Telomeres are repeating sequences that allow cells to distinguish the ends of chromosomes from broken DNA ends. Telomeres prevent DNA damage – they allow the cell to distinguish the end of a chromosome from a chromosome break. They are repeat sequences

Telomerase is an RNA+protein enzyme that solves the end replication problem by elongating telomeres Holoenzyme = multipart active enzyme (apoenzyme+cofactor) A nuclear holoenzyme composed of Human Telomerase Reverse Transcriptase (hTERT), a DNA Polymerase Telomerase Associated RNA (TERC), a 451-Nucleotide RNA that is the template for telomeres Responsible for elongating Telomeres Telomere = a sequence at the end of chromosome that protects DNA and serve as a biological clock for cells. TTAGGG in Humans

Normal somatic human cells do not express telomerase, so they have a limited lifespan The Hayflick Limit = the number of times a cell can divide before telomeres are critically short

Telomerase is essential in human stem Cells in culture and in its absence cells senesce and eventually die Telomerase works in stem cells to keep the ends of chromosomes long enough to prevent DNA damage mechanisms to become activated The shelterin complex (6 subunit protein) recognizes TTAGGG repeats as distinct from broken DNA High shelterin presence is common on long telomeres and is associated with blocked telomerase. Less shelterin allows telomoerase to become active. When telomeres do become too short, the DNA Damage Response is activated and the p53 checkpoint prevents a new cell cycle from beginning, leading to senescence and aging

One would think that a telomerase knockout mouse would not be able to survive… Image showing knockout of telomerase RNA

…but this is not the case! Telomerase knockout mice survive for at least six generations and can develop tumors This suggests that telomerase might not be necessary for tumors to grow, even if it is present. Cells were injected with rasv12 and T-angtigen (oncogenes) to create tumors in nude mice. Each mouse was injected twice. KO = Mutant generation 1 Telomerase negative cells can become immortal cells.

Telomerase is re-expressed in 90% of human tumors Active telomerase allows differentiated cells to avoid crisis and divide indefinitely. Proven by adding hTERT cDNA to cells just before crisis in vitro. hTERT cDNA cells to escape crisis. This shows All components of Telomerase are expressed except hTERT One cause of crisis is critically short telomeres Telomerase activity is enough to create immortal cells

Telomerase re-expression immortalizes cells and contributes to cancer 3’ overhang allows telomerase to bind and add complimentary DNA. Shelterin prevents telomere from being recognized as DNA break

Because it is present in so many tumors, telomerase is an attractive drug target Difficult because Telomerase is expressed in normal stem cells necessary for life (e.g. Bone Marrow) These cells divide more slowly than cancer cells and should be slightly more resistant to inhibitors because telomerase is less necessary [trim]Treatments that may be effective include use of small inhibitors (BIBR1532 – tested in mice), stabilization of G-Quadruplex using ligands (telomestatin), gene therapy, and immunotherapy

Targeting Telomerase could be an issue since it is necessary for stem cells figure shows that most stem cells have lower levels of telomerase than cancer cells, so they should be less affected by treatments targeting telomerase.

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