Seminario 10 Telomeros

PE R S PE C TI V E

Telomeres, Telomerase, and Human Disease

Focus on Rese arch

Telomeres, Telomerase, and Human Disease Steven E. Artandi, M.D., Ph.D.

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elomeres, the genetic segments that appear at a chromosome’s ends, have been known since the 1930s to have special properties that protect these ends. They were first isolated in the 1970s and 1980s and shown to be made up of DNA repeats that, when transferred to the ends of artificial chromosomes in yeast, could protect them from degradation. At the same time, a newly discovered enzyme called telomerase was found to add telomere repeats to the ends of chromosomes with the use of a dedicated RNA template. This mechanism of telomere addition by telome-

rase solved the “end-replication problem” — the inability of conventional DNA polymerases to replicate fully the ends of chromosomes. The inactivation of telomerase components led to telomere shortening and eventual senescence in yeast. Next week, Elizabeth Blackburn, Carol Greider, and Jack Szostak will receive the 2006 Albert Lasker Award for Basic Medical Research for making these seminal observations. These fundamental discoveries in chromosome and cell biology reverberate beyond the basic-science arena; they have ramifications for human health and dis-

TERT

3'

Telomere

AAUCCCAAUC G TTAGGGTTA G G A GGGTTAGGGT T T C A C C CAATCCCA

5'

TERC Figure 1. Structure and Function of Telomerase. Telomerase comprises two subunits, telomerase reverse transcriptase (TERT) and the telomerase RNA component (TERC). Telomerase adds telomere repeats to the ends of chromosomes by reverse transcribing the sequence in the template region of TERC (boxed) from RNA to DNA in six-nucleotide increments (sequences shown in red). Telomerase can continue elongating telomeres through a ratcheting mechanism, by repeatedly dissociating from the newly synthesized telomere, realigning, and adding another six nucleotides at a time. Telomerase up-regulation is critical in cancer, allowing cancer cells to divide an unlimited number of times.

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ease. More recent investigations show that telomeres and telomerase are central to the biology of cancer, stem cells, aging, and the inherited syndrome dyskeratosis congenita. The initial connection between telomerase and cancer was established when most human tumors were found to express telomerase, whereas some normal tissues and cultures of normal cells did not. Telomerase comprises two principal subunits: telomerase reverse transcriptase (TERT), the protein catalytic subunit, and the telomerase RNA component (TERC). When cultured in vitro, many primary human cells lack sufficient TERT to maintain telomeres. Consequently, telomeres shorten progressively with cell division, eventually causing cellular senescence, as a subgroup of telomeres lose their ability to protect the ends of chromosomes and are therefore recognized by the cell’s DNA-repair machinery as damaged DNA (see Figure 1). Overexpression of TERT in primary human cells is sufficient to prevent telomere attrition and enables these otherwise normal cells to proliferate indefinitely — behavior reminiscent of that of cancer cells in humans. Indeed, in cancer, telomerase expression serves just that function, endowing cells with an infinite replicative capacity. Telomerase appears to be upregulated late in tumor development, which may explain why telomeres shorten considerably during tumorigenesis. This telomere shortening can profoundly

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PE R S PE C T IV E

Telomeres, Telomerase, and Human Disease

compromise chromosomal stability and may contribute to the widespread genomic changes seen in cancer in humans. Preclinical models of advanced cancers in humans in which telomerase is expressed suggest that telomerase inhibition is a viable strategy for compromising tumor growth. In normally regenerating tissues, stem cells and progenitor cells express telomerase and intact telomere function is required for tissue homeostasis. In telomeraseknockout mice, critical telomere shortening induces programmed cell death and impairs the function of actively dividing tissues, including the bone marrow, testis, and gastrointestinal tract. The requirement for functional telomeres in tissue maintenance may be related to the essential role of intact

telomeres in stem-cell self-renewal, which has been most clearly demonstrated with hematopoietic stem cells in transplantation experiments. Whether this requirement extends to other types of stem cells has yet to be determined. Although the need for intact telomere function in stem cells may partially explain the pattern of telomerase expression in human tissues, telomerase was recently found to activate stem cells through a mechanism that does not require its telomere-lengthening function. Therefore, both telomeres and telomerase may have profound effects on stem cells and progenitor cells in mammalian tissues. The gradual loss of telomere sequences in cultured primary human fibroblasts suggests a connection to aging. Indeed, with

Telomeres shorten with successive cell divisions in the absence of telomerase

advancing age, telomere shortening is seen in many human tissues and is often described as a “mitotic clock” that reflects the number of cell divisions that have occurred in the tissue’s history. Critical telomere shortening in human tissues may therefore activate senescence responses or lead to cell depletion, either of which could contribute to impaired tissue function in the elderly (see Figure 2). Rather than impairing all cells within a tissue, dysfunctional telomeres may contribute to aging through interference with stem-cell function. Accelerated telomere shortening also accompanies aging in progerias such as Werner’s syndrome. The likelihood of a causal link between telomere shortening and aging was strengthened by the recent findings that the DNA helicase protein

DNA-damage response causes cell senescence or cell death

Cancer, aging, impaired stem-cell function, and dyskeratosis congenita

Loss of telomere cap leads to DNAdamage response

Telomeres Chromosome fusion

Chromosomes align during metaphase

Fused chromosome is stretched during anaphase

Chromosomal breakage resulting in genomic instability

Figure 2. Cellular Effects of Telomere Shortening. In cells that lack sufficient telomerase, telomeres shorten progressively with cell division because of the inability of DNA polymerase to replicate fully the ends of chromosomes. If a chromosome’s protective telomere cap is impaired, a DNA-damage response is triggered, causing cellular senescence or cell death. Alternatively, the loss of telomere protection permits inappropriate joining of telomeres to yield fused chromosomes, which are highly vulnerable to breakage, resulting in genomic instability. Both of these responses may contribute to cancer, aging, impaired stem-cell function, and the inherited syndrome dyskeratosis congenita.

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PE R S PE C TI V E

Telomeres, Telomerase, and Human Disease

that is mutant in Werner’s syndrome is required for efficient telomere replication and for telomere stability. These findings suggest that telomere dysfunction may be partially responsible for the premature aging seen in Werner’s syndrome and, by extension, for certain aspects of normal human aging. Mutations in telomerase components underlie the pathophysiology of dyskeratosis congenita, an inherited syndrome characterized by aplastic anemia, oral leukoplakia, nail dystrophy, and abnormal skin pigmentation. In its autosomal dominant form, dyskeratosis congenita is associated

with mutations in TERC or TERT. An X-linked form of the syndrome is caused by mutations in the protein dyskerin, which binds TERC and is also involved in ribosome biogenesis. Consistent with this link to telomerase is the fact that telomeres are shorter in patients with the syndrome than in normal subjects. Recently, some patients with aplastic anemia but no other signs of dyskeratosis congenita were found to have mutations in either TERC or TERT. These findings indicate that the bone marrow failure seen both in some patients with aplastic anemia and in patients with dyskeratosis congenita is caused by telomere dys-

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function, presumably through the impairment of hematopoietic stemcell function. The basic-science discoveries of Blackburn, Greider, and Szostak have paved the way for these findings linking telomeres and telomerase to human disease. Undoubtedly, additional connections among the telomere, its sustaining enzyme, and human biology will be discovered in the future. Harnessing these insights may well lead to improvements in the treatment of cancer and other disorders associated with aging. Dr. Artandi is an assistant professor of hematology at the Stanford University School of Medicine, Stanford, CA.

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