A deficiency of cysteine impairs fibrillin-1 deposition: implications for the pathogenesis of cystathionine beta-synthase deficiency

Recent therapeutic approaches to cystathionine beta-synthase-deficient homocystinuria

Cystathionine beta-synthase (CBS)-deficient homocystinuria (HCU) is the most common inborn error of sulfur amino acid metabolism. The pyridoxine non-responsive form of the disease manifests itself by massively increasing plasma and tissue concentrations of homocysteine, a toxic intermediate of methionine metabolism that is thought to be the major cause of clinical complications including skeletal deformities, connective tissue defects, thromboembolism and cognitive impairment. The current standard of care involves significant dietary interventions that, despite being effective, often adversely affect quality of life of HCU patients, leading to poor adherence to therapy and inadequate biochemical control with clinical complications. In recent years, the unmet need for better therapeutic options has resulted in development of novel enzyme and gene therapies and exploration of pharmacological approaches to rescue CBS folding defects caused by missense pathogenic mutations. Here, we review scientific evidence and current state of affairs in development of recent approaches to treat HCU.

Combination of Panel-based Next-Generation Sequencing and Clinical Findings in Congenital Ectopia Lentis Diagnosed in Chinese Patients

PURPOSE

To evaluate the diagnostic yield of congenital ectopia lentis (EL) in a Chinese cohort by combining panel-based next-generation sequencing with clinical findings.

DESIGN

A cohort study.

METHODS

In total, 175 patients with congenital EL and their available family members (n = 338) were enrolled. All patients with congenital EL underwent genetic testing. Genotype-phenotype analyses were conducted to assess the biometric and structural ocular manifestations of congenital EL.

RESULTS

In total, 175 patients with congenital EL and 338 of their relatives were included in this study. In these patients, 92.57% (162 of 175) of disease-related variants were detected in FBN1 (83.43%), CPAMD8 (1.71%), COL4A5 (0.57%), ADAMTSL4 (3.43%), LTBP2 (1.71%), and CBS (2.29%). Based on genetic and clinical findings, the primary diagnostic rate was increased to 40.57% from 19.43% with the exception of the 91 diagnoses of potential Marfan syndrome, with a new diagnostic strategy for congenital EL, thus having been developed. Within this group of patients harboring FBN1 mutations, 16.44% (19 of 141) probands were diagnosed with EL syndrome and 2.13% (3 of 141) were diagnosed with Marfan syndrome.

CONCLUSIONS

The results of this cohort study expand the genomic landscape associated with congenital EL in Chinese cohorts. FBN1 mutations represent the most common cause of congenital EL in this population, and we have developed a new diagnostic strategy for congenital EL subtypes via the use of a well-designed panel-based next-generation sequencing that can be used to efficiently and precisely diagnose patients with congenital EL in a cost-effective manner.

The Role of Trace Elements in Pseudoexfoliation Syndrome: A Cross-sectional Study

Purpose

Pseudoexfoliation syndrome (PXF) is an age-related condition, characterized by deposition of whitish flake-shaped materials in the anterior segment of the eye. Although it occurs all over the world, a considerable racial variation exists. According to the high frequency of PXF in Iran and the importance of prevention and early treatment, we evaluated the plasma level of iron, zinc, copper, and magnesium in patients with PXF.

Methods

In this study, 83 individuals were enrolled; 40 patients with cataract and PXF as the case group and 43 age- and sex-matched individuals with cataract but without PXF as the control group. The serum levels of the mentioned microelements were compared in two groups.

Results

In the case group, 25 (62.5%) male and 15 (37.5%) female subjects participated. In the control group, the corresponding figures were 22 (51.2%) and 21 (48.8%), respectively. The mean age of the case group was 66.07 ± 9.46 and that for the control group was 66.88 ± 8.04 years. Regarding the case group, the serum levels of iron, zinc, copper, and magnesium were 60.58 ± 21.04, 84.7 ± 14.37, 120.23 ± 14.43, and 2.11 ± 0.23, respectively. These serum levels in the control group were 89.07 ± 26.06, 97.51 ± 17.42, 123.33 ± 19.01, and 2.14 ± 0.16. The serum levels of iron and zinc were significantly lower in the case group than the control group (P< 0.0001); however, such a difference was not observed in terms of copper and magnesium serum levels.

Conclusion

Our study demonstrated that the serum iron and zinc levels were lower in PXF patients. Nutritional deficiency may be a cause of zonular weakness in these patients. Heme is a cofactor for the enzyme which contributes to the biosynthesis of fibrillin, the major protein in zonular fibers. Therefore, iron can play a substantial role in the biosynthesis of the fibrils and also in the zonular stability.

