Familial chondrocalcinosis due to calcium pyrophosphate dihydrate crystal deposition in English families

"Lessons from Rare Forms of Osteoarthritis"

Osteoarthritis (OA) is one of the most prevalent conditions in the world, particularly in the developed world with a significant increase in cases and their predicted impact as we move through the twenty-first century and this will be exacerbated by the covid pandemic. The degeneration of cartilage and bone as part of this condition is becoming better understood but there are still significant challenges in painting a complete picture to recognise all aspects of the condition and what treatment(s) are most appropriate in individual causes. OA encompasses many different types and this causes some of the challenges in fully understanding the condition. There have been examples through history where much has been learnt about common disease(s) from the study of rare or extreme phenotypes, particularly where Mendelian disorders are involved. The often early onset of symptoms combined with the rapid and aggressive pathogenesis of these diseases and their predictable outcomes give an often-under-explored resource. It is these "rarer forms of disease" that William Harvey referred to that offer novel insights into more common conditions through their more extreme presentations. In the case of OA, GWAS analyses demonstrate the multiple genes that are implicated in OA in the general population. In some of these rarer forms, single defective genes are responsible. The extreme phenotypes seen in conditions such as Camptodactyly Arthropathy-Coxa Vara-pericarditis Syndrome, Chondrodysplasias and Alkaptonuria all present potential opportunities for greater understanding of disease pathogenesis, novel therapeutic interventions and diagnostic imaging. This review examines some of the rarer presenting forms of OA and linked conditions, some of the novel discoveries made whilst studying them, and findings on imaging and treatment strategies.

Modificazioni morfo-funzionali della cartilagine nella senescenza e nell'osteoartrosi

La cartilagine articolare è un tessuto connettivo avascolare, aneurale che ricopre le superfici articolari. La funzione di assorbimento delle sollecitazioni meccaniche, a protezione dell'osso subcondrale, rende la supeficie articolare idonea a sostenere il carico.

Le funzioni inerenti le modalità di assorbimento della sollecitazione meccanica, che fanno sì che la deformazione sia reversibile, dipendono in larga parte dalle caratteristiche della cartilagine, intesa come struttura altamente organizzata. Nell'osteoartrosi umana e nei suoi modelli animali l'alterazione strutturale dei proteoglicani cartilaginei rappresenta l'evento centrale.

Vengono discusse, alla luce delle acquisizioni più recenti, le implicazioni sulle proprieta fisico-chimiche e morfo-strutturali della cartilagine articolare riguardanti le caratteristiche di base dei proteoglicani, la struttura dei collageni, l'organizzazione della matrice extracellulare e le sue modificazioni nella senescenza ed in corso di osteoartrosi con le relative conseguenze sulle proprietà biomeccaniche del disco intervertebrale. Le conoscenze relative alle alterazioni della struttura proteoglicanica e lo sviluppo di nuovi metodi di determinazione dei markers biochimici del danno cartilagineo potrebbero migliorare la comprensione delle relazioni fra senescenza ed osteoartrosi, nonchè il riconoscimento delle modificazioni più precoci e la valutazione della risposta terapeutica.

Epidemiology of calcium pyrophosphate crystal arthritis and basic calcium phosphate crystal arthropathy

Calcium pyrophosphate crystal deposition (CPPD) is common and mainly associates with increasing age and osteoarthritis (OA). Recent studies suggest that CPPD occurs as the result of a generalized articular predisposition and may also associate with low cortical bone mineral density. The epidemiology of basic calcium phosphate (BCP) crystal deposition is poorly understood. Although periarticular BCP crystal deposits occurs at all ages and in both sexes, intra-articular BCP crystal deposition tends to associate with increasing age and OA. Calcium pyrophosphate and BCP crystals frequently coexist in joints with OA.

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.

