Molecular Basis of Disease

Molecular Basis of Disease - DNA's Oopsie Daisies

• Point Mutations (Single Base):
  • Missense: Altered amino acid. E.g., Sickle cell ($GAG \rightarrow GTG$).
  • Nonsense: Premature STOP codon. E.g., β-thalassemia.
  • Silent: No amino acid change.
• Frameshift Mutations:
  • Insertion/deletion (not multiple of 3). Alters reading frame. E.g., Tay-Sachs.
• Trinucleotide Repeats:
  • Expansion of 3-base repeats. E.g., Huntington's (CAG), Fragile X (CGG).
• Chromosomal Alterations:
  • Deletions: E.g., Cri-du-chat (5p-).
  • Duplications.
  • Translocations: E.g., Philadelphia t(9;22) in CML.
• Example: Cystic Fibrosis (CFTR mutations, e.g., ΔF508).

⭐ Sickle cell anemia results from a single GAG to GTG point mutation in the β-globin gene, leading to HbS.

Molecular Basis of Disease - Silent Gene Tweaks

• Epigenetics: Gene expression changes, no DNA sequence alteration.
• DNA Methylation:
  • Cytosine (CpG islands) methylation → gene silencing.
  • Example: Fragile X (FMR1 hypermethylation).
• Histone Modifications:
  • Acetylation (HATs) → euchromatin → ↑ expression.
  • Deacetylation (HDACs) → heterochromatin → ↓ expression.
  • Methylation (HMTs) → context-dependent activation/repression.
• Genomic Imprinting:
  • Parent-of-origin specific monoallelic expression.

  ⭐ Prader-Willi (paternal 15q11-13 deletion/silencing) vs. Angelman (maternal UBE3A/ 15q11-13 deletion/mutation).

• X-Inactivation (Lyonization):
  • Random inactivation of one X in females (XX) via XIST lncRNA_.

Molecular Basis of Disease - Cellular Commotion Control

• Key Signaling Pathways & Roles:
  • Receptor Tyrosine Kinases (RTKs): e.g., EGFR, VEGFR. Activate Ras-MAPK, PI3K/AKT/mTOR pathways.
  • G-Protein Coupled Receptors (GPCRs)
  • JAK-STAT Pathway
  • Control cell growth, differentiation, survival, apoptosis.
• Dysregulation in Cancer:
  • Oncogenes: RAS, MYC (promote uncontrolled growth).
  • Tumor Suppressors: TP53 (guardian of genome), RB (cell cycle control); loss of function leads to unchecked proliferation.
• Clinical Examples:
  • EGFR mutations: Lung adenocarcinoma.
  • HER2/neu amplification: Breast cancer.
  • RAS mutations: Pancreatic, colorectal cancers.

⭐ The Philadelphia chromosome t(9;22) creates the BCR-ABL fusion gene, a constitutively active tyrosine kinase driving Chronic Myeloid Leukemia (CML).

Molecular Basis of Disease - Disease Detective Tools

• PCR (Polymerase Chain Reaction): Amplifies DNA/RNA.
  • RT-PCR: RNA detection (e.g., viral infections).
  • qPCR: Quantifies nucleic acids (e.g., viral load, gene expression).
• FISH (Fluorescence In Situ Hybridization): Detects gene presence/location in cells.
  • Applications: Aneuploidy, HER2 amplification, BCR-ABL translocation, genetic disorders.

  ⭐ FISH can detect HER2/neu gene amplification in breast cancer, guiding targeted therapy with Trastuzumab.

• Microarrays: Profile expression of thousands of genes.
  • Applications: Cancer gene expression profiling, disease signatures.
• NGS (Next-Generation Sequencing): High-throughput DNA/RNA sequencing.
  • Applications: Diagnosing genetic disorders (e.g., CF), cancer mutations, minimal residual disease.
• CRISPR-Cas9: Precise genome editing.
  • Applications: Research, potential gene therapy (e.g., thalassemias).

High‑Yield Points - ⚡ Biggest Takeaways

• Single gene disorders follow Mendelian inheritance patterns: autosomal dominant/recessive, X-linked.
• Trinucleotide repeat disorders (e.g., Huntington's) often show anticipation.
• Mitochondrial DNA disorders exhibit maternal inheritance and heteroplasmy.
• Genomic imprinting involves parent-of-origin specific gene expression (e.g., Prader-Willi/Angelman).
• Epigenetic modifications (e.g., methylation) alter gene expression without DNA sequence changes.
• Cancer involves activated oncogenes (e.g., RAS) and inactivated tumor suppressor genes (e.g., TP53).
• Apoptosis is programmed cell death mediated by caspases via intrinsic or extrinsic pathways.