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Mike was referred for genetic counseling because he was concerned about his extensive family history of colon cancer. That family history was highly suggestive of hereditary nonpolyposis colon cancer (HNPCC). This predisposition is inherited as an autosomal dominant trait, and those who carry the mutant allele have a 75% chance of developing colon cancer by age 65. Mike was counseled about the inheritance of this condition, the associated cancers, and the possibility of genetic testing (on an affected family member). Mike’s aunt elected to be tested for one of the genes that may be altered in this condition and discovered that she did have an altered MSH2 gene. Other family members are in the process of being tested for this mutation.
Seventy-five percent of people who carry the mutant allele will get colon cancer by age 65. This is an example of incomplete penetrance. What could cause this?
To discuss: The cause of incomplete penetrance in case of colon cancer.
Introduction: Cancer can be defined as a group of diseases, which involve abnormal growth of cells, and these cells have the ability to spread and invade other regions of the body. Colon cancer is defined as the uncontrolled growth of cells of the large intestine specifically colon which is the end region of the large intestine. Most colon cancers are benign but in some cases, it can be malignant as well. The cancer property is shown by adenomatous polyps that are cancerous tumors present in the large intestine.
In the given case, Mr. M has a family history of nonpolyposis colon cancer. About 75% of the population inheriting the mutant allele expresses it and the gene mutation responsible for the colon cancer is present on the MSH2 gene.
Explanation of Solution
Penetrance is the term used for describing the percentage of the population that expresses symptoms of the inherited allele. In the given case study, the penetrance is an example of incomplete penetrance. This is because, 75% of the population expresses the inherited allele for colon cancer. In complete inheritance, 100% of the population expresses the inherited allele. The mutation in the MSH2 requires certain trigger factors to turn normal cells into cancerous cells. Some people do not experience this type of cancer even after the presence of the mutated gene because of the choice of a healthy lifestyle. A healthy lifestyle includes low consumption of preservatives or any other mutagens; as a result, the rate of mutation becomes low.
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Chapter 12 Solutions
Human Heredity: Principles and Issues (MindTap Course List)
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