DNA damage response (DDR) senses and resolves DNA damage to preserve the integrity of the genome through a multi-complex pathway. DDR utilizes two main repair mechanisms, including homologous recombination (HR) and non-homologous end-joining (NHEJ) [1]. HR uses a homologous template to restore genetic information in a highly accurate manner. On the other hand, NHEJ does not require a homologous sequence and the two DNA ends rejoin each other [1]. NHEJ is comparatively more error-prone, resulting in the loss of nucleotides on both sides of the DNA break [2,3]. DDR is commonly activated by DNA damage agents that cause double-stranded breaks. Additionally, the ends of linear chromosomes also resemble chromosome breaks, which attract the recruitment of DDR machinery [4]. In healthy cells, chromosome ends are capped by a long stretch of repetitive DNA sequences called telomeres. Telomere ends are occupied with a protein complex named shelterin that prevents chromosome fusions and induction of double-strand break (DSB) responses at chromosome ends [5]. Occupation of telomeres by shelterin proteins inhibits NHEJ to prevent persistent DNA damage responses [6].

Critically shortened telomeres from the end replication problem or inhibition of shelterin protein-binding evoke DDR in cells that eventually leads to senescence or apoptosis [7]. The DDR is initiated by the formation of the sensory complex (DNA-PK, ATM/ATR) and the accumulation of phosphorylated histone H2A.X (γH2A.X) at the damage sites. This activates the p53 pathway, which induces cell-cycle arrest [8]. The Ku complex, made up of the Ku70 and Ku80 proteins, forms a heterodimer and initiates the NHEJ process, while DNA-dependent protein kinase (DNA-PK) subunits are recruited to form DNA-PK holoenzyme at the damage sites [9]. DNA-PK bridges bring the two DNA ends together by creating a long-range synapse. Subsequently, XRCC4, XRCC4-like factor, and DNA ligase 4 (Lig4) ligate the DNA ends [10,11]. In clinic, DDR inhibitors are used to increase the synthetic lethality, especially in tumors with a reduced repertoire of DDRs, making these cells vulnerable to specific DDR inhibition [12] (Table 1). However, the main limitations are developing resistance to these inhibitors and lacking biomarkers to predict drug responses [13].

In cancer, enhanced oncogene-driven cell proliferation and chronic inflammation apply extra pressure on the DNA replication stress and repair mechanism [33, 34, 35]. Structural changes to DNA alter the DNA damage response and reduce the protection role of shelterin proteins at telomeric repeats [7]. Therefore, targeting the telomere proteins and DNA damage response pathway is a promising strategy for developing novel anti-cancer agents for several reasons [36]. Firstly, activation of DDR initiates proliferation arrest by inducing cell-cycle arrest through ATM-ATR and p53 [37]. Secondly, cancer cells harbor shorter telomeres and are prone to DNA damage due to high proliferation rate, oxidative stress, and inefficient telomeres capping [38]. Thirdly, cancer-specific mutations and differential expression of shelterin proteins [39] (Figure 1), as well as the acquisition of non-telomeric roles of telomerase and shelterin in cancer [40∗, 41, 42, 43∗], which was explained as a protein-counting mechanism, imbalance the stoichiometry of these proteins on telomeres and lead to vulnerable telomere ends.

The sensitivity and specificity of DDR inhibitors can be enhanced when combined with telomere-associated inhibitors to make them more cancer-specific to reduce the drug resistance in tumor cells and the cytotoxicity in healthy cells. This review will discuss the reactivation mechanisms of telomerase in cancer, telomere maintenance roles of shelterin proteins, their interactions with DDR complexes, and how this interaction may lead to the development of cancer-specific inhibitors.