Telomeres, together with the enzyme Telomerase are mentioned a lot lately in association with both aging and with cancer. It seems that their gradual shortening during the lifetime of a cell eventually results in damage to DNA and thus the genes of which they are a part. This has led to a great deal of research into Telomeres and Telomerase, some of which will be outlined in this section of the site. First of all however, let us get some of the basic definitions out of the way.
Definition of a Telomere
A telomere is a region of highly repetitive DNA at the end of a linear chromosome that functions as a disposable buffer. Every time linear eukaryotic chromosomes are replicated during late S-phase the DNA polymerase complex is incapable of replicating all the way to the end of the chromosome; if it were not for telomeres, this would quickly result in the loss of useful genetic information, which is needed to sustain a cell’s activities. http://en.wikipedia.org/wiki/Telomere
Definition of Telomerase
Telomerase is the reverse transcriptase responsible for the extension of telomeric repeat sequences in most species studied. If telomerase activity is diminished or absent, telomeres will shorten. Shortened telomeres appear to lead to cell senescence. Eventually telomeric sequences can shorten to the point where they are not long enough to support the telomere-protein complex protecting the ends and the chromosomes become unstable. These shortened ends become 'sticky' and promote chromosome rearrangements. Some rearrangements may contribute to the development of cancers. http://www.genlink.wustl.edu/teldb/tel.html
Role, Function and Compesition of Telomeres
In the majority of Prokaryotes, the chromosomes are circular. This means that they do not have ends that are prone to damage or ‘premature replication termination.’ Note that a small number of bacteria, Borrelia and Streptomyces for example, do possess linear chromosomes (together with circular plasmids). These chromosomes are very different however, from those which are found in Eukaryotic cells (such as those in humans.
A telomere consists of repeating base sequences (in humans this is a repeating string of TTAGGG bases between 3 to 20 kilobases in length). In addition to the telomere itself, there is a 100-300 kilobase sequence associated with the telomere, which is located between the telomere and the rest of the chromosome.
As replication of DNA occurs during the lifetime of an organism, the telomeres gradually shorten. This shortening eventually results in damage to the chromosomes themselves and to the genes that they contain. The results of this are either a reduction in the cells ability to express its genes and a descent into cellular senescence (cellular dormancy) or in some cases the over or under-expression of specific genes. When this chromosomal damage leads to either an over or under-expression of certain genes, then cellular functionality can be compromised. In some cases the organisms survival can also be compromised as cellular replication can become unchecked. These cells thus become cancerous and can lead to the death of the organism.
Role, Function and Compesition of Telomeres
During DNA replication, The DNA unzips and a complimentary strand is formed against the unzipped sections. Telomeres shorten during this process due to the ‘lagging strand’ phenomenon.
Basically DNA replication does not begin at the end of the DNA, but in the centre. DNA Helicase unzips the DNA forming Replication bubbles. An RNA Primer or Primase then attaches to each DNA strand and replication begins in the 3-5 direction, thus forming a new strand in the 5 -3 direction.
DNA polymerases move and replicate the DNA in the 3 to the 5 direction (thus making a replica strand in the 5 to 3 direction). Note that 3 refers to the 3 OH group of the sugar and the 5 refers to the 5 phosphate group of the nucleotide.
The leading strand of DNA is the strand that is oriented in the 3-5 direction. The primer of complementary molecules that binds to the first few of the exposed bases ends with a 3 sugar group. The phosphate of a new nucleotide can be attached here by DNA polymerase. DNA polymerase then continues along the strand, synthesising a new strand as it goes. An animation of this process can be seen below, provided by the Nobel e-museum.
http://www.nobel.se/medicine/educational/dna/a/replication/replication_ani.html.
The lagging strand however, faces more problems when it comes to replication. Because DNA polymerase can only attach to the 3 sugar group and thus move in the 3 to the 5 direction, it needs therefore to move in small jumps (in the opposite direction to overall replication) to replicate the lagging strand. See below animation. This is again provided by the Nobel e-museum.
http://www.nobel.se/medicine/educational/dna/a/replication/lagging_ani.html
Note that the small segments of DNA and primers which are produced on the lagging strand are called Okazaki fragments. Another DNA polymerase enzyme is then recruited to remove the primers and to replace them with DNA. Finally DNA Ligase seals the gaps between the Okazaki fragments.
DNA Replication
The problem with Telomere shortening occurs because, in order to change the RNA primers into DNA, there must be another DNA segment in front of the primer. There is a DNA segment ahead of the primer at every section of the strand, except where the last primer attaches (the end of the telomere). This means that this final primer cannot be replaced with DNA. It does get removed by various enzymes however, but in the process, the telomere shortens.
