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Telomeres, telomerase, and aging: Origin of the theory



In 1971 I published a theory in which I first formulated the DNA end replication problem and explained how it could be solved. The solution to this problem also provided an explanation for the Hayflick Limit, which underpins the discovery of in vitro and in vivo cell senescence. I proposed that the length of telomeric DNA, located at the ends of chromosomes consists of repeated sequences, which play a buffer role and should diminish in dividing normal somatic cells at each cell doubling. I also proposed that the loss of sequences containing important information that could occur after buffer loss could cause the onset of cellular senescence. I also suggested that for germline cells and for the cells of vegetatively propagated organisms and immortal cell populations like most cancer cell lines, an enzyme might be activated that would prevent the diminution of DNA termini at each cell division, thus protecting the information containing part of the genome. In the last few years, most of my suggestions have been authenticated by laboratory evidence. the DNA sequences that shorten in dividing normal cells are telomeres and the enzyme that maintains telomere length constant in immortal cell populations is telomerase.
Experimental Gerontology, Vol. 31, No. 4, pp. 443-448, 1996
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Historical Perspective
Institute of Biochemical Physics of the Russian Academy of Sciences, Chernahovskogo 5, kv. 94,
Moscow 125319 Russia
1971 I published a theory in which I first formulated the DNA end replication
problem and explained how it could be solved. The solution to this problem also provided an
explanation for the Hayrick Limit, which underpins the discovery of in vitro and in vivo cell
senescence. I proposed that the length of telomeric DNA, located at the ends of chromosomes
consists of repeated sequences, which play a buffer role and should diminish in dividing
normal somatic cells at each cell doubling. I also proposed that the loss of sequences
containing important information that could occur after buffer loss could cause the onset of
cellular senescence. I also suggested that for germline cells and for the cells of vegetatively
propagated organisms and immortal cell populations like most cancer cell lines, an enzyme
might be activated that would prevent the diminution of DNA termini at each cell division,
thus protecting the information containing part of the genome. In the last few years, most of
my suggestions have been authenticated by laboratory evidence. The DNA sequences that
shorten in dividing normal cells are telomeres and the enzyme that maintains telomere length
constant in immortal cell populations is telomerase.
Key Words: marginotomy, telomere, telomerase, DNA end-underreplication problem, Hayflick
Limit, cancer cells, normal ceils
THIS IS the history of a theory about the Hayflick Limit and the DNA end-underreplication
problem, which, when first published in 1971, went almost unnoticed for 20 years until recent
research results dramatically proved its validity.
In the last few years an enormous amount of information has been published on the identi-
fication and behavior of telomeres and telomerase and the role of each in normal cell senescence
and cancer cell immortality both in vitro and in vivo. This essay describes the historical
developments that led to this remarkable series of discoveries.
The importance of chromosome ends, called telomeres, was first recognized by McClintock
(1941) and by Muller (1938) who defined telomeres as the functional ends of chromosomes as
J Fax: (011) (7095) 214-9269; E-mail:
(Received 9 November
distinct from random broken ends. Beginning from about 1966 I had understood that the
replication of linear DNA by DNA polymerase would result in the loss of terminal sequences
unless some mechanism existed that would maintain the ends of chromosomes. The mechanism
by which this terminal sequence loss could be explained was called end-underreplication.
In 1971, I published a paper in which I described the existence of the DNA end-
underreplication problem and proposed a theory for its solution. However, it was not until the
last few years that laboratory studies have provided confirmation of the theory that I proposed
25 years ago.
It has been discovered that the ends of linear chromosomes consist of a repeated sequence of
bases whose length decreases with each cell division. It is thought that when the chromosome
shortening reaches a critical length, further events prevent the cell from dividing. In immortal
cancer cells, an enzyme is produced that adds new sequences onto the ends of chromosomes at
each DNA replication, thus maintaining the chromosome length constant. In this way cancer
cells divide indefinitely and, thus, achieve immortality. It is quite gratifying to see one's theory
ultimately supported by laboratory results, and I would like to explain how my theory arose.
I formulated the theory of telomere shortening when I was a postdoctoral student at the
Gamelaya Institute of the Academy of Sciences of the USSR in 1966. Academician Gamelaya,
a vaccinologist and for whom the Institute was named, was a colleague of the great Russian
biologist, Elie Metchnikov, the discoverer of phagocytosis and the theory that aging results from
inadequate fermentation activities of the intestinal flora.
