Genetics Quiz: Oncogenes And Tumor Suppressors

What this quiz covers

This quiz focuses on Oncogenes And Tumor Suppressors, giving you a quick way to practice the rules, question types, and explanations that matter most for Genetics.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

Question 1

1. tumor suppressor gene, because its inactivation requires a loss-of-function event. (correct answer)
2. proto-oncogene, because truncation often leads to constitutive activation of signaling domains.
3. growth factor receptor, because a truncated receptor mimics the ligand-bound state.
4. housekeeping gene, because disruption of basic metabolism is a primary driver of tumorigenesis.

Explanation: When analyzing how mutations contribute to cancer, you need to understand the distinction between tumor suppressor genes and oncogenes, and how different types of mutations affect each category differently. A frameshift mutation early in the coding sequence that creates a severely truncated, non-functional protein represents a complete loss-of-function event. Tumor suppressor genes normally act as cellular "brakes" that prevent uncontrolled cell division. When these genes are inactivated through loss-of-function mutations, cells lose their ability to regulate growth properly, contributing to cancer development. This matches exactly what we see with this frameshift mutation. Looking at why the other options don't fit: Option B is incorrect because proto-oncogenes typically contribute to cancer through gain-of-function mutations that increase their activity, not through truncation that eliminates function. Option C misunderstands how growth factor receptors work—truncated receptors don't mimic ligand binding; they're more likely to be completely inactive. Option D is wrong because while housekeeping genes are important for basic cellular functions, their disruption typically leads to cell death rather than the uncontrolled proliferation characteristic of cancer. The key principle here is that cancer results from losing growth control mechanisms (tumor suppressors) or gaining growth-promoting signals (oncogenes). Since this mutation eliminates protein function entirely, it can only contribute to cancer by removing a growth control mechanism—which is exactly what tumor suppressor genes provide. Remember: loss-of-function mutations typically affect tumor suppressors, while gain-of-function mutations typically create oncogenes from normal proto-oncogenes.

Question 2

1. Oncogene mutations are predominantly frameshift and nonsense mutations, while TSG mutations are typically gene amplifications.
2. Both oncogenes and TSGs are characterized by a wide variety of inactivating mutations distributed throughout their coding sequences.
3. TSG mutations are typically found in the germline, while oncogene mutations are almost exclusively somatic.
4. Oncogene mutations are often recurrent, specific missense mutations in key functional domains, while TSG mutations are diverse, inactivating mutations spread across the gene. (correct answer)

Explanation: When analyzing cancer genetics, you need to understand that oncogenes and tumor suppressor genes (TSGs) have fundamentally different roles and therefore exhibit distinct mutational signatures in tumors. Oncogenes are genes that, when activated or overexpressed, promote cell growth and division. Since only one copy needs to be altered to contribute to cancer (dominant effect), mutations tend to be very specific. These are typically precise missense mutations that either enhance protein function or occur in regulatory regions that increase expression. You'll often see the same mutations recurring across different patients because only certain changes actually activate the oncogene effectively. In contrast, TSGs normally act as cellular brakes, preventing uncontrolled growth. Cancer requires loss of this protective function, and since cells have two copies of each gene, both typically need to be inactivated (recessive effect). This means any mutation that destroys gene function will contribute to cancer - frameshift mutations, nonsense mutations, deletions, or missense mutations that disrupt protein structure. These inactivating mutations can occur anywhere throughout the gene, creating a diverse mutational landscape. Option A reverses the actual pattern. Option B incorrectly suggests both gene types show diverse mutations - this ignores oncogenes' requirement for specific activating changes. Option C confuses inheritance patterns with mutational types; both gene classes can have germline or somatic mutations. The correct answer is D because it captures this key distinction: oncogenes show recurrent, specific mutations in functional domains, while TSGs show diverse, scattered inactivating mutations. Remember: oncogenes need precise activation; TSGs just need to be broken in any way possible.

