Autosomal dominant familial calcium pyrophosphate dihydrate deposition disease is caused by mutation in the transmembrane protein ANKH

Vascular Calcification: Key Roles of Phosphate and Pyrophosphate

Cardiovascular complications due to accelerated arterial stiffening and atherosclerosis are the leading cause of morbimortality in Western society. Both pathologies are frequently associated with vascular calcification. Pathologic calcification of cardiovascular structures, or vascular calcification, is associated with several diseases (for example, genetic diseases, diabetes, and chronic kidney disease) and is a common consequence of aging. Calcium phosphate deposition, mainly in the form of hydroxyapatite, is the hallmark of vascular calcification and can occur in the medial layer of arteries (medial calcification), in the atheroma plaque (intimal calcification), and cardiac valves (heart valve calcification). Although various mechanisms have been proposed for the pathogenesis of vascular calcification, our understanding of the pathogenesis of calcification is far from complete. However, in recent years, some risk factors have been identified, including high serum phosphorus concentration (hyperphosphatemia) and defective synthesis of pyrophosphate (pyrophosphate deficiency). The balance between phosphate and pyrophosphate, strictly controlled by several genes, plays a key role in vascular calcification. This review summarizes the current knowledge concerning phosphate and pyrophosphate homeostasis, focusing on the role of extracellular pyrophosphate metabolism in aortic smooth muscle cells and macrophages.

The combined prevalence of classified rare rheumatic diseases is almost double that of ankylosing spondylitis

BACKGROUND

Rare diseases (RDs) affect less than 5/10,000 people in Europe and fewer than 200,000 individuals in the United States. In rheumatology, RDs are heterogeneous and lack systemic classification. Clinical courses involve a variety of diverse symptoms, and patients may be misdiagnosed and not receive appropriate treatment. The objective of this study was to identify and classify some of the most important RDs in rheumatology. We also attempted to determine their combined prevalence to more precisely define this area of rheumatology and increase awareness of RDs in healthcare systems. We conducted a comprehensive literature search and analyzed each disease for the specified criteria, such as clinical symptoms, treatment regimens, prognoses, and point prevalences. If no epidemiological data were available, we estimated the prevalence as 1/1,000,000. The total point prevalence for all RDs in rheumatology was estimated as the sum of the individually determined prevalences.

RESULTS

A total of 76 syndromes and diseases were identified, including vasculitis/vasculopathy (n = 15), arthritis/arthropathy (n = 11), autoinflammatory syndromes (n = 11), myositis (n = 9), bone disorders (n = 11), connective tissue diseases (n = 8), overgrowth syndromes (n = 3), and others (n = 8). Out of the 76 diseases, 61 (80%) are classified as chronic, with a remitting-relapsing course in 27 cases (35%) upon adequate treatment. Another 34 (45%) diseases were predominantly progressive and difficult to control. Corticosteroids are a therapeutic option in 49 (64%) syndromes. Mortality is variable and could not be determined precisely. Epidemiological studies and prevalence data were available for 33 syndromes and diseases. For an additional eight diseases, only incidence data were accessible. The summed prevalence of all RDs was 28.8/10,000.

CONCLUSIONS

RDs in rheumatology are frequently chronic, progressive, and present variable symptoms. Treatment options are often restricted to corticosteroids, presumably because of the scarcity of randomized controlled trials. The estimated combined prevalence is significant and almost double that of ankylosing spondylitis (18/10,000). Thus, healthcare systems should assign RDs similar importance as any other common disease in rheumatology.

Multifocal Calcific Periarthritis with Distinctive Clinical and Radiological Features in Patients with CD73 Deficiency

OBJECTIVE

Arterial calcification due to deficiency of CD73 (ACDC) is a hereditary autosomal recessive ectopic mineralization syndrome caused by loss-of-function mutations in the 5'-nucleotidase Ecto (NT5E) gene. Periarticular calcification has been reported but the clinical characterization of arthritis as well as the microstructure and chemical composition of periarticular calcifications and synovial fluid crystals has not been systematically investigated.

METHODS

Eight ACDC patients underwent extensive rheumatological and radiological evaluation over a period of 11 years. Periarticular and synovial biopsies were obtained from four patients. Characterization of crystal composition was evaluated by compensated polarized light microscopy, Alizarin red staining for synovial fluid along with x-ray diffraction and x-ray micro tomosynthesis for periarticular calcification.

RESULTS

Arthritis in ACDC patients has a clinical presentation of mixed erosive-degenerative joint changes with a median onset of articular symptoms at 17 years of age and progresses over time to the development of fixed deformities and functional limitations of small peripheral joints with eventually, larger joint and distinct axial involvement later in life. We have identified calcium pyrophosphate (CPP) and calcium hydroxyapatite (CHA) crystals in synovial fluid specimens and determined that CHA crystals are the principal component of periarticular calcifications.

CONCLUSION

This is the largest study in ACDC patients to describe erosive peripheral arthropathy and axial enthesopathic calcifications over a period of 11 years and the first to identify the composition of periarticular calcifications and synovial fluid crystals. ACDC should be considered among the genetic causes of early-onset osteoarthritis, as musculoskeletal disease signs may often precede vascular symptoms.

Differences in intracellular localisation of ANKH mutants that relate to mechanisms of calcium pyrophosphate deposition disease and craniometaphyseal dysplasia

ANKH mutations are associated with calcium pyrophosphate deposition disease and craniometaphyseal dysplasia. This study investigated the effects of these ANKH mutants on cellular localisation and associated biochemistry. We generated four ANKH overexpression-plasmids containing either calcium pyrophosphate deposition disease or craniometaphyseal dysplasia linked mutations: P5L, E490del and S375del, G389R. They were transfected into CH-8 articular chondrocytes and HEK293 cells. The ANKH mutants dynamic differential localisations were imaged and we investigated the interactions with the autophagy marker LC3. Extracellular inorganic pyrophosphate, mineralization, ENPP1 activity expression of ENPP1, TNAP and PIT-1 were measured. P5L delayed cell membrane localisation but once recruited into the membrane it increased extracellular inorganic pyrophosphate, mineralization, and ENPP1 activity. E490del remained mostly cytoplasmic, forming punctate co-localisations with LC3, increased mineralization, ENPP1 and ENPP1 activity with an initial but unsustained increase in TNAP and PIT-1. S375del trended to decrease extracellular inorganic pyrophosphate, increase mineralization. G389R delayed cell membrane localisation, trended to decrease extracellular inorganic pyrophosphate, increased mineralization and co-localised with LC3. Our results demonstrate a link between pathological localisation of ANKH mutants with different degrees in mineralization. Furthermore, mutant ANKH functions are related to synthesis of defective proteins, inorganic pyrophosphate transport, ENPP1 activity and expression of ENPP1, TNAP and PIT-1.

