Chapter 07: Tumor Suppressor Genes - Study Notes

Chapter 07: Tumor Suppressor Genes

Introduction

  • This chapter focuses on tumor suppressor genes, their functions, and their roles in cancer development.

Retinoblastoma

  • Familial Retinoblastoma: Involves a genetic predisposition where individuals inherit one mutant Rb allele.
  • Sporadic Retinoblastoma: Occurs without a family history; requires two somatic mutations of the Rb gene.
Mutational Mechanism
  • Genotype of Fertilized Egg: Contains one mutant Rb allele inherited from a parent.
  • Somatic Mutation: Small mutations in every cell are termed somatic mutations. In sporadic cases, both alleles must be mutated for the disease to manifest.
  • Bilateral Disease: Characterized by two mutant Rb gene alleles (usually familial).
  • Unilateral Disease: Characterized by one mutant Rb allele and one normal allele (typically sporadic).
Cell Cycle Phases and Retinoblastoma
  • S Phase: Chromosome replication occurs; if a cell has a mutant Rb allele, it may undergo mitotic recombination.
  • G2 and M Phases: Further detail into how defects can lead to loss of heterozygosity in daughter cells, resulting in loss of functional Rb gene copies.
  • Loss of Heterozygosity: Events like non-disjunction during mitosis result in cells lacking functional Rb gene copies, which is critical in the development of cancer.

Ras and Tumor Suppressor Pathways

  • Inactive Ras Signal: Illustrates the role of Ras in the signaling pathway. Ras-GAP activates the hydrolysis of GTP to GDP, promoting inactivation.
  • Arginine Finger in Ras: This structural component in Ras facilitates the hydrolysis.
  • Downstream Effectors: Once active, Ras interacts with various downstream effectors to propagate the growth signal.

Wnt Signaling Pathway

  • Components of Wnt Pathway: Involves proteins like LRP, Frizzled, and Dishevelled, which are critical for signaling in cellular functions.
  • Inactive GSK-3β: Without active GSK-3β due to Wnt signaling, beta-catenin accumulates and translocates to the nucleus, affecting gene transcription via Tcf/Lef.
  • Progenitor Cells: This pathway is crucial for regulating cellular proliferation, differentiation, and ensuring correct cell migration in gut crypts.
Impact of Mutations on Signaling
  • APC or β-catenin Mutations: Their mutations disrupt normal regulation leading to progenitor-like phenotypes and accumulate at sites of polyp formation.

NRF2 and Cellular Stress Response

  • Normal Conditions: NRF2 associates with KEAP1 preventing its translocation to the nucleus, promoting the degradation of NRF2 via the proteasome.
  • Stress Conditions: Exposure to reactive oxygen species (ROS), carcinogens or compounds like sulforaphane disrupts the KEAP1-NRF2 interaction, allowing NRF2's accumulation.
  • Antioxidant Response: Once in the nucleus, NRF2 initiates the transcription of antioxidant and detoxification genes, which are defenses against oxidative stress.

Conclusion

  • The chapter reinforces the roles of different tumor suppressor genes and pathways in cancer biology, emphasizing how alterations lead to tumorigenesis.

Figures and Illustrations

  • Figures from Various Pages:
    • Illustrations of retinoblastoma, mutations' effect on the Rb gene, Ras signaling pathway, Wnt signaling pathway, and NRF2 interactions provided various visual aids for understanding complex processes.
    • These figures are accompanied by sources and permissions from established authors, enhancing the credibility of depicted pathways and processes.

Reference

  • Weinberg, R.A. (2023). The Biology of Cancer (3rd ed.). W. W. Norton & Company.