https://www.washingtonpost.com/news/speaking-of-science/wp/2016/01/27/scientists-open-the-black-box-of-schizophrenia-with-dramatic-genetic-finding/?postshare=6141453998947337&tid=ss_gp
The researchers, chiefly from the Broad Institute, Harvard Medical School and Boston Children's Hospital, found that a person's risk of schizophrenia is dramatically increased if they inherit variants of a gene important to "synaptic pruning" -- the healthy reduction during adolescence of brain cell connections that are no longer needed.
In patients with schizophrenia, a variation in a single position in the DNA sequence marks too many synapses for removal and that pruning goes out of control. The result is an abnormal loss of gray matter.