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“Having at our fingertips detailed information about when and where specific gene products are expressed in the brain brings new hope for understanding how this process can go awry in schizophrenia, autism and other brain disorders,” said NIMH Director Thomas R. Insel, M.D.

Among key findings in the prefrontal cortex:Individual genetic variations are profoundly linked to expression patterns. The most similarity across individuals is detected early in development and again as we approach the end of life.Different types of related genes are expressed during prenatal development, infancy, and childhood, so that each of these stages shows a relatively distinct transcriptional identity. Three-fourths of genes reverse their direction of expression after birth, with most switching from on to off.Expression of genes involved in cell division declines prenatally and in infancy, while expression of genes important for making synapses, or connections between brain cells, increases. In contrast, genes required for neuronal projections decline after birth – likely as unused connections are pruned.By the time we reach our 50s, overall gene expression begins to increase, mirroring the sharp reversal of fetal expression changes that occur in infancy.Genetic variation in the genome as a whole showed no effect on variation in the transcriptome as a whole, despite how genetically distant individuals might be. Hence, human cortexes have a consistent molecular architecture, despite our diversity.

Among key findings:Over 90 percent of the genes expressed in the brain are differentially regulated across brain regions and/or over developmental time periods. There are also widespread differences across region and time periods in the combination of a gene’s exons that are expressed.Timing and location are far more influential in regulating gene expression than gender, ethnicity or individual variation.Among 29 modules of co-expressed genes identified, each had distinct expression patterns and represented different biological processes. Genetic variation in some of the most well-connected genes in these modules, called hub genes, has previously been linked to mental disorders, including schizophrenia and depression.Telltale similarities in expression profiles with genes previously implicated in schizophrenia and autism are providing leads to discovery of other genes potentially involved in those disorders.Sex differences in the risk for certain mental disorders may be traceable to transcriptional mechanisms. More than three-fourths of 159 genes expressed differentially between the sexes were male-biased, most prenatally. Some genes found to have such sex-biased expression had previously been associated with disorders that affect males more than females, such as schizophrenia, Williams syndrome, and autism.

Now, researchers led by Mustafa Sahin, MD, PhD, of Children's Department of Neurology, provide evidence that mutations in one of the TSC's causative genes, known as TSC2, prevent growing nerve fibers (axons) from finding their proper destinations in the developing brain.

Sahin and colleagues showed that when mouse neurons were deficient in TSC2, their axons failed to land in the right places.

Further investigation showed that the axons' tips, known as "growth cones," did not respond to navigation cues from a group of molecules called ephrins.

f brain connectivity is the biological problem that gives rise to autism disorders will  effective treatments and cures be developed targeting the connectivity issues?

'People have started to look at autism as a developmental disconnection syndrome - there are either too many connections or too few connections between different parts of the brain,' says Sahin.

Sahin hopes that the brain's miswiring can be corrected by drugs targeting the molecular pathways that cause it.

It turns out that in all three of the primary cortices -- visual, auditory and somatosensory -- we did not see the typical response trial after trial in the individuals with autism. Instead, we saw considerable variability -- sometimes a strong response, sometimes a weak response. The fact that we did not see precise responses in autism was a really important result. It suggests that there is something fundamental that is altered in the cortical responses in autism. This variability in the brain response might also possibly explain why individuals with autism find visual stimulation, touch and sound to be so strong and overwhelming.

We know from genetic research that many of the neurobiological changes that occur in autism have to do with changes at the level of the synapse, the way that information is transmitted from one neuron to another.