Question 3

1. Loss of D is like a TSG loss; Gain of B is like an oncogene activation. (correct answer)
2. Loss of C is like a TSG loss; Gain of D is like an oncogene activation.
3. Loss of A is like a TSG loss; Gain of C is like an oncogene activation.
4. Loss of B is like a TSG loss; Gain of A is like an oncogene activation.

Explanation: The correct answer is A. This question requires applying the concepts of 'brakes' (tumor suppressors) and 'gas pedals' (oncogenes) to a novel pathway. Tumor suppressors inhibit pro-growth/pro-survival pathways. In this pathway, Protein D inhibits Protein A, which starts the pro-survival cascade. Therefore, Protein D is acting as a brake, and its loss of function would be analogous to inactivating a tumor suppressor. Oncogenes promote growth/survival. Proteins A, B, and C are all positive regulators in the pathway. A gain-of-function mutation in any of them would lead to excessive pro-survival signaling, analogous to oncogene activation. Choice A correctly identifies Protein D as the tumor suppressor analog and Protein B as a proto-oncogene analog.

• B is incorrect because loss of C would inhibit survival, which is anti-cancerous. Gain of D would strengthen the inhibition of the pathway, also an anti-cancerous effect.
• C is incorrect because loss of A would block the pro-survival signal, which is not analogous to a TSG loss that promotes cancer.
• D is incorrect because loss of B would block the pro-survival signal, not act as a TSG loss.

Question 4

1. The classification of a gene as an oncogene or tumor suppressor can be dependent on the specific cellular context and tissue type. (correct answer)
2. All genes that function as oncogenes in one cancer type will eventually be found to be tumor suppressors in another.
3. NOTCH1 must be a 'caretaker' gene, as its function is primarily related to maintaining genomic stability across different tissues.
4. Gain-of-function mutations and loss-of-function mutations in the same gene must ultimately activate the same downstream signaling pathways.

Explanation: The correct answer is A. This example of NOTCH1 (and other genes like it) illustrates a crucial concept in cancer genetics: the function of a gene product is highly dependent on the cellular environment, including the tissue type, developmental stage, and the presence of other mutations. A signaling pathway that promotes proliferation in one context (e.g., T-cell development) might promote differentiation or apoptosis in another (e.g., keratinocytes). Therefore, the simple binary classification of oncogene vs. tumor suppressor is an oversimplification, and context is critical.

• B is incorrect as this is an overgeneralization. Many core oncogenes (like KRAS) and tumor suppressors (like RB1) have consistent roles across a vast majority of cancer types.
• C is incorrect because the primary role of NOTCH signaling is in cell fate decisions, proliferation, and differentiation, which is more characteristic of a 'gatekeeper' role, not a 'caretaker' role involved in DNA repair.
• D is incorrect because it is illogical. A gain-of-function and loss-of-function mutation in the same gene would have opposing effects on its direct downstream targets and pathways.

Question 5

1. It demonstrates that some tumor suppressors can be activated by a single gain-of-function mutation.
2. It shows that a single mutated allele can functionally inactivate the wild-type allele, mimicking a two-hit scenario. (correct answer)
3. It proves that p53 is actually an oncogene, as a single mutation can promote cancer.
4. It suggests that epigenetic silencing, not mutation, is the primary mechanism of p53 inactivation.

Explanation: The correct answer is B. The classic view of tumor suppressors is that they are recessive, requiring two loss-of-function 'hits'. A dominant-negative mutation complicates this. The mutation itself is a loss-of-function for the individual protein, but because p53 functions as a tetramer, the mutant protein 'poisons' the complex, inactivating the function of the remaining wild-type proteins. Therefore, a mutation in just one allele can have a dominant effect at the cellular level, functionally mimicking the outcome of a homozygous loss (a two-hit scenario).

