**Molecular Diagnosis and Precision Therapeutic Approaches for Telomere Biology Disorders**

Rosario Perona, Laura Iarriccio, Laura Pintado-Berninches, Javier Rodriguez-Centeno, Cristina Manguan-Garcia, Elena Garcia, Blanca Lopez-Ayllón and Leandro Sastre

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/65353

#### **Abstract**

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76 Telomere - A Complex End of a Chromosome

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Telomeres are nucleo-protein structures located at the end of chromosomes that protect them from degradation. Telomeres length is maintained by the ac‐ tivity of the telomerase complex. These structures are protected by a special‐ ized protein complex named shelterin. In the absence of telomerase activity and/or protection telomeres are shortened after each round of DNA replica‐ tion. When a critical size is reached, telomeres are recognized as damaged DNA by the cell p53-dependent DNA-repair system. Persistent activation of this pathway finally results in cell apoptosis or senescence.

There are a number of rare hereditary diseases caused by the presence of short‐ ened telomeres, collectively named telomeropathies or telomere biology disor‐ ders. In these diseases, cell proliferation is impaired, which results in premature aging and dysfunction of highly proliferative tissues (bone morrow, skin and other epithelia). Among them are Dyskeratosis congenita, the Hoyer‐ aal-Hreidarsson, Revesz and Coats plus syndromes, Aplastic anemia, Idiopath‐ ic pulmonary fibrosis and nonalcoholic, noninfectious liver disease. Mutations present in the genes coding for component of the telomerase and shelterin complexes and other proteins involved in telomere replication are the cause of these diseases. Clinical manifestations, causative mutations, diagnosis and pos‐ sible therapeutic approaches to these diseases will be discussed in this chapter.

**Keywords:** telomere, telomere biology disorders, telomeropathies, pulmonary fibrosis, bone marrow failure

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

### **1. Introduction**

Eukaryotic chromosomes are capped at their ends by specialized nucleo-protein structures, named telomeres that protect them from degradation. Human telomeres have a specific nucleotide sequence composed by thousand of repetitions of the TTAGGG hexanucleotide [1]. A protein complex, named shelterin associates to this DNA region to form the telomere-specific chromatin structure. Telomeres protects the chromosomal ends from degradation and are, therefore, essential for chromosomal and genome stability [2]. In their absence chromosomal ends are recognized as damaged DNA by the cell and can be degraded or recombined with other chromosomal ends resulting in the fusion and reorganization of chromosomes [3]. The maintenance of telomeres is, therefore, of critical importance for the genetic stability of cells and organisms.

Replication of telomeric DNA requires the contribution of a specific enzymatic machinery. DNA polymerases responsible for replication of the rest of the chromosomal DNA cannot completely synthesize telomeric DNA. DNA polymerases always require a primer molecule that cover the 5' end of the DNA and are not able to complete the synthesis of the lagging strand of lineal DNA molecules, such as chromosomes. This end-replication problem results in the shortening of each telomere by 50-100 nucleotides at each DNA replication cycle and, therefore, at each cell division [4]. In most eukaryotic organisms, including humans, telomere length is maintained by the activity of the telomerase complex that elongates the telomeres by a replication-independent mechanism [5]. The complex is formed by a protein with reversetranscriptase activity (TERT) and one RNA with a region of homology to the telomere DNA that is used as template for elongation [6]. Telomerase activity is, therefore, required for unlimited cell proliferation. Telomerase components and, in particular the TERT gene, are expressed to high relative levels during embryonic development allowing high cell prolifera‐ tion rates. Expression of the TERT gene is, however, repressed in most human adult cells [7]. TERT expression is found only in germinal cells, in stem cells, specially in those of highly proliferative tissues such as bone marrow and epithelia and in lymphocytes [8]. The rest of the cells express very low TERT levels and their telomeres get progressively shorter after each cell division. When telomeres reach a critical size get unprotected and are recognized as damaged DNA. The ATM and ATR kinases, that regulate cellular responses to DNA damage are recruited to critically-short telomeres and activate the p53-dependent pathway that results in cell cycle arrest [3]. Prolonged arrest would finally induce apoptotic cell death or cellular senescence. Actually, most tissue-specific stem cells do not express enough TERT protein to completely replicate their telomeres at each cell division and their proliferative capacity decreases with the age of the organism [9]. With time, stem cell exhaustion impairs tissue renewal. Because of this reason, telomere shortening has been recognized as one of the hallmarks of human aging [10].

Telomere replication is also involved in the acquisition of the unlimited proliferative capacity that characterizes tumor cells [7]. Telomerase expression and activity is induced in about 85% of tumors, which allows tumor cells to completely elongate their telomeres at each cell division. In the other about 15% of tumors, telomeres length is maintained by a telomerase-independent mechanism, know as Alternative Lengthening of the Telomeres (ALT) that elongates telomeres through DNA recombination mechanisms [11].

The importance of telomere homeostasis is further enforced by the existence of a number of rare hereditary diseases that are caused by the presence of shortened telomeres, collectively named telomeropathies, short telomere syndromes or telomere biology disorders [12]. These diseases are caused by mutations in genes coding for proteins involved in telomere lengthening (telomerase complex and related proteins) or in the maintenance of telomere structure (shelterin complex). These diseases are commonly characterized by the presence of very short telomeres in the cells of the affected patients. The molecular pathology of these diseases, their diagnosis and emerging therapies will be summarized in this chapter. It is necessary to enforce the importance of telomere homeostaxis for healthy life since excessively long telomeres are also causative of disease. Recent reports have associated the presence of long telomeres to increased frequency of cancers such as melanoma or glioma [13]. Mutations in the promoter region of the TERT gene that increase gene expression are frequently found in these and other tumors [14, 15]. In addition, mutations in the coding region of genes coding for protein of the shelterin complex have been found in human tumors [16, 17].
