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Ch. 24 - Cancer Genetics
Klug - Concepts of Genetics 12th Edition
Klug12th EditionConcepts of Genetics ISBN: 9780135564776Not the one you use?Change textbook
All textbooksKlug 12th EditionCh. 24 - Cancer GeneticsProblem 28b
Chapter 24, Problem 28b

The table in this problem summarizes some of the data that have been collected on mutations in the BRCA1 tumor-suppressor gene in families with a high incidence of both early-onset breast cancer and ovarian cancer.
Table summarizing BRCA1 gene mutations showing codon changes, coding effects like frameshift or stop codons, and absence in control chromosomes.
Examine the types of mutations that are listed in the table, and determine if the BRCA1 gene is likely to be a tumor-suppressor gene or an oncogene.

Verified step by step guidance
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Step 1: Understand the role of tumor-suppressor genes and oncogenes. Tumor-suppressor genes typically prevent uncontrolled cell growth and mutations in these genes often result in loss of function, leading to cancer. Oncogenes, on the other hand, promote cell growth and mutations in these genes often result in gain of function, leading to cancer.
Step 2: Analyze the types of mutations listed in the table. The mutations include frameshift mutations, nonsense mutations (e.g., Gln→Stop), and missense mutations (e.g., Met→Arg). Frameshift and nonsense mutations often lead to loss of function, while missense mutations can alter protein function.
Step 3: Examine the coding effects of the mutations. Frameshift mutations and nonsense mutations typically result in truncated or nonfunctional proteins, which is consistent with the loss of function seen in tumor-suppressor genes. Missense mutations may also impair protein function depending on the specific amino acid change.
Step 4: Consider the frequency of these mutations in control chromosomes. The table shows that these mutations are not found in control chromosomes, suggesting that they are specific to individuals with a high incidence of early-onset breast and ovarian cancer. This supports the idea that these mutations are pathogenic.
Step 5: Conclude based on the evidence. Since the mutations in BRCA1 lead to loss of function and are associated with cancer, this strongly suggests that BRCA1 is a tumor-suppressor gene rather than an oncogene.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Tumor-Suppressor Genes

Tumor-suppressor genes are crucial for regulating cell growth and preventing uncontrolled cell division. When these genes are mutated, their ability to control cell proliferation is compromised, leading to an increased risk of cancer. The BRCA1 gene is a well-known tumor-suppressor gene, and mutations in this gene are associated with a higher incidence of breast and ovarian cancers.
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Mapping Genes

Types of Mutations

Mutations can be classified into several types, including point mutations, insertions, deletions, and frameshifts. Point mutations involve a change in a single nucleotide, while insertions and deletions can alter the reading frame of the gene, potentially leading to a nonfunctional protein. Understanding the specific types of mutations in the BRCA1 gene is essential for assessing its role in cancer predisposition.
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Mutations and Phenotypes

Oncogenes vs. Tumor-Suppressor Genes

Oncogenes and tumor-suppressor genes play opposing roles in cell regulation. Oncogenes promote cell division and survival, often leading to cancer when activated. In contrast, tumor-suppressor genes inhibit cell growth and repair DNA damage. Determining whether BRCA1 functions as a tumor-suppressor gene or an oncogene is critical for understanding its role in cancer development.
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Cancer Mutations
Related Practice
Textbook Question

A study by Bose and colleagues (1998). Blood 92:3362-3367] and a previous study by Biernaux and others (1996). Bone Marrow Transplant 17:(Suppl. 3) S45–S47] showed that BCR-ABL fusion gene transcripts can be detected in 25 to 30 percent of healthy adults who do not develop chronic myelogenous leukemia (CML). Explain how these individuals can carry a fusion gene that is transcriptionally active and yet does not develop CML.

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Textbook Question

Those who inherit a mutant allele of the RB1 tumor-suppressor gene are at risk for developing a bone cancer called osteosarcoma. You suspect that in these cases, osteosarcoma requires a mutation in the second RB1 allele, and you have cultured some osteosarcoma cells and obtained a cDNA clone of a normal human RB1 gene. A colleague sends you a research paper revealing that a strain of cancer-prone mice develops malignant tumors when injected with osteosarcoma cells, and you obtain these mice. Using these three resources, what experiments would you perform to determine:

(a) Whether osteosarcoma cells carry two RB1 mutations

(b) Whether osteosarcoma cells produce any pRB protein

(c) If the addition of a normal RB1 gene will change the cancer-causing potential of osteosarcoma cells?

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Textbook Question

The table in this problem summarizes some of the data that have been collected on mutations in the BRCA1 tumor-suppressor gene in families with a high incidence of both early-onset breast cancer and ovarian cancer.

Note the coding effect of the mutation found in kindred group 2082. This results from a single base-pair substitution. Draw the normal double-stranded DNA sequence for this codon (with the 5' and 3' ends labeled), and show the sequence of events that generated this mutation, assuming that it resulted from an uncorrected mismatch event during DNA replication.

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Textbook Question

The table in this problem summarizes some of the data that have been collected on mutations in the BRCA1 tumor-suppressor gene in families with a high incidence of both early-onset breast cancer and ovarian cancer.

Although the mutations listed in the table are clearly deleterious and cause breast cancer in women at very young ages, each of the kindred groups had at least one woman who carried the mutation but lived until age 80 without developing cancer. Name at least two different mechanisms (or variables) that could underlie variation in the expression of a mutant phenotype, and propose an explanation for the incomplete penetrance of this mutation. How do these mechanisms or variables relate to this explanation?

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Textbook Question

Researchers have identified some tumors that have no recurrent mutations or deletions in known oncogenes or tumor-suppressor genes and no detectable epigenetic alterations. However, these tumors often have large chromosomal deletions. What are some possible explanations that could account for the genetic causes behind these tumors?

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Textbook Question

Although cancer is not a contagious disease in humans or other vertebrates, there have been rare cases in which cancers have spread from one organism to another. Describe three cases of these contagious cancers and what conditions might have led to their appearance.

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