Comparison of three imaging techniques in diagnosis of chondrocalcinosis of the knees in calcium pyrophosphate deposition disease

OBJECTIVE

To study the role of different imaging modalities, ultrasonography, conventional radiography (CR) and CT, in visualization of chondrocalcinosis of the knees in patients with CPDD.

METHODS

Twenty-five patients (14 males and 11 females) with CPDD were enrolled in the study. Diagnosis was made according to D.J. McCarty classification criteria. All patients had arthritis of the knee and underwent aspiration of SF from the knee and microscopic investigation of SF samples. Diagnosis of CPDD was crystal proven. Three imaging methods were performed in patients: CR, CT and US of the knees.

RESULTS

CR of the knee confirmed cartilage calcification (CC) in 13 patients, CT in 18 patients and US in 25 patients. No difference in age or disease duration between patients with CC detected by different imaging methods was found.

CONCLUSION

US appeared to be a helpful tool, possibly better than CR or CT, in revealing CC in patients with CPDD. Informativity of CT and CR in the detection of CC is almost equal.

Honokiol: an effective inhibitor of tumor necrosis factor-α-induced up-regulation of inflammatory cytokine and chemokine production in human synovial fibroblasts

In this study, we investigated the mechanisms underlying the anti-inflammatory effects of honokiol in tumor necrosis factor (TNF)-α-stimulated rheumatoid arthritis synovial fibroblasts (RASFs). RASFs pre-treated with honokiol (0-20 μM) were stimulated with TNF-α (20 ng/ml). The levels of prostaglandin E2 (PGE2), nitric oxide (NO), soluble intercellular adhesion molecule-1 (sICAM-1), transforming growth factor-β1 (TGF-β1), monocyte chemotactic protein-1 (MCP-1), and macrophage inflammatory protein-1α (MIP-1α) in supernatants were determined by enzyme-linked immunosorbent assay (ELISA) and Griess assay. In addition, protein expression levels of cyclooxygenase-2 (COX-2), inducible nitric oxide synthase (iNOS), and phosphorylated Akt, nuclear factor kappa B (NFκB), and extracellular signal-regulated kinase (ERK)1/2 were determined by western blot. The expression of NFκB-p65 was assessed by immunocytochemical analysis. TNF-α treatment significantly up-regulated the levels of PGE2, NO, sICAM-1, TGF-β1, MCP-1, and MIP-1α in the supernatants of RASFs, increased the protein expression of COX-2, iNOS, and induced phosphorylation of Akt, IκB-α, NFκB, and ERK1/2 in RASFs. TNF-α-induced expression of these molecules was inhibited in a dose-dependent manner by pre-treatment with honokiol. The inhibitory effect of honokiol on NFκB-p65 activity was also confirmed by immunocytochemical analysis. In conclusion, honokiol is a potential inhibitor of TNF-α-induced expression of inflammatory factors in RASFs, which holds promise as a potential anti-inflammatory drug.

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.

Testing of the OMERACT 8 Draft Validation Criteria for a Soluble Biomarker Reflecting Structural Damage in Rheumatoid Arthritis: A Systematic Literature Search on 5 Candidate Biomarkers

Objective.

To test the OMERACT 8 draft validation criteria for soluble biomarkers by assessing the strength of literature evidence in support of 5 candidate biomarkers.

Methods.

A systematic literature search was conducted on the 5 soluble biomarkers RANKL, osteoprotegerin (OPG), matrix metalloprotease (MMP-3), urine C-telopeptide of types I and II collagen (U-CTX-I and U CTX-II), focusing on the 14 OMERACT 8 criteria. Two electronic voting exercises were conducted to address: (1) strength of evidence for each biomarker as reflecting structural damage according to each individual criterion and the importance of each individual criterion; (2) overall strength of evidence in support of each of the 5 candidate biomarkers as reflecting structural damage endpoints in rheumatoid arthritis (RA) and identification of omissions to the criteria set.

Results.

