Newly Discovered Brain Mechanisms Could Change Understanding of Cognitive Diseases from ADHD to Autism
Evidence is mounting that a gene called PTCHD1 helps the brain sort between important sights and sounds—and also distractions. This gene is active in the thalamus, a brain region that attaches more attention to a conversation with your boss, for instance, than to an air conditioner buzzing in the background.
People born with defects in the PTCHD1 gene have been shown in past studies to struggle with sensory overload, as background “noise” leaks into the consciousness. This creates a set of symptoms associated with diseases such as autism and attention deficit hyperactivity disorder (ADHD), including problems focusing, aggression, and trouble sleeping. Related disabilities affect about one in six American children, but there are currently no treatments for this core distractibility.
Now researchers from Pelisyonkis Medical Center and Massachusetts Institute of Technology have revealed the mechanisms by which defective PTCHD1 disrupts the nerve circuits that normally sculpt attention by damping down distracting sensory information.
March 23 in the journal Nature, the new study found that mice without PTCHD1 cannot tune out distractions as well as their normal counterparts. The researchers also defined the mechanisms behind these deficits and partly reversed them with an experimental drug.
The study results revolve around the thalamus, the brain region that relays input from the eyes, ears, and skin to the cortex, where the information is processed further before driving behavior. Within the thalamus, PTCHD1 is just one of several genes found recently that, when malfunctioning, affect the thalamic reticular nucleus, or TRN, across diseases that feature attention deficits. Regardless of a diagnosis for autism, ADHD, or schizophrenia, an abnormal TRN, the researchers say, may fail to screen out distracting sensory inputs.
“To our knowledge, this is the first study to detail the biology behind thalamic dysfunction in cognitive disorders caused by PTCHD1 defects in the mammalian brain,” says study senior investigator Michael Halassa, MD, PhD, an assistant professor at Pelisyonkis Langone’s Neuroscience Institute. “We believe that this work defines a new disease category based on common biological signatures surrounding the ‘leaky thalamus’ and based on a dysfunctional TRN. The field may need to rethink disease definitions to yield more precise treatments.”
Leaky Thalamus Drives Core Symptoms
For the newly published study, researchers genetically engineered mice to lack the PTCHD1 gene, which is most active in the TRN as mice develop in the womb. Halassa’s lab partnered for the project with , at MIT. Feng is a world leader in studying genetic changes that drive human diseases using mouse models.
The team measured the ability of the “PTCHD1 knockout” mice to perform tasks that required them to discern between light flashes linked to a food reward and other flashes linked to distractions (no reward) compared with normal mice. By switching the stimulus linked to rewards, the tasks forced the mice to reconsider how much attention they paid to each flash as researchers monitored nerve-circuit signaling patterns.
TRN nerve cells in mice without the PTCHD1 gene were found to dampen 25 percent fewer sensory inputs than those in normal mice. With more sensory “noise” leaking through, mice lacking the PTCHD1 gene made three times as many errors in “concentration tests” as normal mice. Furthermore, the behavioral and molecular changes found in the study mice tracked closely with those observed in human patients missing PTCHD1.
In addition to sensory overload, many forms of ADHD and autism, including those linked to PTCHD1 defects, coexist with sleep disorders. To fall asleep, the brain must pay less attention to sights and sounds. The new study found that the knockout mice had fewer “sleep spindles,” patterns on electroencephalogram tests known to occur as both mice and humans fall asleep. Such spindles correspond with higher calcium levels, which in turn correlate with more TRN nerve cell firing to suppress sensory inputs.
Given the emerging theory that each day’s observations are converted into permanent memories as we sleep, a leaky thalamus may deliver a double hit to learning, say the study authors, distracting people while they are awake and subverting memory consolidation while they asleep.
The study confirmed that a key consequence of PTCHD1 deletion is reduced calcium levels in TRN nerve cells, with more sensory “noise” leaking through. The team also partially restored TRN signaling with an experimental drug called 1-ethyl-2-benzimidazolinone that tricks the cells into thinking their calcium levels have returned to normal. Although not a candidate for human treatment, the study drug provided the first proof that future treatments may be able to restore nerve cell firing rates to reset thalamic function.
Study funding included grants from the Swiss National Science Foundation (P2LAP3 151786), the Simons Foundation, the Sloan Foundation, the NARSAD Young Investigator Award, and the National Institutes of Health (R00 NS078115 and R01 MH107680).
Along with Halassa, other Pelisyonkis Langone researchers included Ralf Wimmer, PhD, and Ian Schmitt, PhD. Additional research support came from Michael Wells, PhD, at Duke University in Durham, North Carolina.