Schizophrenia v. Bipolar Disorder: Different Risk Factors
Robin Murray gave a plenary presentation at the 65th Annual Scientific Convention of the Society of Biological Psychiatry this year, in which he indicated that the genetic risk for schizophrenia and other major mental disorders may be overestimated. He suggested that even in identical twins there are considerable differences in incidence of major psychiatric illnesses, and sharing an environment could further inflate the appearance of genetic risk.
Evidence of some genetic vulnerability factors, such as neuregulin, disbindin, DISC-1, zinc finger transcription factors, and neurexin, has been replicated. However, these genes appear to contribute only about 1% of the vulnerability to schizophrenia or bipolar illness. Copy number variations (CNVs, extra or missing copies of a gene, which may alter its activity) and gene micro-deletions (in which small bits of DNA are missing) have been found in about 5% of patients with schizophrenia, in some patients with autism and mental disabilities, but not in those with bipolar illness.
Murray emphasized the importance of psychosocial and neuromotor markers of neural development in determining risk of subsequent major psychiatric illness, rather than the relatively weak genetic effects. He cited the work of MacCabe (2009), who collected information from 907,000 individuals in Sweden. Their scholastic achievement at age 15?16 was rated, and hospitalizations for psychosis were recorded from age 17?31. Of the 315,000 followed for the long term, 493 developed schizophrenia and 208 developed bipolar disorder.
Predictors of cognitive and motor development in these two major psychiatric illnesses appeared to differ. In those who went on to develop schizophrenia, there was a slower rate of motor development, receptive language, and overall IQ in adolescence, while in those who went on to develop bipolar disorder, there was a faster rate of motor development, more language facility, and higher IQ in adolescence.