Cystathionine β-synthase deficiency in the E-HOD registry-part I: pyridoxine responsiveness as a determinant of biochemical and clinical phenotype at diagnosis

Cystathionine β-synthase (CBS) deficiency has a wide clinical spectrum, ranging from neurodevelopmental problems, lens dislocation and marfanoid features in early childhood to adult onset disease with predominantly thromboembolic complications. We have analysed clinical and laboratory data at the time of diagnosis in 328 patients with CBS deficiency from the E-HOD (European network and registry for Homocystinurias and methylation Defects) registry. We developed comprehensive criteria to classify patients into four groups of pyridoxine responsivity: non-responders (NR), partial, full and extreme responders (PR, FR and ER, respectively). All groups showed overlapping concentrations of plasma total homocysteine while pyridoxine responsiveness inversely correlated with plasma/serum methionine concentrations. The FR and ER groups had a later age of onset and diagnosis and a longer diagnostic delay than NR and PR patients. Lens dislocation was common in all groups except ER but the age of dislocation increased with increasing responsiveness. Developmental delay was commonest in the NR group while no ER patient had cognitive impairment. Thromboembolism was the commonest presenting feature in ER patients, whereas it was least likely at presentation in the NR group. This probably is due to the differences in ages at presentation: all groups had a similar number of thromboembolic events per 1000 patient-years. Clinical severity of CBS deficiency depends on the degree of pyridoxine responsiveness. Therefore, a standardised pyridoxine-responsiveness test in newly diagnosed patients and a critical review of previous assessments is indispensable to ensure adequate therapy and to prevent or reduce long-term complications.

Bone fragility in patients affected by congenital diseases non skeletal in origin

BACKGROUND

Bone tissue represents a large systemic compartment of the human body, with an active metabolism, that controls mineral deposition and removal, and where several factors may play a role. For these reasons, several non-skeletal diseases may influence bone metabolism. It is of a crucial importance to classify these disorders in order to facilitate diagnosis and clinical management. This article reports a taxonomic classification of non-skeletal rare congenital disorders, which have an impact on bone metabolism METHODS: The International Osteoporosis Foundation (IOF) Skeletal Rare Diseases Working Group (SRD-WG), comprised of basic and clinical scientists, has decided to review the taxonomy of non-skeletal rare disorders that may alter bone physiology.

RESULTS

The taxonomy of non-skeletal rare congenital disorders which impact bone comprises a total of 6 groups of disorders that may influence the activity of bone cells or the characteristics of bone matrix.

CONCLUSIONS

This paper provides the first comprehensive taxonomy of non-skeletal rare congenital disorders with impact on bone physiology.

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 β-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.

Elastin modulation and modification by homocysteine: a key factor in the pathogenesis of Pseudoexfoliation syndrome?

BACKGROUND

Pseudoexfoliation syndrome (PXF) is an idiopathic, elastogenesis-associated systemic disease characterised by amyloid-like material aggregates in the eye. Elevated plasma and aqueous humour (aqH) homocysteine (Hcy) is reportedly associated with PXF. This study is aimed to probe Hcy-mediated alterations in elastin expression.

METHODOLOGY

Lens level of Hcy (total Hcy (tHcy)), mRNA expression of Eln, CBS and MTR in lens capsule, protein expression of elastin in aqH were estimated by enzyme immunoassay, quantitative PCR and western blot, respectively in PXF, PXF with glaucoma (PXF-G) cases, in comparison with cataract-alone disease controls. Human lens epithelial cells (hLECs) were exposed to Hcy and homocysteine thiolactone (HCTL) to evaluate elastin expression in vitro. Furthermore, elastin recombinant protein was incubated with Hcy and HCTL to assess secondary and tertiary structural modifications based on circular dichroism spectroscopy, spectrophotometric and SEM studies.

RESULTS

The lens tHcy was significantly high in PXF (p=0.02) and PXF-G (p=0.009). Eln expression was elevated in PXF and PXF-G (p=0.0007). Elastin level in aqH was elevated in PXF (p=0.01) and PXF-G (p=0.002). Hcy (200 µM) and HCTL (1 µM) promoted elastin expression at mRNA level by 36-fold (p=0.02) and 10-fold (p=0.05), respectively, and at protein level by nearly two-fold in cultured hLECs. Secondary structure changes in elastin protein caused by Hcy were evident from 34.11% drop in α-helix and 6.17% gain in β-sheet. Fluorescence, spectral assays and SEM analyses showed aggregation and amyloid formation of elastin with homocysteinylation.