In human blood cells, the range of lengths of the telomeres varies between 8000 base pairs at birth, to 1,500 base pairs in the elderly. During cellular division, an average of 30 to 200 base pairs are removed from the ends of the telomeres.
In normal cases, the cells of a human can divide between 50 to 70 times, with the telomeres shortening with each division, until the cells either commit suicide through a process known as Apoptosis, become Senescent (dormant) or transform to cancerous cells due to genetic damage.
The process of Cellular senescence due to Telomere Shortening
As a cells telomeres shorten during multiple cellular divisions, DNA damage occurs and the cell, recognising this damage shuts itself down. Below is a simplified diagram of how this occurs.
Telomere shortening and aging
DNA damage results in an activation of the p53 gene. p53 then activates p21, which blocks the actions of a number of CDK’s (Cycline Dependent Kinases). Note that CDK’s are involved in the regulation of the cell cycle, transcription and mRNA processing. The blocking of certain CDK’s prevents the phosphorylation of pRb. This lack of hyperphosphorylated pRb results in a failure of expression of several critical genes, which are involved in cellular division. Cellular division then stops.
Telomerase and Telomere extension
A study was conducted (as so many aging studies are) using the nematode Caenorhabditis elegans (Joeng KS, Song EJ, Lee KJ, Lee J (2004). "Long lifespan in worms with long telomeric DNA". Nature Genetics 36 (6): 607-11.). This study indicates that by lengthening the Telomere, longevity can be increased.
Two distinct groups of Nematodes were engineered. The only difference between the two groups, was the length of the Telomere. The group with the longer telomere’s lived approximately 20 percent longer than the group with the shorter telomeres. Also, it was observed that the Nematode’s with the longer telomeres possessed a greater resistance to the effects of heat exposure.
Telomerase is the natural enzyme which promotes telomere repair. It is however not active in most cells. It certainly is active though in stem cells, germ cells, hair follicles and (worryingly) in 90 percent of cancer cells. Telomerase functions by adding bases to the ends of the telomeres. As a result of this telomerase activity, these cells seem to possess a kind of immortality.
In 1990 a team at Geron Corp in Menlo Park, California led by Serge Lichtsteiner and Andrea Bodnar,in association with the University of texas Southwestern Medical center, managed to activate the production of telomerase in cells that do not usually produce this enzyme. The results were that the telomeres started to lengthen. By the time that they released the results of their study, their cells had divided 20 or more times than would normally be expected before senescence would set in. These cells also seemed to retain their normal gene expression and showed no signs of becoming cancerous.
I will now try to explain in more detail the details and results of the teams study.
Basically, telomerase is present in all cell types, however the human gene for the catalytic protein telomerase transcriptase (hTRT) is present only in immortal cells (such as stem cells or cancerous cells). Scientists from the teams observed that lengthening of telomeres in retinal pigment epithelial cells, foreskin fibroblasts and vascular endothelial cells by introducing the hTRT gene into these cells, results in a resumption of telomerase activity. This resumption resulted in the extended longevity of the cultured cells, as discussed previously.
Varying Telomere decline
One interesting study by Peter Lansdorp of the Terry Fox Laboratory, Canada, observed in human fibroblasts that the rate of telomere shortening as cells divide varies between different telomeres. This variation is between 50 to 150 base pairs per cell division. It is worth noting that the telomeres that are shorter initially. For example in humans the 17p telomere, are not necessarily the ones to be destroyed first! It does seem that it is the shortening of specific telomeres that are linked to a cells decline into senescence, apoptosis or its transformation into a cancer. Martens et al, 1998, observed that the shortening of telomeres 1p, 5p and 22p, but not that of 17p, showed a statistical correlation with the descent into cellular senescence.
Despite all of the above outlined observations, it is a statement of fact that the mean telomere length of a species does not always relate to the longevity of that species. For example Katuo et al, 1999, noted that of all studied primates, humans seem to have both the shortest telomeres and the longest lifespan! Bassham et al, 1998, also observed that the long lived frog Xenopus Laevis displayed a great variation in telomere length and that telomere length could even diminish between parent and offspring, with no detectable consequences.
To summarise, it seems that it has not yet been determined whether the instability of a chromosome and its eventual deterioration is a result of general telomere shortening or the shortening of specific telomeres. It has however been found that by artificially lengthening the telomeres of cells, which do not normally have a mechanism to lengthen their own telomeres, the cells longevity and ability to function normally for longer, does increase. More research therefore needs to be conducted in order to determine the exact significance of telomere length in the aging of an organism and the exact consequences of artificially stimulating telomere repair.
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