• A is incorrect because the R248W mutation is a loss-of-function (loss of DNA binding), not a gain-of-function. The dominant effect comes from interference, not from acquiring a new oncogenic activity.
• C is incorrect because p53's fundamental role is tumor suppression. This specific mutational mechanism doesn't change its classification from a tumor suppressor to an oncogene.
• D is incorrect because the scenario explicitly describes a genetic mutation, not epigenetic silencing, as the cause of inactivation.

Question 6

1. Cancer is a dominant trait, as the tumorigenic phenotype eventually re-emerges in the hybrid cells.
2. Normal cells contain genes that can suppress tumorigenicity, and the loss of these genes restores the cancer phenotype. (correct answer)
3. The cancer cell's oncogenes are initially silenced by factors from the normal cell, but these oncogenes can become reactivated over time.
4. The process of cell fusion itself induces widespread mutations that can randomly lead to a reversion to the cancer phenotype.

Explanation: The correct answer is B. This experiment provides fundamental evidence for the existence of tumor suppressor genes. The initial fusion creates a hybrid cell that is non-tumorigenic because the normal cell contributes wild-type copies of tumor suppressor genes, which complement the defects in the cancer cell. The re-emergence of tumorigenicity upon the loss of specific chromosomes from the normal parent cell strongly implies that these lost chromosomes carried the crucial tumor suppressor genes.

• A is incorrect because the initial result (suppression of tumorigenicity) shows that at the cellular level, the cancer phenotype is recessive to the normal phenotype.
• C is incorrect because the data points to the loss of genetic material (chromosomes) from the normal cell, not the reactivation of oncogenes from the cancer cell, as the primary cause for the reversion.
• D is incorrect because the loss of specific chromosomes, not random mutations, is correlated with the phenotype reversion, suggesting a deterministic role for genes on those chromosomes.

Question 7

1. It directly provides a strong proliferative or survival signal to the cell.
2. It causes immediate cell cycle arrest at the G2/M checkpoint.
3. It leads to an increased rate of mutation throughout the genome, accelerating the inactivation of gatekeepers and activation of oncogenes. (correct answer)
4. It promotes angiogenesis by upregulating the expression of vascular endothelial growth factor (VEGF).

Explanation: The correct answer is C. Caretaker genes are responsible for maintaining genomic integrity (e.g., DNA repair, chromosome segregation). Their loss of function does not directly promote cell growth. Instead, it creates a state of genomic instability, leading to a much higher rate of mutations (an increased mutator phenotype). This accelerates the accumulation of other mutations required for cancer, including the 'hits' that inactivate gatekeeper tumor suppressors and activate proto-oncogenes.

• A is incorrect because this describes the function of an activated oncogene or the loss of a gatekeeper tumor suppressor, not a caretaker. Caretakers do not directly regulate cell proliferation.
• B is incorrect because loss of a DNA repair gene would lead to a failure to arrest the cell cycle in response to damage, not cause an arrest itself.
• D is incorrect because while angiogenesis is a hallmark of cancer, it is not the primary or direct consequence of losing a caretaker gene's function.

Question 8

1. EGFR is a tumor suppressor that is inactivated, and PTEN is a proto-oncogene that is activated.
2. Both EGFR and PTEN are proto-oncogenes that undergo gain-of-function mutations.
3. EGFR is a proto-oncogene that is activated, and PTEN is a tumor suppressor that is inactivated. (correct answer)
4. Both EGFR and PTEN are tumor suppressors that undergo loss-of-function mutations.

Explanation: The correct answer is C. This question requires understanding how mutation type reveals gene function. Mutation 1 is a gain-of-function mutation (constitutive activity) in a growth-promoting gene (EGFR), which classifies EGFR as a proto-oncogene. Mutation 2 is a nonsense mutation, which leads to a non-functional protein. This is a loss-of-function mutation. Since PTEN's normal role is to antagonize a pro-growth pathway (acting as a brake), its inactivation promotes cancer, classifying PTEN as a tumor suppressor.

