Cancer biology essentials: oncogenes vs tumor suppressors, RB and APC pathways, colon cancer progression, and targeted therapy explained

Overview

This MIT OpenCourseWare lecture introduces cancer as a stepwise degeneration driven by mutations that disrupt normal cell and tissue behavior. It classifies cancer genes into oncogenes, tumor suppressors, and caretaker genes, and uses concrete examples to illustrate how mutations shift normal growth control toward cancer.

Key takeaways

Students learn how proto-oncogenes become oncogenes, the loss of tumor suppressor function promotes cancer, and how caretaker gene failures compromise genome integrity. The talk also covers how a single pathway, the G1 to S transition, integrates growth signals to decide cell division, highlighting the RB gene as a tumor suppressor and the concept of loss of heterozygosity.

Introduction to cancer genetics

This lecture from MIT OpenCourseWare explains that cancer is a progressive disease caused by mutations that deregulate normal cell and tissue behavior. It frames cancer as a stepwise degeneration and introduces three classes of genes implicated in cancer: oncogenes, tumor suppressors, and caretaker genes. Oncogenes promote growth and survival when mutated to constitutively active forms, whereas tumor suppressors inhibit growth or promote cell death and are typically lost or inactivated in cancer. Caretaker genes maintain genome integrity by repairing DNA and ensuring proper chromosome segregation; loss of function in these genes elevates mutation rates and drives cancer development. A BRCA1 example is used to illustrate how caretaker gene defects can foster oncogenic mutations or loss of tumor suppressors, fueling cancer progression.

RB, E2F and the start of the cell cycle

The talk then delves into a specific signaling pathway that governs the G1 to S transition, known as start. It outlines how a G1 cyclin, in complex with a cyclin-dependent kinase (CDK), phosphorylates the RB protein. In its hypophosphorylated state, RB binds E2F and represses G1S cyclin transcription; when RB is phosphorylated, E2F is liberated, G1S cyclin expression is activated, and cells commit to S phase. RB is identified as a tumor suppressor, with the lecture highlighting its historical role as the first cloned tumor suppressor and its connection to retinoblastoma. The speaker emphasizes the concept that tumor suppressors act recessively at the cellular level, yet the organism shows a dominant inheritance pattern for predisposition in many cancers, using retinoblastoma to illustrate this nuance and the idea of loss of heterozygosity as a driver of tumor formation.

Colon cancer and the APC/WNT axis

Moving to tissue level, the lecture uses the colon as an example to show how tissue architecture, stem cell niches, and cell turnover shape tumor development. A key point is that dysregulation of the WNT signaling pathway disrupts the renewal of colon epithelium. The APC gene, a tumor suppressor, is a central component of the WNT destruction complex that keeps beta-catenin in check. Loss of APC function yields constitutive WNT signaling, causing cells to remain in the crypt and accumulate mutations, forming benign polyps that can progress to carcinoma. The talk introduces familial adenomatous polyposis as a hereditary form linked to APC mutations and explains how sequential mutations in oncogenes and tumor suppressors drive progression from benign lesions to malignant cancer.

Targeted cancer therapies

The final sections touch on targeted therapies, including the example of chronic myelogenous leukemia (CML) driven by the BCR-ABL fusion from a translocation. The kinase becomes constitutively active, and inhibitors such as Gleevec can effectively treat CML by blocking its activity. This case illustrates how understanding cancer signaling pathways leads to targeted interventions, in contrast to traditional chemotherapy. The lecturer notes that more therapies exist beyond surgery and chemotherapy, with ongoing exploration into pathway-specific strategies to curb cancer progression.

Takeaway

The lecture ties cellular level mechanisms to tissue level outcomes and highlights the importance of classifying cancer genes, understanding mutation types, and applying pathway-targeted approaches to treatment. It also sets up next discussions on invasion and metastasis, and further cancer therapies in future sessions.