The more ApoE has been studied, though, the more it has been associated not only with Alzheimer's but also with the ability of the brain to heal from all manner of trauma. People with ApoE4 variants who hit their heads in car accidents, for example, are more likely to have permanent damage or to die than those who have other variants. And a series of small studies suggests that athletes with ApoE4 variants who get hit in the head are more likely to recover slowly and to suffer greater dementia later in life. It is not entirely clear how ApoE affects brain recovery, but the gene is involved in the inflammatory response of the brain after injury, and people with the ApoE4 variant appear to take longer to clear their brains of a particular protein called amyloid, which floods in following head trauma.
A 1997 study of 30 boxers found that those who had taken a lot of blows to the head and had an ApoE4 copy scored significantly worse on tests of brain impairment than similar fighters who did not have an ApoE4 copy. The ApoE4 variant is present in less than 25% of the general population, but it was present in all three of the boxers in the study who were severely impaired. A 2000 study of 53 active pro football players concluded that three factors caused some players to score lower than others on tests of brain function: 1) age, 2) having been hit in the head a lot and 3) possessing an ApoE4 variant.
Last year, during the NFL's concussion controversy, doctors from Boston University made news with research on dozens of cases of brain damage in deceased football players and boxers. What escaped the news was the genetic data the researchers had for nine of the athletes. Five of them, or 56%, had at least one ApoE4 variant, more than twice the proportion found in the general population.
Two years after the study of the 53 football players, Barry Jordan, one of its authors and until a year ago the chief medical officer of the New York State Athletic Commission, considered requiring ApoE screening for all boxers in the state but then backed off. Doctors agree that more work is needed to understand how ApoE4 affects brain recovery before a genetic test should become common practice.