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The cell biology of senescence is reviewed, with particular emphasis on models and theories derived from work with cultured human -fibrbblas: ts. The. biochemistry-and. cell biology of Werner's Syndrome is reviewed, and- the relationship of this genetic disease-to the natural ageing process is. examined... Similarities and'differenc. es between Werner s Syndrome fibroblast cultures . 'arid: those derived " from normal individuals are given special attention. The work presented in this thesis was undertaken to determine whether cells derived from patients suffering from Werner's Syndrome behaved differently in culture to those derived from normal donors. Such a phenotype could be used in the long term to assist current attempts to clone the Werner'-s Syndrome gene. The cell kinetic aspects of -the -Syndrome were studied' because an important feature of the disease is that fibroblasts derived from Werner's Syndrome patients are known to grow extremely poorly in vitro. The data presented show that Werner's Syndrome fibroblasts exhibit a severely restricted in vitro lifespan due to a three to five fold increase in the rate at which the cells exit irreversibly from the cell cycle and become senescent. It is also shown that this behaviour is not due to abnormalities in hyaluronic acid metabolism in vitro despite the fact that elevated levels of hyaluronic acid are diagnostic for Werner's Syndrome patients. Finally, it is demonstrated that cultures of Werner's Syndrome T lymphocytes do not show a reduced li fespan which is highly suggestive that the condition is restricted to a subset of cellular lineages. The effect of these findings on theoretical models of cellular senescence is discussed, as is the practical impact of this data on the ongoing attempt to characterise the Werner's Syndrome gene.

The French-Canadian variant of Leigh syndrome (LSFC) is a distinct and particularly severe presentation of Leigh syndrome characterized by the onset of unpredictable acidotic crises leading to death of 80% of them before the age of five. This autosomal recessive disorder is caused by mutations in LRPPRC, encoding an mRNA binding protein of the same name with a high affinity for mitochondrial transcripts. As a result of the mutations, levels of LRPPRC are decreased in all tissues and cause a severe deficiency of complex IV of the respiratory chain, with a deeper involvement of brain and liver. To gain better knowledge on the pathophysiology of this disease, and of the impact of the OXPHOS defect on the liver, our research consortium developed a mouse model of the disease harboring a liver specific inactivation of Lrpprc (H-Lrpprc). The goal of this thesis is to investigate the in vivo consequences of hepatic Lrpprc inactivation and to test potential therapy for mitochondrial diseases. The characterization of this model and the analysis of the mitochondrial phenotype are presented in Chapter 2 (Cuillerier et al, Human Molecular Genetics, 2017). Despite this severe phenotype, H-Lrpprc mice show no signs of overt liver failure and maintain energy levels, suggesting mechanisms are in place to sustain residual complex IV function. The underlying compensatory mechanisms granting these mice a remarkable resilience were explored and are presented in Chapter 4 (Cuillerier et al, Communications Biology, 2021). Along this project, we developed a protocol, and the optimized conditions of this method are described in Chapter 3 (Cuillerier and Burelle, JoVE, 2019). Although great progress has been made, there are currently no effective or curative treatments for LSFC and mitochondrial diseases. Recently, extensive pre-clinical and clinical studies supported the emergence and safety of mesenchymal stem cells therapy in the treatment of various diseases. Following transplantation, MSCs promote repair through various mechanisms including secretion of cytokines/exosomes, and transfer of mitochondria directly to target cells with impaired mitochondria offering a possibility to replace mutant dysfunctional organelles, which is relevant in the context of genetic mitochondrial diseases. Based on this, the objective of the last chapter of this thesis is to test the therapeutic potential of MSCs for genetic mitochondrial disorders using MSC-based approaches and LSFC as a disease model. Unfortunately, we encountered several obstacles along the way, including the departure of our main collaborator and stem cell expert, and delays in experimental procedures due to the COVID-19 pandemic. Consequently, this study was not completed at the moment of submission of this thesis, and is therefore presented as a pilot study in the form of a manuscript in Chapter 5. Overall, these projects unveiled alterations of mitochondrial functions that go beyond OXPHOS, a complex network of compensatory mechanisms in place to palliate these defects, and finally, encouraging preliminary results suggest MSC therapy could be beneficial for the treatment of mitochondrial diseases.

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With the discovery and development of CRISPR, the technology that might allow us to modify the human germline is at our fingertips, and, consequently, serious practical and ethical consideration is warranted. In the following paper, I examine the ethics of using CRISPR in this way and argue that modifying the human germline for the purpose of preventing serious genetic disease is, in principle, ethically acceptable and ought to be allowed. I present several arguments to this effect including arguments that rely on the principles of beneficence and autonomy. I also examine the larger societal implications of human germline modification. I then respond to six of the most prominent objections that have been raised against CRISPR and germline gene modification before concluding with a brief discussion of the biggest challenge that we face as we move forward with CRISPR, that of limiting the use of this promising and incredibly versatile technology.