Telomeres and Human Aging

Barbara Gilchrest, MD

Telomeres and Human Aging

In this year’s session on “The Science of Aging Skin” Dr. Barbara Gilchrest asked the question:  “Why do we age and what does aging mean?”  After posing this rhetorical question she went on to explain that there are two essential components to aging. The first is that there is a genetic program that assures that cells likely to be damaged over many decades stop dividing after a finite number of divisions and will not carry on their dysregulated program.  The second component is “wear and tear” and refers to the environmental insults to individual cells and organized tissue that cause cells to advance to a senescent state more rapidly.

When we speak about aging of the skin we refer to two components: intrinsic and extrinsic aging.

Intrinsic aging refers to the clinical, histologic, and physiologic changes in sun-protected skin of older adults. This is also called chronological aging.

Extrinsic aging, which is also referred to as “photoaging”, refers to the clinical, histologic, and physiologic changes in habitually sun-exposed skin of older adults.  Photoaging occurs primarily on the face.  There are striking differences between chronological aging and photoaged skin in chronically sun-exposed areas.

Intrinsic aging has a minor impact on the appearance of the skin but over time results in multiple functional deficits such as slow wound healing and the loss of immune competence observed in older skin. By contrast photoaging has a major impact on the appearance of the skin, results in a further loss of immune function, and probably exaggerates the loss of other cellular functions.  Most importantly, photoaging is strongly associated with photocarcinogenesis.

Extrinsic aging is not only the result of UV light but also cigarette smoke, which accelerates the aging process. The effect of cigarette smoking on aging skin has been repeatedly documented since the 1960’s.  Very recently the role of air pollution in extrinsic aging of the skin has been documented in middle-aged women.  The study involved a comparison between women living in the countryside away from highways vs. those living in cities closer to highways.  The women experiencing air pollution near roadways were found to have accelerated skin aging.

Current Concept: “The processes of aging and photoaging are consequences of safeguarding the genome”

Dr. Gilchrest contends that nature is concerned about the genome with little regard to the actual aging process. Cancer is the failure of this safeguard mechanism.

Research in the later half of the 20^th century has identified and documented major mechanisms of aging.   These include “signaling imbalance” due to the inter-related factors of retinoic acid deficiency, corrected by retinoic acid replacement (Voorhees); increased activity of NFκB (a transcription factor that contributes to dysfunction of senescent cells); oxidative stress due in part to aerobic metabolism; UV damage; and other cumulative DNA damage to cells of body, particularly the skin.  More recently telomere shortening, a newly understood aspect of aging, has led to a greater understanding of the aging process.  The Nobel prize in 2009 was given to 3 scientists whose work beginning in the 1980s greatly advanced our understanding of telomeres. Appreciating their role in the aging process promises to lead to novel therapies for aging.

The Role of Telomeres:

Telomeres (Figures 1 and 2) are the terminal portions of chromosomes, in man about 10,000 DNA base pairs that shorten to around 6,000-7,000 base pairs with age.  Telomeres are composed of a repeating base pair sequence of TTAGGG and its complement.  Telomeres form a loop structure that caps the end of DNA strands that are otherwise interpreted as double stand breaks. Without telomeres you get chromosomal fusion, mutations and cell death.  (Figure 2)

Critically short telomere lengths cause cells to go into a senescent state such that the cell stops dividing after 50 – 60 post natal cell divisions.  Nothing is capable of stimulating the cell to divide at that point.

In addition to their role as a “biologic cellular clock” in which telomeres limit the number of cell divisions (Harley et al. Nature 1990), it has been found that telomeres also trigger DNA repair responses   (Karlseder et al. Science, 1998).

The Role of Telomerase:

Telomerase is the enzyme complex responsible for lengthening telomeres by adding TTAGGG sequences to the tips of chromosomes.  Telomerase is expressed in germline cells, stem cells and >90% of malignant cells. Telomerase is also expressed transiently in S (DNA synthesis) phase in normal cells (Masutomi et al. Cell, 2003).  Telomerase slows but does not prevent telomere shortening in normal cells. However, its absence in genetically engineered mice is associated with acceleration of the intrinsic aging program.

Telomerase Activation

Telomerase activation in otherwise normal cells immortalizes the cells, which then divide indefinitely but are not malignant because these cells are still subject to the local environmental commands.  In an animal you get a younger animal but promote carcinogenesis because you remove the essential telomerase shortening that turns off cell division and limits the life of the environmentally mutated cell.

Increased Telomerase Activity in Combination
with Cancer Resistance Delays Aging in Mice

Experiments were performed in which mice were genetically modified and made transgenic for TERT (catalytic component of telomerase) and also over-expressed the tumor suppressors p53, p16 and/or p19 ARF. The combination of increasing telomerase activity while overexpressing cancer resistance gene activity resulted in mice with an increase in median and maximum lifespan without an increase in cancer, an increase in telomere length and an improvement in clinical and molecular aging markers, including in the skin. {Tomas-Loba et al. Cell, 2008}

Compared to old wild type or other control mice Telomerase Reactivation Reverses Tissue Degeneration in Aged Telomerase-Deficient Mice

Dr. Gilchrest described a study involving 4^th generation telomerase-deficient adult mice which were infertile with widespread tissue atrophy. Four weeks of conditional telomerase expression (TERT knock-in) resulted in: skin fibroblast proliferation increased telomere length, organ cellularity, including brain; increased fertility and litter size; increased olfactory responses; increased survival time and no carcinogenesis and a reduction in DNA damage signaling in tissues. (Jaskelioff et al. Nature, 2011)

 Telomeres Are Strongly Implicated in Human Aging

 Telomeres shorten with age in vitro (50-150 base pairs per mitosis) and in vivo. {Lindsay et al. Mut Res, 1991; Vaziri et al. PNAS, 1994; Dimri et al. PNAS, 1995}. Telomere shortening correlates with progression of age-associated diseases such as diabetes.  Telomere length (measured in peripheral blood lymphocytes) correlates with longevity in persons >60 years old.  {Cawthon et al. Lancet, 2003;  Valdes et al. Lancet, 2005}.  Progeria, Werner syndrome and other progeroid syndromes are characterized by short telomeres.

New information on Telomeres:

Over the last few years new information regarding the activation of telomeres and telomere associated proteins involved in DNA repair have been uncovered.  Telomeres are replicated throughout S phase and DNA damage repair proteins associate with telomeres during S phase and are activated. Homologous recombination proteins help to reconstitute the protective telomeric t-loop in the G₂ phase  (Masutomi et al. Cell, 2003; Crabbe et al. Science, 2004; Verdun & Karlseder. Cell, 2006; Verdun & Karlseder. Nature, 2007)

Interpretation for telomere-based activation of DNA damage repair proteins during S phase:

According to Dr. Gilchrest, cumulative evidence regarding the role of telomere-based activation of DNA repair proteins suggests that this process might function as the cell’s final “quality check” before dividing.  This pathway is activated during replication in which telomere-based signaling first acts to reduce DNA damage, to slow senescence, and to protect the genome.  If acute damage is overwhelming or many cycles of cell division make cumulative damage likely, cells are pushed to apoptosis or senescence.  Cancer develops when this mechanism fails.