My chief was Dr. Lev A. Zilber, a man of great intellect and one of the originators of the idea
that viruses may cause cancer. Zilber was a "Renaissance" man who spent several years of
incarceration in the Gulag Archipelago as an alleged Japanese spy. When Zilber was freed he
returned to the Gamelaya under the direction of the same person who was his tormentor in the
Gulag. In a dispute with the director, Zilber hurled a black marble blotter base at the face of the
director, a high ranking KGB officer, but he missed. This occurred during the Khrushchev thaw
and the director was soon replaced. The directors of the Gameleya then changed several times
with some having been appointed more for their achievements as KGB members than as
members of the scientific community.
In Zilber's Department of Immunology and Oncology, I worked in two laboratories. One was
the antibody biosynthesis laboratory of Aaron E. Gurvich who was coinventor of immunosor-
bents and also coined this term instead of immunoadsorbent. I also studied in the laboratory of
Gary I. Abelev who discovered the presence of alpha-fetoprotein in some tumors. Abelev, like
Zilber who died in 1966, struggled with another director, a KGB Colonel, to save Zilber's
Department. The struggle ended when Abelev moved his laboratory to another institute in
Moscow. Both Gurvich and Abelev helped me greatly by providing a letter of recommendation
to Andrey N. Belozersky, the Academician of our Academy of Sciences. In that letter they asked
Belozersky to submit my strange theory on telomeres to Doklady, which is the local Proceedings
of the Academy of Sciences.
My theory looked strange because it stated that a portion of genomic DNA was regularly lost
with each round of DNA replication and that the losses occurred at the ends of chromosomes.
The fact that DNA in eukaryotic chromosomes was linear was not entirely settled at that time.
But, Belozersky gave me a recommendation and my paper appeared in Doklady in 1971. Thus,
my "Theory of Marginotomy" began its official existence.
But, that is getting ahead of the story.
Until the Autumn of 1966, I never thought about the replication of the DNA double helix
termini. This changed when I attended a lecture by Alexander Y. Friedenstein, a cell biologist
at Moscow University. His lecture was on the new phenomenon discovered by Leonard Hay-
flick, in which it was reported that normal human cells, unlike immortal, abnormal or cancer
cells, have a limited capacity for replication. This was, in fact, the second time that I heard about
this phenomenon. I first heard about it at the Gameleya Institute of Epidemiology and Micro-
biology of the Academy of Medical Sciences when I was a postdoctoral student but it made no
strong impression. But, when I heard about it for the second time at Moscow University I was
simply thunder-struck by the novelty and beauty of the Hayflick Limit. I thought about this as
I returned home from the University and walked along the quiet Moscow streets that were paved
with gold-colored leaves on that early evening in late Fall as I made my way to the subway
The Theory of Marginotomy came to me in that Moscow subway station. I heard the deep roar
of an approaching train coming out from the tunnel into the station itself. I imagined the DNA
polymerase to be the train moving along the tunnel that I imagined to be the DNA molecule. I
thought that this polymerase cannot begin to copy from the very beginning because there is a
dead zone between the front end of the polymerase molecule and its catalytic center. This is
analogous to the dead zone between the front end of a subway car standing at the beginning of
the subway platform and the nearest entrance door to the first car. After this serendipitous
underground brainstorm, which happened in the Fall of 1966, I wrote to Hayflick to ask some
questions about his discovery and he sent additional unpublished data to me. I then spent several
years thinking about this idea before publishing it in the central journal of our Academy of
Sciences, Doklady Academii Nauk SSSR (Proceedings of the Academy of Sciences of the
USSR) (Olovnikov, 1971).
So, it was the Hayflick Limit that started me thinking about an explanation for his finding and
its possible link with the DNA end replication problem. Hayflick's finding was for me like
Ariadne's thread was for Theseus, who followed it to escape from the labyrinth. During the
subsequent months, as I commuted to my laboratory via that subway station and saw the
approaching train, my mind returned repeatedly to thoughts about how the Hayflick Limit might
be tied to the DNA end-replication problem.
In 1972 I had an opportunity to present my theory at the 9th International Congress of
Gerontology in Kiev in the Ukraine.
The director of the Gamaleya Institute at the time that I was on its staff was rumored to be
a colonel in the KGB. He forbid all of his staff from participating in the Congress. I believe that
this was done in connection with the intent to keep the Russian biochemist and gerontologist,
Zhores Medvedev, from attending the same Congress. I well remember my deep indignation
when the director personally told me that the publication of my abstract in the Congress
Proceedings was sufficient and that he forbid me to attend in person. Immediately after leaving
his office I arranged to take my vacation and left for the Congress in Kiev.
