JAK-STAT Signaling Pathway — MCQs
Gene not involved in SCID:
Which of the following statements about the secondary immune response is false?
Which is TRUE about JAK2 V617F mutation?
Knudson two-hit hypothesis is classically exemplified by
All the following hormones have receptors on the plasma membrane of target tissues except:
In the mitogen activated protein kinase pathway, the activation of RAS is counteracted by
JAK-STAT pathway is seen in which of the following?
Select the correct sequence of events in the cAMP signaling pathway.
Phospholipid associated with the mechanism of hormone action is
What is the primary function of G-proteins in cellular signaling?
JAK-STAT Signaling Pathway Indian Medical PG Practice Questions and MCQs
Question 1: Gene not involved in SCID:
- A. BTK (Correct Answer)
- B. ZAP70
- C. IL2RG
- D. JAK3
Explanation: ***BTK*** - **Bruton's tyrosine kinase (BTK)** is associated with **X-linked agammaglobulinemia (XLA)**, a primary immunodeficiency characterized by the absence of mature B cells and significantly reduced antibody production. While it causes severe immune deficiency, it is not a direct cause of **SCID**. - XLA results in recurrent bacterial infections due to an inability to produce antibodies, rather than the severe combined T and B cell dysfunction seen in SCID. *ZAP70* - **ZAP70** deficiency is a cause of **SCID**. It leads to impaired T-cell receptor signaling, resulting in profound functional T-cell lymphopenia. - Patients with ZAP70 deficiency have normal numbers of CD4 T cells but very low or absent CD8 T cells, and their T cells are functionally impaired, leading to severe immunodeficiency. *IL2RG* - The **IL2RG** gene encodes the common gamma chain (γc), a crucial component of several **interleukin receptors (IL-2, IL-4, IL-7, IL-9, IL-15, IL-21)**. [1] - Mutations in IL2RG cause **X-linked SCID (X-SCID)**, the most common form of SCID, leading to a block in T-cell and NK-cell development due to defective cytokine signaling. [1] *JAK3* - **Janus kinase 3 (JAK3)** is a tyrosine kinase that associates with the **common gamma chain (γc)** and is essential for cytokine signaling downstream of the γc-containing receptors. [1] - **JAK3 deficiency** results in an **autosomal recessive form of SCID**, clinically indistinguishable from X-SCID, with impaired T-cell and NK-cell development due to defective cytokine signaling. [1] **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 247-248.
Question 2: Which of the following statements about the secondary immune response is false?
- A. The lag period is absent or significantly shorter.
- B. There is a negative phase in the response.
- C. Only T-dependent antigens are recognized.
- D. Immune response against a subsequent antigenic challenge is absent. (Correct Answer)
Explanation: ***Immune response against a subsequent antigenic challenge is absent.*** - This statement is **false** because the secondary immune response is characterized by a **much stronger and faster** immune response upon subsequent exposure to the same antigen. - The presence of **memory cells** ensures that the immune system is highly prepared to combat the antigen more efficiently than during the primary response. *The lag period is absent or significantly shorter.* - This statement is **true** for the secondary immune response. The **memory B and T cells** can be rapidly activated, reducing the time needed to mount an effective response. - Unlike primary responses that can take 5-10 days to produce antibodies, secondary responses typically produce antibodies within **1-3 days**. *There is a negative phase in the response.* - This statement is **false** for the secondary immune response. The **negative phase** is characteristic of the **primary immune response**, not the secondary response. - The negative phase in primary response refers to a transient drop in antibody concentration after initial antigen exposure due to antigen-antibody complex formation. However, the **secondary response shows immediate and robust antibody production** without this negative phase due to pre-existing memory cells. - While this statement is technically false, the question asks for THE false statement, and Option D is more obviously and fundamentally false. *Only T-dependent antigens are recognized.* - This statement is **partially false** but has some truth in context. While **T-dependent antigens** generate the most robust secondary responses with strong memory cell formation, the immune system doesn't ONLY recognize T-dependent antigens. - **T-independent antigens** can elicit responses but typically generate weaker, shorter-lived immunity without strong memory formation. The classical, robust secondary immune response with anamnestic features is predominantly associated with T-dependent antigens.
