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The different phases of the cell cycle. In the first phase (G1) the cell grows. When it has reached a certain size it enters the phase of DNA-synthesis (S) where the chromosomes are duplicated. During the next phase (G2) the cell prepares itself for division. During mitosis (M) the chromosomes are separated and segregated to the daughter cells, which thereby get exactly the same chromosome set up. The cells are then back in G1 and the cell cycle is completed. |
The normal cell cycle
In most tissues of the body, cells multiply through a process known as the cell cycle. Before cells can multiply and divide into other cells, they have to make exact copies of their DNA. DNA is the genetic code that is in all the cells of our bodies and is exactly the same code in each cell no matter what tissue the cell is from. Chromosomes are made up of the genes of our cells and our genes are made up of strands of DNA. Each cell of our body has two copies of each gene, one inherited from our mother and one from our father. The nucleus of the cell houses our chromosomes and genes.
Normally, most cells are not actively growing and dividing and are in the G0 or resting phase of the cell cycle and have a diploid or 2N DNA content. Cells in the G1 phase are actively cycling but like G0 cells have a "diploid" or 2N DNA content. A small percentage of cells in normal tissues are undergoing DNA synthesis (making a copy of their DNA) and are in the S phase of the cell cycle (have a DNA content between 2N and 4N). A few cells have completed their DNA synthesis and doubled their amount of DNA and are in the G2 phase of the cell cycle (have a 4N or tetraploid DNA content). After cells double their DNA, they undergo mitosis (M phase) dividing into two daughter cells that are exact genetic copies of each other and have a DNA content of 2N.
The cell cycle Resting G0 cells receive a signal to replicate and enter the cell cycle at G1 with a 2N DNA content (46 total chromosomes). The G1 Phase prepares the cell for duplication. When those preparations are complete AND no genetic mistakes are detected, the cell enters S Phase (DNA synthesis). During S Phase, all DNA in the cell is duplicated. Upon completion of S Phase, there are a total of 92 chromosomes and the cell has a 4N DNA content. Following S Phase, cells move into G2 Phase where the duplicated DNA is checked for errors. G2 cells then undergo cell division (mitosis - M Phase) and divide into two daughter cells, each with a normal DNA content of 2N. These daughter cells can then undergo DNA synthesis and multiply again or can enter G0 (the resting phase) of the cell cycle. |
Nowell's hypothesis
There are many events or steps that occur in Barrett's esophagus that lead to the development of cancer. A few of these events are known but most are not. Most of the known events appear to occur early, before high-grade dysplasia or cancer actually develops. No one knows what the late events are that give cells the ability to leave their normal growth boundaries and become a cancer.
It is now widely accepted that the development of most cancers is due to something called genomic or genetic instability. This theory was first proposed by Dr. Peter Nowell in 1976. The theory is that for some unknown reason, perhaps due to environmental factors or inherited factors, some cells in the body develop genetic abnormalities that give them the ability to outgrow genetically normal cells. These abnormal cells grow and expand into a clone of cells (a group of cells having the same genetic make-up) and may replace their neighboring normal cells. Eventually one of the abnormal clones may undergo another genetic change that leads to the development of a sub-clonal population with the expansion of this cell line into its own large clone of cells. As multiple genetic abnormalities occur, multiple sub-clones develop or evolve. Eventually, one of these sub-clones may acquire the necessary combination of genetic abnormalities to become a cancer.
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| Ball diagram of Nowell's hypothesis The green balls represent cells that have developed a genetic abnormality and are expanding or growing into a clone of cells. One of these cells develops a second genetic abnormality, illustrated by a blue ball, seen to expand into its own clone of cells or subclones of the green population. A third genetic mistake is made, illustrated by a dark red ball, with clonal expansion of this cell population. Eventually, another genetic mistake is made in one of the cells of the dark red population that allows that cell to become a cancer. |
Cell cycle checkpoint genes
Incredibly, genetic mistakes are rarely made in the duplication of a cell's DNA. If a genetic mistake is made, for example - caused by exposure of a cell to radiation, there are genes (called cell cycle checkpoint genes) that control the cell cycle and prevent cells from dividing into two daughter cells. These cell cycle checkpoint genes insure that abnormal clones of cells will not be produced by the tissues of our bodies under normal circumstances.
The p53 gene
The p53 gene was the first cell cycle checkpoint gene to be discovered in humans. It is referred to as a tumor suppressor gene because its normal function is to suppress the development of tumors by detecting genetic mistakes in G1 cells resulting in arrested cell growth (cell cycle arrest) or destruction (programmed cell death) of the cells with the mistake. When genetic abnormalities develop in the p53 gene leading to loss of its normal function, tumors more readily develop because cells with genetic mistakes are allowed to divide and pass the mistake on to daughter cells.
