Keynote Conference - Interevent

Symp #30 Telomeres
Chair Maria Isabel Cano
Saccharomyces cerevisiae as a model for the study of telomere-mediated replicative senescence
Maria Teresa Teixeira
Emilie Fallet, Pascale Jolivet, Julien Soudet, Zhou Xu, Kamar Serhal and Maria Teresa Teixeira
Institut de Biologie Physico-Chimique, FRE3354 CNRS/UPMC Biologie Moléculaire et Cellulaire des Eucaryotes - 13 rue
Pierre et Marie Curie, 75005 Paris, France ; ERC-STG-2010 D-END
In the absence of telomere length maintenance, telomeres shorten progressively with every replication cycle due to
the DNA-end replication problem. This leads to a permanent cell cycle arrest called replicative senescence. In humans,
this process is involved in the aging of certain organs and in suppression of cancer. Telomeres can be re-elongated by
telomerase or more rarely by homologous recombination (HR) in cells that proliferate indefinitely such as unicellular
eukaryotes, stem cells and cancer cells of multicellular eukaryotes. In telomerase-deficient yeast cells, replicative
senescence is defined as an arrest in G2/M after 60-80 generations as telomeres shorten 2-4 nt/cell division. Our
analysis of the DNA-end replication problem formally demonstrate that telomere shortening occurs during the
synthesis of the leading strand and depends on the length of the 3’-protruding end of chromosomes. Together with
the study of factors that regulate the resection and the synthesis of the 5’ strand, our data support a precise molecular
model of the DNA replication of telomeres. Senescence in yeast depends on the DNA damage checkpoints, similar to
other eukaryotes. A mathematical modeling of the distribution of telomere length and analysis of meiotic products
suggests that the shortest telomere in a cell may have a determinant role in the onset of senescence. Accordingly, the
DNA damage checkpoints recognize a very short telomere in senescent cells, triggering a replication fork regression
and sister chromatid HR. We propose that these pathways counteract the telomere shortening rate allowing a few
additional cell divisions before definitive arrest
Telomere dysfunction in human disease
Rodrigo Calado
University of São Paulo, Ribeirão Preto, Brazil
Telomeres and telomere repair are basic molecular features of cells possessing linear DNA chromosomes and defects
in them result in various diseases. Severe deficiencies result in dyskeratosis congenita, a congenital aplastic anemia
with associated mucocutaneous abnormalities. Mutations in TERT, the catalytic component, and TERC, the RNA
template, can behave as risk factors for the development of bone marrow failure, pulmonary fibrosis, and hepatic
cirrhosis. Both penetrance and organ specificity are variable and not well understood. Chromosome instability is a
result of critical shortening of telomeres and cancer.
Searching for a CST-like complex at Leishmania spp. telomeres
Maria Isabel Cano
Instituto de Biociências, Depto. de Genética, UNESP-Botucatu, São Paulo-Brazil, 18618-970, micano@ibb.unesp.br
In most eukaryotes telomere binding proteins play crucial roles by interacting with several other regulators to ensure
proper telomere maintenance and to form high order complexes. The CST complex, mainly formed by RPA-like
proteins, is being considered a second telomere capping mode occurring from budding yeast to higher eukaryotes.
The role of CST in chromosome-end protection couples the conventional replication machinery and telomere
functions and highlights the complexity of the end- protection process. Leishmania spp. telomeres are composed by
TTAGGG repeats which are maintained by telomerase. The basic Leishmania telomeric complex is formed by the
proteins RPA-1 and Rbp38, which bind in vitro and in vivo, with high affinity to the G-rich telomeric strand, and by the
TRF orthologue representing a shelterin component of this protozoan. Using a large scale search on the tri-tryps
database we were able to confirm that the Leishmania spp. genome, like other trypanosomatids, lacks all of the
conserved telomere-end-binding proteins found in other eukaryotes, such as the key components of the CST (e.g.
CDC13 and CTC) and the shelterin (POT1) complexes. Thus, we speculate that the Leishmania RPA-1 homologue may
play the same roles as POT1/CDC13 at parasite telomeres. In this report we used different approaches to show that
RPA-1 interacts with both Rbp38 and with telomerase. And also that the putative Leishmania CST-like complex meets
the TRF orthologue by physical interactions between Rbp38 and TRF. We speculate whether these protein interactions
reflect the entire telomeric complex or the presence of functionally distinct subcomplexes at parasite telomeres.
Supported by: FAPESP, CNPq
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