Rucete ✏ Campbell Biology In a Nutshell
Unit 3 GENETICS — Concept 18.5 Cancer Results from Genetic Changes That Affect Cell Cycle Control
Cancer arises when cells lose control over growth and division due to genetic mutations. Normal cellular controls that regulate growth become disrupted, often involving genes known as proto-oncogenes and tumor-suppressor genes.
Genes Associated with Cancer
- Proto-oncogenes: Promote normal cell division.
- Tumor-suppressor genes: Inhibit cell division.
- Mutations can convert proto-oncogenes into oncogenes, leading to overactive growth signals.
- Loss of tumor-suppressor gene function removes growth inhibition, enabling uncontrolled cell division.
How Proto-Oncogenes Become Oncogenes
- Epigenetic Changes: Chromatin modifications activate genes inappropriately.
- Translocations: Gene moved near active promoter, increasing expression.
- Gene Amplification: Extra gene copies produce too much protein.
- Point Mutations:
- Regulatory region mutations increase expression.
- Coding region mutations yield hyperactive or stable proteins.
Tumor-Suppressor Genes and Their Functions
- Repair damaged DNA.
- Maintain cell attachment to surroundings.
- Inhibit cell cycle progression.
- Loss of these functions promotes cancer development.
Key Examples: Ras and p53 Genes
- Ras proto-oncogene:
- Encodes protein in growth signaling pathways.
- Mutations cause constant growth signals.
- Seen in ~30% of human cancers.
- p53 tumor-suppressor gene:
- Triggers DNA repair, cell cycle arrest, or apoptosis.
- Loss leads to unchecked division and tumor growth.
- Inactivated in over 50% of cancers.
The Multistep Model of Cancer Development
- Cancer arises through multiple mutations over time.
- Example: Colorectal cancer progresses from polyps to tumors due to mutations in genes like APC, SMAD4, p53, and ras.
- Age increases risk as mutations accumulate.
Inherited Cancer Risk
- Inherited mutations raise cancer susceptibility:
- BRCA1 and BRCA2: Breast cancer risk.
- APC and HNPCC: Colon cancer syndromes.
Viruses and Cancer
- Some viruses contribute to cancer by:
- HPV: Cervical cancer.
- Epstein-Barr, HTLV-1: Associated with specific cancers.
Modern Approaches and Treatment
- Genomic analysis guides diagnosis and therapy:
- Breast cancer classified by receptor status (ER, PR, HER2).
- Targeted drugs like Herceptin treat specific mutations.
In a Nutshell
Cancer results from genetic alterations disrupting normal controls over cell division. Proto-oncogenes and tumor-suppressor genes are commonly mutated, leading to uncontrolled growth. Cancer develops progressively through accumulated genetic mutations, influenced by heredity, environmental factors, and viruses. Modern genomics-based approaches offer more targeted, personalized cancer treatments.