Early- and late-diagnosed autism have different genetic signatures

Autism diagnosed early in life has different characteristics and genetic influences from later-diagnosed cases, new research suggests.

Researchers from the University of Cambridge showed that polygenic factors differ depending on age at diagnosis for autism. Earlier-diagnosed and later-diagnosed autism are linked to different developmental trajectories, and the link between autism and attention deficit hyperactivity disorder (ADHD) increases with later diagnosis. These results suggest that an autism diagnosis is influenced by more than just differences in healthcare or awareness.

'The term "autism" likely describes multiple conditions,' said Dr Varun Warrier, senior author of the study published in Nature. 'Understanding how the features of autism emerge not just in early childhood but later in childhood and adolescence could help us recognise, diagnose, and support autistic people of all ages.'

The researchers looked at data from four large international birth cohorts, conducting a genome-wide association study. They identified multiple genetic variants associated with early or late-diagnosed autism, and there was little overlap between the two groups.

The results indicated that many factors may contribute to the age at diagnosis for autism. The authors calculated that 11 percent of the variance in age at diagnosis is caused by genetic factors, which they noted is similar to the below 15 percent caused by socioeconomic or clinical differences.

The study also reported that diagnosis age for autism is partly heritable and that later-diagnosed autism is correlated with other mental-health conditions such as depression, post-traumatic stress disorder and self-harm.

The authors acknowledge that factors such as the use of self-reported data, the limited diversity of genetic data available, as well as large variations between the information available for the different cohorts, may have had an effect on their findings.

'It makes me hopeful that even more subgroups will come to light, and each will find an appropriate diagnostic label,' said cognitive neuroscientist Professor Uta Frith from University College London, who was not involved in the study. 'It is time to realise that "autism" has become a ragbag of different conditions. If there is talk about an "autism epidemic", a "cause of autism" or a "treatment for autism", the immediate question must be, which kind of autism?'