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Site of p53 action on the cell cycle If a G1 cell makes a genetic mistake, the protein made by the p53 gene does not allow that cell to enter S phase and copy its DNA. The abnormal G1 cell will usually undergo programmed cell death. This prevents cells with genetic abnormalities from dividing and undergoing clonal expansion and evolution. |
p53 gene abnormalities are detected in up to 95% of Barrett's associated cancers indicating that loss of function of the p53 gene is a necessary step in the progression to cancer in Barrett's esophagus. Loss of function of the p53 gene in Barrett's esophagus is one of the earliest known genetic events in the development of cancer and it is closely tied to abnormalities that develop in the cell cycle of Barrett's cells. These abnormalities can be detected by a test called flow cytometry (a test that measures the amount of DNA in a cell).
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| In most people, genetic abnormalities in the p53 gene develop only in cells of a particular tissue, such as in Barrett's cells, and do not occur in other cells of the body. These types of genetic abnormalities are referred to as somatic genetic abnormalities because you are not born with them and cannot pass them on to your children. Somatic genetic abnormalities may occur through environmental exposure, such as tobacco use, for example. |
Increased 4N fraction
The earliest abnormality in the cell cycle of Barrett's cells that can be detected by flow cytometry is an increase in the percentage of cells that have doubled their amount of DNA (4N cells). It has been shown that this increase in percentage of 4N cells corresponds to loss of function of the p53 gene. These 4N cells appear to be unstable and lead to the development of aneuploid cells, cells that have multiple genetic errors or mistakes. Frequently in Barrett's high-grade dysplasia and cancer, multiple different aneuploid cell populations representing multiple sub-clones can be detected. There is evidence that these populations develop by clonal evolution and expansion similar to that proposed by Nowell.
Increased numbers of 4N cells and abnormalities in the p53 gene are some of the earliest abnormalities detected in the Barrett's lining and can be detected BEFORE high-grade dysplasia or cancer develops. In fact, abnormalities in the p53 gene can be detected in some apparently normal diploid (2N) Barrett's cells prior to the development of increased 4N cells, aneuploidy or high-grade dysplasia in Barrett's esophagus. One large study that followed Barrett's patients with and without a p53 gene abnormality reported that patients with a p53 gene abnormality had a significantly increased chance of developing cancer as compared to Barrett's patients who did NOT have a p53 gene abnormality. In this study, patients with a p53 gene abnormality also developed increased numbers of 4N cells, aneuploidy, and high-grade dysplasia much more frequently than patients without a p53 gene abnormality.
The p16 gene
In addition to the p53 gene, other genes are inactivated, or their functions lost, in the progression to cancer in Barrett's esophagus. Another gene, called p16, is frequently abnormal in Barrett's esophagus. p16 is also a tumor suppressor gene and, like p53, inhibits the transition of cells from G1 to S phase. Abnormalities in the p16 gene are the earliest known genetic abnormalities in Barrett's esophagus and can be detected prior to the development of p53 gene abnormalities, increased 4N cells, aneuploid cells, or high-grade dysplasia. Recently, it has been shown that there are several types of p16 gene abnormalities that can affect one or both copies of the p16 gene. What was surprising about this study, was that more than 85% of all Barrett's linings have at least one of these abnormalities in their p16 gene. In fact, the number of abnormalities in the p16 gene increases with the increase in the length of the Barrett's lining. Very short Barrett's linings tend NOT to have p16 gene abnormalities, intermediate length Barrett's linings tend to have only one abnormality in the p16 gene, and very long Barrett's linings tend to have 2 abnormalities. This has led investigators to hypothesize that the Barrett's lining starts out as a single clone of cells that develops p16 gene abnormalities very early on, causing the Barrett's to spread along the esophagus. The greater the number of p16 gene abnormalities, the greater the ability of these cells to spread and grow and extend the length of the Barrett's segment. In addition to causing spread of the Barrett's lining, p16 gene abnormalities may make it easier for other gene abnormalities to develop in the Barrett's lining or for these other gene abnormalities to spread over wide areas of the Barrett's lining. However, as most patients' Barrett's linings have at least one p16 gene abnormality and most patients with Barrett's do NOT develop cancer during their lifetime, including most people with long Barrett's linings, many more genetic errors must take place before a cancer can develop.
Other genes
Other genes that tend to develop abnormalities in the progression to cancer include genes on chromosome 5, 13 and 18. Clearly, there are many other genes involved in the progression to cancer in Barrett's esophagus that are yet to be identified and their roles characterized.