CONCLUSION

The study reveals that lens accumulation of Hcy associated with hyperhomocysteinaemia is characteristic of PXF that augments elastin expression. Hcy causes structural changes promoting elastin aggregation, thereby contributing to defective elastin in PXF and PXF-G.

NGS panel analysis in 24 ectopia lentis patients; a clinically relevant test with a high diagnostic yield

BACKGROUND

Several genetic causes of ectopia lentis (EL), with or without systemic features, are known. The differentiation between syndromic and isolated EL is crucial for further treatment, surveillance and counseling of patients and their relatives. Next generation sequencing (NGS) is a powerful tool enabling the simultaneous, highly-sensitive analysis of multiple target genes.

OBJECTIVE

The aim of this study was to evaluate the diagnostic yield of our NGS panel in EL patients. Furthermore, we provide an overview of currently described mutations in ADAMTSL4, the main gene involved in isolated EL.

METHODS

A NGS gene panel was analysed in 24 patients with EL.

RESULTS

A genetic diagnosis was confirmed in 16 patients (67%). Of these, four (25%) had a heterozygous FBN1 mutation, 12 (75%) were homozygous or compound heterozygous for ADAMTSL4 mutations. The known European ADAMTSL4 founder mutation c.767_786del was most frequently detected.

CONCLUSION

The diagnostic yield of our NGS panel was high. Causative mutations were exclusively identified in ADAMTSL4 and FBN1. With this approach the risk of misdiagnosis or delayed diagnosis can be reduced. The value and clinical implications of establishing a genetic diagnosis in patients with EL is corroborated by the description of two patients with an unexpected underlying genetic condition.

Plasma homocysteine levels and genetic polymorphisms in folate metablism are associated with breast cancer risk in chinese women

Background

Folate plays a pivotal role in DNA synthesis, repair, methylation and homocysteine (Hcy) metabolism. Therefore, alterations in the folate-mediated one-carbon metabolism may lead to abnormal methylation proliferation, increases of tumor/neoplasia and vein thrombosis/cardiovascular risk. The serine hydroxymethyhransferase (SHMT), methionine synthase (MS), methionine synthase reductase (MTRR) and cystathionine beta synthase (CBS) regulate key reactions in the folate and Hcy metabolism. Therefore, we investigated whether the genetic variants of the SHMT, MS, MTRR and CBS gene can affect plasma Hcy levels and are associated with breast cancer risk.

Methods

Genotyping was performed by PCR-RFLP method. Plasma Hcy levels were measured by the fluorescence polarization immunoassay on samples of 96 cases and 85 controls.

Results

(a) The SHMT 1420 T, MS 2756G, MTRR 66G allele frequency distribution showed significant difference between case and controls (p < 0.01 ~ 0.05). (b) The concentration of plasma Hcy levels of SHMT 1420TT was significantly lower than that of the wild type, while the plasma Hcy levels of MS 2756GG, CBS 699TT/1080TT significantly higher than that of the wild type both in case and controls. The plasma Hcy levels of MTRR 66GG was significantly higher than that of wild type in cases. The plasma Hcy levels of the same genotype in cases were significantly higher than those of controls except SHMT 1420CC, MS 2756AA, MTRR 66GG; (c) Multivariate Logistic regression analysis showed that SHMT C1420T (OR = 0.527, 95% CI = 0.55 ~ 1.24), MS A2756G (OR = 2.32, 95% CI = 0.29 ~ 0.82), MTRR A66G (OR = 1.84, 95% CI = 0.25 ~ 1.66) polymorphism is significantly associated with breast cancer risk. And elevated plasma Hcy levels were significantly linked to increased risk of breast cancer (adjusted OR = 4.45, 95% CI = 1.89-6.24 for the highest tertile as compared with the lowest tertile).

Conclusions

The current study results seem to suggest a possibility that SHMT C1420T mutation may be negatively correlated with breast cancer susceptibility; while MS A2756G and MTRR A66G mutation may be positively associated with breast cancer risk. SHMT C1420T, MS A2756G, MTRR A66G, CBS C1080T, CBS C699T locus mutation may be factors affecting plasma levels of Hcy. The plasma Hcy levels could be metabolic risk factor for breast cancer risk to a certain extent.

