| Summary: | <p>Maintaining replicative immortality is a crucial step in the development of cancers and is achieved by activating a telomere lengthening mechanism. While the majority of cancers rely on upregulation of telomerase, approximately 10-15% of cancers achieve telomere length maintenance through a telomerase-independent mechanism referred to as the Alternative Lengthening of Telomeres (ALT) pathway. ALT cancers exhibit a number of characteristic markers and recent work has sought to understand the mechanisms that underly the generation of these markers and of ALT telomere maintenance. </p>
<p>Previous work has also highlighted the striking loss of the chromatin remodelling protein ATRX and its histone chaperon partner DAXX in ALT cancers, and the ability of ATRX and DAXX to suppress the ALT pathway. Until now, however, it has remained unclear as to the mechanisms by which ATRX loss drives ALT development given that ATRX loss alone is generally insufficient to generate the ALT phenotype. </p>
<p>In this thesis a novel marker of ALT, the presence of Replication Protein A coincident with telomeric sequences, is shown. This marker is dependent on the presence of the endonuclease Mus81, which is known to cleave stalled replication forks. In conjunction, evidence is presented showing that the telomeric replication stress or lesions on telomeric DNA when combined with loss of ATRX or DAXX are able to trigger ALT pathway induction. This induction can be triggered both through artificial means with genotoxic agents, but also through loss of the histone lysine methyltransferase SETD2, which is responsible for the methylation of Histone H3.3 at lysine 36. Together these observations provide critical understanding of a possible route of ALT development, given the frequent co-occurrence of histone H3.3 G34R/V mutations, which are known to be detrimental to SETD2 activity, and ATRX/DAXX mutations in ALT cancers. </p>
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