Huntington’s Disease: More is Less:
The biology of codon
repeats, and bioethical dilemmas in gene testing.
Part I. Engage:
Go to www.woodrow.org/teachers/esi/2002/Biology/Projects/p8/ethics.htm#activity and read only the scenario paragraph about Sue (paragraph reproduced below).
Sue is a 19-year-old college
student. Desperate for a little extra spending money, Sue responds to an ad in
the local paper and volunteers to donate a blood sample to a company setting up
a genetic database. On her follow-up visit, Sue is shocked to learn that
she carries the gene for Huntington's Disease. As she
researches the disease, she realizes that one of her parents must also express
the
If you were Sue, you too would want to find out something about Huntington’s Disease (HD). So, visit this website for the basics: www.ygyh.org/hd/whatisit.htm .
What happens to a person with HD?
How is this disease acquired?
What is the cause of HD:
Is there a cure for HD?
Sue is puzzled because she has learned that HD is an inherited disease. Her parents are both well, but one of her parents may yet develop HD in later life. Yet none of her elderly grandparents have signs of HD. Let’s assume that all the fathers in her family are the TRUE fathers and Sue is not adopted. How could she get the disease without one of her grandparents having the disease?
For help with this mystery, go to www.ygyh.org/hd/inherited.htm and follow the section on “How does a person get Huntington’s disease?”
Now explain in your own words, how Sue could have the HD gene and disease without any of her grandparents having HD disease. Your explanation should include these terms: CAG repeats, codon expansions, intermediate number of repeats.
Part II. Explore:
A. Laboratory Activity.
Sue has shared her news with her parents and her brother. The family meets with a medical geneticist and a genetic counselor. Sue’s mother and her brother decide they want to be tested for the HD gene but her father is adamant that he does not want to know if he has HD.
At the Your Genes Your Health site for HD, the section on “How is it Diagnosed? explains the two-step process for detecting HD in a person.
Step one – PCR of the HD site is done for the purpose of:
Step two – Electrophoresis of the PCR products is done for the purpose of:
If a person has HD, the HD gene site is larger than normal because of greater number of trinucleotide (CAG) repeats within that site. Hence the DNA PCR product from a person with HD will be larger than the PCR products from person who does NOT have HD. During electrophoresis, the HD DNA moves slower than normal HD DNA because the abnormal HD gene has a larger than normal number of trinucleotide (CAG) repeats within its gene sequence (thus a larger than normal PCR product).
You are a laboratory technician in the BBD Pathology Lab. You and your coworkers are going to test today’s PCR products from DNA samples taken from Sue’s brother (P1) and mother (P2) to see if there are any cases of HD. You have Sue’s sample as the positive control, and another sample which is known to be a negative control.
This is actually a simulation of the gene test involving
only the electrophoresis portion of the test (not the first part, the
PCR). For an explanation with animations
of the full diagnostic test, go to www.ygyh.org/hd/diagnosis.htm
and click on Diagnostic gene testing.
Materials:
Agarose gel
Electrophoresis chamber
Electrophoresis buffer
Power supply
PCR product sample containing normal huntingtin DNA, the negative control (-).
PCR product sample containing abnormal huntingtin DNA, labeled + for positive control.
PCR product from Patient samples, numbered as P1 and P2.
Micropipettors
Procedure:
1) Obtain three 0.5mL tubes, one containing the normal negative control (N), one containing the positive control (+), and one containing your Patient sample (P). Record the number of your particular patient sample______.
2) Place a gel into the electrophoresis chamber. Fill the reservoirs with buffer, and then add just enough buffer to cover the gel.
3) Load 10-15μL of each sample into a separate well with a micropipette. Label the wells you used in the sketch below.
4) Note how your gel is oriented with respect to the positive and negative poles. (Remember- Run to red.)
5) Place the cover on the chamber and plug the electrodes into to the power supply, black to black, red to red.
6) Turn on power supply; look for bubbles rising from the wires at each end to ensure the electrophoresis box is working correctly.
7) Run your gel at 120-125 volts until the dye is about ¾ of the way down the gel. This should take about 10-15 minutes.
8) Turn off the power supply and disconnect the electrodes.
9) Use the report form to record your results and diagnosis.
What is the difference between diagnostic and predictive testing for HD?
B. (Option
for advanced biology students)
For more detail about the mechanism of codon expansions, go to www.woodrow.org/teachers/esi/2002/Biology/Projects/p8/intro.htm#top
What is “replication slippage?”
It turns out that though the DNA and protein alternations that cause HD are known, the cause of the codon expansions is unknown. Scientists construct conceptual models to explain observations like a codon expansion, in order that the models can be tested with further experimentation.
Go through the explanation of two current models of how codon expansion may occur at www.stanford.edu/group/hopes/causes/mutation/q4.html and then www.stanford.edu/group/hopes/causes/mutation/q5.html
List below the names of the two models to explain how codon expansions happen:
1.
2.
Is either model currently regarded as preferred? Why or why not?
Part III. Extend
One of the consequences of any disease that runs in families is that if one person finds out he/she has a genetic disease, then other family members are automatically involved.
Thus ethical dilemmas arise as gene testing becomes available and used.
The four principles used in discussing bioethical dilemmas are:
www.uq.edu.au/oppe/PDFS/Ethics_primer.pdf
List the four principles:
1.
2.
3.
4.
From Sue’s scenario at (www.woodrow.org/teachers/esi/2002/Biology/Projects/p8/ethics.htm#activity ) look at the ethical questions.
Sue's head is spinning from all the issues she must now face.
1. Sue’s father does not want to be tested but her mother and her brother are very eager to be tested. What if Sue’s mother turns out to be negative for HD? What will this mean?
2. Recently Sue and her boyfriend, Steve, have discussed marriage. Does her boyfriend have a right to know of her status before he makes plans to marry and have children? She knows that if she chooses to have children, there is a 50% chance that her child will inherit the disease. As far as she is concerned, she has two options- she can take measures against reproduction to ensure that she can not pass the gene for Huntington's disease to offspring or she can choose to conceive, knowing that pre-zygotic testing may be able to tell her whether or not her child has the disease. However, she is on the fence when it comes to terminating a pregnancy. What are the pros and cons of both options? Are there any other options Sue has not considered?
Choose either question, name a dilemma and link the dilemma to one or more of the ethical principles.
Read more about a researcher working on Huntington’s Disease http://publications.nigms.nih.gov/findings/sept08/hunting.asp