The search identified 111 articles. The strength of evidence in support of these biomarkers reflecting structural damage was low for all biomarkers and was rated highest for U-CTX-II [score of 6.5 (numerical rating scale 0–10)]. The lowest scores for retention of specific criteria in the draft set went to criteria that refer to the importance of animal studies, correlations with other biomarkers reflecting damage, and an understanding of the metabolism of the biomarker.

Conclusion.

Evidence in support of any of the 5 tested biomarkers (MMP-3, CTX-I, CTX-II, OPG, RANKL) was inadequate to allow their substitution for radiographic endpoints in RA. Three of the criteria in the draft criteria set might not be required, but few omissions were identified.

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.

Biomechanical signals suppress proinflammatory responses in cartilage: early events in experimental antigen-induced arthritis

Although biomechanical signals generated during joint mobilization are vital in maintaining integrity of inflamed cartilage, the molecular mechanisms of their actions are little understood. In an experimental model of arthritis, we demonstrate that biomechanical signals are potent anti-inflammatory signals that repress transcriptional activation of proinflammatory genes and augment expression of anti-inflammatory cytokine IL-10 to profoundly attenuate localized joint inflammation.

Biochemical and genetic analysis of ANK in arthritis and bone disease

Mutations in the progressive ankylosis gene (Ank/ANKH) cause surprisingly different skeletal phenotypes in mice and humans. In mice, recessive loss-of-function mutations cause arthritis, ectopic crystal formation, and joint fusion throughout the body. In humans, some dominant mutations cause chondrocalcinosis, an adult-onset disease characterized by the deposition of ectopic joint crystals. Other dominant mutations cause craniometaphyseal dysplasia, a childhood disease characterized by sclerosis of the skull and abnormal modeling of the long bones, with little or no joint pathology. Ank encodes a multiple-pass transmembrane protein that regulates pyrophosphate levels inside and outside tissue culture cells in vitro, but its mechanism of action is not yet clear, and conflicting models have been proposed to explain the effects of the human mutations. Here, we test wild-type and mutant forms of ANK for radiolabeled pyrophosphate-transport activity in frog oocytes. We also reconstruct two human mutations in a bacterial artificial chromosome and test them in transgenic mice for rescue of the Ank null phenotype and for induction of new skeletal phenotypes. Wild-type ANK stimulates saturable transport of pyrophosphate ions across the plasma membrane, with half maximal rates attained at physiological levels of pyrophosphate. Chondrocalcinosis mutations retain apparently wild-type transport activity and can rescue the joint-fusion phenotype of Ank null mice. Craniometaphyseal dysplasia mutations do not transport pyrophosphate and cannot rescue the defects of Ank null mice. Furthermore, microcomputed tomography revealed previously unappreciated phenotypes in Ank null mice that are reminiscent of craniometaphyseal dysplasia. The combination of biochemical and genetic analyses presented here provides insight into how mutations in ANKH cause human skeletal disease.

Epidemiology of crystal arthropathy

Gout is an inflammatory arthritis mediated by the crystallization of uric acid within the joints and often is associated with hyperuricemia. Data suggest that the overall disease burden of gout remains substantial and may be increasing. Identifying and characterizing modifiable risk factors for gout is a major step in preventing and managing this painful condition. As more scientific data on the risk factors and comorbidities of gout become available, their integration into gout prevention and care strategies may become essential. This article reviews the relevant epidemiologic data, with a focus on recent progress and data on other crystal arthropathies.

Genetics of chondrocalcinosis

Rapid developments in genetic analysis have enabled the dissection of a variety of arthropathies that are inherited in a Mendelian manner. These disorders include calcium crystal arthropathies such as calcium pyrophosphate dihydrate deposition (CPPD) disease and hydroxyapatite deposition disease. In CPPD disease, mutations in a recently discovered gene, ANKH, have been demonstrated in five affected families and may also be associated with the idiopathic deposition of calcium pyrophosphate dihydrate crystals. The product of ANKH appears to be involved in cellular transport of inorganic pyrophosphate (PPi) and mutations in ANKH have been shown to have a significant impact on the regulation of intra- and extracellular levels of PPi. In families with hydroxyapatite deposition disease, no gene locus has yet been linked to the disorder.