Changes in bone mineral density and body composition of children with well-controlled homocystinuria caused by CBS deficiency

Homocystinuria due to cystathionine β-synthase (CBS) deficiency is an inherited disorder of the metabolism of methionine. Clinical manifestations include mental retardation, dislocation of the optic lens, vascular lesions, arterial and venous thromboembolism, skeletal abnormalities, and osteoporosis. Most homocystinuria patients diagnosed in adulthood have severe osteoporosis, and homocystinuria is frequently mentioned as a cause of osteoporosis. Good control of plasma homocysteine may prevent or delay some of these complications. However, the effectiveness of bone mineral density (BMD) gain or fracture prevention has not been addressed. Here, we describe changes in BMD and body composition in 5 CBS deficiency patients who were diagnosed at young age and were managed with good metabolic control. We found that the BMD of each region was within the normal range. BMD gain was adequate and the patients had no significant change in skeletal morphology.

Defective homocysteine metabolism: potential implications for skeletal muscle malfunction

Hyperhomocysteinemia (HHcy) is a systemic medical condition and has been attributed to multi-organ pathologies. Genetic, nutritional, hormonal, age and gender differences are involved in abnormal homocysteine (Hcy) metabolism that produces HHcy. Homocysteine is an intermediate for many key processes such as cellular methylation and cellular antioxidant potential and imbalances in Hcy production and/or catabolism impacts gene expression and cell signaling including GPCR signaling. Furthermore, HHcy might damage the vagus nerve and superior cervical ganglion and affects various GPCR functions; therefore it can impair both the parasympathetic and sympathetic regulation in the blood vessels of skeletal muscle and affect long-term muscle function. Understanding cellular targets of Hcy during HHcy in different contexts and its role either as a primary risk factor or as an aggravator of certain disease conditions would provide better interventions. In this review we have provided recent Hcy mediated mechanistic insights into different diseases and presented potential implications in the context of reduced muscle function and integrity. Overall, the impact of HHcy in various skeletal muscle malfunctions is underappreciated; future studies in this area will provide deeper insights and improve our understanding of the association between HHcy and diminished physical function.

Structure and function of the mammalian fibrillin gene family: implications for human connective tissue diseases

Fibrillins and latent transforming growth factor β binding proteins (LTBPs) are components of the extracellular matrix of connective tissue. While fibrillins are integral to the 10nm microfibrils, and often associated with elastin, all family members are likely to have an additional role in regulating the bioavailability of transforming growth factor β (TGBβ). Both fibrillins and LTBPs are large glycoproteins, containing a series of calcium binding epidermal growth factor domains as well as a number of copies of a unique 8 cysteine domain found only in this protein superfamily. There are three mammalian fibrillins and four LTBPs. Fibrillin monomers link head to tail in microfibrils which can then form two and three dimensional structures. In some tissues elastin is recruited to the fibrillin microfibrils to provide elasticity to the tissue. LTBPs are part of the TGBβ large latent complex which sequesters TGBβ in the extracellular matrix. Fibrillin-1 appears to bind to LTBPs to assist in this process and is thus involved in regulating the bioavailability of TGBβ. Mutation of fibrillin genes results in connective tissue phenotypes which reflect both the increased level of active TGBβ and the structural failure of the extracellular matrix due to the absence or abnormality of fibrillin protein. Fibrillinopathies include Marfan syndrome, familial ectopia lentis, familial thoracic aneurysm (mutations of FBN1) and congenital contractural arachnodactyly (mutation of FBN2). There are no diseases currently associated with mutation of FBN3 in humans, and this gene is no longer active in rodents. Expression patterns of fibrillin genes are consistent with their role in extracellular matrix structure of connective tissue. FBN1 expression is high in most cell types of mesenchymal origin, particularly bone. Human and mouse FBN2 expression is high in fetal cells and has more restricted expression in mesenchymal cell types postnatally. FBN3 is expressed early in development (embryonic and fetal tissues) in humans. The fibrillins are thus important in maintaining the structure and integrity of the extracellular matrix and, in combination with their sequence family members the LTBPs, also contribute to the regulation of the TGFβ family of major growth factors.

Frequent epigenetic silencing of the folate-metabolising gene cystathionine-beta-synthase in gastrointestinal cancer

Background

Both gastric and colorectal cancers (CRC) are the most frequently occurring malignancies worldwide with the overall survival of these patients remains unsatisfied. Identification of tumor suppressor genes (TSG) silenced by promoter CpG methylation uncovers mechanisms of tumorigenesis and identifies new epigenetic biomarkers for early cancer detection and prognosis assessment. Cystathionine-beta-synthase (CBS) functions in the folate metabolism pathway, which is intricately linked to methylation of genomic DNA. Dysregulation of DNA methylation contributes substantially to cancer development.

