Look at the below mutations which all occur in a single cell. Categorize each protein involved as an oncogene or tumor suppressor and describe the consequence of the mutation described below. Then based on the changes, decide whether you think they cell would be able to grow and divide in the presence and absence of growth signals, and whether it would undergo apoptosis in the presence of stress. Explain your reasoning

 

A cell was found to contain the following mutations:

 

o Mutation in the GTP binding domain of Ras which inhibits GTP hydrolysis to GDP.

o A mutation in the promotor of the Apaf1 gene that leads to 10 fold decrease expression of

the gene.

o A chromosomal deletion of the region of the genome around and including one copy of the

ATM gene.

o The E2F1 protein is mutated and cannot bind to DNA.

o Mutation in the Mdm2 protein, that leads it to tightly bind p53 even when it is

phosphorylated

Mutation in the GTP binding domain of a cell, would not undergo apoptosis in the presence of stress and would be able to divide in the presence of the external growth factor.

·     As the apoptosis signaling is reduced by 10 folds there would be several folds reduction in the chances of the cell to undergo apoptosis in the presence of the stress. A mutation in the Apaf1 gene would not affect the cell division process and would promote the cell to grow and divide in the presence of external growth signals.

·     ATM is responsible for the protection of the cell from the oxidative damage from stress and other factors, a reduction in the ATM expression due to the deletion of gene inhibits the cell from undergoing apoptosis and arresting of the cell cycle. The ATM gene is not linked with the binding of the external growth signals and would promote the cell cycle progression and division of the cell.

·     In the presence of stress, the transcription factor is not able to bind to the DNA and recognize the cellular damage caused due to the oxidative stress and the cell would not undergo apoptosis.

·     A mutation resulting in the ability of the MDM2 to bind to the phosphorylated form of p53 and carry out the breakdown of the p53, thereby promoting cell survival and reducing the chances of undergoing apoptosis.

TSG (tumor suppressor gene) is a type of gene that helps in the suppression of a cell to become a tumor and works by regulation of the cell division and replication. The loss of function of the TSG results in the development of the normal cell into cancerous. Oncogene is a gene that can produce a protein that can turn in a normal cell to cancerous. A gene is termed oncogene if its gain of function results in the formation of cancer.

• In normal conditions, the guanine exchange factor (GEF) exchanges the GDP (guanine diphosphate) with GTP attached to the Ras, resulting in the activation of Ras protein. Ras protein in its activated form activates the downstream signaling process to initiate cell cycle progression. A mutation in the GTP binding domain of Ras that results in the inhibition of the GTP hydrolysis to turn Ras protein back to its inactivated state or off state. It is functioning as an oncogene in this case. The consequence of this mutation results in the continuous activation of the cell cycle signaling and the cell would continue to grow even in absence of growth signals, as the Ras-GTP signaling remains activated even in the absence of the external signaling. An activated Ras-GTP (guanine triphosphate) molecule inhibits the expression of apoptotic proteins and increases the survival rate of the cell by inhibiting apoptosis. In the case of stress, the cell would not undergo apoptosis in the presence of stress.
• APAF1 (apoptotic protease activating factor 1) gene is responsible for the production of a protein that contains domains that help in the formation of the apoptosome. The formation of apoptosome results in the dimerization of the caspase-9, resulting in the apoptosis of the cell. A mutation in the promoter region of the Apaf1 gene results in the reduction of 10 folds in its expression in the cell, reduces the cell to undergo apoptosis and increases the survival rate of the cell. The mutation involves the loss of function of the Apaf 1 gene and is defined as a TSG. A reduction in the Apaf1 signaling improves the survival rate of the cell and promotes cell cycle progression. As the apoptosis signaling is reduced by 10 folds there would be several folds reduction in the chances of the cell to undergo apoptosis in the presence of the stress. A mutation in the Apaf1 gene would not affect the cell division process and would promote the cell to grow and divide in the presence of external growth signals.
• ATM gene is responsible for controlling the cell cycle replication and DNA (deoxyribonucleic acid) repair. It works by preventing the cell to progress further in the cell cycle until the damaged DNA is repaired and also activates the p53 to inhibit the cell cycle progression. Each cell contains 2 copies of the ATM gene obtained one from father and other from mother. Deletion of the one copy of ATM gene results in the reduction in the ability to repair the DNA damage in the nucleus and cell promotes the cell cycle progression. This mutation involves the loss of function of the ATM gene and is defined as a TSG. ATM (ataxia telangiectasia mutated) is responsible for the protection of the cell from the oxidative damage from stress and other factors, a reduction in the ATM expression due to the deletion of gene inhibits the cell from undergoing apoptosis and arresting of the cell cycle. The ATM gene is not linked with the binding of the external growth signals and would promote the cell cycle progression and division of the cell.
• E2F1 gene encodes protein E2F1 which acts as the transcription factor. It is responsible for controlling transcription and regulates cell cycle progression. It is essential for the transition of the G1 to the S phase. It is also responsible for inducing apoptosis via p53 mediated pathway. It is upregulated in the case of DNA damage and helps in repairing the damaged DNA through ATM/ATR pathway. It works by binding to the other factors and proteins and then binding to the DNA for the regulation of the cell cycle. A mutation in the gene resulting in the inability of the E2F1 protein to bind to the DNA, can reduce its ability to control the apoptosis and would promote cell survival. In the presence of stress, the transcription factor is not able to bind to the DNA and recognize the cellular damage caused due to the oxidative stress and the cell would not undergo apoptosis. E2F1 is responsible for the progression of the cell cycle and cell development, the inability of E2F1 to bind to the DNA would result in the reduced ability for the progression cell cycle. Since the transcription factor is unable to bind to the DNA, there would be no effect of the external growth signal on the cell cycle.
• Mdm2 gene encodes a ubiquitin ligase protein and promotes the formation of a tumor by degrading the TS proteins and progression of a normal cell into a cancerous cell. Damage in the DNA promotes the phosphorylation of the p53 which promotes the arresting of the cell cycle and DNA repair. The phosphorylated form of the p53 is unable to bind to the MDM2 and thereby preventing the proteolytic degradation of p53. The phosphorylated form of p53 promotes the inhibition of the cell cycle and apoptosis. A mutation resulting in the ability of the MDM2 to bind to the phosphorylated form of p53 and carry out the breakdown of the p53, thereby promoting cell survival and reducing the chances of undergoing apoptosis. The cells would grow in the presence of the growth signals, as the mutation promotes cell cycle progression and cell development.