Biomarkers in Psychiatry

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Biomarker Basics - Defining the Dots

  • Definition: An objective, quantifiable biological characteristic measured and evaluated as an indicator of normal biologic processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.
  • Key Types:
    • Diagnostic: Aid in confirming/classifying disorders (e.g., CSF Aβ42/tau for Alzheimer's, though not primary for psychiatric Dx).
    • Prognostic: Predict future disease course/outcome (e.g., inflammatory markers in depression).
    • Predictive: Identify likelihood of response to a specific treatment (e.g., genetic markers for antidepressant response).
    • Pharmacodynamic/Monitoring: Measure drug effect on target or disease activity (e.g., drug levels, receptor occupancy).
    • Susceptibility/Risk: Indicate predisposition to a disorder (e.g., APOE ε4 allele for Alzheimer's, some genetic risk scores for schizophrenia).
  • Ideal Characteristics: High sensitivity & specificity, reliable, reproducible, non-invasive, cost-effective, clinically relevant, easy to measure, reflects pathophysiology.
  • Challenges in Psychiatry:
    • Heterogeneity of disorders (many underlying causes for similar symptoms).
    • Symptom overlap across different diagnoses.
    • Complex, multifactorial pathophysiology.
    • Ethical considerations (e.g., genetic screening). Data modalities and opportunities in psychiatric biomarkers

⭐ Currently, no single biomarker is sufficient for diagnosing major psychiatric disorders; diagnosis remains primarily clinical, based on symptom criteria. This is a crucial point for exams, highlighting the limitations despite ongoing research.

Brain Insights - Imaging & Electrical

Neuroimaging Techniques Comparison

Neuroimaging Techniques:

TechniqueTypeMeasuresPsychiatric Relevance
MRIStructuralBrain anatomy, volumeVolumetric changes (schizophrenia: ventricular ↑; depression: hippocampal ↓)
fMRIFunctionalBOLD signal (neuronal activity)Aberrant connectivity (autism, schizophrenia), task activation (mood disorders)
PETFunctionalRadiotracers (receptors, metabolism)Dopamine D2 occupancy (antipsychotics), glucose hypometabolism (dementia)
SPECTFunctionalrCBF, limited receptor imagingrCBF abnormalities (dementia), DAT imaging (Parkinsonism)
  • EEG (Electroencephalography):
    • Records spontaneous cortical electrical activity.
    • Uses: Seizures, sleep disorders, delirium.
    • qEEG: Potential biomarkers in depression, ADHD (altered power spectra).
  • ERPs (Event-Related Potentials):
    • Time-locked EEG responses to events.
    • Key components:
      • P300: ↓ amplitude in schizophrenia, dementia (attention, context updating).
      • MMN: ↓ amplitude in schizophrenia (auditory change detection).

⭐ Reduced P300 amplitude in ERPs is a consistent finding in schizophrenia, reflecting deficits in attention and information processing.

Molecular Clues - Genes, 'Flames & Fluids

  • Genetic Markers:
    • Polygenic Risk Scores (PRS): Aggregate small effects of many DNA variants (SNPs).
    • Key Candidate Genes: BDNF (Brain-Derived Neurotrophic Factor), COMT (Catechol-O-Methyltransferase), SLC6A4 (5-HTTLPR for serotonin transporter).
    • Epigenetics: DNA methylation, histone modifications altering gene activity without DNA sequence change.
  • Inflammatory Markers ('Flames):
    • Pro-inflammatory Cytokines: ↑ Interleukin-6 (IL-6), ↑ Tumor Necrosis Factor-alpha (TNF-α).
    • C-Reactive Protein (CRP): Acute phase reactant, often elevated in mood/psychotic disorders.
    • Microglial activation & Kynurenine pathway dysregulation contributing to neuroinflammation.
  • Neurochemical & Fluid Markers ('Fluids):
    • CSF Metabolites: ↓ Homovanillic acid (HVA - dopamine), ↓ 5-Hydroxyindoleacetic acid (5-HIAA - serotonin).
    • Peripheral: Salivary/urinary cortisol (HPA axis activity), plasma BDNF levels.
    • 📌 Mnemonic: Genes, Inflammation, Neurotransmitters (GIN) for molecular clues.

⭐ Reduced brain-derived neurotrophic factor (BDNF) levels are consistently implicated in Major Depressive Disorder and influence antidepressant treatment response.

Clinical Utility - Bench to Bedside

  • Current Applications:
    • Diagnosis: Aid differential diagnosis (e.g., rule out organic causes); not standalone.
    • Prognosis: Emerging for predicting illness course or treatment response.
    • Treatment Selection: Pharmacogenomics (e.g., CYP2D6, CYP2C19 variants) guides antidepressant choice, ↓ side effects.
  • Limitations:
    • Low specificity/sensitivity for most disorders.
    • Disease heterogeneity; research replication issues.
    • Cost, accessibility.
  • Ethical Aspects:
    • Stigmatization, genetic determinism.
    • Data privacy, informed consent.
  • Future Outlook:
    • Multi-modal biomarkers (genomics, imaging, EEG).
    • AI/ML for personalized psychiatry.

    ⭐ Pharmacogenomic tests (e.g., for CYP2D6, CYP2C19) help tailor antidepressant therapy by predicting drug metabolism, potentially improving efficacy and reducing adverse events.

High‑Yield Points - ⚡ Biggest Takeaways

  • No definitive diagnostic biomarkers exist for most psychiatric illnesses yet.
  • Key research areas: genetic markers (e.g., APOE ε4 for Alzheimer's), neuroimaging, and CSF analysis.
  • ↓ CSF 5-HIAA (serotonin metabolite) is associated with suicidality and impulsivity.
  • Structural neuroimaging (MRI, CT) primarily rules out organic pathology.
  • Functional neuroimaging (fMRI, PET) shows altered brain activity patterns but is mainly a research tool.
  • Pharmacogenomics (e.g., HLA-B*1502 for carbamazepine risk) helps predict drug response/adverse effects.

Practice Questions: Biomarkers in Psychiatry

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Flashcards: Biomarkers in Psychiatry

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_____ theory assumes three sets of receptor systems, each of which functions as an antagonistic pair

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_____ theory assumes three sets of receptor systems, each of which functions as an antagonistic pair

Opponent process

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