Inorganic pyrophosphate in metabolic diseases predisposing to calcium pyrophosphate dihydrate crystal deposition

Calcium Pyrophosphate Crystal Deposition in a Cohort of 57 Patients with Gitelman Syndrome

OBJECTIVE

Gitelman syndrome (GS) is the most frequent salt-wasting genetic tubulopathy and a source of hypokalaemia and hypomagnesemia. Chondrocalcinosis (CC) is a frequent feature of GS. The aim of our study was to determine the prevalence, distribution patterns, clinical phenotypes and risk factors of CC in GS.

METHODS

This prospective study of a cohort of 57 patients with GS included a systematic screening for CC by peripheral joint radiography, cervical spine computerized tomography (CT) and joint ultrasonography. The prevalence of cervical C1-C2 CC by CT was compared between 33 GS patients and sex- and age-matched controls. Clinical and biochemical features were analysed to identify factors associated with CC.

RESULTS

Mean age of patients was 46.5 ± 12.4 years, 66.7% were women, and 93.0% carried SLC12A3 mutations. Mean serum magnesium level was 0.60 ± 0.30 mmol/l. CC was observed in 79% of patients, with the highest prevalence at the cervical spine (81.8%) followed by the knee (52.6%), wrist (50.9%), ankle (38.6%), temporomandibular joint (36.4%), shoulder (33.3%), hip (22.8%), elbow (14.0%) and sclero-choroid (12.1%). Prevalence of CC at the C1-C2 level was higher in the GS cohort than control group (72.7% vs 9.1%) (adjusted odds ratio 21.0, 95% CI 2.8-156.1, p= 0.003). Independent factors associated with CC were low serum magnesium level and age.

CONCLUSION

GS was associated with widespread CC, favoured by aging and hypomagnesemia. The C1-C2 level was the most affected site. Follow-up of this unique cohort will help understanding the clinical consequences of CC, especially the precise characterization of pyrophosphate arthropathy.

The Therapeutic Potential of Exogenous Adenosine Triphosphate (ATP) for Cartilage Tissue Engineering

OBJECTIVE

While mechanical stimuli can be used to enhance the properties of engineered cartilage, a promising alternative may be to directly harness the underlying mechanotransduction pathways responsible. Our initial studies on the adenosine triphosphate (ATP)-purinergic receptor pathway demonstrated that stimulation by exogenous ATP improved tissue growth and properties but elicited matrix turnover under high doses (250 µM) potentially due to the accumulation of extracellular inorganic pyrophosphate (ePPi). Therefore, the purpose of this study was to identify the mechanism of ATP-mediated catabolism and determine a therapeutic dose to maximize the anabolic effect.

DESIGN

Isolated bovine articular chondrocytes were seeded in high-density, 3-dimensional culture supplemented with varying doses of ATP for 4 weeks. The effects on biosynthesis, matrix metalloproteinase 13 (MMP-13) protein activity, and PPi accumulation were determined. Separate monolayer experiments were conducted to determine the effect of ePPi on MMP-13 activity.

RESULTS

High doses of ATP resulted in an increase in ePPi accumulation (by 54%) and MMP-13 activity (by 39%). Monolayer experiments confirmed a link between increased ePPi accumulation and MMP-13 activity, which appeared to require calcium and was inhibited by the MEK1/2 inhibitor U0126. Cultures supplemented with 62.5 to 125 µM ATP favored an anabolic response, which represented the therapeutic dose range.

CONCLUSIONS

A therapeutic dose range of exogenous ATP to improve the properties of engineered cartilage has been identified, and a possible catabolic mechanism involving excess PPi was determined. Future research into PPi signal transduction and pathological crystal formation is necessary to maximize the beneficial effect of exogenous ATP on chondrocyte cultures.