Methodology/Principal Findings

To identify potential TSGs silenced by aberrant promoter methylation in CRC, we analyzed tumor and adjacent tissues from CRC cases using the Illumina Human Methylation45 BeadChip. We identified hypermethylation of the CBS gene in CRC samples, compared to adjacent tissues. Methylation and decreased mRNA expression of CBS were detected in most CRC cell lines by methylation-specific PCR and semiquantitative RT-PCR, as well as in gastric cancer. Treatment with 5-aza-2'-deoxycytidine and/or trichostatin A reversed methylation and restored CBS mRNA expression indicating a direct effect. Aberrant methylation was further detected in 31% of primary CRCs (29 of 96) and 55% of gastric tumors (11 of 20). In contrast, methylation was seldom found in normal tissues adjacent to the tumor. CBS methylation was associated with KRAS mutations in primary CRCs (P = 0.04, by χ²-test). However, no association was found between CBS methylation or KRAS mutations with cancer relapse/metastasis in Stage II CRC patients.

Conclusion

A novel finding from this study is that the folate metabolism enzyme CBS mRNA levels are frequently downregulated through CpG methylation of the CBS gene in gastric cancer and CRC, suggesting that CBS functions as a tumor suppressor gene. These findings warrant further study of CBS as an epigenetic biomarker for molecular diagnosis of gastrointestinal cancers.

Effects of hyperhomocysteinemia during the gestational period on ossification in rat embryo

Severe hyperhomocysteinemia, as seen in classic homocystinuria, is associated with several skeletal malformations and osteopenia. Moreover, hyperhomocysteinemia during pregnancy has been associated with multiple developmental defects in the fetus. This study was undertaken to determine whether offspring of hyperhomocysteinemic mothers have demonstrable changes in bone volume and the epiphyseal growth plate. Ten adult female Sprague-Dawley rats were randomly assigned to the control or experimental group. The experimental group received 100 mg/kg/day of homocysteine in their drinking water for 3 weeks before mating and for the total duration of pregnancy. In each group, three pups per mother were randomly selected. The histomorphometric properties of tibial, radial and vertebral growth plates of newborn rats and the volume fraction of bone were compared between groups. The plasma homocysteine concentration at the end of study was significantly higher in dams in the experimental group (16.42+/-1.5 vs. 4.7+/-1.7 mumol/L, P<0.05). In offspring born to dams given the homocysteine supplement, the volume fraction of bone in the tibia (30.7+/-1.5% vs. 36.8+/-1.9%, P<0.05), radius (29.6+/-1.1% vs. 37.4+/-2%, P<0.05) and vertebra (34.4+/-1.8% vs. 41+/-1.9%, P<0.05) were significantly decreased whereas vertical heights of proliferative (423+/-25.1 vs. 301.8+/-28.1 microm for radius and 131.9+/-5.9 vs. 107.8+/-3.5 microm for vertebra) and hypertrophic zones (213.1+/-12 vs. 163.3+/-7.5 microm for tibia, 153.2+/-7.7 vs. 121.1+/-7.9 microm for radius and 112+/-9.9 vs. 88.4+/-10.1 microm for the vertebra) were increased (P<0.05). The results showed that the administration of homocysteine caused osteopenia in newborn rats. In addition, these data suggest that hyperhomocysteinemia may induce disruption of normal development of epiphyseal cartilage in the rat embryo.

Functional consequences of homocysteinylation of the elastic fiber proteins fibrillin-1 and tropoelastin

Homocystinuria caused by cystathionine-beta-synthase deficiency represents a severe form of homocysteinemias, which generally result in various degrees of elevated plasma homocysteine levels. Marfan syndrome is caused by mutations in fibrillin-1, which is one of the major constituents of connective tissue microfibrils. Despite the fundamentally different origins, both diseases share common clinical symptoms in the connective tissue such as long bone overgrowth, scoliosis, and ectopia lentis, whereas they differ in others. Fibrillin-1 contains approximately 13% cysteine residues and can be modified by homocysteine. We report here that homocysteinylation affects functional properties of fibrillin-1 and tropoelastin. We used recombinant fragments spanning the entire fibrillin-1 molecule to demonstrate that homocysteinylation, but not cysteinylation leads to abnormal self-interaction, which was attributed to a reduced amount of multimerization of the fibrillin-1 C terminus. The deposition of the fibrillin-1 network by human dermal fibroblasts was greatly reduced by homocysteine, but not by cysteine. Furthermore, homocysteinylation, but not cysteinylation of elastin-like polypeptides resulted in modified coacervation properties. In summary, the results provide new insights into pathogenetic mechanisms potentially involved in cystathionine-beta-synthase-deficient homocystinuria.

