The ABCC6 Transporter: A New Player in Biomineralization

Abstract

Pseudoxanthoma elasticum (PXE) is an inherited metabolic disease with autosomal recessive inheritance caused by mutations in the ABCC6 gene. Since the first description of the disease in 1896, alleging a disease involving the elastic fibers, the concept evolved with the further discoveries of the pivotal role of ectopic mineralization that is preponderant in the elastin-rich tissues of the skin, eyes and blood vessel walls. After discovery of the causative gene of the disease in 2000, the function of the ABCC6 protein remains elusive. More than 300 mutations have been now reported and the concept of a dermal disease has progressively evolved toward a metabolic disorder resulting from the remote effects caused by lack of a circulating anti-mineralization factor. Very recently, evidence has accumulated that this anti-mineralizing factor is inorganic pyrophosphate (PPi). This leads to decreased PPi/Pi (inorganic phosphate) ratio that results from the lack of extracellular ATP release by hepatocytes and probably renal cells harboring the mutant ABCC6 protein. However, the mechanism by which ABCC6 dysfunction causes diminished ATP release remains an enigma. Studies of other ABC transporters, such as ABCC7 or ABCC1 could help our understanding of what ABCC6 exact function is. Data and a hypothesis on the possible roles of ABCC6 in acquired metabolic diseases are also discussed.

Keywords: ABC transporter; arterial calcifications; chronic kidney disease; inorganic pyrophosphate; pseudoxanthoma elasticum.

Figure 1

Topology model illustrating the distribution of PXE-associated missense mutations in the ABCC6 protein. Legend: nucleotidE binding domains and the transmembrane helices are shown in grey. Missense mutations are represented in red. The transmembrane domains (TMD0, TMD1, and TMD2) and the linker regions (L0, L1) of the protein are indicated.