After I presented my short paper, there among my listeners was Hayflick. I asked him for his
opinion and he said that it was very interesting but obviously required support from experi-
mental data. Later, I learned that he did not speak to me at length because he was heavily
involved in attempts to learn the fate of his kidnaped friend, Zhores Medvedev, who like me,
had been forbidden to come to the Kiev Congress and had taken his vacation to circumvent the
official denial. Medvedev was later found to have been interrogated by the local police and then
taken by them to the Kiev railroad station and put on a train back to Moscow. He was protected
from harm because of the international outcry made by many Western scientists at the confer-
ence who were organized by Hayflick to come to the aid of their colleague.
More than two decades have passed since my publication. Although I had no doubts about the
accuracy of my proposal, nevertheless it has been an uncertain and lonely 20 years for me
because, like most innovative ideas, it was unacceptable to many biologists. There was, how-
ever, at least one notable exception.
At an April, 1974 session of the annual meeting of the Federation of American Societies for
Experimental Biology (FASEB), chaired by Hayrick, Robin Holliday presented a lecture in
which he described my theory and referred to my first paper (Olovnikov, 1971). Holliday's
lecture was subsequently published, and represents the first reference to my original paper in
Russian on Marginotomy by a Western scientist (Holliday, 1975).
It was not until a letter arrived a few years ago that I began to feel encouraged about my old
speculations. The letter came from Calvin Harley, who was then at McMaster University in
Canada. He described his experimental evidence (Harley
et al.,
1990) that telomere shortening
could explain the Hayrick Limit (Hayrick and Moorhead, 1961). It was a very exciting and
inspiring moment for me. My thoughts returned to that Autumn day when I left the lecture on
the Hayflick Limit and descended into the Moscow subway station. So, for me the telomere
shortening story has its origin with the report of Hayrick and Moorhead.
In the intervening years telomerists have supplied a remarkable amount of scientific data in
support of the Theory of Marginotomy (for reviews, see Harley, 1991; Levy
et al.,
Greider, 1993, 1994; Blackburn, 1994; Harley
et al.,
1993; Rhyu, 1995). However, because
most Western scientists do not read the Russian scientific literature, reference to my theory is
made to my later 1973 paper published in English in the Journal of Theoretical Biology, despite
the fact that I made reference to my original Russian publication (Olovnikov, 1971) on the first
page of my 1973 paper (Olovnikov, 1973). Thus, my original and primary publication was made
in 1971 (Olovnikov, 1971) and before the paper by James Watson in 1972, in which he
independently made a similar proposal about the DNA end-underreplication problem (Watson,
1972). Watson, however, did not come to a conclusion on the significance of the phenomenon
for somatic and germline cells or for cancer and aging.
I also published another paper in 1972 on the same problem in which I discussed incomplete
replication at the termini of chromosomes and DNA shortening due to removal of the RNA
primer from the DNA end and the peculularitfies, of DNA polymerase construction and move-
ment. I used, as an example, lymphocyte proliferation (Olovnikov, 1972).
My 1971 paper in Doklady, written in Russian (Olovnikov, 1971), can be summarized as
follows: I first described the "end-underreplication problem" as incomplete replication of the
ends of the DNA double helix and gave to this phenomenon the name "marginotomy." Now,
I realize that this term is not a good one because it may sound unusual to an English speaker's
ear. The phenomenon is often designated in the current literature as "end-underreplication."
I then described in my 1971 paper (Olovnikov, 1971) that the circle form of bacterial DNA
and of all circular DNA in the prokaryotic world offered a form of protection from "margi-
notomy" or underreplication of the DNA termini because a circle has no ends.
I also described my explanation of the Hayrick Limit, in which I proposed that during each
round of DNA replication that occurs during the doubling of normal somatic cells, a portion of
telomeric DNA is lost because of end-underreplication. In this way the cell can count the
number of cell doublings it has already performed. After the loss of a critical portion of the
telomeric DNA, the cells will change their normal, young phenotype to an old phenotype. This
is the Hayrick Limit that results in the process of cellular senescence (Hayrick and Moorhead,
1961; Hayrick, 1965). My predictions were subsequently confirmed by Harley
et al. (1990)
other researchers and called a mitotic clock or the telomere hypothesis.
I also explained (Olovnikov, 1971) that germ line cells and tumor cells are able to protect their
telomeric DNA from shortening by the expression of a special form of DNA polymerase that
does not exist in normal somatic cells. This polymerase has been identified as telomerase by
Greider and Blackburn (1985, 1989), Morin (1989), and others (Henderson and Larson, 1991).
I also proposed a variant of protection from marginotomy or from DNA end-underreplication by
proposing that a DNA fragment coming from the outside may attach to the telomere end. This
was experimentally confirmed in
by Biessmann
et al.
