Cancer Genetics: Mechanisms and Genetic Basis

Cancer Genetics

Introduction to Cancer

Cancer is a major health concern and the second most common cause of death in Western countries. It is fundamentally a disease of uncontrolled cell division, resulting from the accumulation of genetic mutations. Understanding the genetic basis of cancer is essential for studying its development, progression, and potential treatments.

• Cancer is characterized by abnormal cell proliferation and loss of normal growth control.

• Genetic mutations drive the transformation of normal cells into cancerous cells.

• Both solid tumors and blood cancers (hematological malignancies) exist, differing in their origin and spread.

Types of Cancer: Solid vs. Blood Cancers

Cancers are classified based on their tissue of origin and behavior in the body.

• Solid cancers: Originate in organs or tissues and form localized tumors.

• Blood cancers (hematological malignancies): Begin in bone marrow or immune system cells and spread through blood and lymph, typically without forming solid tumors.

• Examples of blood cancers include leukemia and lymphoma.

Mechanisms of Cancer Development

Cancer arises through multiple genetic and cellular mechanisms that disrupt normal cell regulation.

• Cell cycle deregulation: Loss of control over cell division.

• Apoptosis evasion: Cancer cells avoid programmed cell death.

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

• Tissue invasion and metastasis: Cancer cells invade surrounding tissues and spread to distant sites.

• Immune system evasion: Cancer cells avoid detection and destruction by the immune system.

Genetic Defects Affecting Cell-Cycle Regulation

One of the hallmarks of cancer cells is the loss of control over cell proliferation, which is tightly regulated in normal cells.

• Cell proliferation: The process of cell growth and division, essential for development and tissue repair.

• Cancer cells acquire mutations that disrupt the normal regulation of the cell cycle, leading to uncontrolled growth.

Control of Apoptosis

Apoptosis is a genetically programmed process of cell death that eliminates damaged or unnecessary cells.

• When DNA replication, repair, or chromosome assembly is defective, normal cells halt the cell cycle until the issue is corrected.

• If damage is irreparable, cells initiate apoptosis to prevent propagation of mutations.

• Cancer cells often evade apoptosis, allowing survival and proliferation despite genetic damage.

Example: In the provided image, normal white blood cells are compared to cells undergoing apoptosis, which show grape-like clusters called apoptotic bodies.

Cancer as a Genetic Disease

Cancer is fundamentally a genetic disease, but not all genetic changes are inherited. The distinction between somatic and germline mutations is crucial.

• Both normal and cancer cells contain the same set of genes; differences arise from which genes are turned on or off.

• Cancer cells exhibit high levels of genomic instability, accumulating multiple mutations over time.

Proto-Oncogenes and Oncogenes

Genes that regulate cell growth and division can become cancer-promoting when mutated.

• Proto-oncogenes: Normal genes that promote cell proliferation and differentiation. They act as the "accelerator pedal" for the cell cycle.

• Oncogenes: Mutated or overexpressed proto-oncogenes that drive uncontrolled cell division. Activation typically involves a gain-of-function mutation, requiring only one mutated copy.

• Activation mechanisms: Point mutations (e.g., in RAS genes), gene amplification (e.g., HER2 in breast cancer).

Tumor Suppressor Genes

Tumor suppressor genes act as the "brake pedal" for cell growth, preventing tumor formation.

• Tumor suppressor genes: Inhibit cell growth and division; loss of function leads to cancer.

• Inactivation typically requires loss-of-function mutations in both gene copies (alleles).

• Examples:

  • TP53 ("guardian of the genome"): DNA damage response, apoptosis.

  • RB1: Controls G1/S checkpoint.

  • APC: Regulates Wnt signaling (mutated in colon cancer).

• Knudson’s Two-Hit Hypothesis: Both alleles must be inactivated for loss of function (e.g., RB1 in retinoblastoma).

Genetic Basis of Cancer: Somatic vs. Germline Mutations

Cancer-causing mutations can arise in somatic (non-reproductive) cells or be inherited through germline mutations.

• Somatic mutations: Occur in non-reproductive cells; cause sporadic cancers.

• Germline mutations: Inherited; cause familial/hereditary cancers (e.g., BRCA1 mutations).

• Most cancers are caused by somatic mutations; only 5-10% are associated with inherited germline mutations.

Driver vs. Passenger Mutations

Cancer cells accumulate many mutations, but only a subset directly contribute to cancer development.

• Driver mutations: Provide a growth advantage to tumor cells and are essential for cancer progression.

• Passenger mutations: Accumulated due to genomic instability but do not contribute to the cancer phenotype.

Hereditary Cancer Syndromes

Certain inherited mutations predispose individuals to specific cancer types, often following Mendelian inheritance patterns.

Syndrome	Gene	Cancer Type	Inheritance
Retinoblastoma	RB1	Eye tumors	Autosomal dominant
Familial Adenomatous Polyposis (FAP)	APC	Colon cancer	Autosomal dominant
Lynch Syndrome (HNPCC)	MLH1, MSH2	Colon, endometrial cancer	Autosomal dominant

Genetic Testing for Cancer

Genetic testing can identify mutations associated with hereditary cancer risk or somatic mutations in tumors.

• Requires a DNA sample from blood or saliva.

• Testing is often covered by health insurance.

• Tumor biopsies are used to detect somatic mutations.

• Genetic testing involves sequencing genes and identifying mutations (e.g., 23A>T indicates a substitution of adenine (A) with thymine (T) at position 123 of the coding DNA sequence).

Key Equations and Concepts

• Knudson’s Two-Hit Hypothesis: Both alleles of a tumor suppressor gene must be inactivated for cancer to develop. $\text{Cancer risk} \propto \text{Number of inactivated alleles}$

• Mutation notation: $23A \rightarrow T$ (Adenine at position 23 is replaced by Thymine)

Additional info: The notes expand on the mechanisms of cancer development, the role of genetic mutations, and the importance of genetic testing in cancer diagnosis and risk assessment.