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.

Genetic studies of chondrocalcinosis

PURPOSE OF REVIEW

Our understanding of the causation of the chondrocalcinosis and other disorders characterized by ectopic mineralization is rapidly increasing, and genetic studies have contributed substantially to recent major advances in the field. This review will discuss what is known about the genetics of chondrocalcinosis and what we have learned from genetic studies to date.

RECENT FINDINGS

Chondrocalcinosis is one of a family of conditions associated with ectopic mineralization. This family also includes disorders of mineralization of bone and spinal and other ligaments, and vascular calcification. There has been increasing evidence of the key role of transport and metabolism of inorganic pyrophosphate (PPi) in control of mineralization, and as the likely explanation for the association of a variety of genetic variants with chondrocalcinosis and ectopic mineralization elsewhere. This may be an overly simplistic view of this family of conditions, with recent evidence suggesting that, for example, ANKH variants may not all predispose to chondrocalcinosis by effects on PPi transport, but may also influence chondrocyte maturation.

SUMMARY

Understanding the control of the process of mineralization and its tissue specificity are important steps in the search for rational therapies for these conditions.

Familial calcium pyrophosphate dihydrate deposition disease. A Tunisian kindred

INTRODUCTION

Familial calcium pyrophosphate dihydrate deposition disease (CPDD) is uncommon, with about 50 affected families identified to date in the world. Genetic studies in familial CPDD are focusing on the ANKH gene. We report a new Tunisian kindred with CPDD.

PATIENTS AND METHODS

The development of CPDD in a patient who was only 35 years of age prompted a family study. A medical history, physical examination, and radiographs were performed in 103 family members older than 18 years.

RESULTS

Fifteen family members had CPDD. There were 10 men and five women, with a mean age of 59.4 years. Onset was usually in the third or fourth decade. Four clinical patterns were found: Five patients had pseudogout, five had pseudoosteoarthritis, three had asymptomatic disease, and two had pseudorheumatoid arthritis. Inheritance was autosomal dominant with low penetrance. No associations with specific HLA antigens were found. The disease was mild. These characteristics fit the description of Gaucher type 1 familial CPDD.

CONCLUSION

Inherited autosomal dominant CPDD with low penetrance was found in 15 members of a Tunisian kindred. The disease was mild. We are planning a genetic study including tests for ANKH gene mutations in this kindred.

Autosomal dominant early childhood seizures associated with chondrocalcinosis and a mutation in the ANKH Gene

We describe the pattern of early childhood seizures within a family with autosomal dominant chondrocalcinosis (CCAL, which causes adult-onset arthritis). All affected family members with CCAL experienced seizures in early childhood, usually, but not always, associated with fever. Similarities exist to the syndrome of generalized epilepsy with febrile seizures plus (GEFS+). A mutation within the ANKH gene on chromosome 5p has been found previously in this family; other patients with familial CCAL (but without seizures) have mutations in the same gene. ANKH codes for a transmembrane protein involved in the regulation of extracellular pyrophosphate ion levels, although its precise mechanism of action remains unclear. It is highly expressed in the brain, and its expression may be influenced by seizure activity. The mutation within this family creates a premature initiation codon, adding four amino acids to the N-terminus of the protein. We postulate that this may lead to a gain of function, causing seizure susceptibility as well as chondrocalcinosis. Mutations within this gene may underlie other forms of genetic epilepsy and febrile seizures.

Relative risk of knee chondrocalcinosis in siblings of index cases with pyrophosphate arthropathy

OBJECTIVES

To examine the genetic contribution to common, apparently sporadic, radiographic knee chondrocalcinosis (CC) and pyrophosphate arthropathy (PA).

METHOD

(1).

DESIGN

radiographic sibling study. Comparison of the prevalences of knee CC and PA in siblings of index cases with PA with those in the community. (2).

SUBJECTS

80 index cases with PA listed for total knee replacement; 122 of their siblings aged >or=40 years; and 1729 participants from community knee pain surveys who had undergone knee radiographs. (3).

MAIN OUTCOME MEASURE

odds ratios of knee CC and PA in siblings versus community participants.

RESULTS

The prevalence of knee CC was 13% (15/116) in the siblings and 6.9% (119/1727) in the community participants. The adjusted odds ratio (aOR) was 1.2, 95% confidence interval (CI) 0.6 to 2.3. The main risk factors for knee CC were age, knee pain, and knee OA. The prevalence of knee PA was 7% (9/122) in the siblings and 3.4% (59/1729) among the community participants (aOR = 1.1, 95% CI 0.4 to 2.7). The main risk factors for PA were age and knee pain. The age, sex, and knee pain standardised prevalence of PA in the Nottingham community aged >or=40 was 2.40%.

CONCLUSION

The risk of knee CC and PA in siblings of index cases with PA is no higher than that in the general population. Although rare familial CC is recognised, this study suggests that no major genetic predisposition to CC occurs in common symptomatic knee OA.

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.

Familial calcium pyrophosphate dihydrate deposition disease and the ANKH gene

The crystal deposition arthropathies comprise a host of disorders that may occur idiopathically or as secondary manifestations of associated diseases. Rarely, crystal deposition presents as a familial disorder. Most affected family members display radiographically detectable crystals of calcium pyrophosphate dihydrate in their joint spaces. In genetic studies of familial calcium pyrophosphate dihydrate deposition disease, a region on the short arm of chromosome 5 was found to be genetically linked to the phenotype displayed by several of these families. Among the positional candidates at this locus was ANKH, the human homolog of a gene that is responsible for the phenotype of progressive ankylosis (ank) in the mouse. ANKH codes for a transmembrane protein that appears to regulate the transport of inorganic pyrophosphate. It was analyzed as a potential positional candidate gene for calcium pyrophosphate dihydrate deposition disease, and in several unrelated families, sequence variants were identified that segregated with the calcium pyrophosphate dihydrate deposition disease phenotype among affected members. A discussion of ANKH as the familial calcium pyrophosphate dihydrate deposition disease gene is presented.

Up-regulated expression of cartilage intermediate-layer protein and ANK in articular hyaline cartilage from patients with calcium pyrophosphate dihydrate crystal deposition disease

OBJECTIVE

Excess accumulation of extracellular inorganic pyrophosphate (ePPi) in aged human cartilage is crucial in calcium pyrophosphate dihydrate (CPPD) crystal formation in cartilage matrix. Two sources of ePPi are ePPi-generating ectoenzymes (NTPPPH) and extracellular transport of intracellular PPi by ANK. This study was undertaken to evaluate the role of NTPPPH and ANK in ePPi elaboration, by investigating expression of NTPPPH enzymes (cartilage intermediate-layer protein [CILP] and plasma cell membrane glycoprotein 1 [PC-1]) and ANK in human chondrocytes from osteoarthritic (OA) articular cartilage containing CPPD crystals and without crystals.

METHODS

Chondrocytes were harvested from knee cartilage at the time of arthroplasty (OA with CPPD crystals [CPPD], n = 8; OA without crystals [OA], n = 10). Normal adult human chondrocytes (n = 1) were used as a control. Chondrocytes were cultured with transforming growth factor beta1 (TGFbeta1), which stimulates ePPi elaboration, and/or insulin-like growth factor 1 (IGF-1), which inhibits ePPi elaboration. NTPPPH and ePPi were measured in the media at 48 hours. Media CILP, PC-1, and ANK were determined by dot-immunoblot analysis. Chondrocyte messenger RNA (mRNA) was extracted for reverse transcriptase-polymerase chain reaction to study expression of mRNA for CILP, PC-1, and ANK. NTPPPH and ANK mRNA and protein were also studied in fresh frozen cartilage.