Finally, in my 1971 paper (Olovnikov, 1971), I proposed that the gene of the DNA poly-
merase, which is specific for a germ line cells' immortality, also is expressed in cancer cells,
thus endowing them with immortality also. The enzyme, of course, is now called telomerase and
is now studied by many groups.
One of my reasons for writing this Historical Commentary is to explain to my English-
speaking colleagues the full content of my 1971 paper. I do this because there does not seem to
be a complete understanding of its content as demonstrated by some of the erroneous references
and historical confusion that has now crept into the enormously expanding scientific literature
on the origin of the ideas that led to the discovery of telomere shortening and telomerase.
In particular, I find that my 1971 paper is frequently overlooked in favor of the 1972 paper
by Watson when references are made to the end-replication problem. In the field of cellular
senescence I find many papers that refer to my 1973 paper rather than to my original paper of
1971. I also find authors who do, indeed, refer to my 1971 paper but do not realize that it was
in this same paper that I also proposed the telomeric mitotic counter in senescing cells that
exhibit the Hayrick Limit. Reference to the 1972 paper of Watson is frequently the only
reference made to the origins of the telomerase hypothesis when, in fact, he independently
arrived at this idea that I first speculated upon in my 1971 paper (Olovnikov, 1971).
I am continuing with my theoretical work on the end replication problem and will soon
publish some thoughts on why telomere shortening might benefit normal cells. I prefer to pursue
theoretical ideas because they fascinate me and someone must propose unifying ideas before any
experiments can be designed.
Theoretical work also has the advantage of not requiring great financial resources. My
Western readers are well aware that we were once separated by an Iron Curtain. Today, we are
separated from the other world by a Golden Curtain. Our libraries are starved for recently
published books and journals, and many of our research laboratories are without modern in-
struments or sufficient supplies to conduct meaningful biological research.
Nonetheless, a misty morning does not signify a cloudy day.
BIESSMANN, H., CARTER, S.B., and MASON, J.M. Chromosome ends in
without telomeric DNA
Proc. Natl. Acad. Sci. USA
87, 1758-1761, 1990.
BLACKBURN, E.H. Telomeres: No end in sight.
77, 621-623, 1994.
GREIDER, C.W. Telomerase and telomere-length regulation: Lessons from small eukaryotes to mammals.
Cold Spring
Harb. Syrup. Quant. Biol.
LVIII, 719-723, 1993.
GREIDER, C.W. Mammalian telomere dynamics: Healing, fragmentation shortening and stabilization.
Curr. Opin.
Genet. Dev.
4, 203-211, 1994.
GREIDER, C.W. and BLACKBURN, E.H. Identification of a specific telomere ~terminal transferase activity in
43, 405413, 1985.
GREIDER, C.W. and BLACKBURN, ~E,H. A,telomeric sequence in the RNA of
telomerase required for
telomere repeat synthesis.
337, 331-337, 1989.
HARLEY, C.B. Telomere loss: Mitotic clock or genetic time bomb?
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256, 271-282, 1991.
HARLEY, C.B., FUTCHER, A.B., and GREIDER, C.W. Telomeres shorten during ageing of human fibroblasts. Nature
345, 458-460, 1990.
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HAYFLICK, L. The limited in vitro lifetime of human diploid cell strains. Exp. Cell Res. 37, 614-636, 1965.
HAYFL1CK, L. and MOORHEAD, P.S. The serial cultivation of human diploid cell strains. Exp. Cell. Res. 25,
585~521, 1961.
HENDERSON, E.R. and LARSON, D.D. Telomeres--What's new at the end? Curr. Opin. Genet. Dev. 1, 538-543,
HOLLIDAY, R. Growth and death of diploid and transformed human fibroblasts. Fed. Proc. 34, 51-55, 1975.
LEVY, M.Z., ALLSOPP, R.C., FUTCHER, A.B., GREIDER, C.W. and HARLEY, C.B. Telomere end-replication
problem and cell aging. J. Mol. Biol. 225, 951-960, 1992.
McCLINTOCK, B. The stability of broken ends of chromosomes in Zea Mays. Genetics 41, 234-282, 1941.
MORIN, G.B. The human telomere terminal transferase is a ribonucleoprotein that synthesizes TTAGGG repeats. Cell
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MULLER, H.J. The remaking of chromosomes. Collect. Net 13, 1181-1198, 1938.
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OLOVNIKOV, A.M. Immune response and process of marginotomy in lymphoid cells. Vesmik Acad. Medizinskikh
Nauk SSSR 12, 85-87, 1972.
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polynucleotides and biological significance of the phenomenon. J. Theor. Biol. 41, 181-190, 1973.
