**1. Introduction**

#### **1. 1. Telomeres, telomerases, and p53 interaction**

Genomic instability is probably one of the first phases in carcinogenesis and would thus facilitate the accumulation of multiple mutations. This instability was first described in connection with the discovery of microsatellite instability with the hereditary non-polyposis colon cancer (HNPCC) or Lynch syndrome for example. In most cancers, there is another form of genomic instability, that is, the chromosomal instability (CIN), which we will describe in this review [1]. At molecular level, this instability could result in chromosome rearrangements (number and structure), duplication or segregation anomalies during mitosis, along with DNA repair system dysfunction [1–4]. The CIN may be explained by the mutator hypothesis in hereditary cancers (germline mutations in DNA repair genes as *TP53* or *BRCA* genes for instance, beginning in the precancerous lesions) and by the oncogene-induced DNA replica‐ tion stress in sporadic (non-hereditary) cancers [1]. Because CIN precedes *TP53* and DNA repair genes mutations in sporadic cancer, genomic instability would be the result of activated oncogenes or antioncogenes (growth signaling pathways...) [1]. However, in both cases (sporadic and genetic cancers), CIN may be due also to telomere erosion and is likely linked to multifactorial molecular events.

Telomere dysfunction has been described to be one of the initial phases in genomic instability. Telomeres are found at the tips of the chromosomes and consist of a large number of hexamer (TTAGGG)n repeats capped with the multiprotein shelterin complex. Their main function is to protect chromosomal ends from fusion and nucleotidic degradation, which ensures chromosomal stability. Telomeres are regularly shortened during cell division due to incom‐ plete replication of telomeric DNA. Excessive shortening (a "telomere crisis") activates DNA repair at telomeres leading to CIN and finally cell death. In tumors, telomeres are usually shortened, which makes the cell unstable, but the activation of telomerase (telomere elongation complex generally found only in stem, progenitor, and tumor cells and not in normal somatic cells) maintains the telomere length at a certain level, enabling the survival of these cells [2– 4]. These unstable cells can then continue to proliferate and this results in cancer. The interac‐ tion between telomeres and p53 would thus represent one of the trigger events in carcinogen‐ esis [5]. In physiological terms, telomere shortening represents a real "genotoxic stress" resulting in activation of the DNA damage signaling pathway (and consequently p53 tumor suppressor protein), finally leading to apoptosis or cycle arrest. When p53 is inactivated by mutations, as is the case in ovarian high-grade serous carcinoma, the telomere dysfunction is neither repaired nor signaled, resulting in genomic instability with the accompanying chromosome aberrations, which are a prelude to cancer [2–6].

DNA double-strand breaks can be the result of telomere dysfunction and are one of the most critical DNA alterations at the origin of genomic instability. One of the first response mecha‐ nisms aimed at repairing these breaks is activation of the DNA damage checkpoints. These checkpoints represent the transition state between two cell phases that may be inhibited by genotoxic lesions. There is then a slowing or complete halt in the progression from one phase to the next (checkpoint G1/S or G2/M) or during replication (checkpoint intra-S) [7].

The phosphorylation of ATM (a member of the phosphoinositide 3-kinase or PI3 kinase) occurs first. Then ATM phosphorylates Chk2 p53, H2AX, and 53BP1. Chk2 also phosphorylates p53 (Figure 1) [8–12]. Histone H2AX phosphorylated on residue serine 139, also called γH2AX, is a marker for cellular DNA double-strand breaks. Phosphorylation of H2AX results in the condensation of chromatin each side of the break, which makes γH2AX detectable at nuclear level by immunohistochemistry.

**1. Introduction**

36 Telomere - A Complex End of a Chromosome

**1. 1. Telomeres, telomerases, and p53 interaction**

to multifactorial molecular events.

chromosome aberrations, which are a prelude to cancer [2–6].

