Early arterial calcification does not correlate with bone loss in pseudoxanthoma elasticum

Abstract

Background and aims: Pseudoxanthoma elasticum (PXE; OMIM 264800, prevalence 1/25,000 to 1/50,000) is an autosomal recessive multisystem disease due to deficiency in ABCC6, an ATP-binding cassette, sub-family C transporter. The PXE phenotype is mainly characterized by progressive ectopic calcification of connective tissues (namely skin, retinal Bruch's membrane and peripheral arteries) but the impact of PXE on bone structure is currently unknown. The present study sought to investigate bone mineralization and its potential link with vascular calcification in a large cohort of PXE patients with inherited mutations of the ABCC6 gene.

Methods and results: 96 patients (61 women) matching the PXE criteria participated in this study. Their clinical history and status and bone biological markers were collected. Bone mineral density (BMD) was measured by dual energy X-ray absorptiometry and expressed as T- and Z-scores. Osteoporotic fractures were identified by X-ray, and coronary (CAC) and lower limb arterial calcification (LLAC) scores were determined by CT scan.

Results: 44% of the women were menopausal. Osteopenia was disclosed in 46% (17 women) while 23% (9 women) exhibited osteoporosis, 3 with severe osteoporosis. Fractures of an osteoporotic nature were authenticated in 3 patients (1 woman). Markers of bone remodelling processes (CTX, BSAP and osteocalcin) were within the normal range for our laboratory standards. Severe vitamin D deficiency (<25nmol/L) was found in 15%, while 51% exhibited no vitamin D deficiency (vitamin D≥50nmol/L). LLAC and CAC scores were significantly higher in the patients with a low T- and/or Z-score, although this difference disappeared in multivariate analysis with age as a confounding factor. There was no significant difference in LLAC and CAC between PXE patients with and without osteoporotic fractures. There was no statistically significant association between BMD, LLAC and CAC and any of the bone remodelling factors.

Conclusions: This is the first report on the bone mineralization process in PXE patients. Our data shows that PXE patients are not markedly prone to exaggerated bone demineralization and fracture risk, and prevalence of osteoporosis remains within the normal range for the general population. Furthermore, the relationships between LLAC, but not CAC, and BMD with age are similar to those observed in the general population. Therefore, despite its pivotal role in ectopic calcification, ABCC6 deficiency does not interfere with the bone-vascular axis. The lack of PXE-related disturbances between BMD and arterial calcification also supports vitamin D supplementation in PXE patients with vitamin D deficiency. ClinicalTrials.gov Identifier: NCT01446393.

Keywords: ABCC6; Arterial calcification; Osteoporosis; Pseudoxanthoma elasticum.