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... RS has been employed as a well-established in vitro cell aging model to investigate the normal process of organismal aging [14]. The most extensively studied stimuli leading to RS is telomere attrition caused by the "end-replication problem" during DNA replication [15,16]. It suppresses mitochondrial biogenesis through proliferatoractivated receptor γ coactivator (PGC)-1α and/or PGC-1β through p53, mTOR, or PARP activation [17], generating a telomere-to-mitochondria axis to accelerate the aging process. ...
... Since then, the RS of HDF with a finite cellular replicative life span has been widely used for cellular modeling of human aging. The key underlying mechanism of RS has been explained by telomere attrition, or telomere shortening, which is a result of the "end-replication problem" during repetitive cell division [16]. Despite such inevitable telomere attrition induced by the "end-replication problem" in mitotic cells, the repeating telomere DNA sequences have a protective mechanism involving the shelterin complex, which is composed of six proteins (TPP1, POT1, RAP1, TIN2, TRF1, and TRF2). ...
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While mitochondrial bioenergetic deregulation has long been implicated in cellular senescence, its mechanistic involvement remains unclear. By leveraging diverse mitochondria-related gene expression profiles derived from two different cellular senescence models of human diploid fibroblasts, we found that the expression of mitoribosomal proteins (MRPs) was generally decreased during the early-to-middle transition prior to the exhibition of noticeable SA-β-gal activity. Suppressed expression patterns of the identified senescence-associated MRP signatures (SA-MRPs) were validated in aged human cells and rat and mouse skin tissues and in aging mouse fibroblasts at single-cell resolution. TIN2- and POT1-interaction protein (TPP1) was concurrently suppressed, which induced senescence, accompanied by telomere DNA damage. Lastly, we show that SA-MRP deregulation could be a potential upstream regulator of TPP1 suppression. Our results indicate that mitoribosomal deregulation could represent an early event initiating mitochondrial dysfunction and serve as a primary driver of cellular senescence and an upstream regulator of shelterin-mediated telomere deprotection.
... One of the primary hallmarks is telomere shortening (Lopez-Otin et al., 2013). According to the telomere theory of aging and cellular senescence, cells have a definite number of divisions and define when replication is suitable (Olovnikov, 1996). Telomeres, which have a role in the biological clock, have thousands of tandem DNA repeats, TTAGGG at the end of each linear chromosome (Zia et al., 2021). ...
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Aging is accompanied by many changes in brain and contributes to progressive cognitive decline. In contrast to pathological changes in brain, normal aging brain changes have relatively mild but important changes in structural, biochemical and molecular level. Representatively, aging associated brain changes include atrophy of tissues, alteration in neurotransmitters and damage accumulation in cellular environment. These effects have causative link with age associated changes which ultimately results in cognitive decline. Although several evidences were found in normal aging changes of brain, it is not clearly integrated. Figuring out aging related changes in brain is important as aging is the process that everyone goes through, and comprehensive understanding may help to progress further studies. This review clarifies normal aging brain changes in an asymptotic and comprehensive manner, from a gross level to a microscopic and molecular level, and discusses potential approaches to seek the changes with cognitive decline.
... Importantly, in this interim process of telomere restoration through ALT-driven homologous recombination, the telomere ends of the chromosomes were found closed [73]. Telomere shortening in diploid somatic cells is associated with the linear chromosome end replication problem, cutting telomeres in each cell cycle by~50 bp [121]. This process is the molecular basis underpinning the Hayflick limit [122], permitting somatic cells to replicate only a limited number of times, proportional to the species' lifespan. ...
p>Here, we review the role of the circadian clock (CC) in the resistance of cancer cells to genotoxic treatments in relation to whole-genome duplication (WGD) and telomere-length regulation. The CC drives the normal cell cycle, tissue differentiation, and reciprocally regulates telomere elongation. However, it is deregulated in embryonic stem cells (ESCs), the early embryo, and cancer. Here, we review the DNA damage response of cancer cells and a similar impact on the cell cycle to that found in ESCs—overcoming G1/S, adapting DNA damage checkpoints, tolerating DNA damage, coupling telomere erosion to accelerated cell senescence, and favouring transition by mitotic slippage into the ploidy cycle (reversible polyploidy). Polyploidy decelerates the CC. We report an intriguing positive correlation between cancer WGD and the deregulation of the CC assessed by bioinformatics on 11 primary cancer datasets (rho = 0.83; p < 0.01). As previously shown, the cancer cells undergoing mitotic slippage cast off telomere fragments with TERT, restore the telomeres by ALT-recombination, and return their depolyploidised offspring to telomerase-dependent regulation. By reversing this polyploidy and the CC “death loop”, the mitotic cycle and Hayflick limit count are thus again renewed. Our review and proposed mechanism support a life-cycle concept of cancer and highlight the perspective of cancer treatment by differentiation.</p
... Other notable markers in this group include 53BP1, which co-localizes with γH2AX [47]; MDC1, which facilitates the recruitment of ATM kinase and, thus, promotes further H2AX phosphorylation [48]; Rad17, which reacts to local replication stress [49] and telomere dysfunction-induced foci (TIF) [50]. The downregulation of telomerase and telomere shortening, which can be measured by qPCR or FISH, are also useful markers of replicative cellular aging [51]. ...