Miscellaneous non-inflammatory musculoskeletal conditions. Bartter's and Gitelman's diseases

Bartter's and Gitelman's syndromes are two different genetic renal diseases, but are both characterised by hypokalaemia and metabolic alkalosis. Bartter's syndrome is characterised by multiple gene mutations (Na-K-2Cl cotransporter; K(+) channels renal outer medullary potassium channel (ROMK); Cl channels, chloride channel Kb (ClCNKb); regulatory protein Barttin; and Ca(2+) -sensing receptor, CaSR) at the thick ascending limb of Henle's loop, while Gitelman's syndrome is caused by a mutation in the gene encoding the renal thiazide sensitive Na(+)-Cl(-) cotransporter, located in the apical membrane of the distal convoluted tubule. The co-existence of hypokalaemia with hypomagnesaemia and hypocalciuria represents the biochemical hallmark of Gitelman's syndrome that distinguishes it from Bartter's syndrome. Calcium pyrophosphate deposition (CPPD) including chondrocalcinosis has been frequently reported in association with Bartter's syndrome. Some authors postulate that these cases were probably due to Gitelman's syndrome and not due to Bartter's syndrome as all patients had hypomagnesaemia. This electrolyte disorder seems to induce CCP crystal deposition. To date, no cases of CPPD have been reported in patients who had Bartter's syndrome without hypomagnesaemia. CPPD may be found in other conditions associated with hypomagnesaemia, such as short bowel syndrome or tacrolimus therapy in liver transplantation patients. As acute CPP crystal arthropathy or pseudogout can be the onset presentation of Gitelman's syndrome, CPPD should be considered a major feature of this disease. Rheumatologists should be aware of the association between Gitelman's syndrome and CPPD, and should consider this metabolic disorder when CPPD occurs in younger patients.

Purine receptors modulate chondrocyte extracellular inorganic pyrophosphate production

OBJECTIVE

Extracellular inorganic pyrophosphate (ePPi) plays a key role in the regulation of normal and pathologic mineralization. The purpose of this work was to evaluate the role of P1 and P2 purine receptors in modulating ePPi production by articular chondrocytes.

METHODS

Porcine cartilage explants and chondrocyte monolayers were cultured in the presence of P1 agonists, or a P2 agonist or antagonist and inhibitors of P2 signaling. Ambient media ePPi concentrations were measured after 48-96h.

RESULTS

The P1 agonists NECA and CGS 21680 significantly decreased ePPi concentrations surrounding chondrocytes and cartilage explants. The P2 agonist, ADP, increased ePPi levels, and the P2 antagonist, suramin, decreased ePPi concentrations. Thapsigargin and 1,2 bis-(2-aminophenoxy)ethane-N,N,N'N'-tetra acetic acid (BAPTA), which dampen Ca(2+)-related P2 signaling, suppressed the response to ADP.

CONCLUSIONS

Purine receptors are important regulators of ePPi production by chondrocytes. P1 receptor stimulation diminishes and P2 receptor stimulation enhances ePPi production. Alterations in receptor signaling or aberrations of extracellular purine nucleotide metabolism resulting in abnormal quantities or proportions of P1 and P2 receptor ligands could foster changes in ePPi production that in turn affect mineralization. We propose a homeostatic role for extracellular purine nucleotides and purine receptors in stabilizing ePPi concentrations.

Long-term follow-up of bone mineral density in childhood hypophosphatasia

OBJECTIVE

Hypophosphatasia (HP; MIM 241510) is an inborn error of bone metabolism, characterized by a genetic defect in the gene of the tissue-non-specific alkaline phosphatase TNSALP. Long-term data on bone mineral density measurements are not available.

METHODS

We have analyzed changes of bone mineral density (pQCT and DXA) prospectively during 4years of follow-up in a cohort of 6 patients with childhood HP.

RESULTS

At diagnosis hypermineralization of the trabecular bone in the metaphyseal area of long bones in affected children was noted. During 4 years of follow-up a gradual, significant decrease of mineralization was noted in the radial metaphyses. In contrast, BMC by DXA and total body DXA values were stable in comparison to healthy controls. During follow-up a systemic hyperprostaglandinism was documented in the majority of the patients. Non-steroidal anti-inflammatory drug treatment was evaluated by measuring prostaglandin excretion in the urine.

CONCLUSIONS

Metaphyseal hypermineralization in childhood HP, which might be a compensation for a mechanically incompetent bony structure, decreased over time. There might be a pathophysiological link to continually elevated systemic prostaglandins.

