Cancer as a Genetic Disease

Genetic Nature of Cancer

Cancer is fundamentally a genetic disease of somatic cells, characterized by the accumulation of mutations in specific genes that regulate cell growth, division, and survival. These mutations disrupt normal cellular processes, leading to uncontrolled proliferation and tumor formation. - Somatic mutations occur in body cells and are not inherited, while germline mutations can be passed to offspring and predispose individuals to cancer. - Key genes involved: Oncogenes, tumor suppressor genes, and DNA repair genes. - Multiple mutations are required for a cell to become cancerous, supporting the "multi-hit" model of tumorigenesis.

Incidence and Relevance

The risk of developing cancer is significant, with approximately 50% of men and 33% of women affected during their lifetime. Cancer is not a single disease but a collection of related diseases, each defined by the specific genes mutated and the cell type of origin.

Cell Division and Tumorigenesis

Normal vs. Cancer Cell Division

Normal cell division is tightly regulated by checkpoints and signaling pathways, ensuring proper growth and repair. In cancer, these controls are lost, resulting in unchecked cell division and accumulation of mutations. - Normal cells undergo apoptosis (programmed cell death) or are targeted by the immune system when damaged. - Cancer cells evade these mechanisms, continue dividing, and may undergo metastasis, spreading to other parts of the body.

Cellular Pathways Mutated in Cancer

Growth Regulation and Cell Cycle

The cell cycle is regulated by checkpoints that ensure DNA integrity and proper division. Genes that block cell cycle progression are tumor suppressors (brakes), while genes that promote division are proto-oncogenes (accelerators). - Oncogenes arise from gain-of-function mutations in proto-oncogenes, leading to constant activation. - Tumor suppressor genes are inactivated by loss-of-function mutations, removing growth inhibition.

Phenotypic Consequences of Cancer Mutations

Loss of Growth Control and Metastasis

Cancer cells exhibit several phenotypic changes due to accumulated mutations: - Loss of growth regulation - Evasion of apoptosis - Increased migration and invasion - Ability to metastasize and colonize distant tissues

Types of Cancer Genes

Oncogenes vs. Tumor Suppressor Genes

Oncogenes: - Proto-oncogenes normally drive cell cycle progression. - Gain-of-function mutations convert them to oncogenes, causing constant cell division. - Usually dominant mutations. Tumor Suppressor Genes: - Normally block cell cycle progression, repair DNA, or promote apoptosis. - Loss-of-function mutations remove these brakes, allowing uncontrolled growth. - Usually recessive mutations.

Genomic Alterations in Cancer

Karyotype Abnormalities

Cancer cells often display structural abnormalities in their chromosomes, such as deletions, translocations, and duplications, which contribute to tumorigenesis.

Multi-Step Model of Cancer Development

Tumorigenesis is a multi-step process, requiring mutations in several genes over time. Each cancer arises from a single cell (clonal origin), and its progeny gain a growth advantage.

Hallmarks of Cancer

Key Steps in Malignant Transformation

Hanahan and Weinberg (2011) identified ten hallmarks required for a normal cell to become a malignant tumor: - Activation of oncogenes - Inactivation of tumor suppressor genes - Altered metabolism (Warburg effect) - Genome instability - Evasion of apoptosis - Avoidance of immune destruction - Immortality - Inflammation - Vascularization (angiogenesis) - Metastasis

Inherited vs. Sporadic Cancer

Inherited Cancer Predisposition

A subset of cancers are inherited, typically due to autosomal dominant mutations in oncogenes or tumor suppressor genes. Individuals inherit one mutant allele and acquire additional mutations somatically.

Sporadic Cancer

Most cancers are caused by spontaneous or environmental mutations, with only about 5% attributable to inherited alleles.

Types of Mutations Causing Cancer

Mutation Classes

Cancer can result from various types of mutations: - Missense mutations: Change a single amino acid. - Nonsense mutations: Introduce a premature stop codon. - Frameshift mutations: Alter the reading frame. - Deletions: Remove part or all of a gene. - Chromosome rearrangements: Translocations, exchanges of large DNA segments.

Translocations and Cancer

Philadelphia Chromosome (Chronic Myeloid Leukemia): - Reciprocal translocation between chromosomes 9 and 22 creates a BCR-ABL fusion gene, producing a constitutively active kinase. Burkitt's Lymphoma: - Translocation of c-MYC from chromosome 8 to the IgV region on chromosome 14 results in overproduction of c-MYC protein, promoting cell growth.

Cancer Therapy

Mechanistic Basis of Therapies

Cancer treatments exploit the molecular basis of the disease: - Surgery: Removal of tumors when feasible. - Radiation: Damages DNA in dividing cells. - Chemotherapy: Targets rapidly dividing cells, often by inducing DNA damage (e.g., cisplatin). - Immunotherapy: Uses antibodies against proteins expressed by cancer cells (e.g., Herceptin, Keytruda). - Targeted therapies: Drugs like Gleevec block specific proteins (e.g., ABL kinase in CML) to inhibit growth and survival signals.

Summary Table: Selected Cancers with Inherited Predispositions

Key Equations and Concepts

Multi-Hit Model of Cancer

The probability of cancer increases with the number of mutations required: where is the probability of a mutation per cell division and is the number of required mutations.

Dominant vs. Recessive Mutations

- Oncogenes: Dominant gain-of-function mutations. - Tumor suppressor genes: Recessive loss-of-function mutations (both alleles must be inactivated).

Conclusion

Cancer is a complex genetic disease involving multiple mutations in key regulatory genes. Understanding the molecular basis of cancer has enabled the development of targeted therapies and improved diagnostic strategies. Additional info: The notes expand on the mechanistic basis of cancer therapies and the multi-step model of tumorigenesis for completeness.