Colorectal Cancer, TP53

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Expand Collapse Colorectal Cancer  - General Description Colorectal Cancer (CRC) is cancer that initiates in the colon or rectum-the lower part of the digestive system in the body. During digestion, food moves through the stomach and small intestine into the colon. The colon absorbs water and nutrients from food, and stores waste matter (stool) that moves from the colon through the rectum before leaving the body.

Most CRC's and rectal cancers are adenocarcinomas, meaning that they originate in cells that make and release mucus and other fluids. CRC often begins as a growth called a polyp, which may form on the inner wall of the colon or rectum. Over time, some polyps become cancerous. This highlights the importance of colonoscopy screening to find and remove polyps before they become cancerous.

CRC is the fourth most common type of cancer diagnosed in the U.S. Deaths from CRC have decreased with the use of colonoscopies and fecal occult blood tests, which check for blood in the stool. Disparities in survival have been observed between African American and other populations. This may be due to factors such as access to colonoscopy screening, or to other factors not yet identified.

Because of its prevalence, scientists have studied CRC extensively, even creating models of how cancer develops using CRC as an example. There are also families with a very high incidence of CRC occurrence. When these families were studied, certain conditions that create instability in the whole genome were identified that predispose people to CRC. These include what is called the Chromosomal Instability pathway (CIN), as well as MicroSatellite Instability pathway (MSI). These can also occur as spontaneous (uninherited) conditions in some patients. Between 6-10% of CRC's are found to have MSI. Some CRC tumors have been found to have a lot of mutations, or as physicians call it, a "very high mutational load". Some also express a ligand called PD-L1.

These are now recognized features of some CRC's, and immunological treatments may be recommended in these cases. MGH has one of the most extensive Immuno-oncology clinical trials portfolios of any US hospital. Testing for features such as CIN, MSI, a high mutational burden, and the expression of PD-L1 can be conducted at the MGH genetics laboratory, as well as at other large academic centers. Genetic instability such as CIN or MSI lead to the activation of oncogenes such as KRAS, and the inactivation of tumor suppressors such as PTEN, both of which promote tumor growth.

Other genetic alterations in how the DNA in cells is organized have been found to contribute to CRC in families and individuals. These are called epigenetic changes. Normal DNA has methyl groups added in specific regions that regulate gene expression. When the genes that suppress growth-called tumor suppressors-are methylated abnormally, this prevents the production of tumor suppressor proteins important in controlling or stopping cell growth. When tumor suppressor genes are missing, unregulated growth occurs, contributing to the development of cancer. Some tumor suppressor proteins that are frequently inactivated in CRC are APC, TP53, or loss of one arm of chromosome 18 that contains a tumor suppressor.

The study of families with a high prevalence of CRC have lead scientists to discover genetic changes that contribute to the development of CRC in sporadic cases occurring in patients. Mutations in the genes encoding the following proteins have now been associated with subsets of CRC; ALK, AKT, APC, beta-catenin, BRCA1 and BRCA2, BRAF, EGFR, ERBB2 (HER2), ERBB3 (HER3), IDH2, KRAS, MET, NRAS, PI3K, ROS, PTEN, SMO,TP53, TRK 1, 2 and 3, and others that are still being identified. Information on these specific genes is available on this website if you select the gene you want to know more about.

Distinct familial syndromes of CRC such as Lynch syndrome have been studied in patients, leading to the identification of other mechanisms contributing to the development of cancer. Before a cell can divide into two daughter cells, DNA has to be replicated so both daughter cells will have a full complement of chromosomes. DNA replication requires an enzyme called DNA Polymerase. DNA Polymerase occasionally makes errors while it is replicating DNA. Cells therefore have a "proof-reading" process that detects mistakes when they occur during DNA replication. DNA Polymerase mistakes mean that incorrect nucleotides have been incorporated into the DNA, causing mutations. Mistakes in the DNA sequence are repaired in a process called mismatch repair (MMR).
MMR involves a complex of multiple proteins. In Lynch syndrome, one or more of the proteins involved in MMR is mutated, and the mistakes in the DNA do not get corrected. Mutations in MMR proteins are not only found in familial cases of CRC, but also in patients with sporadic (non-inherited) CRC. Defects in MMR also contribute to microsatellite instability (MIS), described above. The accumulation of these mutations can lead to cancer.

