Fibrillin-2 defects impair elastic fiber assembly in a homocysteinemic chick model

Genome-wide association study of Helicobacter pylori serological status in Latin American children

BACKGROUND

Few genome-wide association studies (GWAS) on Helicobacter pylori infection susceptibility have been conducted for admixed populations from developing countries. Here, we performed a GWAS to identify genetic factors associated with H. pylori serostatus in a cohort of admixed children from a large Latin American urban center.

METHODS

A cross-sectional study involving 1161 children from 4 to 11 years old living in poor areas of Salvador, in northeastern Brazil. Logistic regression analysis was performed to detect associations between single-nucleotide variants (SNVs) and H. pylori seropositivity, assuming an additive genetic model. Enrichment analyses were conducted using the MAGMA v1.10 software.

RESULTS

We found 22 SNVs to be suggestively associated (p < 10-5 ) with H. pylori seropositivity. The most suggestive SNV was the rs77955022 (p = 4.83e-07) located in an intronic region of EXOC3 at 5p15.33. The second most suggestively associated SNV was rs10914996 (p = 8.97e-07), located in an intergenic region at 1p34.3. Furthermore, we were able to replicate three SNVs (p < 0.05) in the Study of Health in Pomerania (SHIP) cohort: the rs2339212 and rs4795970, both located at 17q12 near TMEM132E, as well as the rs6595814, an intronic variant of FBN2 at 5q23.3. The enrichment analysis indicated the participation of genes and metabolic pathways related to the regulation of the digestive system and gastric acid secretion in the risk of seropositivity for H. pylori.

CONCLUSIONS

Additional studies are required to validate these association findings in larger population samples and to get insight into the underlying physiological mechanisms.

Plasma and Aorta Biochemistry and MMPs Activities in Female Rabbit Fed Methionine Enriched Diet and Their Offspring

This study investigated whether a high Met diet influences biochemical parameters, MMPs activities in plasma, and biochemical and histological remodeling in aorta, in both pregnant female rabbits and their offspring. Four female rabbit groups are constituted (each n = 8), nonpregnant control (NPC), pregnant control (PC) that received normal commercial chow, nonpregnant Met (NPMet), and pregnant Met (PMet) that received the same diet supplemented with 0,35% L-methionine (w/w) for 3 months (500 mg/d). All pregnant females realize 3 successive pregnancies. Plasma results showed that Met excess increased Hcy, raised CRP in NPMet and decreased it in PMet, enhanced significantly proMMP-2 and proMMP-9 activities in NPMet, and reduced them in PMet. Aorta showed a rise in collagen level, essentially in PMet, a reduction of elastin content in both PMet and NPMet, and a significant decrease in lipid content in PMet, with histological changes that are more pronounced in NPMet than PMet. Met excess enhanced proMMP-9 activities in NPMet while it decreased them in PMet. PMet newborn presented increase in uremia and CRP and significant rise of active MMP-2 and MMP-9 forms. In aorta, media and adventitia thickness increased, total lipids content decreased, proMMP-9 activity decreased, and proMMP-2 activity increased.

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.

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.