Effective NSAID treatment indicates that hyperprostaglandinism is affecting the clinical severity of childhood hypophosphatasia

Background

Hypophosphatasia (HP) is an inborn error of bone metabolism characterized by a genetic defect in the gene encoding the tissue-nonspecific alkaline phosphatase (TNSALP). There is a lack of knowledge as to how the variability and clinical severity of the HP phenotype (especially pain and walking impairment) are related to metabolic disturbances or impairments, subsequent to the molecular defect.

Methods

We analyzed the changes in clinical symptoms and the prostaglandin (PG) metabolism in response to treatment with non-steroidal anti-inflammatory drugs (NSAIDs) in six children affected by childhood HP. In addition, by exposing HP fibroblasts to pyridoxal phosphate and/or calcium pyrophosphate in vitro, we analyzed whether the alterations in PG levels are sequelae related to the metabolic defect.

Results

Childhood HP patients, who often complain about pain in the lower limbs without evident fractures, have systemic hyperprostaglandinism. Symptomatic anti-inflammatory treatment with NSAIDs significantly improved pain-associated physical impairment. Calcium pyrophosphate, but not pyridoxal phosphate, induced cyclooxygenase-2 (COX-2) gene expression and PG production in HP and normal fibroblasts in vitro.

Conclusion

Clinical features of childhood HP related to pain in the lower legs may be, at least in part, sequelae related to elevated PG levels, secondary to the primary metabolic defect. Consequently, NSAID treatment does improve the clinical features of childhood HP.

Association of sporadic chondrocalcinosis with a -4-basepair G-to-A transition in the 5'-untranslated region of ANKH that promotes enhanced expression of ANKH protein and excess generation of extracellular inorganic pyrophosphate

OBJECTIVE

Certain mutations in ANKH, which encodes a multiple-pass transmembrane protein that regulates inorganic pyrophosphate (PPi) transport, are linked to autosomal-dominant familial chondrocalcinosis. This study investigated the potential for ANKH sequence variants to promote sporadic chondrocalcinosis.

METHODS

ANKH variants identified by genomic sequencing were screened for association with chondrocalcinosis in 128 patients with severe sporadic chondrocalcinosis or pseudogout and in ethnically matched healthy controls. The effects of specific variants on expression of common markers were evaluated by in vitro transcription/translation. The function of these variants was studied in transfected human immortalized CH-8 articular chondrocytes.

RESULTS

Sporadic chondrocalcinosis was associated with a G-to-A transition in the ANKH 5'-untranslated region (5'-UTR) at 4 bp upstream of the start codon (in homozygotes of the minor allele, genotype relative risk 6.0, P = 0.0006; overall genotype association P = 0.02). This -4-bp transition, as well as 2 mutations previously linked with familial and sporadic chondrocalcinosis (+14 bp C-to-T and C-terminal GAG deletion, respectively), but not the French familial chondrocalcinosis kindred 143-bp T-to-C mutation, increased reticulocyte ANKH transcription/ANKH translation in vitro. Transfection of complementary DNA for both the wild-type ANKH and the -4-bp ANKH protein variant promoted increased extracellular PPi in CH-8 cells, but unexpectedly, these ANKH mutants had divergent effects on the expression of extracellular PPi and the chondrocyte hypertrophy marker, type X collagen.

CONCLUSION

A subset of sporadic chondrocalcinosis appears to be heritable via a -4-bp G-to-A ANKH 5'-UTR transition that up-regulates expression of ANKH and extracellular PPi in chondrocyte cells. Distinct ANKH mutations associated with heritable chondrocalcinosis may promote disease by divergent effects on extracellular PPi and chondrocyte hypertrophy, which is likely to mediate differences in the clinical phenotypes and severity of the disease.

Upregulated ank expression in osteoarthritis can promote both chondrocyte MMP-13 expression and calcification via chondrocyte extracellular PPi excess

OBJECTIVE

In idiopathic chondrocalcinosis and in osteoarthritis (OA), increased extracellular PP(i) (ecPP(i)) promotes calcification. In chromosome 5p-associated familial chondrocalcinotic degenerative arthropathy, certain mutations in the membrane protein ANK may chronically raise ecPP(i) via enhanced PP(i) channeling. Therefore, we assessed if dysregulated wild-type ANK expression could contribute to pathogenesis of idiopathic degenerative arthropathy through elevated ecPP(i).

