Defects in Signal Transduction and Disease Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for Defects in Signal Transduction and Disease. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Defects in Signal Transduction and Disease Indian Medical PG Question 1: If there is a Gs alpha subunit gain-of-function mutation, this results in
- A. Increased cAMP (Correct Answer)
- B. Decreased cAMP
- C. Increased GTPase activity
- D. Decreased IP₃
Defects in Signal Transduction and Disease Explanation: ***Increased cAMP***
- A **gain-of-function mutation** in the **Gs alpha subunit** means it remains in its active, GTP-bound state for longer.
- The activated Gs alpha subunit stimulates **adenylyl cyclase**, leading to persistently high levels of **cAMP**.
*Decreased cAMP*
- This would result from a **loss-of-function** mutation in the Gs alpha subunit or a gain-of-function in an inhibitory G protein (Gi), not a Gs gain-of-function.
- A decrease in cAMP would inhibit downstream signaling pathways, which is the opposite of what occurs with Gs activation.
*Increased GTPase activity*
- **GTPase activity** is responsible for hydrolyzing GTP to GDP, which inactivates the G alpha subunit.
- A gain-of-function mutation often implies **reduced GTPase activity**, causing the G protein to stay active longer, not increased activity.
*Decreased IP*
- **IP3 (inositol trisphosphate)** is a secondary messenger produced via the activation of **phospholipase C**, typically by Gq proteins.
- Gs alpha subunit mutations primarily affect the **adenylyl cyclase/cAMP pathway**, not the inositol phosphate pathway.
Defects in Signal Transduction and Disease Indian Medical PG Question 2: Which of the following is a genetic disorder that does not primarily affect ion channels?
- A. Cystic fibrosis
- B. Liddle's syndrome
- C. Hypokalemic periodic paralysis
- D. Tay-Sachs disease (Correct Answer)
Defects in Signal Transduction and Disease Explanation: ***Cystic fibrosis***
- Primarily caused by mutations in the **CFTR gene**, which encodes a **chloride channel**, but it is not classified as a classic channelopathy.
- The disease mainly affects **mucus production** rather than direct dysfunction of ion channels in the traditional sense. [1]
*Hypokalemic periodic paralysis*
- This condition is directly related to **ion channel dysfunction**, specifically affecting **sodium channels** in muscle cells.
- It causes episodic **muscle weakness** and hypokalemia due to improper ion transport.
*Liddle's syndrome*
- A genetic disorder resulting from mutations affecting **epithelial sodium channels**, leading to **hypertension**.
- It exemplifies classic channelopathy by causing dysregulation of sodium reabsorption in the kidneys.
*Tay-sach's disease*
- A ***gangliosidosis*** caused by a deficiency in the enzyme **Hexosaminidase A**, rather than ion channel dysfunction.
- It results in the accumulation of **GM2 gangliosides** leading to neurological degeneration, not affecting ion channels directly.
**References:**
[1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 120-122.
Defects in Signal Transduction and Disease Indian Medical PG Question 3: What is the primary function of G-proteins in cellular signaling?
- A. Signal transducers (Correct Answer)
- B. Mediators of hormone action
- C. Molecules that bind hormones
- D. Intracellular signaling molecules
Defects in Signal Transduction and Disease Explanation: ***Signal transducers***
- G-proteins act as **molecular switches**, converting extracellular signals received by G protein-coupled receptors (GPCRs) into intracellular responses.
- They bind **GTP** in their active state and **hydrolyze it to GDP** to become inactive, regulating downstream effectors like enzymes and ion channels.
*Mediators of hormone action*
- While G-proteins are involved in the action of many hormones, this describes a *result* of their function rather than their fundamental role.
- Their primary function is to transduce signals, which then mediates hormone effects.
*Molecules that bind hormones*
- **Receptors**, not G-proteins, are primarily responsible for binding hormones or other ligands.
- G-proteins are activated *after* a receptor binds a ligand and undergoes a conformational change.
*Intracellular signaling molecules*
- This statement is true, but it's a broad category. **Signal transducers** specifically highlights their role in converting one form of signal to another.
- Many molecules operate intracellularly, but G-proteins' unique role is in linking receptor activation to effector modulation.
Defects in Signal Transduction and Disease Indian Medical PG Question 4: The gene most commonly involved in endometrial carcinoma is:
- A. PTEN (Correct Answer)
- B. BRAF
- C. KRAS
- D. Mismatch repair genes
Defects in Signal Transduction and Disease Explanation: ***PTEN***
- **PTEN** (phosphatase and tensin homolog) is a **tumor suppressor gene** frequently inactivated in **endometrial carcinoma**, particularly in cases of **endometrioid histology**.
- Loss of PTEN function leads to uncontrolled cell proliferation and survival by activating the **PI3K/Akt signaling pathway**, contributing to tumor development.
*BRAF*
- **BRAF mutations** are most commonly associated with **melanoma** and certain types of **thyroid cancer**, specifically papillary thyroid carcinoma.
- While BRAF mutations can be found in a small subset of other cancers, they are not a primary driver or common gene in endometrial carcinoma.
