Genetic Mechanisms of Cancer: Cell Cycle, Mutations, and Therapeutic Targets

Cancer Genetics and Cell Cycle Regulation

Introduction to Cancer Genetics

Cancer is a group of genetic diseases characterized by uncontrolled cell division and the accumulation of mutations. Understanding the genetic and cellular mechanisms underlying cancer is essential for developing effective therapies and prevention strategies.

• Cancer arises from mutations in genes that regulate cell growth and division.

• Both somatic and germ-line mutations can contribute to cancer, but most environmental factors cause somatic mutations.

Gene Silencing and Editing: CRISPR vs. siRNA

Mechanisms and Applications

• siRNA (small interfering RNA):

  • Induces gene knockdown (reduces gene expression).

  • Effects are temporary (lasting months).

• CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats):

  • Induces gene knockout (permanently disrupts gene function).

  • Effects are permanent.

Example: siRNA can be used for temporary suppression of a disease gene in research, while CRISPR can be used to create permanent gene knockouts in model organisms.

Cellular Organization and Differentiation

Stem Cells and Specialized Cells

• All multicellular organisms are composed of specialized cells derived from stem cells.

• Examples of specialized cells: muscle, fat, bone, blood, epithelial, nervous, immune, and sex cells.

Example: A fertilized frog egg divides into millions of cells in about 36 hours, demonstrating rapid cell division during development.

Cell Cycle and Checkpoints

Regulation of Cell Division

• The cell cycle is tightly regulated by checkpoints (G1, G2, and M).

• Checkpoints ensure cells only divide when conditions are appropriate.

• If checkpoints are disabled, cells may divide uncontrollably, leading to cancer.

Key Checkpoints:

• G1 checkpoint: Monitors cell size, nutrients, and DNA integrity.

• G2 checkpoint: Ensures DNA replication is complete and accurate.

• M checkpoint: Ensures chromosomes are properly attached to the spindle before division.

Failure to meet checkpoint requirements leads to apoptosis (programmed cell death).

Cancer: Genetic and Cellular Basis

Hallmarks and Causes of Cancer

• Cancer cells evade normal growth controls by:

  • Producing their own division signals (autocrine stimulation).

  • Losing contact inhibition (continue dividing when crowded).

  • Avoiding apoptosis (programmed cell death).

• Genetic changes in cancer cells include:

  • Uncontrolled cell division

  • Loss of anchorage and increased mobility (metastasis)

  • Angiogenesis (growth of blood vessels into the tumor)

  • Immunosuppression (evading the immune system)

• Major causes of DNA damage: heredity, viruses, UV radiation, chemicals, smoking, and errors during cell division.

Types of Genes Involved in Cancer

• Proto-oncogenes: Normal genes that promote cell division; mutations (oncogenes) act dominantly to drive cancer.

• Tumor-suppressor genes: Genes that inhibit cell division or promote apoptosis; mutations act recessively to remove growth inhibition.

• DNA-repair genes: Mutations increase genomic instability and cancer risk.

Genomic Instability and Tumor Progression

• Cancer cells have a higher mutation rate due to defective DNA repair systems.

• Genomic instability leads to chromosomal abnormalities and epigenetic changes.

• Clonal evolution: Tumors originate from a single cell; additional mutations create subclones, increasing genetic diversity within the tumor.

Therapeutic Approaches and Targeted Therapy

Cancer Treatment Options

• Hormone therapy

• Surgery

• Bone marrow transplantation

• Chemotherapy (targets dividing cells)

• Radiation therapy

• Targeted therapy (e.g., Gleevec, Her2/Herceptin)

• Immunotherapy

Targeted Therapy Example: HER2 and Herceptin

• HER2: A growth factor receptor often amplified in breast cancer.

• Herceptin: An antibody that binds HER2, blocking its activity and leading to cancer cell destruction.

• FISH analysis: Used to detect HER2 gene amplification in tumor cells.

Whole-Genome Sequencing in Cancer

• Comparing normal and cancer cells from the same patient reveals driver mutations (those that cause cancer).

• Identifying druggable targets is a major goal of personalized cancer therapy.

Tables

Comparison of CRISPR and siRNA

Feature	siRNA	CRISPR
Effect	Knockdown	Knockout
Duration	Temporary (months)	Permanently alters DNA

Major Types of Cancer in the USA (by Percentage)

Cancer Type	Percentage (Male)	Percentage (Female)
Prostate	36%	-
Breast	-	32%
Lung	14%	13%
Colon and rectum	10%	12%
Urinary	8%	-
Leukemia and lymphomas	7%	6%
All other	13%	13%

Key Terms and Concepts

• Apoptosis: Programmed cell death, a mechanism to remove damaged or unneeded cells.

• Metastasis: Spread of cancer cells to distant organs.

• Angiogenesis: Formation of new blood vessels to supply the tumor.

• Genomic instability: Increased tendency of genome alteration during cell division.

• Driver mutations: Genetic changes that contribute directly to cancer development.

Additional info: Epigenetic changes, such as DNA methylation and histone modification, can also contribute to cancer by altering gene expression without changing the DNA sequence.