Frataxin Restoration in the Nervous System: Possibilities for Gene Therapy
Authors: David R. Lynch, Elizabeth Kichula, Hong Lin
Journal: Molecular Therapy (2018)

Friedreich ataxia (FRDA) is a progressive neurodegenerative disease of children and young adults resulting from deficiency of the mitochondrial protein frataxin, which is encoded by a nuclear gene. A variety of therapies have been proposed to treat or reverse FRDA, mainly based on augmentation of mitochondrial function or increasing levels of frataxin production.1Strawser C.J. Schadt K.A. Lynch D.R. Therapeutic approaches for the treatment of Friedreich's ataxia.Expert Rev. Neurother. 2014; 14: 949-957Crossref PubMed Scopus (33) Google Scholar Of these therapies, one of the more promising is gene therapy, based on impressive results following treatment of a cardiac mouse model of FRDA with AAV9-FXN. In mice with a severe complete tissue-specific knockout of cardiac frataxin, this agent reverses cardiomyopathy leading to near complete biochemical and behavioral reversal.2Perdomini M. Belbellaa B. Monassier L. Reutenauer L. Messaddeq N. Cartier N. Crystal R.G. Aubourg P. Puccio H. Prevention and reversal of severe mitochondrial cardiomyopathy by gene therapy in a mouse model of Friedreich's ataxia.Nat. Med. 2014; 20: 542-547Crossref PubMed Scopus (141) Google Scholar Such results provide the initial impetus for investigation of cardiac gene therapy in humans with FRDA; however, such interventions would ideally act both in the heart as well as the nervous system. In this issue of Molecular Therapy, Piguet and colleagues3Piguet F. de Montigny C. Vaucamps N. Reutenauer L. Eisenmann A. Puccio H. Rapid and complete reversal of sensory ataxia by gene therapy in a novel model of Friedreich ataxia.Mol. Ther. 2018; 26 (this issue): 1940-1952Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar have extended the previous work from their laboratory on gene therapy in mouse models of FRDA to include the nervous system. In this work, they create a new model of frataxin deficiency using Cre-lox technology to produce a frataxin knockout in parvalbumin-containing neurons, in order to target the large dorsal root ganglion (DRG) cells that are an early site of neuronal degeneration in FRDA. Clinically, FRDA patients present with difficulty walking and loss of coordination, initially in the lower extremities.4Pandolfo M. Friedreich ataxia: the clinical picture.J. Neurol. 2009; 256: 3-8Crossref PubMed Scopus (290) Google Scholar Over time, this progresses to more diffuse ataxic symptoms, affecting both fine motor function as well as speech. The disease is characterized by early injury to proprioceptive neurons, more progressive cerebellar disease, and deterioration of the corticospinal tracts.5Koeppen A.H. Friedreich's ataxia: pathology, pathogenesis, and molecular genetics.J. Neurol. Sci. 2011; 303: 1-12Abstract Full Text Full Text PDF PubMed Scopus (286) Google Scholar, 6Morral J.A. Davis A.N. Qian J. Gelman B.B. Koeppen A.H. Pathology and pathogenesis of sensory neuropathy in Friedreich's ataxia.Acta Neuropathol. 2010; 120: 97-108Crossref PubMed Scopus (58) Google Scholar Individuals commonly die of coexistent cardiomyopathy.7Tsou A.Y. Paulsen E.K. Lagedrost S.J. Perlman S.L. Mathews K.D. Wilmot G.R. Ravina B. Koeppen A.H. Lynch D.R. Mortality in Friedreich ataxia.J. Neurol. Sci. 2011; 307: 46-49Abstract Full Text Full Text PDF PubMed Scopus (183) Google Scholar At the molecular level, expanded GAA repeats in the first intron of the FXN gene lead to decreased levels of frataxin, consequently causing a deficiency of iron sulfur cluster-containing enzymes and mitochondrial dysfunction.8Campuzano V. Montermini L. Moltò M.D. Pianese L. Cossée M. Cavalcanti F. Monros E. Rodius F. Duclos F. Monticelli A. et al.Friedreich's ataxia: autosomal recessive disease caused by an intronic GAA triplet repeat expansion.Science. 