• A is incorrect because it reverses the classifications. EGFR is the proto-oncogene and PTEN is the tumor suppressor.
• B is incorrect because the mutation in PTEN is a loss-of-function, not a gain-of-function.
• D is incorrect because the mutation in EGFR is a gain-of-function, not a loss-of-function.

Question 9

1. E2F is constitutively free and active, leading to inappropriate entry into S phase regardless of growth signals. (correct answer)
2. E2F is permanently bound by a non-functional Rb protein, leading to cell cycle arrest in G1.
3. E2F is targeted for degradation because its normal binding partner, Rb, is absent.
4. E2F activity is still properly regulated by CDK phosphorylation, but it now controls apoptosis instead of proliferation.

Explanation: When you encounter questions about tumor suppressor genes like RB1, focus on understanding what happens when the normal "brakes" on cell division are removed. The Rb protein normally acts as a gatekeeper at the G1/S checkpoint, preventing cells from replicating their DNA until appropriate growth signals are received. In normal cells, Rb binds to and sequesters E2F transcription factors. Only when CDKs phosphorylate Rb in response to proper growth signals does Rb release E2F, allowing transcription of S-phase genes. This creates a tightly controlled checkpoint. With biallelic loss-of-function mutations in RB1, no functional Rb protein is produced. This means E2F transcription factors are never sequestered—they're constitutively free to activate S-phase genes regardless of whether the cell has received appropriate growth signals. This leads to uncontrolled progression into S phase, a hallmark of cancer. Answer B incorrectly suggests that non-functional Rb still binds E2F, but loss-of-function mutations typically result in no protein production or completely inactive protein that cannot bind anything. Answer C misunderstands the relationship—E2F doesn't require Rb for stability, only for regulation. Answer D incorrectly claims CDK regulation remains intact and that E2F switches to controlling apoptosis, neither of which occurs. Remember that tumor suppressor gene mutations typically result in loss of normal growth controls. When you see "biallelic loss-of-function," think complete loss of the protein's regulatory function, leading to the pathway being "always on."

Question 10

1. In both patients, a truncated, partially functional APC protein is produced, leading to a gain of function.
2. The first patient has a complete loss of APC protein, while the second patient has overexpression of APC protein.
3. In both patients, the result is a significant reduction or complete absence of functional APC protein. (correct answer)
4. The first patient's genetic mutations are heritable, while the second patient's epigenetic changes are not.

Explanation: The correct answer is C. The key concept is that different mechanisms can lead to the same functional outcome. APC is a tumor suppressor. The first patient has a 'two-hit' scenario with genetic mutations: a nonsense mutation creates a non-functional protein from one allele, and deletion (LOH) removes the other. This results in a complete loss of functional APC protein. The second patient has epigenetic silencing via promoter hypermethylation, which prevents transcription of the APC gene. This also results in a loss of functional APC protein. Thus, the functional outcome is the same in both cases.

• A is incorrect because these events lead to a loss, not a gain, of function. A nonsense mutation and epigenetic silencing both eliminate protein production.
• B is incorrect because hypermethylation leads to gene silencing and thus a loss of protein expression, not overexpression.
• D is incorrect because the question asks about the functional consequence for the protein in the tumor, not heritability. Furthermore, the genetic mutations described are somatic (in the tumor) and thus not heritable unless they were also present in the germline.

Question 11

1. It creates a novel MYC-IgH fusion protein with enhanced transcriptional activity.
2. It causes a loss of function of the MYC gene, which is a tumor suppressor.
3. It inactivates the tumor suppressor function of the IgH locus on chromosome 14.
4. It leads to the constitutive, high-level expression of the normal MYC protein in B-cells. (correct answer)