ELASTIN DEFECTS IN THE LUNGS OF AVIAN AND MURINE MODELS OF HOMOCYSTEINEMIA

Homocysteinemia in animals is associated with disruption of the elastic fiber component of the extracellular matrix, resulting in vascular complications. The authors have utilized both avian and murine models to investigate the effects of homocysteinemia on lung development and repair following injury. Days old chicks were fed a diet containing 2% methionine for 3 weeks. Pregnant mice were given 2% methionine in the diet and feeding continued for up to 6 weeks after birth. The lungs were removed and examined for defects in elastin fiber formation. Methionine levels were elevated 20-fold in the serum from chicks receiving the methionine and 10-fold in pregnant mice. The elastic fibers in the parabronchi and air capillaries of chicks receiving methionine were thin and clearly disrupted. In the 2% methionine neonatal pups, normal lung development was prevented and the alveoli were significantly enlarged. However, after the pups reached 10 days of age the 2% methionine lungs did not differ histologically from the normal controls. Fetal mice reflected the same serum methionine levels as the dams fed the 2% methionine diet, yet after birth the serum levels of the neonates returned to control levels within 3 days. The authors found that the high serum methionine levels of the dams were not transferred to the milk, allowing the pups to reverse the histopathology observed early and then develop normally. The ability of the lung to replace elastin following elastase injury was not different in mice raised on the 2% methionine diet compared to controls. The studies show that continuous exposure of the developing lung to high circulating levels of methionine/homocysteine can result in major disruptions of elastic fibers and lung architecture. However, young mammals such as the mouse are protected from extended lung pathology because toxic levels of methionine are not transferred through the mothers milk.

Increased endothelin-1 reactivity and endothelial dysfunction in carotid arteries from rats with hyperhomocysteinemia

BACKGROUND AND PURPOSE

There are interactions between endothelin-1 (ET-1) and endothelial vascular injury in hyperhomocysteinemia (HHcy), but the underlying mechanisms are poorly understood. Here we evaluated the effects of HHcy on the endothelin system in rat carotid arteries.

EXPERIMENTAL APPROACH

Vascular reactivity to ET-1 and ET(A) and ET(B) receptor antagonists was assessed in rings of carotid arteries from normal rats and those with HHcy. ET(A) and ET(B) receptor expression was assessed by mRNA (RT-PCR), immunohistochemistry and binding of [(125)I]-ET-1.

KEY RESULTS

HHcy enhanced ET-1-induced contractions of carotid rings with intact endothelium. Selective antagonism of ET(A) or ET(B) receptors produced concentration-dependent rightward displacements of ET-1 concentration response curves. Antagonism of ET(A) but not of ET(B) receptors abolished enhancement in HHcy tissues. ET(A) and ET(B) receptor gene expressions were not up-regulated. ET(A) receptor expression in the arterial media was higher in HHcy arteries. Contractions to big ET-1 served as indicators of endothelin-converting enzyme activity, which was decreased by HHcy, without reduction of ET-1 levels. ET-1-induced Rho-kinase activity, calcium release and influx were increased by HHcy. Pre-treatment with indomethacin reversed enhanced responses to ET-1 in HHcy tissues, which were reduced also by a thromboxane A(2) receptor antagonist. Induced relaxation was reduced by BQ788, absent in endothelium-denuded arteries and was decreased in HHcy due to reduced bioavailability of NO.

CONCLUSIONS AND IMPLICATIONS

Increased ET(A) receptor density plays a fundamental role in endothelial injury induced by HHcy. ET-1 activation of ET(A) receptors in HHcy changed the balance between endothelium-derived relaxing and contracting factors, favouring enhanced contractility.

The role of hyperhomocysteinemia as well as folate, vitamin B(6) and B(12) deficiencies in osteoporosis: a systematic review

Hyperhomocysteinemia (HHCY) has been suggested as a new risk factor for osteoporosis. Recent epidemiological, clinical and experimental studies provide a growing body of data, which is reviewed in this article. Epidemiological and (randomized) clinical trials suggest that HHCY increases fracture risk, but has minor effects on bone mineral density. Measurement of biochemical bone turnover markers indicates a shift of bone metabolism towards bone resorption. Animal studies confirm these observations showing a reduced bone quality and stimulation of bone resorption in hyperhomocysteinemic animals. Homocysteine (HCY) has been found to accumulate in bone by collagen binding. Cell culture studies demonstrate that high HCY levels stimulate osteoclasts but not osteoblasts, indicating again a shift of bone metabolism towards bone resorption. Regarding B-vitamins, only a few in vivo studies with equivocal results have been published. However, two large cell culture studies confirm the results obtained with exogenous HCY administration. In addition, HHCY seems to have adverse affects on extracellular bone matrix by disturbing collagen crosslinking. In conclusion, existing data suggest that HHCY (and possibly B-vitamin deficiencies) adversely affects bone quality by a stimulation of bone resorption and disturbance of collagen crosslinking.

