma tHcy levels (although still abnormally high) does not refute the concept that hyperhomocysteinemia is a risk factor for CVD because almost all patients may have had long-standing elevated plasma tHcy levels within a range that makes them prone to develop atherosclerosis. Instead, a potentially detrimental effect of hyperhomocysteinemia on CVD in patients with ESRD may be obscured by the influence of hypoalbuminemia, whatever the cause, because hypoalbuminemia and its causes are strong predictors of mortality.

CONCLUSION

Our findings imply that nutritional status and serum albumin level, as well as the presence of inflammation and DM, should be taken into consideration when evaluating tHcy as a risk factor for CVD in patients with ESRD.

Riboflavin is a determinant of total homocysteine plasma concentrations in end-stage renal disease patients

The effect of thiamine (vitamin B(1)) or riboflavin (vitamin B(2)) availability on fasting total homocysteine (tHcy) plasma levels in end-stage renal disease patients is unknown. A cross-sectional study was performed in a population of non-vitamin supplemented patients maintained on continuous ambulatory peritoneal dialysis. Red blood cell availability of thiamine (alpha-ETK) and of riboflavin (alpha-EGR), along with other predictors of tHcy plasma levels, was considered in the analysis. There was a linear association of alpha-EGR with tHcy plasma concentrations (P = 0.009), which was not observed for alpha-ETK. Among red blood cell vitamins, alpha-EGR was the only predictor of tHcy levels (P = 0.035), whereas alpha-ETK, red blood cell pyridoxal-5-phosphate supply (alpha-EGOT) and red blood cell folate levels had no effect. The risk for having a high tHcy plasma levels within the fourth quartile (plasma tHcy >38.3 micromol/L) was increased by an alpha-EGR > median (odds ratio, 4.706; 95% confidence interval, 1.124 to 19.704; P = 0.026). By way of contrast, alpha-ETK had no effect in these analyses. Independent predictors of tHcy plasma levels were serum albumin, alpha-EGR, red blood cell folate, and certain MTHFR genotypes. A logistic regression analysis showed that the MTHFR genotype is a predictor for having a tHcy plasma concentration within the fourth quartile. In summary, riboflavin availability, as measured by alpha-EGR, is a determinant of fasting tHcy plasma levels in peritoneal dialysis patients. This finding may have implications for tHcy lowering therapy in individuals with end-stage renal disease.

Methylenetetrahydrofolate reductase genotype, vitamin B12, and folate influence plasma homocysteine in hemodialysis patients

Hyperhomocysteinemia, a well-recognized cardiovascular risk factor, is frequent in hemodialysis (HD) patients. A common polymorphism in the 5,10-methylenetetrahydrofolate reductase (MTHFR) gene, C-->T substitution at nucleotide 677, is associated with homocysteine (Hcy) level elevation. We examined whether three factors involved in the methionine cycle could influence plasma Hcy concentrations in HD patients: MTHFR polymorphism; vitamin B12, an essential cofactor; and folate, the substrate. In a cross-sectional study, serum vitamin B12, folate, and plasma Hcy were measured and MTHFR genotyping was performed in 534 HD patients. Effects of MTHFR genotypes, vitamin B12, and folate on plasma Hcy levels were examined in 450 HD patients not administered vitamin B12 or folate. To examine the effect of vitamin B12 on plasma Hcy concentrations, we compared plasma Hcy concentrations in HD patients with and without vitamin B12 supplementation. To examine whether functional vitamin B12 deficiency exists even in HD patients with normal vitamin B12 concentrations, 15 HD patients (serum vitamin B12 concentrations, 250 to 2,100 pg/mL) were treated with vitamin B12 (mecobalamin, 1.5 mg/d) for 8 weeks. Serum concentrations of methylmalonic acid (MMA) and vitamin B12 were measured. Hcy levels were higher and folate levels were lower in patients with the TT and CT genotypes compared with patients with the CC genotype. Analysis of covariance to determine independent predictors of high Hcy levels identified low serum vitamin B12 and folate levels and high albumin (Alb) levels in CC-genotype patients, low folate levels and high Alb levels in CT-genotype patients, and low folate levels in TT-genotype patients. Plasma Hcy levels were lower in CC- and CT-genotype patients with vitamin B12 supplementation than in those without supplementation. Vitamin B12 supplementation for 8 weeks significantly reduced MMA concentrations in HD patients with normal serum vitamin B12 concentrations. These results indicate that MTHFR genotype influences the correlation of Hcy level with vitamin B12 and folate levels in HD patients. Functional vitamin B12 deficiency may exist, even in HD patients with normal vitamin B12 concentrations. The efficacy of vitamin B12 and folate supplementation on plasma Hcy levels may depend on MTHFR genotype.

Folate supplementation in peritoneal dialysis patients with normal erythrocyte folate: effect on plasma homocysteine

The possible role of folate supplementation in reducing hyperhomocysteinemia in dialysis patients has been reported in several recent papers. However, scant data are available for peritoneal dialysis patients; besides, none of these studies investigated either the role of intraerythrocyte folate concentration or the presence of side effects caused by folate administration. Sixty-six peritoneal dialysis patients with hyperhomocysteinemia (>15 micromol/l) and normal folate status (as assessed by erythrocyte folate level >600 nmol/l) were randomly allocated to receive either oral folate (5 mg/day) or no vitamin supplementation. After 2 months of therapy, patients were requested to answer a questionnaire investigating the occurrence of symptoms possibly related to folate supplementation. Twenty-nine treated patients and 30 untreated controls completed the study. In the treated patients, serum and erythrocyte folate increased significantly (p < 0.0001) (respectively from 10.6 +/- 4.9 to 237 +/- 231 nmol/l and from 1,201 +/- 297 to 2,881 +/- 294 nmol/l) to levels at the uppermost limit of detection by laboratory methods. Serum vitamin B(12) levels did not change. Plasma homocysteine levels decreased from 54 +/- 32 to 23 +/- 14 micromol/l after folate supplementation and remained unchanged in the control group. After 4 months of folate therapy, homocysteine concentration was within the normal range in 5 patients (17%) and below 30 micromol/l in the other 21 (72%). Folate therapy resulted in a decrease in homocysteine of more than 50% in 45% of the patients and decrease of more than 20% in a further 38%. No significant symptoms were reported. Thus, serum and erythrocyte folate increase confirms that normal folate levels are inadequate in dialysis patients, even if serum and erythrocyte levels before folate supplementation cannot predict the effect on homocysteine plasma levels.

Increased prevalence of combined MTR and MTHFR genotypes among individuals with severely elevated total homocysteine plasma levels

The prevalence of the methionine synthase (MTR) 2756A-->G polymorphism among individuals with severely elevated total homocysteine (tHcy) plasma levels is unknown. Therefore, 1,716 subjects, including 415 hemodialysis patients, 179 peritoneal dialysis patients, 733 kidney graft recipients, and 389 healthy subjects, were investigated. The distribution of MTR 2756A-->G, as well as 5,10-methylenetetrahydrofolate reductase (MTHFR) 677C-->T/1298A-->C, genotypes among study participants with extremely high tHcy plasma levels (>90th percentile) was compared with the genotype distribution of subjects with very low tHcy plasma levels (<10th percentile). The prevalence of MTR 2756AG and GG genotypes alone did not differ between individuals with extremely high or extremely low tHcy levels (P = 0.7402; odds ratio [OR], 1.076; 95% confidence interval [CI], 0.697 to 1.662). Conversely, combined MTR and MTHFR genotypes (MTR 2756AG and 2756GG and MTHFR 677TT/1298AA and 677CT/1298AC) were found more often in the highest (n = 34) compared with the lowest plasma tHcy decile (n = 19; P = 0.0252; OR, 1.983; 95% CI, 1.079 to 3.643). The number of patients with the wild-type MTR and MTHFR genotype was three times greater in the lowest compared with the highest decile (17 versus 6 patients, respectively; P = 0.0155; OR, 0.330; 95% CI, 0.126 to 0.861). In summary, our study shows that the 2756A-->G transition of MTR in combination with MTHFR 677TT/1298AA and 677CT/1298AC can be associated with extremely high tHcy plasma levels.

An MTHFR variant, homocysteine, and cardiovascular comorbidity in renal disease

BACKGROUND

It is unclear whether total serum homocysteine (tHcy) and the C677T mutation of methylenetetrahydrofolate reductase (MTHFR) are associated with cardiovascular disease (CVD) in patients with end-stage renal disease (ESRD).

