A genetic screen for behavioral mutations that perturb dopaminergic homeostasis in mice

Abstract

Disruption of dopaminergic (DA) systems is thought to play a central role in the addictive process and in the pathophysiology of schizophrenia. Although inheritance plays an important role in the predisposition to these disorders, the genetic basis of this is not well understood. To provide additional insight, we have performed a modifier screen in mice designed to identify mutations that perturb DA homeostasis. With a genetic background sensitized by a mutation in the dopamine transporter (DAT), we used random chemical mutagenesis and screened for mutant mice with locomotor abnormalities. Four mutant lines were identified with quantitatively elevated levels of locomotor activity. Mapping of mutations in these lines identified two loci that alter activity only when dopamine levels are elevated by a DAT mutation and thus would only have been uncovered by this type of approach. One of these quantitative trait loci behaves as an enhancer of DA neurotransmission, whereas the other may act as a suppressor. In addition, we also identified three loci which are not dependent on the sensitized background but which also contribute to the overall locomotor phenotype.Keywords: Dopamine, ENU, Locomotor, mouse, phenotypedriven screen Disorders involving dopaminergic (DA) neurotransmission have been implicated in a variety of neurological disorders including schizophrenia, attention deficit hyperactivity disorder, Parkinson's disease and drug addiction (Castellanos & Tannock 2002;Lotharius & Brundin 2002;Sawa & Snyder 2002;Wise 2004). The range of possible molecular and cellular mechanisms that might contribute to alterations in dopamine-regulated behavior is very large. For example, alterations in the density or effectiveness of synaptic inputs to DA neurons could be as important as could similar changes on the output side. At the molecular level, alterations in the metabolism or catabolism of dopamine or in the efficiency of signal transduction downstream of the receptors could conceivably have important effects. Given the prevalence and inheritance patterns of diseases involving DA transmission, it is likely that multiple genes influence the vulnerability to each disease. A variety of techniques including human genetic mapping, the construction of mouse transgenic models and invertebrate forward genetic screens are being used to uncover underlying factors. To complement the above approaches, we have developed an N-ethyl-N-nitroso-urea (ENU) mutagenesis screen in mice designed to uncover mutations that perturb DA homeostasis.In mice, ENU is a highly potent mutagen. Male mice (G0) treated with ENU are bred to produce first generation (G1) mice harboring a fixed set of mutations. This treatment can raise the mutation rate 300-fold above the spontaneous background mutation rate (Hitotsumachi et al. 1985;Russell et al. 1982). Although mutation rates vary from gene to gene, specific locus tests suggest that new alleles at any locus will be generated in one of 500-1500 G1 animals, making a directed screen for mutant phenotypes an ...

N -Ethyl- N -Nitrosourea Mutagenesis: Boarding the Mouse Mutant Express

“…For example, small-scale genetic screens have recently identified mice with altered responsiveness to serotonin or dopamine (56,63). Such pharmaceutically oriented strategies may be of particular value in uncovering the often mystifying molecular and physiologic mechanisms of drug action and thereby may facilitate development of individually tailored treatments for human patients.…”

“…In addition, there has been some interest in devising screens that uncover subtler deficits in specific neurotransmission systems. Thus, we have devised a screen to assess serotonin responsiveness in the mouse, while in independent efforts others have assessed dopaminergic responsiveness (56,63). The results from these "pharmacologically" and genetically sensitized screens have been promising.…”

A Genetic Screen for Modifiers of the Delta1-Dependent Notch Signaling Function in the Mouse

“…These already have proven to be highly efficient in the fly and recently also in the mouse (Cormier et al 2000;Carpinelli et al 2004;Curtis 2004;Murcia et al 2004;Speca et al 2006). N-ethyl-N-nitrosourea (ENU) mutagenesis has been used as a powerful tool for the establishment of mutant mouse lines that model diseases and for the elucidation of protein function Nolan et al 2000).…”

Uncovering the uncharacterized and unexpected: Unbiased phenotype‐driven screens in the mouse

“…One such screen to discover genes that influence the dopaminergic response took advantage of a mouse background that is already hyperactive due to homozygous mutation of the dopamine transporter (DAT) gene. This screen identified four mutant lines with altered locomotor activity and showed the phenotype of two lines was only visible on the DAT null background, indicating they would not have been identified in a non-sensitized screen (Speca et al, 2006). As has been true in other model organisms, enhancer and suppressor screens promise to be powerful in the mouse.…”