1996; 271: 1423-1427Crossref PubMed Scopus (2297) Google Scholar, 9Rötig A. de Lonlay P. Chretien D. Foury F. Koenig M. Sidi D. Munnich A. Rustin P. Aconitase and mitochondrial iron-sulphur protein deficiency in Friedreich ataxia.Nat. Genet. 1997; 17: 215-217Crossref PubMed Scopus (872) Google Scholar As the loss of frataxin is ubiquitous, the exact rationale for the selective loss of cardiac cells and specific neurons (large DRG neurons, dentate nucleus of cerebellum) remains unclear. The new model presented in this issue of Molecular Therapy selectively knocks out frataxin production early in the large DRG neurons and later in the parvalbumin-containing cells of the brain, particularly the Purkinje cells of the cerebellum and scattered cortical interneurons. This produces a severe ataxia, based initially on the loss of the sensory input followed by the subsequent effects on Purkinje neurons; the early sensory based ataxia is reversed by restoration of frataxin expression in these cells by AAV9-FXN. This provides proof of concept for gene therapy targeting the nervous system in FRDA, a crucial step in the move toward a successful treatment for this disease. The work also elegantly identifies a series of other phenomena that may help understand the neurobiology of FRDA. First, using this model they identify higher levels of frataxin in the large DRG neurons compared with other DRG components, perhaps explaining the predisposition of these cells to die in FRDA. They also establish a lumbar to cervical gradient of expression of frataxin in DRG, matching the gradient of DRG dysfunction in FRDA. Consequently, this approach provides a rationale at the level of frataxin expression for one component of the selective neuronal vulnerability in FRDA. Furthermore, even with deprivation of frataxin for weeks, DRG neurons do not die, they only become highly dysfunctional. This has implications for novel treatments, including gene therapy, as it remains unclear up to what point in the disease process gene therapy and frataxin restoration could be expected to be beneficial in causing not only a pause in future neurodegeneration, but in actual improvement and recovery of injured neurons. Finally, this model provides a second component to the ataxia, with knockout of frataxin in the Purkinje cells of the cerebellum. This matches the two components of ataxia of FRDA, with an early onset sensory ataxia followed by an ataxia of cerebellar origin. While Purkinje cells have not been thought to be a large source of neurodegeneration in humans with FRDA, recent studies suggest the possibility of loss of synapses on Purkinje cells as a mechanism in FRDA, which shows the relevance of this model to human disease.10Lin H. Magrane J. Rattelle A. Stepanova A. Galkin A. Clark E.M. Dong Y.N. Halawani S.M. Lynch D.R. Early cerebellar deficits in mitochondrial biogenesis and respiratory chain complexes in the KIKO mouse model of Friedreich ataxia.Dis. Model. Mech. 2017; 10: 1343-1352Crossref PubMed Scopus (40) Google Scholar, 11Kemp K.C. Cook A.J. Redondo J. Kurian K.M. Scolding N.J. Wilkins A. Purkinje cell injury, structural plasticity and fusion in patients with Friedreich's ataxia.Acta Neuropathol. Commun. 2016; 4: 53Crossref PubMed Scopus (27) Google Scholar While the experiments here provide proof of concept for nervous system gene therapy in FRDA, there are multiple potential confounders affecting its translation to human gene therapy. First, in contrast to the mouse generated here, FRDA is not a complete knockout, but rather a knock down of frataxin expression. This leads to a more slowly progressive illness in humans compared to mice, with the temporal course of progression in humans being roughly 100 times longer than in these mice. Consequently, the downstream effects of frataxin deficiency may differ substantially, with possible compensatory mechanisms in humans that are sufficient to ameliorate some, but not all, aspects of the phenotype. It is thus difficult to precisely compare the