METHODS

A cross-sectional sample of 459 patients with ESRD on chronic dialysis was assessed to determine whether tHcy and the C677T mutation are associated with CVD prevalence in multiple logistic regression. As CVD mortality is high, we examined the relationship between homozygosity and duration of dialysis.

RESULTS

Mean tHcy was higher in patients without a history of CVD (35.2 micromol/L vs. 30.4 micromol/L, P = 0.02). In multivariate models, CVD was negatively associated with tHcy and positively associated with TT genotype, male gender, and body mass index. Mean tHcy levels were higher among those with the TT genotype compared with those with the CC genotype when adjusted for age, folate, creatinine, and albumin (37.9 micromol/L vs. 31.9 micromol/L, P = 0.005). Among whites, the prevalence of the TT genotype was higher in those having undergone less than one year of dialysis (P = 0.002).

CONCLUSIONS

The C677T genotype of MTHFR is associated with CVD in ESRD and may be a more meaningful marker than tHcy for abnormal homocysteine metabolism in ESRD. Prospective data from ongoing clinical trials are needed to improve our understanding of these findings. Screening for this polymorphism may help guide prevention measures.

Hyperhomocysteinemia in chronic renal failure patients: relation to nutritional status and cardiovascular disease

A moderate increase in plasma total homocysteine (tHcy) is considered to be an independent risk factor for cardiovascular disease (CVD) in the general population. Almost all chronic renal failure (CRF) patients have plasma concentration of tHcy that is elevated 3 to 4 times above normal. The prevalence of CVD, diabetes mellitus, malnutrition and hypoalbuminemia is high in CRF patients. Previous investigations have focused on the role of vitamin status on plasma tHcy in CRF patients, but little information exists on the influence of nutritional status and hypoalbuminemia on plasma tHcy in CRF, although a substantial fraction of tHcy (>70%) is protein-bound, mainly to albumin. Our study in patients with end-stage renal disease showed that more than 90% of the patients had elevated plasma tHcy levels, which were higher in patients with normal nutritional status than in malnourished patients. Moreover, plasma tHcy was inversely correlated with subjective global nutritional assessment (high values denote malnutrition) and positively correlated with serum albumin and protein intake. Hence, it seems likely that serum-albumin is a strong determinant of plasma tHcy in CRF patients and this may contribute to the lower tHcy levels in malnourished patients. Patients with diabetes mellitus had lower serum-albumin and plasma tHcy than non-diabetic patients, irrespective whether they were malnourished or not. Patients with CVD had lower (although still elevated) plasma tHcy levels than those without CVD. An explanation may be that the prevalence of diabetes mellitus, malnutrition and hypoalbuminema, i.e. factors that decrease tHcy, was higher in patients with CVD, which may explain why they had less elevated values. Assuming that hyperhomocysteinemia carries an independent risk of CVD, this implies that almost all CRF patients are exposed to this risk. CRF patients with CVD had a higher prevalence of malnutrition, hypoalbuminemia and diabetes mellitus, which was associated with a lower plasma Hcy level. This may explain why plasma tHcy was lower (although still abnormally high) in patients with CVD than in patients without CVD. The lower tHcy levels in CVD patients do not contradict the assumption that hyperhomocysteinemia is a risk factor for CVD since almost all patients are exposed to this risk, and other factors might be present that confound the relationship between the absolute tHcy levels and CVD. Our findings imply that nutritional status and serum albumin, as well as the presence of diabetes mellitus, should be taken into consideration when evaluating tHcy as a risk factor for CVD in CRF patients.

Hyperhomocysteinemia is related to residual glomerular filtration and folate, but not to methylenetetrahydrofolate-reductase and methionine synthase polymorphisms, in supplemented end-stage renal disease patients undergoing hemodialysis