Homocysteine oxidative stress and relation to bone mineral density in post-menopausal osteoporosis

BACKGROUND AND AIMS

Mildly elevated homocysteine (Hcy) and oxidative stress are novel and potentially modifiable risk factors for post-menopausal osteoporosis. We hypothesized that imbalance of oxidant/ antioxidant status and increased Hcy concentration stimulates osteoporotic activity, leading to increased collagen I breakdown in post-menopausal women.

METHODS

Patients were divided into 2 groups (NOP and OP). Group NOP had normal bone mineral density (BMD) and group OP low BMD. Thirty-four (69%) were in group OP and 15 (31%) in group NOP. Serum Total Antioxidant Status (TAS) and Total Peroxide (TPx) levels were determined with new automated methods. The study included measurement of Deoxypyridinoline (DPD).

RESULTS

In OP patients plasma t-Hcy, urine deoxypyridinoline and plasma TPx were significantly higher than those in NOP controls. In addition, OP patients also had lower TAS levels than controls, which represent the oxidative imbalance. Pearson's correlation analysis revealed a negative correlation between t-Hcy and TAS (p<0.038). A significant negative correlation was also found between TAS level and BMD values for the spine in OP patients (p<0.035). In contrast, a positive correlation between t-Hcy and TPx in OP patients was demonstrated significantly, r=0.52, p<0.029.

CONCLUSIONS

We show that the OP group had reduced TAS, whereas the elevated TPx was different from that in the NOP group. Slightly elevated homocysteinemia may contribute to increasing TPx and reducing TAS in the OP group. However, our results suggest a weak but negative relationship between TAS and BMD. Further investigations are needed to examine the relationship of oxidative stress as an endogenous bioactive agent to bone loss in post-menopausal women. Since oxidative stress is the imbalance between total oxidants and antioxidants in the body, any single oxidant/ antioxidant parameter may not reflect overall oxidative stress. Further studies are needed to understand the underlying mechanisms of these findings.

Modification of the Structure and Function of Fibrillin-1 by Homocysteine Suggests a Potential Pathogenetic Mechanism in Homocystinuria

Homocystinuria, a disorder originating in defects in the methionine metabolism, is characterized by an elevated plasma concentration of homocysteine. Most patients have a defect in the cystathionine-beta-synthase, the key enzyme in the conversion of homocysteine to cysteine. Many abnormalities in the connective tissue of patients with homocystinuria resemble those seen in Marfan syndrome, caused by mutations in fibrillin-1. These observations led to the hypothesis that the structure and function of fibrillin-1 is compromised in patients with homocystinuria. To test this hypothesis we produced recombinant human fibrillin-1 fragments spanning the central portion of the molecule (8-Cys/transforming growth factor-beta binding domain 3 to calcium binding EGF domain 22) and extensively analyzed the potential of homocysteine to modify structural and functional properties of these proteins. Circular dichroism spectroscopy revealed moderate changes of their secondary structures after incubation with homocysteine. Equilibrium dialysis demonstrated a number of high affinity calcium binding sites in the tandemly repeated calcium binding epidermal growth factor-like domains 11-22. Calcium binding of homocysteine-modified fragments was completely abolished. Incubation of the recombinant proteins with homocysteine rendered the analyzed calcium binding EGF domains as well as the 8-Cys/transforming growth factor-beta binding domain 3 significantly more susceptible to proteolytic degradation. Furthermore, data were obtained demonstrating that homocysteine can covalently modify fibrillin-1 via disulfide bonds. These data strongly suggest that structural and functional modifications as well as degradation processes of fibrillin-1 in the connective tissues of patients with homocystinuria play a major role in the pathogenesis of this disorder.

Increased osteoclast activity in the presence of increased homocysteine concentrations

BACKGROUND

Increased plasma homocysteine (HCY) may be an independent risk factor for osteoporotic fractures and therefore may also adversely affect bone metabolism. We analyzed the effect of HCY on human osteoclast (OC) activity.

