Telomerase is an eternally young topic
For a long time, for a short time..., or the Nobel Prize for future immortality
Valery Yudin, "Weekly PHARMACY"Photos and illustrations: nobelprize.org
Humanity has been subject to many different diseases throughout its history, including hundreds of incurable or dangerous ones. However, it is not so much the possibility of getting sick that scares us the most. We are most concerned about the changes in the body that occur over time. These changes are called old age... And then you involuntarily remember Faina Georgievna Ranevskaya, who once said: "Getting old is boring, but this is the only way to live for a long time."
More than a century ago, in 1889, the German zoologist August Weismann (Friedrich Leopold August Weismann) in his "Essay on questions of Heredity and biology of kinship" ("Essays upon Heredity and Kindred Biological Problems") for the first time tried to explain this problem from a scientific point of view – the problem of aging. Being a supporter of the theory of evolution, A. Weisman also considered old age to be the result of evolution: ageless organisms are not only not useful, but also harmful, since they take the place of the young. That is why, in his opinion, evolution should have led living organisms to aging.
His idea of aging as a result of evolution was debunked by another scientist, English biologist Peter Brian Medawar, who in 1951 made a report to the Royal Society of London called "An Unresolved Problem in Biology" ("An Unresolved Problem in Biology"). In this report, he noted that animals in nature rarely live to old age, so the course of evolution can in no way influence the aging process. (In 1960, Medavar, together with the Australian virologist Frank Macfarlane Burnet, received the Nobel Prize in Physiology or Medicine "For the discovery of artificial immune tolerance".)
However, these descriptive theories could not explain anything specific about the causes of aging. The decoding of the DNA structure in 1953 could not clarify much, but only posed even more questions to scientists with many unknowns, which gave rise to more and more theories of aging. (In 1962, scientists – British biologist Francis Crick, American biologist James Dewey Watson and British-New Zealand scientist Maurice Hugh Frederick Wilkins were awarded the Nobel Prize in Physiology or Medicine for deciphering the structure of DNA.)
In those years, scientists were inclined to believe that human cells capable of proliferation in the body can be reproduced indefinitely in culture. However, if this were actually the case, it would mean that people grow old and die not in accordance with their inherent cellular degradation program, but due to extracellular processes occurring at some higher physiological level (Kvitko O.V., Koneva I.I. et al., 2000). And it was only 40 years later that scientists began to come to more or less general views on the theory of aging.
One of these theories was laid down almost half a century ago: in 1961, the American scientist Leonard Hayflick, professor of anatomy at the University of California in San Francisco, discovered that even under ideal conditions cells are able to divide only a limited number of times, and when approaching the limit they show signs of aging (Hayflick L., Moorhead P.S., 1961). Such a division limit was established for the cells of almost all multicellular organisms. The maximum number of divisions varies and depends on the type of cells, and varies even more depending on the organism. So, it was found out that for most human cells this limit is 52 divisions. Moreover, it turned out that with an increase in the age of the donor, the number of divisions that the cells of the body were able to perform significantly decreased. That is, in the body of any living being there is something like a "biological clock", a division counter, which sets a limit on the total number of cell divisions of our body ... (Hayflick L., 1998) This limit was called the Hayflick limit.
However, how does this very "counter" work? The hypothesis about his work in 1971 was put forward by the Russian Soviet scientist Alexey Olovnikov. Based on the data that had appeared by that time on the principles of DNA synthesis in cells, he proposed the theory of marginotomy, which would explain the mechanism of operation of such a "counter". According to A. Olovnikov, in the matrix synthesis of polynucleotides, DNA polymerase is not able to fully reproduce the linear matrix, the replica is always shorter in its initial part. Thus, with each cell division, its DNA is shortened, which limits the proliferative potential of cells and, obviously, is the "counter" of the number of divisions and, accordingly, the lifespan of the cell (Olovnikov A.M., 1971)
It took more than 10 years before A. Olovnikov's guess was confirmed by a series of discoveries made in the late 1980s by three scientists: Elizabeth H. Blackburn, Jack W. Szostak and Carol W. Greider. These three were awarded in 2009 . Nobel Prize in Physiology or Medicine for the fact that they managed to figure out the mechanism by which chromosomes are copied completely during cell division, and they also found out how chromosomes are protected from degradation.