Explanation: When you encounter questions about chromosomal translocations and cancer, focus on understanding how gene regulation changes rather than assuming the gene itself must be damaged. Translocations often cause problems by putting normal genes in the wrong regulatory environment. The t(8;14) translocation in Burkitt's lymphoma demonstrates a classic mechanism of oncogenesis through gene dysregulation. The MYC proto-oncogene normally produces a transcription factor that promotes cell division, but its expression is tightly controlled. When the translocation moves MYC next to the immunoglobulin heavy chain enhancer, this powerful regulatory sequence drives continuous, high-level MYC expression in B-cells. Since these are the same cells that normally use the IgH enhancer to produce large amounts of antibodies, MYC gets inappropriately overexpressed, leading to uncontrolled cell proliferation and cancer. Answer D correctly identifies this mechanism - the normal MYC protein is produced at abnormally high levels due to the new regulatory environment. Answer A is wrong because no fusion protein forms; the MYC coding sequence remains intact and separate from IgH sequences. Answer B incorrectly classifies MYC as a tumor suppressor when it's actually a proto-oncogene that promotes cell division. Answer C misunderstands the mechanism - the IgH enhancer continues functioning normally, but now it's inappropriately activating MYC instead of just immunoglobulin genes. Remember: in translocation-induced cancers, look for regulatory changes rather than protein changes. The "wrong gene, wrong place, wrong time" principle often explains the oncogenic mechanism.

Question 12

1. The molecular mechanism of cell transformation is different, as the germline mutation is a gain-of-function whereas the somatic mutations are loss-of-function.
2. Sporadic cases typically present as bilateral tumors at an early age, while hereditary cases are usually unilateral and occur later in life.
3. The first hit in the hereditary form makes the RB1 gene a dominant oncogene, while in the sporadic form it remains a recessive tumor suppressor.
4. In both cases, the tumor cells are homozygous or hemizygous for the loss of RB1 function, but the probability of this occurring is much higher in the hereditary form. (correct answer)

Explanation: When you encounter questions about retinoblastoma, you're dealing with Knudson's "two-hit hypothesis" for tumor suppressor genes. The key insight is that RB1 requires the loss of both functional copies to cause cancer, but the timing and probability of these losses differs dramatically between hereditary and sporadic cases. In hereditary retinoblastoma, patients inherit one defective RB1 allele in every cell. They only need one additional somatic mutation in a retinal cell to lose all RB1 function. In sporadic cases, patients start with two normal RB1 alleles, so the same retinal cell must suffer two independent somatic mutations. Both scenarios ultimately produce the same result: complete loss of RB1 function in tumor cells, making answer D correct. Let's examine why the other options miss the mark. A is wrong because both germline and somatic RB1 mutations are loss-of-function—there's no gain-of-function involved. B reverses the clinical presentation: hereditary cases typically appear as bilateral tumors in young children (high probability of multiple affected cells), while sporadic cases are usually unilateral and occur later (low probability of two hits in the same cell). C contains a fundamental error—RB1 remains a tumor suppressor gene in both scenarios; the "first hit" doesn't magically transform it into an oncogene. Remember this pattern: for tumor suppressor genes, hereditary cases have a "head start" with one inherited mutation, making cancer much more likely than sporadic cases requiring multiple independent hits.

Question 13

1. The tumor contains a single, heterozygous missense mutation in GENE-X that is absent from the patient's normal tissue.
2. The tumor shows high-level amplification of the chromosomal region containing GENE-X.
3. The tumor expresses a fusion transcript involving GENE-X and a transcription factor gene, resulting from a chromosomal translocation.
4. The tumor has a homozygous deletion of the GENE-X locus, while normal tissue is heterozygous. (correct answer)

