Telomere Fragility and MiDAS: Managing the Gaps at the End of the Road

Abstract

Telomeres present inherent difficulties to the DNA replication machinery due to their repetitive sequence content, formation of non-B DNA secondary structures, and the presence of the nucleo-protein t-loop. Especially in cancer cells, telomeres are hot spots for replication stress, which can result in a visible phenotype in metaphase cells termed "telomere fragility". A mechanism cells employ to mitigate replication stress, including at telomeres, is DNA synthesis in mitosis (MiDAS). While these phenomena are both observed in mitotic cells, the relationship between them is poorly understood; however, a common link is DNA replication stress. In this review, we will summarize what is known to regulate telomere fragility and telomere MiDAS, paying special attention to the proteins which play a role in these telomere phenotypes.

Keywords: DNA replication; Mitotic DNA synthesis; replication stress; telomere fragility; telomeres.

Figure 1

Examples of Telomere Aberrations Detected on Metaphase Spreads. Cartoons of a chromosome with normal telomere ends (a), shortened telomeres (light green) (b), a signal-free end or telomere loss (c), a telomere–telomere fusion or dicentric chromosome (d), fragile telomeres (e,f), mitotic DNA synthesis (g), and a telomere containing DNA-damage-response proteins such as γH2AX or 53BP1 (h). Green = telomeres; purple = centromeres; red = EdU foci; yellow and gray = DDR proteins. Created in BioRender.

Figure 2

Schematic of DNA Synthesis During MiDAS. When a replication fork collapses following replication stress (star), a single-ended double-strand break forms. After resection, the 3′ overhang of the recipient strands (red/orange) can invade the donor stands (blue/purple). After D-loop formation, if lagging-strand synthesis takes place on the invading stand, no new synthesis will occur for the donor strand, resulting in conservative DNA repair synthesis. If instead lagging-strand synthesis occurs on the extruded donor D-loop strand, following D-loop migration structure specific nucleases (red arrows) will resolve the holiday junction. This results in new synthesis for both the donor and receipt and is semi-conservative DNA repair synthesis. Created in BioRender.

Figure 3

Speculative Model of the Relationship between Telomere Fragility and MiDAS. A normal chromatid end is folded into the protective T-loop structure by shelterin, which prevents DDR and chromosome fusions. When a replication fork stalls, by 8-oxo-guanine for example, this triggers a DDR. If replication is not completed by the end of G2, the under-replicated gap will persist when the cell enters mitosis. In order to fill the gap and maintain genome stability, cells can conduct telomere MiDAS in prophase. If MiDAS fails or is incomplete, under-replicated gaps will likely lead to mitotic segregation errors in the cell. When arrested in metaphase, however, this altered telomere structure can be observed with FISH, resulting in doublets or smeared signals. Created in BioRender.