RESULTS

Basal ePPi elaboration and NTPPPH activity in conditioned media from CPPD chondrocytes were elevated compared with normal chondrocytes, and tended to be higher compared with OA chondrocytes. Basal expression of mRNA for CILP (chondrocytes) and ANK (cartilage) was higher in both CPPD chondrocytes and CPPD cartilage extract than in OA or normal samples. PC-1 mRNA was less abundant in CPPD chondrocytes and cartilage extract than in OA chondrocytes and extract, although the difference was not significant. CILP, PC-1, and ANK protein levels were similar in CPPD, OA, and normal chondrocytes or cartilage extracts. Both CILP and ANK mRNA expression and ePPi elaboration were stimulated by TGFbeta1 and inhibited by IGF-1 in chondrocytes from all sources.

CONCLUSION

CILP and ANK mRNA expression correlates with chondrocyte ePPi accumulation around CPPD and OA chondrocytes, and all respond similarly to growth factor stimulation. These findings suggest that up-regulated CILP and ANK expression contributes to higher ePPi accumulation from CPPD crystal-forming cartilage.

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.

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

Familial autosomal dominant calcium pyrophosphate dihydrate (CPPD) chondrocalcinosis has previously been mapped to chromosome 5p15. We have identified a mutation in the ANKH gene that segregates with the disease in a family with this condition. ANKH encodes a putative transmembrane inorganic pyrophosphate (PPi) transport channel. We postulate that loss of function of ANKH causes elevated extracellular PPi levels, predisposing to CPPD crystal deposition.

Corticosteroids alter the differentiated phenotype of articular chondrocytes

Experimental evidence suggests that recommended dosages of some corticosteroids used clinically as antiinflammatory agents for treating arthropathies damage articular cartilage, but low dosages may be chondroprotective. The purpose of this study was to evaluate how different concentrations of methylprednisolone affect chondrocyte function and viability. Articular cartilage and chondrocytes were obtained from young adult horses, 1.5-3.5 years of age. Corticosteroid-induced changes in collagen expression were studied at the transcriptional level by Northern blot analyses and at the translational level by measuring [3H]-proline incorporation into [3H]-hydroxyproline. Fibronectin mRNA splicing patterns were evaluated with ribonuclease protection assays. Cytotoxicity was studied using erythrosin B dye exclusion. Steady-state levels of type II procollagen mRNA decreased without concurrent changes in type I procollagen expression as the medium methylprednisolone concentrations were increased from 1 x 10(1) to 1 x 10(8) pg/ml, dropping below 10% of control values by 1 x 10(5) pg/ml. Cytotoxicity occurred as methylprednisolone levels were increased further from 1 x 10(8) to 1 x 10(9) pg/ml. Changes in total collagen (protein) synthesis were less pronounced, but also demonstrated significant suppression between 1 x 10(4) and 1 x 10(8) pg/ml. Corticosteroid-induced changes in fibronectin isoform levels were evaluated in articular cartilage samples without in vitro culture. The cartilage-specific (V + C)(-) isoform was suppressed in both normal and inflamed joints by a single intraarticular injection (0.1 mg/kg) of methylprednisolone. Combined, these data indicate that methylprednisolone suppresses matrix protein markers of chondrocytic differentiation. Decreased and altered chondrocyte expression of matrix proteins likely contributes to the pathogenesis of corticosteroid-induced cartilage degeneration.

