- Infection with high-risk strains of human papillomavirus (HPV) is a primary cause of cervical cancer. The virus integrates into the host genome and expresses two key proteins, E6 and E7. The E6 protein binds to the p53 protein and targets it for ubiquitin-mediated degradation. The E7 protein binds to the retinoblastoma (Rb) protein and disrupts its interaction with the E2F transcription factor. How does the action of these viral proteins relate to the functions of oncogenes and tumor suppressors?
Answer: E6 and E7 are viral oncogenes whose actions functionally mimic the biallelic inactivation of the TP53 and RB1 tumor suppressor genes.
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.
- The chromosomal translocation t(8;14), often found in Burkitt's lymphoma, places the MYC proto-oncogene from chromosome 8 next to the strong immunoglobulin heavy chain (IgH) enhancer on chromosome 14. The MYC coding sequence itself is not mutated. How does this event lead to oncogenesis?
Answer: It leads to the constitutive, high-level expression of the normal MYC protein in B-cells.
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.
- A researcher identifies a family with a high incidence of various cancers occurring at a young age, consistent with Li-Fraumeni syndrome. Genetic sequencing reveals a germline missense mutation in one allele of the TP53 gene in all affected individuals. At the cellular level, for a tumor to develop, what is the most likely additional event required regarding the TP53 gene, and why?
Answer: Loss or inactivation of the remaining wild-type TP53 allele, because TP53 is a tumor suppressor that requires two 'hits' for its function to be eliminated.
The correct answer is B. TP53 is a classic tumor suppressor gene. In hereditary cancer syndromes like Li-Fraumeni, an individual inherits one non-functional allele (the 'first hit'). According to Knudson's two-hit hypothesis, a second somatic event ('second hit') must occur in a cell to inactivate the remaining wild-type allele. This second hit, often a mutation or loss of heterozygosity, leads to a complete loss of tumor suppressor function in that cell, paving the way for tumorigenesis.
* A is incorrect because TP53 is a tumor suppressor, not an oncogene. Oncogenes typically require one gain-of-function mutation, not two.
* C is incorrect because while some TP53 mutations can have a dominant-negative effect (where the mutant protein interferes with the wild-type protein), complete tumor progression typically still involves the loss of the wild-type allele (loss of heterozygosity), making the cell homozygous for the mutant or null allele. The 'two-hit' model is the canonical explanation.
* D is incorrect because the critical second event involves the TP53 gene itself, not a separate oncogene. While oncogene activation is part of cancer progression, it is not the required 'second hit' that completes the inactivation of the TP53 tumor suppressor pathway.
- A frameshift mutation occurring very early in the coding sequence of a gene is discovered in the genome of a cancer cell. This mutation is predicted to result in a severely truncated, non-functional protein. This type of mutation is most likely to contribute to cancer if the affected gene is a(n):
Answer: tumor suppressor gene, because its inactivation requires a loss-of-function event.
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.
- Based on the pathway described in the passage, a loss-of-function mutation in which protein would be most analogous to the inactivation of a tumor suppressor gene, and a gain-of-function mutation in which protein would be most analogous to the activation of an oncogene?
Answer: Loss of D is like a TSG loss; Gain of B is like an oncogene activation.
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.
- A patient with colon cancer has their tumor sequenced. The report indicates a nonsense mutation in one allele of the APC gene and loss of heterozygosity (LOH) at the APC locus, resulting in the absence of the second allele. A second patient's tumor shows two wild-type APC alleles but dense CpG island hypermethylation in the promoter region of the APC gene. Which statement best describes the functional consequence for the APC protein in both patients?
Answer: In both patients, the result is a significant reduction or complete absence of functional APC protein.
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.
- A researcher is studying a novel gene, CGR1. They create a mouse model where one allele of CGR1 is knocked out (*CGR1*+/-). These mice show a modest increase in cellular proliferation but do not develop tumors. However, when these mice are exposed to a mutagen, they develop tumors at a high rate, and analysis of these tumors consistently reveals mutations that create a premature stop codon in the remaining wild-type CGR1 allele. Based on these observations, CGR1 is best classified as:
Answer: A classic tumor suppressor gene that follows the two-hit hypothesis for tumorigenesis.
The correct answer is C. The experimental evidence strongly supports the classification of CGR1 as a classic tumor suppressor gene. The first 'hit' is the engineered knockout of one allele. This predisposes the mice to cancer but is not sufficient on its own. The second 'hit' is the somatic loss-of-function mutation (premature stop codon) in the remaining wild-type allele, which is observed in all tumors. This progression perfectly illustrates Knudson's two-hit hypothesis.
