ACDC results from deficiency of CD73, a glycosyl phosphatidylinositol (GPI)-anchored protein that is expressed widely in different tissues such as the colon, kidney, brain, liver, heart and lung. Within the vasculature, CD73 functions in several cell types, including endothelial cells, vascular smooth muscle cells and fibroblasts, by modulating purinergic signaling. Impairment of that signaling results in increased alkaline phosphatase activity within CD73-deficient fibroblasts in vitro [3
] (). The increased alkaline phosphatase, which promotes calcification [7
] in part by reducing pyrophosphate concentrations (), can be rescued by adenosine, the product of CD73 catalysis [3
]. We hypothesize that, in ACDC, the specific vessels (and also the joint capsules) that calcify are determined by their contingent of adenosine receptors; studies in the A2b receptor knockout mouse illustrate that different vascular beds have different A2b adenosine receptor expression patterns [8
Fig. 7 A model of the purinergic pathway in vascular cells. ATP is converted to pyrophosphate and AMP by ENPP1. CD73 converts AMP to inorganic phosphate and adenosine, which binds to plasma membrane receptors that induce intracellular signaling. This lowers (more ...)
We now demonstrate that CD73 deficiency, due to bi-allelic mutations of NT5E
, involves calcification of the internal elastic lamina of affected vessels. This previously unrecognized finding has relevance for Marfan syndrome, Loeys-Dietz syndrome, some forms of isolated aortic aneurysms [9
], chronic renal failure [10
] associated with longstanding diabetes, or Monkeberg arteriosclerosis [12
], and other disorders [13
] in which the genesis and repair of elastic fibers play critical roles.
Calcification of elastic fibers also plays a critical role in the pathology of PXE [14
], which involves small vessels and the possibility of a circulating factor [15
]. PXE results from bi-allelic mutations affecting the transport protein ABCC6 [16
]. ABC membrane transporters of the C class (specifically, ABCC4, ABCC5 and ABCC8) carry nucleotides out of cells in an ATP-dependent fashion [17
]; the ligand for ABCC6, however, has not been identified. We propose that adenosine or AMP could be the ligand for ABCC6, whose function would involve delivering adenosine to cells of small vessels.
Two lines of evidence support this possibility. First, PXE and ACDC closely resemble each other with respect to the histopathological findings of broken and calcified elastic fibers (Figs. -). Second, on clinical grounds, PXE has been mistaken for another disorder of adenosine production, Generalized Arterial Calcification of Infancy (GACI). This autosomal recessive disorder due to biallelic mutations in ENPP1
() presents with calcification of large and medium-sized arteries and results in arterial stenosis, myocardial ischemia, and death in early childhood [20
]. The mechanism of calcification in GACI is considered to be reduced production of PPi from ATP because of deficient ENPP1. However, in GACI there may also be a paucity of local adenosine because of reduced substrate (AMP) for the downstream enzyme, CD73. The greater severity of clinical and pathological findings in GACI compared with ACDC could be explained by the dual reduction of pyrophosphate concentrations in GACI, i.e., from decreased production by ENPP1 and from reduced adenosine signaling (). In 2010, Le Boulanger et al. described a documented (genetically confirmed) PXE patient whose sibling died in infancy with the clinical diagnosis of GACI [21
]. Apparently, PXE can present with such severity that it is mistaken for GACI.
Knowledge of the involvement of adenosine in preventing vascular calcification allows for therapeutic interventions Specifically, the reversal of cellular calcification by adenosine suggests that therapy for arterial medial calcification due to decreased inhibition of alkaline phosphatase might be attempted using adenosine analogs, metabolizable or not. Presumably, the adenosine pathway could also be targeted in cases of diabetic arterial calcification of medium sized vessels, such as Monkeberg arteriosclerosis, and for small vessels, as in PXE.