Know Your Brain: Chronic Traumatic Encephalopathy (CTE)

Background

comparison of a healthy brain and brain afflicted with CTE, from the Boston University Center for the Study of Traumatic Encephalopathy

In 1928, physician and researcher Harrison Martland published a scientific paper titled Punch DrunkIn it, he described 23 cases of boxers who had started to display neurological symptoms after experiencing the repetitive head trauma that goes hand in hand with their sport. They sometimes developed symptoms that resembled Parkinson's disease, like tremors and abnormalities in gait, as well as more general types of cognitive deterioration. About a decade later, another researcher gave a new name to Martland's punch drunk syndrome, calling it dementia pugilistica.

A year before Martland's popularization of the term punch drunk syndrome, physicians Michael Osnato and Vincent Gilberti had published a review of cases of what was known at the time as postconcussion neurosis---a neurological disorder that emerged after a concussion. Osnato and Gilberti concluded that concussions could be associated with subsequent neurodegeneration, or the degeneration and death of neurons. Because the pathology they saw in the cases they studied resembled the effects of a type of brain inflammation known as encephalitis, Osnato and Gilberti decided this disorder should be called traumatic encephalitis, which soon was modified to traumatic encephalopathy.

In 1940, researchers Bowman and Blau coined the term chronic traumatic encephalopathy when describing the case of a 28-year old professional boxer who had been unable to get commisioned to continue boxing because he was suffering from a number of symptoms including paranoia, depression, memory deficits, and impaired cognition. Bowman and Blau added the word chronic to Osnato and Gilberti's original terminology because this patient's case had not improved over the course of 18 months. They thus called the condition chronic traumatic encephalopathy, or CTE.

Although the first reports of CTE described boxers, it wasn't long before similar symptoms were reported in American football players---and players of any sport that involved the potential for multiple head injuries. It wasn't until 2005, however, that widespread attention was focused on American football as a potential cause of CTE. This attention followed the publication of a report by neuropathologist Bennet Omalu and colleagues after the examination of the brain of former NFL player Mike Webster. Webster had died of a heart attack but had suffered from memory problems and depression late in his life. Upon autopsy, it was found that Webster's brain showed signs of degeneration and the researchers concluded that Webster had suffered from CTE. Autopsies of the brains of a number of other football players have resulted in similar observations.

What is CTE?

CTE is a neurological condition thought to be the consequence of repetitive head trauma, although other risk factors must also be at play since not everyone who experiences repetitive head trauma develops CTE. Symptoms associated with CTE generally begin to appear years (sometimes decades) after trauma and may include: problems with cognition like memory and attentional deficits; behavioral abnormalities like paranoia, aggression, and impulsivity; mood disturbances like depression, anxiety, and suicidal thoughts; and movement problems like tremor and other Parkinsonian symptoms. In the majority of cases, the symptoms of CTE are progressive---meaning they get worse over time.

Despite a long list of recognized symptoms, however, there are no widely accepted diagnostic criteria that define what CTE should look like (although at least two sets of diagnostic criteria have been proposed). Sometimes CTE is defined specifically as the pathological changes that occur in the brains of patients, while the presentation of symptoms is called traumatic encephalopathy syndrome.

What causes CTE?

Typically, CTE is associated with repeated concussions and subconcussive blows (i.e. trauma that doesn't result in clinical symptoms). The evidence is not clear at this point as to how many instances of head trauma are required to cause CTE, or if it could be caused by one incident. Also, not everyone who experiences repetitive head trauma will develop CTE, which suggests that other factors must also be involved. But researchers are still working to identify those other risk factors.

Populations who are at risk for frequent head trauma are also most likely to develop CTE, as CTE has been observed in: boxers, American football players, professional hockey players, professional wrestlers, victims of physical abuse, military personnel, and so on. It's important to emphasize that, as mentioned above, head trauma does not have to result in clinical symptoms to increase the risk of CTE. Someone who takes frequent blows to the head may be at greater risk of developing CTE, even if those blows don't result in concussive symptoms.

What happens in the brain in CTE?

The pathological features of CTE in the brain are perhaps better defined than the overt symptoms of CTE. The principal feature is the accumulation of a protein called tau into insoluble clusters, also known as aggregates. This process is thought to begin when tau protein becomes hyperphosphorylated, which means that multiple chemical groups called phosphoryl groups have attached to tau to the point where no more can attach to the molecule. At this point, tau, which normally interacts with and helps to maintain the stability of microtubules in the cell, disassociates from the microtubules. Then, the hyperphosphorylated tau protein forms the aggregates mentioned above in neurons and astrocytes surrounding blood vessels in the brain. The clusters of tau are called neurofibrillary tangles when they appear in neurons and are often called astrocytic tangles when they appear in astrocytes.