Explanation: When analyzing somatic mutations in tumors, you need to understand the fundamental difference between tumor suppressors and oncogenes. Tumor suppressors normally prevent cancer by controlling cell division, so cancer develops when these genes are lost or inactivated. Oncogenes, conversely, promote cell growth and become cancer-causing when they're overactivated or overexpressed. The strongest evidence for GENE-X being a tumor suppressor is option D: homozygous deletion in the tumor while normal tissue remains heterozygous. This pattern shows complete loss of gene function in cancer cells, which is the hallmark mechanism of tumor suppressor inactivation. The "two-hit hypothesis" explains that both copies of a tumor suppressor must be lost for cancer to develop. Option A represents only partial loss (one mutated copy remains), which typically isn't sufficient to inactivate a tumor suppressor completely. Option B shows high-level amplification, which suggests GENE-X is being overproduced—this pattern indicates oncogene activation, not tumor suppressor loss. Option C describes a fusion transcript from chromosomal translocation, another classic mechanism of oncogene activation where the gene gains new, cancer-promoting functions. Remember this key distinction: tumor suppressors are lost in cancer (deletions, inactivating mutations), while oncogenes are gained or overactivated (amplifications, activating mutations, translocations). When you see complete gene loss like homozygous deletion, think tumor suppressor. When you see gene overexpression or novel fusion proteins, think oncogene.

Question 14

1. E6 and E7 are viral oncogenes whose actions functionally mimic gain-of-function mutations in cellular proto-oncogenes like RAS or MYC.
2. E6 and E7 are viral proteins that cause loss-of-function mutations in the host's TP53 and RB1 genes, respectively.
3. E6 and E7 are viral oncogenes whose actions functionally mimic the biallelic inactivation of the TP53 and RB1 tumor suppressor genes. (correct answer)
4. E6 and E7 are viral tumor suppressors that induce apoptosis in the host cell to promote viral replication and spread.

Explanation: The correct answer is C. p53 and Rb are key tumor suppressor proteins. By targeting them for degradation or inactivation, the viral proteins E6 and E7 effectively eliminate their function. This has the same outcome as if the cell had sustained 'two hits'—biallelic loss-of-function mutations—in its endogenous TP53 and RB1 genes. Therefore, these viral proteins (which are considered viral oncogenes) functionally mimic the loss of tumor suppressors, thereby removing critical cell cycle checkpoints.

• A is incorrect because while E6 and E7 are oncogenes, their mechanism is the inactivation of tumor suppressors, not the activation of cellular proto-oncogenes. Mimicking a RAS mutation would involve activating a signaling pathway directly.
• B is incorrect because the viral proteins do not cause mutations in the host's DNA. They act at the protein level to eliminate the tumor suppressor proteins themselves.
• D is incorrect because E6 and E7 promote cell survival and proliferation (by inactivating p53 and Rb), not apoptosis. Their classification is oncogenic, not tumor suppressive.

Question 15

1. Both the BRCA1 and PIK3CA mutations are oncogenic and were likely inherited from a parent.
2. The germline BRCA1 mutation is a loss-of-function in a tumor suppressor, while the somatic PIK3CA mutation is a gain-of-function in an oncogene. (correct answer)
3. The BRCA1 mutation is a 'caretaker' oncogene, and the PIK3CA mutation is a 'gatekeeper' tumor suppressor.
4. Both mutations are likely somatic events that occurred independently in the breast tissue, leading to cancer.

Explanation: The correct answer is B. This question requires classifying two different cancer-associated genes. BRCA1 is a 'caretaker' tumor suppressor gene involved in DNA repair; a pathogenic variant represents a loss of function. The term 'germline' indicates it was inherited. PIK3CA is a proto-oncogene encoding a catalytic subunit of PI3-kinase; activating mutations are a gain of function that promotes cell growth and survival. The term 'somatic' indicates this mutation occurred in the tumor tissue and was not inherited.

• A is incorrect because the BRCA1 mutation is a loss-of-function in a tumor suppressor, not an oncogenic (gain-of-function) mutation. Also, only the BRCA1 mutation was germline (inherited).
• C is incorrect as it misclassifies both genes. BRCA1 is a caretaker tumor suppressor (not oncogene), and PIK3CA is a gatekeeper proto-oncogene (not tumor suppressor).
• D is incorrect because the BRCA1 mutation is explicitly described as germline, meaning it was inherited and is present in all of the patient's cells, not just the tumor.

