Researchers have identified specific genes containing rare mutations that significantly increase the likelihood of developing attention deficit hyperactivity disorder. These findings, published in the journal Nature, suggest that distinct biological mechanisms involving nerve cell communication play a major role in the condition. The study links these genetic alterations to broader life outcomes, including educational attainment and socioeconomic status.
Attention deficit hyperactivity disorder is a neurodevelopmental condition that affects a significant portion of the global population. Previous research indicates that the disorder is highly heritable, meaning it is largely passed down through families.
Most genetic studies have focused on common variants, which are small changes in DNA found in many people that individually have a tiny effect on risk. However, these common changes do not account for the full genetic picture of the disorder. A gap remained in understanding how rare but powerful genetic mutations might contribute to the diagnosis.
To address this, a large international team led by researchers from Aarhus University in Denmark and the Broad Institute of MIT and Harvard initiated a massive genetic analysis. The lead authors include Ditte Demontis, Jinjie Duan, and Anders D. Børglum, who sought to pinpoint specific genes that carry a high risk. They aimed to uncover the biological machinery that malfunctions when these specific genetic errors occur.
The team utilized the iPSYCH cohort, which is a large collection of genetic data derived from the Danish population. They performed whole-exome sequencing on nearly 9,000 individuals diagnosed with attention deficit hyperactivity disorder and approximately 54,000 individuals without the diagnosis.
Whole-exome sequencing focuses on the protein-coding regions of the genome, where mutations are most likely to disrupt biological function. This targeted approach allows scientists to filter out the noise of the non-coding genome and focus on changes that alter the structure of proteins.
The analysis revealed three specific genes where rare variants were significantly more common in people with the disorder compared to the control group. These genes are named MAP1A, ANO8, and ANK2. The study indicates that carrying a rare variant in these genes can increase the risk of developing the condition by up to 15 times. This effect size is substantially larger than what is typically seen with common genetic variants.
The researchers then sought to understand the function of these specific genes within the human body. To do this, they analyzed gene expression patterns in various types of brain tissue. They discovered that these genes are highly active in the brain during fetal development and continue to be important into adulthood. This suggests that the genetic roots of the disorder may be established very early in life.
Specifically, the variants appear to affect dopaminergic and GABAergic neurons. These distinct types of nerve cells are essential for regulating attention, motivation, and impulse control. Dopaminergic neurons allow the brain to process rewards and maintain focus. GABAergic neurons act as a braking system for neural activity, helping to prevent overexcitement in the brain.
To further explore these mechanisms, the investigators examined how the proteins produced by these genes interact with other proteins. They utilized induced pluripotent stem cells to grow human nerve cells in the laboratory for this purpose. This allowed them to observe the cellular machinery in a controlled environment. The experiments showed that MAP1A, ANO8, and ANK2 interact with a wider network of proteins.
These protein networks are often implicated in other neurodevelopmental conditions. The study found significant overlap with genes associated with autism and schizophrenia. This suggests a shared biological foundation across different psychiatric diagnoses. Disruption in these networks appears to affect the cytoskeleton, which provides structure to cells, and ion channels, which help cells communicate electrically.
MAP1A encodes a protein involved in the assembly of microtubules, which are structural components of the cell. ANK2 and ANO8 encode proteins involved in transporting calcium ions across the cell membrane. The proper flow of calcium is essential for the transmission of signals between neurons. When these genes are mutated, the structural integrity and signaling capabilities of the neurons may be compromised.
Beyond the diagnosis itself, the team investigated how these rare variants correlate with life outcomes. They linked the genetic data to Danish national registries containing detailed information on education and employment. The analysis showed that individuals with the disorder who carry these rare deleterious variants had lower educational attainment on average. They were also more likely to have a lower socioeconomic status compared to those without the variants.
The definitions of low socioeconomic status included receiving social security payments, early retirement benefits, or experiencing long-term unemployment. Individuals with the disorder and these specific mutations were five to seven times more likely to fall into these categories than the general population. This association held true even when the researchers accounted for other factors.
The study further assessed intellectual function in a subgroup of adults. The data revealed a link between these rare mutations and cognitive performance. For each rare high-risk variant an individual carried, their IQ score was lower by approximately 2.25 points. This provides evidence that the genetic architecture of the disorder directly influences cognitive domains.
The authors also looked at how these rare variants interact with the more common genetic risks. They found that the risks act additively. An individual might have a background load of common risk variants, and the presence of a rare variant adds another layer of susceptibility on top of that. The combined weight of these genetic factors pushes the individual past the threshold for diagnosis.
Ditte Demontis explains the importance of the findings regarding brain development. “Our findings support that disturbances in brain development and function are central to the development of ADHD,” she says. The identification of specific cell types provides a clearer target for future biological research.
While this study identifies three specific genes, the authors estimate that many more rare risk variants remain to be discovered. The current sample size allowed for the statistical confirmation of only the strongest signals. Future research will require even larger cohorts to map the remaining genes involved in the disorder.
The study also notes that these rare variants explain only a small fraction of the total cases. Most individuals with the diagnosis do not carry these specific high-impact mutations. The majority of the risk in the general population is still driven by the accumulation of many common variants.
Additionally, the overlap with intellectual disability was significant in the cohort. Approximately 18 percent of the individuals with the disorder also had a diagnosis of intellectual disability. However, the associations with education and socioeconomic status held true even when excluding those with intellectual disabilities. This confirms that the impact on life outcomes is not solely due to cognitive impairment.
Uncovering these mechanisms opens new avenues for understanding the biology of the brain. The discovery of causal genes with high-effect variants provides a foothold for deeper mechanistic studies. Anders D. Børglum notes that these genes “give us insight into some of the fundamental biological processes, which can guide the design of deeper mechanistic studies.”
This could eventually lead to the identification of new targets for pharmaceutical treatment. By understanding exactly which ion channels or structural proteins are affected, drug developers might create more precise therapies. Currently, treatments focus on symptoms, but genetic insights offer the potential for addressing underlying causes.
Jinjie Duan emphasizes that this is just the start of this line of inquiry. “We are only at the beginning of uncovering these rare high-effect variants,” Duan says. The team’s calculations suggest that many other genes will be implicated as sample sizes grow.
The study, “Rare genetic variants confer a high risk of ADHD and implicate neuronal biology,” was authored by Ditte Demontis, Jinjie Duan, Yu-Han H. Hsu, Greta Pintacuda, Jakob Grove, Trine Tollerup Nielsen, Janne Thirstrup, Makayla Martorana, Travis Botts, F. Kyle Satterstrom, Jonas Bybjerg-Grauholm, Jason H. Y. Tsai, Simon Glerup, Martine Hoogman, Jan Buitelaar, Marieke Klein, Georg C. Ziegler, Christian Jacob, Oliver Grimm, Maximilian Bayas, Nene F. Kobayashi, Sarah Kittel-Schneider, Klaus-Peter Lesch, Barbara Franke, Andreas Reif, Esben Agerbo, Thomas Werge, Merete Nordentoft, Ole Mors, Preben Bo Mortensen, Kasper Lage, Mark J. Daly, Benjamin M. Neale & Anders D. Børglum.