Question 3: Which is TRUE about JAK2 V617F mutation?
- A. Seen in ET.
- B. Rare in CML.
- C. Common in PV but not specific.
- D. Seen in PV, ET, and PMF but rare in CML. (Correct Answer)
Explanation: ***Seen in PV, ET, and PMF but rare in CML.*** - The **JAK2 V617F mutation** is a hallmark of classic Philadelphia chromosome-negative **myeloproliferative neoplasms (MPNs)**, including **polycythemia vera (PV)** (~95%), **essential thrombocythemia (ET)** (~50-60%), and **primary myelofibrosis (PMF)** (~50-60%) [1]. - It is **rare in chronic myeloid leukemia (CML)**, which is defined by the **BCR-ABL1 fusion gene** [1]. - This option provides the **most comprehensive and accurate** description of JAK2 V617F distribution across myeloproliferative disorders. *Seen in ET.* - While the **JAK2 V617F mutation** is indeed found in approximately 50-60% of patients with **essential thrombocythemia (ET)**, this statement is **incomplete** [2]. - It fails to mention the mutation's presence in other MPNs (PV and PMF) and its rarity in CML, making it a partial truth rather than the best answer. *Rare in CML.* - This statement is **medically accurate** - JAK2 V617F is indeed **rare in CML**, as CML is characterized by the **BCR-ABL1 translocation** [1]. - However, this option is **incomplete** as it omits crucial information about the mutation's presence in PV, ET, and PMF. - Knowing where the mutation IS found is more clinically useful than only knowing where it's rare. *Common in PV but not specific.* - This statement is **technically correct** - the **JAK2 V617F mutation** is found in about 95% of patients with **polycythemia vera (PV)**, making it very **common** in this condition [2]. - The phrase "**not specific**" is also accurate because the mutation is found in other MPNs (ET and PMF), not exclusively in PV [1]. - However, this option is less complete than the correct answer because it doesn't describe the full distribution pattern across all major MPNs or mention its rarity in CML. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of White Blood Cells, Lymph Nodes, Spleen, and Thymus, p. 624. [2] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 614-615.
Question 4: Knudson two-hit hypothesis is classically exemplified by
- A. Crohn disease
- B. Ulcerative colitis
- C. Retinoblastoma (Correct Answer)
- D. Melanoma
Explanation: ***Retinoblastoma*** - The **Knudson two-hit hypothesis** was **originally formulated** based on studies of **retinoblastoma** by Alfred Knudson in 1971 [1]. - It posits that **two separate mutational events** are required to inactivate **both alleles** of the **Rb tumor suppressor gene** in the same cell, leading to tumor formation [1], [2]. - This explains both **hereditary** (germline mutation + somatic mutation) and **sporadic** (two somatic mutations) forms of retinoblastoma [1], [2]. - Retinoblastoma remains the **paradigmatic example** of this hypothesis and tumor suppressor gene inactivation [2]. *Crohn disease* - This is an **inflammatory bowel disease**, not a neoplasm, with complex etiology involving genetic susceptibility, environmental factors, and immune dysregulation. - Its pathogenesis does **not follow the Knudson two-hit hypothesis**, which specifically relates to tumor suppressor gene inactivation in cancer. *Ulcerative colitis* - Similar to Crohn disease, **ulcerative colitis** is an **inflammatory bowel disease** with multifactorial etiology, not a neoplastic condition. - While chronic UC can increase colorectal cancer risk through accumulated mutations, the disease itself does **not represent the two-hit hypothesis model**. *Melanoma* - **Melanoma** is a skin cancer often linked to **UV radiation** and mutations in oncogenes like **BRAF** and tumor suppressors like **PTEN** and **CDKN2A**. [3] - While some familial melanomas involve tumor suppressor genes, melanoma is **not the classic example** used to illustrate the Knudson hypothesis—**retinoblastoma holds that distinction**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, p. 300. [2] 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. 227-228. [3] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Neoplasia, pp. 297-298.
