Wednesday, April 2, 2008

Does the fountain of youth spring from our chromosomes?

The search for immortality has long been a quest of the human spirit. Whether it manifests as a belief in some sort of spiritual afterlife or in prolonging our mortal lives, humanity seems to find the ending of consciousness a horrid thought. Naturally, the mechanisms for why people grow old and die would gain a huge amount of attention by both researchers and the non-scientific community. Many scientists believe the key to mortality has already been found, and it is located at the ends of our chromosomes. Research has discovered that regions of repetitive DNA stretches called telomeres found on the ends of our DNA strands are cut shorter every time they are copied. Eventually the telomeres are worn away and genes near the end of the chromosomes are lost which contain protein instructions the body desperately needs to survive. Some believe that it is this slow yet eventual erosion of the ends of our chromosomes that leads to aging. The discoveries in this area will have a huge impact on more than just showing the way to a possible fountain of youth. Cancer research and cloning may also hinge on developments in the field of telomere research.

Telomeres exist as the body’s way of solving a problem with DNA replication. DNA is replicated by the use of an enzyme called DNA polymerase. DNA polymerase functions to copy our chromosomal DNA, using an existing DNA "parental" strand as a template. The polymerase performs this feat by attaching nucleotides to polymerize a new "daughter" DNA strand in complement to the parental strand. Adenine (A) is added to the new strand complementary to thymine (T) while guanine (G) is added complementary to cytosine (C), and visa versa. There are two main problems with the capabilities of the DNA polymerase. The first is that it can’t start from scratch. There must be a segment of the new strand from which the polymerase can begin attaching new nucleotides. The use of primers easily solves this problem. These primers are RNA fragments that bind by random assortment complementary to sites on the parent strand of DNA, and must be in place before the DNA polymerase can begin copying the parent strand.

The second problem caused by DNA polymerase during replication is much more difficult for cells to surpass. DNA polymerase can only work in a 5’ to 3’ direction. The terms 5’ and 3’ refer to the sugar molecule in the sugar/phosphate DNA backbone. The numbers relate to the carbon in that ring of sugar. 5’ is the fifth carbon in that ring. 3’ is the third. In order for the polymerase to attach a new complementary nucleotide, an alcohol (-OH) group must be available on the 3’ carbon of the sugar molecule. That is the site where the polymerase attaches the phosphate group of the next nucleotide. This phosphate group is subsequently attached to the 5’ carbon of the new nucleotide’s sugar group. The polymerase can therefore only work from the direction of the previously attached 5’ carbon to the 3’ carbon, which has the –OH group available for the attachment of the next nucleotide. The problem with this unidirectional movement lies with the primers, for they can’t stay in the new strand because they are RNA, and not DNA. Removal of these RNA primers is really not a problem when they are located in the middle of the new daughter strand. There will be a 5’ carbon available for a DNA polymerase to fill in the gap that remained after primer removal. However, the problem lies at the beginning each chromosome. A primer was necessary to provide a 5’ carbon for the beginning of synthesis, yet once it is removed, an upstream 5’ carbon is not available from which a polymerase can attach nucleotides and fill in the gap. Therefore, because the nucleotides are are not replaced after removal of the first primer at the beginning of every chromosome, every time the chromosome replicates the daughter strand will be shorter than the parental strand. Studies have shown that the length of a chromosome shortens by about 50 nucleotides every time it replicates. The damage isn’t huge compared to the overall length of a chromosome, but it does mean the chromosome is mortal in that it is slowly being eaten away at the ends with every cell division. If any of these 50 nucleotides contains the instruction to begin the transcription of a gene, that gene and the protein it encodes will never be usable by the body again.

Replication shortens the chromosome

The body’s natural cure to this dilemma is the production of expendable nucleotides at the 3’ end of every chromosome. These "cannon fodder" nucleotides are called telomeres. Telomeres are repetitive hexameric (6 base pair) sequences of DNA. In humans this repeated G-rich sequence is AGGGTT. These sequences are 1000-1700 base pairs long at the beginning of a mammalian life. Cells seldom survive past about 50 divisions in vitro, which most researchers ascribe to the deletion of too many genes in the process of replication.