New insights into endogenous mechanisms of protection against arterial calcification

Cardiovascular complications due to accelerated atherosclerosis and arterial stiffening are the leading cause of morbidity and mortality in the Western society. Both pathologies are frequently associated with vascular calcification. Deposits of calcium phosphate salts, mainly in form of hydroxyapatite, is the hallmark of vascular calcification. Calcification is frequently observed in atherosclerotic lesions (intimal calcification) associated with vascular smooth muscle cells (VSMCs) and macrophages. By contrast, medial calcification, occurring in the elastic region of the arteries, is almost exclusively associated with VSMCs, and is common in arteriosclerosis related to aging, diabetes, and chronic kidney disease. In extracellular fluids, a range of endogenous low- and high-molecular weight calcification inhibitors are present, including osteopontin, matrix-Gla proteins and Fetuin A. Moreover, pyrophosphate deficiency plays a key role in vascular calcification. Pyrophosphate is produced by extracellular hydrolysis of ATP and is degraded to phosphate by tissue non-specific alkaline phosphatase. Loss of function in the enzymes and transporters involved in the extracellular pyrophosphate metabolism leads to excessive deposition of calcium-phosphate salts. This review summarizes the current knowledge about endogenous mechanisms of protection against calcification in the aortic wall, focusing on the role of extracellular pyrophosphate metabolism in vascular smooth muscle cells and macrophages.

NLRP3 inflammasome activation in inflammaging

The process of aging is associated with the appearance of low-grade subclinical inflammation, termed inflammaging, that can accelerate age-related diseases. In Western societies the age-related inflammatory response can additionally be aggravated by an inflammatory response related to modern lifestyles and excess calorie consumption, a pathophysiologic inflammatory response that was coined metaflammation. Here, we summarize the current knowledge of mechanisms that drive both of these processes and focus our discussion the emerging concept that a key innate immune pathway, the NLRP3 inflammasome, is centrally involved in the recognition of triggers that appear during physiological aging and during metabolic stress. We further discuss how these processes are involved in the pathogenesis of common age-related pathologies and highlight potential strategies by which the detrimental inflammatory responses could be pharmacologically addressed.

Structural biology of solute carrier (SLC) membrane transport proteins

The human solute carriers (SLCs) comprise over 400 different transporters, organized into 65 families ( http://slc.bioparadigms.org/ ) based on their sequence homology and transport function. SLCs are responsible for transporting extraordinarily diverse solutes across biological membranes, including inorganic ions, amino acids, lipids, sugars, neurotransmitters and drugs. Most of these membrane proteins function as coupled symporters (co-transporters) utilizing downhill ion (H⁺ or Na⁺) gradients as the driving force for the transport of substrate against its concentration gradient into cells. Other members work as antiporters (exchangers) that typically contain a single substrate-binding site with an alternating access mode of transport, while a few members exhibit channel-like properties. Dysfunction of SLCs is correlated with numerous human diseases and therefore they are potential therapeutic drug targets. In this review, we identified all of the SLC crystal structures that have been determined, most of which are from prokaryotic species. We further sorted all the SLC structures into four main groups with different protein folds and further discuss the well-characterized MFS (major facilitator superfamily) and LeuT (leucine transporter) folds. This review provides a systematic analysis of the structure, molecular basis of substrate recognition and mechanism of action in different SLC family members.

Role of pyrophosphate in vascular calcification in chronic kidney disease

Vascular calcification is a pathology characterized by the deposition of calcium-phosphate in cardiovascular structures, mainly in the form of hydroxyapatite crystals, resulting in ectopic calcification. It is correlated with increased risk of cardiovascular disease and myocardial infarction in diabetic patients and in those with chronic kidney disease (CKD). Vascular smooth muscle cells are sensitive to changes in inorganic phosphate (Pi) levels. They are able to adapt and modify some of their functions and promote changes which trigger calcification. Pi is regulated by parathyroid hormone and 1,25-dihydroxyvitamin D. Changes in the transport of Pi are the primary factor responsible for the regulation of Pi homeostasis and the calcification process. Synthesis of calcification inhibitors is the main mechanism by which cells are able to prevent vascular calcification. Extracellular pyrophosphate (PPi) is a potent endogenous inhibitor of calcium-phosphate deposition both in vivo and in vitro. Patients with CKD show lower levels of PPi and increased activity of the enzyme alkaline phosphatase. Numerous enzymes implicated in the metabolism of PPi have been associated with vascular calcifications. PPi is synthesized from extracellular ATP by nucleotide pyrophosphatase/phosphodiesterase from extracellular ATP hydrolysis. PPi is hydrolyzed into Pi by tissue-nonspecific alkaline phosphatase. ATP can be hydrolyzed to Pi via the ectonucleoside triphosphate diphosphohydrolase family. All these enzymes must be in balance, thereby preventing calcifications. However, diseases like CKD or diabetes induce alterations in their levels. Administration of PPi could open up new treatment options for these patients.

Combined approach for finding susceptibility genes in DISH/chondrocalcinosis families: whole-genome-wide linkage and IBS/IBD studies

Twelve families with exuberant and early-onset calcium pyrophosphate dehydrate chondrocalcinosis (CC) and diffuse idiopathic skeletal hyperostosis (DISH), hereafter designated DISH/CC, were identified in Terceira Island, the Azores, Portugal. Ninety-two (92) individuals from these families were selected for whole-genome-wide linkage analysis. An identity-by-descent (IBD) analysis was performed in 10 individuals from 5 of the investigated pedigrees. The chromosome area with the maximal logarithm of the odds score (1.32; P=0.007) was not identified using the IBD/identity-by-state (IBS) analysis; therefore, it was not investigated further. From the IBD/IBS analysis, two candidate genes, LEMD3 and RSPO4, were identified and sequenced. Nine genetic variants were identified in the RSPO4 gene; one regulatory variant (rs146447064) was significantly more frequent in control individuals than in DISH/CC patients (P=0.03). Four variants were identified in LEMD3, and the rs201930700 variant was further investigated using segregation analysis. None of the genetic variants in RSPO4 or LEMD3 segregated within the studied families. Therefore, although a major genetic effect was shown to determine DISH/CC occurrence within these families, the specific genetic variants involved were not identified.

