Mitochondrial retinopathies

The retina is an exquisite target for defects of oxidative phosphorylation (OXPHOS) associated with mitochondrial impairment. Retinal involvement occurs in two ways, retinal dystrophy (retinitis pigmentosa) and subacute or chronic optic atrophy, which are the most common clinical entities. Both can present as isolated or virtually exclusive conditions, or as part of more com-plex, frequently multisystem syndromes. In most cases, mutations of mtDNA have been found in association with mitochondrial retinopathy. The main genetic abnormalities of mtDNA include mutations associated with neurogenic muscle weakness, ataxia and retinitis pigmentosa (NARP) sometimes with earlier onset and increased severity (maternally inherited Leigh syndrome, MILS), single large-scale deletions determining Kearns–Sayre syndrome (KSS, of which retinal dystrophy is a cardinal symptom), and mutations, particularly in mtDNA-encoded ND genes, associated with Leber hereditary optic neuropathy (LHON). However, mutations in nuclear genes can also cause mito-chondrial retinopathy, including autosomal recessive phenocopies of LHON, and slowly progressive optic atrophy caused by dominant or, more rarely, recessive, mutations in the fusion/mitochondrial shaping protein OPA1, encoded by a nuclear gene on chromosome 3q29

AAV9-based gene therapy partially ameliorates the clinical phenotype of a mouse model of Leigh syndrome

Leigh syndrome (LS) is the most common infantile mitochondrial encephalopathy. No treatment is currently available for this condition. Mice lacking $\textit{Ndufs4}$, encoding NADH: ubiquinone oxidoreductase iron-sulfur protein 4 recapitulates the main findings of complex I related LS, including severe multisystemic complex I deficiency and progressive neurodegeneration. In order to develop a gene therapy approach for LS, we used here an AAV2/9 vector carrying the human $\textit{NDUFS4}$ coding sequence (hNDUFS4). We administered AAV2/9-$\textit{hNDUFS4}$ by intravenous (IV) and/or intracerebroventricular (ICV) routes to either newborn or young $\textit{Ndufs4}$$^{-/-}$ mice. We found that IV administration alone was only able to correct the complex I deficiency in peripheral organs, while ICV administration partially corrected the deficiency in the brain. However, both treatments failed to improve the clinical phenotype or to prolong the lifespan of $\textit{Ndufs4}$$^{-/-}$ mice. In contrast, combined IV and ICV treatments resulted, along with increased complex I activity, in the amelioration of the rotarod performance, and in a significant prolongation of the lifespan. Our results indicate that extraneurological organs play an important role in LS pathogenesis, and provide an insight into current limitations of AAV-mediated gene therapy in multisystem disorders. These findings warrant future investigations to develop new vectors able to efficiently target multiple organs.This work was supported by the Core Grant from the MRC, ERC advanced grant FP7- 322424 and NRJ-Institut de France Grant (to MZ) and by the grant [GR-2010-2306- 756] from the Italian Ministry of Health (to CV)

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing has led and will allow to identify additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients’ specimen, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actors underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I, III and IV

RCC1L (WBSCR16) isoforms coordinate mitochondrial ribosome assembly through their interaction with GTPases

Mitochondrial translation defects can be due to mutations affecting mitochondrial-or nuclear-encoded components. The number of known nuclear genes involved in mitochondrial translation has significantly increased in the past years. RCC1L (WBSCR16), a putative GDP/GTP exchange factor, has recently been described to interact with the mitochondrial large ribosomal subunit. In humans, three different RCC1L isoforms have been identified that originate from alternative splicing but share the same N-Terminus, RCC1LV1, RCC1LV2 and RCC1LV3. All three isoforms were exclusively localized to mitochondria, interacted with its inner membrane and could associate with homopolymeric oligos to different extent. Mitochondrial immunoprecipitation experiments showed that RCC1LV1 and RCC1LV3 associated with the mitochondrial large and small ribosomal subunit, respectively, while no significant association was observed for RCC1LV2. Overexpression and silencing of RCC1LV1 or RCC1LV3 led to mitoribosome biogenesis defects that resulted in decreased translation. Indeed, significant changes in steady-state levels and distribution on isokinetic sucrose gradients were detected not only for mitoribosome proteins but also for GTPases, (GTPBP10, ERAL1 and C4orf14), and pseudouridylation proteins, (TRUB2, RPUSD3 and RPUSD4). All in all, our data suggest that RCC1L is essential for mitochondrial function and that the coordination of at least two isoforms is essential for proper ribosomal assembly

Human diseases associated with defects in assembly of OXPHOS complexes

The structural biogenesis and functional proficiency of the multiheteromeric complexes forming the mitochondrial oxidative phosphorylation system (OXPHOS) require the concerted action of a number of chaperones and other assembly factors, most of which are specific for each complex. Mutations in a large number of these assembly factors are responsible for mitochondrial disorders, in most cases of infantile onset, typically characterized by biochemical defects of single specific complexes. In fact, pathogenic mutations in complex-specific assembly factors outnumber, in many cases, the repertoire of mutations found in structural subunits of specific complexes. The identification of patients with specific defects in assembly factors has provided an important contribution to the nosological characterization of mitochondrial disorders, and has also been a crucial means to identify a huge number of these proteins in humans, which play an essential role in mitochondrial bioenergetics. The wide use of next generation sequencing (NGS) has led to and will allow the identifcation of additional components of the assembly machinery of individual complexes, mutations of which are responsible for human disorders. The functional studies on patients\u2019 specimens, together with the creation and characterization of in vivo models, are fundamental to better understand the mechanisms of each of them. A new chapter in this field will be, in the near future, the discovery of mechanisms and actions underlying the formation of supercomplexes, molecular structures formed by the physical, and possibly functional, interaction of some of the individual respiratory complexes, particularly complex I (CI), III (CIII), and IV (CIV)