Ageing of the conduit arteries

Conduit arteries become stiffer with age due to alterations in their morphology and the composition of the their major structural proteins, elastin and collagen. The elastic lamellae undergo fragmentation and thinning, leading to ectasia and a gradual transfer of mechanical load to collagen, which is 100-1000 times stiffer than elastin. Possible causes of this fragmentation are mechanical (fatigue failure) or enzymatic (driven by matrix metallo proteinases (MMP) activity), both of which may have genetic or environmental origins (fetal programming). Furthermore, the remaining elastin itself becomes stiffer, owing to calcification and the formation of cross-links due to advanced glycation end-products (AGEs), a process that affects collagen even more strongly. These changes are accelerated in the presence of disease such as hypertension, diabetes and uraemia and may be exacerbated locally by atherosclerosis. Raised MMP activity, calcification and impaired endothelial function are also associated with a high level of plasma homocysteine, which itself increases with age. Impaired endothelial function leads to increased resting vascular smooth muscle tone and further increases in vascular stiffness and mean and/or pulse pressure. The effect of increased stiffness, whatever its underlying causes, is to reduce the reservoir/buffering function of the conduit arteries near the heart and to increase pulse wave velocity, both of which increase systolic and pulse pressure. These determine the peak load on the heart and the vascular system as a whole, the breakdown of which, like that of any machine, depends more on the maximum loads they must bear than on their average. Reversing or stabilising the increased arterial stiffness associated with age and disease by targeting any or all of its causes provides a number of promising new approaches to the treatment of systolic hypertension and its sequelae, the main causes of mortality and morbidity in the developed world.

Alpha1D-adrenoceptor-induced relaxation on rat carotid artery is impaired during the endothelial dysfunction evoked in the early stages of hyperhomocysteinemia

Hyperhomocysteinemia is a known risk factor for cardiovascular diseases, but the underlying mechanisms of this pathology are complex. We aimed to evaluate the effect of hyperhomocysteinemia in vasorelaxations induced by alpha(1D)-adrenoceptor agonists. Vascular reactivity of rat carotid artery to the alpha-adrenoceptor agonist, phenylephrine, was enhanced in hyperhomocysteinemia. Mechanical removal of endothelium did not modify the carotid responsiveness to phenylephrine, compared to control. Phenylephrine induces endothelium-dependent relaxation, in the presence of 5-methyl urapidil (alpha(1A)-adrenoceptor antagonist). We hypothesised that endothelial-relaxant alpha(1)-adrenoceptors are impaired by hyperhomocysteinemia. Incubation with prazosin (selective alpha(1)-adrenoceptor antagonist) or BMY7378 (8-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-8-azaspiro[4,5]decane-7, 9-dione dihydrochloride) (selective alpha(1D)-adrenoceptor antagonist), similarly inhibited phenylephrine-induced relaxations in both control and hyperhomocysteinemic carotids. Immunohistochemistry showed enhanced immunoreactivity for eNOS and iNOS in hyperhomocysteinemic rats. In carotid arteries from hyperhomocysteinemic rats there was a decrease in superoxide dismutase activity and enhanced superoxide anion production. We conclude that alpha(1D)-adrenoceptors mediate endothelium-dependent relaxation triggered by phenylephrine in rat carotid artery and affect the final tone. Furthermore, the enhanced phenylephrine-induced contraction in carotid artery due to hyperhomocysteinemia is endothelium-dependent and involves a loss of the inhibitory effect of relaxant alpha(1D)-adrenoceptors by reducing NO biodisponibility.

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.

