Gene discovery in a putative mouse model of ADHD

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DESCRIPTION (provided by applicant): Abnormal thyroid levels during gestation can have devastating effects on brain development and cognition. Resistance to thyroid (RTH) syndrome is a heritable condition caused by mutations in the TRbeta gene that typically result in elevated thyroid hormones, short stature, and tachycardia. More than half of RTH patients have attention deficit hyperactivity disorder (ADHD), with the incidence about 50% higher among males. Although the etiology of ADHD is unknown, considerable evidence implicates deficiencies in the dopaminergic and noradrenergic neurotransmitter systems. A normally functioning thyroid system is critical for proper development of the catecholaminergic systems, and thyroid abnormalities can result in behavioral and neurochemical features consistent with ADHD. We have recently found that a TRbeta transgenic mouse bearing a human mutant thyroid beta1 receptor reproduces all of the key symptoms of ADHD, such as juvenile hyperactivity, deficits in sustained and selective attention, impulsivity, and reduced catecholamine levels. Interestingly, the TRbeta transgenic mice have normal levels of thyroid hormones, thyroid stimulating hormone (TSH), and suppression of TSH. This is intriguing because it raises the possibility that modest developmental thyroid dysfunction may contribute to a larger proportion of ADHD cases than previously thought. In addition to the core symptoms of the disorder, mice demonstrate many of the more subtle features of ADHD, e.g., the hyperactivity dissipates in adulthood, the penetrance is greater among males than among females, and the deficit in sustained attention is attenuated with greater reinforcement levels. Another interesting feature of the TRbeta transgenic mice is that the hyperactivity phenotype depends on the maternal genotype, independent of the mouse's own genotype. This suggests a possible biological or behavioral basis for maternal or environmental effects on ADHD subtypes. This high degree of analogy between complex human behavioral disorders in an animal model is unparalleled for a complex, multigenic behavioral disorder. We propose to use microarray technology to examine differential gene expression in wild-types vs. transgenics, males vs. females, and offspring of transgenic dams vs. offspring of wild-type dams, in pups, adolescents, and adults. The TRbeta transgenic mouse model provides us with a rare opportunity to discover genes downstream of TRbeta activity that are able to produce all of the core symptoms and many adjunct features of ADHD-genes that may be differentially expressed in a large number of children with ADHD. In addition, we have an unprecedented opportunity to discover how the relationship between gene expression and behavior differs according to diagnostic subtype, gender, treatment refractoriness, and environmental (e.g., maternal) conditions.