The tau aggregates in CTE form in the cerebral cortex, primarily at the depth of the invaginations of the cortical surface known as the cortical sulci. These aggregates may also form in other layers of the cortex, some regions of the hippocampus, and in other subcortical nuclei.

What effect these clusters have exactly is still uncertain, as while their presence is correlated with the severity of neurodegeneration, it has not been clearly demonstrated to cause it. Still, neurofibrillary tangles are thought to be able to disrupt cellular communication, which could lead to detrimental effects on the cell. They also have the ability to pass from one affected neuron to other unaffected neurons, which seems to indicate a potential for the pathology to spread within the brain. 

Aggregates of tau are found in other neurodegenerative diseases like Alzheimer's disease as well, and some hallmarks of other neurodegenerative diseases, like the amyloid plaques commonly seen in Alzheimer's disease, also occur in CTE. But the distribution of tau in CTE, as well as the absence of defining features of another neurodegenerative disease is what allows for the diagnosis of CTE. For example, if tau-associated degeneration occurs in certain regions of the hippocampus alongside the formation of amyloid plaques, it would be indicative of Alzheimer's disease rather than CTE.

While tau deposits are the primary microscopic sign of CTE, there are also more evident signs, like reduced brain weight, atrophy of the cerebral cortex (especially in the frontal and temporal lobes), atrophy of various other regions of the brain like the hippocampus and amygdala, enlargement of the ventricles, and thinning of the corpus callosum

Prevalence of CTE

CTE has received a great deal of media attention over the past several years, and this has led to some misunderstandings about the prevalence of the disorder. For example, in 2017 a story about CTE in National Football League (NFL) players received a lot of media attention, with headlines reporting that CTE was found in 99% of brains of NFL players that had been studied. This study, however, used brains that had been donated to be studied for CTE, regardless of whether or not symptoms had emerged during the players' lives. This introduces a potential source of bias, as relatives of players may have donated the players' brains because of concern about symptoms that had arisen during the players' lives. In other words, many of the brains involved in the study may have been donated because of concerns about CTE, making it less surprising that almost all of the brains showed signs of CTE.

Due in part to the potential biases surrounding brain donation for CTE study, the actual prevalence of CTE is difficult to estimate. One study that included a larger brain bank found CTE in 31.8% of the brains of individuals with a history of repetitive head trauma, and no cases among 198 brains without such a history. Larger studies are underway now to try to get a better sense of how prevalent CTE is in the general population.

Read more about the neuroscience of traumatic brain injury.

References:

Asken BM, Sullan MJ, DeKosky ST, Jaffee MS, Bauer RM. Research Gaps and Controversies in Chronic Traumatic Encephalopathy: A Review. JAMA Neurol. 2017 Oct 1;74(10):1255-1262. doi: 10.1001/jamaneurol.2017.2396.

Montenigro PH, Corp DT, Stein TD, Cantu RC, Stern RA. Chronic traumatic encephalopathy: historical origins and current perspective. Annu Rev Clin Psychol. 2015;11:309-30. doi: 10.1146/annurev-clinpsy-032814-112814. Epub 2015 Jan 12.

Know Your Brain: Huntington's disease

Background

George Huntington was not a prolific researcher. In fact, he only published three scientific papers in his career. The first of these, however, published in 1872 when Huntington was just 22 years old, would lead to his name being found in most neuroscience textbooks today.

In that paper, titled On chorea, Huntington discussed a disorder called chorea that had been known since the Middle Ages. The word chorea comes from the Geek word choreia, which means to dance, and it was used to describe a disorder characterized by involuntary, spasmodic movements of the limbs that bore some resemblance to an odd dance. 

There are a number of different types of chorea, each linked to a different underlying cause. George Huntington wrote the best description to that point of a particular form of chorea that began in adulthood and was inherited, inexorably progressive, and always fatal. Within a few decades, the chorea Huntington described was widely-recognized as a unique disorder, which---in appreciation for Huntington's clear and accurate depiction of the disease---was called Huntington's chorea. The name was eventually changed to Huntington's disease (HD), as it became apparent that the condition involved more than just chorea. Not all patients with HD develop chorea and even those who do experience a number of other symptoms that are not related to movement.

In the 20th century, the hereditary nature of HD became even clearer as our understanding of common patterns of inheritance improved. In the 1980s, a gene was identified that seemed to be the cause of HD. With this discovery, HD joined a short list of diseases caused by the mutation of a single gene.

What is Huntington's disease?

The symptoms of HD can appear at any age, but typically emerge in middle age (the average age of onset is 40 years). At first, patients will often experience subtle changes in personality, cognition, and movement. For example, a patient might become irritable, have trouble remembering things, or be especially restless or fidgety. These symptoms, however, are usually not enough to lead to a diagnosis. 