METHODS

We cultured peripheral blood mononuclear cells from 17 healthy male donors [median (SD) age, 30 (5) years] for 20 days with 25 microg/L macrophage-colony-stimulating factor (days 0-11), 20 microg/L receptor-activator of nuclear factor-kappaB ligand (days 6-20), and 4 different concentrations of HCY (0, 10, 50, and 100 micromol/L; days 0-20). For control purposes, cysteine and glutathione were tested in equimolar concentrations. OCs were identified as large, multinucleated cells with tartrate-resistant acid phosphatase (TRAP) activity and surface vitronectin receptors. We quantified OC activity by measuring TRAP activity. We analyzed cathepsin K (CP-K) activity in 9 donor samples and estimated the dentine-resorbing activity on standard dentine slices in 3 samples.

RESULTS

After 20 days of culture, most cells were fully differentiated OCs. TRAP activity increased with increasing HCY concentrations (P < 0.001). HCY concentrations of 10, 50, and 100 micromol/L stimulated TRAP activity by 20%, 15%, and 42%. Additionally, HCY stimulated CP-K activity (P = 0.005): in the presence of 100 micromol/L HCY, CP-K activity was approximately 38% higher than in controls (P = 0.002). Bone-resorbing activity was significantly increased in cultures with 50 and 100 micromol/L HCY. Cysteine and glutathione significantly decreased TRAP and CP-K activity.

CONCLUSIONS

Increased HCY concentrations specifically stimulate OC activity in vitro, suggesting a mechanistic role of HCY for bone resorption. Future studies clarifying the mechanistic role of increased HCY concentrations in osteoporosis could have interesting therapeutic implications.

Cystathionine beta synthase deficiency affects mouse endochondral ossification

Cystathionine beta synthase (CBS) is a crucial regulator of plasma concentrations of homocysteine. Severe hyperhomocysteinemia due to CBS deficiency confers diverse clinical manifestations, notably characteristic skeletal abnormalities. To investigate this aspect of hyperhomocysteinemia, we analyzed the skeleton of CBS-deficient mice, a murine model of severe hyperhomocysteinemia. Radiography, Alcian Blue/Alizarin Red S-stained whole skeletal preparations, and histological comparisons were used to determine the extent, pattern, and distribution of skeletal abnormalities in CBS-deficient mice. Disruption of the murine CBS gene leads to skeletal abnormalities, notably kyphoscoliosis, with temporal shortening of long bones due to impaired cartilage differentiation, albeit to differing degrees.

Molecular effects of homocysteine on cbEGF domain structure: insights into the pathogenesis of homocystinuria

Homocystinuria is an inborn error of methionine metabolism that results in raised serum levels of the highly reactive thiol-containing amino acid homocysteine. Homocystinurics often exhibit phenotypic abnormalities that are similar to those found in Marfan syndrome (MFS), a heritable connective tissue disorder that is caused by reduced levels of, or defects in, the cysteine-rich extracellular matrix (ECM) protein fibrillin-1. The phenotypic similarities between homocystinuria and MFS suggest that elevated homocysteine levels may result in an altered function of fibrillin-1. We have used recombinant calcium binding epidermal growth factor-like (cbEGF) domain fragments from fibrillin-1, and an unrelated protein Notch1, to analyse the effects of homocysteine on the native disulphide (cystine) bonds of these domains. We show using analytical reverse phase, high performance liquid chromatography (HPLC), electrospray ionisation mass spectrometry (ESI-MS) and limited proteolysis that homocysteine attacks intramolecular disulphide bonds causing reduction of cystine and domain misfolding, and that the effects of homocysteine are dependent on its concentration. We also identify the importance of calcium binding to cbEGF domains for their stabilisation and protection against homocysteine attack. Collectively, these data suggest that reduction of intramolecular cbEGF domain disulphide bonds by homocysteine and the resulting disruption of this domain fold may contribute to the change in connective tissue function seen in homocystinuria. Furthermore, since we show that the effects of homocysteine are not unique to fibrillin-1, other cbEGF-containing proteins may be implicated in the pathogenic mechanisms underlying homocystinuria.

Homocysteine and carotid intima-media thickness: a critical appraisal of the evidence

This review examines the relationship between hyperhomocysteinemia, a risk factor for vascular disease, and carotid intima-media thickness (CIMT), a valid marker of generalized atherosclerosis and future vascular disease risk. The relationship between two important determinants of hyperhomocysteinemia in the general population-folate status and the 677C --> T methylenetetrahydrofolate reductase (MTHFR) polymorphism-and CIMT is also covered.

METHODS

We searched literature databases for articles examining homocysteine and CIMT published before September 2003.

