Genetic markers for ADHD influence early toddler activity levels

A toddler's activity levels are influenced by genetic differences, with some of the same genetic markers also linked to Attention Deficit and Hyperactivity Disorder (ADHD), according to new research from the University of Surrey. 

In the first study of its kind, an international team of scientists, led by Professor Angelica Ronald, analysed data from nearly 80,000 infants from multiple national and international cohort studies, to discover the genetic differences that influence toddler activity levels. Genetic differences (or genetic variation) refer to the differences in DNA sequences between individuals. 

Previous studies indicate that toddler activity levels vary widely between children. Environmental factors are known to play a role, however, this is the first study to identify the genetic markers involved. 

Professor Angelica Ronald, lead author of the study and Professor of Psychology and Genetics at the University of Surrey, said: 

"From 12 to 36 months is a very exciting time for parents and children, with a lot of new skills being learnt. During this time, we know that toddlers vary widely in how much they move around. Some children are happy to sit still whereas others are livelier, they run around and move a lot. Activity levels are important and likely influence a wide variety of experiences including learning, exploring the wider environment, development of motor skills and the type of parenting that children receive. 

"We were interested to learn about the underlying biology of early activity levels and whether these behaviors share some of the same genetic influences that affect things late in life such as diagnoses of ADHD and educational outcomes. Our results can be used to think about how better to identify young children who would benefit from early interventions for their ADHD-like behaviors." 

The team of scientists were able to map and identify the specific genetic differences influencing toddler activity levels. Ten percent of the variation in toddler activity levels can be explained by common genetic differences discovered by the researchers. The genetic markers they discovered were found to overlap strongly with those that influence likelihood of ADHD which is diagnosed later in childhood or in adulthood. 

As a specific example, markers in the gene RHEBL1 were identified as being associated with lower toddler activity levels. These same genetic markers also affect how active the RHEBL1 gene is in the cortex, a major region of the brain. In other words, the same DNA variations that seem to influence lower toddler activity levels also appear to turn up the activity of the RHEBL1 gene in this brain region. This finding opens up new avenues for understanding how a child's early activity levels are shaped by their biology. The findings could help scientists better understand the relationship between movement, brain development and learning. 

Anja Hollowell, who was the first author of the study and completed the work while at the University of Surrey and Birkbeck, and is now based at UCL, explained: 

"Our findings show that very early activity levels are influenced by many of the same genetic markers that influence later ADHD. Earlier identification of ADHD would be very helpful for providing timely support to children and their families. 

"We did not know until now that the genetic basis of ADHD was shared with these early behavioral traits." 

As part of the same study, the team also discovered genetic variants associated with other key toddler behaviors, namely, sociability, emotionality and shyness. They identified multiple genes that were associated with these behaviors. 

Interestingly, the genetic markers associated with toddler emotionality were also found to be associated with neurotic personality traits in adulthood. In addition, the genetic variation associated with toddler emotionality and with low sociability overlapped with the genetic variation known to be associated with likelihood of autism. 

Professor Ronald added: 

"For the first time, we have identified the genetic variation associated with these early behavioral traits in toddlers. Our work highlights the significance of 'nature' in a young child's behavior. This complements the existing research on environmental influences on early childhood. These new results advance our understanding of the early pathways influencing individual differences in child development." 

This study was published in Nature Human behavior.