Control of Cellular Aging, Tissue Function, and Cancer by p53 Downstream of Telomeres

Abstract

Telomeres, the nucleoprotein complex at the ends of eukaryotic chromosomes, perform an essential cellular role in part by preventing the chromosomal end from initiating a DNA-damage response. This function of telomeres can be compromised as telomeres erode either as a consequence of cell division in culture or as a normal part of cellular ageing in proliferative tissues. Telomere dysfunction in this context leads to DNA-damage signaling and activation of the tumor-suppressor protein p53, which then can prompt either cellular senescence or apoptosis. By culling cells with dysfunctional telomeres, p53 plays a critical role in protecting tissues against the effects of critically short telomeres. However, as telomere dysfunction worsens, p53 likely exacerbates short telomere-driven tissue failure diseases, including pulmonary fibrosis, aplastic anemia, and liver cirrhosis. In cells lacking p53, unchecked telomere shortening drives chromosomal end-to-end fusions and cycles of chromosome fusion-bridge-breakage. Incipient cancer cells confronting these telomere barriers must disable p53 signaling to avoid senescence and eventually up-regulate telomerase to achieve cellular immortality. The recent findings of highly recurrent activating mutations in the promoter for the telomerase reverse transcriptase (TERT) gene in diverse human cancers, together with the widespread mutations in p53 in cancer, provide support for the idea that circumvention of a telomere-p53 checkpoint is essential for malignant progression in human cancer.

Figure 1.

Telomere shortening in cell culture limits the proliferation of primary cells. After a defined number of population doublings in culture, short telomeres in fibroblasts trigger a DNA-damage response that acts through p53 to cause cellular senescence and a halt on proliferation. To proceed past senescence, cells must inactivate p53 and Rb, or up-regulate telomerase to achieve telomere maintenance. Once cells escape senescence, continued telomere shortening will lead to a period of genomic instability termed crisis, which is characterized by cell death and chromosomal instability. Expression of telomerase will stabilize the genome and allow continued proliferation of the culture.

Figure 2.

Shelterin, the telomere-binding complex, protects chromosome ends and controls telomerase function. Telomeric DNA is bound by the double-stranded DNA-binding proteins TRF1 and TRF2 and by the single-stranded DNA-binding protein Pot1. Rap1 binds to TRF2 and functions as a transcriptional regulator. TIN2 nucleates the complex by connecting TRF1 and TRF2 to the Tpp1/Pot1 subcomplex. Each component of shelterin serves a specialized function in telomere maintenance, with TRF2 and Pot1 involved in suppression of DNA-damage signaling, and TRF1 involved in facilitating replication through difficult to replicate telomeric sequences. Telomerase is recruited to the complex through its interaction with Tpp1; once at the telomere, its action is negatively regulated by Pot1. Telomerase is a ribonucleoprotein composed of a noncoding RNA TERC that serves as a scaffold for protein components TERT, TCAB, dyskerin, NOP10, and NHP2. Dyskerin and its associated proteins promote TERC biogenesis and stability, whereas TCAB facilitates trafficking of telomerase to the telomere. TERT is the catalytic subunit and TERC encodes the template for reverse transcription.

Figure 3.

Pathways connecting telomeres and p53. Telomeres shorten with aging or with extended passage in culture, leading to eventual telomere uncapping. Uncapped telomeres lose their protective function, and telomere-dysfunction-induced foci, or TIFs, composed of DNA-damage proteins, are recruited to uncapped telomeres. Dysfunction telomeres can signal through the ATM and ATR kinases to phosphorylate p53. Phosphorylation stabilizes p53 by inhibiting its interaction with MDM2, and p53 then transcriptionally up-regulates target genes mediating G₁ checkpoint arrest, senescence, and apoptosis.

Figure 4.

Telomere dysfunction promotes cancer through cycles of chromosome fusion-bridge-breakage. In a p53-null background, telomeres are able to shorten over repeated cell divisions without causing senescence. After a critical length is reached, telomere uncapping occurs leading to covalent ligation of sister chromatids in this example. During mitosis, chromosome migration to opposite spindle poles results in chromosomal breakage. The broken chromosome is now susceptible to recombination with other chromosomes, causing nonreciprocal translocations and leading to copy number alterations. After many cycles, up-regulation of telomerase by malignant cells would allow stabilization of the karyotype, and the tumor with a newly altered genome would be able to proceed in its evolution.