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Pathology- Multiple Sclerosis (MS)
This guide summarizes key information on Multiple Sclerosis to aid understanding and effective study.
I. Definition & Epidemiology:
  • Definition: MS is a relapsing-remitting, demyelinating disease of the Central Nervous System (CNS). Neurological disturbances affect different CNS areas at different times. Think intermittent attacks in different locations.
  • Epidemiology:
    • Peak onset: 20-30 years old.
    • Slightly more common in females.
    • Significant geographical variation: higher incidence at higher latitudes (farther from the equator), near absence near the equator. This hints at environmental factors.
II. Aetiology & Pathogenesis:
  • Aetiology (Cause): Unknown. Leading hypothesis: immune-mediated demyelination triggered by a childhood infection in genetically susceptible individuals. Think infection + genetics = trigger.
  • Pathogenesis (Mechanism):
    • Demyelination episodes cause acute neurological deficits (symptoms) developing over days and lasting weeks.
    • Early stages: Complete recovery is typical.
    • Progression: Slower recovery, residual deficits, axonal death, and permanent disability due to extensive axonal loss. Think initial recovery, then progressive damage.
III. Clinical Presentation: (Note the association between location of demyelination and symptoms)
  • Optic Nerve: Blurred vision, loss of color vision.
  • Cerebellum: Vertigo, incoordination.
  • Brainstem: Eye movement disorders.
  • Spinal Cord: Patchy numbness/tingling, progressing to paraplegia (paralysis of lower limbs), incontinence, and sexual dysfunction.
IV. Macroscopic & Microscopic Findings:
  • Macroscopy (Gross Examination): Well-circumscribed grey plaques in CNS white matter. Common locations: optic nerves, periventricular white matter, brainstem, cervical spinal cord. Think visible lesions in specific areas.
  • Histopathology (Microscopic Examination):
    • Active plaques: Inflammatory infiltrate, myelin sheath destruction.
    • Established plaques: Complete myelin loss, reduced oligodendrocytes (myelin-producing cells), relatively normal or slightly reduced axon numbers (initially).
V. Prognosis:
  • Most patients experience progressive disease.
  • Significant complications due to disability: pneumonia, urinary tract infections, pressure sores. Think disability leads to secondary complications.
Key Concepts to Remember:
  • Relapsing-remitting: Periods of attacks followed by periods of remission (partial or complete recovery).
  • Demyelination: Damage to the myelin sheath surrounding nerve fibers, impairing signal transmission.
  • Geographical variation: Strong evidence for environmental influence.
  • Progressive axonal loss: The ultimate cause of permanent disability.
  • Location-specific symptoms: The location of the demyelination dictates the presenting symptoms.
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Pathology-Alzheimer's Disease
I. Definition & Epidemiology:
  • Definition: Alzheimer's disease (AD) is a neurodegenerative disorder clinically characterized by dementia and histopathologically by neuronal loss in the cerebral cortex, accompanied by amyloid plaques and neurofibrillary tangles. Crucially, it's the most common cause of dementia.
  • Epidemiology:
    • Incidence dramatically increases with age (5% >65 years old, 20% >80 years old).
    • Represents a significant socioeconomic burden on healthcare systems.
II. Aetiology & Pathogenesis:
  • Aetiology (Cause): The cause is unknown in most cases. A small percentage are familial, linked to genetic mutations in the amyloid precursor protein (APP) gene on chromosome 21.
  • Pathogenesis (Mechanism):
    • AD is considered a "proteinopathy," focusing on the abnormal accumulation of amyloid-beta (Aβ) and tau proteins.
    • Aβ peptides are derived from APP through secretase enzymes.
    • The exact mechanism by which Aβ and tau accumulation leads to neuronal loss remains unclear—this is a key area of ongoing research.
III. Clinical Presentation:
  • Early Stages: Begins with memory loss, especially recent memory and new learning difficulties. Progressive decline in daily activities (finances, shopping).
  • Middle Stages: Loss of motor skills impacts dressing, cooking, and cleaning.