Disruption of biomineralization pathways in spinal tissues of a mouse model of diffuse idiopathic skeletal hyperostosis

Equilibrative nucleoside transporter 1 (ENT1) mediates passage of adenosine across the plasma membrane. We reported previously that mice lacking ENT1 (ENT1(-/-)) exhibit progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis (DISH) in humans. Here, we investigated mechanisms underlying aberrant mineralization in ENT1(-/-) mice. Micro-CT revealed ectopic mineralization of spinal tissues in both male and female ENT1(-/-) mice, involving the annulus fibrosus of the intervertebral discs (IVDs) of older mice. IVDs were isolated from wild-type and ENT1(-/-) mice at 2months of age (prior to disc mineralization), 4, and 6months of age (disc mineralization present) and processed for real-time PCR, cell isolation, or histology. Relative to the expression of ENTs in other tissues, ENT1 was the primary nucleoside transporter expressed in wild-type IVDs and mediated the functional uptake of [(3)H]2-chloroadenosine by annulus fibrosus cells. No differences in candidate gene expression were detected in IVDs from ENT1(-/-) and wild-type mice at 2 or 4months of age. However, at 6months of age, expression of genes that inhibit biomineralization Mgp, Enpp1, Ank, and Spp1 were reduced in IVDs from ENT1(-/-) mice. To assess whether changes detected in ENT1(-/-) mice were cell autonomous, annulus fibrosus cell cultures were established. Compared to wild-type cells, cells isolated from ENT1(-/-) IVDs at 2 or 6months of age demonstrated greater activity of alkaline phosphatase, a promoter of biomineralization. Cells from 2-month-old ENT1(-/-) mice also showed greater mineralization than wild-type. Interestingly, altered localization of alkaline phosphatase activity was detected in the inner annulus fibrosus of ENT1(-/-) mice in vivo. Alkaline phosphatase activity, together with the marked reduction in mineralization inhibitors, is consistent with the mineralization of IVDs seen in ENT1(-/-) mice at older ages. These findings establish that both cell-autonomous and systemic mechanisms contribute to ectopic mineralization in ENT1(-/-) mice.

Matrix stiffness promotes cartilage endplate chondrocyte calcification in disc degeneration via miR-20a targeting ANKH expression

The mechanical environment is crucial for intervertebral disc degeneration (IDD). However, the mechanisms underlying the regulation of cartilage endplate (CEP) calcification by altered matrix stiffness remain unclear. In this study, we found that matrix stiffness of CEP was positively correlated with the degree of IDD, and stiff matrix, which mimicked the severe degeneration of CEP, promoted inorganic phosphate-induced calcification in CEP chondrocytes. Co-expression analysis of the miRNA and mRNA profiles showed that increasing stiffness resulted in up-regulation of miR-20a and down-regulation of decreased ankylosis protein homolog (ANKH) during inorganic phosphate-induced calcification in CEP chondrocytes. Through a dual luciferase reporter assay, we confirmed that miR-20a directly targets 3'-untranslated regions of ANKH. The inhibition of miR-20a attenuated the calcium deposition and calcification-related gene expression, whereas the overexpression of miR-20a enhanced calcification in CEP chondrocytes on stiff matrix. The rescue of ANKH expression restored the decreased pyrophosphate efflux and inhibited calcification. In clinical samples, the levels of ANKH expression were inversely associated with the degeneration degree of CEP. Thus, our findings demonstrate that the miR-20a/ANKH axis mediates the stiff matrix- promoted CEP calcification, suggesting that miR-20a and ANKH are potential targets in restraining the progression of IDD.

Pyrophosphate: a key inhibitor of mineralisation

Inorganic pyrophosphate has long been known as a by-product of many intracellular biosynthetic reactions, and was first identified as a key endogenous inhibitor of biomineralisation in the 1960s. The major source of pyrophosphate appears to be extracellular ATP, which is released from cells in a controlled manner. Once released, ATP can be rapidly hydrolysed by ecto-nucleotide pyrophosphatase/phosphodiesterases to produce pyrophosphate. The main action of pyrophosphate is to directly inhibit hydroxyapatite formation thereby acting as a physiological 'water-softener'. Evidence suggests pyrophosphate may also act as a signalling molecule to influence gene expression and regulate its own production and breakdown. This review will summarise our current understanding of pyrophosphate metabolism and how it regulates bone mineralisation and prevents harmful soft tissue calcification.

Vascular Calcification Revisited: A New Perspective for Phosphate Transport

Elevated serum phosphorus has emerged as a key risk factor for pathologic calcification of cardiovascular structures, or vascular calcification (VC). To prevent the formation of calciumphosphate deposits (CPD), the body uses adenosine-5’-triphosphate (ATP) to synthesize inhibitors of calcification, including proteins and inhibitors of low molecular weight. Extracellular pyrophosphate (PPi) is a potent inhibitor of VC, which is produced during extracellular hydrolysis of ATP. Loss of function in the enzymes and transporters that are involved in the cycle of extracellular ATP, including Pi transporters, leads to excessive deposition of calcium-phosphate salts. Treatment of hyperphosphatemia with Pi-binders and Injection of exogenous PPi are the effective treatments to prevent CPD in the aortic wall. The role of sodium phosphate cotransporters in ectopic calcification is contradictory and not well defined, but their important role in the control of intracellular Pi levels and the synthesis of ATP make them an important target to study.

Inflammation: a culprit for vascular calcification in atherosclerosis and diabetes

It is today acknowledged that aging is associated with a low-grade chronic inflammatory status, and that inflammation exacerbates age-related diseases such as osteoporosis, Alzheimer's disease, atherosclerosis and type 2 diabetes mellitus (T2DM). Vascular calcification is a complication that also occurs during aging, in particular in association with atherosclerosis and T2DM. Recent studies provided compelling evidence that vascular calcification is associated with inflammatory status and is enhanced by inflammatory cytokines. In the present review, we propose on one hand to highlight the most important and recent findings on the cellular and molecular mechanisms of vascular inflammation in atherosclerosis and T2DM. On the other hand, we will present the effects of inflammatory mediators on the trans-differentiation of vascular smooth muscle cell and on the deposition of crystals. Since vascular calcification significantly impacts morbidity and mortality in affected individuals, a better understanding of its induction and development will pave the way to develop new therapeutic strategies.

The association between ANKH promoter polymorphism and chondrocalcinosis is independent of age and osteoarthritis: results of a case-control study

Introduction

Chondrocalcinosis (CC) most commonly results from calcium pyrophosphate crystal deposition (CPPD). The objective of this study is to examine the association between candidate single-nucleotide polymorphisms (SNPs) and radiographic CC.

Methods

SNPs in ankylosis human (ANKH), high ferritin (HFE), tissue non-specific alkaline phosphatase (TNAP), ecto-neucleotide pyrophosphatase 1 (ENPP1), and transferrin (TE) genes were genotyped in participants of the Genetics of Osteoarthritis and Lifestyle (GOAL) and Nottingham Osteoarthritis Case-Control studies. Adjusted genotype odds ratio (aOR_GENOTYPE), the OR for association between one additional minor allele and CC, was calculated and adjusted for age, gender, body mass index (BMI), and osteoarthritis (OA) by using binary logistic regression. Statistical significance was set at P ≤0.003 after Bonferroni correction for multiple tests.

Results

The -4bpG > A polymorphism in the 5′ untranslated region (5′ UTR) of ANKH associated with CC after Bonferroni correction. This was independent of age, gender, OA, and BMI; aOR_GENOTYPE (95% confidence interval, or CI) was 1.39 (1.14-1.69) (P = 0.001). rs3045 and rs875525, two other SNPs in ANKH, associated with CC; aOR_GENOTYPE (95% CI) values were 1.31 (1.09-1.58) (P = 0.005) and 1.18 (1.03-1.35) (P = 0.015), respectively; however, this was non-significant after Bonferroni correction.