RESULTS

We identified 54 studies. Observational studies generally failed to demonstrate a relationship between homocysteine and CIMT in homocystinuric, uremic, hypercholesterolemic or non-insulin-dependent diabetes mellitus patients or in subjects with insulin insensitivity. Weak associations, but usually only in certain sub-populations were found in vascular disease patients and in population-based studies. B vitamins reduce the progression of CIMT in renal transplant recipients and vascular disease patients as demonstrated by two trials. The majority of studies demonstrated increased CIMT in individuals with the MTHFR 677TT genotype. Folate status showed no relation to CIMT.

DISCUSSION

In non-patient populations, hyperhomocysteinemia is weakly associated with CIMT. The association of the 677 C--> T MTHFR polymorphism with CIMT further supports this finding. Lastly, folate levels may need to reach a critically low status before an association can be found between folate and CIMT. Larger trials in various population types are needed to determine whether folate alone or in combination with Vitamins B6 and B12 will slow down or even reverse atherosclerotic progression.

Perturbations in homocysteine-linked redox homeostasis in a murine model for hyperhomocysteinemia

Elevated plasma levels of homocysteine are a risk factor for cardiovascular diseases, neural tube defects, and Alzheimer's disease. The transsulfuration pathway converts homocysteine to cysteine, and approximately 50% of the cysteine in glutathione is derived from homocysteine in human liver cells, which suggests the hypothesis that defects in the transsulfuration pathway perturb redox homeostasis. To test this hypothesis, we examined a murine model for hyperhomocysteinemia in which the gene encoding the first enzyme in the transsulfuration pathway, cystathionine beta-synthase (CBS), has been disrupted. Limited metabolite profiling and CBS expression studies in liver, kidney, and brain reveal tissue-specific differences in the response to Cbs disruption. Homozygous disruption of Cbs lowered cysteine concentration in all three organs. Glutathione concentration was diminished in liver and brain, thus affecting the redox buffering capacity in these organs, whereas the approximately twofold higher glutathione synthesis capacity in kidney helped preserve the glutathione pool size despite loss of the transsulfuration pathway in this organ. In contrast, disruption of a single Cbs allele elicited only minor redox perturbations. Furthermore, the Cbs+/- genotype did not confer a significant disadvantage compared with the Cbs+/+ genotype in hepatocytes challenged by oxidative stress from exposure to tertiary butylhydroperoxide. These studies provide evidence that homozygous disruption of Cbs perturbs redox homeostasis and reduces cysteine levels, raising the possibility that these changes may be important in the etiology of the clinical manifestations of CBS deficiency.

Chemical and biomechanical characterization of hyperhomocysteinemic bone disease in an animal model

BACKGROUND

Classical homocystinuria is an autosomal recessive disorder caused by cystathionine beta-synthase (CBS) deficiency and characterized by distinctive alterations of bone growth and skeletal development. Skeletal changes include a reduction in bone density, making it a potentially attractive model for the study of idiopathic osteoporosis.

METHODS

To investigate this aspect of hyperhomocysteinemia, we supplemented developing chicks (n = 8) with 0.6% dl-homocysteine (hCySH) for the first 8 weeks of life in comparison to controls (n = 10), and studied biochemical, biomechanical and morphologic effects of this nutritional intervention.

RESULTS

hCySH-fed animals grew faster and had longer tibiae at the end of the study. Plasma levels of hCySH, methionine, cystathionine, and inorganic sulfate were higher, but calcium, phosphate, and other indices of osteoblast metabolism were not different. Radiographs of the lower limbs showed generalized osteopenia and accelerated epiphyseal ossification with distinct metaphyseal and suprametaphyseal lucencies similar to those found in human homocystinurics. Although biomechanical testing of the tibiae, including maximal load to failure and bone stiffness, indicated stronger bone, strength was proportional to the increased length and cortical thickness in the hCySH-supplemented group. Bone ash weights and IR-spectroscopy of cortical bone showed no difference in mineral content, but there were higher Ca2+/PO4(3-) and lower Ca2+/CO3(2-) molar ratios than in controls. Mineral crystallization was unchanged.

CONCLUSION

In this chick model, hyperhomocysteinemia causes greater radial and longitudinal bone growth, despite normal indices of bone formation. Although there is also evidence for an abnormal matrix and altered bone composition, our finding of normal biomechanical bone strength, once corrected for altered morphometry, suggests that any increase in the risk of long bone fracture in human hyperhomocysteinemic disease is small. We also conclude that the hCySH-supplemented chick is a promising model for study of the connective tissue abnormalities associated with homocystinuria and an important alternative model to the CBS knock-out mouse.