Targeted ablation of Abcc1 or Abcc3 in Abcc6−/− mice does not modify the ectopic mineralization process

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Abstract
Pseudoxanthoma elasticum (PXE) is a heritable disorder characterized by ectopic mineralization of connective tissues, with considerable intra- and interfamiliar phenotypic variability. PXE is caused by mutations in the ABCC6 gene, which encodes a transporter protein, MRP6, and targeted ablation of Abcc6 in mice recapitulates the manifestations of PXE. In this study, we examined the hypothesis that the expression of other members of the Abcc family may be altered in Abcc6 null mice, possibly explaining the phenotypic variability because of the functional overlap of these transporters. Analysis of the transcript levels of Abcc1-10 and 12 in the liver of Abcc6 (-/-) mice by quantitative RT-PCR indicated that the levels of other C family mRNAs were not significantly different from wild-type mice. Next, we developed Abcc6/1(-/-) and Abcc6/3(-/-) double null mice and examined them for tissue mineralization. Histopathologic examination, coupled with computerized morphometric analysis, and chemical assay of calcium x phosphate product in the muzzle skin of Abcc1(-/-) and Abcc3(-/-) mice did not reveal evidence of mineralization. Abcc6/1(-/-) and Abcc6/3(-/-) double knock-out mice exhibited connective tissue mineralization similar to that in Abcc6 (-/-) mice. These results emphasize the importance of the Abcc6 gene in the ectopic mineralization process and further suggest that other members of the Abcc family, particularly Abcc1 and Abcc3, do not modulate the effects of Abcc6 in this mouse model.