Discovery of the new genes - New Technology - Mouse models for mitochondrial pathologies Mouse models for mitochondrial pathologies
Mitochondrial disorders are currently defined as clinical phenotypes associated with abnormalities of the mitochondrial energy metabolism, i.e. oxidative phosphorylation (OXPHOS). OXPHOS is carried out in the inner mitochondrial membrane by the five enzymatic complexes of the respiratory chain. OXPHOS disorders are usually associated with specific biochemical defects of one or more respiratory chain complexes, and/or with peculiar morphological changes in the muscle biopsy of affected individuals. One of the main difficulties that challenges the investigators interested in mitochondrial disorders is the complex pathogenetic mechanisms that lead to the different clinical and biochemical phenotypes described in humans. To proceed further in the understanding of the physiopathology of these conditions, we must obtain an integrated view of the fundamental pathogenetic processes underlying the disease phenotypes. To this scope a few in vivo models have been developed recently, such as conditional KO mice for Tfam, the most important transcription (and replication-initiation) factor of mtDNA, or heteroplasmic mice carrying a large-scale deletion of the mtDNA, similar to what is found in Kearns-Sayre syndrome or Progressive External Ophthalmoplegia (PEO). However, because of the great complexity of OXPHOS, the dual genetic system controlling this metabolic pathway, and the heterogeneity of its abnormalities, more models must be produced, that can recapitulate, at least in part, the different phenotypes and etiologies documented in humans. Last but not least, the production of animal models will also open the possibility to test new therapeutic approaches to these disorders, for which no effective treatment is at the moment available.
Our group has already shown that Surf1, a gene located on chromosome 9q34, is a major gene responsible for most of the cases of Leigh Syndrome due to cytochrome c oxidase (COX) deficiency (1,2), LSCOX, one of the most common disorders of the mitochondrial respiratory chain in infancy. Loss-of-function of SURF-1 protein seems to be specifically associated with LSCOX, although a proportion of cases must be due to abnormalities in gene(s) other than SURF-1. SURF-1 was the first nuclear gene to be consistently mutated in a major category of respiratory chain defects. To better understand the role of Surf1p and the pathogenesis of LSCOX, we created a constitutive knockout (KO) mouse model for Surf1. The murine phenotype is characterized by the following hallmarks. 1) High post-implantation embryonic lethality, affecting approximately 90% of the -/- individuals; 2) early-onset mortality of post-natal individuals; 3) highly significant deficit in muscle strength and motor performance; 4) profound and isolated defect of COX activity in skeletal muscle and liver, and, to a lesser extent, heart and brain; 5) reduced histochemical reaction to COX and mitochondrial proliferation in skeletal muscle; 6) no obvious abnormality in brain morphology, reflecting the virtual absence of overt neurological symptoms. Further characterization of the phenotype is necessary. We also created a loxable recombinant model that determines the production of a null Surf1 allele with a minimum structural perturbation of the region, similar to the spontaneous human mutations associated to LS-COX. Such model shows some of the biochemical and histological abnormalities of the previous model, loosing the embryonic lethality. Additional investigation on these animals will include (i) clinical and neuropathological studies on the effects of pharmacologically induced epilepsy, (ii) the investigation of apoptotic markers in different tissues of the KO animals (iii) the assessment of motor performance and neuropathology during the aging process. The availability of different Surf1 KO animal models will allow us to better understand the role of this protein in the assembly of COX and try different therapeutic strategies.
References 1. Zhu Z, Yao J, Johns T, Fu K, De Bie I, Macmillan C, Cuthbert AP, Newbold RF, Wang J, Chevrette M, Brown GK, Brown RM, Shoubridge EA.
SURF1, encoding a factor involved in the biogenesis of cytochrome c oxidase, is mutated in Leigh syndrome. Nat Genet. 1998 Dec; 20(4):337-43. 2. Tiranti V, Hoertnagel K, Carrozzo R, Galimberti C, Munaro M, Granatiero M, Zelante L, Gasparini P, Marzella R, Rocchi M, Bayona-Bafaluy MP, Enriquez JA, Uziel G, Bertini E, Dionisi-Vici C, Franco B, Meitinger T, Zeviani M. Mutations of SURF-1 in Leigh disease associated with cytochrome c oxidase deficiency. Am J Hum Genet. 1998 Dec;63(6):1609-21. 3. Inoue K, Nakada K, Ogura A, Isobe K, Goto Y, Nonaka I, Hayashi JI. Generation of mice with mitochondrial dysfunction by introducing mouse mtDNA carrying a deletion into zygotes. Nat Genet. 2000 Oct;26(2):176-81. 4. Agostino A, Invernizzi F, Tiveron C, Fagiolari G, Prelle A, Lamantea E, Giavazzi, A, Battaglia G, Tiranti V, Zeviani M. Absence of Surf1 protein causes high embryonic lethality, mitochondrial disease and specific COX deficiency in mice. Hum. Mol. Gen.
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