The solution to this riddle was found at the ends of chromosomes, areas that were called telomeres (the term telomere, from Greek. telos – the end and meros – the part, proposed by Herman Joseph Moeller in 1932), as well as the enzyme that forms them, telomerase: long spirals of DNA molecules carrying our gene information are "packed" into chromosomes, at the ends of which are the very telomeres located there as protective "caps" (fig. 2a).
E. Blackburn and J. Shostak discovered that the unique sequence of DNA contained in chromosomes is protected from degradation by means of these "caps". K. Greider and E. Blackburn also discovered telomerase, an enzyme that completes telomeres at the ends of the chromosome, which inevitably shorten every time it divides (Fig. 2b).
Scientists have also noted that the shorter the telomeres, the older the cell, and vice versa: if the activity of telomerase, which completes telomeres, is high and the same telomere length is constantly maintained, the cell does not age. According to the latter "scenario", cancer cells develop, which, as scientists suggest, are practically immortal; and certain hereditary diseases, on the contrary, are characterized by the presence of defective telomerases, which leads to rapid cell aging. Thus, the Nobel Prize recognized the identification of this mechanism as a discovery of fundamental importance that can stimulate the development of new therapeutic strategies.
The mysterious telomere Chromosomes contain our genome, which is carried by DNA molecules.
Back in the 1930s, the American geneticist Hermann Joseph Muller (Hermann Joseph Muller, winner of the Nobel Prize in Physiology and Medicine in 1946 "For the discovery of the appearance of mutations under the influence of X-ray irradiation"), and then Barbara McClintock (Barbara McClintock, winner of the Nobel Prize in Physiology and Medicine in 1983 "For the discovery of transposing genetic systems") noticed that the structures at the ends of the chromosomes, the so-called telomeres, prevented the chromosomes from interlocking with each other. They suggested that telomeres may have a protective role, but how they work remained a mystery.
After scientists in the mid-twentieth century learned to understand how genes are copied, another problem arose: during cell division, the DNA helix is "stitched" into two chains and copied by the DNA polymerase enzyme (see Fig. 1). However, the end of one of the two DNA strands cannot be copied by DNA polymerase, since this is prevented by the presence of an AR site at its end. (AP-site, from apurin-apirimidine site; cleavage of the N-glycoside bond of deoxynucleotides in DNA leads to its occurrence. Due to the inability to form canonical base pairs, AP sites effectively block the action of DNA polymerases. Many DNA polymerases stop when they reach the AP sites of the DNA matrix, after which they dissociate.) It is logical to assume that, as a result, the length of the chromosome should decrease every time the cell divides. However, this does not actually happen. Why?
This question was answered when this year's Nobel Laureates established how the telomere functions and discovered the enzyme telomerase that copies it.
Telomere protects chromosomes At the beginning of her research career, E. Blackburn was engaged in mapping DNA sequences, studying the chromosomes of freshwater ciliated infusoria of the genus Tetrahymena, some species of which are often used as model organisms in biological and medical research.
The scientist found repeating DNA sequences in the chromosome of this infusoria, which can be written as CCCCAA. The function of this sequence was unclear. At the same time, another scientist, J. Shostak, made an observation according to which a linear DNA molecule, like a mini–chromosome, introduced into a yeast cell, rapidly degraded (Fig. 2b).
Both scientists met at a conference in the 1980s, where E. Blackburn presented the results of her work. They interested J. Shostak, and both researchers decided to perform a common experiment in which barriers between such two very distant species of organisms as infusoria and yeast would be overcome. So, E. Blackburn isolated the above-mentioned CCCCAA sequence from the DNA of the tetrachymena infusoria, and J. Shostak attached this sequence to a mini-chromosome, which he placed in a yeast cell (see Figure 2b). The results, published in 1982, were striking – the CCCCAA sequence of telomere DNA protected the mini-chromosomes from degradation. Since DNA telomeres from one organism (tetrachymena infusoria) protected the chromosomes of a completely different organism (yeast), this experiment demonstrated the existence of a previously unknown fundamental mechanism. Later it became obvious that the DNA of the telomere with its characteristic sequence is present in most plants and animals – from amoeba to man.
