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.

Plasma phospholipid dysregulation in patients with cystathionine-ß synthase deficiency.

BACKGROUND & AIMS: Patients with cystathionine ß-synthase deficiency (CBSD) exhibit high circulating levels of homocysteine and enhanced lipid peroxidation. We have characterized the plasma lipidome in CBSD patients and related lipid abnormalities with reactions underlying enhanced homocysteine levels. METHODS AND RESULTS: Using an ultra-high-performance liquid chromatography-electrospray ionization-quadrupole-time of flight-mass spectrometry method, plasma lipids were determined with an untargeted lipidomics approach in 11 CBSD patients and 11 matched healthy subjects (CTRL). Compared to CTRL, CBSD patients had a higher medium and long-chain polyunsaturated fatty acids (PUFA) content in phosphatidylethanolamine (PE) and lysophosphatidylethanolamine (LPE) species (p < 0.02), and depletion of phosphatidylcholine (PC; p = 0.02) and of lysophosphatidylcholine (LPC; p = 0.003) species containing docosahexaenoic acid (DHA), suggesting impaired phosphatidylethanolamine-N-methyltransferase (PEMT) activity. PEMT converts PE into PC using methyl group by S-adenosylmethionine (SAM) thus converted in S-adenosylhomocysteine (SAH). Whole blood SAM and SAH concentrations by liquid chromatography tandem mass spectrometry were 1.4-fold (p = 0.015) and 5.3-fold (p = 0.003) higher in CBSD patients than in CTRL. A positive correlation between SAM/SAH and PC/PE ratios (r = 0.520; p = 0.019) was found. CONCLUSIONS: A novel biochemical abnormality in CBSD patients consisting in depletion of PC and LPC species containing DHA and accumulation of PUFA in PE and LPE species is revealed by this lipidomic approach. Changes in plasma SAM and SAH concentrations are associated with such phospholipid dysregulation. Given the key role of DHA in thrombosis prevention, depletion of PC species containing DHA in CBSD patients provides a new direction to understand the poor cardiovascular outcome of patients with homocystinuria.

Metabolic Serendipities of Expanded Newborn Screening.

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

An unusually high plasma concentration of homocysteine resulting from a combination of so-called "secondary" etiologies.

The metabolism of homocysteine is complex and involves many enzymes as well as vitamin-derived cofactors. Any dysregulation of this metabolism may lead to hyperhomocysteinemia (HHCy) which is responsible for many clinical disorders including thromboembolic events. HHCy may result from very different etiologies and is generally classified into three groups according to homocysteine concentrations: moderate (<30 µmol/L), intermediate (30-100 µmol/L) or major (>100 µmol/L). Major HHCy cases are generally due to monogenic defects of key enzymes involved in homocysteine metabolism, such as cystathionine-ß-synthase or 5,10-methylene-tetrahydrofolate reductase, or to any defect in vitamin B12 absorption, transport or metabolism. By contrast, moderate and intermediate HHCy tend to result from so-called "secondary" etiologies (e.g. tobacco, drugs, alcohol, vitamin deficiencies or pathological contexts). Here we describe the case of a patient with an unusually high plasma homocysteine concentration (1562 µmol/L) which was only explained by a combination of such secondary etiologies, among them chronic renal failure, hypothyroidism, the homozygous C677T MTHFR variant, a novel heterozygous variant of the MSR gene, and a vitamin deficiency. In addition, this patient exhibited a spectacular decline in homocysteine concentrations (returning to normal) after betaine and vitamin administration. In conclusion, this case highlights that major HHCy may also result from the combination of secondary etiologies, with vitamin deficiency as a triggering factor.

Novel method for l-methionine determination using l-methionine decarboxylase and application of the enzyme for l-homocysteine determination.