Conclusions

This study validates the association between a functional polymorphism in the 5′ UTR of ANKH and CC and shows for the first time that this is independent of age and OA – the two key risk factors for CC. It shows that other SNPs in ANKH may also associate with CC. This supports the role of extracellular inorganic pyrophosphate in the pathogenesis of CC. The findings of this hospital-based study require replication in a community-based population.

Loss of equilibrative nucleoside transporter 1 in mice leads to progressive ectopic mineralization of spinal tissues resembling diffuse idiopathic skeletal hyperostosis in humans

Diffuse idiopathic skeletal hyperostosis (DISH) is a noninflammatory spondyloarthropathy, characterized by ectopic calcification of spinal tissues. Symptoms include spine pain and stiffness, and in severe cases dysphagia and spinal cord compression. The etiology of DISH is unknown and there are no specific treatments. Recent studies have suggested a role for purine metabolism in the regulation of biomineralization. Equilibrative nucleoside transporter 1 (ENT1) transfers hydrophilic nucleosides, such as adenosine, across the plasma membrane. In mice lacking ENT1, we observed the development of calcified lesions resembling DISH. By 12 months of age, ENT1(-/-) mice exhibited signs of spine stiffness, hind limb dysfunction, and paralysis. Micro-computed tomography (µCT) revealed ectopic mineralization of paraspinal tissues in the cervical-thoracic region at 2 months of age, which extended to the lumbar and caudal regions with advancing age. Energy-dispersive X-ray microanalysis of lesions revealed a high content of calcium and phosphorus with a ratio similar to that of cortical bone. At 12 months of age, histological examination of ENT1(-/-) mice revealed large, irregular accumulations of eosinophilic material in paraspinal ligaments and entheses, intervertebral discs, and sternocostal articulations. There was no evidence of mineralization in appendicular joints or blood vessels, indicating specificity for the axial skeleton. Plasma adenosine levels were significantly greater in ENT1(-/-) mice than in wild-type, consistent with loss of ENT1--a primary adenosine uptake pathway. There was a significant reduction in the expression of Enpp1, Ank, and Alpl in intervertebral discs from ENT1(-/-) mice compared to wild-type mice. Elevated plasma levels of inorganic pyrophosphate in ENT1(-/-) mice indicated generalized disruption of pyrophosphate homeostasis. This is the first report of a role for ENT1 in regulating the calcification of soft tissues. Moreover, ENT1(-/-) mice may be a useful model for investigating pathogenesis and evaluating therapeutics for the prevention of mineralization in DISH and related disorders.

ATP and arterial calcification

BACKGROUND

Arterial calcification (AC) is a major health problem associated with extreme morbidity and a shortened survival. It is currently without any effective treatment. ATP and the purinergic system in general are now emerging as being important in the pathogenesis of AC and potentially provide a new focus for novel therapies.

METHODS

This review systematically analyses and discusses the current literature examining the relevance of the purinergic system to AC. Particular emphasis is given to the enzymes associated with ATP metabolism and their role in maintaining a balance between promotion and inhibition of arterial mineralization. Points of controversy are highlighted, and areas for future research are suggested.

CONCLUSION

The potential roles of ATP and the purinergic system in AC are beginning to be elucidated. While further work is necessary, current knowledge suggests that several components of the purinergic system could be targeted to develop new treatments for AC.

Novel ANKH amino terminus mutation (Pro5Ser) associated with early-onset calcium pyrophosphate disease with associated phosphaturia

This report describes a 32-year-old woman presenting since childhood with progressive calcium pyrophosphate disease (CPPD), characterized by severe arthropathy and chondrocalcinosis involving multiple peripheral joints and intervertebral disks. Because ANKH mutations have been previously described in familial CPPD, the proband's DNA was assessed at this locus by direct sequencing of promoter and coding regions and revealed 3 sequence variants in ANKH. Sequences of exon 1 revealed a novel isolated nonsynonymous mutation (c.13 C>T), altering amino acid in codon 5 from proline to serine (CCG>TCG). Sequencing of parental DNA revealed an identical mutation in the proband's father but not the mother. Subsequent clinical evaluation demonstrated extensive chondrocalcinosis and degenerative arthropathy in the proband's father. In summary, we report a novel mutation, not previously described, in ANKH exon 1, wherein serine replaces proline, in a case of early-onset severe CPPD associated with metabolic abnormalities, with similar findings in the proband's father.

Treatment and management of pseudogout: insights for the clinician

Pseudogout and the associated calcium pyrophosphate dihydrate (CPPD)- crystal-related arthropathies are common conditions that present particular management problems in clinical practice as they often affect older patients with multiple medical comorbidities. The epidemiology, metabolic and endocrine disease associations, and routine investigations used in the diagnostic workup are briefly reviewed. Current treatment approaches that are mainly directed at relieving the symptoms of joint inflammation are outlined. Unlike gout, there are no agents available that have been shown to decrease crystal load in CPPD-related joint disease. Recent novel insights into the pathogenesis of crystal-induced joint inflammation and subsequent joint degeneration are also discussed. The potential of colchicine as a prophylactic agent in managing recurrent attacks and the likely mechanisms of its effects on the NACHT, LRR and PYD domains-containing protein 3 (NALP-3) inflammasome of the innate immune system are highlighted. The use of agents that directly target the inflammasome, in particular drugs which inhibit the interleukin 1 pathway, in the treatment of severe, refractory pseudogout is also discussed. Finally, there is particular emphasis on the likely pathogenic role of CPPD crystal deposition in degenerative joint disease and the use of targeted anticrystal therapies as potential disease-modifying drugs.

Genetics and mechanisms of crystal deposition in calcium pyrophosphate deposition disease

Calcium pyrophosphate deposition (CPPD) disease (common in older adults) can be asymptomatic, associated with osteoarthritis, or can present as acute/chronic inflammatory arthritis. Due to the phenotypic complexity of CPPD, the European League Against Rheumatism (EULAR) recently made recommendations on terminology, diagnosis, and management based on available research evidence and expert consensus. There are no disease-modifying treatments for CPPD disease, and therapy remains nonspecific with the use of anti-inflammatory and analgesic drugs. For years, it has been known that inorganic phosphate and pyrophosphate regulate the formation of CPP or hydroxyapatite crystals. The discovery of ANKH (human homologue of progressive ankylosis) mutations in familial CPPD disease confirmed the importance of phosphate/pyrophosphate homeostasis in CPPD, with ANKH being a regulator of inorganic pyrophosphate transport. Despite progress in our understanding of the function of ANKH, much remains to be investigated. This review summarizes the genetic basis of this disease and focuses on the challenges of research in this area.