* A is incorrect because the mutations observed in the tumors are loss-of-function (premature stop codon), not gain-of-function, which is characteristic of oncogenes.
* B is incorrect because the loss of one allele was not sufficient to cause tumors, only a mild phenotype. Therefore, the gene is not haploinsufficient in the context of tumorigenesis.
* D is incorrect because CGR1 behaves like a tumor suppressor (loss of function leads to tumors), not a proto-oncogene. Dominant-negative effects typically involve missense mutations, not knockout or nonsense mutations.
- Some mutations in the TP53 gene, such as R248W, result in a mutant p53 protein that not only loses its own tumor-suppressive DNA-binding function but also forms tetramers with wild-type p53 protein produced from the other allele, inactivating the entire complex. This phenomenon is known as a dominant-negative effect. How does this mechanism challenge a simplified view of tumor suppressor genes?
Answer: It shows that a single mutated allele can functionally inactivate the wild-type allele, mimicking a two-hit scenario.
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.
- Comparative genomic hybridization (CGH) analysis of a neuroblastoma tumor sample reveals a highly amplified region on chromosome 2, which contains the MYCN gene. No point mutations are found within the MYCN coding sequence. This genomic alteration strongly suggests that MYCN functions as which type of gene in this context?
Answer: An oncogene that is activated by an increase in gene dosage.
The correct answer is B. Gene amplification is a mechanism that increases the copy number of a gene, leading to overexpression of its protein product. This increase in gene dosage is a classic gain-of-function mechanism for activating proto-oncogenes. MYCN is a known proto-oncogene, and its amplification is a key driver in neuroblastoma. The overabundance of the MYCN transcription factor protein promotes uncontrolled cell growth and proliferation.
* A is incorrect because the observation is gene amplification (copy number gain), not loss. Tumor suppressors are inactivated by loss, not gain.
* C is incorrect because while genomic instability is a hallmark of cancer, the primary role of MYCN is as a transcription factor promoting growth, not DNA repair. Amplification of a DNA repair gene would not typically be a primary cancer driver.
* D is incorrect because haploinsufficiency relates to the loss of one copy of a tumor suppressor gene, which is the opposite of the gene amplification observed here.
- A pharmaceutical company is developing two new cancer drugs. Drug A is a small molecule designed to restore the normal folding and DNA-binding ability of a specific mutant p53 protein. Drug B is a monoclonal antibody that binds to and inhibits the extracellular domain of an overexpressed HER2 receptor. Which statement most accurately contrasts the therapeutic strategies of Drug A and Drug B?
Answer: Drug A attempts to reactivate a tumor suppressor, while Drug B aims to inhibit an oncogene product.
The correct answer is B. This question requires identifying the class of gene being targeted. p53 is a classic tumor suppressor gene. Mutations in TP53 are loss-of-function. Drug A, by trying to restore its function, is a strategy to reactivate a tumor suppressor. HER2 (ERBB2) is a receptor tyrosine kinase and a well-known proto-oncogene. Its overexpression (a gain-of-function mechanism) drives cancer. Drug B, by inhibiting the receptor, targets this oncogenic product. Therefore, Drug A targets a tumor suppressor and Drug B targets an oncogene.
* A is incorrect as it reverses the roles of the two targets. HER2 is the oncogene, and p53 is the tumor suppressor.
* C is incorrect because Drug A targets a loss-of-function mutation in a tumor suppressor, not a gain-of-function mutation.
* D is incorrect as it misclassifies both genes. Drug A targets a tumor suppressor (not a proto-oncogene), and Drug B targets an oncogene (not a tumor suppressor).
- Analysis of a lung adenocarcinoma reveals a specific point mutation in the KRAS gene, G12C, which locks the KRAS protein in a GTP-bound, active state. This tumor does not have any mutations in upstream growth factor receptors or downstream kinases like RAF or MEK. How does this single KRAS mutation contribute to uncontrolled cell proliferation?
Answer: It is a gain-of-function mutation that causes the cell to behave as if it is constantly receiving a growth signal, even in its absence.
The correct answer is C. KRAS is a proto-oncogene that encodes a G-protein involved in signal transduction. The G12C mutation is a classic gain-of-function mutation. It impairs the GTPase activity of the KRAS protein, so it cannot hydrolyze GTP to GDP. This traps KRAS in a permanently 'on' state, leading to constitutive activation of downstream pro-growth pathways (like the RAF-MEK-ERK pathway) independent of upstream signals from growth factor receptors.