Hereditary disorders mimicking and/or causing premature osteoarthritis

Osteoarthritis is the most common joint disease, causing considerable disability and impairment of quality of life. Hereditary osteochondrodysplasias and some inborn errors of metabolism may mimic or cause premature osteoarthritis. Osteochondrodysplasias usually cause joint deformities, such as coxa vara or genu varum, which can cause abnormal biomechanics. In most of these disorders, the articular cartilage is originally defective as a result of genetically determined collagen or matrix protein abnormalities, or the deposition of mucopolysaccharides. In the case of inborn errors of metabolism, the pathological process affects healthy articular structures, causing secondary osteoarthritis. In alkaptonuria, the pathological deposition of polymerized homogenistic acid causes defective changes in cartilage, articular capsule and tendons. In Wilson's disease, the premature osteoarthritis might be caused by the copper deposition. It is worth paying attention to these rare disorders, even when they are mild or incomplete, because early diagnosis can lead to prevention and effective treatment. In addition, research is discovering the specific gene defects and molecular abnormalities that are responsible for disease expression. This may in turn lead to opportunities for prenatal diagnosis; thus, genetic counselling and gene replacement therapy may be a realistic possibility in the near future.

Familial and clinical aspects of calcium pyrophosphate deposition disease

The mechanisms involved in calcium pyrophosphate dehydrated deposition disease (CPPDD) are unknown and those families with the disease, described in different countries, provide a fertile file for genomic research. Genomic DNA studies in these kindred with secondary or primary form of CPPDD provide a shortcut for trying to investigate the biomolecular basis of the disease. Mutations in the COL2A1 gene have been identified in one family with spondyloepiphyseal dysplasia and secondary deposits of pyrophosphate and apatite crystalline deposits. In another kindred with CPPDD due to precocious osteoarthritis, the phenotype was linked to markers of chromosome 8p. In four other kindreds (British, Argentinean, French, and the United States), the phenotypes were linked to a precise region of chromosome 5p. Two possible genes located in this region that are expressed in the articular cartilage, but of unknown articular physiologic role are being investigated as possible CPPDD genes. From the clinical point of view, CPPDD spectrum of clinical and radiographic manifestations is enlarging, especially those related to spine involvement or pseudo tumoral forms. At the end, the present review of a current therapeutic approach for CPPDD is discussed.

Refinement of the chromosome 5p locus for familial calcium pyrophosphate dihydrate deposition disease

Familial calcium pyrophosphate dihydrate deposition disease (CPPDD) is a disease of articular cartilage that is radiographically characterized by chondrocalcinosis due to the deposition of calcium-containing crystals in affected joints. We have documented the disease in an Argentinean kindred of northern Italian ancestry and in a French kindred from the Alsace region. Both families presented with a common phenotype including early age at onset and deposition of crystals of calcium pyrophosphate dihydrate in a similar pattern of affected joints. Affected family members were karyotypically normal. Linkage to the short arm of chromosome 5 was observed, consistent with a previous report of linkage of the CPPDD phenotype in a large British kindred to the 5p15 region. However, recombinants in the Argentinean kindred have enabled us to designate a region<1 cM in length between the markers D5S416 and D5S2114 as the CPPDD locus.

Pseudogout in a young patient

Calcium pyrophosphate dihydrate (CPPD) crystal deposition disease is conventionally classified into cases that are hereditary, idiopathic (sporadic) or associated with other disorders. In the idiopathic form, the disease usually occurs in middle-aged or elderly patients. An earlier age of disease onset is observed in the hereditary form and the form associated with other disorders. Therefore, the occurrence of CPPD crystal deposition disease in a young patient merits thorough investigation for an underlying cause such as haemochromatosis, hyperparathyroidism, Wilson's disease, hypophosphatasia or hypomagnesaemia and requires a family study to investigate a possible hereditary cause. We report a case of a young female patient who presented with pseudogout at the age of 24 years; no associated diseases or familial occurrence were found despite a follow-up of more than 12 years.