For many years, clinical studies have suggested that blood levels of l-methionine and L-homocysteine correlate with health status or homocystinuria/hypermethioninemia. l-Methionine in a solution containing 0%, 10%, or 20% human serum was detected in 10-200 µM using l-methionine decarboxylase (MetDC). Spike and recovery tests showed that the enzymatic assay could accurately and reproducibly determine the increases in l-methionine in serum samples. These results suggest that our enzymatic method using MetDC is useful for primary screening of hypermethioninemia or homocystinuria based on serum l-methionine concentration. Additionally, we confirmed that l-methionine (100 nmol) in solution was degraded to less than the detection limit by incubation at 37ºC for 10 min using 2 U of MetDC. Therefore, l-homocysteine in serum samples can be detected with equivalent sensitivity using l-methionine γ-lyase (MGL), in solutions that either did not contain l-methionine or contained l-methionine preincubated with MetDC.Abbreviations: DTT: dithiothreitol; IPTG: isopropyl-ß-d-thiogalactopyranoside; KPB: potassium phosphate buffer; MBTH: 3-methyl-2-benzothiazolinonehydrazone; mdc: the gene coding l-methionine decarboxylase; MetDC: l-methionine decarboxylase; mgl: the gene coding l-methionine γ-lyase; MGL: l-methionine γ-lyase; PLP: pyridoxal 5'-phosphate.

Folate, genomic stability and colon cancer: The use of single cell gel electrophoresis in assessing the impact of folate in vitro, in vivo and in human biomonitoring.

Intake of folate (vitamin B9) is strongly inversely linked with human cancer risk, particularly colon cancer. In general, people with the highest dietary intake of folate or with high blood folate levels are at a reduced risk (approx. 25%) of developing colon cancer. Folate acts in normal cellular metabolism to maintain genomic stability through the provision of nucleotides for DNA replication and DNA repair and by regulating DNA methylation and gene expression. Folate deficiency can accelerate carcinogenesis by inducing misincorporation of uracil into DNA, by increasing DNA strand breakage, by inhibiting DNA base excision repair capacity and by inducing DNA hypomethylation and consequently aberrant gene and protein expression. Conversely, increasing folate intake may improve genomic stability. This review describes key applications of single cell gel electrophoresis (the comet assay) in assessing genomic instability (misincorporated uracil, DNA single strand breakage and DNA repair capacity) in response to folate status (deficient or supplemented) in human cells in vitro, in rodent models and in human case-control and intervention studies. It highlights an adaptation of the SCGE comet assay for measuring genome-wide and gene-specific DNA methylation in human cells and colon tissue.

Metabolism of sulfur compounds in homocystinurias.

BACKGROUND AND PURPOSE: Homocystinurias are rare genetic defects characterized by altered fluxes of sulfur compounds including homocysteine and cysteine. We explored whether the severely perturbed sulfur amino acid metabolism in patients with homocystinurias affects the metabolism of hydrogen sulfide. EXPERIMENTAL APPROACH: We studied 10 treated patients with a block in the conversion of homocysteine to cysteine due to cystathionine ß-synthase deficiency (CBSD) and six treated patients with remethylation defects (RMD) and an enhanced flux of sulfur metabolites via transsulfuration. Control groups for CBSD and RMD patients consisted of 22 patients with phenylketonuria on a low-protein diet and of 12 healthy controls respectively. Plasma and urine concentrations of selected sulfur compounds were analysed by HPLC and LC-MS/MS. KEY RESULTS: Patients with CBSD exhibited plasma concentrations of monobromobimane-detected sulfide similar to appropriate controls. Urinary homolanthionine and thiosulfate in CBSD were increased significantly 1.9 and 3 times suggesting higher hydrogen sulfide synthesis by γ-cystathionase and detoxification respectively. Surprisingly, patients with RMD had significantly lower plasma sulfide levels (53 and 64% of controls) with lower sulfite concentrations, and higher taurine and thiosulfate levels suggesting enhanced cysteine oxidation and hydrogen sulfide catabolism respectively. CONCLUSION AND IMPLICATIONS: The results from this study suggest that severe inherited defects in sulfur amino acid metabolism may be accompanied by only moderately perturbed hydrogen sulfide metabolism and lends support to the hypothesis that enzymes in the transsulfuration pathway may not be the major contributors to the endogenous hydrogen sulfide pool. LINKED ARTICLES: This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Classical Homocystinuria in a Juvenile Patient.

