14 November 2007. The relationship between schizophrenia and neurodevelopment may have just gotten stronger. In this week’s PNAS, researchers in the United States and Finland, led by Alcino Silva and Tyrone Cannon, University of California, Los Angeles, report that an inducible, reversible DISC1 C-terminal fragment, which is believed to mimic the effects of a mutation found in some families with schizophrenia and other psychiatric disorders, causes schizophrenia-like symptoms when turned on in young but not in adult mice. The findings suggest that disruption to DISC1 during brain development may contribute to disease pathology.
DISC1 was originally discovered when a translocation in the gene was linked to an extended Scottish family with increased risk for mental illness, particularly schizophrenia and bipolar disorder (see Millar et al., 2000). Since then, DISC1 has been a focus for researchers and has led to several mouse models of disease. What is interesting about this new model is that the gene can be turned on and off at will simply by adding or removing an inducer—the estrogen analog tamoxifen.
First author Weidong Li and colleagues coupled a DNA fragment coding the C-terminal fragment of DISC1 (amino acids 671-852) to a piece of DNA that codes for a mutant estrogen receptor ligand binding domain (LBD) that binds tamoxifen but not estrogen. The chimeric protein is normally silenced by chaperone proteins, but on addition of tamoxifen the LBD undergoes a conformational change that frees it from the chaperones. Once the construct was established in transgenic mice, the researchers were able to “turn on” the DISC1 fragment by simply giving the animals the estrogen analog.
Li and colleagues confirmed by Western blot analysis that the transgene, under the control of the neuronal α-calmodulin kinase II promoter, was expressed in the brain (cortex, hippocampus, striatum, and cerebellum). They also confirmed that the mice appeared and behaved normally—in the absence of tamoxifen. When the drug was administered, however, the animals’ behavior changed.
The researchers found that when the transgene was turned on at postnatal day 7, the animals exhibited behavioral deficits that have correlates in patients with schizophrenia. They had spatial working memory deficits (more frequently made incorrect choices in a delayed non-match to place task), showed symptoms of depression (gave up earlier in a forced swim test), and were asocial compared to wild-type mice (showed less interest in sniffing out a new mouse introduced into the same chamber). Interestingly, delaying administration of tamoxifen until the mice reached adulthood spared them from these deficits.
Bringing in Nudel and Lis1
How can this C-terminal fragment of DISC1 induce such profound behavioral changes? The researchers believe that the fragment binds to DISC1 partners, such as Nudel and Lis1 (see SRF related news story), preventing them from interacting with the full-length, functional DISC1. This type of dominant-negative effect has been proposed to explain human DISC1 effects and has served as the basis for a transgenic mouse model recently described by Akira Sawa and colleagues at Johns Hopkins University, Baltimore, Maryland (see SRF related news story).
To test this dominant-negative theory, Li and colleagues immunoprecipitated the chimeric protein and tested for the presence of Nudel and Lis1. They found that the chimera did, indeed, bind these proteins, which lends some credence to the idea. Furthermore, the researchers found that turning on the transgene led to morphological and functional changes in the brain, which is in keeping with interfering with Nudel and Lis1, proteins that play important roles in neurodevelopment. Li and colleagues found that animals given tamoxifen at postnatal day 7 had reduced dendritic complexity in that the number of branch points and intersections in hippocampal neurons was reduced. Hippocampal slices also exhibited reduced baseline neurotransmission, though long-term potentiation appeared to be unaffected.
How does this model relate to the human condition? Collaborating with colleagues in Finland, the researchers found that a DISC1 haplotype previously linked to risk for schizophrenia, working memory problems, and reduced gray matter density (see Cannon et al., 2005) quadruples the likelihood that the carrier will have deficits in sociability. The authors conclude that their findings “proved a critical functional link between the histological ramifications of altered DISC1 and the reduced gray matter density in schizophrenia that is known to vary with genetic proximity to affected individuals in monozygotic and dizygotic twins discordant for this disorder and to be associated with schizophrenia-related haplotypes of DISC1."—Tom Fagan.
Li W, Zhou Y, Jentsch JD, Brown RAM, Tian X, Ehninger D, Hennah W, Peltonen L, Lonnqvist J, Huttunen MO, Kaprio J, Trachtenberg JT, Silva AJ, Cannon TD. Specific developmental disruption of disrupted-in-schizophrenia-1 function results in schizophrenia-related phenotypes in mice. PNAS. 2007 Nov 13;104:18280-18285. Abstract