Textbook Question
How do we know that malignant tumors arise from a single cell that contains mutations?
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19. Cancer Genetics
Overview of Cancer
1:46 minutesProblem C.3a
Textbook Question
For the retinal cancer retinoblastoma, the inheritance of one mutated copy of RB1 from one of the parents is often referred to as a mutation that produces a 'dominant predisposition to cancer.' This means that the first mutation does not produce cancer but makes it very likely that cancer will develop. Define the 'two-hit hypothesis' for retinoblastoma.
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The 'two-hit hypothesis' is a model proposed to explain the development of retinoblastoma and other cancers caused by tumor suppressor gene mutations. It suggests that two genetic 'hits' or mutations are required to inactivate both copies of a tumor suppressor gene, such as RB1, leading to cancer.
Step 1: Understand that individuals with a hereditary predisposition to retinoblastoma inherit one mutated copy of the RB1 gene from a parent. This is the first 'hit,' which is present in all cells of the body (germline mutation).
Step 2: The second 'hit' occurs as a somatic mutation in a retinal cell during the individual's lifetime. This mutation inactivates the remaining functional copy of the RB1 gene in that specific cell.
Step 3: When both copies of the RB1 gene are inactivated in a retinal cell, the cell loses its ability to regulate the cell cycle properly, leading to uncontrolled cell division and tumor formation.
Step 4: The 'dominant predisposition' arises because individuals with one inherited mutation (first hit) are much more likely to acquire the second mutation in a retinal cell, making the development of retinoblastoma highly probable.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Two-Hit Hypothesis
The two-hit hypothesis, proposed by Alfred Knudson, suggests that cancer develops through a two-step process involving mutations in tumor suppressor genes. In the case of retinoblastoma, the first 'hit' is a hereditary mutation in one copy of the RB1 gene, which predisposes individuals to cancer. The second 'hit' is typically a somatic mutation that occurs in the other copy of the gene, leading to the loss of function necessary for tumor suppression and resulting in cancer development.
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Translation:Wobble Hypothesis
Tumor Suppressor Genes
Tumor suppressor genes are critical for regulating cell growth and preventing uncontrolled cell division. The RB1 gene, which encodes the retinoblastoma protein, is a key tumor suppressor that helps control the cell cycle. When both copies of the RB1 gene are mutated, the regulatory function is lost, allowing cells to proliferate uncontrollably, which can lead to cancer.
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Dominant Inheritance
Dominant inheritance refers to a pattern where only one mutated copy of a gene is sufficient to express a trait or predisposition to a condition. In retinoblastoma, inheriting one mutated RB1 gene from a parent results in a dominant predisposition to develop cancer, as the individual is at a higher risk of acquiring the second mutation necessary for tumor formation. This concept is crucial for understanding the genetic basis of familial retinoblastoma.
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