Mapping a mouse limbic seizure susceptibility locus on chromosome 10

The importance of genetic factors in epilepsy is well accepted, but the specific genes responsible have been identified for only a few rare Mendelian epilepsy syndromes in humans (Winawer & Shinnar, 2005; Baulac & Baulac, 2010). The difficulty in finding genes results in part from the fact that most epilepsy is “complex,” with multiple genetic, nongenetic, and possibly interacting causes, and temporal lobe epilepsy (TLE) exemplifies this problem (Mathern et al., 1996; Cendes et al., 1998; Semah et al., 1998; Engel, 2001; Fuerst et al., 2001; Kobayashi et al., 2001; Stephen et al., 2001; Ottman, 2005; Santos et al., 2002; Vadlamudi et al., 2003; Scharfman & Pedley, 2006). Epilepsy is also heterogeneous because of the variety of underlying biologic processes that may give rise to seizure susceptibility and act as potential therapeutic targets. Mapping seizure susceptibility loci in mice provides a framework for identifying potentially novel candidate genes for complex human disorders (Frankel, 2009). Prolific breeding and short gestation expedite genetic linkage methods (Palmer & Phillips, 2002; Phillips et al., 2002), and the availability of genetic maps and sequence data for multiple inbred strains also facilitate genetic research in mice. Conservation of genes from mouse to human genomes allows genetic discoveries in the mouse to be translated for study in human populations. Inbred mouse strains differ in susceptibility to develop seizures and seizure-induced hippocampal damage in response to many different convulsant stimuli, and these differences have been shown to be heritable ((Neumann & Collins, 1991, 1992; Ferraro et al., 1995, 1997; Schauwecker & Steward, 1997; Ferraro et al., 1998; Gershenfeld et al., 1999; Hain et al., 2000; Schauwecker, 2002; McKhann et al., 2003; Frankel, 2009). The A/J strain has been shown to be more susceptible than B6 to most (though not all) types of convulsant stimuli (Kosobud & Crabbe, 1990; Mathis et al., 1994; Gershenfeld et al., 1999; Golden et al., 2001; Ferraro et al., 2002), and has recently been described to have sleep-related spontaneous seizures (Strohl et al., 2007). Genetic variation among strains is presumed to be a result of functional polymorphisms in genes that affect the threshold for neuroexcitability in response to exogenous stimuli (Frankel et al., 2001). Identifying these genes can provide insight into epilepsy pathophysiology and lead to improved and novel therapeutic strategies. Quantitative trait loci (QTL) are genomic loci that make a quantitative contribution to a trait, such as seizure susceptibility. QTL mapping can identify these loci, and eventually the causative DNA-sequence polymorphisms. This powerful strategy is designed specifically to investigate oligogenic models, in which several genes of small to moderate effect act to influence a trait. Chromosome substitution strains (CSS) are strains in which a single chromosome from one inbred strain (donor) has been transferred onto a second strain (host) by repeated backcrossing, and provide an invaluable resource for mapping mouse QTLs. Chromosome substitution strain analysis provides quick and efficient localization at the chromosomal level without the need for cross-breeding or genotyping (Nadeau et al., 2000). Using C57BL/6J (B6, host) × A/J (donor) CSS, we recently identified contributions to susceptibility to pilocarpine-induced seizures on mouse chromosomes 10 and 18 (Winawer et al., 2007). Pilocarpine, a muscarinic cholinergic agonist, induces initial severe continuous limbic seizures (status epilepticus) in rodents, followed by cell loss similar to that observed in TLE, and delayed onset of chronic spontaneous limbic seizures (Turski et al., 1989; Turski, 2000; Leite et al., 2002; Borges et al., 2003). Prior to our report (Winawer et al., 2007), limbic seizures had not been studied in the A/J strain, and susceptibility to pilocarpine-induced seizures had not been studied with QTL mapping. Herein we report fine mapping of a pilocarpine seizure susceptibility locus on mouse chromosome 10 (Ch10), using F2 mouse populations and interval-specific congenic strains. (See Methods for breeding scheme details.)