Matrix Gla protein is involved in elastic fiber calcification in the dermis of pseudoxanthoma elasticum patients

Abstract

Mature MGP (Matrix g-carboxyglutamic acid protein) is known to inhibit soft connective tissues calcification. We investigated its possible involvement in pseudoxanthoma elasticum (PXE), a genetic disorder whose clinical manifestations are due to mineralization of elastic fibers. PXE patients have lower serum concentration of total MGP compared to controls (Po0.001). Antibodies specific for the noncarboxylated (Glu-MGP) and for the g-carboxylated (Gla-MGP) forms of MGP were assayed on ultrathin sections of dermis from controls and PXE patients. Normal elastic fibers in controls and patients were slightly positive for both forms of MGP, whereas Gla-MGP was more abundant within control's than within patient's elastic fibers (Po0.001). In patients' calcified elastic fibers, Glu-MGP intensively colocalized with mineral precipitates, whereas Gla-MGP precisely localized at the mineralization front. Data suggest that MGP is present within elastic fibers and is associated with calcification of dermal elastic fibers in PXE. To investigate whether local cells produce MGP, dermal fibroblasts were cultured in vitro and MGP was assayed at mRNA and protein levels. In spite of very similar MGP mRNA expression, cells from PXE patients produced 30% less of Gla-MGP compared to controls. Data were confirmed by immunocytochemistry on ultrathin sections. Normal fibroblasts in vitro were positive for both forms of MGP. PXE fibroblasts were positive for Glu-MGP and only barely positive for Gla-MGP (Po0.001). In conclusion, MGP is involved in elastic fiber calcification in PXE. The lower ratio of Gla-MGP over Glu-MGP in pathological fibroblasts compared to controls suggests these cells may play an important role in the ectopic calcification in PXE.

Low serum vitamin K in PXE results in defective carboxylation of mineralization inhibitors similar to the GGCX mutations in the PXE-like syndrome

“…The immunoreaction for ucMGP, the inactive form of MGP, was slightly more intense on PXE elastic fibers compared with controls (Tables 3 and 4). Moreover, as already reported, 20 it was confirmed that ucMGP antibodies were associated with either bulky and finely dispersed mineral precipitates inside elastic fibers (Figure 5c), whereas cMGP antibodies nicely and precisely localized at the border of the mineralized areas of calcified elastic fibers (Figure 5d). …”

“…In general, PXE fibroblasts were less positive for cMGP compared with controls (Table 3). 20 In the extracellular space, the distribution of positive immunostaining for cMGP and ucMGP gave results almost identical to those in patients with the PXE-like syndrome (Figure 5c and d). Irrespective of the calcification degree, elastic fibers were the only extracellular matrix component positive for both antibodies.…”

“…Recently reported findings already point toward a role of MGP in classic PXE, but neither the involvement of other Gla proteins nor the mechanism for such involvement have been studied yet. [19][20][21][22] Because of the remarkable phenotypical similarities of the PXE-like syndrome and PXE and the absence in PXE of GGCX or VKORC1 mutations, which both have the potential of disrupting the VK cycle, we wondered whether the VK cycle could also be disturbed by another mechanism in PXE. More precisely, we hypothesized that disturbances in Gla proteins in PXE patients could be linked to VK, the co-factor in the VK cycle and an essential mediator of Gla-protein carboxylation.…”

Gene expression profiling of ABC transporters in dermal fibroblasts of pseudoxanthoma elasticum patients identifies new candidates involved in PXE pathogenesis

“…12,13 Indeed, several studies reported alterations in systemic blood components in PXE patients and mouse models due to MRP6 deficiency. [14][15][16][17][18][19][20] Accordingly, PXE was assumed to be primarily a metabolic disorder. 12,[21][22][23] In contrast, results of several studies also point to a local role for ABCC6/MRP6 as expression of the protein was detected in dermal fibroblasts from healthy controls whereas it was found to be absent in fibroblasts from PXE patients.…”

Modifier genes in pseudoxanthoma elasticum: novel insights from the Ggcx mouse model

“…Importantly, cMGP is not detected in lesional skin from PXE patients, which supports its possible implication in the abnormal mineralization process seen in the disease (see Fig. 1) [20,21].…”