Oddly, these telomeres are not encoded in the initial DNA resulting from egg fertilization. What this means is that the telomeres must be added later in development. In 1985 Elizabeth Blackburn and Carol Greider discovered a new DNA polymerase which can add telomeres to DNA. This polymerase, called telomerase, is a ribonucleoprotein present in the very early stages of development. Telomerase activity stops in later development, as it is only required to put the telomeres in place once. Ribonucleoproteins contain RNA, which telomerase uses as a template to synthesize the hexameric DNA telomeres. Because telomerase is a polymerase that copies an RNA template (its own) into DNA, it is a reverse transcriptase. A reverse transcriptase is so named because it is capable of writing codes of DNA from an RNA template which is the reverse of transcription. Reverse transcriptases have gained a lot of fame because they are used by retroviruses, notably HIV, for viral replication.

Telomerase binds to the 3’ end of a chromosome and lines its own RNA template so that a few of its RNA base pairs are complementary to that of the strand. Another segment of the ribozyme hangs over the edge providing a template for the synthesis of the telomeres (CCUAAC). Telomerase synthesizes the hexomeric sequence and then translocates to a new 3’ recognition site, which is within the hexanucleotide it just produced, and repeats the procedure. A normal DNA polymerase and primer can then complete the complementary strand’s 5’ end with all of the new hexomeric repeats--all except the last bit of course. The exact details of telomerase function are currently under research, but its currently understood mechanism as a DNA polymerase that carries its own template appears quite unique and phenomenal.

Could the "Fountain of Youth," simply be a shot of telomerase? Some research hints that this might be a good start to combat aging. For example, recent studies have shown that mice deficient in the gene for encoding telomerase RNA (mTR) developed liver cirrhosis sooner and regenerated much slower than normal mice. These same mice also showed improved liver function upon receiving gene delivery of telomerase. In the future, it may be possible to induce telomerase to reset aging cells back to their chromosomal state during a person’s young and vibrant 20’s. However, in most cases the addition of telomerase into somatic cells late in development would be a death sentence. Cell death (apoptosis) is often a good thing in the body. If some cells didn’t die, some tissues would never stop growing. Apoptosis is a crucial tool used by the body to maintain proper development. Certain cells must die at certain times or else the entire organism will perish.

Another aging-related subject that telomere research might prove helpful to is cloning research. Cloning researchers have found that unfortunately the telomeres of cloned animals (such as the famed cloned sheep named "Dolly") are much shorter than a counterpart of the same developmental "age". Even though cloning technology has attained successful birth rates as high as 80%, most of these clones die before even reaching adulthood. Shortened telomeres appear to be the most likely cause of these deaths. Research seeks to uncover a means of safely extending the telomeres of the clones. Some may hope that the solution to the clone problem will eventually bring about a magic youth potion to humanity.

Telomerases might be cancer's Achilles heel

Besides the prevention of age-related health problems, another motivating drive for telomerase research is to develop effective cancer treatments. Scientists are attempting to destroy the telomeres by eradicating telomerase activity in cancer cells. The purpose is to limit the number of divisions possible in these cells. Normal somatic cells have no telomerase present in them because the expression of the telomerase gene is shut down early in life. Because these cells live a long time in the body, the telomeres created early in life are long enough to serve them for the number of divisions they need to make during the lifetime of the organism. However, cancer cells are defined by unbridled cell division, and therefore it is the telomerase which allows cancer cells to continue their unhindered proliferation and subsequent immortality. One of the mutations that leads to a cell becoming cancerous is one that disrupts the cells ability to shut down telomerase expression. Cancer researchers have become very interested in designing drugs that target and inactivate telomerase, for if telomerase could be inactivated this would lead to cancer cells becoming mortal again and stop them in their tracks.

Although using telomere research for finding a treatment for cancer is a popular concept that everyone supports, the idea of significantly extending life is much more controversial. With the population of Earth bulging proudly over 6 billion souls one has to ponder if human immortality would be a blessing at this point in time. Endless life could be to society what cell immortality is to the body.

No comments: