Telomere Maintenance Mechanisms in Soft Tissue Sarcomas

Abstract

1.1 Telomeres and tumorigenesis 1.1.1 Telomere structure Human chromosomes, which are composed of linear double-stranded DNA, are capped at their ends by nucleoprotein complexes called telomeres (reviewed in (Martinez & Blasco, 2011)). This telomeric cap prevents end-to-end fusion events between chromosomes, and protects chromosome ends from being recognized as double-stranded breaks by the DNA damage machinery in the cell. Telomeric DNA is composed of tandem 5’TTAGGG repeats at each end of the chromosome, extending up to tens of kilobases. Poorly understood endprocessing reactions after replication result in a 3’ overhang on the G-rich strand, which invades the upstream double-stranded telomeric DNA to form a displacement (D)-loop such that the end of the chromosome is buried thereby preventing detection by the cellular DNA damage response (Greider, 1999). Telomeric DNA is in complex with a number of proteins that act to stabilize the structure and mediate telomeric functions of capping and length regulation (Figure 1). Double-stranded (ds) telomeric repeats are bound directly by TRF1 (Zhong et al., 1992) and TRF2 (Bilaud et al., 1997; Broccoli et al., 1997), while the POT1-TPP1 heterodimer binds to the single-stranded (ss) telomeric DNA (de Lange, 2005; Lei et al., 2002). The dsand ss-telomeric DNA complexes are linked through their interaction with TIN2 (Abreu et al., 2010; Kim et al., 1999; O'Connor et al., 2006), together forming the telomeric shelterin complex (de Lange, 2005). Additional proteins necessary for telomeric function are recruited by interactions with components of the shelterin complex (reviewed in (Martinez & Blasco, 2011)). Mammalian telomeres have been shown to contain characteristics of heterochromatin, including the presence of homologues of the heterochromatin binding protein HP1 (Koering et al., 2002; Sharma et al., 2003), enriched tri-methylation of histone H3 lysine 9 (H3K9) and histone H4 lysine 20 (H4K20) (Garcia-Cao et al., 2004), and methylation of CpG dinucleotides in subtelomeric DNA repeats (Gonzalo et al., 2006). Studies in telomerase knockout mice demonstrated that as telomeres become shorter the heterochromatic marks are lost and replaced by marks characteristic of open chromatin, such as increased acetylation of histone tails (Benetti et al., 2007), suggesting that a minimum telomere length is necessary to maintain the appropriate chromatin structure at chromosome ends. Alterations in the level of epigenetic modifications, such as tri-methylation of H3K9 and H4K20 via knockout of the relevant modifying enzymes, led to increased telomere length