Abstract 807 Genetic Basis of Disease I Platform, Monday, 5/3

ATP-binding cassette (ABC) transporters are members of a superfamily of membrane proteins involved in the transport of a variety of molecules across biological membranes. There are four known human peroxisome membrane proteins in the ABC transporters: PMP70, ALDP, ALDP-related (ALDR) and PMP70-related (P70R). Mutations in ALD are responsible for X-ALD and result in accumulation of very long chain fatty acids (VLCFA) due to impaired β-oxidation in peroxisomes. The function of the other 3 peroxisomal ABC transporters is unknown. To better understand PMP70 function we targeted the gene in mice by standard methods. Offspring of heterozygote matings occur in the expected Mendelian ratios. Absence of the PMP70 mRNA and protein confirmed complete inactivation of this gene in the PMP70-/- animals. Immunohistochemistry on PMP70-/- fibroblasts showed that peroxisomes are present and contain normal amounts of PTS1 and PTS2 targeted matrix proteins. Thus, PMP70 is not required in peroxisome biogenesis. Immunoblots of the PMP70-/- mouse liver and kidney showed a specific increase (>20X) of P70R suggesting this may compensate for the absence of PMP70. Steady state levels of P70R mRNA were unchanged indicating the mechanism for P70R accumulation in PMP70-/- is post transcriptional. Additionally we found a dramatic (10X) reduction in hepatic glycogen of the PMP70-/- animals in the fed state. Glycogen synthase mRNA and activity levels are not different from control thus the mechanism for this may relate to increased glycogen utilization. Finally, the PMP70-/- mice also showed a striking dicarboxylic aciduria in both the fed and fasted states. Because oxidation of dicarboxylic acids (DCA) normally occurs mainly in peroxisomes, we speculate that PMP70 may be involved in the uptake of these metabolites into peroxisomes and that accumulation of DCA may have regulatory effects that explain the other phenotypic features of the PMP70-/- mice.