Classical homocystinuria, also known as cystathionine beta synthase deficiency, is a rare disorder of methionine metabolism, leading to an abnormal accumulation of homocysteine and its metabolites in blood and urine. A young child with homocystinuria is discussed, who presented with behavioral abnormalities, involuntary movement, mental retardation, and decreased vision since birth. The diagnosis of homocystinuria was not made at initial presentation. Subtle phenotypic features with developmental delay and MRI brain finding of bilateral medially dislocated lens, eventually provided the first indication at five years of age. Laboratory screening with plasma amino acid profile by ion exchange chromatography (IEC) showed elevated homocystine and methionine, and low cystine in plasma in the absence of vitamin B12, and folate deficiency; giving the diagnosis of classical homocysteinuria.

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

PURPOSE OF REVIEW: The current review highlights the varied effects of medical foods high in leucine (Leu) and devoid of valine (Val) and isoleucine (Ile) in the management of methylmalonic acidemia (MMA) and propionic acidemia and cobalamin C (cblC) deficiency, aiming to advance dietary practices. RECENT FINDINGS: Leu is a key metabolic regulator with a multitude of effects on different organ systems. Recent observational studies have demonstrated that these effects can have unintended consequences in patients with MMA as a result of liberal use of medical foods. The combination of protein restriction and medical food use in MMA and propionic acidemia results in an imbalanced branched-chain amino acid (BCAA) dietary content with a high Leu-to-Val and/or Ile ratio. This leads to decreased plasma levels of Val and Ile and predicts impaired brain uptake of multiple essential amino acids. Decreased transport of methionine (Met) across the blood-brain barrier due to high circulating Leu levels is of particular concern in cblC deficiency in which endogenous Met synthesis is impaired. SUMMARY: Investigations into the optimal composition of medical foods for MMA and propionic acidemia, and potential scenarios in which Leu supplementation may be beneficial are needed. Until then, MMA/propionic acidemia medical foods should be used judiciously in the dietary management of these patients and avoided altogether in cblC deficiency.

DNA damage in homocystinuria: 8-oxo-, 8-dihydro-2'-deoxyguanosine levels in cystathionine-ß-synthase deficient patients and the in vitro protective effect of N-acetyl­L­cysteine

Introduction: Homocysteine (Hcy) tissue accumulation occurs in a metabolic disease characterized biochemically by cystathionine ß-synthase (CBS) deficiency and clinically by mental retardation, vascular problems, and skeletal abnormalities. Previous studies indicate the occurrence of DNA damage secondary to hyperhomocysteinemia and it was observed that DNA damage occurs in leukocytes from CBS-deficient patients. This study aimed to investigate whether an oxidative mechanism could be involved in DNA damage previously found and investigated the in vitro effect of N-acety-L-cysteine (NAC) on DNA damage caused by high Hcy levels. Methods: We evaluated a biomarker of oxidative DNA damage in the urine of CBS­deficient patients, as well as the in vitro effect of NAC on DNA damage caused by high levels of Hcy. Moreover, a biomarker of lipid oxidative damage was also measured in urine of CBS deficient patients. Results: There was an increase in parameters of DNA (8-oxo-7,8-dihydro-2'- deoxyguanosine) and lipid (15-F2t-isoprostanes levels) oxidative damage in CBS-deficient patients when compared to controls. In addition, a significant positive correlation was found between 15-F2t-isoprostanes levels and total Hcy concentrations. Besides, an in vitro protective effect of NAC at concentrations of 1 and 5 mM was observed on DNA damage caused by Hcy 50 µM and 200 µM. Additionally, we showed a decrease in sulfhydryl content in plasma from CBS-deficient patients when compared to controls. Discussion: These results demonstrated that DNA damage occurs by an oxidative mechanism in CBS deficiency together with lipid oxidative damage, highlighting the NAC beneficial action upon DNA oxidative process, contributing with a new treatment perspective of the patients affected by classic homocystinuria.

