A new genetic locus mapped from behavioral variation in mice: audiogenic seizure prone (ASP)

Animal models of inherited epilepsy

A significant proportion of the childhood epilepsies have a genetic component. Therefore, animal models that can be bred for seizure expression may provide important information regarding the mechanisms by which molecular defects result in the neuronal hyperexcitability states collectively termed "epilepsy." Because of the rate and ease of breeding, rodent models are the most commonly used. The genetically epilepsy-prone rat has motor seizures in response to auditory stimuli. It is likely that the seizures are generated in the inferior colliculus because of an abnormality in the noradrenergic system. The seizure predisposition is inherited as an autosomal dominant trait. The genetic absence epilepsy rat has age-related spontaneous seizures characterized by motor arrest and head drops that are correlated with generalized spike-wave on the electroencephalogram (EEG). The seizure generating mechanism appears to be located in the lateral thalamic nuclei. The epileptic mongolian gerbil demonstrates behavioral arrest followed by myoclonic, tonic, and tonic-clonic seizures in response to unfamiliar environments. The underlying neuroanatomy involves hippocampal-cortical interactions indicative of a partial epilepsy. The tottering mouse has absence and myoclonic seizures, a 6- to 7-Hz ictal spike-wave EEG, and noradrenergic hyperinnervation that are linked to a mutation on chromosome 8. Hippocampal network hyperexcitability has been found with normal neuronal intrinsic properties. Stargazer is a mouse mutant with almost identical clinical and electrographic features as found in tottering. However, the genetic defect is located on chromosome 15 and no abnormalities of norepinephrine have been discovered. The El mouse demonstrates ictal automatisms in response to vestibular stimulation. Metabolic and structural abnormalities have been found in the hippocampus. Linkage to chromosomes 9 and 2 have been reported recently. The dilute brown agouiti mouse demonstrates motor seizures in response to auditory stimuli. Chromosomes 4 and 17 are linked to seizure expression. Thus, a variety of models exist to study the genetic, biochemical, structural and electrophysiological mechanisms that underlie the predisposition and expression of the inherited epilepsies.

A novel gene causing a mendelian audiogenic mouse epilepsy

Frings mice are a model of generalized epilepsy and have seizures in response to loud noises. This phenotype is due to the autosomal recessive inheritance of a single gene on mouse chromosome 13. Here we report the fine genetic and physical mapping of the locus. Sequencing of the region led to identification of a novel gene; mutant mice are homozygous for a single base pair deletion that leads to premature termination of the encoded protein. Interestingly, the mRNA levels of this gene in various tissues are so low that the cDNA has eluded detection by standard library screening approaches. Study of the MASS1 protein will lead to new insights into regulation of neuronal excitability and a new pathway through which dysfunction can lead to epilepsy.

Nonallelism for the audiogenic seizure prone (Asp1) and the aryl hydrocarbon receptor (Ahr) loci in mice

Previous studies showed an association between the Ahr locus on Chr 12 and a major gene, Asp1, that influences susceptibility to audiogenic seizures (AGS) in mice. Although the association was thought to involve close linkage, a pleiotropic effect of the Ahr locus on AGS susceptibility was not excluded. Two congenic strains, D2.B6N-Asp1b and the D2N.B6N-Ahrb1, were used to evaluate further the association between the Ahr and Asp1 loci. Both strains are genetically identical to the AGS susceptible DBA/2 (D2) strain except for a small amount of C57BL/6N (B6N) genome surrounding the Ahr locus and encompassing the Asp1 locus. The AGS susceptibility of both congenic strains is similar and significantly lower than that of the D2 strain. We found that the Ahr/Asp1 critical region encompasses 5.5-7.0 cM from the proximal microsatellite marker D12Mit153 to the distal marker D12Mitl12. The D2N.B6N-Ahrb1 expresses B6 alleles for all markers within the critical region, whereas the D2.B6N-Asp1b expresses the B6 allele only at the Asp1 locus. Furthermore, we determined that the D2.B6N-Asp1b mouse expresses both the D2 phenotype and genotype at the Ahr locus, i.e., zoxazolamine paralysis and T to C and G to A transition mutations in the Ahr cDNA at bp sites 3330 and 3336, respectively. We therefore conclude that the Ahr and Asp1 loci are nonallelic and that the Ahr gene is excluded as a candidate for Asp1.

Cloning, characterization, and chromosomal localization of rec1.3, a member of the G-protein-coupled receptor family highly expressed in brain

During a project to identify G-protein-coupled receptors (GPCR) expressed within taste buds, we have isolated a novel receptor-like sequence. The full length sequence of this receptor (rec1.3) has been obtained in both cow and mouse. Rec1.3 bears little sequence similarity to any GPCR whose ligand is known: the closest identity (33%) is to the orphan receptor edg-1. In cow, rec1.3 is expressed most prominently in the brain, with moderate expression in testis and tongue; in the mouse the expression is more widespread. No specific binding for a range of ligands was detected when the mouse coding sequence was expressed in eukaryotic cells. In situ hybridization showed that rec1.3 is widely expressed throughout the mouse brain and is highly expressed in localized regions of the hindbrain, midbrain and hypothalamus. The rec1.3 gene was localized to the centromeric region of chromosome 4 in mouse, a region associated with neonatal seizures.

Analysis of B10.D2 recombinant congenic mouse strains shows that audiogenic and beta-CCM-induced seizures depend on different genetic mechanisms

The rate of occurrence of audiogenic seizures and seizures induced by the benzodiazepine receptor inverse agonist methyl-beta-carboline-3-carboxylate (beta-CCM) were analyzed in several recombinant congenic strains of mice bred from B10.D2 and DBA/2J. Although both types of seizures have similar behavioral patterns and might involve GABAergic mechanisms, no correlation was observed between the occurrence of the two types of seizures across the strains, suggesting that these two types of seizures depend on different genetic mechanisms.

Confirmation of the influence of a chromosome 7 locus on susceptibility to audiogenic seizures

Mouse strain differences in susceptibility to audiogenic seizures (AGS) are due to allelic differences at multiple genetic loci. Three loci that influence susceptibility to AGS have been mapped on Chromosomes (Chr) 12, 4, and 7 (Asp-1, Asp-2, and Asp-3, respectively). Here we report evidence that confirms linkage of Asp-3 to c on Chr 7 and parental effects on AGS susceptibility, but not genomic imprinting of Asp-3.

Genetic dissection of susceptibility to audiogenic seizures in inbred mice

Mice of some inbred strains, such as 21-day-old DBA/2J mice, have generalized convulsions when exposed to intense auditory stimulation. Analysis of susceptibility to audiogenic seizures in BXD recombinant inbred strains has demonstrated the influence of at least three loci. One locus, Asp-1, is located on chromosome 12 between Ah and D12Nyu1; another locus, Asp-2, is on chromosome 4, tightly linked to b. Here we report evidence that Asp-2 is located within an 8-centimorgan segment distal to b and that Asp-3 is linked to Mtv-1 on chromosome 7. We also present evidence that these three loci account for most of the heritable variation in susceptibility to audiogenic seizures in crosses of DBA/2J and C57BL/6J mice and that susceptibility to audiogenic seizures is influenced by genomic imprinting. Thus, genomic imprinting may complicate linkage and mapping studies and should be considered in analyses of complex modes of inheritance.

Mapping of two genes that influence susceptibility to audiogenic seizures in crosses of C57BL/6J and DBA/2J mice

The difference in susceptibility to audiogenic seizures (AGS) between C57BL/6J and DBA/2J inbred strains of mice is due to multiple genetic factors. AGS susceptibility was tested in 21-day-old mice from classical crosses, BXD recombinant inbred (RI) strains, a congenic DBA/2N.B6N-Ahb inbred strain and crosses between the BXD RI strains and DBA/2J. Analysis of these data reveals that the variation in AGS susceptibility between these two strains results from allelic differences at three or more loci. Most of the variation is due to allelic differences at two loci. The first, Asp-1 (formerly Ias), is a major gene located on chromosome 12, between Ah and D12 Nyul. The second, Asp-2 (formerly asp), is a minor gene located on chromosome 4, tightly linked to b. The negative correlation of brain stem Ca2(+)-ATPase activity and AGS susceptibility in the BXD RI strains suggests that the strain difference in Ca2(+)-ATPase activity is inherited as a polygenic trait and that Asp-1 and Asp-2 are linked to, or identical to, factors that influence Ca2(+)-ATPase activity.

Length of the chromosomal segment marked by galactose-1-phosphate uridyl transferase and soluble aconitase and conserved since divergence of lineages leading to mouse and man

Standard linkage testing crosses and ovarian teratoma mapping were used to estimate the length of the chromosomal segment that is marked by galactose-1-phosphate uridyl transferase and soluble aconitase and that has been conserved since divergence of lineages leading to mouse and man. These experiments were also used to determine whether theRb(4·6)2BnrRobertsonian translocation suppresses recombination on the proximal portion of mouse Chromosome 4. The estimated length of the conserved segment marked by galactose-1-phosphate uridyl transferase and soluble aconitase in mouse and man was estimated to be 24 cM. It was also shown thatRb(4·6)2Bnrstrongly suppressed recombination on the centromeric portion of mouse Chromosome 4.