The importance of accurately replicating DNA following various types of mistakes or damage is reflected in the multiple pathways cells have for correcting or repairing broken DNA. Actual breaks in the DNA strands can happen due to exposure to radiation or other DNA damaging agents. In the case of the occurrence of breaks in DNA, there are also mechanisms for detecting these breaks. Double strand breaks (DSB's) in the DNA can be repaired via several mechanisms, including Non-Homologous End Joining (NHEJ) or Homologous Repair (HR). Many proteins are involved in DSB repair. Mutations in any of the many proteins involved in either of these repair pathways (see BRCA1 and BRCA2 genes) lead to damaged DNA, which results in DNA that is incorrectly replicated, causing mutations that contribute to the development of cancer. DNA repair machinery in the cell is important in keeping the genome stable and accurate.

Testing for the mutations and genomic conditions that contribute to the development or progression of CRC is available at MGH in the sophisticated CLIA certified genomic testing lab, and in other large Centers and some private testing companies used by physicians. Validated treatments, Immune therpies, as well as clinical trials investigating improved targeted and immunologic therapies are available to patients at MGH.

NIH/NCI Cancer Website www.cancer.gov 2017

Colorectal Cancer (CRC) is cancer that initiates in the colon or rectum-the lower part of the digestive system in the body. During digestion, food moves through the stomach and small intestine into the colon. The colon absorbs water and nutrients from food, and stores waste matter (stool) that moves from the colon through the rectum before leaving the body.

Most CRC's and rectal cancers are adenocarcinomas, meaning that they originate in cells that make and release mucus and other fluids. CRC often begins as a growth called a polyp, which may form on the inner wall of the colon or rectum. Over time, some polyps become cancerous. This highlights the importance of colonoscopy screening to find and remove polyps before they become cancerous.

CRC is the fourth most common type of cancer diagnosed in the U.S. Deaths from CRC have decreased with the use of colonoscopies and fecal occult blood tests, which check for blood in the stool. Disparities in survival have been observed between African American and other populations. This may be due to factors such as access to colonoscopy screening, or to other factors not yet identified.

Because of its prevalence, scientists have studied CRC extensively, even creating models of how cancer develops using CRC as an example. There are also families with a very high incidence of CRC occurrence. When these families were studied, certain conditions that create instability in the whole genome were identified that predispose people to CRC. These include what is called the Chromosomal Instability pathway (CIN), as well as MicroSatellite Instability pathway (MSI). These can also occur as spontaneous (uninherited) conditions in some patients. Between 6-10% of CRC's are found to have MSI. Some CRC tumors have been found to have a lot of mutations, or as physicians call it, a "very high mutational load". Some also express a ligand called PD-L1.

These are now recognized features of some CRC's, and immunological treatments may be recommended in these cases. MGH has one of the most extensive Immuno-oncology clinical trials portfolios of any US hospital. Testing for features such as CIN, MSI, a high mutational burden, and the expression of PD-L1 can be conducted at the MGH genetics laboratory, as well as at other large academic centers. Genetic instability such as CIN or MSI lead to the activation of oncogenes such as KRAS, and the inactivation of tumor suppressors such as PTEN, both of which promote tumor growth.

Other genetic alterations in how the DNA in cells is organized have been found to contribute to CRC in families and individuals. These are called epigenetic changes. Normal DNA has methyl groups added in specific regions that regulate gene expression. When the genes that suppress growth-called tumor suppressors-are methylated abnormally, this prevents the production of tumor suppressor proteins important in controlling or stopping cell growth. When tumor suppressor genes are missing, unregulated growth occurs, contributing to the development of cancer. Some tumor suppressor proteins that are frequently inactivated in CRC are APC, TP53, or loss of one arm of chromosome 18 that contains a tumor suppressor.

The study of families with a high prevalence of CRC have lead scientists to discover genetic changes that contribute to the development of CRC in sporadic cases occurring in patients. Mutations in the genes encoding the following proteins have now been associated with subsets of CRC; ALK, AKT, APC, beta-catenin, BRCA1 and BRCA2, BRAF, EGFR, ERBB2 (HER2), ERBB3 (HER3), IDH2, KRAS, MET, NRAS, PI3K, ROS, PTEN, SMO,TP53, TRK 1, 2 and 3, and others that are still being identified. Information on these specific genes is available on this website if you select the gene you want to know more about.

Distinct familial syndromes of CRC such as Lynch syndrome have been studied in patients, leading to the identification of other mechanisms contributing to the development of cancer. Before a cell can divide into two daughter cells, DNA has to be replicated so both daughter cells will have a full complement of chromosomes. DNA replication requires an enzyme called DNA Polymerase. DNA Polymerase occasionally makes errors while it is replicating DNA. Cells therefore have a "proof-reading" process that detects mistakes when they occur during DNA replication. DNA Polymerase mistakes mean that incorrect nucleotides have been incorporated into the DNA, causing mutations. Mistakes in the DNA sequence are repaired in a process called mismatch repair (MMR).
MMR involves a complex of multiple proteins. In Lynch syndrome, one or more of the proteins involved in MMR is mutated, and the mistakes in the DNA do not get corrected. Mutations in MMR proteins are not only found in familial cases of CRC, but also in patients with sporadic (non-inherited) CRC. Defects in MMR also contribute to microsatellite instability (MIS), described above. The accumulation of these mutations can lead to cancer.

The importance of accurately replicating DNA following various types of mistakes or damage is reflected in the multiple pathways cells have for correcting or repairing broken DNA. Actual breaks in the DNA strands can happen due to exposure to radiation or other DNA damaging agents. In the case of the occurrence of breaks in DNA, there are also mechanisms for detecting these breaks. Double strand breaks (DSB's) in the DNA can be repaired via several mechanisms, including Non-Homologous End Joining (NHEJ) or Homologous Repair (HR). Many proteins are involved in DSB repair. Mutations in any of the many proteins involved in either of these repair pathways (see BRCA1 and BRCA2 genes) lead to damaged DNA, which results in DNA that is incorrectly replicated, causing mutations that contribute to the development of cancer. DNA repair machinery in the cell is important in keeping the genome stable and accurate.

Testing for the mutations and genomic conditions that contribute to the development or progression of CRC is available at MGH in the sophisticated CLIA certified genomic testing lab, and in other large Centers and some private testing companies used by physicians. Validated treatments, Immune therpies, as well as clinical trials investigating improved targeted and immunologic therapies are available to patients at MGH.

NIH/NCI Cancer Website www.cancer.gov 2017

Colorectal Cancer (CRC) is cancer that initiates in the colon or rectum-the lower part of the digestive system in the body. During digestion, food moves through the stomach and small intestine into the colon. The colon absorbs water and nutrients from food, and stores waste matter (stool) that moves from the colon through the rectum before leaving the body.

Most CRC's and rectal cancers are adenocarcinomas, meaning that they originate in cells that make and release mucus and other fluids. CRC often begins as a growth called a polyp, which may form on the inner wall of the colon or rectum. Over time, some polyps become cancerous. This highlights the importance of colonoscopy screening to find and remove polyps before they become cancerous.

CRC is the fourth most common type of cancer diagnosed in the U.S. Deaths from CRC have decreased with the use of colonoscopies and fecal occult blood tests, which check for blood in the stool. Disparities in survival have been observed between African American and other populations. This may be due to factors such as access to colonoscopy screening, or to other factors not yet identified.

Because of its prevalence, scientists have studied CRC extensively, even creating models of how cancer develops using CRC as an example. There are also families with a very high incidence of CRC occurrence. When these families were studied, certain conditions that create instability in the whole genome were identified that predispose people to CRC. These include what is called the Chromosomal Instability pathway (CIN), as well as MicroSatellite Instability pathway (MSI). These can also occur as spontaneous (uninherited) conditions in some patients. Between 6-10% of CRC's are found to have MSI. Some CRC tumors have been found to have a lot of mutations, or as physicians call it, a "very high mutational load". Some also express a ligand called PD-L1.

These are now recognized features of some CRC's, and immunological treatments may be recommended in these cases. MGH has one of the most extensive Immuno-oncology clinical trials portfolios of any US hospital. Testing for features such as CIN, MSI, a high mutational burden, and the expression of PD-L1 can be conducted at the MGH genetics laboratory, as well as at other large academic centers. Genetic instability such as CIN or MSI lead to the activation of oncogenes such as KRAS, and the inactivation of tumor suppressors such as PTEN, both of which promote tumor growth.

Other genetic alterations in how the DNA in cells is organized have been found to contribute to CRC in families and individuals. These are called epigenetic changes. Normal DNA has methyl groups added in specific regions that regulate gene expression. When the genes that suppress growth-called tumor suppressors-are methylated abnormally, this prevents the production of tumor suppressor proteins important in controlling or stopping cell growth. When tumor suppressor genes are missing, unregulated growth occurs, contributing to the development of cancer. Some tumor suppressor proteins that are frequently inactivated in CRC are APC, TP53, or loss of one arm of chromosome 18 that contains a tumor suppressor.

The study of families with a high prevalence of CRC have lead scientists to discover genetic changes that contribute to the development of CRC in sporadic cases occurring in patients. Mutations in the genes encoding the following proteins have now been associated with subsets of CRC; ALK, AKT, APC, beta-catenin, BRCA1 and BRCA2, BRAF, EGFR, ERBB2 (HER2), ERBB3 (HER3), IDH2, KRAS, MET, NRAS, PI3K, ROS, PTEN, SMO,TP53, TRK 1, 2 and 3, and others that are still being identified. Information on these specific genes is available on this website if you select the gene you want to know more about.

Distinct familial syndromes of CRC such as Lynch syndrome have been studied in patients, leading to the identification of other mechanisms contributing to the development of cancer. Before a cell can divide into two daughter cells, DNA has to be replicated so both daughter cells will have a full complement of chromosomes. DNA replication requires an enzyme called DNA Polymerase. DNA Polymerase occasionally makes errors while it is replicating DNA. Cells therefore have a "proof-reading" process that detects mistakes when they occur during DNA replication. DNA Polymerase mistakes mean that incorrect nucleotides have been incorporated into the DNA, causing mutations. Mistakes in the DNA sequence are repaired in a process called mismatch repair (MMR).
MMR involves a complex of multiple proteins. In Lynch syndrome, one or more of the proteins involved in MMR is mutated, and the mistakes in the DNA do not get corrected. Mutations in MMR proteins are not only found in familial cases of CRC, but also in patients with sporadic (non-inherited) CRC. Defects in MMR also contribute to microsatellite instability (MIS), described above. The accumulation of these mutations can lead to cancer.

The importance of accurately replicating DNA following various types of mistakes or damage is reflected in the multiple pathways cells have for correcting or repairing broken DNA. Actual breaks in the DNA strands can happen due to exposure to radiation or other DNA damaging agents. In the case of the occurrence of breaks in DNA, there are also mechanisms for detecting these breaks. Double strand breaks (DSB's) in the DNA can be repaired via several mechanisms, including Non-Homologous End Joining (NHEJ) or Homologous Repair (HR). Many proteins are involved in DSB repair. Mutations in any of the many proteins involved in either of these repair pathways (see BRCA1 and BRCA2 genes) lead to damaged DNA, which results in DNA that is incorrectly replicated, causing mutations that contribute to the development of cancer. DNA repair machinery in the cell is important in keeping the genome stable and accurate.

Testing for the mutations and genomic conditions that contribute to the development or progression of CRC is available at MGH in the sophisticated CLIA certified genomic testing lab, and in other large Centers and some private testing companies used by physicians. Validated treatments, Immune therpies, as well as clinical trials investigating improved targeted and immunologic therapies are available to patients at MGH.

NIH/NCI Cancer Website www.cancer.gov 2017

Colorectal Cancer (CRC) is cancer that initiates in the colon or rectum-the lower part of the digestive system in the body. During digestion, food moves through the stomach and small intestine into the colon. The colon absorbs water and nutrients from food, and stores waste matter (stool) that moves from the colon through the rectum before leaving the body.

Most CRC's and rectal cancers are adenocarcinomas, meaning that they originate in cells that make and release mucus and other fluids. CRC often begins as a growth called a polyp, which may form on the inner wall of the colon or rectum. Over time, some polyps become cancerous. This highlights the importance of colonoscopy screening to find and remove polyps before they become cancerous.

CRC is the fourth most common type of cancer diagnosed in the U.S. Deaths from CRC have decreased with the use of colonoscopies and fecal occult blood tests, which check for blood in the stool. Disparities in survival have been observed between African American and other populations. This may be due to factors such as access to colonoscopy screening, or to other factors not yet identified.

Because of its prevalence, scientists have studied CRC extensively, even creating models of how cancer develops using CRC as an example. There are also families with a very high incidence of CRC occurrence. When these families were studied, certain conditions that create instability in the whole genome were identified that predispose people to CRC. These include what is called the Chromosomal Instability pathway (CIN), as well as MicroSatellite Instability pathway (MSI). These can also occur as spontaneous (uninherited) conditions in some patients. Between 6-10% of CRC's are found to have MSI. Some CRC tumors have been found to have a lot of mutations, or as physicians call it, a "very high mutational load". Some also express a ligand called PD-L1.

These are now recognized features of some CRC's, and immunological treatments may be recommended in these cases. MGH has one of the most extensive Immuno-oncology clinical trials portfolios of any US hospital. Testing for features such as CIN, MSI, a high mutational burden, and the expression of PD-L1 can be conducted at the MGH genetics laboratory, as well as at other large academic centers. Genetic instability such as CIN or MSI lead to the activation of oncogenes such as KRAS, and the inactivation of tumor suppressors such as PTEN, both of which promote tumor growth.

Other genetic alterations in how the DNA in cells is organized have been found to contribute to CRC in families and individuals. These are called epigenetic changes. Normal DNA has methyl groups added in specific regions that regulate gene expression. When the genes that suppress growth-called tumor suppressors-are methylated abnormally, this prevents the production of tumor suppressor proteins important in controlling or stopping cell growth. When tumor suppressor genes are missing, unregulated growth occurs, contributing to the development of cancer. Some tumor suppressor proteins that are frequently inactivated in CRC are APC, TP53, or loss of one arm of chromosome 18 that contains a tumor suppressor.

The study of families with a high prevalence of CRC have lead scientists to discover genetic changes that contribute to the development of CRC in sporadic cases occurring in patients. Mutations in the genes encoding the following proteins have now been associated with subsets of CRC; ALK, AKT, APC, beta-catenin, BRCA1 and BRCA2, BRAF, EGFR, ERBB2 (HER2), ERBB3 (HER3), IDH2, KRAS, MET, NRAS, PI3K, ROS, PTEN, SMO,TP53, TRK 1, 2 and 3, and others that are still being identified. Information on these specific genes is available on this website if you select the gene you want to know more about.

Distinct familial syndromes of CRC such as Lynch syndrome have been studied in patients, leading to the identification of other mechanisms contributing to the development of cancer. Before a cell can divide into two daughter cells, DNA has to be replicated so both daughter cells will have a full complement of chromosomes. DNA replication requires an enzyme called DNA Polymerase. DNA Polymerase occasionally makes errors while it is replicating DNA. Cells therefore have a "proof-reading" process that detects mistakes when they occur during DNA replication. DNA Polymerase mistakes mean that incorrect nucleotides have been incorporated into the DNA, causing mutations. Mistakes in the DNA sequence are repaired in a process called mismatch repair (MMR).
MMR involves a complex of multiple proteins. In Lynch syndrome, one or more of the proteins involved in MMR is mutated, and the mistakes in the DNA do not get corrected. Mutations in MMR proteins are not only found in familial cases of CRC, but also in patients with sporadic (non-inherited) CRC. Defects in MMR also contribute to microsatellite instability (MIS), described above. The accumulation of these mutations can lead to cancer.

The importance of accurately replicating DNA following various types of mistakes or damage is reflected in the multiple pathways cells have for correcting or repairing broken DNA. Actual breaks in the DNA strands can happen due to exposure to radiation or other DNA damaging agents. In the case of the occurrence of breaks in DNA, there are also mechanisms for detecting these breaks. Double strand breaks (DSB's) in the DNA can be repaired via several mechanisms, including Non-Homologous End Joining (NHEJ) or Homologous Repair (HR). Many proteins are involved in DSB repair. Mutations in any of the many proteins involved in either of these repair pathways (see BRCA1 and BRCA2 genes) lead to damaged DNA, which results in DNA that is incorrectly replicated, causing mutations that contribute to the development of cancer. DNA repair machinery in the cell is important in keeping the genome stable and accurate.

Testing for the mutations and genomic conditions that contribute to the development or progression of CRC is available at MGH in the sophisticated CLIA certified genomic testing lab, and in other large Centers and some private testing companies used by physicians. Validated treatments, Immune therpies, as well as clinical trials investigating improved targeted and immunologic therapies are available to patients at MGH.

NIH/NCI Cancer Website www.cancer.gov 2017

PubMed ID's
2188735, 23897299, 20965415
Expand Collapse TP53  - General Description
CLICK IMAGE FOR MORE INFORMATION
The p53 (TP53) gene produces a protein, P53 which has many complex functions within the cell. It has been called the “guardian of the genome” for reasons that have to do with these complex functions. Normal, non-cancerous cells have tightly regulated pathways that control cell growth, mediating cessation of growth or even cell death when circumstances warrant it. P53 is at the center of these pathways, acting as a “tumor suppressor” in responding to circumstances in the cell that require a cessation of growth. Perhaps for this reason, P53 is one of the most commonly mutated genes across all cancer types.

P53 itself regulates the expression of several genes that are involved in growth arrest or “cell cycle arrest”. Growth arrest is important for stopping the cell from normal growth and cell division so that if, for instance, there has been damage to the DNA from UV irradiation or some other insult causing DNA damage, the cessation of the cell cycle allows DNA repair to take place before the cell resumes growth. If the damage to the DNA is too extensive to repair, or if other factors such as oncogenic stress impact the cell, P53 then has roles in other processes that are part of the cell’s repertoire of responses. These include processes such as apoptosis (programmed cell death), senescence (irreversible cell cycle arrest), autophagy (regulated destruction of selected proteins within the cell, leading to cell death), and some important metabolic changes in the cell (see graphic above, adapted with permission).

P53 is itself acted upon by proteins in the cell that detect DNA damage or oncogenic stress (see graphic depicting P53 at the center of a number of cellular responses). In the case of DNA damage to the cell, P53 is acted upon by a protein called ATM and another designated CHK2 (see glossary for more information). These proteins activate P53 to regulate the changes that will cause growth arrest. Interestingly, these two genes themselves are found to be mutated and have altered function in certain cancers. The fact that both P53 and the genes that trigger P53’s response and initiation of growth arrest are mutated in some cancers highlights the importance of P53 to normal cell growth. P53 is found to be mutated in over half of cancers studied, including ovarian cancer, colon and esophageal cancer, and many other types of cancer. Because p53 plays so many complex roles in the cell, we do not depict it in a simple graphic as we have with other proteins on this web site in which genetic alterations have been found in specific tumors that lead to dysregulation of these proteins. Rather, P53 as a negative regulator of cell growth under important circumstances plays this role at the center of a complex network of pathways within the cell. Many of the proteins involved in the pathways that regulate P53 and its responses are also found to be genetically altered in some cancers.

As we have seen, the P53 protein has many functions in the cell, and because of these many roles, its location in the nucleus or cytoplasm varies, depending on the function and when it exerts its effect during the cell cycle. One important protein that regulates P53 is called HDM2/MDM2, depicted in the graphic above. The HDM2/MDM2 protein contains a p53 binding domain, and once bound to p53, it inhibits the activation of the P53 protein, and thereby prevents P53 from regulating growth arrest, even when there is damage to the DNA. Some cancers have been found to overexpress HDM2/MDM2, meaning there is an excess of the protein which binds to P53, preventing it from exerting its important role in regulating growth arrest. Cell division that occurs despite damage to the DNA can lead to cancer. Interestingly, those cancers that have been found to over-express HDM2/MDM2 typically are not found to have p53 mutations. This provides scientists with evidence that by whatever means, either through increasing the amount of the P53 inhibitor HDM2/MDM2, or, through mutations in P53 that prevent the normal activities of the protein, the normal function of P53 is important in preventing cancer. MDM2 was named after its discovery in studies on laboratory mice. The human version of the gene is designated HumanDM2, or HDM2. Genetic alterations leading to over-expression of MDM2 are observed most commonly in sarcomas, but have also been observed in endometrial cancer, colon cancer, and stomach cancer.

Source: Molecular Genetics of Cancer, Second Edition
Chapter No. 2, Section No. 12
Leif W. Ellisen, MD, PhD
The p53 (TP53) gene produces a protein, P53 which has many complex functions within the cell. It has been called the “guardian of the genome” for reasons that have to do with these complex functions. Normal, non-cancerous cells have tightly regulated pathways that control cell growth, mediating cessation of growth or even cell death when circumstances warrant it. P53 is at the center of these pathways, acting as a “tumor suppressor” in responding to circumstances in the cell that require a cessation of growth. Perhaps for this reason, P53 is one of the most commonly mutated genes across all cancer types.

