Behavioral characteristics exhibit a strong genetic component, as evidenced by similarities among family members, and particularly between identical twins, in behavior, personality, and cognitive ability. Because behavior is typically genetically complex, mouse models in which the importance of one or a few genes may be examined in isolation are invaluable tools to investigate the molecular mechanisms underlying behaviors or their disorders; many human behaviors have measurable correlates in mice. Several behavioral gene discoveries made in the Center for the Genetics of Host Defense, including Scn10a and Tomt (below), began as observations of atypical mouse behavior, such as circling, head tossing, or sustaining odd postures. We continue to monitor mutagenized mice for uncharacteristic behavior, to determine if those behaviors are heritable, and to map the mutations associated with them. In addition, together with our collaborators, we are using the forward genetic approach to identify genes that protect against addiction (in collaboration with the Self Laboratory) and abnormal fear responses (in collaboration with Quinghua Liu). The genetic screens involve observing and measuring behaviors of mutagenized mice before and after the presentation of a stimulus (for example, a choice between a familiar versus a novel environment). We also screen for mutations that result in autistic-like behavior; these screens involve measuring behaviors such as nesting, grooming, and vocalizations in mutagenized mice (in collaboration with Maria Charnour). Motor function, the ability to control and coordinate the movements of the muscles, is also under surveillance in a genetic screen that tests the ability of mutagenized mice to balance on a rotating rod (in collaboration with Juan Pascual).
Mice with a dominant mutation of Scn10a called Possum displayed a striking, inducible behavior in which scruffing the mice (grasping the loose skin at the back of the neck, a routine technique used to immobilize mice for various procedures) resulted in arrest of all movement, assumption of a rigid posture, and transient apnea. The induced tonic immobility lasted approximately one to five minutes, after which mice abruptly resumed normal activity. Scn10a encodes the pore-forming α-subunit of the voltage-gated sodium channel Nav1.8, which is expressed in sensory neurons of the trigeminal and dorsal root ganglia (DRG). The Possum mutation resulted in increased Nav1.8 channel currents, and increased excitability of small DRG neurons from Possum heterozygous mice. Dominant missense mutations adjacent to the cytoplasmic leaflet of the plasma membrane lipid bilayer in other sodium channels have also been reported to cause channel hyperactivation. The Possum mutation is located at the cytoplasmic interface of Nav1.8, in one of the pore-lining helices, and may hold the channel in a permanently active position.
Nav1.8 propagates pain signals induced by cold, heat, and mechanical stimuli in humans. Possum mice were hypersensitive to cold pain; they displayed normal pain responses to heat and mechanical stimuli. However, noxious cold, heat, or mechanical stimuli, and induced fear failed to induce tonic immobility in Possum mice. Cardiac abnormalities (vagus nerve-stimulated sinus bradycardia and irregular RR intervals on electroencephalogram [EEG]) were also not responsible for scruffing-induced immobility. Thus, the mechanistic basis of the behavior is yet unknown. However, Possum mice displayed altered CNS activity as measured by EEG during tonic immobility, suggesting that Nav1.8 may modulate the activity of CNS neurons and thereby contribute to this induced behavior.
The Possum phenotype and causative Scn10a mutation are reported in (Blasius et al. Proc.Natl.Acad.Sci.U.S.A. 108, 19413-19418).
The neurobehavioral phenotype called “add” was striking for two reasons: first, the causative mutation was attributed to an unannotated gene, now called Tomt or Comt2, the expression of which was firmly established by our data; second, based upon our findings in Comt2 mutant mice, mutations in the orthologous human gene were found to cause nonsyndromic deafness in humans.
The add phenotype was first observed as a behavioral abnormality in which homozygotes exhibited circling, head tossing, and repetitive short-lasting arching of the neck (“stargazing”); mice were also noted to be more aggressive than their wild type littermates. Genetic mapping narrowed the add critical region to 1.1 Mb containing 30 annotated genes, none of which contained mutations. However, in earlier Ensembl releases (prior to v41), a total of 33 genes were listed, and one of these genes (denoted “similar to catecholamine O-methyltransferase” and here called Comt2) was withdrawn from annotation for lack of evidence. In add mice, Comt2 contained a single base pair transversion (G to T) in the second of four exons, resulting in substitution of an arginine for a leucine residue at position 48 of the 258 aa polypeptide chain. The mutation significantly reduced the methyltransferase activity of Comt2. The Comt2 protein was found to be highly expressed in sensory hair cells of the inner ear (Figure 1), and consequently, the hypomorphic add mutation caused vestibular impairment, profound sensorineuronal deafness, and progressive degeneration of the organ of Corti.
The human COMT2 gene is located at the autosomal recessive deafness-63 (DFNB63) locus on chromosome 11q13.4. Based on our findings in add mice, we screened DNA from human families and identified a nonsense mutation in COMT2 that causes nonsyndromic deafness. This finding was confirmed by Ahmed et al, who reported four additional homozygous missense mutations causing autosomal recessive deafness in humans (Ahmed et al. Nat.Genet. 40, 1335-1340).
The add phenotype and causative Comt2 mutation are reported in (Du et al. Proc.Natl.Acad.Sci.U.S.A. 105, 14609-14614).