Glomerular filtration is one of the major determinants of plasma total homocysteine (tHcy). To evaluate the respective roles of residual glomerular filtration (by measuring a specific protein marker, cystatin C), genetic polymorphisms and nutritional status in tHcy blood levels in end-stage renal disease patients (ESRD) under hemodialysis and supplemented with folate, we measured tHcy, folate, vitamin B12 (B12), creatinine, cystatin C, albumin and C-reactive protein and determined the polymorphism of methylenetetrahydrofolate reductase (MTHFR) (C677T and A1289C) and of methionine synthase (MS) (A2756G) in 114 ESRD patients before hemodialysis and 76 control subjects. All patients received a folate supplementation of 700 microg/day. Hyperhomocysteinemia was observed in all patients and exceeded the upper normal limit by 2-fold in 52.4% of the patients. Serum folate was significantly increased and the B12 level was not different from controls. Folate, Cystatin C and creatinine were significantly correlated to tHcy, while no correlation was found between tHcy, albumin and C-reactive protein. No difference in genotype frequency between ESRD patients and controls was found for MTHFR A1289C and MS A2756G. The MTHFR 677TT genotype was less frequent and was associated with a significantly higher tHcy level in patients. Folate and residual glomerular filtration estimated by cystatin C and creatinine levels were two independent determinants of tHcy in ESRD patients. These data suggest that hyperhomocysteinemia is a consequence as well as a complicating factor of renal failure.

Association of two MTHFR polymorphisms with total homocysteine plasma levels in dialysis patients

The effect of the combined 5,10-methylenetetrahydrofolate reductase (MTHFR) 677C-->T and 1298A-->C genotype on total homocysteine (tHcy), folate, and vitamin B(12) plasma levels was investigated in 983 subjects, including 415 hemodialysis patients, 179 peritoneal dialysis patients, and 389 healthy individuals. Mean tHcy plasma concentrations were 27.2 +/- 15.8 micromol/L in hemodialysis patients, 25.4 +/- 19.1 micromol/L in peritoneal dialysis patients, and 8.9 +/- 3.5 micromol/L in healthy individuals. Hyperhomocysteinemia (tHcy > 15 micromol/L) was detected in 81.6% of patients and 2.6% of controls. Multiple stepwise regression analysis showed that the MTHFR 677C-->T/1298A-->C genotype (CC/AA, CC/AC, CC/CC, CT/AA, CT/AC, TT/AA), vitamin use, age, folate and vitamin B(12) plasma level were significant predictors of tHcy plasma levels. Analysis of variance showed that this effect of MTHFR genotypes on tHcy level was caused by significantly greater tHcy levels in 677TT/1298AA hemodialysis and peritoneal dialysis patients versus other genotypes. Compound heterozygous controls (677CT/1298AC genotype) had significantly greater tHcy levels compared with 677CC/1298AA controls. There was no major effect of MTHFR polymorphisms on folate and vitamin B(12) plasma concentrations. This study shows that the MTHFR 677TT/1298AA genotype, but not the 677CT/1298AC genotype, is a significant predictor of tHcy plasma levels in dialysis patients.

Efficacy of folinic versus folic acid for the correction of hyperhomocysteinemia in hemodialysis patients

The effectiveness of intravenous folinic acid or intravenous folic acid for the treatment of hyperhomocysteinemia of hemodialysis patients is unknown. In a randomized, controlled, double-blind trial, 66 hemodialysis patients were administered either 15 mg of folic acid or an equimolar amount (16.1 mg) of folinic acid intravenously three times weekly. Normalization of total homocysteine (tHcy) plasma levels after 4 weeks of treatment was achieved in 10 patients (30.3%) in the folic-acid group and 6 patients (18.2%; P: = 0.389) in the folinic-acid group (normalization at any time during the study period in 39.4% and 33.3% of the patients; P: = 0.798). The relative reduction in tHcy plasma levels at week 4 was 32.2% in the folic-acid group and 34.1% in the folinic-acid group. A high baseline tHcy plasma concentration (P: = 0.00001), methylenetetrahydrofolate reductase (MTHFR) 677TT/1298AA genotype (P: = 0.03540), and low red blood cell folate concentrations (P: = 0.02285) were associated with a better relative response to treatment. Normalization of tHcy plasma levels was dependent on a lower baseline tHcy level (P: = 0.01976), younger age (P: = 0.00896), and MTHFR 677TT/1298AA or 677CT/1298AC genotypes (P: = 0.00208 and P: = 0.02320, respectively). A 4-week course of intravenous folinic acid is not superior to intravenous folic acid in reducing elevated tHcy plasma levels in hemodialysis patients. The response to treatment is predicted by tHcy plasma level, red blood cell folate content, and MTHFR genotype.