The enzyme that builds telomeres After the discovery of telomeres, the question arose about their nature.
It was necessary to establish a mechanism by which they would be built at the ends of the chromosome. K. Grader, who worked under the guidance of the already mentioned E. Blackburn, conducted a study to find out whether some previously unknown enzyme was involved in the formation of telomere DNA. On Christmas Day, 1984, K. Greider found signs of enzymatic activity in the cell extract. The enzyme discovered by E. Blackburn and K. The grader was called telomerase. After its isolation and purification, scientists found that it consists not only of protein, but also of RNA, which contains the same CCCCAA sequence as the telomere. Thus, RNA serves as a template for the construction of the telomere, while the protein component of the enzyme is needed directly for enzymatic activity (see Figure 2b). Telomerase extends the DNA of the telomere, providing a platform that in turn allows DNA polymerases to copy the chromosome along its entire length, without losing genetic information. Thus, the chromosome is not shortened during copying.
Telomere Anti-aging cells Now scientists have begun to investigate what role the telomere plays in the cell.
J. Shostak and co-workers found that in yeast cells with mutations, the telomere gradually shortens; such cells grow very poorly, and then stop dividing altogether. E. Blackburn and co-workers observed a similar pattern in the tetrahymene infusoria if telomerase RNA was mutated. In both cases, this led to premature cellular aging. On the contrary, normally functioning telomeres prevented chromosome shortening and delayed cellular aging. Later, K. Grader and her colleagues found that a similar mechanism works in human cells, where telomerase interfered with the shortening of chromosomes. Thanks to intensive research in this area, we now know that the DNA sequence in the telomere "attracts" proteins that form a protective "cap" around the fragile DNA tips.
An important element of the puzzle: aging, diseases and standard of living These discoveries had a strong resonance in the scientific community.
According to many scientists, the shortening of the telomere can be the cause not only of cellular aging, but also of aging of the entire organism as a whole. At the same time, the aging process, as it turned out a little later, is much more complex, and it is now believed to depend on many different factors, among which the telomere is one of many. However, research in this area continues with the same intensity.
However, most cells do not divide that often, so their chromosomes are not at risk of shortening during mitosis, and therefore they do not require high telomerase activity. On the contrary, cancer cells have the ability to divide indefinitely and still retain the necessary telomere length (as it turned out, due to the high activity of the enzyme telomerase). Due to this, methods of treating oncological and other diseases based on inhibition of telomerase activity are being developed, which will be discussed in more detail below.
However, the telomere, as it turned out, can be considered not only in the medical and biological aspect, but also, no matter how surprising it may sound, in the socio-economic, as well as in terms of quality of life.
The same E. Blackburn and colleagues from the University of California San Francisco (University of California, San Francisco) during a pilot study revealed, for example, that mothers caring for their seriously ill children had shorter telomeres in the phase of the highest point of emotional stress (today.ucsf.edu; Njajou O.T., Cawthon R.M., Damcott C.M., 2007).
Another study published in 2006 in the journal "Aging Cell", which was conducted by Dr. Tim Spector (Dr. Tim Spector) with colleagues from the London hospital "St. Thomas's Hospital" and the New Jersey Medical School (New Jersey Medical School), showed that people from developing countries have shorter telomeres than those who lives in the developed (www.reuters.com ; news.bbc.co.uk ; Cherkas L. F., Aviv A., Valdes A.M., 2006).
As mentioned above, several studies are currently underway, including clinical trials, in which the effectiveness of drugs is evaluated, the mechanism of action of which is somehow directed at the activity of the telomerase enzyme. Thus, the American company Geron Corporation conducts four clinical trials with human participation to study drugs that exhibit inhibitory properties against telomerase, as well as the possibility of human immunization with a vaccine that would promote the production of antibodies to cells with hyperactive telomerase. Another American company, Merck & Co, received from Geron licensing rights to approve an application for an investigational drug (Investigation New Drug – IND) for another similar vaccine.