DESIGN

Using cells with genetic alterations in expression of ANK and the PP(i)-generating nucleotide pyrophosphatase phosphodiestrase (NPP) PC-1, we examined how increased ANK expression elevates ecPPI, testing for codependent effects with PC-1. We also evaluated the effects of ANK expression on chondrocyte growth, matrix synthesis, and MMP-13 expression and we immunohistochemically examined ANK expression in situ in human knee OA cartilages.

RESULTS

Using cells expressing defective ANK, as well as PC-1 knockout cells, we demonstrated that ANK required PC-1 (and vice versa) to raise ecPP(i) and that the major ecPP(i) regulator TGFbeta required both ANK and PC-1 to elevate ecPP(i). Upregulation of wild-type ANK by transfection in normal chondrocytes not only raised ecPP(i) 5-fold to approximately 100nM but also directly stimulated matrix calcification and inhibited collagen and sulfated proteoglycans synthesis. In addition, upregulated ANK induced chondrocyte MMP-13, an effect that also was stimulated within 2h by treatment of chondrocytes with 100nM PP(i) alone. Finally, ANK expression was upregulated in situ in human knee OA cartilages.

CONCLUSION

Elevation of ecPP(i) by ANK critically requires the fraction of cellular PP(i) generated by PC-1. The upregulation of ANK expression in OA cartilage and the capacity of increased ANK expression to induce MMP-13 and to promote matrix loss suggest that increased ANK expression and ecPP(i) exert noxious effects in degenerative arthropathies beyond stimulation of calcification.

Steroid treatment and the development of scoliosis in males with duchenne muscular dystrophy

BACKGROUND

Scoliosis due to progressive muscle weakness occurs in almost all males with Duchenne muscular dystrophy, and it progresses relentlessly. Previous studies have shown that corticosteroid treatment slows the decline in muscle strength and stabilizes muscle strength in patients with this disease. We hypothesized that steroids may also attenuate the development of scoliosis. The purpose of this study was to compare the prevalence of scoliosis in male patients with Duchenne muscular dystrophy who received steroids with a control group of such patients who did not.

METHODS

A group of seven to ten-year-old boys with Duchenne muscular dystrophy who were able to walk were enrolled in a nonrandomized comparative study to determine the effect of deflazacort (a derivative of prednisone) on muscle strength and pulmonary function. Thirty patients were treated with deflazacort (treatment group), and twenty-four were not (control group). The patients were matched for age and pulmonary function at baseline. To assess the development of scoliosis, the patients in each group were followed for at least five years. Survival curves were plotted to determine the chance of scoliosis of >/==" BORDER="0">20 degrees developing. The difference between the groups with respect to the chance of scoliosis developing was determined with Kaplan-Meier analysis.

RESULTS

A curve of >/==" BORDER="0">20 degrees developed during the follow-up period in sixteen (67%) of the twenty-four patients in the control group but in only five (17%) of the thirty patients in the treatment group. Fifteen of the twenty-four patients in the control group underwent spine surgery, at a mean age of thirteen years, whereas only five of the thirty patients in the treatment group underwent spine surgery, at a mean age of fifteen years. Kaplan-Meier analysis demonstrated a significant difference between the two groups with regard to development of scoliosis of >/==" BORDER="0">20 degrees (p < 0.001). Cataracts developed in ten patients in the treatment group, and stress fractures developed in three patients in the treatment group. Patients in the treatment group weighed a mean of 3.7 kg more than did those in the control group.

CONCLUSIONS

Steroid treatment slows the progression of scoliosis in males with Duchenne muscular dystrophy; however, longer-term evaluation will be necessary to determine if the treatment prevents the development of scoliosis or just delays its onset. At the very least, steroid treatment delays the need for spinal surgery.

LEVEL OF EVIDENCE

Therapeutic study, Level II-1 (prospective cohort study). See Instructions to Authors for a complete description of levels of evidence.

UK community prevalence of knee chondrocalcinosis: evidence that correlation with osteoarthritis is through a shared association with osteophyte

OBJECTIVES

(1) To estimate the prevalence of chondrocalcinosis (CC) in the community and to characterise its compartmental distribution within the knee. (2) To investigate the associations between CC and individual radiographic features of osteoarthritis (OA) at the tibiofemoral joint (TFJ) and patellofemoral joint (PFJ).