The early symptoms then progress into symptoms that allow for a clear diagnosis of HD. These include: conspicuous movement problems like chorea, impaired coordination and balance, abnormal eye movements, and muscle rigidity. Also, cognitive symptoms can become debilitating, and can involve difficulty focusing, a tendency to become fixated on a thought, lack of impulse control, and lack of awareness. Psychiatric symptoms like depression, insomnia, and fatigue are common as well.

huntington's disease causes significant neurodegeneration in the basal ganglia (highlighted structures here in the middle of the brain).

HD is a neurodegenerative disorder, meaning it is characterized by the degeneration and death of neurons. Thus, symptoms are accompanied by pathological changes to the brain, including neuronal loss in specific brain regions. The caudate and putamen (both part of the basal ganglia) are the two areas where cell loss is the most prominent, and damage to them is thought to be the cause of some of the movement problems HD patients experience. Other areas of the brain, however, like the substantia nigra, cerebral cortex, hippocampus, cerebellum, hypothalamus, and thalamus are also affected. The disease is invariably fatal; the average time from diagnosis to death is around 20 years

What causes Huntington's disease?

HD is a rare disorder, occurring in the western world at a rate of about 4-10 cases per every 100,000 people. As mentioned above, the disease is inherited, and can be traced back to a mutation in a single gene called huntingtin (HTT). The HTT gene contains a DNA sequence that consists of three nucleotides (cytosine, adenosine, and guanine, or CAG) in repetition---a pattern known as a trinucleotide repeat. 

Some degree of trinucleotide repetition in the HTT gene is normal and will not result in HD. When the gene is mutated, however, excess CAG repeats (above 35) can occur. The higher the number of repeats, the greater the risk of developing HD. For example, 36-39 repeats leads to an increased risk of HD, but also the possibility that the onset of the disease will be so late in life that noticeable symptoms may not appear before death due to other causes. 40 or more repeats, however, is a fully penetrant mutation, meaning that all people with the mutation will develop disease. As the number of repeats increases, the age of onset is more likely to be younger, with 70 or more repeats causing the disease to appear during youth. The largest repeat length seen so far has been 250, but it is very uncommon to see repeat lengths greater than 80.

The mutation in the HTT gene that leads to HD is known as an autosomal dominant mutation. This means that the gene variation that encodes for the mutant protein is dominant, and will be expressed if it's inherited. If one parent has HD, their child has a 50% chance of inheriting the mutated gene, and thus of developing HD.

The function of the huntingtin protein (which is produced by the HTT gene) is not fully understood, but it is thought to play important roles in embryonic development. There are also indications it is an important regulator of signaling pathways in the cell. It's expressed throughout the body, but is especially prevalent in the central nervous system.

An excess number of CAG repeats in the HTT gene leads to the production of a mutated form of huntingtin protein. The characteristics of these mutated proteins cause them to be likely to be cleaved by cellular enzymes. The cleaved fragments then have a propensity to group together, forming clusters within neurons that are not easily removed by brain enzymes. It has been hypothesized that these protein clusters (which are similar in some ways to the protein aggregates seen in Alzheimer's, Parkinson's, and other neurodegenerative diseases) may play a role in the neuronal damage seen in HD.

Additionally, mutant huntingtin seems to also be able to recruit other, normal proteins, into these clusters. In this way, huntingtin protein is thought to spread its abnormal state to healthy proteins, which might cause normal cell functions to be disrupted. Finally, some have suggested that mutated huntingtin may also have direct toxic effects on neurons.

As with other neurodegenerative diseases, however, the exact way HD leads to neurological damage is not fully understood, and the effect the protein aggregates have on the brain is still not completely clear. Some researchers, for example, have even argued that huntingtin aggregates are part of the brain's defense mechanism as it tries to cope with other pathogenic changes that are occurring. In general, though, the evidence supports the hypothesis that huntingtin aggregates are toxic and play some role in the progression of disease.

As huntingtin protein deposits accumulate in the brain of an HD patient, areas of the brain also begin to display neurodegeneration. As mentioned above, the basal ganglia (e.g. caudate, putamen) are strongly affected, but other regions like the substantia nigra, cerebral cortex, hippocampus, cerebellum, hypothalamus, and thalamus experience degeneration as well.

There is no cure for HD. There are a number of medications, however, that can be taken to treat the symptoms of the disease. These vary depending on the symptoms being targeted, and range from drugs liked tetrabenazine to treat chorea to selective serotonin reuptake inhibitors for depression. While drugs like tetrabenazine are effective in treating symptoms of HD, some also have significant side effects. This often leaves patients without great options for treatment. The sparsity of treatment options combined with the inevitably fatal nature of the disease contributes to the suicide rate being about 5 to 10 times higher in HD patients.

Reference (in addition to linked text above):

Walker FO. Huntington's disease. Lancet. 2007 Jan 20;369(9557):218-28.