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Theses

Genome instability : from genome content variations to gene expression plasticity

Abstract : Most animal cells are diploid, containing two copies of each chromosome. Establishment of proper bipolar mitotic spindle containing two centrosomes, one at each pole contributes to accurate chromosome segregation. This is essential for the maintenance of genome stability, tissue and organism homeostasis. However, numerical deviations to the diploid set are observed in healthy tissues. Polyploidy is the doubling of the whole chromosome set and aneuploidy concerns the gain or loss of whole chromosomes. Importantly, whole genome duplications and aneuploidy have also been associated to pathological conditions. For example, variations to genome content are associated with chromosome instability and cancer development, however their exact contribution to cancer genome remains poorly understood.In the first part of my PhD project, I investigated the consequences of polyploidy during cell division. I found that the presence of extra DNA and extra centrosomes generated invariably multipolar spindles. Then I identified contributors to the multipolar status using in vivo approaches in Drosophila neural stem cells and in vitro culture of cancer cells. Further I combined DNA and spindle perturbations with computer modelling and found that in polyploid cells, the presence of excessive DNA acts as a physical barrier blocking spindle pole coalescence and bipolarity. Indeed, laser ablation to disrupt and increase in microtubule stability and length to bypass the DNA-barrier could rescue bipolar spindle formation. This discovery challenges the current view that suggested extra-centrosomes as only contributor to spindle multipolarity and provides a rational to understand chromosome instability typical of polyploid cells.The aim of the second part of my PhD project was to generate a novel tool to quantitively probe chromosome loss in vivo in Drosophila tissues. Aneuploidy has been observed in various physiological tissues, however the frequency of this error remained highly debatable. In addition, tools developed so far to assess aneuploidy lack a temporal dimension. To circumvent this, I used the expression of a GFP report gene driven by the GAL4/UAS system and its inhibition by GAL80. In principle, the random loss of the chromosome carrying the GAL80 sequence leads to GFP appearance in aneuploid cells that can therefore be followed in live tissues. I found that chromosome loss was extremely infrequent in most tissues of the wild type fly. This tool combined with fluorescent marker and/or tested in various genetic background, might help understanding mechanisms behind aneuploidy genesis and outcome in vivo.While developing this tool, I discovered that in the larval brain, GFP cells where not a by-product of chromosome loss but rather an unexpected mis-regulation in the expression of the GAL80 gene. These results have strong implications for the Drosophila community as it can result in false positive in clonal experiments. Further, I discovered a mosaicism and plasticity of the Drosophila brain in neural stem cells for gene expression which differs from other organs and that is influenced by environmental stimuli. This possibly reflects a certain level of plasticity in the brain necessary for neuronal diversity, adaptation and survival.
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Submitted on : Friday, September 24, 2021 - 2:14:27 PM
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Alix Goupil. Genome instability : from genome content variations to gene expression plasticity. Agricultural sciences. Université Paris sciences et lettres, 2021. English. ⟨NNT : 2021UPSLS053⟩. ⟨tel-03353899⟩

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