  • Late Stages: Agitation, restlessness, wandering, and disinhibition emerge, causing distress for family and caregivers. Eventually, speech loss, immobility, and incontinence occur.
IV. Macroscopic & Microscopic Findings:
  • Macroscopy (Gross Anatomy):
    • Reduced brain weight (often <1000g).< />pan>
    • Cortical atrophy, particularly in the temporal lobe and hippocampus.
  • Histopathology (Microscopic Anatomy):
    • Key Features: Abundant neuritic plaques and neurofibrillary tangles in the cerebral cortex, alongside neuronal and synaptic loss.
    • Neuritic Plaques: Spherical collections of distorted neuronal processes around a central amyloid core, primarily composed of Aβ protein.
    • Neurofibrillary Tangles: Intracellular accumulations of paired helical filaments within neurons; mainly composed of tau protein.
V. Prognosis:
  • Death typically occurs approximately 10 years post-diagnosis, often due to complications like pneumonia.
Key Concepts to Master:
  • Amyloid-beta (Aβ) plaques: Extracellular deposits, key hallmark of AD.
  • Neurofibrillary tangles: Intracellular accumulations of tau protein, another hallmark of AD.
  • APP (Amyloid Precursor Protein): A protein implicated in familial forms of AD.
  • Tau protein: A microtubule-associated protein whose abnormal accumulation contributes to neurofibrillary tangles.
  • Proteinopathy: A disease caused by misfolded or aggregated proteins.
  • The relationship between Aβ and tau accumulation and neuronal death needs further study.
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Pathology-Parkinson's Disease
I. Definition & Epidemiology:
  • Definition: Parkinson's disease is a neurodegenerative disorder. Clinically, it's characterized by parkinsonism (a group of motor symptoms); histologically (microscopically), it's defined by neuronal loss in the brain and the presence of Lewy bodies (abnormal protein aggregates) concentrated in the substantia nigra. Crucially, parkinsonism itself is not diagnostic of Parkinson's Disease.
  • Epidemiology: Primarily affects the elderly. Prevalence is approximately 1% in individuals over 60 years old.
II. Aetiology (Causes):
  • Mostly Unknown: The cause of most Parkinson's cases remains unclear.
  • Genetic Factors (Rare Cases): Rarely, inherited mutations in the PARK1 gene (chromosome 4), which codes for α-synuclein (a protein component of Lewy bodies), are implicated.
III. Pathogenesis (Disease Mechanism):
  • Dopamine Deficiency: Neurons in the substantia nigra project to the putamen and globus pallidus (basal ganglia structures crucial for movement). These neurons release dopamine, a neurotransmitter essential for controlling movement. In Parkinson's, dopamine release is significantly reduced.
  • Movement Disorder: The lack of dopamine leads to the characteristic movement disorders.
IV. Presentation (Symptoms):
  • Parkinsonism: The classic triad of symptoms includes:
    • Tremor: Involuntary shaking.
    • Rigidity: Stiffness and resistance to movement.
    • Bradykinesia: Slowness of movement.
  • Important Note: Parkinsonism can result from various causes (drugs, toxins, infections, trauma) and isn't solely indicative of Parkinson's disease.
V. Macroscopic & Microscopic Findings:
  • Macroscopy (Gross Examination): Shows pallor (loss of color) in the substantia nigra and locus ceruleus (brain regions).
  • Histopathology (Microscopic Examination): Reveals:
    • Loss of pigmented neurons in the substantia nigra.
    • Presence of Lewy bodies within remaining neurons.
VI. Prognosis & Treatment:
  • Treatment: Dopamine-replacement therapies can alleviate parkinsonism symptoms. However, these treatments do not slow or stop the disease's progression.
  • Variable Progression: The rate of disease progression varies significantly between individuals.
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Pathology- Huntington's Disease
I. Definition & Epidemiology:
  • Definition: HD is an inherited, neurodegenerative disorder resulting from a mutation in the HTT gene. This mutation leads to the production of a dysfunctional huntingtin protein.​
  • Epidemiology:
    • Prevalence: 5-10 per 100,000 globally, with geographical variation.
    • Onset: Typically 35-45 years, but can occur at any age.
    • Sex: Affects men and women equally.
    • Inheritance: Autosomal dominant (only one affected copy of the gene is needed to develop the disease).
II. Genetics:
  • HTT Gene: Contains a CAG trinucleotide repeat sequence.
    • Normal HTT: Less than 36 CAG repeats.
    • Mutant HTT: More than 36 CAG repeats. The number of repeats directly correlates with disease severity and age of onset (more repeats = earlier onset, greater severity).
  • Anticipation: The number of CAG repeats tends to increase in subsequent generations, leading to earlier onset and more severe disease in offspring. This phenomenon is known as anticipation.
III. Pathogenesis:
  • Huntingtin Protein (HTT): The protein encoded by the HTT gene has various cellular functions and is expressed throughout the body, but is most concentrated in the brain and testes.
  • Mechanism of Disease: Mutated huntingtin protein is cytotoxic (toxic to cells), particularly affecting neurons in the caudate nucleus and putamen (basal ganglia structures crucial for movement control).
IV. Clinical Presentation:
  • Early Symptoms: Uncontrolled, jerky, involuntary movements (chorea).
  • Progressive Disease: Over time, HD leads to progressive motor, neuropsychiatric (mood disorders, cognitive issues), and cognitive decline, ultimately culminating in dementia.
V. Macroscopic & Microscopic Findings:
  • Macroscopy: Significant atrophy (shrinking) of the caudate nucleus and putamen. Cortical atrophy may also be present.
  • Histopathology: Marked neuronal loss in the caudate nucleus. Surviving neurons contain excessive amounts of the mutated huntingtin protein.
VI. Prognosis:
  • Survival: Average survival is approximately 20 years from symptom onset, but varies depending on the number of CAG repeats (more repeats = shorter survival).
  • Cause of Death: Often pneumonia or cardiac failure due to mutated huntingtin expression in cardiac muscle.
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Pathology-Motor Neurone Disease (MND)
I. Definition & Epidemiology:
  • Definition: MND is a group of neurodegenerative diseases causing selective loss of motor neurons. This means the nerve cells that control voluntary muscle movement are progressively destroyed.
  • Epidemiology:
    • Rare disease: Annual incidence 1-5 per 100,000.
    • Slight male predominance.
    • Typical onset: 50-70 years old.
II. Aetiology (Causes):
  • Mostly Idiopathic: In most cases (90%), the cause is unknown.
  • Familial (Inherited): Approximately 10% of cases are inherited, showing a genetic component.
  • Genetic Links: Several genes associated with familial MND have been identified, including SOD1, TDP-43, and FUS.
III. Pathogenesis (Disease Mechanisms):
  • Poorly Understood: The precise mechanisms driving MND remain largely unclear, even with insights from familial cases.
  • RNA Metabolism Dysfunction: A leading hypothesis points to defects in RNA metabolism as a crucial factor in motor neuron degeneration. This is based on the properties of TDP-43 and FUS proteins.
  • TDP-43 & FUS: Both are RNA/DNA-binding proteins with similar structures. Their dysfunction is strongly implicated in MND development.
IV. Clinical Presentation:
  • Muscular Symptoms: Asymmetrical muscle weakness and wasting (atrophy), muscle twitching (fasciculations), and muscle stiffness (spasticity) in limbs are common early signs.
  • Bulbar Symptoms: Difficulty with swallowing (dysphagia), chewing, speaking (dysarthria), coughing, and breathing (dyspnea) are characteristic as the disease progresses and affects the muscles controlling these functions.
  • Cognitive Changes: Cognitive impairment can also occur in some cases.
V. Macroscopic & Microscopic Findings:
  • Macroscopy (Gross Examination): The anterior roots of the spinal cord (which carry motor neuron axons) are atrophied (shrunken).
  • Histopathology (Microscopic Examination):
    • Selective loss of motor neurons in the motor cortex (brain) and anterior horns of the spinal cord is the defining feature.
    • In sporadic (non-inherited) MND, remaining motor neurons often contain abnormal protein inclusions that include ubiquitin and TDP-43.
VI. Prognosis:
  • Progressive & Fatal: MND is typically progressive, leading to death within a few years.
  • Cause of Death: Aspiration pneumonia (lung infection from inhaling food or saliva due to swallowing difficulties) is a frequent cause of death.




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Pathology- Creutzfeldt-Jakob Disease (CJD)
I. Definition & Epidemiology:
  • Definition: CJD is a rare, fatal spongiform encephalopathy caused by the accumulation of misfolded prion protein (PrP) resistant to normal cellular breakdown. It's the most common human prion disease.
  • Epidemiology: Extremely rare, with an annual incidence of approximately 1 per 1,000,000.
II. Etiology (Causes):
  • Sporadic CJD: Spontaneous, random misfolding of normal PrP into the abnormal PrP form. The exact mechanism remains unclear.
  • Familial CJD: Inherited mutations in the PRNP gene increase the likelihood of PrP misfolding.
  • Variant CJD (vCJD): Believed to be transmitted via consumption of beef contaminated with PrPSc from cattle with bovine spongiform encephalopathy (BSE, "mad cow disease").
III. Pathogenesis (Disease Mechanism):
  • Misfolded Protein Propagation: The presence of abnormal PrPSc acts as a template, causing normal PrPC to misfold into the abnormal form.
  • Exponential Growth & Cell Death: This process leads to an exponential increase in PrPSc, ultimately causing neuronal cell death and the characteristic brain damage.
IV. Clinical Presentation:
  • Sporadic CJD: Typically affects middle-aged and elderly individuals. Onset is marked by rapidly progressing neurological symptoms.
  • Variant CJD (vCJD): Affects younger individuals (<30 years old). initially presents with psychiatric symptoms, followed by cerebellar ataxia (problems coordination and balance) dementia. < />pan>Key difference from sporadic CJD.
V. Histopathology (Microscopic Findings):
  • Sporadic CJD & vCJD: Both show spongiform changes (vacuolation of grey matter), neuronal loss (death of nerve cells), and gliosis (scarring in the brain).
  • Variant CJD (vCJD only): The presence of numerous "florid plaques" composed of amyloid forms of PrPSc is a key distinguishing neuropathological feature. These are absent in other CJD forms.
VI. Prognosis:
  • Currently, no effective treatment exists for CJD. The disease is invariably fatal.​

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Pathology-Guillain-Barré Syndrome (GBS)
I. Definition & Epidemiology:
  • Definition: Guillain-Barré syndrome (GBS) is an acute demyelinating polyneuropathy. This means it's a rapid onset disorder affecting many nerves, characterized by damage to the myelin sheath (the protective covering around nerves). It typically appears 1-2 weeks after a respiratory or gastrointestinal infection.
  • Rarity: GBS is a rare disease with an annual incidence of only 1-2 cases per 100,000 people.​​
II. Etiology (Causes):
  • Infectious Triggers: The most common triggers are infections caused by:
    • Campylobacter jejuni
    • Mycoplasma
    • Cytomegalovirus (CMV)
    • HIV
    • Varicella-zoster virus (VZV)
    • Epstein-Barr virus (EBV)
  • Other Associations: GBS can also be associated with:
    • Vaccination (although this is rare and the benefits of vaccination far outweigh the risk)
    • Surgery
    • Malignancy (cancer)
  • Idiopathic Cases: In a significant number of cases, no clear cause can be identified.
III. Pathogenesis (Mechanism):
  • Cross-Reactivity: The leading theory suggests that the immune system, while fighting off an infection, mistakenly attacks the myelin sheath of peripheral nerves. This happens because an antigen (a protein on the surface of the pathogen) is similar to an antigen on the nerve myelin, leading to an immune "cross-reaction".
  • Demyelination & Polyneuropathy: This attack on the myelin causes demyelination (loss of the myelin sheath), resulting in an acute polyneuropathy (a condition affecting many peripheral nerves).
IV. Clinical Presentation:
  • Initial Symptoms: GBS typically begins with sudden onset of tingling and numbness in the fingers and toes (distal paresthesia).
  • Progressive Weakness: Over several weeks, the muscle weakness spreads proximally (from the extremities towards the trunk and center of the body), a hallmark characteristic of GBS.
  • Major Risk: Progressive ventilatory failure (difficulty breathing) is the most serious complication and a leading cause of death. This necessitates close monitoring and potential respiratory support.
V. Prognosis:
  • Recovery Rate: Approximately 85% of patients make a complete or near-complete recovery.
  • Long-Term Disability: About 10% of patients are unable to walk without assistance one year after diagnosis.
Key Concepts for Understanding and Studying:
  • Acute vs. Chronic: GBS is acute, meaning it develops rapidly. Contrast this with chronic conditions which develop slowly over time.
  • Polyneuropathy: Understand that "poly" means many and "neuropathy" refers to nerve dysfunction. Therefore, polyneuropathy means widespread nerve dysfunction.
  • Demyelination: Focus on the concept of immune system cross-reactivity leading to damage of the myelin sheath and subsequent nerve dysfunction.
  • Ascending Weakness: The progressive spread of weakness from the periphery (fingers and toes) to the center of the body is a critical diagnostic feature.
  • Ventilatory Failure: Recognize this as the most significant threat to life in GBS patients.
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Pathology- Myasthenia Gravis
I. Definition:
  • Myasthenia gravis (MG) is an autoimmune disease. This means the body's own immune system mistakenly attacks healthy tissues.
  • Specifically, autoantibodies (self-attacking antibodies) target the nicotinic acetylcholine receptor (nAChR) at the neuromuscular junction. This is the point where nerves meet muscles.​
II. Epidemiology (Disease Frequency and Distribution):
  • Rare: Annual incidence is low (20 per 100,000).
  • Age and Gender: More common in women under 50 and men over 50. Note the age difference between genders.
III. Aetiology (Cause):
  • Unknown Trigger: The precise cause of autoantibody production is unclear. This is a crucial point to remember – we don't fully understand why the immune system attacks the nAChR.
  • Thymus Involvement: A significant link exists between MG and the thymus gland (located in the chest). Up to 75% of patients have thymus abnormalities:
    • Thymoma: A tumor of the thymus.
    • Hyperplasia: Enlargement of the thymus.
  • Thymus as a Source?: It's hypothesized that the abnormal thymus may be the site where these autoantibodies are produced.
IV. Pathogenesis (Mechanism of Disease):
  • Neuromuscular Junction: Recall that the nAChR is crucial for muscle contraction. Acetylcholine, a neurotransmitter, binds to it, triggering muscle activation.
  • Autoantibody Interference: The autoantibodies bind to the nAChR, preventing acetylcholine from binding effectively. This reduces the muscle's ability to contract.
  • Result: Impaired muscle contraction and weakness.
V. Presentation (Symptoms):
  • Key Symptom: Muscle Fatigue: Weakness that worsens with activity and improves with rest is the hallmark of MG. This is a fundamental characteristic.
  • Muscle Groups Affected (in order of typical involvement):
    1. Extraocular muscles (eye muscles) – leading to diplopia (double vision) and ptosis (drooping eyelids).
    2. Bulbar muscles (muscles controlling swallowing and speech) – difficulty swallowing (dysphagia) and speech impairment (dysarthria).
    3. Facial muscles
    4. Neck muscles
    5. Limb girdle muscles (shoulders and hips)
    6. Trunk muscles
  • Diagnostic Challenge: Symptoms can be subtle and easily mistaken for other conditions, making diagnosis difficult. This highlights the importance of considering MG in patients with unexplained muscle weakness.
VI. Prognosis (Outcome):
  • Generally Favorable: Most patients respond well to treatment. The disease is typically relapsing (symptoms come and go) but not progressive (does not steadily worsen over time).
  • Thymoma Exception: Patients with aggressive thymoma may have a poorer prognosis due to the cancerous nature of the tumor.
Key Concepts to Master:
  • Autoimmune nature: MG is caused by the body attacking its own nAChRs.
  • Neuromuscular junction dysfunction: The problem lies in the communication between nerves and muscles.
  • Muscle fatigue: The defining symptom.