Question 5: All the following hormones have receptors on the plasma membrane of target tissues except:
- A. Epinephrine
- B. Glucagon
- C. Estradiol (Correct Answer)
- D. Thyrotropin
Explanation: ***Estradiol*** - **Estradiol** is a **steroid hormone** derived from cholesterol, making it **lipid-soluble**. - Due to its lipid solubility, estradiol can readily pass through the **plasma membrane** and bind to **intracellular receptors** in the cytoplasm or nucleus. *Epinephrine* - **Epinephrine** is a **catecholamine hormone** and is **water-soluble**. - Water-soluble hormones cannot cross the lipid bilayer of the plasma membrane and thus bind to **receptors located on the cell surface**. *Glucagon* - **Glucagon** is a **peptide hormone** and is **water-soluble**. - Like other peptide hormones, it binds to **specific receptors embedded in the plasma membrane** to elicit its cellular effects via second messenger systems. *Thyrotropin* - **Thyrotropin**, also known as **Thyroid-Stimulating Hormone (TSH)**, is a **glycoprotein hormone** and is **water-soluble**. - TSH exerts its action by binding to **receptors on the plasma membrane** of thyroid follicular cells.
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
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.
Question 7: JAK-STAT pathway is seen in which of the following?
- A. Calcitonin
- B. Aldosterone
- C. Vasopressin
- D. Leptin (Correct Answer)
Explanation: ***Leptin*** - **Leptin** binding to its receptor activates the **JAK-STAT pathway**, regulating appetite and metabolism. - This pathway involves the phosphorylation of **STAT proteins**, which then translocate to the nucleus to induce gene expression. *Calcitonin* - **Calcitonin** activates **G protein-coupled receptors**, leading to an increase in intracellular cyclic AMP (cAMP). - Its primary role is in **calcium homeostasis**, lowering blood calcium levels. *Aldosterone* - **Aldosterone** is a steroid hormone that binds to **intracellular mineralocorticoid receptors**. - This complex then acts as a **transcription factor**, affecting gene expression in the kidneys to regulate sodium and potassium balance. *Vasopressin* - **Vasopressin** (ADH) binds to **G protein-coupled receptors** (V1 and V2 receptors). - V2 receptor activation in the kidney leads to increased **cAMP** and insertion of aquaporins, regulating water reabsorption.
Question 8: Select the correct sequence of events in the cAMP signaling pathway.
- A. Adenylyl cyclase converts ATP to cAMP, which activates PKA. (Correct Answer)
- B. PKA is activated before cAMP is formed.
- C. Adenylyl cyclase activates PKA before producing cAMP.
- D. cAMP directly activates adenylyl cyclase to produce more cAMP.
Explanation: ***Adenylyl cyclase converts ATP to cAMP, which activates PKA.*** - **Adenylyl cyclase** is an enzyme that catalyzes the conversion of **ATP (adenosine triphosphate)** into **cyclic AMP (cAMP)**, a crucial second messenger. - Subsequently, **cAMP** binds to and activates **Protein Kinase A (PKA)**, which then phosphorylates various target proteins to mediate cellular responses. *PKA is activated before cAMP is formed.* - **cAMP formation** is a prerequisite for **PKA activation**; PKA cannot be activated independently before cAMP is produced. - The binding of **cAMP** to the regulatory subunits of **PKA** is what causes the dissociation and activation of its catalytic subunits. *Adenylyl cyclase activates PKA before producing cAMP.* - **Adenylyl cyclase's** sole function in this pathway is to synthesize **cAMP** from ATP; it does not directly activate PKA. - **PKA activation** is mediated by **cAMP**, not directly by adenylyl cyclase. *cAMP directly activates adenylyl cyclase to produce more cAMP.* - While **cAMP** is a critical messenger, it does not directly activate **adenylyl cyclase** to produce more of itself in a positive feedback loop. - **Adenylyl cyclase** is typically activated by **G-protein coupled receptors (GPCRs)** binding to their ligands, which then stimulate G proteins to activate adenylyl cyclase.
Question 9: Phospholipid associated with the mechanism of hormone action is
- A. Phosphatidylcholine
- B. Phosphatidylethanolamine
- C. Plasmalogen
- D. Phosphatidylinositol (Correct Answer)
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**.
Question 10: 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
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.
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