METHODS

From three community questionnaire studies investigating the prevalence of knee pain, standing anteroposterior and skyline radiographs were obtained on 1727 subjects (1084 women, 643 men; mean age 63.7; 999 (58%) with knee pain). A single observer recorded the presence and site of CC and graded osteophyte and joint space narrowing (JSN) using a line atlas. "OA" was globally defined as the presence of definite osteophyte and definite JSN. Minimum joint space width (JSW) was measured to 0.1 mm with a metered dial caliper

RESULTS

(1) The crude prevalence of CC was 7.0% (95% confidence interval (CI) 5.8 to 8.2). This showed a strong association with age. The age adjusted odds ratio (aOR) for CC in women v men was 0.79 (95% CI 0.52 to 1.12). The age, sex, and knee pain standardised estimate for those aged >40 in Nottingham, UK was 4.5%. Patellofemoral CC was seen in only nine cases, all with tibiofemoral CC. (2) The age-sex aOR for the association between CC and OA was 2.08 at the PFJ (1.38 to 3.12) and 2.00 (1.11 to 3.60) at the TFJ. There was no association between measured JSW and CC at either the PFJ or TFJ. Both total osteophyte score and total number of sites with osteophyte were positively associated with CC; aOR for the upper quartile was 2.40 (1.48 to 3.90) and 1.94 (1.15 to 3.26), respectively. An association between CC and diuretic use was also demonstrated (aOR=2.07, 1.02 to 4.19).

CONCLUSIONS

In this large UK community study the age, sex, and knee pain adjusted prevalence of CC was 4.5%. There was a strong age association, but no sex predisposition. Patellofemoral CC was uncommon. An association between OA and CC was confirmed, but this appears to operate through an association with osteophyte rather than JSN. The new association between CC and diuretic use might theoretically be explained by diuretic induced hypomagnesaemia.

Mutations in ANKH cause chondrocalcinosis

Chondrocalcinosis (CC) is a common cause of joint pain and arthritis that is caused by the deposition of calcium-containing crystals within articular cartilage. Although most cases are sporadic, rare familial forms have been linked to human chromosomes 8 (CCAL1) or 5p (CCAL2) (Baldwin et al. 1995; Hughes et al. 1995; Andrew et al. 1999). Here, we show that two previously described families with CCAL2 have mutations in the human homolog of the mouse progressive ankylosis gene (ANKH). One of the human mutations results in the substitution of a highly conserved amino acid residue within a predicted transmembrane segment. The other creates a new ATG start site that adds four additional residues to the ANKH protein. Both mutations segregate completely with disease status and are not found in control subjects. In addition, 1 of 95 U.K. patients with sporadic CC showed a deletion of a single codon in the ANKH gene. The same change was found in a sister who had bilateral knee replacement for osteoarthritis. Each of the three human mutations was reconstructed in a full-length ANK expression construct previously shown to regulate pyrophosphate levels in cultured cells in vitro. All three of the human mutations showed significantly more activity than a previously described nonsense mutation that causes severe hydroxyapatite mineral deposition and widespread joint ankylosis in mice. These results suggest that small sequence changes in ANKH are one cause of CC and joint disease in humans. Increased ANK activity may explain the different types of crystals commonly deposited in human CCAL2 families and mutant mice and may provide a useful pharmacological target for treating some forms of human CC.

"Pseudoseptic" pseudogout associated with hypomagnesemia in liver transplant patients

Hypomagnesemia has been associated with deposition of calcium pyrophosphate dihydrate crystals in articular structures, causing pseudogout, also known as calcic gout. Occasionally, pseudogout may mimic septic arthritis; this "pseudoseptic" attack may be of especial concern in the immunocompromised host, such as transplant recipient patients, who may be indeed at risk of developing septic arthritis. We report the cases of two patients in whom pseudogout developed after liver transplantation. Synovial fluid appearance and leukocyte counting in synovial fluid mimicked septic arthritis, but calcium pyrophosphate dihydrate crystals were observed. Magnesium depletion before transplantation and further tacrolimus-induced renal magnesium leakage were probably working in these patients.