Leptin concentrations and SCD-1 indices in classical homocystinuria: Evidence for the role of sulfur amino acids in the regulation of lipid metabolism.

BACKGROUND: We describe body composition, lipid metabolism and Stearoyl-CoA desaturase-1 (SCD-1) indices in patients with classical homocystinuria (HCU). METHODS: Eleven treated HCU patients and 16 healthy controls were included. Body composition and bone mineral density were assessed by dual X-ray absorptiometry. Sulfur amino acids (SAA) and their derivatives (total homocysteine, cysteine, methionine, S-adenosylmethionine, S-adenosylhomocysteine, and glutathione), lipids (free fatty acids, acylcarnitines, triglycerides and lipoproteins), glucose, insulin, leptin, adiponectin, and isoprostanes were measured in plasma. Insulin resistance was evaluated by HOMA-IR. To estimate liver SCD-1 activity, SCD-16 [16:1(n-7)/16:0] and SCD-18 [18:1(n-9)/18:0] desaturation indices were determined. RESULTS: In HCU patients, SCD-16 index was significantly reduced (p=0.03). A trend of an association of SCD-16 index with cysteine was observed (r=0.624, p=0.054). HCU patients displayed lower lean mass (p<0.05), with no differences in fat mass percentage. Leptin and low-density lipoprotein concentrations were lower in HCU patients (p<0.05). Femur bone mineral density Z-scores were correlated with plasma cysteine (r=0.829; p=0.04) and total homocysteine (r=-0.829; p=0.04) in HCU patients. CONCLUSIONS: We report alterations in leptin and SCD-1 in HCU patients. These results agree with previous findings from epidemiologic and animal studies, and support a role for SAA on lipid homeostasis.

Engineering and Characterization of an Enzyme Replacement Therapy for Classical Homocystinuria.

Homocystinuria due to loss of cystathionine beta-synthase (CBS) causes accumulation of homocysteine and depletion of cysteine. Current treatments are suboptimal, and thus the development of an enzyme replacement therapy based on PEGylated human truncated CBS (PEG-CBS) has been initiated. Attenuation of potency was observed, which necessitated a screen of several PEG-CBS conjugates for their efficacy to correct and maintain the plasma metabolite profile of murine homocystinuria after repeated administrations interrupted with washouts. We found that CBS coupling with maleimide PEG inconsistently modified the enzyme. In contrast, the PEG-CBS conjugate with 20 kDa N-hydroxysuccinimide-PEG showed very little loss of potency likely due to a reproducible PEGylation resulting in species modified with five PEGs per subunit on average. We developed assays suitable for monitoring the extent of CBS PEGylation and demonstrated a sustainable partial normalization of homocystinuria upon continuous PEG-CBS administration via osmotic pumps. Taken together, we identified the PEG-CBS conjugate suitable for manufacturing and clinical development.

Newborn screening for remethylation disorders and vitamin B12 deficiency-evaluation of new strategies in cohorts from Qatar and Germany.

