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Title: Investigation into the silencing of heterochromatin in mammalian cells
Author: Natisvili, Theona
ISNI:       0000 0004 7228 6765
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2016
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Much of the mammalian genome is composed of repetitive elements, consisting of tandem and interspersed repeats. Heterochromatinisation of pericentromeric tandem satellite repeats plays an important role in centromere formation and maintenance of genome stability. Additionally, several human diseases are caused by microsatellite repeat expansion. Among different classes of interspersed repeats, retro-elements LINEs and SINEs are considered to contribute to genome evolution; however they also pose a potential threat to genome stability. Hence it appears essential that repetitive elements are transcriptionally silenced, which occurs via heterochromatinisation. To further understand heterochromatin formation, this thesis investigated: (i) the frataxin (FXN) gene which contains a GAA-repeat expansion causing FXN silencing which in turn leads to the neurodegenerative disease Friedreich’s ataxia (FRDA); (ii) the effect of proteasome inhibition on the transcription of heterochromatinised DNA repeats. Here, the silenced FXN gene was shown to be upregulated by treatment of FRDA patients with the HDAC inhibitor, nicotinamide. This upregulation was accompanied by reduction of the heterochromatin histone modification H3K9me3. Moreover, the proteasome was found at the heterochromatinised FXN gene in cells derived from patients, consistent with the hypothesis that, in FRDA, proteasomal degradation of a stalled RNAPII might contribute to FXN silencing. Although proteasome inhibition did not upregulate FXN transcription, it caused transcriptional activation of pericentromeric satellite repeats which appeared to be independent of cell-cycle perturbations and was accompanied by delocalisation of HP1α from chromocentres without detectable changes in the levels of histone modifications. Proteasome inhibition also increased the number of chromocentres per cell consistent with destabilisation of chromocentres. This effect of proteasome inhibition in upregulation of pericentromeric repeats was shown to be at least partly dependent on activation of heat shock transcription factor and binding of this factor to a consensus sequence within the pericentromeric satellite repeat region.
Supervisor: Festenstein, Richard Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral