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.
IQ-related risks were notably different for the two disorders. People who fell within the two standard deviations below the mean IQ had almost 4-fold higher rates of schizophrenia, while those who scored higher than two standard deviations above the mean were at decreased risk. In contrast, those with IQs two standard deviations above the mean were at 3-fold increased risk for bipolar disorder, with a smaller subgroup showing mildly increased risk with lower IQs as well.
These data are partially consistent with observations of increased obstetrical complications in schizophrenia, including neonatal insults to the brain (during delivery) resulting in brain damage, small growth, and a 7-fold increased incidence of neonatal hypoxia (low blood oxygen concentration). In contrast there is generally little evidence for an increase in obstetrical complications in bipolar illness. Instead, being highly creative and an exceptional, enthusiastic student appears associated with a slightly increased risk for bipolar illness. Other risk factors for schizophrenia included: living in a bigger city, experiencing childhood abuse and neglect, social isolation, adverse life events (of the intrusive variety, such as being in a car accident, and not of the loss variety, such as losing a job or significant other, which are more associated with bipolar illness).
Editor’s Note: We have postulated that the increased intelligence and creativity seen in those with bipolar disorder (or at risk for it) could be related to the findings that a common variant in the gene for brain-derived neurotrophic factor (BDNF) that functions more efficiently (val 66 val proBDNF) is a risk factor for bipolar disorder. The poorer functioning allele (val 66 met proBDNF) is a risk factor for mild cognitive dysfunction in both patients with bipolar disorder and schizophrenia, as well as in normal volunteer controls.
Murray formulated the hypothesis that environmental events could alter the dopamine system, which has been closely linked to psychosis and its treatment with dopamine antagonists (antipsychotics). He cited data indicating that dopamine synthesis, as measured by PET scan, was increased in those with prodromal psychosis. Since dopamine is associated with reward learning and the salience of ideas and objects, he reasoned that increases in dopamine could account for delusions, because the increases in dopamine might involve assigning salience to unimportant stimuli.
Murray noted that pre-term hypoxia leads to a 50% increase in the size of the ventricles and a decrease in hippocampal volume, and other investigators have shown that ventral/hippocampal lesions lead to sensitization of the dopamine systems. Tony Grace has postulated that decreased hippocampal excitatory output reverses the break on dopamine release in the n. accumbens, and increases dopamine and cell firing in the midbrain ventral tegmental dopamine neurons that synapse in the n. accumbens.
Such a model may also have relevance for the pathophysiology of bipolar disorder. Those experiencing a first psychotic episode have higher than normal levels of cortisol in their blood, and increased cortisol is associated with decreased hippocampal volume.
Editor’s note: These findings may intersect with data that show that BDNF appears decreased in the hippocampus of patients with depression, rodents subjected to defeat stress, and cocaine-sensitized animals. Increases in BDNF have been noted in the n. accumbens of depressed patients who died via suicide compared with controls (Krishnan et al. 2008). Moreover, increases in BDNF in the n. accumbens are also seen in animals experiencing defeat stress-induced depressive-like behaviors, and in cocaine sensitization paradigms.
Thus, deficient hippocampal function as marked by decreased volume in those with schizophrenia, or decreased hippocampal BDNF in those with mood disorders, could be associated with disinhibition of firing of dopamine neurons in the ventral/tegmental area and increased release of dopamine in that area of the brain along with increases in BDNF as well. This BDNF formulation is also consistent with the environmental vulnerabilities Murray describes acting to alter dopamine functioning in the psychoses. Environmental stressors and exposure to abused substances could cause hippocampal volumetric and functional deficits in concert with n. accumbens hyperactivity.
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