Difference in the homocysteine-lowering effect of folic acid in haemodialysis patients with and without occlusive vascular disease

BACKGROUND

Hyperhomocysteinaemia has been identified as an independent cardiovascular risk factor and is found in more than 85% of patients on maintenance haemodialysis. Previous studies have shown that folic acid can lower circulating homocysteine in dialysis patients. We evaluated prospectively the effect of increasing the folic acid dosage from 1 to 6 mg per dialysis on plasma total homocysteine levels of haemodialysis patients with and without a history of occlusive vascular artery disease (OVD).

METHODS

Thirty-nine stable patients on high-flux dialysis were studied. Their mean age was 63 +/-11 years and 17 (43%) had a history of OVD, either coronary and/or cerebral and/or peripheral occlusive disease. For several years prior to the study, the patients had received an oral post-dialysis multivitamin supplement including 1 mg of folic acid per dialysis. After baseline determinations, the folic acid dose was increased from 1 to 6 mg/dialysis for 3 months.

RESULTS

After 3 months, plasma homocysteine had decreased significantly by approximately 23% from 31.1 +/- 12.7 to 24.5 +/- 9 micromol/l (P = 0.0005), while folic acid concentrations had increased from 6.5 +/- 2.5 to 14.4+/-2.5 microg/l (P < 0.0001). However, the decrease of homocysteine was quite different in patients with and in those without OVD. In patients with OVD, homocysteine decreased only marginally by approximately 2.5% (from 29.0 +/- 10.3 to 28.3 +/- 8.4 micromol/l, P = 0.74), whereas in patients without OVD there was a significant reduction of approximately 34% (from 32.7+/-14.4 to 21.6+/-8.6 micromol/l, P = 0.0008). Plasma homocysteine levels were reduced by > 15% in three patients (18%) in the group with OVD compared with 19 (86%) in the group without OVD (P = 0.001), and by > 30% in none of the patients (0%) in the former group compared with 13 (59%) in the latter (P = 0.001).

CONCLUSIONS

These results indicate that the homocysteine-lowering effect of folic acid administration appears to be less effective in haemodialysis patients having occlusive vascular disease than in those without evidence of such disease.

Recent insights into the molecular genetics of the homocysteine metabolism

The homocysteine plasma level is determined by non-genetic and genetic factors. In recent years evidence has accumulated that the total homocysteine plasma level of patients under different forms of renal replacement therapy is influenced by a common mutation at nucleotide position 677 of the gene coding for 5,10-methylenetetrahydrofolate reductase (MTHFR 677C-->T). Furthermore, compound heterozygosity for the 677T allele and a novel A-->C polymorphism at nucleotide position 1298 of MTHFR is suggested to correlate with a decrease of folate plasma concentrations. Because polymorphisms of genes coding for proteins involved in the metabolism of homocysteine may contribute to elevated total homocysteine plasma concentrations, molecular genetic analyses of the homocysteine pathways experienced a drift towards screening for candidate genes with a putative relationship to total homocysteine plasma levels. One example is the cloning of the FOLR1 gene coding for the folate-binding protein (Folbp1), which has recently been inactivated in mice, thus representing an elegant model to investigate the consequence on the homocysteine metabolism. Furthermore, the recent characterization of the CUBN gene encoding the intrinsic factor-vitamin B12 receptor (cubilin) provides a basis to identify the causative mutations in patients suffering from a hereditary syndrome of hyperhomocysteinemia that presents with megaloblastic anemia and proteinuria. This review focuses on recent insights into the molecular genetics of MTHFR, FOLR1, and CUBN, and their relationships to the metabolism of the amino acid homocysteine.

Uremic toxins and peritoneal dialysis

Uremic toxicity is related in part to the accumulation of toxic substances, the nature of which has only partly been characterized. Because of the use of a highly permeable membrane and better preservation of the residual renal function, it could be anticipated that some of these uremic toxins are more efficiently cleared across the peritoneal membrane, and that the plasma and tissue levels of these compounds are lower than in hemodialysis patients. This article analyzes the generation and removal of several uremic toxins in peritoneal dialysis patients. The following uremic toxins are discussed: beta2-microglobulin, advanced glycation end products, advanced oxidation protein products, granulocyte inhibitory proteins, p-Cresol, and hyperhomocysteinemia. Some recent studies are reviewed suggesting that uremic toxins are involved in the progression of renal failure and are at least partially removed by peritoneal dialysis. We conclude that, although the plasma levels of some of these compounds are lower in peritoneal dialysis versus hemodialysis patients, it does not mean that the peritoneal dialysis patient is "better" protected against the numerous disturbances caused by these toxins.

