Genetics of Common Diseases

Polygenic & Multifactorial Inheritance - Complex Trait Tango

• Polygenic vs. Multifactorial:
  • Polygenic Inheritance: Multiple genes contribute, effects are additive; no significant environmental role.
  • Multifactorial Inheritance: Multiple genes + environmental factors interact. Underlies most common chronic diseases (e.g., diabetes, hypertension).
• Liability/Threshold Model:
  • Individuals possess a 'liability' (genetic + environmental predisposition) to a trait, often normally distributed.
  • Disease manifests if liability crosses a critical biological threshold.
• Recurrence Risk (RR): Factors that ↑ RR (📌 SNoSC):
  • ↑ Proband Severity.
  • ↑ No. of affected family members.
  • Affected relative is of the less commonly affected Sex.
  • Consanguinity (higher chance of sharing predisposing genes).
• Heritability ($h^2$):
  • Proportion of total phenotypic variance in a population due to genetic variation.
  • Estimated via twin studies: $h^2 = 2 \times (r_{MZ} - r_{DZ})$, where $r_{MZ}$ and $r_{DZ}$ are concordance rates in monozygotic and dizygotic twins, respectively.

⭐ Recurrence risk in multifactorial diseases is typically higher for relatives of an affected individual of the less commonly affected sex (e.g., pyloric stenosis in females).

Type 2 Diabetes Mellitus Genetics - Sweet Gene Story

• T2DM: Multifactorial disease; interplay of genetic predisposition & environment (obesity, diet, physical inactivity).

• Key Susceptibility Genes (examples):

  • TCF7L2: Insulin secretion/processing. (📌 Strongest common risk gene)
  • PPARG: Adipogenesis, insulin sensitivity.
  • KCNJ11: Beta-cell KATP channel, insulin secretion.
  • CAPN10: Possible roles in insulin action/secretion.

• T2DM vs. MODY (Maturity Onset Diabetes of the Young)

  Feature	T2DM	MODY
  Genetics	Polygenic	Monogenic (Autosomal Dominant)
  Usual Onset	Typically >30 yrs, often associated with obesity	Typically <25 yrs, often non-obese
  Key Genes (e.g.)	TCF7L2, PPARG	HNF1A (MODY3), GCK (MODY2)

⭐ Variants in TCF7L2 confer the strongest common genetic risk for Type 2 Diabetes across multiple populations.

Cardiovascular Disease Genetics - Heartfelt Genes

• Essential Hypertension (HTN):
  • Significant genetic contribution.
  • Candidate genes: RAAS pathway (e.g., ACE, AGT).
• Coronary Artery Disease (CAD):
  • Polygenic nature; multiple genes contribute small effects.
  • Key loci from GWAS: 9p21.
  • Lipid metabolism genes: APOE variants, PCSK9.
  • Target LDL < 70 mg/dL (high-risk), < 55 mg/dL (very high-risk).
• Familial Hypercholesterolemia (FH):
  • High-impact monogenic disorder predisposing to early CAD.
  • Mutations: LDLR (most common), APOB, PCSK9.
  • Target LDL < 100 mg/dL (primary), < 70 mg/dL (with ASCVD).

⭐ The 9p21 chromosomal locus is a significant genetic risk factor for coronary artery disease, independent of traditional cardiovascular risk factors.

Common Cancer Genetics - Rogue Cell Blueprints

• Sporadic Cancers (e.g., Breast, Colorectal, Prostate, Lung): Primarily driven by an accumulation of somatic mutations.
• Low-Penetrance Alleles: Common genetic variants, identified by Genome-Wide Association Studies (GWAS), contribute small, additive risk to cancer development.
  • Polygenic Risk Scores (PRS) aggregate these effects to estimate an individual’s overall susceptibility.
• Somatic vs. Germline Mutations:
  • Somatic: Acquired, present only in tumor cells; responsible for most sporadic cancers.
  • Germline: Inherited, present in all cells; significantly ↑ predisposition (e.g., BRCA1/2, Lynch syndrome - high-penetrance mutations).
• Key Pathways Often Affected: DNA repair, cell cycle control, apoptosis.

⭐ Most common cancers arise from an accumulation of somatic mutations, but inherited germline variants can significantly increase predisposition.

High‑Yield Points - ⚡ Biggest Takeaways

• Common diseases (e.g., diabetes, hypertension, CAD) show multifactorial inheritance.
• Polygenic Risk Scores (PRS) predict individual genetic susceptibility to complex diseases.
• GWAS identify common SNPs (variants) associated with population disease risk.
• Heritability quantifies genetic contribution to phenotypic variation for a trait.
• TCF7L2 is a key susceptibility gene for Type 2 Diabetes pathogenesis.
• The APOE ε4 allele significantly increases risk for Alzheimer's disease.
• Familial hypercholesterolemia (AD disorder) leads to premature CAD.