17 September 2007. Schizophrenia researchers may be turned on by a new transgenic mouse. In the September Molecular Psychiatry, Mikhail Pletnikov and colleagues at Johns Hopkins University report that they have engineered mice with a mutant human DISC1 (hDISC1) transgene that can be turned on simply by feeding the mice an antibiotic. Many researchers believe that DISC1, or disrupted in schizophrenia, is the most promising of a growing list of schizophrenia susceptibility genes, and the new mouse should help them study how the timing of mutant DISC1 expression might influence the pathophysiology underlying schizophrenia and related diseases.
DISC1 was discovered in an extended Scottish family with a history of mental illness (see SRF related news story). In affected family members a chromosomal rearrangement results in the truncation of the DISC1 gene. Pletnikov and colleagues mimicked this natural genetic rearrangement, inserting a truncated human DISC1 gene, under the control of the tetracycline responsive element (TRE), into normal mice. The well-known TRE element is turned on by the tetracycline transactivator, tTA. The researchers crossed the DISC1 transgenic animals with a second transgenic line harboring a tTA gene driven by the neuron-specific calcium-calmodulin kinase II (CaMKII) promoter. In double transgenic offspring, neuron-produced tTA activates the mutant DISC1 gene. The whole system can be turned off by simply adding doxycycline to mouse chow, since doxycycline prevents activation of the TRE by tTA.
In keeping with the neuron-specific expression of CaMKII, Pletnikov and colleagues found that human DISC1 expression was restricted to mouse forebrain where it turned up as early as embryonic day 15 (ED 15). Human DISC1 was absent from the brainstem and the cerebellum, and was not detected in astrocytes or microglia, the other two major cell types in the brain. The mutant DISC1 appeared to have no effect on early neurodevelopment, since the architecture and morphology of the brain was normal, as was body weight and breeding and nesting behavior. However, at 9 months the volume of the brain lateral ventricles was significantly increased in double transgenic mice compared to those expressing tTA alone. The growth of lateral ventricles is also a feature of a transgenic mouse recently engineered by Akira Sawa’s group at Johns Hopkins (see SRF related news story), suggesting that this is a feature common to DISC1 mouse models. The authors believe this is due to a decrease in dendritic arborization, rather than neurodegeneration. In support of this, Pletnikov and colleagues found that there was significant loss of neurite complexity in primary cortical neurons isolated from the double transgenic mice and an associated decrease in levels of SNAP-25, a pre-synaptic protein marker. The authors also report that a putative dominant-negative mechanism of the neuronal effects of mutant human DISC1 could be related to its interaction with endogenous mouse DISC1, leading to decreased levels of mouse DISC1 and LIS1, a DISC1 protein partner. Both proteins are involved in neurodevelopment. Barbara Lipska and colleagues at the National Institutes of Health, Bethesda, Maryland, have also found reductions in LIS1 in brain tissue taken from people with schizophrenia (see SRF related news story).
The researchers subjected the mice to a range of tests to judge if they might have behavioral changes that relate to schizophrenia. Unlike the mice developed in the Sawa lab, these mice were no different from controls when tested for prepulse inhibition of the acoustic startle response. They also appeared to have normal olfaction and showed no differences in anxiety level. Male double transgenic mice did show greater spontaneous locomotor activity than controls, however, and they engaged more frequently in aggressive social interactions than controls. In contrast, female mice had some difficulty in a water maze test of spatial memory. It is not clear why these behaviors were sex-dependent, though there are significant sex differences in schizophrenia. One potential weakness of the model, the authors note, is that the mice were bred in a hybrid genetic background, which is likely to increase phenotypic variability. Nevertheless, “the opportunity to regulate expression of mutant hDISC1 is an advantage of the model, facilitating study of the timing of the effects of mutant DISC1 on brain and behavior development,” write the authors.—Tom Fagan.
Pletnikov MV, Ayhan Y, Nikolskaia O, Xu Y, Ovanesov M, Huang H, Mori S, Moran TH, Ross CA. Inducible expression of mutant human DISC1 in mice is associated with brain and behavioral abnormalities reminiscent of schizophrenia. Molecular Psychiatry. 2007. Abstract