Iron accumulation in tissues of magnesium-deficient rats with dietary iron overload

The mineral imbalances in magnesium-deficient rats with dietary iron overload were studied. Forty-four male Wistar rats were divided into six groups and fed six diets, two by three, fully crossed: magnesium adequate or deficient, and iron deficient, adequate, or excess. The concentrations of iron, magnesium, calcium, and phosphorus in tissues of the rats were measured. The results were as follows: (1) The excess iron intake reinforced the iron accumulation in liver and spleen of magnesium deficient rats; (2) The saturation of iron binding capacity was enormously elevated in the magnesium deficient rats fed excess iron; and (3) Dietary iron deprivation diminished the degree of calcium deposition in kidney of magnesium deficient rats. These results suggest that magnesium-deprived rats have abnormal iron metabolism losing homeostatic regulation of plasma iron, and magnesium deficient rats with dietary iron overload may be used as an experimental hemochromatosis model.

Tumoral calcium pyrophosphate dihydrate deposition disease mimicking a cervical spine neoplasm: case report

A case of tumoral calcium pyrophosphate dihydrate crystal deposition disease involving the upper cervical spine is reported. It presented clinically, radiographically, and by preliminary intraoperative pathological evaluation as a possible malignant soft tissue tumor. An aggressive resection of the lesion was performed. This case differs from previous reports of calcium pyrophosphate dihydrate crystal deposition disease of the cervical spine by the size, location, and radiographic appearance of the lesion. We suggest that radical surgical procedures should not be performed without consideration of this diagnosis in lesions with similar presentations. A brief review of spinal and tumoral calcium pyrophosphate dihydrate crystal deposition is presented.

Causal link between nucleotide pyrophosphohydrolase overactivity and increased intracellular inorganic pyrophosphate generation demonstrated by transfection of cultured fibroblasts and osteoblasts with plasma cell membrane glycoprotein-1. Relevance to calcium pyrophosphate dihydrate deposition disease

OBJECTIVE

In subjects with idiopathic calcium pyrophosphate dihydrate (CPPD) deposition disease, cartilage chondrocytes elaborate increased amounts of PPi. The mechanism of the intracellular PPi elevation is not known. Plasma membrane 5'-nucleotide phosphodiesterase I/nucleotide pyrophosphohydrolase (NTPPPH) activity also is elevated in chondrocytes and dermal fibroblasts of patients with idiopathic CPPD deposition disease. NTPPPH, as an ecto-enzyme, could act within certain intracellular compartments. Thus, we hypothesized a potential causal link between increased NTPPPH activity and increased intracellular PPi.

METHODS

Transformed simian fibroblasts (COS cells) and human osteoblasts (U2OS cells) were transfected with the 5'-nucleotide phosphodiesterase I ecto-enzyme plasma cell membrane glycoprotein-1 (PC-1), recently shown to be expressed in cartilage, osteoblasts, and fibroblasts.

RESULTS

Transfection with PC-1 markedly up-regulated 5'-nucleotode phosphodiesterase I activity and increased intracellular PPi concentrations by increasing the capacity of cells to generate PPi. Importantly, this did not require supplementation with exogenous nucleotides.

CONCLUSION

Cellular overexpression of PC-1 produces NTPPPH overactivity and increased intracellular PPi generation in vitro. These findings support the potential importance of NTPPPH overactivity in PPi generation, both inside and outside the cell, in some subjects with CPPD deposition disease.

Diseases associated with calcium pyrophosphate deposition disease

Although many metabolic and endocrine diseases have been reported to predispose to calcium pyrophosphate dihydrate crystal deposition, the validity of many of these associations remains unclear. A critical review of the literature relating to these associations, with illustrative cases and data derived from the authors' own experience, is presented. It is concluded that there is good evidence to associate hypophosphatasia, hypomagnesemia, and hyperparathyroidism with chondrocalcinosis and acute attacks of "pseudogout." Meta-analysis also suggests a small but significant association between hypothyroidism and chondrocalcinosis. Hemochromatosis stands alone in clearly associating not only with chondrocalcinosis but also with structural change and chronic arthropathy. The biochemical mechanisms that may produce these various associations are discussed. Recommendations are made concerning appropriate screening for metabolic and endocrine disease in patients with chondrocalcinosis.