P53 itself regulates the expression of several genes that are involved in growth arrest or “cell cycle arrest”. Growth arrest is important for stopping the cell from normal growth and cell division so that if, for instance, there has been damage to the DNA from UV irradiation or some other insult causing DNA damage, the cessation of the cell cycle allows DNA repair to take place before the cell resumes growth. If the damage to the DNA is too extensive to repair, or if other factors such as oncogenic stress impact the cell, P53 then has roles in other processes that are part of the cell’s repertoire of responses. These include processes such as apoptosis (programmed cell death), senescence (irreversible cell cycle arrest), autophagy (regulated destruction of selected proteins within the cell, leading to cell death), and some important metabolic changes in the cell (see graphic above, adapted with permission).

P53 is itself acted upon by proteins in the cell that detect DNA damage or oncogenic stress (see graphic depicting P53 at the center of a number of cellular responses). In the case of DNA damage to the cell, P53 is acted upon by a protein called ATM and another designated CHK2 (see glossary for more information). These proteins activate P53 to regulate the changes that will cause growth arrest. Interestingly, these two genes themselves are found to be mutated and have altered function in certain cancers. The fact that both P53 and the genes that trigger P53’s response and initiation of growth arrest are mutated in some cancers highlights the importance of P53 to normal cell growth. P53 is found to be mutated in over half of cancers studied, including ovarian cancer, colon and esophageal cancer, and many other types of cancer. Because p53 plays so many complex roles in the cell, we do not depict it in a simple graphic as we have with other proteins on this web site in which genetic alterations have been found in specific tumors that lead to dysregulation of these proteins. Rather, P53 as a negative regulator of cell growth under important circumstances plays this role at the center of a complex network of pathways within the cell. Many of the proteins involved in the pathways that regulate P53 and its responses are also found to be genetically altered in some cancers.

As we have seen, the P53 protein has many functions in the cell, and because of these many roles, its location in the nucleus or cytoplasm varies, depending on the function and when it exerts its effect during the cell cycle. One important protein that regulates P53 is called HDM2/MDM2, depicted in the graphic above. The HDM2/MDM2 protein contains a p53 binding domain, and once bound to p53, it inhibits the activation of the P53 protein, and thereby prevents P53 from regulating growth arrest, even when there is damage to the DNA. Some cancers have been found to overexpress HDM2/MDM2, meaning there is an excess of the protein which binds to P53, preventing it from exerting its important role in regulating growth arrest. Cell division that occurs despite damage to the DNA can lead to cancer. Interestingly, those cancers that have been found to over-express HDM2/MDM2 typically are not found to have p53 mutations. This provides scientists with evidence that by whatever means, either through increasing the amount of the P53 inhibitor HDM2/MDM2, or, through mutations in P53 that prevent the normal activities of the protein, the normal function of P53 is important in preventing cancer. MDM2 was named after its discovery in studies on laboratory mice. The human version of the gene is designated HumanDM2, or HDM2. Genetic alterations leading to over-expression of MDM2 are observed most commonly in sarcomas, but have also been observed in endometrial cancer, colon cancer, and stomach cancer.

Source: Molecular Genetics of Cancer, Second Edition
Chapter No. 2, Section No. 12
Leif W. Ellisen, MD, PhD
Expand Collapse TP53  in Colorectal Cancer
Genetic alterations in TP53 are common in CRC. TP53 us a tumor suppressor, and when it is mutated or genetically altered, it cannot perform its normal function in the cell, which is to suppress growth.

Genetic alterations in TP53 are common in CRC. TP53 us a tumor suppressor, and when it is mutated or genetically altered, it cannot perform its normal function in the cell, which is to suppress growth.

PubMed ID's
17512720, 17060676
Expand Collapse No mutation selected
The mutation of a gene provides clinicians with a very detailed look at your cancer. Knowing this information could change the course of your care. To learn how you can find out more about genetic testing please visit http://www.massgeneral.org/cancer/news/faq.aspx or contact the Cancer Center.
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Trial Status: Showing Results: 1-10 of 39 Per Page:
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