temporal events of neurodegeneration between mice and human. Thus, the observation that DRG cells and axons remain alive after loss of frataxin may not be translatable to FRDA patients. Studies on the survival course of large DRG neurons in humans would be helpful, but are difficult to perform. Consequently, the ideal target of gene therapy in humans may not be the DRG neuron. Although the classical neuropathological lesion of FRDA is loss of the large DRG neurons and their axons in the dorsal columns, most of the loss of DRG may occur early, even before presentation, as sensory conduction levels in humans with FRDA are usually absent at presentation in the lower extremities and rarely change with time.12Quercia N. Somers G.R. Halliday W. Kantor P.F. Banwell B. Yoon G. Friedreich ataxia presenting as sudden cardiac death in childhood: clinical, genetic and pathological correlation, with implications for genetic testing and counselling.Neuromuscul. Disord. 2010; 20: 340-342Abstract Full Text Full Text PDF PubMed Scopus (17) Google Scholar, 13Santoro L. Perretti A. Lanzillo B. Coppola G. De Joanna G. Manganelli F. Cocozza S. De Michele G. Filla A. Caruso G. Influence of GAA expansion size and disease duration on central nervous system impairment in Friedreich's ataxia: contribution to the understanding of the pathophysiology of the disease.Clin. Neurophysiol. 2000; 111: 1023-1030Crossref PubMed Scopus (35) Google Scholar, 14Cruz-Martínez A. Palau F. Central motor conduction time by magnetic stimulation of the cortex and peripheral nerve conduction follow-up studies in Friedreich's ataxia.Electroencephalogr. Clin. Neurophysiol. 1997; 105: 458-461Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar Instead, areas such as the dentate nucleus of the cerebellum, the corticospinal tracts, and even the cerebellar cortex have been more recently implicated in disease progression. In addition, imaging studies now demonstrate even further heterogeneity in the neuroanatomical changes in FRDA, with axonal abnormalities and white matter lesions being identified.15Rezende T.J. Silva C.B. Yassuda C.L. Campos B.M. D'Abreu A. Cendes F. Lopes-Cendes I. França Jr., M.C. Longitudinal magnetic resonance imaging study shows progressive pyramidal and callosal damage in Friedreich's ataxia.Mov. Disord. 2016; 31: 70-78Crossref PubMed Scopus (37) Google Scholar Thus, in human FRDA the exact target in the nervous system is not yet obvious, creating challenges in designing the appropriate gene therapy vectors. Interestingly, the new model presented in this paper exhibits frataxin-dependent cerebellar loss, and thus can serve to some degree as a tool in investigating cerebellar features of FRDA. Overall, while the data from the present study do not provide a protocol for gene therapy in FRDA, they reveal a detailed paradigm for further studies. Many obstacles beyond those identified in the present study will appear in the translation of gene therapy from animal concept to human therapy. Still, the present work provides proof that gene therapy may be useful long term in FRDA, and hope for all those with this disorder. D.R.L. drafted the original manuscript. H.L. and E.K. provided critical review. D.R.L. and H.L. each receive grant support from the Muscular Dystrophy Association (MDA), NIH, FDA, and Friedreich Ataxia Research Alliance. D.R.L. has received clinical trial support from Horizon, Reata, and Takeda Pharmaceuticals. Rapid and Complete Reversal of Sensory Ataxia by Gene Therapy in a Novel Model of Friedreich AtaxiaPiguet et al.Molecular TherapyMay 10, 2018In BriefFriedreich ataxia, a rare incurable mitochondrial disease, is characterized primarily by sensory and spinocerebellar ataxia. In this issue of Molecular Therapy, Puccio et al. (2018) generated a new mouse model that recapitulates the neuropathophysiology and demonstrate the proof-of-concept for the potential of gene therapy to treat the FA neuropathy. Full-Text PDF Open Archive