Phosphate/pyrophosphate and MV-related proteins in mineralisation: discoveries from mouse models

During the process of matrix vesicle (MV)-mediated initiation of mineralisation, chondrocytes and osteoblasts mineralise the extracellular matrix by promoting the seeding of basic calcium phosphate crystals of hydroxyapatite (HA) along the collagen fibrils. This orchestrated process is carefully regulated by the balanced action of propagators and inhibitors of calcification. The primary antagonistic regulators of extracellular matrix mineralisation are phosphate (Pi) and inorganic pyrophosphate (PPi). Studies in mouse models and in humans have established critical roles for Pi/PPi homeostasis in biomineralisation. In this review, we present the regulators of Pi/PPi, as derived from animal models, and discuss their clinical relevance to physiological and pathological mineralisation.

Activation of nuclear factor-kappa B accelerates vascular calcification by inhibiting ankylosis protein homolog expression

Vascular calcification is a major risk factor of cardiovascular mortality, particularly for patients with end-stage renal disease and diabetes. Although chronic inflammation is one of the etiologic factors, the underlying mechanism is not fully understood. To clarify this, we studied how nuclear factor-kappa B (NF-κB) induction, a mediator of inflammation, might promote vascular calcification. Activation of NF-κB by tumor necrosis factor (TNF) promoted inorganic phosphate-induced calcification in human aortic smooth muscle cells. Pyrophosphate (an inhibitor of calcification) efflux to the extracellular matrix was suppressed along with the decreased expression of ankylosis protein homolog (ANKH), a transmembrane protein that controls pyrophosphate efflux of cells. The restoration of ANKH expression in these cells overcame the decreased pyrophosphate efflux and calcification. Tristetraprolin, a downstream product of NF-κB activation, may mediate destablization of ANKH mRNA since its knockdown by shRNA increased ANKH expression and decreased calcification. Furthermore, a rat chronic renal failure model, with increased serum TNF levels, activated NF-κB and decreased ANKH levels. In contrast, the inhibition of NF-κB maintained ANKH expression and attenuated vascular calcification both in vivo and in vitro. Both human calcified atherosclerotic lesions and arteries from patients with chronic kidney disease had activated NF-κB and decreased ANKH expression. Thus, TNF-activated NF-κB promotes inflammation-accelerated vascular calcification by inhibiting ankylosis protein homolog expression and consequent pyrophosphate secretion.

ANKH and susceptibility to and severity of ankylosing spondylitis

OBJECTIVE

Unconfirmed reports describe association of ankylosing spondylitis (AS) with several candidate genes including ANKH. Cellular export of inorganic pyrophosphate is regulated by the ANK protein, and mutant mice (ank/ank), which have a premature stop codon in the 3' end of the ank gene, develop severe ankylosis. We tested the association between single-nucleotide polymorphisms (SNP) in these genes and susceptibility to AS in a population of patients with AS. We investigated the role of these genes in terms of functional (BASFI) and metrological (BASMI) measures, and the association with radiological severity (mSASSS).

METHODS

Our study was conducted on 355 patients with AS and 95 ethnically matched healthy controls. AS was defined according to the modified New York criteria. Four SNP in ANKH (rs27356, rs26307, rs25957, and rs28006) were genotyped. Association analysis was performed using Cochrane-Armitage and linear regression tests for dichotomous and quantitative variables. Analyses of Bath Ankylosing Spondylitis Disease Activity Index (BASDAI), BASFI, and mSASSS were controlled for sex and disease duration.

RESULTS

None of the 4 markers showed significant single-locus disease associations (p > 0.05), suggesting that ANKH was not a major determinant of AS susceptibility in our population. No association was observed between these SNP and age at symptom onset, BASDAI, BASFI, BASMI, or mSASSS.

CONCLUSION

These results confirm data in white Europeans that ANKH is probably not a major determinant of susceptibility to AS. ANKH polymorphisms do not markedly influence AS disease severity, as measured by BASMI and mSASSS.

[Crystal arthropathies]

Crystal arthropathies represent a heterogenic group of skeletal diseases associated with the deposition of mineralised material within joints and periarticular soft tissues. Gout is the most common and pathogenetically best understood crystal arthropathy, followed by basic calcium phosphate and calcium pyrophosphate dihydrate deposition diseases, and, in very rare cases, calcium oxalate crystal arthropathy. These crystals are responsible for different rheumatic syndromes, including acute or chronic synovial inflammation, and also contribute to cartilage degeneration. This review gives an overview of the pathological and clinical changes of these arthropathies.

Bisphosphonates: the first 40 years

The first full publications on the biological effects of the diphosphonates, later renamed bisphosphonates, appeared in 1969, so it is timely after 40years to review the history of their development and their impact on clinical medicine. This special issue of BONE contains a series of review articles covering the basic science and clinical aspects of these drugs, written by some of many scientists who have participated in the advances made in this field. The discovery and development of the bisphosphonates (BPs) as a major class of drugs for the treatment of bone diseases has been a fascinating story, and is a paradigm of a successful journey from 'bench to bedside'. Bisphosphonates are chemically stable analogues of inorganic pyrophosphate (PPi), and it was studies on the role of PPi as the body's natural 'water softener' in the control of soft tissue and skeletal mineralisation that led to the need to find inhibitors of calcification that would resist hydrolysis by alkaline phosphatase. The observation that PPi and BPs could not only retard the growth but also the dissolution of hydroxyapatite crystals prompted studies on their ability to inhibit bone resorption. Although PPi was unable to do this, BPs turned out to be remarkably effective inhibitors of bone resorption, both in vitro and in vivo experimental systems, and eventually in humans. As ever more potent BPs were synthesised and studied, it became apparent that physico-chemical effects were insufficient to explain their biological effects, and that cellular actions must be involved. Despite many attempts, it was not until the 1990s that their biochemical actions were elucidated. It is now clear that bisphosphonates inhibit bone resorption by being selectively taken up and adsorbed to mineral surfaces in bone, where they interfere with the action of the bone-resorbing osteoclasts. Bisphosphonates are internalised by osteoclasts and interfere with specific biochemical processes. Bisphosphonates can be classified into at least two groups with different molecular modes of action. The simpler non-nitrogen containing bisphosphonates (such as etidronate and clodronate) can be metabolically incorporated into non-hydrolysable analogues of ATP, which interfere with ATP-dependent intracellular pathways. The more potent, nitrogen-containing bisphosphonates (including pamidronate, alendronate, risedronate, ibandronate and zoledronate) are not metabolised in this way but inhibit key enzymes of the mevalonate/cholesterol biosynthetic pathway. The major enzyme target for bisphosphonates is farnesyl pyrophosphate synthase (FPPS), and the crystal structure elucidated for this enzyme reveals how BPs bind to and inhibit at the active site via their critical N atoms. Inhibition of FPPS prevents the biosynthesis of isoprenoid compounds (notably farnesol and geranylgeraniol) that are required for the post-translational prenylation of small GTP-binding proteins (which are also GTPases) such as rab, rho and rac, which are essential for intracellular signalling events within osteoclasts. The accumulation of the upstream metabolite, isopentenyl pyrophosphate (IPP), as a result of inhibition of FPPS may be responsible for immunomodulatory effects on gamma delta (γδ) T cells, and can also lead to production of another ATP metabolite called ApppI, which has intracellular actions. Effects on other cellular targets, such as osteocytes, may also be important. Over the years many hundreds of BPs have been made, and more than a dozen have been studied in man. As reviewed elsewhere in this issue, bisphosphonates are established as the treatments of choice for various diseases of excessive bone resorption, including Paget's disease of bone, the skeletal complications of malignancy, and osteoporosis. Several of the leading BPs have achieved 'block-buster' status with annual sales in excess of a billion dollars. As a class, BPs share properties in common. However, as with other classes of drugs, there are obvious chemical, biochemical, and pharmacological differences among the various BPs. Each BP has a unique profile in terms of mineral binding and cellular effects that may help to explain potential clinical differences among the BPs. Even though many of the well-established BPs have come or are coming to the end of their patent life, their use as cheaper generic drugs is likely to continue for many years to come. Furthermore in many areas, e.g. in cancer therapy, the way they are used is not yet optimised. New 'designer' BPs continue to be made, and there are several interesting potential applications in other areas of medicine, with unmet medical needs still to be fulfilled. The adventure that began in Davos more than 40 years ago is not yet over.

