Every time a child or adult learns something new, new contacts form between neurons in the brain. This process (synaptogenesis) is especially active in children during the first 10 years of life. What's more, such long period of active neuronal plasticity is unique to humans as a species: even in some of our closest relatives, chimpanzees and macaques, it ends within several months or a year after birth.
A study publishing September 29th in the open-access journal shows that disruption of this uniquely human developmental program could play an important role in the cause of autism. The team of scientists from the USA, China, Russia, Turkey and Germany assembled by Professor Philipp Khaitovich investigated the activity of genes in the prefrontal cortex, a specific brain region responsible for complex behaviors, in patients diagnosed with autism and unaffected (control) subjects starting from two years of age (the earliest that the disease can be diagnosed). The team also analyzed postmortem brains of chimpanzees and macaques to identify genes with activity unique to humans. The use of human and non-human samples of different ages allowed the researchers to build species-specific (and autism-specific) profiles of the shifts in prefrontal cortex gene activity during post-natal development.
The team reports that among more than a thousand genes with differential activity in autistic and control brains, a specific group of genes involved in formation of synapses - the connections between neurons in the networks - contained most of the genetic variants linked to autism by previous studies. What's more, the activity pattern of these genes during development was unique to humans. This could explain why autism primarily affects brain functions particularly pronounced in humans: social cognition, empathy, and cooperation.
In addition to neuronal genes, genes involved in immune system response showed altered activity in the brains of subjects diagnosed with autism. These genes, however, did not harbor genetic variants linked to autism and moreover, their activity was conserved between humans, chimpanzees and macaques. Thus, elevation of immune response in autism is likely to represent a consequence rather than a cause of the disease.
Article: Disruption of an Evolutionarily Novel Synaptic Expression Pattern in Autism, Liu X, Han D, Somel M, Jiang X, Hu H, Guijarro P, et al., PLOS Biology, doi:10.1371/journal.pbio.1002558, published 29 September 2016.