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Oncology -Cancer Genetics
This study guide summarizes the provided text on the genetic factors contributing to cancer development. Understanding this material requires grasping the interplay between genes, mutations, and environmental influences.
I. Cancer Development: The Big Picture
Cancer development involves alterations in three main categories of genes:
A. Tumor Suppressor Genes:
The contribution of genetic mutations to cancer varies:
This study guide summarizes the provided text on the genetic factors contributing to cancer development. Understanding this material requires grasping the interplay between genes, mutations, and environmental influences.
I. Cancer Development: The Big Picture
- High Mortality: Malignancy (cancer) accounts for 77 million deaths annually worldwide.
- Preventability: At least 50% of cancer cases are potentially preventable through lifestyle and environmental modifications.
- Monoclonal Origin: Most cancers originate from a single cell that accumulates sufficient mutations in key genes, leading to uncontrolled proliferation. Heterogeneity increases as the cancer progresses.
Cancer development involves alterations in three main categories of genes:
A. Tumor Suppressor Genes:
- Function: Inhibit cell growth and proliferation. They act like "brakes" on cell division.
- Mechanism: Both copies of a tumor suppressor gene must be inactivated (loss of function) for the effect to manifest (recessive). This can be through inherited (germline) or acquired mutations.
- Example: p53 Gene: A crucial regulator of cell cycle and genome integrity. Mutations are found in ~75% of human cancers and are associated with Li-Fraumeni syndrome.
- Function: Normally promote cell growth and division. They act like the "accelerator" on cell division.
- Mechanism: A mutation in only one allele is sufficient to cause uncontrolled growth (dominant).
- Example: Ras Gene: A G-protein involved in signal transduction. Mutated Ras remains active even without proper signals, leading to uncontrolled cell growth. Mutations are implicated in ~30% of cancers.
- Function: Repair damaged DNA. Their malfunction accelerates the accumulation of mutations in tumor suppressor and proto-oncogenes.
- Mechanism: Loss of function allows mutations to accumulate unchecked.
- Example: ATM Gene: Involved in DNA damage detection and cell cycle regulation. Mutations cause ataxia-telangiectasia, characterized by cerebellar ataxia, increased cancer risk (lymphomas/leukemias), and hypersensitivity to ionizing radiation.
The contribution of genetic mutations to cancer varies:
- High Penetrance Genes (5%+ of fatal cancers): These genes, when mutated, significantly increase the risk of specific cancers. They are usually dominantly inherited.
- Examples:
- BRCA1/2: Predispose to breast and ovarian cancers (and others). High lifetime risk for female carriers.
- RB1: Associated with retinoblastoma (eye tumor).
- APC: Linked to familial adenomatous polyposis (FAP), leading to numerous colonic adenomas and subsequent cancer.
- Examples:
- Genes with Modest Effects: These genes may interact with environmental factors to influence cancer risk. Examples include interactions with tumor viruses.
- Sporadic Cancers: These cancers arise from somatic mutations caused by carcinogens like aromatic hydrocarbons and UV radiation.
- Epigenetics: Modifications to the genetic code that influence gene expression without changing the DNA sequence itself.
- Post-translational Modifications: Changes to proteins after they are synthesized, which can affect their function and contribute to cancer development.
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