Telomere Length Regulation

Abstract

The number of (TTAGGG)_n repeats at the ends of chromosomes is highly variable between individual chromosomes, between different cells and between species. Progressive loss of telomere repeats limits the proliferation of pre-malignant human cells but also contributes to aging by inducing apoptosis and senescence in normal cells. Despite enormous progress in understanding distinct pathways that result in loss and gain of telomeric DNA in different cell types, many questions remain. Further studies are needed to delineate the role of damage to telomeric DNA, replication errors, chromatin structure, liquid-liquid phase transition, telomeric transcripts (TERRA) and secondary DNA structures such as guanine quadruplex structures, R-loops and T-loops in inducing gains and losses of telomere repeats in different cell types. Limitations of current telomere length measurements techniques and differences in telomere biology between species and different cell types complicate generalizations about the role of telomeres in aging and cancer. Here some of the factors regulating the telomere length in embryonic and adult cells in mammals are discussed from a mechanistic and evolutionary perspective.

Keywords: development; lifespan; quadruplex DNA; telomerase; telomere length measurements; telomere length regulation; telomere replication; tumor suppression.

Figure 1

Q-FISH shows extreme variability in the length of telomere repeats in human (A) and mouse (B) chromosomes. (A) Human metaphase chromosomes stained with DAPI (blue) from a cultured fibroblast following hybridization with Cy3 labeled (CCCTAA)₃ PNA (yellow). Note the large variation in telomere fluorescence intensity at individual chromosome ends, discrepancies in telomere fluorescence between sister chromatids and occasional telomere fluorescence spots outside chromosomes*. (B) Mouse metaphase chromosomes from a cultured skin fibroblast hybridized sequentially with a mixture of different fluorescent probes: first Cy3 labeled (CCCTAA)₃ PNA (B, top left panel) and FITC labeled CGGCATTGTAGAACAGTG PNA specific for mouse minor satellite sequences (B, bottom left panel) followed by staining of DNA with DAPI, image acquisition and hybridization with FITC labeled chromosome paint probe specific for respectively chr2 and chr11 (B, bottom right panel). Telomere length was analyzed using the TFL-Telo software (B, top right panel) (28). Note the very short telomeres on the short arm of chr “16” and the very long telomeres on the long arm of chr “22” (arbitrary chromosome numbers and fluorescence intensity values). For details see (29, 30).

Figure 2

Telomeres in a human lymphocyte are not randomly distributed in the nucleus. Shown are optical sections through the interphase nucleus of a human T lymphocyte following formaldehyde fixation and fluorescence in situ hybridization with fluorescently labeled (CCCTAA)₃ PNA (shown in yellow/green). DNA is counterstained with DAPI (shown in red). A stack of images, acquired at separate focal planes, was processed using deconvolution microscopy (40). Note that telomeres appear to cluster at the interface between DNA bright areas, presumably reflecting heterochromatin and DNA weakly stained areas, presumably representing euchromatin (Chavez and Lansdorp unpublished observations).

Figure 3

High mobility of very short telomeres in cultured mouse embryonic stem cells. Viable cells, tagged with Venus-TRF1, were imaged at a fixed position over 10 minutes. The position of individual fluorescent telomere spots was recorded every 10 seconds. Two categories of telomere spots were observed: low intensity spots and high intensity spots. The recorded position of each spot at each time interval was used to calculate the travel distance of individual telomeres. See Supplementary Information and Supplementary Movie 3 for details.