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Cellular senescence is defined as irreversible cell cycle arrest caused by various processes that render viable cells non-functional, hampering normal tissue homeostasis. It has many endogenous and exogenous inducers, and is closely connected with age, age-related pathologies, DNA damage, degenerative disorders, tumor suppression and activation, wound healing, and tissue repair. However, the literature is replete with contradictory findings concerning its triggering mechanisms, specific biomarkers, and detection protocols. This may be partly due to the wide range of cellular and in vivo animal or human models of accelerated aging that have been used to study senescence and test senolytic drugs. This review summarizes recent findings concerning senescence, presents some widely used cellular and animal senescence models, and briefly describes the best-known senolytic agents.
... To explain this link between somatic cell proliferation and aging, it was hypothesized that a small amount of terminal DNA sequence might be lost during each round of DNA replication, in part due to short RNA primers that initiate DNA replication [5]. In an initial step towards addressing this hurdle, known as the "end replication problem" [6,7], telomeres were identified as a simple repetitive sequence based on analysis of a tiny rDNA chromosome that is abundant in the macronucleus of Tetrahymena thermophila [8]. Tetrahymena possesses a germline micronucleus with five chromosomes that The T-loop can unfold to reveal a 3′ single-stranded overhang that allows for de novo telomere repeat addition by C. elegans telomerase. ...
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Transgenerational inheritance can occur at telomeres in distinct contexts. Deficiency for telomerase or telomere-binding proteins in germ cells can result in shortened or lengthened chromosome termini that are transmitted to progeny. In human families, altered telomere lengths can result in stem cell dysfunction or tumor development. Genetic inheritance of altered telomeres as well as mutations that alter telomeres can result in progressive telomere length changes over multiple generations. Telomeres of yeast can modulate the epigenetic state of subtelomeric genes in a manner that is mitotically heritable, and the effects of telomeres on subtelomeric gene expression may be relevant to senescence or other human adult-onset disorders. Recently, two novel epigenetic states were shown to occur at C. elegans telomeres, where very low or high levels of telomeric protein foci can be inherited for multiple generations through a process that is regulated by histone methylation.Together, these observations illustrate that information relevant to telomere biology can be inherited via genetic and epigenetic mechanisms, although the broad impact of epigenetic inheritance to human biology remains unclear.
... With each DNA replication, telomeric DNA shortens by around 50-200 bp due to the end replication problem (ERP) of semiconservative DNA replication where DNA polymerase can synthesize only the leading strand continuously while the lagging strand is synthesized with the help of an RNA primer and short DNA fragments (Okazaki fragments). Those are finally stitched together by DNA ligase while the most distal RNA primer cannot be replaced by new DNA and its removal thus leaves an ss overhang of around 100-200 nucleotides [9]. In addition, oxidative stress can accelerate telomere shortening [10]. ...
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Due to their close connection with senescence, aging, and disease, telomeres and telomerase provide a unique and vital research route for boosting longevity and health span. Despite significant advances during the last three decades, earlier studies into these two biological players were impeded by the difficulty of achieving real-time changes inside living cells. As a result of the clustered regularly interspaced short palindromic repeats (CRISPR)-associated system’s (Cas) method, targeted genetic studies are now underway to change telomerase, the genes that govern it as well as telomeres. This review will discuss studies that have utilized CRISPR-related technologies to target and modify genes relevant to telomeres and telomerase as well as to develop targeted anti-cancer therapies. These studies greatly improve our knowledge and understanding of cellular and molecular mechanisms that underlie cancer development and aging.
