Brian Luke's lab has moved to the IMB in Mainz. Please note the new contact details in the left column!
Telomere structure and chromosome end protection
Telomeres make up the ends of linear chromosomes and are composed of repetitive G rich DNA elements bound to telomere-specific binding proteins. Whereas DNA ends resulting from broken or damaged DNA often leads to cell death; telomeres avoid being detected as DNA damage due to their intrinsic protected state. The specific telomere binding proteins play an active role in keeping telomeres "capped" by preventing nucleases from accessing the chromosome ends as well as actively inhibiting DNA damage checkpoint proteins. Interestingly, telomeres have been shown to form a fold-back or loop structure that has been proposed to participate in telomere protection. In humans the loop is referred to as a t-loop and interestingly yeast have also been shown to loop back their telomeres, suggesting that telomere looping is an evolutionarily conserved feature. Using high-throughput genetic screening techniques in yeast we are gaining first insights into understanding the molecular mechanisms behind telomere looping and so far we have identified over 100 genes (including the Rpd3 histone deacetylase complex) that are important for loop formation (Figure 1). Characterization of these various mutants with respect to telomere protection and maintenance will be a future challenge for the lab.
Non-coding RNA at telomeres (TERRA) function and regulation
Despite their heterochromatic state telomeres get transcribed into a non-coding Telomeric Repeat-containing RNA (TERRA). TERRA is conserved from yeast to man although its function remains unknown. We have taken advantage of a transcriptionally inducible yeast telomere in order to probe TERRA function. In this manner we can turn transcription on and off in a controlled manner and analyze such parameters as: telomere length, telomere capping and cellular senescence. Using this system we have seen that telomeric transcription causes telomere shortening and leads to rapid cellular senescence. We are currently testing the model that telomeric transcription results in DNA replication dependent telomere loss. Furthermore, we have characterized many proteins involved in endogenous TERRA regulation including the SIR complex as well as the telomere binding proteins, Rif1 and Rif2. We are currently characterizing other novel proteins that are involved in the in vivo regulation of TERRA.
Klermund J, Bender K, Luke B: High nutrient levels and TORC1 activity reduce cell viability following prolonged telomere dysfunction and cell cycle arrest. Cell Reports, 2014; In Press (full article)
Balk B, Dees M, Bender K and Luke B: The differential processing of telomeres in response to increased telomeric transcription and RNA-DNA hybrid accumulation. RNA Biol. 2014 Feb 5;11(2) (see Abstract)
Balk B*, Maicher A*, Klermund J, Luke-Glaser S, Dees M, Bender K and Luke B: Telomeric RNA-DNA hybrids affect telomere length dynamics and senescence. Nat. Struct. Mol. Biol, 2013, DOI: 10.1038. (see Abstract)
Poschke H*, Dees M*, Chang M, Kaderali L, Amberkar S, Rothstein R and Luke B: Rif2 promotes a telomere fold-back structure through Rpd3L recruitment in budding yeast. PloS Genetics. 2012 Sep;8(9):e1002960.(see Abstract)
Luke-Glaser S, Luke B: The budding yeast helicase Mph1 impairs growth when telomere integrity is compromised. PloS One. 2012 July 27;7(7):e42028. (see Abstract)
Maicher A, Kastner L, Dees M and Luke B: Deregulated telomere transcription causes replication dependent telomere shortening and promotes cellular senescence. Nucleic Acids Research. 2012 Aug;40(14):6649-59. (see Abstract)
Iglesias N, Redon S, Pfeiffer V, Dees M, Lingner J*, Luke B*: Subtelomeric repetitive elements determine TERRA regulation by Rap1/Rif and Rap1/Sir complexes in yeast. EMBO Rep. 2011, 12:587-593 (see Abstract).
Luke B, Panza A, Redon S, Iglesias N, Li Z, and Lingner J: The Rat1p 5’ to 3’ exonuclease degrades telomeric transcripts and promotes telomere elongation in S. cerevisiae. Mol Cell. 2008, 32:465-477 (see Abstract).
Maicher A, Lockhart A, Luke B: Breaking new ground: Digging into TERRA function. Biochim Biophys Acta, 2014, Apr 1; 1839(5):387-394 (see Abstract)
Luke-Glaser S, Poschke H and Luke B: Getting in (and out of) the loop: regulating higher order telomere structures. Frontiers in Oncology. 2012 Nov 30;2:180.
Maicher A*, Kastner L*, and Luke B: Telomeres and Disease - Enter TERRA. RNA Biology. 2012 Jun 1:9(6):843-9. (see Abstract).
Screening mutants: we introduced our looping readout into 4800 viable yeast mutants and screened for mutants that grew on FOA (unable to loop) as depicted in the lower box. We scored 112 genes that are required for telomere looping (click image to enlarge).
TERRA gets transcribed at all telomeres. We are testing the model that upon DNA replication TERRA needs to be removed in order to prevent replication fork collapse and subsequent telomere loss (click image to enlarge).