Case Study

CHAPTER 5 Beyond Mendel’s Laws

Long QT Syndrome

Roger Maxwell is very health-conscious. He runs, swims, and hikes; follows a low-carbohydrate diet; and generally feels great. He sees a physician when he needs to, in addition to annual physical exams at the large company where he is an engineer. He’d never allow himself to get so out of shape that heart disease would be a risk. Because of his strict adherence to this healthy lifestyle, Roger is surprised when a medical intern, gazing at his yearly electrocardiogram (ECG) at his work physical, clearly picks up on something.

“What? What are you looking at?” Roger blurts out while buttoning up his shirt.

“Oh, it’s probably nothing.” But she doesn’t look like it’s probably nothing.

“The heart murmur? My mom’s been telling me about it since childhood. Not a big deal. The doctors called it something last year, something I never heard of.”

“Did you check it out?” asks the intern.

“Nah. It wasn’t bothering me, so I forgot about it. Why? What’s wrong?”

“Well, maybe you should ask the doctor to explain it again and suggest what to do.”

“About what?”

“The doctor will explain it. Please don’t worry, though,” says the intern as she rushes off to the next patient.

Roger’s electrocardiograms had in fact been showing that he has had long QT syndrome, and not a heart murmur, for many years. The doctor explains that this is a problem with the heart’s rhythm, and not its valves. Roger goes home and Googles long QT syndrome right away. What he finds concerns him enough to alert his relatives.

Long QT syndrome is a lengthening in the time that it takes the ventricles (the lower two heart chambers) to recover after a contraction, called the QT interval on an electrocardiogram. This delay is called torsade de pointes, and it causes lightheadedness upon standing or even fainting, as blood pressure drops when the heart rhythm becomes abnormal. If the arrhythmia turns into the more erratic condition called ventricular fibrillation, it can be deadly. Some cases of sudden cardiac arrest in people who apparently do not have heart disease are in fact due to long QT syndrome. This may have been the case with Roger’s aunt, Amelia, his mother’s sister, who died at age 34 of what was thought to be a heart attack, but, now Roger realizes, was more likely an arrhythmia. Still, with only one affected relative, Roger had never thought of his aunt’s early demise as a family history, especially since his mother is healthy.

Roger reads that in people with some forms of long QT syndrome, fatal arrhythmia can be triggered by intense emotions or a sudden loud sound. The first recorded case of the condition was a little girl, who collapsed dead when her teacher suddenly yelled at her. Her older brother had died in a similar circumstance. Suddenly, Roger remembers that his daughter Sheila faints very easily. She even passed out once at a rock concert because she got so excited. He’d never panicked over it because his mother fainted easily, too. A pattern was emerging.

Long QT syndrome is caused by mutations in any of at least 10 genes that encode either proteins that form parts of ion channels (potassium, sodium, or calcium) or proteins that affect the functioning of these channels. Ion channels control the spread of nerve impulses and the resulting muscle contraction. The time for the heart’s recovery after a beat, called repolarization, extends the period when ions are trapped inside heart muscle cells because the channels are blocked, too slow to open, or too quick to close in people who are at elevated risk due to inheriting a mutation. People with long QT syndrome can experience arrhythmia if they take certain drugs that prolong the QT interval. These drugs include certain antibiotics, antidepressants, and diuretics (“water pills”).

Roger reads about the different genes and drug combinations that cause long QT syndrome on Wikipedia, and then he scans www.genetests.org  to find labs that test for all of them. Only then does he make an appointment with a cardiologist, and he arranges to have his blood and that of his daughter and mother sent to one of the labs. Two weeks later, he learns that they all have a dominant mutation in a gene called HERG (for “human ether-a-go-go”) that causes long QT syndrome type 2 (LQT2). Even healthy family members could have inherited the mutation, because 15 percent of people with long QT syndrome do not have symptoms. The concern is their increased risk of developing symptoms in the future—perhaps suddenly.

QUESTIONS FOR RESEARCH AND DISCUSSION

Discuss how incomplete penetrance, variable expressivity, pleiotropy, and genetic heterogeneity can affect the severity of a disease in a family.

A person can lower risk of preventable types of cardiovascular disease by exercising regularly and following a diet low in saturated fats and simple carbohydrates and high in fruits and vegetables. Why are these approaches ineffective against long QT syndrome?

Explain how the molecular bases of the various forms of long QT syndrome make genetic heterogeneity very likely.

Synesthesia Revisited

Peter Maxwell is lucky. In addition to his beautiful red shiny hair and musical gifts, he has escaped the family’s long QT syndrome and is a synesthete. However, he isn’t entirely convinced that he has his genes to thank that musical notes appear colored to him. Instead, he worries that it was his experimentation with LSD in the 1960s and its persistent effects that have caused what appears to be synesthesia—he can’t remember when he first noticed it.

List the evidence that Peter’s synesthesia is not the result of his LSD use.

Discuss how genome-wide association studies and brain imaging have contributed to our understanding of synesthesia.

Suggest an experiment that can distinguish whether synesthesia is inherited or a learned behavior.

Lewis, R. (12/2009). Case Workbook for Human Genetics, 9th Edition. [VitalSource Bookshelf Online]. Retrieved from https://digitalbookshelf.southuniversity.edu/#/books/0077461576/