Aging is basically the inevitable and progressive decline of cells, tissues, and organisms with the passage of time. A gradual process of aging in cells, tissues, and organs accompanied by the steady waning of function are normal incidents in the lifespan of an organism. Therefore there is an urgent need to find apt interventions that slow down aging and reduce or postpone the incidence of debilitating age-related diseases. There are many antiaging strategies in development which include procedures such as augmentation of autophagy, elimination of senescent cells, transfusion of plasma from young blood, intermittent fasting, enhancement of adult neurogenesis, physical exercise, antioxidant intake, and stem-cell therapy. Mitochondrial dysfunction, telomere attrition, genomic instability, epigenetic alterations, and stem-cell exhaustion are the molecular and cellular hallmarks of aging. Manipulation of specific signaling pathways and cellular reprogramming has been shown to dramatically affect the aging process. Cellular reprogramming has enabled a new era of regenerative medicine, allowing the conversion of terminally differentiated somatic cells into pleuripotent cells via somatic cell nuclear transfer or forced expression of Yamanaka factors (Oct4, Sox2, klf4, and c-myc). Targeting forced expression of Yamanaka factors is a quite effective approach because these hallmarks are in some instances regulated through epigenetic mechanisms. During cellular reprogramming, these aging hallmarks are restored to a youthful state. Thus epigenetic reprogramming via targeting Yamanaka factors may represent an effective strategy for developing antiaging therapies.
The self-replicating machine has high utility by virtue of its universal construction properties and its productive capacity for exponential growth. Their capacity is unrivalled. They can be deployed to the Moon to industrialize it using local in-situ resources in the short term to open up the solar system and thence deployed on interstellar spacecraft to explore the entire Galaxy by exploiting in-situ stellar system resources. Nevertheless, there are significant concerns regarding the inherent safety of self-replicating machines. We consider the general problem of runaway population growth in physical self-replicating machines to prevent the grey goo problem, the number of offspring spawned by self-replicating machines may be controlled at a genetic level. We adopt a biologically-inspired approach based on telomeres, DNA endcaps that are progressively shortened during cellular replication. This acts as a counter that imposes a limit to the number of replication cycles (Hayflick limit). By examining the biological process in detail, we can obtain some insights in implementing similar mechanisms in self-replicating machines. In particular, we find that counting mechanisms are vulnerable to cancerous runaway.
Aging is associated with numerous changes in the human mind and brain, even in the absence of significant disease. In this review, we summarize some of the research exploring the causes and consequences of age-related brain changes. We discuss research investigating cognitive functions that typically decline as we age, as well as functions that often improve. We also discuss age-related changes in brain structure and brain activity that are associated with these behavioral changes. Finally, we summarize some of the research on the genetics of aging as well as cellular and molecular mechanisms that play a role in the aging process.
Adrenocortical carcinoma (ACC) is one of the deadliest endocrine malignancies and telomere maintenance by activated telomerase is critically required for ACC development and progression. Because telomerase reverse transcriptase (TERT) and regulator of telomere elongation helicase 1 (RTEL1) play key roles in telomere homeostasis, we determined their effect on ACC pathogenesis and outcomes. Analyses of TCGA and GEO datasets showed significantly higher expression of RTEL1 but not TERT in ACC tumors, compared to their benign or normal counterparts. Furthermore, gains/amplifications of both TERT and RTEL1 genes were widespread in ACC tumors and their expression correlated with their gene copy numbers. Higher expression of either TERT or RTEL1 was associated with shorter overall and progression-free survival (OS and PFS) in the TCGA ACC patient cohort, and higher levels of both TERT and RTEL1 mRNA predicted the shortest patient OS and PFS. However, multivariate analyses showed that only RTEL1 independently predicted patient OS and PFS. Gene set enrichment analysis further showed enrichments of wnt/β-catenin, MYC, glycolysis, MTOR, and DNA repair signaling pathways in ACC tumors expressing high TERT and RTEL1 mRNA levels. Taken together, TERT and RTEL1 promote ACC aggressiveness synergistically and may serve as prognostic factors and therapeutic targets for ACC.
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The isolation and characterization of 25 strains of human diploid fibroblasts derived from fetuses are described. Routine tissue culture techniques were employed. Other than maintenance of the diploid karyotype, ten other criteria serve to distinguish these strains from heteroploid cell lines. These include retention of sex chromatin, histotypical differentiation, inadaptability to suspended culture, non-malignant characteristics in vivo, finite limit of cultivation, similar virus spectrum to primary tissue, similar cell morphology to primary tissue, increased acid production compared to cell lines, retention of Coxsackie A9 receptor substance, and ease with which strains can be developed. Survival of cell strains at - 70 °C with retention of all characteristics insures an almost unlimited supply of any strain regardless of the fact that they degenerate after about 50 subcultivations and one year in culture. A consideration of the cause of the eventual degeneration of these strains leads to the hypothesis that non-cumulative external factors are excluded and that the phenomenon is attributable to intrinsic factors which are expressed as senescence at the cellular level. With these characteristics and their extremely broad virus spectrum, the use of diploid human cell strains for human virus vaccine production is suggested. In view of these observations a number of terms used by cell culturists are redefined.