* A is incorrect because this is a gain-of-function, not loss-of-function, mutation. Loss-of-function is characteristic of tumor suppressors, not oncogenes like KRAS.
* B is incorrect because the cell is heterozygous for the mutation (one mutant, one wild-type allele). A dominant-negative effect is not the primary mechanism here; rather, the mutant protein is constitutively active on its own. The presence of the wild-type protein is irrelevant to the mutant's signaling.
* D is incorrect because the mutation's direct effect is on protein signaling activity, not on the epigenetic regulation of other genes. While downstream effects on gene expression occur, the primary mechanism is not epigenetic silencing.
- The discovery of a somatic mutation in a tumor is the first step in determining its significance. Which of the following findings would provide the strongest evidence that a newly discovered gene, GENE-X, is a tumor suppressor rather than an oncogene?
Answer: The tumor has a homozygous deletion of the GENE-X locus, while normal tissue is heterozygous.
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.
- A key distinction between oncogenes and tumor suppressor genes (TSGs) lies in their mutational patterns in cancer. If you were to sequence a large panel of diverse tumor types, which finding would be most consistent with the established properties of these gene classes?
Answer: Oncogene mutations are often recurrent, specific missense mutations in key functional domains, while TSG mutations are diverse, inactivating mutations spread across the gene.
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.
- The NOTCH1 gene presents a complex case in cancer biology. In some hematopoietic cancers, gain-of-function mutations in NOTCH1 are oncogenic. However, in certain solid tumors like squamous cell carcinomas, loss-of-function mutations in NOTCH1 are common, suggesting it acts as a tumor suppressor. What is the most important conclusion that can be drawn from these context-dependent roles?
Answer: The classification of a gene as an oncogene or tumor suppressor can be dependent on the specific cellular context and tissue type.
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.
- Hereditary retinoblastoma is caused by a germline mutation in one RB1 allele, followed by a somatic mutation in the second allele. Sporadic retinoblastoma requires two independent somatic mutations in both RB1 alleles in the same retinal cell. Which of the following accurately compares these two scenarios?
Answer: 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.
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.
- The Retinoblastoma (Rb) protein controls the G1/S checkpoint by binding to and inhibiting E2F transcription factors. When Rb is phosphorylated by cyclin-dependent kinases (CDKs), it releases E2F, which then activates genes required for S phase. In a cancer cell with a biallelic loss-of-function mutation in the RB1 gene, what is the expected status of E2F and its effect on the cell cycle?
Answer: E2F is constitutively free and active, leading to inappropriate entry into S phase regardless of growth signals.
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."
- A cell biologist compares two types of mutations. Mutation 1 is a missense mutation in the kinase domain of the EGFR gene, a receptor tyrosine kinase, that causes the receptor to be active without its ligand. Mutation 2 is a nonsense mutation in the PTEN gene, which encodes a phosphatase that antagonizes the PI3K signaling pathway. Which statement correctly classifies these genes based on these mutations?
Answer: EGFR is a proto-oncogene that is activated, and PTEN is a tumor suppressor that is inactivated.
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.
- Loss of function of 'caretaker' tumor suppressor genes, such as those involved in mismatch repair (e.g., MSH2) or double-strand break repair (e.g., BRCA1), differs from the loss of 'gatekeeper' tumor suppressors (e.g., APC, RB1). What is the primary mechanism by which the loss of a caretaker gene contributes to cancer development?
Answer: It leads to an increased rate of mutation throughout the genome, accelerating the inactivation of gatekeepers and activation of oncogenes.
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.
- When a highly metastatic cancer cell is fused with a normal, non-cancerous fibroblast, the resulting hybrid cell often loses its ability to form tumors. However, upon continued culture, some of these hybrid cells regain their tumorigenic potential, and this change is frequently correlated with the loss of specific chromosomes derived from the normal fibroblast parent. What is the most direct conclusion from this classic experiment?
Answer: Normal cells contain genes that can suppress tumorigenicity, and the loss of these genes restores the cancer phenotype.
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.
- A patient with breast cancer undergoes genetic testing. The report shows a germline pathogenic variant in the BRCA1 gene. Analysis of the tumor tissue reveals a somatic activating mutation in the PIK3CA gene. Which statement accurately describes the implications of these two findings?
Answer: 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.
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.
