Structural Chromosome Instability: Types, Origins, Consequences, and Therapeutic Opportunities

Abstract

Chromosomal instability (CIN) refers to an increased rate of acquisition of numerical and structural changes in chromosomes and is considered an enabling characteristic of tumors. Given its role as a facilitator of genomic changes, CIN is increasingly being considered as a possible therapeutic target, raising the question of which variables may convert CIN into an ally instead of an enemy during cancer treatment. This review discusses the origins of structural chromosome abnormalities and the cellular mechanisms that prevent and resolve them, as well as how different CIN phenotypes relate to each other. We discuss the possible fates of cells containing structural CIN, focusing on how a few cell duplication cycles suffice to induce profound CIN-mediated genome alterations. Because such alterations can promote tumor adaptation to treatment, we discuss currently proposed strategies to either avoid CIN or enhance CIN to a level that is no longer compatible with cell survival.

Keywords: DNA damage; DNA repair; chromosome aberrations; chromosome bridges; chromosome instability; lagging chromosomes; micronuclei; ultra-fine bridges.

Figure 1

The multiple origins and tight associations of chromosome instability (CIN) phenotypes. (A) Under-replicated DNA (UR-DNA) formed during S phase and not resolved before mitosis leads to ultra-fine bridges (UFBs) when sister chromatids are pulled in opposite directions during anaphase. Breakage of these bridges can eventually lead to micronuclei (MN) formation in the following cell cycle. (B) Unrepaired one-ended double-strand breaks (DSBs) formed during S or M phase due to replication fork collapse can be visualized as chromosome breaks during mitosis. Improper repair of breaks by end-joining mechanisms can create fusions among non-homologous chromosomes, giving rise to radial chromosomes. Radial chromosomes form bulky chromosome bridges due to multiple centromeres and unequal pulling toward opposite poles. Similarly to UFBs, breakage of these bridges can also lead to MN formation. Breaks that are not fixed and lack a centromere give rise to acentric fragments, while breaks that activate the DNA damage response and DNA repair during mitosis (dotted arrow) can form lagging chromosomes. Both acentric fragments and lagging chromosomes are well known sources of MN. (C) Fully duplicated chromosomes mostly lead to normal cell division in mitosis and absence of structural CIN. However, under certain circumstances, they can experience lagging at the metaphase plate during anaphase, usually due to kinetochore-microtubule attachment problems. These lagging chromosomes can form whole chromosome micronuclei in the next cell cycle.

Figure 2

Mechanisms that inhibit or promote chromosome instability (CIN). Top (blue lines): Mechanisms involved in inhibiting or restraining CIN during the different cell cycle stages. Non-homologous end joining (NHEJ) can be utilized throughout the cell cycle but is required for double-strand break (DSB) repair in G1 phase due to the absence of homologous recombination (HR). Translesion synthesis (TLS), HR, and multiple endonuclease-dependent mechanisms, such as the BTRR complex and structure-specific endonucleases (SSEs), can prevent the accumulation of CIN during mitosis. DNA bridge resolution and mitotic DNA synthesis (MiDAS) during M phase can resolve multiple types of DNA bridges. Broken chromosome fragments can be incorporated in M phase via chromosome tethering mechanisms. Replication or mitotic catastrophe in S or M phase, respectively, when exacerbated, usually leads to cell death, thus preventing the accumulation of cells with CIN. Bottom (orange arrows): Cells that enter mitosis with chromosome abnormalities due to failed repair mechanisms present alternative mechanisms of resolution that can potentiate CIN and carcinogenesis, such as micronuclei (MN) breakage via chromothripsis. Some forms of DNA damage lead to 53BP1 nuclear bodies, whose implications are still largely unknown. In contrast, some MN or gross mitotic abnormalities, such as multinucleation, can lead to cell death.

Figure 3

The multifaceted role of CIN in carcinogenesis. Normal tissue (bottom, green shade) can become tumorigenic under circumstances directly or indirectly associated with the induction of replication stress (yellow shade). At that point, tumor cells commonly present a low level of chromosome instability (CIN), which can serve as a fuel for early steps of cellular transformation and thus enable the carcinogenic process (yellow shade). As the tumor progresses (orange shade), cells become increasingly genetically unstable. Under these conditions, CIN can promote the fast acquisition of multiple tumor characteristics such as drug resistance and metastasis. Eventually, if CIN levels are very high, cells can reach a threshold in which they are no longer viable (red shade). Surviving cells in these stages are highly genetically unstable, with an increased probability of being multinucleated after subsequent cycles of aberrant mitosis finalization. Such a scenario has good chances of triggering cell death due either to improper gene expression or to sub-optimal S and M phase finalization.

Figure 4

The potent combination of chemotherapy and immunotherapy. (A) Breast Cancer Susceptibility Protein (BRCA)-deficient tumor cells treated with Poly(ADP ribose) Polymerase inhibitor (PARPi) exhibit under-replicated DNA and unrepaired double strand breaks in S phase, which trigger bulky chromosome bridges and micronuclei formation, eventually causing cell death. Micronuclei also trigger the cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) immune pathway, which activates dendritic (DC) and T cells, which are recruited to the tumor. However, their anti-tumor activity is blocked due to Programmed Death-Ligand 1 (PD-L1) expression. (B) PARPi treatment combined with immunotherapy such as anti-PD-L1 antibodies also leads to bulky chromosome bridges, micronuclei and cGAS-STING pathway activation. However, under these conditions, the PD-L1 antibodies can block PD-L1, leading to an increased anti-tumor cell response mediated by T-cells.