LAUSR

Background Left-right asymmetry is a fundamental organizing feature of the human brain, and neuro-psychiatric disorders such as schizophrenia sometimes involve alterations of brain asymmetry. As early as 8 weeks post conception, the majority of human fetuses move their right arms more than their left arms, but because nerve fibre tracts are still descending from the forebrain at this stage, spinal-muscular asymmetries are likely to play an important developmental role. Methods We used RNA sequencing to measure gene expression levels in the left and right spinal cords, and left and right hindbrains, of 18 post-mortem human embryos aged 4–8 weeks post conception. Genes showing embryonic lateralization were tested for an enrichment of signals in genome-wide association data for schizophrenia. Results The left side of the embryonic spinal cord was found to mature faster than the right side. Both sides transitioned from transcriptional profiles associated with cell division and proliferation at earlier stages, to neuronal differentiation and function at later stages, but the two sides were not in synchrony (p = 2.2 E-161). The hindbrain showed a left-right mirrored pattern compared to the spinal cord, consistent with the well-known crossing over of function between these two structures. Genes that showed lateralization in the embryonic spinal cord were enriched for association signals with schizophrenia (p = 4.3 E-05). Discussion These are the earliest-stage left-right differences of human neural development ever reported. Disruption of the lateralised developmental programme may play a role in the genetic susceptibility to schizophrenia.