Genomic instability is probably one of the first phases in carcinogenesis and would thus facilitate the accumulation of multiple mutations. This instability was first described in connection with the discovery of microsatellite instability with the hereditary non-polyposis colon cancer (HNPCC) or Lynch syndrome for example. In most cancers, there is another form of genomic instability, that is, the chromosomal instability (CIN), which we will describe in this review [1]. At molecular level, this instability could result in chromosome rearrangements (number and structure), duplication or segregation anomalies during mitosis, along with DNA repair system dysfunction [1–4]. The CIN may be explained by the mutator hypothesis in hereditary cancers (germline mutations in DNA repair genes as *TP53* or *BRCA* genes for instance, beginning in the precancerous lesions) and by the oncogene-induced DNA replica‐ tion stress in sporadic (non-hereditary) cancers [1]. Because CIN precedes *TP53* and DNA repair genes mutations in sporadic cancer, genomic instability would be the result of activated oncogenes or antioncogenes (growth signaling pathways...) [1]. However, in both cases (sporadic and genetic cancers), CIN may be due also to telomere erosion and is likely linked

Telomere dysfunction has been described to be one of the initial phases in genomic instability. Telomeres are found at the tips of the chromosomes and consist of a large number of hexamer (TTAGGG)n repeats capped with the multiprotein shelterin complex. Their main function is to protect chromosomal ends from fusion and nucleotidic degradation, which ensures chromosomal stability. Telomeres are regularly shortened during cell division due to incom‐ plete replication of telomeric DNA. Excessive shortening (a "telomere crisis") activates DNA repair at telomeres leading to CIN and finally cell death. In tumors, telomeres are usually shortened, which makes the cell unstable, but the activation of telomerase (telomere elongation complex generally found only in stem, progenitor, and tumor cells and not in normal somatic cells) maintains the telomere length at a certain level, enabling the survival of these cells [2– 4]. These unstable cells can then continue to proliferate and this results in cancer. The interac‐ tion between telomeres and p53 would thus represent one of the trigger events in carcinogen‐ esis [5]. In physiological terms, telomere shortening represents a real "genotoxic stress" resulting in activation of the DNA damage signaling pathway (and consequently p53 tumor suppressor protein), finally leading to apoptosis or cycle arrest. When p53 is inactivated by mutations, as is the case in ovarian high-grade serous carcinoma, the telomere dysfunction is neither repaired nor signaled, resulting in genomic instability with the accompanying

DNA double-strand breaks can be the result of telomere dysfunction and are one of the most critical DNA alterations at the origin of genomic instability. One of the first response mecha‐ nisms aimed at repairing these breaks is activation of the DNA damage checkpoints. These checkpoints represent the transition state between two cell phases that may be inhibited by genotoxic lesions. There is then a slowing or complete halt in the progression from one phase

to the next (checkpoint G1/S or G2/M) or during replication (checkpoint intra-S) [7].

**Figure 1.** Relationship between telomeres and the DNA damage response pathway. DNA double-strand breaks may be the result of telomere dysfunction with activation of the DNA damage checkpoints. The apical kinase ATM is first activated. Then ATM activates the DNA damage mediators, H2AX and 53BP1. ATM also phosphorylates the down‐ stream kinase Chk2, which phosphorylates p53. ATM may directly activate the effector p53. In non-cancerous situa‐ tion, this pathway leads to checkpoint arrest, cellular senescence, or apoptosis. In cancerous situation, this cascade may activate tumor proliferation to invasive carcinoma. On the left side, you can see the serous carcinogenic sequence from benign fallopian tube (no p53 mutation) to serous tubal intraepithelial carcinoma (the precancerous lesion with p53 signature) and after that ovarian cancer (with also *TP 53* mutation) [15, 17].

Strong expression of γH2AX was detected in most precancerous lesions and was considered to represent an antitumor function. Experimental studies have shown that H2AX-deficient mice were at greater risk of immunodeficiency, infertility, and were also more sensitive to ionizing radiation, all of which can be explained by increased chromosomal instability related to reduced capability of the DNA repair systems. When these mice were also p53 deficient (knock-out), there was an increased risk of tumors [13, 14].

As it has been demonstrated that telomere shortening occurs early during carcinogenesis (being found in 89% of preinvasive lesions of the bladder, cervix, colon, and also the esopha‐ gus) [5], we investigated genetic instability in precancerous ovarian lesions, which has been described very recently.