Circadian rhythms and temporal structure of the susceptibility to tonic-clonic seizures in the mouse

A circadian paradigm of susceptibility to tonic-clonic convulsions was established for two strains of mice. This research dealt with rhythmometric methods for estimating the main circadian parameters characterizing the temporal structure of these behavioral phenomena. Such experiments provided a quantitative approach for studying some neurobiological mechanisms regulating the seizure susceptibility.

Audiogenic seizure susceptibility of C57BL/6 in equilibrium DBA/2 allophenic mice

Allophenic mice composed of cells from a strain (DBA/2) susceptible to sound-induced seizures and cells from a strain (C57BL/6) resistant to them were produced by embryo aggregation techniques. Twenty-eight allophenic mice were tested for audiogenic seizure susceptibility. The results were compared with the genotypic composition of the coat melanocytes. For those animals with a predominance of one genotype or the other in the coat, their seizure phenotype was the same as that of the strain most represented in the coat. In contrast, those animals with major contributions of both genotypes in the coat demonstrated the entire spectrum of susceptibility phenotypes. Such results are likely to be a manifestation of a relatively small target tissue for the genes influencing the development of audiogenic seizure susceptibility.

A searching procedure for transformations and models in a classical Mendelian cross breeding study

Adopting Wright's four criteria (homogeneity, normality, additivity of effects, and parsimony of parameters needed) concerning a good scale of measurement, a systematic procedure is presented to find an adequate transformation for the data of a classical Mendelian cross. By using Cavalli's least-squares fitting procedure, fits of all possible genetical (sub)models are compared to each other. The model yielding the best fit with as few parameters as possible is selected. More complex models are accepted only if they fit significantly better than simpler ones. Four worked examples are presented from a classical cross study in rats: time till onset of a learned bar-press response under a continuous-reinforcement schedule and protein content, acetylcholinesterase activity, and butyrylcholinesterase activity in the occipital cortex. Using several transformations, invariance of resulting models was found if the criterion of homogeneity of variance was met.

Increase in aggressiveness of male mice after irradiation of paternal spermatozoa with 600 R of gamma-rays as dependent on fertility

The agonistic behavior of unexperienced pairs of NMRI male mice was determined by counting the bites received from and delivered to the opponent within 24 hr. The first 10 min of agonistic encounters was recorded by video tape to analyze the frequency and duration of ten behavioral traits. Each pair consisted of two F1 males, one of which was derived from paternal spermatozoa irradiated with 600 R of gamma-rays (600-F1), while the other stemmed from a sham-treated father (control-F1). The 600-F1 males exhibited higher aggressiveness than their control F1 counterparts, in which the sterile and semisterile males (presumed to be translocation heterozygotes) showed a higher level of agonistic behavior and overall activity than the normally fertile F1 males of the same group. 600-F1 males released more urine drops than the control males. No significant differences between 600-F1 and control-F1 males or between fertile and sterile plus semisterile males were found for learning ability.

Distribution of crossing-over in mouse chromosomes

In devising maps of the positions of gene loci along chromosomes, by measurements of crossing-over, it is usual to assume that the chiasmata which give rise to the crossing-over are randomly located, unless there is evidence to the contrary. If chiasmata are in fact preferentially located at certain points or regions this not only affects mapping, but also is of interest for population genetics since it may favour the development during evolution of blocks of genes relatively undisturbed by crossing-over. Preferential localization of chiasmata is well known in a number of disparate organisms (Darlington 1965; John & Lewis, 1965), but has so far been little studied in mammals. Evidence is presented suggesting tha chiasmata are in fact non-randomly located in certain chromosomes of the mouse.

Effects of situational variables on performance of inbred mice in active- and passive-avoidance situations

A group of experiments were conducted to assess the interaction between environmental factors and specific genetic loci upon the performance of mice in avoidance learning situations. The performance of all mice was better in passive-avoidance than in active-avoidance situations. The performance of C57BL/6J mice was generally superior to that of DBA/2J mice. C57BL/6J mice usually avoided foot-shocks, while DBA/2J mice attempted to escape them. DBA/2J mice performed poorly in the presence of mild foot-shocks, while C57-BL/6J mice performed poorly in the presence of intense foot-shocks. Analysis of the data suggested that the loci involved probably exerted their influence by affecting the subject's response to its environment.