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Telomeric Repeat Containing RNA: Biogenesis, Composition and Function

Final Report Summary - TERRA (Telomeric Repeat Containing RNA: Biogenesis, Composition and Function)

In human cells, the genetic information is stored on 46 chromosomes. Chromosomes consist of DNA as well as proteins and RNA, which package the DNA into the chromosomes. The proteins also regulate which genes from the DNA are expressed in a given organ. The physical ends of the chromosomes are known as telomeres. They are crucial for the chromosomes as they protect the DNA extremities from degradation and fusion events. In addition, telomeres regulate the lifespan of cells. Telomeres shorten with every chromosome duplication event, which precedes cell division. Therefore, most somatic cells in our body can divide only 40-60 times. After this, the short telomeres instruct the cell to stop growing and dividing. This state is called cellular senescence. This telomere-mediated limitation of cell growth suppresses tumor formation. Indeed, tumor cells overcome this growth barrier through activation of an enzyme called telomerase which adds new telomere repeats to the ends of the telomeres. Through telomerase, tumor cells gain an immortal phenotype.
How can the telomere fulfill all its functions? So-far we know that in addition to the telomeric DNA, the proteins that bind telomeric DNA are crucially important for telomere function. In addition, we discovered in the year of 2007 that telomeres contain a long RNA molecule termed TERRA. TERRA is transcribed from the telomeric DNA and stays partially associated with the telomeric DNA protein complex. Thus, this RNA is not transported from the nucleus to cytoplasm as the better-known messenger RNAs, which are translated into proteins. Instead TERRA is staying in the nucleus and fulfills its function as noncoding RNA. Being telomere-associated we now demonstrate that TERRA mediates several of the above discussed telomere functions.
During the tenure of this grant, we determined the exact nucleotide structure of TERRA molecules as expressed from many chromosome ends. Thus we now understand which DNA sequences at the telomere are used to synthesize TERRA and what activities modify it post transcription. We find that TERRA levels are much lower during S phase of the cell cycle than during the growth phase that precedes cell division. We have identified proteins that associate with TERRA and therefore mediate TERRA function. These functions include the regulation of telomere length, the mediation of cellular senescence and telomeric chromatin remodeling. For example, we have demonstrated that telomerase, the enzyme which counteracts telomere shortening at chromosome ends and is responsible for the immortality of cancer cells is inhibited upon binding to TERRA. We also demonstrated that hnRNPA1 which is associated with a large fraction of TERRA circumvents this inhibition. Still, TERRA induction also promotes telomere shortening through increasing the activity of the exonuclease 1 enzyme at chromosome ends which accelerates telomere shortening and cellular senescence by actively removing nucleotides from chromosome ends. TERRA also associates with the lysine demethylase enzyme LSD1. Importantly, TERRA transcription is increased upon telomere shortening and loss of the telomere binding protein TRF2. Our data indicate that increased TERRA promotes LSD1 recruitment and that LSD1 then activates the MRE11 nuclease at chromosome ends leading to DNA end processing in the event of cellular senescence. SUV39H1 is another enzyme that we identified in association with TERRA during cellular senescence. SUV39H1 modifies histone proteins at telomeres during senescence. Altogether, our data demonstrate that TERRA orchestrates functions of proteins during cellular senescence. Finally, we have identified proteins promoting TERRA maturation (components of the so-called THO complex). These proteins are critical for telomeres as they prevent TERRA from interfering with telomere protection and DNA replication.