Autosomal recessive mental retardation, deafness, ankylosis, and mild hypophosphatemia associated with a novel ANKH mutation in a consanguineous family

CONTEXT

Mutations in ANKH cause the highly divergent conditions familial chondrocalcinosis and craniometaphyseal dysplasia. The gene product ANK is supposed to regulate tissue mineralization by transporting pyrophosphate to the extracellular space.

OBJECTIVE

We evaluated several family members of a large consanguineous family with mental retardation, deafness, and ankylosis. We compared their skeletal, metabolic, and serological parameters to that of the autosomal recessive progressive ankylosis (ank) mouse mutant, caused by a loss-of-function mutation in the murine ortholog Ank.

PARTICIPANTS

The studied patients had painful small joint soft-tissue calcifications, progressive spondylarthropathy, osteopenia, mild hypophosphatemia, mixed hearing loss, and mental retardation.

RESULTS

After mapping the disease gene to 5p15, we identified the novel homozygous ANK missense mutation L244S in all patients. Although L244 is a highly conserved amino acid, the mutated ANK protein was detected at normal levels at the plasma membrane in primary patient fibroblasts. The phenotype was highly congruent with the autosomal recessive progressive ankylosis (ank) mouse mutant. This indicates a loss-of-function effect of the L244S mutation despite normal ANK protein expression. Interestingly, our analyses revealed that the primary step of joint degeneration is fibrosis and mineralization of articular soft tissues. Moreover, heterozygous carriers of the L244S mutation showed mild osteoarthritis without metabolic alterations, pathological calcifications, or central nervous system involvement.

CONCLUSION

Beyond the description of the first human progressive ankylosis phenotype, our results indicate that ANK influences articular soft tissues commonly involved in degenerative joint disorders. Furthermore, this human disorder provides the first direct evidence for a role of ANK in the central nervous system.

Pathophysiology of articular chondrocalcinosis--role of ANKH

Calcium pyrophosphate (CPP) crystal deposition (CPPD) is associated with ageing and osteoarthritis, and with uncommon disorders such as hyperparathyroidism, hypomagnesemia, hemochromatosis and hypophosphatasia. Elevated levels of synovial fluid pyrophosphate promote CPP crystal formation. This extracellular pyrophosphate originates either from the breakdown of nucleotide triphosphates by plasma-cell membrane glycoprotein 1 (PC-1) or from pyrophosphate transport by the transmembrane protein progressive ankylosis protein homolog (ANK). Although the etiology of apparent sporadic CPPD is not well-established, mutations in the ANK human gene (ANKH) have been shown to cause familial CPPD. In this Review, the key regulators of pyrophosphate metabolism and factors that lead to high extracellular pyrophosphate levels are described. Particular emphasis is placed on the mechanisms by which mutations in ANKH cause CPPD and the clinical phenotype of these mutations is discussed. Cartilage factors predisposing to CPPD and CPP-crystal-induced inflammation and current treatment options for the management of CPPD are also described.

[Osteoarthritis in hereditary metabolic diseases]

Primary osteoarthritis (OA) of peripheral joints is a common disease mainly occurring after the age of 50. It is important to distinguish primary from secondary OA. Younger age at disease onset, rapid progression, unusual disease manifestations and co-morbidities are signs of secondary OA. This review outlines an important group of secondary OA. Hereditary metabolic diseases can exhibit joint involvement. For some of these diseases, correct diagnosis is critical, since appropriate therapy influences not only joint function and quality of life, but can also prevent relevant end-organ damage.

Analysis of meniscal degeneration and meniscal gene expression

BACKGROUND

Menisci play a vital role in load transmission, shock absorption and joint stability. There is increasing evidence suggesting that OA menisci may not merely be bystanders in the disease process of OA. This study sought: 1) to determine the prevalence of meniscal degeneration in OA patients, and 2) to examine gene expression in OA meniscal cells compared to normal meniscal cells.

METHODS

Studies were approved by our human subjects Institutional Review Board. Menisci and articular cartilage were collected during joint replacement surgery for OA patients and lower limb amputation surgery for osteosarcoma patients (normal control specimens), and graded. Meniscal cells were prepared from these meniscal tissues and expanded in monolayer culture. Differential gene expression in OA meniscal cells and normal meniscal cells was examined using Affymetrix microarray and real time RT-PCR.

RESULTS

The grades of meniscal degeneration correlated with the grades of articular cartilage degeneration (r = 0.672; P < 0.0001). Many of the genes classified in the biological processes of immune response, inflammatory response, biomineral formation and cell proliferation, including major histocompatibility complex, class II, DP alpha 1 (HLA-DPA1), integrin, beta 2 (ITGB2), ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1), ankylosis, progressive homolog (ANKH) and fibroblast growth factor 7 (FGF7), were expressed at significantly higher levels in OA meniscal cells compared to normal meniscal cells. Importantly, many of the genes that have been shown to be differentially expressed in other OA cell types/tissues, including ADAM metallopeptidase with thrombospondin type 1 motif 5 (ADAMTS5) and prostaglandin E synthase (PTGES), were found to be expressed at significantly higher levels in OA meniscal cells. This consistency suggests that many of the genes detected in our study are disease-specific.

CONCLUSION

Our findings suggest that OA is a whole joint disease. Meniscal cells may play an active role in the development of OA. Investigation of the gene expression profiles of OA meniscal cells may reveal new therapeutic targets for OA therapy and also may uncover novel disease markers for early diagnosis of OA.