Poor long-term survival after coronary angiography in patients with renal insufficiency

Cardiovascular disease is common among dialysis patients, but much less is known regarding non-dialysis-dependent renal insufficiency (NDDRI) and its association with cardiac disease. We undertook a study to assess the impact of renal insufficiency on survival post-coronary angiography by comparing three groups of patients: dialysis-dependent patients, patients with NDDRI (creatinine > 2.3 mg/dL), and a reference group with creatinine levels less than 2.3 mg/dL and not on dialysis therapy. We used a prospective cohort that consisted of all patients undergoing coronary angiography in Alberta, Canada, from January 1, 1995, to December 31, 1997. Of the 16,989 patients, 196 patients (1.2%) were on dialysis therapy, 262 patients (1.5%) had NDDRI, and 16,531 patients (97.3%) formed the reference group. Mortality rates 1 year after angiography were 30.2% for patients with NDDRI, 15.8% for dialysis patients, and 4.1% for the reference group. Compared with the reference group, crude 4-year survival was significantly worse for dialysis patients and those with NDDRI, with hazard ratios of 4.05 (95% confidence interval, 3.02 to 5.42) and 7.32 (95% confidence interval, 5.97 to 8.97), respectively. Even after adjusting for clinical risk factors, survival remained worse for dialysis patients and those with NDDRI, with hazard ratios of 2.59 (95% confidence interval, 1.92 to 3.49) and 2.51 (95% confidence interval, 2.02 to 3.12), respectively. We conclude that renal insufficiency, both dialysis dependent and non-dialysis dependent, is an independent risk factor for increased mortality and poor long-term survival among patients undergoing coronary angiography.

Therapeutic potential of total homocysteine-lowering drugs on cardiovascular disease

An elevated total homocysteine (tHcy) plasma concentration is associated with increased morbidity and mortality due to cardiovascular disease in the general population and in patients with impaired renal function. The prevalence of hyperhomocysteinaemia (plasma levels above 15 micromol/l) in the general population is less than 5% and can be as high as 50% in patients with vascular disease. In patients with renal insufficiency, elevated tHcy plasma levels are detected in 50 - 100% of the patients. Total homocysteine plasma levels can be lowered or normalised by folic acid and/or vitamin B(6) and vitamin B(12) supplementation. In patients with advanced chronic renal insufficiency or end-stage renal disease, hyperhomocysteinaemia is partially resistant to folic acid or vitamin therapy. However, higher tHcy plasma levels may also reflect tissue damage and the increase in Hcy after an acute incident such as stroke or myocardial infarction may be necessary for tissue repair mechanisms. This implies, that lowering tHcy may even be harmful to some patients. Currently, prospective studies are underway to clarify whether folate supplementation, with or without additional other vitamins, improves cardiovascular disease morbidity and mortality in the general population, as well as in renal failure patients. While population-wide screening for and treatment of hyperhomocysteinaemia is generally not recommended, treatment of high risk patients may be considered.

A C677T mutation in the methylenetetrahydrofolate reductase gene modifies serum cysteine in dialysis patients

Patients undergoing hemodialysis have impaired metabolism of such sulfur-containing amino acids as cysteine (Cys) and homocysteine (Hcy), which may lead to accelerated atherosclerosis. Considering that Cys is mainly synthesized from Hcy, a common C677T mutation in the methylenetetrahydrofolate reductase (MTHFR) gene may affect the serum total Cys (tCys) concentration, as well as total Hcy (tHcy) concentration, through reduced remethylation of Hcy to methionine, even in hemodialysis patients. To identify the independent determinants for the tCys concentration in dialysis patients, we determined MTHFR C/T genotypes and serum concentrations of tHcy, tCys, and vitamins as cofactors in 464 hemodialysis patients. Serum tCys concentration was positively associated with serum tHcy concentration and negatively associated with the MTHFR mutation, although the mutation correlated positively with serum tHcy concentration. Slopes of regression lines relating tHcy and tCys concentrations differed between the MTHFR genotypes, and the relationship was strengthened with a decreasing number of T alleles. Additionally, serum concentrations of folate and vitamin B(12) correlated positively with tCys concentration, whereas they correlated negatively with tHcy concentration. These findings suggest that the MTHFR mutation is an independent predictor for serum tCys concentrations in hemodialysis patients and that a tCys-decreasing effect of the mutation may arise largely from its attenuation of the positive Cys-Hcy correlation. The tCys-increasing effect of folate and vitamin B(12) appears to be linked to their enhancement of Hcy remethylation.