Markers of osteoarthritis: a review of the literature

There may be a great potential in the use of diagnostic "markers" of osteoarthritis in synovial fluid to diagnose the disease in an earlier stage and perhaps assess the severity of the disease and monitor the effect of a treatment. In the present study, potential markers are characterized, discussed, and grouped according to the latest knowledge on the etiology, pathogenesis, and pathology of osteoarthritis. They are grouped according to their origin as either cartilage degradation products, or related to the mechanisms of cartilage degradation, or related to chrondrocytic anabolic activity during disease, or related to genetic disorders. Also potential markers that have not yet been studied clinically or experimentally are discussed. Examples of the progress that has been made in human medicine approaching reliable diagnostic markers that should also be tried in veterinary medicine are described.

Expression of the murine plasma cell nucleotide pyrophosphohydrolase PC-1 is shared by human liver, bone, and cartilage cells. Regulation of PC-1 expression in osteosarcoma cells by transforming growth factor-beta

A bone and cartilage enzyme with both 5'-nucleotide phosphodiesterase I and nucleotide pyrophosphohydrolase (NTPPPH) activity modulates physiologic mineralization and pathologic chondrocalcinosis by generating inorganic pyrophosphate. We hypothesized that, as for alkaline phosphatase, expression of an NTPPPH gene can be shared by cells from bone, cartilage, and liver and by certain leukocytes. Recently, we demonstrated the hepatocyte and murine plasma cell membrane glycoprotein PC-1 to have both 5'-nucleotide phosphodiesterase I and NTPPPH activity. We detected polypeptides cross-reactive with PC-1 in human U20S osteosarcoma cells, articular chondrocytes, homogenized human knee cartilages, human knee synovial fluids, hepatoma cells, and murine plasmacytoma cells. Constitutive low abundance PC-1 mRNA expression was detected in U20S cells and chondrocytes by a nested RNA-PCR assay and by Northern blotting. TGF beta is known to substantially increase NTPPPH activity in primary osteoblast cultures. We demonstrated that TGF beta 1 increased NTPPPH activity and the level of PC-1 mRNA and immunoprecipitable [35S]-methionine-labeled PC-1 polypeptides in U20S cells. The identification of PC-1 as an NTPPPH expressed in cells derived from bone and cartilage may prove useful in furthering the understanding of the role of NTPPPH i n physiologic and pathologic mineralization.

Cloning of the chicken alpha 3(IX) collagen chain completes the primary structure of type IX collagen

Type IX collagen is composed of three genetically distinct polypeptides that contain several collagenous and non-collagenous domains. The alpha 2(IX) chain also contains a covalently bound glycosaminoglycan side chain. Type IX collagen is located on the surface of collagen fibrils of both hyaline cartilage and vitreous humor, such that one of the collagenous domains (COL3) projects from the surface of the fibril in a periodic manner. We have cloned and sequenced a full-length cDNA for the chicken alpha 3(IX) collagen chain from a cartilage cDNA library. Together with the sequence of the alpha 1(IX) and alpha 2(IX) chains, this completes the primary structure of type IX collagen for one species. These sequences will be useful to better understand the mechanism of triple-helix formation in type IX collagen and the nature of type II and type IX collagen interactions in fibril formation.

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

Inorganic pyrophosphate (PPi) levels were estimated by radiometric assay in urine and in synovial fluid (SF) from asymptomatic, nonarthritic knees of patients with untreated metabolic disease and normal controls. SF PPi was significantly elevated in patients with hyperparathyroidism (mean +/- SEM 19 +/- 3 microM; n = 9), hemochromatosis (23 +/- 5 microM; n = 6), and hypomagnesemia (27 +/- 0.1 microM; n = 2) compared with normal subjects (10 +/- 0.5 microM, n = 50), and was low in patients with hypothyroidism (4.2 +/- 2.3 microM; n = 11) (P less than 0.05 all comparisons). Urinary PPi was elevated only in those with hypophosphatasia. Local elevation of ionic PPi may be relevant to the mechanism of crystal formation in metabolic diseases predisposing to calcium pyrophosphate dihydrate (CPPD) crystal deposition. The finding of low SF PPi levels in patients with hypothyroidism further questions the association between this condition and CPPD.