The platform for creating such vaccines is being tested (currently jointly with Merck & Co.) in three directions. According to the first of them, an adenovirus/plasmid–based vaccine is being considered; the second direction of the study is an attempt to create a vaccine based on autogenic dendritic cells (autologous dendritic cell based vaccine) called GRNVAC1 (formerly TVAX), which in the first phase of clinical trials in prostate cancer demonstrated a significant increase in T-cell response (www.geron.com ). The third direction concerns the creation of a vaccine based on embryonic stem cells of dendrite derivatives developed by Geron and currently undergoing preclinical testing. These vaccination methods are aimed at ensuring that the human immune system itself learns to attack cancer cells expressing telomerase.
The next approach used for drug development is the creation of telomerase inhibitors. Thus, the effect of the drug GRN163L, developed by Geron, is aimed at stopping the spread of a cancerous tumor by binding telomerase. This drug candidate is currently undergoing the initial phase of human clinical trials.
However, not everything is so simple. Despite the fact that in vitro it was indeed possible to achieve the death of a population of cancer cells, nevertheless, there are several "buts", for example, the existence of an ALT mechanism (from the English Alternative Lengthening of Telomeres - alternative telomere lengthening), which complicates the effectiveness of such therapy (Bryan T.M., Englezou A., Gupta J., 1995; Henson J.D., Neumann A.A., Yeager T.R., 2002). However, the same "Geron" claims that its already mentioned drug from the group of telomerase inhibitors GRN163L has shown high efficacy, causing the death of cancer stem cells. The mechanism of its action lies in the fact that it binds directly to the RNA matrix of telomerase, thereby blocking its ability to further synthesize telomeres, which can, as a "cap", protect the chromosome of a cancer cell from degradation. At the same time, even a mutation of the telomerase RNA matrix is not able to restore its activity, scientists say. This approach, aimed specifically at inhibiting the enzyme's RNA matrix, according to E. Blackburn, is promising for the creation of medicines in the future. And not only for the treatment of cancerous tumors – telomerase inhibitors can also help to cope with some hereditary diseases that, as it has now become known, are caused by defective telomerase, including certain forms of congenital aplastic anemia, a disease in which the growth and maturation of all three cell lines in the bone marrow is sharply suppressed or even absent. Some inherited dermatological and pulmonary diseases are also caused by telomerase defects.
However, interest is aroused not only by inhibitors, but also by activators of the telomerase enzyme – companies still do not give up hope of trying to influence a person's life expectancy or at least the rate of his aging (www.sierrasci.com ). And here their attempts are fully justified: conducted in 2009 a study of telomerase activity in Ashkenazi Jews has shown that centenarians have hyperactive telomerase (www.livescience.com ). So, since 2001, several compounds have been discovered that activated the production of this enzyme. The first was the California company "Geron", which discovered a substance called TA-65, which the scientists of this company isolated from the plant Astragalus similar (Astragalus propinquus). This substance has shown its effectiveness in activating telomerase. In April 2007, Geron transferred the rights to research its effectiveness in humans to the New York company Telomerase Activation Sciences Inc. - TA Sciences (www.tasciences.com ; www.lef.org ). In November 2007, the American company Sierra Sciences LLC. also stated that it had discovered a substance called C0057684 that caused a high level of telomerase activity in somatic cells; in 2009, the same company announced the discovery of 62 more compounds that have a similar effect (Tanglao, Shawna, et al., 2008; www.sierrasci.com ).
To be continued… Discoveries made by E. Blackburn, K. Greider and J.
Shostak, contributed to the formation of our understanding of new aspects of cell division, shed light on the mechanisms of disease development and prompted the development of potentially new therapeutic methods, about the likely practical application of which, we are sure, the Weekly PHARMACY will write more than once.
Eternal Youth Portal www.vechnayamolodost.ru30.11.2009