Effect of MTHFR 1298A-->C and MTHFR 677C-->T genotypes on total homocysteine, folate, and vitamin B(12) plasma concentrations in kdiney graft recipients

The effect of 5,10-methylenetetrahydrofolate reductase (MTHFR) 677C-->T and 1298A-->C on total homocysteine (tHcy), folate and vitamin B(12) levels was investigated in 733 kidney graft recipients. The six major genotype combinations were used as grouping variables, and age, gender, BMI, serum creatinine, and creatinine clearance and ln-folate, ln-vitamin B(12), or logarithmus naturalis tHcy (ln-tHcy) were used as covariates in three ANCOVA and multiple stepwise linear regression models. Hyperhomocysteinemia was present in 49.7% of the patients. The allele frequency of MTHFR 677T and 1298C was 0.319 and 0.326. MTHFR genotype and all other variables were significant predictors of ln-tHcy (higher tHcy plasma levels for MTHFR 677TT/1298AA versus all other five genotype groups: P < 0. 05). BMI, creatinine clearance, ln-tHcy, and MTHFR genotype influenced ln-folate (lower folate levels for MTHFR 677TT/1298AA versus all other genotype groups: P < 0.05). Creatinine clearance and ln-tHcy were the only predictors of ln-vitamin B(12) levels. In a prespecified subgroup analysis (n = 496), the MTHFR genotype also influenced tHcy levels and compound heterozygous patients had significantly lower folate levels as compared with MTHFR 677CC/1298AA and 677CC/1298CC. This study shows that the MTHFR 677TT/1298AA and 677CT/1298AC genotypes are significant predictors of tHcy and folate plasma levels.

Effect of high dose folic acid therapy on hyperhomocysteinemia in hemodialysis patients: results of the Vienna multicenter study

Homocysteine is associated with atherosclerosis and enhanced cardiovascular risk. In previous studies, treatment with folic acid up to 15 mg/d failed to correct hyperhomocysteinemia in the majority of end-stage renal disease patients. A dose of 30 or 60 mg of folic acid per day was compared with 15 mg/d in an attempt to normalize hyperhomocysteinemia in 150 hemodialysis patients. In a randomized, double-blind, multicenter study, 144 patients completed the 4-wk treatment period and 121 patients completed the 6-mo follow-up. Total homocysteine plasma levels were reduced by 32.1% (15 mg/d), 29. 9% (30 mg/d), or 37.8% (60 mg/d) with no significant differences found between the three treatment groups. Baseline total homocysteine plasma concentration was an independent predictor of the response to folic acid therapy (P = 0.0001), whereas the 5, 10-methylenetetrahydrofolate reductase polymorphisms (MTHFR 677C --> T and 1298A --> C) had no influence. Nevertheless, patients with the MTHFR 677TT genotype more frequently attained normal total homocysteine plasma levels than patients with the CC or CT genotype (P = 0.025). In response to 60 mg of folic acid per day, TT genotype patients had lower folate plasma levels compared to CC or CT genotype patients (P = 0.016). After completion of the 4-wk treatment period with 30 or 60 mg of folic acid per day, there was a marked rebound of total homocysteine plasma levels at the end of the follow-up in patients with the MTHFR 677TT genotype, which even exceeded baseline values in several patients (P = 0.0001). This study clearly demonstrates that doses of 30 or 60 mg of folic acid per day are not more effective than 15 mg/d in reducing hyperhomocysteinemia in regular hemodialysis patients. Patients with the MTHFR 677TT genotype are more likely to realize normal total homocysteine plasma levels. Folic acid at 30 or 60 mg/d but not 15 mg/d results in a rebound of total homocysteine plasma concentrations when treatment is stopped.