Question 16

1. Cancer is a dominant trait, as the tumorigenic phenotype eventually re-emerges in the hybrid cells.
2. Normal cells contain genes that can suppress tumorigenicity, and the loss of these genes restores the cancer phenotype. (correct answer)
3. The cancer cell's oncogenes are initially silenced by factors from the normal cell, but these oncogenes can become reactivated over time.
4. The process of cell fusion itself induces widespread mutations that can randomly lead to a reversion to the cancer phenotype.

Explanation: The correct answer is B. This experiment provides fundamental evidence for the existence of tumor suppressor genes. The initial fusion creates a hybrid cell that is non-tumorigenic because the normal cell contributes wild-type copies of tumor suppressor genes, which complement the defects in the cancer cell. The re-emergence of tumorigenicity upon the loss of specific chromosomes from the normal parent cell strongly implies that these lost chromosomes carried the crucial tumor suppressor genes.

• A is incorrect because the initial result (suppression of tumorigenicity) shows that at the cellular level, the cancer phenotype is recessive to the normal phenotype.
• C is incorrect because the data points to the loss of genetic material (chromosomes) from the normal cell, not the reactivation of oncogenes from the cancer cell, as the primary cause for the reversion.
• D is incorrect because the loss of specific chromosomes, not random mutations, is correlated with the phenotype reversion, suggesting a deterministic role for genes on those chromosomes.

Question 17

1. E6 and E7 are viral oncogenes whose actions functionally mimic gain-of-function mutations in cellular proto-oncogenes like RAS or MYC.
2. E6 and E7 are viral proteins that cause loss-of-function mutations in the host's TP53 and RB1 genes, respectively.
3. E6 and E7 are viral oncogenes whose actions functionally mimic the biallelic inactivation of the TP53 and RB1 tumor suppressor genes. (correct answer)
4. E6 and E7 are viral tumor suppressors that induce apoptosis in the host cell to promote viral replication and spread.

Explanation: The correct answer is C. p53 and Rb are key tumor suppressor proteins. By targeting them for degradation or inactivation, the viral proteins E6 and E7 effectively eliminate their function. This has the same outcome as if the cell had sustained 'two hits'—biallelic loss-of-function mutations—in its endogenous TP53 and RB1 genes. Therefore, these viral proteins (which are considered viral oncogenes) functionally mimic the loss of tumor suppressors, thereby removing critical cell cycle checkpoints.

• A is incorrect because while E6 and E7 are oncogenes, their mechanism is the inactivation of tumor suppressors, not the activation of cellular proto-oncogenes. Mimicking a RAS mutation would involve activating a signaling pathway directly.
• B is incorrect because the viral proteins do not cause mutations in the host's DNA. They act at the protein level to eliminate the tumor suppressor proteins themselves.
• D is incorrect because E6 and E7 promote cell survival and proliferation (by inactivating p53 and Rb), not apoptosis. Their classification is oncogenic, not tumor suppressive.

Question 18

1. It demonstrates that some tumor suppressors can be activated by a single gain-of-function mutation.
2. It shows that a single mutated allele can functionally inactivate the wild-type allele, mimicking a two-hit scenario. (correct answer)
3. It proves that p53 is actually an oncogene, as a single mutation can promote cancer.
4. It suggests that epigenetic silencing, not mutation, is the primary mechanism of p53 inactivation.

Explanation: The correct answer is B. The classic view of tumor suppressors is that they are recessive, requiring two loss-of-function 'hits'. A dominant-negative mutation complicates this. The mutation itself is a loss-of-function for the individual protein, but because p53 functions as a tetramer, the mutant protein 'poisons' the complex, inactivating the function of the remaining wild-type proteins. Therefore, a mutation in just one allele can have a dominant effect at the cellular level, functionally mimicking the outcome of a homozygous loss (a two-hit scenario).

