**3. Factors promoting telomere instability**

**2.1. Telomere or chromosome end loss**

122 Telomere - A Complex End of a Chromosome

**2.2. Telomere dysfunction and erosion**

which became critically short, so it cannot function properly.

telomeric repeats (see [26, 29] for details).

As previously noted, true telomere loss is due to chromosome breakage at one or both ends of the chromosome, and can generate chromosome instability, both by allowing degradation of the ends of chromosomes and promoting chromosome fusions. Fusion can occur between sister chromatids, or between different chromosomes if telomeres are lost in more than one of the chromosomes of a given cell. Chromosome fusion results in chromosome instability through the abovementioned BFB cycles [26, 27, 29], when chromosomes, after telomere loss, repeatedly fuse and break for many cell generations. BFB cycles can continue for multiple cell generations, leading to extensive chromosomal rearrangements, and terminate when the unstable chro‐ mosome eventually acquires a new telomere and so becomes stable [26, 27, 29]. BFB cycles involving sister chromatid fusions result in several types of chromosome rearrangements, including terminal deletions, inverted duplications, DNA amplification, duplicative and nonreciprocal translocations, and dicentric chromosomes, all of which have been associated with human cancer. Chromosomes lacking one telomere remain unstable until they are capped, and lost telomeres after a BFB cycle can be acquired by several mechanisms, including nonreciprocal translocation, duplication/translocation, subtelomeric duplication, or direct

telomere addition [26, 27, 29]. For a detailed description of BFB cycles see [26, 27].

As previously stated, dysfunctional telomeres arise when they lose their end-capping function or become critically short (a phenomenon called telomere erosion or attrition), which causes chromosomal termini to behave like a DSB [9, 31]. In effect, dysfunctional (uncapped or shorten) telomeres are sensed as true DSB, according to the presence of DNA damage response proteins at telomeres in senescent cells or shelterin deficient cells [32]. Therefore, dysfunctional telomeres act as DSB, interfering with the correct rejoining of broken ends. Both telomeres and DSB are DNA ends, and as such, both recruit many of the same proteins. As previously mentioned, proteins governing the DNA damage response are intimately involved in the regulation of telomeres, which undergo processing and structural changes that elicit a transient DNA damage response [19]. Chromosomes with dysfunctional telomeres tend to fuse with one another, producing dicentrics, which can give rise to the abovementioned BFB cycles [26, 27]. It must be taken into account that telomere shortening does not always mean telomere dysfunction. Only when telomeric repeats loss gives rise to a defective telomere structure a dysfunctional telomere appears. Thus, telomere erosion refers to a dysfunctional telomere

Telomere dysfunction at the chromosomal level is commonly assessed applying the telomere FISH technique to metaphase chromosomes [26, 29, 30]. A normal metaphase chromosome exhibits four telomeric signals, two at each end (one per chromatid). When the telomere becomes dysfunctional, one or more of these telomeric signals are lost or duplicated [26, 29, 30]. The presence of chromosome ends with undetectable telomeric hybridization signals has been shown to be a good indicator of critically short and probably dysfunctional telomeres in mammalian cells [33-36]. It is important to mention that not all telomere involving chromo‐ somal aberrations imply telomere dysfunction, but only those ones directly involving terminal Several factors can promote telomere instability. Telomere instability due to true telomere loss can be generated by any mutagen which breaks the chromosome and induces terminal deletions, as shown by several studies (see [26] for review). This kind of instability gives rise to the so-called "incomplete chromosome elements", which comprise chromosomes without one or both telomeres (incomplete chromosomes) and the acentric fragments resulting from the breakage event (termed "terminal fragments") [26, 29, 30].

Telomere instability due to telomere dysfunction can be generated in several ways [26]. Alterations in the shelterin complex or other telomere-binding proteins [7, 8, 41], some DNA damage response proteins required for proper telomere protection [42], the structure of telomeric DNA (loss of telomeric sequences, see below), the structure or activity of telomerase [43], TERRA [15-18] or the enzymes helicases [44, 45] can give rise to dysfunctional telomeres. All these factors are involved in the production of telomere-related chromosomal aberrations. These aberrations have been described in detail elsewhere [26, 29, 30] and thus they will not be considered in the present chapter. Moreover, dysfunctional telomeres may result as a consequence of mutagen-induced telomeric DNA damage [26].

Referring to the relationship between telomere shortening and dysfunction, we must bear in mind that, as previously mentioned, telomere length is maintained by a dynamic process of telomere shortening and lengthening. Telomeres lose approximately 20-300 bp of repeat sequences every cell division mainly due to the "end replication problem" [9]. This is the most obvious mechanism for the loss of telomeric repeat sequences, i.e., attrition due to the failure to compensate for the gradual loss of these repeats during cell division, and is termed repli‐ cative erosion or replicative shortening, which leads to replicative senescence of cells. Thus, telomeres regulate the replicative life span of somatic cells, acting as a "mitotic clock". There is another kind of telomere shortening, termed "stress dependant shortening", which is produced by stress-inducing factors like radiation, oncogenes, oxidative damage within telomeric DNA, chromosome end-specific exonuclease activity, and the lack of telomerase activity [46-49]. Stress dependant shortening can lead to the loss of large blocks of telomeric repeat sequences through different mechanisms, including recombination, problems encoun‐ tered during DNA synthesis or inefficient DNA repair. In addition, telomere shortening is accelerated by active oxygen species and ultraviolet radiation, which are thought to be major environmental causes of human telomere shortening [46]. In the next section, we will sum‐ marize our current knowledge concerning the main data available about telomere instability induced by anticancer drugs on mammalian cells.
