Rescue of ABCC6 Disease-Causing Mutations in Mouse Liver
OCTOBER 28, 2011
by Sharon Terry, Executive Director, PXE International, and
Christine Vocke, Director of Education and Information, PXE International
This lay summary first appeared in the October 2011 eNewsletter.
Le Saux O, Fülöp K, Yamaguchi Y, Iliás A, Szabó Z, Brampton CN, Pomozi V, Huszár K, Arányi T, Váradi A. Expression and in vivo rescue of human ABCC6 disease-causing mutants in mouse liver. PLoS One. 2011;6(9):e24738. Free article in PubMed.
Mutations in the ABCC6 gene cause problems in the ABCC6 protein. This sometimes means there is no protein created, or the protein’s structure is incorrect (structural problems), or there are problems with it traveling to the membrane of the cell where it should do its work (functional trafficking problems). These problems with the protein are all categories of ‘loss of function’ – the protein doesn’t function properly. The normal function of the protein would be to transport a substance (we don’t know what that substance is) across the cell membrane. The loss of function ultimately causes mineralization of the mid-dermis of the skin, the mid-laminar layer of the arteries, and a membrane in the eye called the Bruch’s membrane.
The authors listed above, led by András Váradi, PhD, DSc, with significant research done by Olivier Le Saux, PhD, established a complex experimental strategy to determine the effect of mutations in the human ABCC6 gene. As stated above, these mutations cause structural and functional changes. To assess the effects of the mutations, the scientists studied mice and cell cultures (cells grown on a plate in a lab).
The scientists first looked for mutations that caused problems in the protein, but still allowed some of the protein’s function to remain. Although these altered proteins allowed some transport activity, they did not move through the cell to the correct location. The team studied several mutations that met these requirements. Genes with these select disease-causing mutations were injected into adult, normal (wild type) mice, through a vein in the tail. The team then looked at how the select mutations affected the liver of the mice, since the ABCC6 gene is very active in the liver. They found that two mutations, R1138Q and R1314W, allowed significant transport activity. V1298F was the only mutation that went to the proper location in the mouse liver and inserted itself in the membrane. The rest of the mutations mostly accumulated in the intracellular space (space inside the cell), and did not travel to the cell membrane. Both R1138Q and R1314W inserted themselves into the endoplasmic reticulum, an organ structure within the cell (organelle) that produces the protein and fat needed by the cell. This is not where it is supposed to be.
The group then tested whether a drug called 4-PBA (4-phenylbutyrate), approved for clinical use in other diseases, could restore their intracellular trafficking to the cell membrane in cell cultures, and in mouse liver. The trafficking of R1314W to the cell membrane was significantly improved by 4-PBA treatment, thus potentially restoring some of its normal function in the cell. This work demonstrates the feasibility of a therapy that could cause the mutated ABCC6 protein to behave somewhat normally, and allow some function. These results might have implications for human treatment therapy. This work is very preliminary. Mice are good models, but they are not humans and so more mouse research needs to be done, and then human studies will follow.
In order to understand if the protein arriving in the right place is in fact a potential therapy, there has to be something to measure as pseudoxanthoma elasticum (PXE) progresses in the body. This is called a biomarker, and we do not yet have a good biomarker for PXE. While scientists search for a potential therapy for PXE, every person affected must contribute their clinical information so that we can find the best biomarker, and be ready to measure the effects of a potential therapy in PXE. It would be a travesty if we get to the point of having a therapy and don’t yet have a biomarker!
To contribute your clinical information by completing the online Pseudoxanthoma Elasticum epidemiological survey, please click the "I am interested in participating" button.