This is a useful model from Pletnikov, Ross, and colleagues, but like all models, it has some limitations. Since DISC1 is known to have a strong role in development and physiology, the development of inducible mutants is necessary to separate the two.

In the TeT-off system used in the paper, mice must be treated with doxycycline for their entire lives to keep the expression of this gene off. Doxycycline must be used at high levels and may have side effects when used this long. The TeT-on system is better because doxycycline is only used transiently for 1 week for maximum induction then washed away. The TeT-on system is also available for the same promoter used in the paper, that of the CaMKII gene.

The phenotype of reduced neurite length was obtained from in vitro neuron cultures, which are prone to artifacts. There are ways of labeling these neurons in vivo for measuring neurite length and spines. The brain phenotype was obtained by MRI. There are ways, such as adding manganese, of enhancing active pathways. This has been done in the bird brain to map song pathways.

The behavioral phenotype was similar to the recent paper from the Sawa group (Hikida et al., 2007) in that it also analyzed a transgenic mouse expressing the same C-terminal truncation of the human DISC1 gene, using the same CaMKII promoter. An important difference in the findings was a reduction of murine DISC1 (50 percent at protein level) in the Pletnikov et al. mice but none in the Sawa group mice. This issue is important because of a recent paper in Cell by the Song group (Duan et al., 2007). In that paper, RNAi was used to reduce wild-type native murine DISC1. Individual neurons with targeted DISC1 knockdown showed accelerated neurite development, greater synapse formation and enhanced excitability. Hippocampal granule cells showed accelerated morphological integration resulting in mispositioning. Unfortunately, in the Song paper they analyzed only cells with complete or no knockdown of DISC1. Partial knockdown vectors were made that achieved 75 percent reduction at the protein level but were not analyzed. Only then would it be possible to compare these morphological data with those from Pletnikov et al., which was a 50 percent reduction. Another difference was that the Song group found that DISC1 needed to interact with Nudel. Pletnikov et al. found normal levels of Nudel in the mice but lower LIS1, which could explain the brain development phenotype.

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