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We have recovered terminal chromosome deletions of the X chromosome of Drosophila [Df(1)RT; RT = receding tips] that break in various positions of the yellow gene (y) region and delete all distal DNA sequences. Terminal DNA fragments are heterogeneous in length. Molecular cloning and sequencing of the terminal DNA fragments revealed that the broken ends of the deleted chromosomes do not carry any telomeric DNA sequences, yet the broken chromatids do not fuse to one another. Moreover, we confirmed by sequence analysis of 49 independently cloned terminal DNA fragments from two RT lines collected at different times that they lose DNA sequences from their distal ends at a rate of 70-75 base pairs per fly generation. We calculate that the rate of loss from these ends is consistent with the removal of an octanucleotide RNA primer at each round of DNA replication in the germ line.
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The terminus of a DNA helix has been called its Achilles' heel. Thus to prevent possible incomplete replication and instability of the termini of linear DNA, eukaryotic chromosomes end in characteristic repetitive DNA sequences within specialized structures called telomeres. In immortal cells, loss of telomeric DNA due to degradation or incomplete replication is apparently balanced by telomere elongation, which may involve de novo synthesis of additional repeats by novel DNA polymerase called telomerase. Such a polymerase has been recently detected in HeLa cells. It has been proposed that the finite doubling capacity of normal mammalian cells is due to a loss of telomeric DNA and eventual deletion of essential sequences. In yeast, the est1 mutation causes gradual loss of telomeric DNA and eventual cell death mimicking senescence in higher eukaryotic cells. Here, we show that the amount and length of telomeric DNA in human fibroblasts does in fact decrease as a function of serial passage during ageing in vitro and possibly in vivo. It is not known whether this loss of DNA has a causal role in senescence.
The observed concatemers of T7 DNA are consistent with replication schemes resulting in double-helical molecules with 3´ ended tails. Right-ended and left-ended molecules can then join to form dimers which on further replication similarly form larger concatemers.
Three possible explanations are presented for the differences in growth potential between human diploid fibroblasts of finite life-span and permanent transformed lines: 1) Only diploid cells have a molecular clock mechanism which counts cell divisions prior to senescence. Two hypothetical examples of such mechanisms are described; however, the available evidence argues against a clock mechanism for aging in fibroblasts. 2) Cells become committed with a given probability to a slow buildup in protein errors, which leads after many divisions to a lethal error catastrophe. It can be shown that speeding up the rate at which the error catastrophe develops, as may occur in transformed cells, can convert a population of finite life-span to one with infinite growth. 3) The growth rate of diploid cells may not depend on the limiting concentration of any one protein. If so, cells with a low level of errors will not have a reduced generation time, and there will be no selection against them. On the other hand the uncontrolled growth of transformed cells may be reduced in rate by the presence of faulty proteins, so that there is continuous selection for those with the fewest errors. Finally, the analogous problem of the mortality of somatic cells and the immortality of the germ line is also briefly discussed.
Since DNA polymerase requires a labile primer to initiate unidirectional 5'-3' synthesis, some bases at the 3' end of each template strand are not copied unless special mechanisms bypass this "end-replication" problem. Immortal eukaryotic cells, including transformed human cells, apparently use telomerase, an enzyme that elongates telomeres, to overcome incomplete end-replication. However, telomerase has not been detected in normal somatic cells, and these cells lose telomeres with age. Therefore, to better understand the consequences of incomplete replication, we modeled this process for a population of dividing cells. The analysis suggests four things. First, if single-stranded overhangs generated by incomplete replication are not degraded, then mean telomere length decreases by 0.25 of a deletion event per generation. If overhangs are degraded, the rate doubles. Data showing a decrease of about 50 base-pairs per generation in fibroblasts suggest that a full deletion event is 100 to 200 base-pairs. Second, if cells senesce after 80 doublings in vitro, mean telomere length decreases about 4000 base-pairs, but one or more telomeres in each cell will lose significantly more telomeric DNA. A checkpoint for regulation of cell growth may be signalled at that point. Third, variation in telomere length predicted by the model is consistent with the abrupt decline in dividing cells at senescence. Finally, variation in length of terminal restriction fragments is not fully explained by incomplete replication, suggesting significant interchromosomal variation in the length of telomeric or subtelomeric repeats. This analysis, together with assumptions allowing dominance of telomerase inactivation, suggests that telomere loss could explain cell cycle exit in human fibroblasts.
Telomeres are specialized chromatin domains located at the ends of chromosomes. They are involved in chromosome replication, stability and localization in the nucleus. In addition to these functions, recent work suggests that telomeres are involved in such superficially diverse cellular phenomena as ageing, cancer, nuclear architecture and nuclear/cellular division.