Pseudoxanthoma elasticum: clinical phenotypes, molecular genetics and putative pathomechanisms

Abstract

Pseudoxanthoma elasticum (PXE), a prototype of heritable multisystem disorders, is characterised by pathologic mineralisation of connective tissues, with primary clinical manifestations in the skin, eyes and the cardiovascular system. The causative gene was initially identified as ABCC6 which encodes an ABC transporter protein (ABCC6) expressed primarily in the liver and the kidneys. The critical role of ABCC6 in ectopic mineralisation has been confirmed by the development of Abcc6(-/-) knock-out mice which recapitulate the features of connective tissue mineralisation characteristic of PXE. Over 300 distinct loss-of-function mutations representative of over 1000 mutant alleles in ABCC6 have been identified by streamlined mutation detection strategies in this autosomal recessive disease. More recently, missense mutations in the GGCX gene, either in compound heterozygous state or digenic with a recurrent ABCC6 nonsense mutation (p.R1141X), have been identified in patients with PXE-like cutaneous findings and vitamin K-dependent coagulation factor deficiency. GGCX encodes a carboxylase which catalyses gamma-glutamyl carboxylation of coagulation factors as well as of matrix gla protein (MGP) which in fully carboxylated form serves as a systemic inhibitor of pathologic mineralisation. Collectively, these observations suggest the hypothesis that a consequence of loss-of-function mutations in the ABCC6 gene is the reduced vitamin K-dependent gamma-glutamyl carboxylation of MGP, with subsequent connective tissue mineralisation. Further progress in understanding the detailed pathomechanisms of PXE should provide novel strategies to counteract, and perhaps cure, this complex heritable disorder at the genome-environment interface.

Conflict of interest statement

Dr. Pfendner is employed by GeneDx, Inc., a commercial laboratory specializing in mutation analysis. The other authors have no financial conflicts to declare.

Figures

Figure 1

Clinical and histopathologic features of a patient with the classic form of PXE (left panel) as well as in a patients with PXE-like cutaneous features and coagulation deficiency (right panel). On the left, the patient demonstrates characteristic yellowish papules on the antecubital fossa, and skin biopsy of these lesions, when evaluated by Hematoxylin and Eosin stain, shows basophilic, elastotic material in the mid dermis (arrow). Special stains for phosphate and calcium (von Kossa and Alizarin Red stains, respectively) reveal mineralization of the elastotic material. In a 15-year old patient with PXE-like clinical features (right panel) there is evidence of yellowish primary lesions similar to those seen in classic PXE, in addition to redundant, loose and sagging skin. Histopathology is similar to that in the patient with the classic form of PXE. Note that the classic form of PXE shows early changes with less mineralization. (Modified from reference 59, with permission).

Figure 2

Schematic representation of the ABCC6 gene (a) and the encoded protein, ABCC6 (b), together with positions of mutations identified in patients with PXE. (a) The ABCC6 gene consists of 31 distinct exons (vertical blocks), some of which encode the two ATP binding domains (green exons). The arrows point to the positions of nonsense, splicing, and small insertion or deletion mutations identified in the gene. Positions of large deletions spanning multiple exons or even the entire gene are indicated below the figure. (b) The ABCC6 putative transporter protein consists of three transmembrane domains each depicting 5, 6 or 6 transmembrane segments (light blue) that traverse the plasma membrane, respectively. The protein domains corresponding to the Walker motifs within the ATP binding domains are highlighted by dark green color. Positions of missense mutations identified in the ABCC6 gene are indicated by arrows. (Modified from reference 21, with permission).

Figure 3

Distribution of missense mutations on the homology model of human ABCC6. For model building, regions aa288-856 (TMD1-ABC1) and aa948-1503 (TMD2-ABC2) were considered and the Sav1866 structure with the nucleotide AMP-PNP (PDB:20NJ) was utilized as a template. The two halves of the molecule are indicated with different colors, pink and olive (only the polypeptide backbones are visualized). PXE-associated missense mutations are highlighted with red color, and two bound nucleotides are indicated by dark green.

Figure 4

Conceptual illustration of the proposed metabolic hypothesis of PXE. Under physiologic conditions, the ABCC6 protein is expressed in high levels in the liver, presumably transporting critical metabolites to the circulation (right panel). In the absence of ABCC6 transporter activity in the liver, changes in the concentration of such substrate molecules in the circulation can take place, and the changes result in mineralization of a number of tissues, such as the eye, the arterial blood vessels, the kidney, and the skin (middle panel). The presence of mineralization is detected in transgenic Abcc6-/- mice, that recapitulate the features of human PXE, by Alizarin Red stain. (Modified from reference 33, with permission).

Figure 5

Detection of mutations in the GGCX and ABCC6 genes in a family with PXE-like cutaneous phenotype in association with vitamin K-dependent multiple coagulation factor deficiency. The proband, whose clinical features are shown in Figure 1, is identified by an arrow. Mutation analysis in the nuclear pedigree identified the presence of the p.R1141X nonsense mutation in the ABCC6 gene and two missense mutations, p.V255M and p.S300F, in the GGCX gene, as indicated on the left. (Modified from reference 59, with permission).

Figure 6

Demonstration that the matrix gla protein (MGP) is under-carboxylated in the lesional skin of a patient with classic form of PXE (left panel) as well in a patient with PXE-like cutaneous features from the family in Figure 5 (middle panel), as compared to skin from a control individual (right panel). Skin biopsies were stained with specific antibodies recognizing either the under-carboxylated (ucMGP) or the carboxylated (cMGP) forms of MGP. The secondary antibodies consisted of biotin-conjugated anti-IgG, recognized by avidin-alkaline phosphatase conjugates, and visualized by incubation with an alkaline phosphatase substrate yielding red color (Modified from reference 59, with permission).

Figure 7

Speculative schematic representation of the activation of blood coagulation factors (Glu) and under-carboxylated matrix gla protein (ucMGP) by γ-glutamyl carboxylation. Lower panel: The active γ-glutamyl carboxylated forms of the coagulation factors (Gla) and carboxylated matrix gla protein (cMGP) are secreted into circulation from hepatocytes and are required for normal blood coagulation and prevention of unwanted mineralization of peripheral tissues, respectively. The γ-glutamyl carboxylase is encoded by the GGCX gene and is dependent on reduced vitamin K (KH2) as a co-factor in the liver. The oxidized forms of vitamin K (KO) and the dietary vitamin K are reduced back to KH2 by enzymatic reactions coupled with conversion of GSH to GSSG. The role of ABCC6, the transmembrane efflux transporter, in this process is currently unknown. Upper panel: One could hypothesize that the ABCC6 protein transports vitamin K, possibly in its glutathione conjugated form, from the liver into circulation and to the peripheral tissues. Consequently, cells in peripheral tissues, such as fibroblasts in the skin, lack the ability to convert inactive ucMGP to active, fully carboxylated cMGP, possibly due to lack of KH2. Thus, deficiency of cMGP in the resident fibroblasts allows pathologic mineralization of connective tissues deposited in the pericellular matrix to proceed, resulting in PXE.