[Crystal-induced activation of the inflammasome: gout and pseudogout]

Crystals in tissues lead the innate immune system to the same kind of acute response seen with pathogens. Via activation of the inflammasome, interleukin-1 (IL-1) is released, which upregulates mediators such as cyclooxygenase, tumor necrosis factor, and IL-8 and induces an acute granulocytic inflammation. Therefore, in addition to nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, and colchicine, IL-1 blockers appear to be effective. Large clinical trials have already been initiated. Such an approach could constitute a valuable alternative for patients with contraindications or insufficient response to NSAIDs. After the attack has subsided, control of uric acid metabolism is central. At least several of the responsible urate transporters have been unraveled, which could lead to more focused therapy in the future. At present, diet and blockade of uric acid synthesis remain the main pillars of therapy. The new xanthine oxidase inhibitor febuxostat constitutes a novel option for patients with renal insufficiency or intolerance to allopurinol.

Calcification of articular cartilage in human osteoarthritis

OBJECTIVE

Hypertrophic chondrocyte differentiation is a key step in endochondral ossification that produces basic calcium phosphates (BCPs). Although chondrocyte hypertrophy has been associated with osteoarthritis (OA), chondrocalcinosis has been considered an irregular event and linked mainly to calcium pyrophosphate dihydrate (CPPD) deposition. The aim of this study was to determine the prevalence and composition of calcium crystals in human OA and analyze their relationship to disease severity and markers of chondrocyte hypertrophy.

METHODS

One hundred twenty patients with end-stage OA undergoing total knee replacement were prospectively evaluated. Cartilage calcification was studied by conventional x-ray radiography, digital-contact radiography (DCR), field-emission scanning electron microscopy (FE-SEM), and synovial fluid analysis. Cartilage calcification findings were correlated with scores of knee function as well as histologic changes and chondrocyte hypertrophy as analyzed in vitro.

RESULTS

DCR revealed mineralization in all cartilage specimens. Its extent correlated significantly with the Hospital for Special Surgery knee score but not with age. FE-SEM analysis showed that BCPs, rather than CPPD, were the prominent minerals. On histologic analysis, it was observed that mineralization correlated with the expression of type X collagen, a marker of chondrocyte hypertrophy. Moreover, there was a strong correlation between the extent of mineralization in vivo and the ability of chondrocytes to produce BCPs in vitro. The induction of hypertrophy in healthy human chondrocytes resulted in a prominent mineralization of the extracellular matrix.

CONCLUSION

These results indicate that mineralization of articular cartilage by BCP is an indissociable process of OA and does not characterize a specific subset of the disease, which has important consequences in the development of therapeutic strategies for patients with OA.

[Crystal arthropathies]

Basic calcium phosphate (BCP) and calcium pyrophosphate dihydrate crystals are the most common types of pathologic crystals, followed by monosodium urate crystals and, in rare cases, calcium oxalate crystals. These crystals have been associated with a variety of quite different rheumatic syndromes. They are responsible for acute synovial inflammation and also contribute to cartilage degradation and bone lesions within the joint. Although understanding of the molecular mechanisms involved in generating the pathologic effects of these crystals has increased, the role of BCP crystals in particular remains poorly understood. The clinical implication of articular deposits of calcium-containing crystals in osteoarthritis is unknown. This review provides an overview of the clinical and pathological changes of these four different types of crystals.

Investigation of calcium crystals in OA knees

For studies on matrix mineralization in osteoarthritis (OA), a clear analytical approach is necessary to identify and to quantify mineralization in the articular cartilage. The aim of this study is to develop an effective algorithm to quantify and to identify cartilage mineralization in the experimental setting. Four patients with OA of the knee undergoing total knee replacement and four control patients were included. Cartilage calcification was studied by digital contact radiography (DCR), field emission scanning electron microscopy (FE-SEM) X-ray element analysis and Raman spectroscopy (RS). DCR revealed mineralization in all OA cartilage specimens. No mineralization was observed in the control cartilage. Patient I showed rhomboid shaped crystals with a mean Ca:P molar ratio of 1.04 indicated the presence of calcium pyrophosphate dihydrate (CPPD) crystals, while Patients II, III and IV presented carbonate-substituted hydroxyapatite (HA). RS also showed the presence of CPPD crystals in Patient I while Patients II, III and IV revealed spectra confirming the presence of HA crystals. In the corresponding chondrocyte cell culture analyzed with SEM, the presence of CPPD crystals in the culture of Patient I and HA crystals in the culture of Patient II, III and IV was confirmed. No mineralization was found in the cell culture of the controls. The differentiation between BCP and CPPD crystals plays an important role, and the techniques presented here provide an accurate differentiation of these two types of crystals. For quantification of articular cartilage mineralization, DCR is a simple and accurate method.

Introduction of a Phe377del mutation in ANK creates a mouse model for craniometaphyseal dysplasia

Craniometaphyseal dysplasia (CMD) is a monogenic human disorder characterized by thickening of craniofacial bones and flaring metaphyses of long bones. Mutations for autosomal dominant CMD have been identified in the progressive ankylosis gene ANKH. Previous studies of Ank loss-of-function models, Ank(null/null) and Ank(ank/ank) mice, suggest that Ank plays a role in the regulation of bone mineralization. However, the mechanism for Ank mutations leading to CMD remains unknown. We generated the first knockin (KI) mouse model for CMD expressing a human mutation (Phe377 deletion) in ANK. Homozygous Ank knockin mice (Ank(KI/KI)) replicate many typical features of human CMD including hyperostosis of craniofacial bones, massive jawbones, decreased diameters of cranial foramina, obliteration of nasal sinuses, fusion of middle ear bones, and club-shaped femurs. In addition, Ank(KI/KI) mice have increased serum alkaline phosphatase and TRACP5b, as reported in CMD patients. Biochemical markers of bone formation and bone resorption, N-terminal propeptide of type I procollagen and type I collagen cross-linked C-terminal telopeptide, are significantly increased in Ank(KI/KI) mice, suggesting increased bone turnover. Interestingly, Ank(KI/KI) bone marrow-derived macrophage cultures show decreased osteoclastogenesis. Despite the hyperostotic phenotype, bone matrix in Ank(KI/KI) mice is hypomineralized and less mature, indicating that biomechanical properties of bones may be compromised by the Ank mutation. We believe this new mouse model will facilitate studies of skeletal abnormalities in CMD at cellular and molecular levels.

The CPPDD-associated ANKH M48T mutation interrupts the interaction of ANKH with the sodium/phosphate cotransporter PiT-1

OBJECTIVE

Numerous dominant human homolog of progressive ankylosis (ANKH) mutations have been identified in familial calcium pyrophosphate dihydrate crystal deposition disease (CPPDD). Due to the dominant nature of these mutations, we investigated whether ANKH interacts with other proteins; and if so, whether any CPPDD-associated ANKH mutation might disrupt such protein interactions.