The C677T methylenetetrahydrofolate reductase gene mutation in hemodialysis patients

Hyperhomocysteinemia, which results from renal impairment, may promote arteriosclerosis and glomerulosclerosis. The combined effect of renal failure and a common mutation (C677T) in the methylenetetrahydrofolate reductase gene that leads to serum homocysteine elevations in dialysis patients was investigated. Genotypes were determined at this locus in 545 dialysis patients and 676 healthy subjects, and serum concentrations of total homocysteine and folate were measured in a subgroup of 464 patients. Multiple regression analysis showed that the TT genotype and low serum folate concentration were independent positive predictors of the serum total homocysteine concentration. The negative slope of a regression line relating homocysteine and folate concentrations was significantly steeper for patients with the TT genotype than for other genotypes. Patients with the TT genotype were significantly younger at the time of the study (54.8 +/- 12.9 versus 59.6 +/- 12.3 yr; P < 0.0001) and at initiation of dialysis (46.6 +/- 16.2 versus 51.2 +/- 15.9 yr; P < 0.02) than those with other genotypes. In patients who were older at the time of the study or at initiation of dialysis, the prevalence of the TT genotype was lower than in control subjects. In the middle quartiles (37.1 to 63.0 yr) for age at the start of dialysis, the prevalence of the TT genotype was lower in patients with a longer duration of dialysis. In this cross-sectional study, genotype and serum total homocysteine concentration were not independent risk factors for vascular disease in dialysis patients. These results indicate that the methylenetetrahydrofolate reductase mutation is a significant independent predictor for serum total homocysteine concentration.

High prevalence of hyperhomocysteinemia in critically ill patients

OBJECTIVE

To test the hypothesis that the prevalence of hyperhomocysteinemia is increased in critically ill patients and correlates with disease severity and mortality in these patients.

DESIGN

A prospective study.

SETTING

Three medical intensive care units at the University of vienna Medical School serving both medical and surgical patients.

PATIENTS

All consecutive admissions (n = 56) during a period of 4 wks. A total of 112 age- and gender-matched healthy individuals constituted the control group.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Blood samples were drawn within 24 hrs after admission for analysis of total homocysteine (tHcy), folate, vitamin B6 levels, and vitamin B12 levels as well as to identify the 677C-->T polymorphism in the gene coding for the enzyme 5,10-methylenetetrahydrofolate reductase. Acute Physiology and Chronic Health Evaluation III scores at admission and 24 hrs after admission as well as 30-day survival were documented in all patients. Hyperhomocysteinemia was more prevalent in critically ill patients (16.1%; 95% confidence interval, 7.6% to 28.3%) compared with age- and gender-matched healthy individuals (5.4%; 95% confidence interval, 2.0% to 11.3%; chi-square test; p = .022). There was no difference in tHcy plasma concentrations in the first 24 hrs after admission to an intensive care unit between survivors and nonsurvivors. The 5,10-methylenetetrahydrofolate reductase 677C-->T polymorphism had no influence on tHcy levels and survival of intensive care unit patients.

CONCLUSIONS

The prevalence of hyperhomocysteinemia is increased in critically ill patients compared to age- and gender-matched healthy individuals. The clinical significance of this finding remains to be determined.

Hyperhomocysteinemia in organ transplantation

An elevated total homocysteine plasma concentration is associated with an increased morbidity and mortality due to cardiovascular disease in the general population, in patients with renal failure and in recipients of kidney or heart transplants. The fasting or post-methionine loading plasma concentration of total homocysteine is elevated in 50-60% of renal transplant recipients with stable graft function and in the majority of heart transplant recipients. Fasting and post-methionine loading hyperhomocysteinemia can be normalized in virtually all renal transplant patients by a combination of folic acid (5 mg/d), vitamin B6 (50 mg/d) and vitamin B12 (0.4 mg/d). In individuals without renal failure much lower doses of folate and vitamin B12 are able to correct hyperhomocysteinemia. Currently, prospective studies are under way to clarify whether folate and vitamin therapy improves cardiovascular disease morbidity and mortality in the general population and in organ transplant recipients. While population wide screening for and treatment of hyperhomocysteinemia is generally not recommended, treatment of high risk patients, including renal failure patients and kidney and heart transplant recipients, can be considered but still represents an experimental therapy.