*KRAS*
- **KRAS mutations** are frequently observed in **colorectal cancer**, **pancreatic cancer**, and **non-small cell lung cancer**.
- Though KRAS can be mutated in various cancers, it is not the most commonly involved gene in endometrial carcinoma.
*Mismatch repair genes*
- Mutations in **mismatch repair (MMR) genes** (e.g., MLH1, MSH2, MSH6, PMS2) are characteristic of **Lynch syndrome** and lead to **microsatellite instability (MSI)**.
- While MSI is observed in a significant subset of endometrial cancers (especially those associated with Lynch syndrome), PTEN mutations are more broadly common across all types of endometrial carcinoma.
Defects in Signal Transduction and Disease Indian Medical PG Question 5: G-protein coupled receptor that does not act through opening of potassium channels is:
- A. Dopamine D2 receptor
- B. Muscarinic M2 receptor
- C. Serotonin 5 HT 1 receptor
- D. Angiotensin 1 receptor (Correct Answer)
Defects in Signal Transduction and Disease Explanation: ***Angiotensin 1 receptor***
- The **angiotensin 1 receptor (AT1R)** is a **Gq-coupled receptor** that primarily activates the **phospholipase C (PLC)** pathway, leading to increased intracellular **calcium** and **IP3/DAG** signaling.
- Its activation mediates vasoconstriction, aldosterone release, and cardiac hypertrophy, none of which involve direct opening of potassium channels.
*Dopamine D2 receptor*
- **Dopamine D2 receptors** are **Gi/o-coupled receptors** that inhibit adenylyl cyclase and **open potassium channels**, leading to **hyperpolarization** and reduced neuronal excitability.
- This action contributes to its **antipsychotic** and **motor control** effects.
*Muscarinic M2 receptor*
- **Muscarinic M2 receptors** are **Gi/o-coupled receptors** found in the heart that cause **bradycardia** by activating **acetylcholine-gated inwardly rectifying potassium (GIRK) channels**, leading to hyperpolarization.
- They also inhibit adenylyl cyclase, reducing cAMP levels and decreasing heart rate and contractility.
*Serotonin 5 HT 1 receptor*
- **Serotonin 5-HT1 receptors** (e.g., 5-HT1A) are **Gi/o-coupled receptors** that, upon activation, **increase potassium conductance** (hyperpolarization) and inhibit adenylyl cyclase.
- This leads to a reduction in neuronal firing and is implicated in the anxiolytic and antidepressant effects of these receptors.
Defects in Signal Transduction and Disease Indian Medical PG Question 6: In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by
- A. Inositol triphosphate
- B. GTPase activating protein (Correct Answer)
- C. Phosphatidyl inositol
- D. Protein kinase C
Defects in Signal Transduction and Disease Explanation: ***GTPase activating protein***
- **GTPase Activating Proteins (GAPs)** facilitate the hydrolysis of **GTP bound to RAS** to GDP, converting active RAS back to its inactive state.
- This inactivation is crucial for turning off the downstream signaling of the **MAPK pathway** and preventing uncontrolled cell proliferation.
*Inositol triphosphate*
- **Inositol triphosphate (IP3)** is a secondary messenger that triggers the release of **intracellular calcium** from the endoplasmic reticulum.
- It is involved in various signaling pathways, but its primary role is not to directly counteract RAS activation.
*Phosphatidyl inositol*
- **Phosphatidylinositol (PI)** is a component of cell membranes and can be phosphorylated to produce various **phosphatidylinositol phosphates (PIPs)**, like **PIP2** and **PIP3**.
- These molecules act as docking sites for signaling proteins but do not directly inactivate RAS.
*Protein kinase C*
- **Protein kinase C (PKC)** is a family of enzymes involved in signal transduction, typically activated by **diacylglycerol (DAG)** and calcium.
- It phosphorylates various proteins, mediating diverse cellular responses, but it does not directly counteract the activation of RAS.
Defects in Signal Transduction and Disease Indian Medical PG Question 7: Which of the following stimulate adenylate cyclase with G-protein coupled action ?
- A. Shiga toxin
- B. Cholera toxin (Correct Answer)
- C. Diphtheria toxin
- D. Pseudomonas toxin
Defects in Signal Transduction and Disease Explanation: ***Cholera toxin***
- Cholera toxin is a **G-protein-activating toxin** that irreversibly activates **adenylate cyclase**.
- This leads to increased intracellular levels of **cAMP**, causing excessive fluid secretion into the intestinal lumen and severe diarrhea.
*Shiga toxin*
- Shiga toxin acts by inactivating the **60S ribosomal subunit**, thereby inhibiting protein synthesis in eukaryotic cells.
- Its primary effect is **cytotoxicity**, not direct stimulation of adenylate cyclase.
*Diphtheria toxin*
- Diphtheria toxin inhibits **protein synthesis** by inactivating **elongation factor-2 (EF-2)** through ADP-ribosylation.
- This toxin specifically targets host cells, leading to cellular death and tissue damage.
*Pseudomonas toxin*
- **Exotoxin A** produced by *Pseudomonas aeruginosa* also inhibits **protein synthesis** by ADP-ribosylating and inactivating **EF-2**, similar to diphtheria toxin.