METHODS

Stable ATDC5 ANKH wt- and ANKH M48T-transfectants were generated. Lysates from these transfectants were used to identify candidate protein interaction with ANKH by coimmunoprecipitation followed by Western blot analysis. The effect of high phosphate on the expression of genes involved in modulating Pi (inorganic phosphate)/PPi (inorganic pyrophosphate) homeostasis in these transfectants was assessed.

RESULTS

We showed that ANKH protein associates with the sodium/phosphate cotransporter PiT-1, and that ANKH M48T mutant protein failed to interact with PiT-1. We also showed that upon high phosphate treatment, the normally coordinated upregulation of endogenous Ank and PiT1 transcript expression was disrupted in ANKH M48T transfectants.

CONCLUSION

Our results suggested that there is a coordinated interrelationship between 2 key participants of Pi and PPi metabolism, ANKH and PiT-1.

Telomerase transduced osteoarthritis fibroblast-like synoviocytes display a distinct gene expression profile

OBJECTIVE

To examine the differential gene expression in telomerase transduced osteoarthritis fibroblast-like synoviocytes (hTERT-OA 13A FLS) and telomerase transduced rheumatoid arthritis FLS (hTERT-RA 516 FLS) and test the hypothesis that longterm culture of hTERT-OA 13A FLS display a disease-specific gene expression profile.

METHODS

Gene expression in passage 8 hTERT-OA 13A FLS and passage 8 hTERT-RA 516 FLS were compared using microarray assays. Differential expression of selected genes was further examined by reverse transcription-polymerase chain reaction (RT-PCR). After continuous expansion in culture for an additional 4 months, gene expression in the longterm cultures of hTERT-OA 13A FLS and hTERT-RA 516 FLS was again examined with microarray and real-time RT-PCR.

RESULTS

hTERT-OA 13A FLS displayed a distinct gene expression profile. While hTERT-RA 516 FLS expressedADAMTS1, ADAMTS3, ADAMTS5, and several carboxypeptidases, hTERT-OA 13A FLS expressed matrix metalloproteinase (MMP)1, MMP3, and several cathepsins at higher levels. Numerous genes classified in the immune response, lipid transport/catabolism, and phosphate transport biological processes were also expressed at higher levels in hTERT-OA 13A FLS. In contrast, numerous genes classified in the positive regulation of cell proliferation, anti-apoptosis, and angiogenesis biological processes were expressed at higher levels in hTERT-RA 516 FLS. Further, of the recently proposed 21 candidate synovial biomarkers of OA, 12 (57%) were detected in our study.

CONCLUSION

The findings indicate that OA FLS may not be a passive bystander in OA and that telomerase transduced OA FLS offer an alternative tool for the study of synovial disease markers and for the identification of new therapeutic targets for OA therapy.

The ANKH ΔE490Mutation in Calcium Pyrophosphate Dihydrate Crystal Deposition Disease (CPPDD) affects tissue non-specific Alkaline Phosphatase (TNAP) activities

ANKH (human homolog of progressive ankylosis) regulates inorganic pyrophosphate (PPi) transport. Dominant ANKH mutations were detected in at least five multiplex families with calcium pyrophosphate dihydrate crystal deposition disease (CPPPD). The objective of this study is to assess the functional consequences of one CPPDD-associated ANKH mutation (ΔE490) in chondrogenic ATDC5 cells. Stable ATDC5 transfectants bearing myc-tagged constructs of wild-type ANKH, mutant ANKH (ΔE490) and neo controls were generated. Upon ITS (insulin, transferrin and selenium) induction, expression of chondrocyte markers including alkaline phosphatase activity in the various transfectants was assessed. The ANKH ΔE490- transfectants had low alkaline phosphatase activities throughout ITS treatment due to lower TNAP protein expression and the presence of intracellular low-molecular-weight inhibitors. Our results suggest that the interplay of ANKH and TNAP activities is tightly regulated.

Microcytosis in ank/ank mice and the role of ANKH in promoting erythroid differentiation

Progressive ankylosis (Ank and the human homolog, ANKH) is a transmembrane protein which regulates transport of inorganic pyrophosphate (PPi). ank/ank mice with a mutated ank gene, have calcification and bone ankylosis of the affected joints. In the course of studying these mutant mice, we found that they have microcytosis. These mutant mice have lower mean red blood cell volume (MCV) and lower hemoglobin content in red cells (mean corpuscular hemoglobin, MCH) than normal mice. Using quantitative real-time PCR analysis, we showed that Ank was expressed in the E/Meg bipotent precursor, BFU-E, CFU-E, but there was no Ank expression in the hemoglobinizing erythroblasts. Stable ANKH transfectants in K562 cells highly expressed two immature erythroid cell markers, E-cadherin and endoglin. Enhanced Erythropoietin (Epo) expression and downregulation of SHP-1 were detected in these transfectants. Consequently, the autocrine Epo-EpoR signaling pathway was activated, as evidenced by higher p-Tyr JAK2, p-Tyr EpoR and p-Tyr STAT5B in the ANKH transfectants. Our results revealed a novel function of ANKH in the promotion of early erythroid differentiation in K562 cells. We also showed that ank/ank mice have lower serum levels of Epo than the normal littermates, and this is the likely cause of microcytosis in these mutant mice.

Cuff tear arthropathy: evidence of functional variation in pyrophosphate metabolism genes

We investigated the role of two genes, ANKH and TNAP, in patients with cuff tear arthropathy. These genes encode proteins which regulate the extracellular concentration of inorganic pyrophosphate, fluctuations of which can lead to calcium crystal formation. Variants were detected by direct sequencing of DNA and their frequencies compared with healthy controls. The effect of variants on protein function was further studied by in vitro approaches. Variant genotypes were observed more frequently in the cases when compared with controls in ANKH (45% and 20%) and TNAP (32% and 9%). Variants in ANKH altered inorganic pyrophosphate (PPi) concentrations in transfected human chondrocytes. There was a higher mean serum concentration of TNAP detected in female patients compared with normal ranges. Cuff tear arthropathy is associated with variants in ANKH and TNAP that alter extracellular inorganic pyrophosphate concentrations causing calcium crystal deposition. This supports a theory that genetic variants predispose patients to primary crystal deposition which when combined with a massive rotator cuff tear leads to the development of arthritis.

Genetic factors in the pathogenesis of CPPD crystal deposition disease

Crystal deposition is a very complex process ruled by numerous factors. A small but important proportion of cases of chondrocalcinosis are monogenic, and many of the genes involved have been identified. These genetic findings strongly point to control of the level of extracellular inorganic pyrophosphate as the primary mechanism for their association with either calcium pyrophosphate dihydrate or hydroxyapatite deposition. However, effects on extracellular inorganic pyrophosphate levels do not explain the mechanism of association in all of these monogenic diseases. Further, there are likely to be several as yet unidentified genes that are important in this common condition. This review highlights what genetic studies have demonstrated about the processes involved in these diverse but related disorders.