  • Thymus gland involvement: A significant association exists between MG and thymic abnormalities.
  • Variable presentation: Diagnosis can be challenging due to subtle or atypical symptoms.
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Oncology - Cancer Risk Factors
This study guide summarizes external factors contributing to cancer risk, categorized for easier understanding and recall.
I. Smoking
  • The Biggest Culprit: Tobacco smoking is the leading known carcinogen and the most significant preventable cause of premature death in developed nations.​
  • Cancer Types Affected: Responsible for a substantial percentage of global cancer cases (15% overall, >30% in men from developed countries). Strongly linked to lung cancer (790% increase in risk for smokers), but also causes mesothelioma, myeloid leukemia, various gastrointestinal cancers (oral, esophageal, gastric, pancreatic), ENT cancers (pharyngeal, laryngeal, nasopharyngeal, nasal cavity, paranasal sinuses), urinary tract cancers (bladder, renal), liver cancer, and cervical cancer.
  • Synergistic Effects: Smoking amplifies the cancer risk associated with other carcinogens like alcohol and asbestos.
  • Global Impact: Increasing global cigarette consumption, particularly among women and in developing countries, is expected to further increase the cancer burden.
  • Cessation & Passive Smoking: Quitting smoking reduces cancer risk, though cessation programs have limited success. Passive smoking also significantly contributes (15-30% of lung cancer in non-smokers).
II. Alcohol Consumption
  • Cancer Links: Alcohol is a causative factor in several cancers:
    • Head and neck cancer (oropharynx): Risk increases linearly with alcohol intake.
    • Esophageal cancer (squamous cell): Risk strongly related to alcohol consumption, even at low levels.
    • Breast cancer: Moderate to heavy intake (>2 units/day) linked to increased incidence (2-10% of cases potentially alcohol-related). Mechanism unclear, possibly through increased estrogen and androgen levels.
    • Hepatocellular carcinoma (HCC): Moderate to heavy drinking increases risk through alcoholic cirrhosis. The link is less clear in the absence of cirrhosis.
  • Confounding Factor: The strong association between alcohol and tobacco use complicates the assessment of each substance's individual contribution to cancer risk. Their effects are synergistic (multiplicative), not just additive.
III. Diet and Obesity
  • Obesity's Role: Adult obesity is a risk factor for numerous solid tumors, including endometrial, post-menopausal breast, kidney, esophageal, and colorectal cancers. It's also implicated in prostate, liver, ovarian, gastric, and pancreatic cancers, potentially contributing to up to 20% of cancer deaths in developed countries. The mechanisms are not fully understood. The obesity epidemic will likely increase the cancer burden further.
  • General Dietary Factors:
    • Vegetables: High vegetable consumption may reduce colon cancer risk (especially distal tumors), but evidence is not strong for other cancers.
    • Fruits: Consistent evidence linking high fruit intake to reduced cancer risk is lacking.
    • Red Meat: High red meat consumption increases colonic and rectal cancer risk.
    • Fiber: The previously suggested link between high-fiber diets and reduced colon cancer risk might be confounded by factors like folate intake, and needs further confirmation.
    • Fat: Research continues into how different dietary fats influence cancer risk, with saturated and trans fats (in meats and some dairy) being of particular concern.
  • Specific Dietary Risk Factors: Dietary changes can significantly impact cancer incidence:
    • Salt Fish: Reducing intake could reduce nasopharyngeal cancer by 33-50% in developing countries.
    • Aflatoxins: (Mycotoxin found in contaminated food) Halving median daily intake could reduce HCC incidence by up to 40% in Africa and Asia.
IV. Exercise
  • Inactivity & Cancer Risk: Physical inactivity increases the risk of many adult cancers, with the strongest evidence for breast and colon cancers.
  • Benefits of Exercise: The benefits of increased exercise are independent of obesity. Exercise also improves outcomes and reduces recurrence rates in cancer patients (strongest evidence in breast cancer).
Key Concepts to Remember:
  • Synergistic effects: Multiple risk factors multiply, rather than simply add to, the risk.
  • Confounding factors: It is difficult to isolate the independent effect of one risk factor when others are also present (e.g., alcohol and smoking).
