Pex13 Mutant Mice as Models for the Peroxisome Biogenesis Disorders

Abstract

Zellweger syndrome (ZS) is the most severe form of a spectrum of disorders resulting from mutations in PEX genes, genes that encode proteins necessary for peroxisome biogenesis. Loss of functional peroxiosmes leads to disruption of multiple metabolic pathways involving the peroxisome, including very long chain fatty acid oxidation and plasmalogen and bile acid synthesis. ZS patients exhibit a range of clinical abnormalities, including facial dysmorphism, cataracts, hypotonia, seizures, psychomotor retardation, and hearing impairment. In terms of tissue pathology, there are also wide ranging effects, including neuronal migration defects, hepatomegaly, retinopathy, and renal cysts. Pex13 encodes a peroxisomal membrane protein that is essential for peroxisome biogenesis. Previous work in this laboratory resulted in the generation of a Pex13-null mouse model for the purpose of investigating the pathogenesis of Zellweger syndrome. The work in the first part of this thesis extends these studies and describes the generation and initial characterisation of tissue-specific Pex13 mouse models. These tissue-specific models are expected be useful tools for analysis of the impact of localised, brain- and liver-specific elimination of peroxisomes on the pathogenesis of ZS. In addition, in the second part of the thesis, a separate and novel hypothesis is addressed as an explanation for the molecular pathogenesis of ZS, through investigating the relationship between reduced peroxisome abundance and microtubule-mediated peroxisome trafficking. Pex13 brain mutant mice were generated by mating the previously generated Pex13-floxed mice with mice expressing Cre recombinase under the control of the neuron-specific rat nestin promoter. Pex13 brain mutant mice displayed growth retardation beginning at day 5 postnatal, with gradual deterioration until death at approximately day 22 postnatal. Other clinical features included contracted postures, under-developed lower body mass, abnormal and unsteady gait, and abnormal motor coordination. In terms of brain metabolic function, these mice exhibited significant defects in plasmalogen synthesis, but, surprisingly, VLCFA levels were similar to those of littermate control mice. Significantly, peroxisome elimination in brain resulted in increased levels of plasmalogen levels in liver of Pex13 brain mutant mice. Consistent with the expected pathology resulting from deficiency of brain peroxisomes, brain mutants exhibited defective neuronal migration characterised by increased cellular density in the intermediate zone of the neocortex. Microarray analysis of total brain RNA from Pex13 brain mutants revealed several functionally-linked pathways associated with the differentially expressed genes, including cell-cell signalling, cell compromise/death, lipid metabolism, cell movement, and serotonin synthesis.