- It does not directly affect adenylate cyclase activity.
Defects in Signal Transduction and Disease Indian Medical PG Question 8: Phospholipid associated with the mechanism of hormone action is
- A. Phosphatidylcholine
- B. Phosphatidylethanolamine
- C. Plasmalogen
- D. Phosphatidylinositol (Correct Answer)
Defects in Signal Transduction and Disease Explanation: ***Phosphatidylinositol*** (Correct)
- **Phosphatidylinositol (PI)** and its phosphorylated derivatives, particularly **PIP2 (phosphatidylinositol 4,5-bisphosphate)**, are critical in signal transduction pathways activated by many hormones.
- Hormones binding to **G protein-coupled receptors** can activate phospholipase C, which cleaves PIP2 into **inositol triphosphate (IP3)** and **diacylglycerol (DAG)**, leading to increased intracellular calcium and protein kinase C activation, respectively.
*Phosphatidylcholine* (Incorrect)
- **Phosphatidylcholine** is a major component of cell membranes and is involved in membrane structure and fluidity.
- While it can be a source of signaling molecules like **lysophosphatidic acid**, it is not primarily associated with the initial intracellular signaling events of hormone action in the same way as phosphatidylinositol.
*Phosphatidylethanolamine* (Incorrect)
- **Phosphatidylethanolamine** is another abundant membrane phospholipid primarily involved in membrane structure and stability.
- It can be a precursor for other lipids, but it does not directly participate in the **second messenger systems** triggered by most hormones as a primary signaling molecule.
*Plasmalogen* (Incorrect)
- **Plasmalogens** are a unique class of phospholipids containing an ether bond at the sn-1 position.
- They are abundant in certain tissues, particularly nervous and cardiovascular tissues, and are thought to have antioxidant properties, but they are not directly involved in the initiating events of **hormone signaling pathways**.
Defects in Signal Transduction and Disease Indian Medical PG Question 9: Which of the following binds to Tyrosine Kinase receptor?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. Prolactin
- D. Growth Hormone
Defects in Signal Transduction and Disease Explanation: ***Insulin***
- **Insulin** is a classic example of a hormone that binds to and activates a **tyrosine kinase receptor**, leading to a cascade of intracellular signaling events for glucose uptake and metabolism.
- The **insulin receptor** is a heterodimeric protein with intrinsic tyrosine kinase activity that phosphorylates itself and other proteins upon insulin binding.
*Glucagon*
- **Glucagon** primarily acts on **G protein-coupled receptors (GPCRs)**, specifically the glucagon receptor, to increase cyclic AMP (cAMP) and activate protein kinase A.
- Its main roles are to stimulate **glycogenolysis** and **gluconeogenesis** in the liver.
*Prolactin*
- **Prolactin** binds to a receptor that is a member of the **cytokine receptor superfamily**, which lacks intrinsic enzyme activity.
- Upon ligand binding, these receptors associate with and activate **Janus kinases (JAKs)**, leading to the JAK-STAT signaling pathway.
*Growth Hormone*
- **Growth hormone (GH)** also binds to a receptor belonging to the **cytokine receptor superfamily** (similar to prolactin), which then associates with and activates **JAKs**.
- This activation subsequently initiates the **JAK-STAT signaling pathway**, mediating its diverse growth-promoting and metabolic effects.
Defects in Signal Transduction and Disease Indian Medical PG Question 10: Which of the following statements about G protein-coupled receptors (GPCRs) is true?
- A. The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.
- B. G proteins can act as either inhibitory or excitatory based on the type of alpha subunit. (Correct Answer)
- C. G proteins bind directly to hormones to become activated.
- D. In the resting state, G proteins are bound to GTP.
Defects in Signal Transduction and Disease Explanation: ***G proteins can act as either inhibitory or excitatory based on the type of alpha subunit.***
- Different classes of Gα subunits (e.g., **Gαs**, **Gαi**, **Gαq**) couple to diverse downstream effectors, leading to either **stimulation** (excitatory) or **inhibition** of cellular processes.
- For example, **Gαs** activates adenylyl cyclase, while **Gαi** inhibits it, demonstrating their opposing roles.
*The three subunits alpha, beta, and gamma must remain together as a complex for G protein to function.*
- Upon activation, the **Gα subunit dissociates** from the **Gβγ dimer**, and both free units can then independently modulate effector molecules.
- For the G protein to function in signal transduction, the α subunit often separates from the βγ dimer to interact with its target enzyme or ion channel.
*G proteins bind directly to hormones to become activated.*
- **GPCRs** (the receptors themselves) bind to hormones or other ligands on the **extracellular side** of the membrane.
- The binding of the ligand to the GPCR induces a conformational change in the receptor, which then activates the associated G protein on the intracellular side.
*In the resting state, G proteins are bound to GTP.*
- In the **resting (inactive) state**, the Gα subunit of the trimeric G protein is bound to **GDP**.
- Activation occurs when the GPCR facilitates the exchange of **GDP for GTP** on the Gα subunit.
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