High levels of homocysteine inhibit lysyl oxidase (LOX) and downregulate LOX expression in vascular endothelial cells

BACKGROUND

Hyperhomocysteinemia, an independent risk factor for cardiovascular disease and atherothrombosis, alters endothelial function through a mechanism not fully understood. Downregulation of lysyl oxidase (LOX), an enzyme involved in extracellular matrix maturation, impairs the endothelial barrier function and could be involved in homocysteine (HC)-induced endothelial dysfunction.

OBJECTIVE

The aim of this study was to analyze the effect of HC on LOX regulation in vascular endothelial cells.

RESULTS

HC at pathophysiological concentrations (35 microM) inhibited LOX activity in porcine aortic endothelial cells. Homocysteine thiolactone and related molecules containing sulfhydryl groups (cysteine), but not methionine or homocystine (non-containing thiol-group) inhibited LOX. In addition, the blockade of HC-sulfhydryl group by N-ethylmaleimide abrogated HC-induced LOX downregulation. This process was triggered by oxidative stress since superoxide dismutase and vitamin C reverted LOX inhibition caused by HC. On the contrary, the effect was not mediated through the induction of endoplasmic reticulum stress. Finally, higher doses of HC (200 microM), common in severe hyperhomocysteinemia, decreased LOX mRNA levels ( approximately 2-fold) and LOX promoter activity in transient transfection experiments.

CONCLUSIONS

These findings suggest that LOX inhibition contributes to the endothelial dysfunction associated with hyperhomocysteinemia. This effect was dependent on a mechanism involving both an inhibition of LOX activity and a reduction of LOX expression.

Microfibril-associated glycoprotein-1 and fibrillin-2 are associated with tropoelastin deposition in vitro

Elastic system fibers consist of microfibrils and tropoelastin. During development, microfibrils act as a template on which tropoelastin is deposited. Microfibril-associated glycoprotein-1 (MAGP-1) and fibrillin-2, the major components of microfibrils, provide the likely template for tropoelastin deposition. In this study, we used the RNA interference (RNAi) technique to establish MAGP-1 and fibrillin-2 gene-specific knock-downs individually in elastin-producing cells (human gingival fibroblasts). We then examined the extracellular deposition of tropoelastin by western blotting. These two genes were specifically suppressed to < 30% of the control level, and this was responsible for the diminution of tropoelastin deposition. An immunofluorescence study also confirmed that RNAi-mediated down-regulation of MAGP-1 or fibrillin-2 led to the loss of tropoelastin immunoreactivity. These results suggest that MAGP-1 and fibrillin-2 are, directly or indirectly, associated with the extracellular deposition of tropoelastin during elastic fiber formation in human gingival fibroblasts in vitro.

Elastic fiber production in cardiovascular tissue-equivalents

Elastic fiber incorporation is critical to the success of tissue-engineered arteries and heart valves. Elastic fibers have not yet been observed in tissue-engineered replacements fabricated in vitro with smooth muscle cells. Here, rat smooth muscle cells (SMC) or human dermal fibroblasts (HDF) remodeled collagen or fibrin gels for 4 weeks as the basis for a completely biological cardiovascular tissue replacement. Immunolabeling, alkaline extraction and amino acid analysis identified and quantified elastin. Organized elastic fibers formed when neonatal SMC were cultured in fibrin gel. Fibrillin-1 deposition occurred but elastin was detected in regions without fibrillin-1, indicating that a microfibril template is not required for elastic fiber formation within fibrin. Collagen did not support substantial elastogenesis by SMC. The quantity of crosslinked elastic fibers was enhanced by treatment with TGF-beta1 and insulin, concomitant with increased collagen production. These additives overcame ascorbate's inhibition of elastogenesis in fibrin. The elastic fibers that formed in fibrin treated with TGF-beta1 and insulin contained crosslinks, as evidenced by the presence of desmosine and an altered elastin labeling pattern when beta-aminopropionitrile (BAPN) was added. These findings indicate that in vitro elastogenesis can be achieved in tissue engineering applications, and they suggest a physiologically relevant model system for the study of three-dimensional elastic structures.

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.