Severe early-onset manifestations of pseudoxanthoma elasticum resulting from the cumulative effects of several deleterious mutations in ENPP1, ABCC6 and HBB: transient improvement in ectopic calcification with sodium thiosulfate.

Pseudoxanthoma elasticum (PXE) is a rare disorder characterized by fragmentation and progressive calcification of elastic fibres in connective tissues. Overlap has been reported between the inherited PXE phenotype associated with ENPP1, ABCC6 or NT5E mutations and acquired PXE clinical manifestations associated with haemoglobinopathies induced by HBB mutations. No treatment is currently available for PXE. A young boy presented with severe early-onset systemic calcifications occurring in the skin as elastosis perforans serpiginosa (EPS) and in the arteries, causing mesenteric and limb ischaemia. Analyses revealed deleterious ABCC6, ENPP1 and HBB mutations. The diagnosis of severe PXE was retained and we have coined the term 'PXE+ syndrome' to describe the cumulative effects of the various mutations in this uncommon phenotype. Given the severity, rapid progression and a potentially fatal prognosis, intravenous sodium thiosulfate (STS) was initiated at 25 g three times weekly for 6 months. Numerous side-effects prompted dosage adjustment to 10 g intravenously daily. Treatment efficacy was evaluated at 6 months. Asthaenia, anorexia and pre-/postprandial pain had subsided, entailing weight gain. Abdominal EPS had diminished. Calcific stenosis of the coeliac and mesenteric arteries was no longer detectable on arterial ultrasonography. Follow-up revealed only transient efficacy of STS. Discontinuation of treatment to evaluate the persistence of effects resulted in relapse of the initial symptomatology after 4 months. STS efficacy is conceivably due to strong antioxidant properties and chelation of calcium to form soluble calcium thiosulfate complexes. This case is suggestive of PXE+ syndrome for which STS may represent potential treatment in severe cases. What's already known about this topic? Generalized arterial calcification of infancy may occur in association with ABCC6 mutations and pseudoxanthoma elasticum (PXE) can be linked to ENPP1 mutations. A PXE-like phenotype has also been reported in a subset of patients with inherited haemoglobinopathies, namely sickle cell disease or ß-thalassaemia, related to HBB mutations. To date, there is still no cure for PXE. What does this study add? We report a severe case of PXE resulting from the cumulative effects of several deleterious mutations in ENPP1, ABCC6 and HBB. We suggest the term 'PXE+ syndrome' to describe such patients. Sodium thiosulfate therapy could represent a potential option in severe cases of PXE+ syndrome.

Decreased levels of nucleotide pyrophosphatase phosphodiesterase 1 are associated with cartilage calcification in osteoarthritis and trigger osteoarthritic changes in mice.

OBJECTIVE: To analyse the function of nucleotide pyrophosphatase phosphodiesterase (NPP1), a member of the pyrophosphate pathway, in osteoarthritis (OA). METHODS: mRNA expression of NPP1, ANK ankylosing protein and tissue non-specific alkaline phosphatase was assessed by quantitative PCR. NPP1 protein levels were analysed in mouse and human cartilage samples. Bone metabolism was analysed by F18-positron emission tomography-scanning and µCT in ttw/ttw mice. Ttw/ttw mice are mice carrying a loss-of-function mutation in NPP1. Calcification of articular cartilage was assessed using von Kossa staining and OA severity using the Mankin score. Cartilage remodelling was investigated by type X collagen immunohistochemistry. RESULTS: Expression of NPP1, but not the other members of this pathway, inversely correlated with cartilage calcification and OA severity in mouse and humans. Proinflammatory cytokines downregulated the expression of NPP1, demonstrating an influence of inflammation on matrix calcification. Ttw/ttw mutant mice, carrying a loss-of-function mutation in NPP1, exhibit increased bone formation process in joints compared with wild types. Ttw/ttw mice also developed spontaneous OA-like changes, evaluated by histological analysis and in vivo imaging. Ectopic calcifications were associated with increased expression of collagen X in the cartilage. CONCLUSION: The authors conclude that OA is characterised by the reactivation of molecular signalling cascades involving proinflammatory cytokines, thereby regulating the pyrophosphate pathway which consequently leads to cartilage ossification, at least in part resembling endochondral ossification.

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.

Fetal hydrops, hyperechogenic arteries and pathological doppler findings at 29 weeks: prenatal presentation of generalized arterial calcification of infancy - a novel mutation in ENPP1.

Prenatal diagnosis of generalized arterial calcification of infancy (GACI) (OMIM #208000) is difficult and rare. There are various known gene mutations in ENPP1 (ectonucleotide pyrophosphatase/phosphodiesterase 1) locus 6q22-q23. We present a case of suspected intrauterine diagnosis at 29 weeks of gestation in a consanguineous couple. The sonographic findings were fetal hydrops (hydrothorax, skin edema, ascites, pericardial effusion and polyhydramnion), echogenic great arteries and pathological Doppler findings. An intrauterine therapy with bisphosphonates was considered, but delayed due to rapid deterioration in fetal Doppler flows with suspected fetal asphyxia. The couple was informed about the most unfavorable prognosis in fetal hydrops, however, they opted for elective delivery. A cesarean section was performed. Early neonatal death occurred due to primary intracranial hemorrhage. Postmortem and genetic testing confirmed a novel mutation in the ENPP1 gene.