BACKGROUND: Newborn screening is a precondition for early diagnosis and successful treatment of remethylation disorders and classical homocystinuria (cystathionine-ß-synthase deficiency). Newborn screening for classical homocystinuria using total homocysteine measurement in dried blood spots has been very successfully performed for many years for newborns from Qatar. METHODS: A new optimized newborn screening strategy for remethylation disorders and homocystinuria was developed and evaluated for newborns from Qatar using total homocysteine measurement as first-tier and methionine, methionine-phenylalanine-ratio and propionylcarnitine as second-tiers. Proposed cut-offs were also retrospectively evaluated in newborn screening samples of 12 patients with remethylation disorders and vitamin B12 deficiency from Qatar and Germany. RESULTS: Over a 12 months period, the proposed strategy led to a decrease in the recall rate in homocysteine screening for Qatar from 1.09% to 0.68%, while allowing for additional systematic inclusion of remethylation disorders and vitamin B12 deficiency into the screening panel for Qatar. In the evaluated period the applied strategy would have detected all patients with classical homocystinuria identified by the previous strategy and in addition 5 children with maternal nutritional vitamin B12 deficiency and one patient with an isolated remethylation disorder. Additional retrospective evaluation of newborn screening samples of 12 patients from Germany and Qatar with remethlyation disorders or vitamin B12 deficiency showed that all of these patients would have been detected by the cut-offs used in the proposed new strategy. In addition, an adapted strategy for Germany using methionine, methionine-phenylalanine-ratio and propionylcarnitine as first-tier, and homocysteine as a second-tier test was also positively evaluated retrospectively. CONCLUSIONS: The proposed strategy for samples from Qatar allows inclusion of remethylation disorders and vitamin B12 deficiency in the screening panel, while lowering the recall rate. An adapted second-tier strategy is presented for screening in Germany and will be prospectively evaluated over the next years in a pilot project named "Newborn Screening 2020".

Simultaneous Determination of Methionine and Homocysteine by on-column derivatization with o-phtaldialdehyde.

A fast and simple HPLC-based assay has been developed for the simultaneous determination of homocysteine (Hcy) and methionine (Met) in plasma and urine samples, utilizing as small volume of sample as 10µL. The assay uses on-column derivatization with o-phthaldialdehyde. The separation of Hcy and Met was achieved in 14min on a reversed phase C-18 column, followed by fluorescence detection (excitation at 348nm and emission at 438nm for Met; excitation at 370nm and emission at 480nm for Hcy). Linearity of the detector response was observed in the range of 2-60 µmol L-1 for Met and 2-40 µmol L-1 for Hcy. The method was successfully applied for Met and Hcy quantification in human and mouse plasma and urine samples from cystathionine ß-synthase deficient and unaffected individuals.

Validation of an HPLC method for total homocysteine quantification in plasma / Validación de un método para la cuantificación de homocisteína total en plasma por HPLC

Introduction. Homocysteine (Hcy) is a nonessential amino acid which links the methionine and the folate cycles. Hcy levels are increased in genetic disorders, such as classic homocystinuria and methylmalonic aciduria combined with homocystinuria. Other monogenic, multifactorial diseases and physiological conditions are also associated with high Hcy levels. The aim of this study is to validate a method to quantify Hcy in plasma. Material and methods. A method to quantify Hcy in plasma by HPLC was validated, by determining the following parameters: specificity, linearity, precision, accuracy, and detection and quantification limits according ICH and EMEA guidelines. Homocysteine was measured in 43 healthy individuals, 2 patients with high levels of methylmalonic acid, and a previously diagnosed patient with classic homocystinuria. Results. The method was able to identify and quantify Hcy without interferences. A linear behavior was observed in a range of 6-100 μM with r2 = 0.9967. The precision and accuracy studies showed variation coefficients under 6%. The limits of detection and quantification were 3.12 μM and 6.25 μM, respectively. The 43 healthy individuals studied had normal Hcy levels. The patient with elevated urinary methylmalonic acid levels and the homocystinuria patient showed high Hcy levels. Conclusion. The method is valid for the quantification of Hcy in plasma as it fulfills the requirements for validation of analytical methods. Its introduction into diagnostic and follow up algorithms is important in genetic diseases where this amino acid is increased (AU)

