| Case A |
Adapted from a case developed by by Anne M. Casper, Department of Molecular Genetics and Microbiology, Duke University. Copyright held by the National Center for Case Study Teaching in Science, University at Buffalo, State University of New York. Used with permission.
Background: Colorectal cancer is one of the leading causes of cancer death in the United States. Each year, about 150,000 Americans are diagnosed with colorectal cancer, and more than 50,000 die from the disease. Colorectal cancer usually begins with a polyp, which can be detected through the use of several tests: double contrast barium enema, flexible sigmoidoscopy, colonoscopy, or CT colonography (virtual colonoscopy). Early detection and and removal of polyps can prevent the development of colon cancer.
About 5% of people who develop colorectal cancer have an inherited genetic susceptibility to the disease. The two most common inherited syndromes linked with colorectal cancers are familial adenomatous polyposis (FAP) and hereditary non-polyposis colorectal cancer (HNPCC). Inherited mutations in a tumor suppressor gene called adenomatous polyposis coli (APC) are responsible for familial adenomatous polyposis (FAP). The APC gene regulates the formation of polyps in the colon.
Case A: Sam called his younger sister Jane to tell her he had just been diagnosed with colon cancer. Although Sam was only 40, he had suffered with diarrhea and found bright red blood in his stool, so his physician recommended a colonoscopy. Because their father had died at the age of 45 from colon cancer, Sam’s doctor recommended that he get additional genetic testing to see if the cause was hereditary. Sam encouraged Jane who was 35 to get a colonoscopy and planned to also call the twins, Mark and Caroline, age 33, to encourage them to get screened for colon cancer as well. Sam also recommended that if he had the mutated APC gene that his siblings also undergo genetic testing.
Procedure: Proteins from Sam’s tumor tissue were analyzed by Western blot, using antibodies specific for the APC protein. What do the results of this test indicate about the nature of his mutation?
- Protein sample: Sam’s tumor tissue
- Protein sample: normal colon tissue
- Antibody: APC
To determine the genetic status of each family member, DNA is isolated from a blood sample, and a DNA dot blot is performed using probes corresponding to the mutated and normal sequence in the APC gene:
- DNA samples: control normal APC, control mutated APC
- DNA samples: Sam, Jane, Mark, Caroline
- Probes: normal APC, mutated APC
Bioinformatics: PCR is used to amplify cDNA containing the protein coding region of the APC gene from tumor cell RNA. Use sequence alignment to compare Sam’s APC sequence to the normal gene sequence. Are there any difference? If so, where? In the MEGA sequence alignment window, change the nucleotide sequence to the amino acid sequence by clicking on the “Translated protein sequence” tab. How does Sam’s translated protein sequence compare to the normal sequence?
- What conclusions can you draw from the results of the DNA analysis?
- What do you notice when you compare Sam’s APC protein translation to the normal APC protein?
- How would you counsel Jane and Sam based on the results of the test? What would you advise Mark and Caroline?
- If someone is predisposed to getting cancer, does that mean that he or she will definitely get cancer someday?
- In a small number of patients whose families appear to have all the classical characteristics of FAP, a mutation cannot be found in the APC cDNA. What are two possible reasons for why mutations may not be found in some patients whose families appear to have FAP?
- How might environmental factors such as diet influence the development of colon cancer?
- Are there risks associated with screening such as colonoscopy? How do these risks compare with the risk of contracting colon cancer?