It seems that over the course of any medical show series on TV, there will inevitably be a character who has been tested for the genetic condition called Huntington’s disease (HD). I’ve seen it a million times between ER, Grey’s Anatomy, and House, but recently I have even seen it even in the non-hospital setting on Breaking Bad. In fact, when I googled it, I even came across a Wikipedia page called “List of Huntington’s disease media depictions.”
So why is Huntington’s such an appealing topic in the media? Well, for starters the progression of the disease is so steep, that it is startling.Huntington’s is a neurodegenerative disorder that affects muscle coordination and leads to cognitive decline and psychiatric problems starting at around age 40. Life expectancy in HD is generally around 20 years following the onset of visible symptoms and there is no treatment or cure. When I was in grad school, I visited the Huntington’s Disease Unit at Terence Cardinal Cooke Health Care Center. I remember meeting a woman who was in her 50s who used to be a law librarian, and also a 40-something year old man who was a former judge in the New York State Supreme Court.Both were obviously very intelligent individuals, but were now experiencing severe muscle jerks called chorea, dementia, and disturbed behavior, among many other neurological problems. I remember leaving the unit that day and calling my husband on my way out to warn him that I would be in a depressed mood.
Huntington’s disease is “100%, or completely, penetrant,” which means that people who have the genetic mutation at birth will definitely develop the disease to some degree at some point in their lives. The interesting thing is that these people are not surprised when it happens to them, because only people who have affected parents will be affected. Affected individuals have a 50% risk of passing the disease mutation to each of their children. So while not all children of affected individuals will inherit the disease, if you do have Huntington’s, you have inevitably seen one of your parents suffer from it before. There are many ethical issues that arise in the realm of genetic testing in Huntington’s families–issues such as when to test an individual, determining the psychological well-being of the individual prior to testing, disclosing this information to partners, and doing prenatal diagnosis with the possibility of selective abortion, to name a few. The combination of the disease course, severity, and these ethical issues obviously makes for good plot lines.
Luckily, most genetic diseases are not 100% penetrant. Individuals who carry the genetic mutation for diseases with “incomplete, or reduced, penetrance” are not guaranteed to develop the disease. Incomplete penetrance often is seen with familial cancer syndromes. For example, many people with a mutation in the BRCA1 or BRCA2 gene will develop breast or ovarian cancer during their lifetime, but some people will not. Individuals with a mutation in a mismatch repair gene associated with Lynch syndrome are likely to develop colon cancer and other GI-related cancers, but this is not definite. Doctors cannot predict which people with these mutations will develop cancer or when the tumors will develop. These familial cancers also may appear to “skip” generations due to incomplete penetrance.
While there is, at this time, nothing that can be done to prevent Huntington’s disease, the good news is that individuals with BRCA or Lynch-related mutations can take actions to reduce their risk of developing cancer. Carriers may consider risk-reducing surgeries: female BRCA carriers may have mastectomies and/or oophorectomies, and some many individuals with Lynch syndrome will have colonic resections. Additionally, these carriers will increase surveillance with more rigorous screenings (think mammograms and colonoscopies), and many will try to alter their lifestyle with diet and exercise.
Isn’t it encouraging to know that all the stuff you’ve inherited from your parents (the good and the bad) is not necessarily your fate?