• A is incorrect because the R248W mutation is a loss-of-function (loss of DNA binding), not a gain-of-function. The dominant effect comes from interference, not from acquiring a new oncogenic activity.
• C is incorrect because p53's fundamental role is tumor suppression. This specific mutational mechanism doesn't change its classification from a tumor suppressor to an oncogene.
• D is incorrect because the scenario explicitly describes a genetic mutation, not epigenetic silencing, as the cause of inactivation.

Question 19

1. tumor suppressor gene, because its inactivation requires a loss-of-function event. (correct answer)
2. proto-oncogene, because truncation often leads to constitutive activation of signaling domains.
3. growth factor receptor, because a truncated receptor mimics the ligand-bound state.
4. housekeeping gene, because disruption of basic metabolism is a primary driver of tumorigenesis.

Explanation: When analyzing how mutations contribute to cancer, you need to understand the distinction between tumor suppressor genes and oncogenes, and how different types of mutations affect each category differently. A frameshift mutation early in the coding sequence that creates a severely truncated, non-functional protein represents a complete loss-of-function event. Tumor suppressor genes normally act as cellular "brakes" that prevent uncontrolled cell division. When these genes are inactivated through loss-of-function mutations, cells lose their ability to regulate growth properly, contributing to cancer development. This matches exactly what we see with this frameshift mutation. Looking at why the other options don't fit: Option B is incorrect because proto-oncogenes typically contribute to cancer through gain-of-function mutations that increase their activity, not through truncation that eliminates function. Option C misunderstands how growth factor receptors work—truncated receptors don't mimic ligand binding; they're more likely to be completely inactive. Option D is wrong because while housekeeping genes are important for basic cellular functions, their disruption typically leads to cell death rather than the uncontrolled proliferation characteristic of cancer. The key principle here is that cancer results from losing growth control mechanisms (tumor suppressors) or gaining growth-promoting signals (oncogenes). Since this mutation eliminates protein function entirely, it can only contribute to cancer by removing a growth control mechanism—which is exactly what tumor suppressor genes provide. Remember: loss-of-function mutations typically affect tumor suppressors, while gain-of-function mutations typically create oncogenes from normal proto-oncogenes.

Question 20

1. It creates a novel MYC-IgH fusion protein with enhanced transcriptional activity.
2. It causes a loss of function of the MYC gene, which is a tumor suppressor.
3. It inactivates the tumor suppressor function of the IgH locus on chromosome 14.
4. It leads to the constitutive, high-level expression of the normal MYC protein in B-cells. (correct answer)

Explanation: When you encounter questions about chromosomal translocations and cancer, focus on understanding how gene regulation changes rather than assuming the gene itself must be damaged. Translocations often cause problems by putting normal genes in the wrong regulatory environment. The t(8;14) translocation in Burkitt's lymphoma demonstrates a classic mechanism of oncogenesis through gene dysregulation. The MYC proto-oncogene normally produces a transcription factor that promotes cell division, but its expression is tightly controlled. When the translocation moves MYC next to the immunoglobulin heavy chain enhancer, this powerful regulatory sequence drives continuous, high-level MYC expression in B-cells. Since these are the same cells that normally use the IgH enhancer to produce large amounts of antibodies, MYC gets inappropriately overexpressed, leading to uncontrolled cell proliferation and cancer. Answer D correctly identifies this mechanism - the normal MYC protein is produced at abnormally high levels due to the new regulatory environment. Answer A is wrong because no fusion protein forms; the MYC coding sequence remains intact and separate from IgH sequences. Answer B incorrectly classifies MYC as a tumor suppressor when it's actually a proto-oncogene that promotes cell division. Answer C misunderstands the mechanism - the IgH enhancer continues functioning normally, but now it's inappropriately activating MYC instead of just immunoglobulin genes. Remember: in translocation-induced cancers, look for regulatory changes rather than protein changes. The "wrong gene, wrong place, wrong time" principle often explains the oncogenic mechanism.