  • Limited evidence: While many associations are noted, the mechanisms and strengths of the relationships are not always well-understood. More research is needed.
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Oncology- Oncogenes and Tumor Suppressor Genes
I. Oncogenes
A. Definition: Genes whose normal function promotes cell proliferation and inhibits apoptosis (programmed cell death). When mutated or overexpressed, they contribute to uncontrolled cell growth, a hallmark of cancer.

B. Key Examples & Mechanisms of Activation:
  • RAS family (HRAS, KRAS, NRAS): Normally activated by growth factor signaling (e.g., EGFR) through GTP binding. Mutations (codons 12, 13, 61) lock RAS in the active GTP-bound state, leading to constitutive signaling, independent of growth factors.
  • Other Oncogenes: BRAF, RET, SRC, ERBB2/3, MYC.
  • Mechanisms of Overexpression (beyond mutation):
    • Gene amplification: Increased gene copy number leads to higher protein levels (e.g., ERBB2 in breast cancer, SRC in colon cancer).
    • Chromosomal fusions: Oncogene fused to a highly active promoter, resulting in overexpression in specific tissues.
    • Deregulation of signaling pathways: Oncogene expression triggered by activation of pathways like RAS and Wnt (e.g., MYC).
II. Tumor Suppressor Genes
A. Definition: Genes that normally restrain cell proliferation, promote apoptosis, or repair DNA damage. Loss of function contributes to cancer development.
B. Two Main Classes:
  1. Regulators of Cell Growth and Apoptosis: These genes directly inhibit cell proliferation or promote apoptosis. Examples include TP53, APC, and PTEN. Loss of function leads to uncontrolled growth, similar to oncogene activation. Note that APC's role is particularly significant in intestinal homeostasis.
  2. DNA Repair Genes: Genes involved in repairing DNA damage. Loss of function leads to a "mutator phenotype" – increased genomic instability and accumulation of mutations, increasing the likelihood of cancer development. Examples include BRCA1/2 (involved in non-homologous end joining) and mismatch repair genes (e.g., MLH1, MSH2).
C. Tumor Suppressor Gene Inactivation:
  • Two-hit hypothesis (previously dominant view): Both copies of a tumor suppressor gene need to be mutated for a significant effect. This often occurs through somatic mutations in one allele, and inheriting a mutated copy (germline mutation) in the other.
  • Germline mutations: Inherited mutations increase cancer risk; individuals with one mutated copy are predisposed to developing cancer in specific tissues upon loss of the second allele (e.g., Li-Fraumeni syndrome (TP53), FAP (APC), Cowden disease (PTEN)). The specific tissue affected often reflects the gene's role in that tissue.
  • Epigenetic inactivation: Mechanisms like methylation can silence tumor suppressor gene expression without altering the DNA sequence itself (e.g., MLH1 in colorectal cancer).
  • Mutant p53: In addition to loss of p53 function, mutations can create a non-functional but stable protein that exhibits oncogenic activity, interacting with other p53 family members (p63, p73) to disrupt their function and promote metastasis.
D. Key Examples & Functions:
  • TP53: The most frequently mutated tumor suppressor gene. Its role in apoptosis, growth arrest, DNA repair, and genomic stability is vital. Loss of p53 function contributes to multiple aspects of tumor progression.
  • APC: Negative regulator of the Wnt signaling pathway, crucial for intestinal homeostasis. Loss of APC contributes to colorectal cancer.
  • PTEN: Involved in cell growth regulation; loss contributes to several cancers.
  • BRCA1/2: Key roles in DNA repair; mutations increase risk of breast, ovarian, and pancreatic cancers.
  • Mismatch repair genes (MSH2, MLH1): Involved in DNA mismatch repair; mutations predominantly increase colorectal cancer risk. MLH1 inactivation via methylation is common in microsatellite instability (MSI) cancers.
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Kembara Xtra 

Facts about medicine and its subtopic such as anatomy, physiology, biochemistry, pharmacology, medicine, pediatrics, psychiatry, obstetrics and gynecology and surgery.