Introducción. La homocisteína (Hcy) es un aminoácido no esencial que enlaza el ciclo de la metionina con el ciclo del folato. Los niveles de Hcy se incrementan en trastornos genéticos como la homocistinuria clásica y la aciduria metilmalónica combinada con homocistinuria. Otras enfermedades monogénicas, multifactoriales y condiciones fisiológicas han sido asociadas con altos niveles de Hcy. El objetivo de este estudio es validar un método para la cuantificación de Hcy en plasma. Material y métodos. Se validó un método para la cuantificación de Hcy en plasma por HPLC. Se determinaron los parámetros de validación: especificidad, linealidad, precisión, exactitud, límite de detección y cuantificación según las guías ICH y EMEA. El método se aplicó en 43 individuos sanos, 2 con ácido metilmalónico elevado y uno con diagnóstico previo de homocistinuria clásica. Resultados. El método permitió la identificación y la cuantificación de la Hcy sin interferencias. Se observó un comportamiento lineal en un rango de 6-100 μM, con un r2 = 0,9967. En el estudio de precisión y exactitud se obtuvieron coeficientes de variación inferiores al 6%. Los límites de detección y cuantificación fueron de 3,12 y 6,25 μM, respectivamente. Los 43 individuos sanos mostraron niveles normales de Hcy. Uno de los pacientes con ácido metilmalónico elevado presentó niveles aumentados, igual que el paciente con homocistinuria. Conclusión. El método validado para la cuantificación de Hcy en plasma cumple con los criterios para la validación de métodos analíticos. Su introducción en los algoritmos de diagnóstico y seguimiento es importante en las enfermedades genéticas que cursan con incremento de este aminoácido (AU)

Hyperhomocysteinemia-induced upper extremity deep vein thrombosis and pulmonary embolism in a patient with methyltetrahydrofolate reductase mutation: a case report and literature review.

The study highlights pulmonary embolism and deep vein thrombosis by methylene tetrahydrofolate reductase (MTHFR) deficiency-related hyperhomocysteinemia occurring in rare locations of left veins superior to the heart extensively. A 59-year-old white man with history of leg pain, smoking, weight loss, benign prostatic hyperplasia, lipoma and panic attack presented with shortness of breath and chest pain for 2 days precipitated by not feeling well for months. The diagnostic workup revealed pulmonary embolism and deep vein thrombosis in the left subclavian vein which extended throughout the left brachiocephalic vein to the superior vena cava and left jugular vein. Further workup showed moderate hyperhomocysteinemia with normal levels of vitamin B6, B12 and folic acid. Methylene tetrahydrofolate reductase genetic study found the patient to be homozygous for G677T variant. He was started on low-molecular-weight heparin and was discharged on oral anticoagulant. No recurrent thrombotic episodes were witnessed after 4 months of follow-up after discharge.

High homocysteine induces betaine depletion.

Betaine is the substrate of the liver- and kidney-specific betaine-homocysteine (Hcy) methyltransferase (BHMT), an alternate pathway for Hcy remethylation. We hypothesized that BHMT is a major pathway for homocysteine removal in cases of hyperhomocysteinaemia (HHcy). Therefore, we measured betaine in plasma and tissues from patients and animal models of HHcy of genetic and acquired cause. Plasma was collected from patients presenting HHcy without any Hcy interfering treatment. Plasma and tissues were collected from rat models of HHcy induced by diet and from a mouse model of cystathionine ß-synthase (CBS) deficiency. S-adenosyl-methionine (AdoMet), S-adenosyl-homocysteine (AdoHcy), methionine, betaine and dimethylglycine (DMG) were quantified by ESI-LC-MS/MS. mRNA expression was quantified using quantitative real-time (QRT)-PCR. For all patients with diverse causes of HHcy, plasma betaine concentrations were below the normal values of our laboratory. In the diet-induced HHcy rat model, betaine was decreased in all tissues analysed (liver, brain, heart). In the mouse CBS deficiency model, betaine was decreased in plasma, liver, heart and brain, but was conserved in kidney. Surprisingly, BHMT expression and activity was decreased in liver. However, in kidney, BHMT and SLC6A12 expression was increased in CBS-deficient mice. Chronic HHcy, irrespective of its cause, induces betaine depletion in plasma and tissues (liver, brain and heart), indicating a global decrease in the body betaine pool. In kidney, betaine concentrations were not affected, possibly due to overexpression of the betaine transporter SLC6A12 where betaine may be conserved because of its crucial role as an osmolyte.