The purpose of this study is to explore the genetics of seizure susceptibility by identifying the loci contributing to susceptibility in a mouse model. This has potentially powerful implications for understanding the genetic contributions to human epilepsy. Using mouse CSS, Ch10 F2
populations, and ISCS, we have identified and fine mapped a locus on distal mouse chromosome 10 (Ch10) that contributes to limbic seizure susceptibility. Mapping of pilocarpine-induced seizures has not been reported previously, and there have been few prior reports of seizure susceptibility candidate loci on mouse Ch10 or syntenic human regions. Gershenfeld et al. reported an association of D10Mit180 (117.59 Mb) with seizure susceptibility in a B6 × A/J cross, in response to β-CCM, a γ-aminobutyric acid (GABA)ergic agent that produces generalized (not limbic) seizures (Gershenfeld et al., 1999
). Subsequent studies of a congenic have suggested that this QTL may play a role in both anxiety-related endophenotypes and seizure susceptibility (Zhang et al., 2005
We have also found evidence for the presence of a locus underlying both seizure susceptibility and anxiety endophenotypes on A/J Ch10 (Ponder et al., 2007
; Winawer et al., 2007
). In an independent dataset we have confirmed the location of this QTL, using a fear-conditioning paradigm (Palmer A, unpublished results). The possibility that a single allele pleiotropically influences both traits should be investigated further, especially in light of the known comorbidity between psychiatric disorders and epilepsy (Winawer & Hesdorffer, 2010
). The region on distal Ch10 may contain one or more loci that play a role in epilepsy and psychiatric comorbidity.
D10Mit180 is located at approximately 117.59 Mb on Ch10, near marker rs13480781, which is located at 117.56 Mb (). Marker rs13480719 maps to a homologous region on human CH12q. This may overlap a locus on chromosome 12q22–23, which has been found to be linked to familial TLE with febrile seizures in a single extended pedigree (Claes et al., 2004
). To our knowledge, no other potential epilepsy genes have been mapped to this region.
We have mapped two seizure susceptibility phenotypes to the same region of mouse Ch10. The first, time in stage 3, is a measure of latency, or duration of partial status. The second, highest stage reached, is a measure of seizure severity. The colocalization of the latency and severity aspects of susceptibility suggests that this QTL has a broad effect on seizure susceptibility across different parameters. Defects in some molecular or biochemical pathways may act to prolong seizures once initiated, whereas others may affect the mechanism of propagation from partial to generalized seizures. Identification of genes that raise risk for one or both mechanisms can give insight into the biology of seizure susceptibility and development of targeted therapies.
These results may be limited, in that they map seizure susceptibility in response to an acute stimulus, not epilepsy. However, identification of genes influencing seizure susceptibility is fundamental to understanding human epilepsy (Noebels, 2003
). For example, mutagenesis in a mouse model identified a single mutation on mouse Ch2, Szt1
, conferring susceptibility to electroconvulsive threshold (ECT) minimal clonic seizures (Yang et al., 2003
). Two of the three known deleted genes—Kcnq2
—are known to be mutated in human epilepsy families (Singh et al., 1998
; Steinlein, 2000
), and Kcnq2
haploinsufficiency was shown to determine the Szt1 susceptibility phenotype. These results demonstrate a direct correspondence between mouse seizure susceptibility genes and human epilepsy genes. Furthermore, screening compounds for seizure susceptibility in rodents has led to the development of many effective anticonvulsant agents (Frankel, 2009
). Even if specific genes identified do not raise risk for epilepsy per se, the information obtained will still be valuable for elucidating the pathophysiology of seizures and therapeutic targets.
The identification of mouse QTLs and quantitative trait genes (QTGs) can provide a framework for identifying novel homologous candidate genes for human epilepsy. This has been demonstrated by the identification of a maximal electroshock seizure threshold (MEST) locus on mouse chromosome 1 (Ch1), fine mapping, selection of a potassium channel candidate gene, Kcnj10
, and subsequent discovery of genetic association in a human epilepsy population (Ferraro et al., 2001
; Buono et al., 2004
; Ferraro et al., 2004
). Mutations in Kcnj10
have also been reported in families with epilepsy, sensorineural deafness, and renal tubulopathy, supporting the gene’s role in cellular homeostasis in humans (Bockenhauer et al., 2009
; Sala-Rabanal et al., 2010
). Improving prior probability by the identification of biologically probable candidates can help minimize the false-positive results that plague human genetic association studies (Cardon & Bell, 2001
; Emahazion et al., 2001
) We have previously demonstrated success translating from mouse QTL to human genetic association with the identification of CSNK1E
, a candidate gene for methamphetamine sensitivity in both mice and humans (Veenstra-Vander-Weele et al., 2006
). QTL mapping is a powerful technique with the potential to identify novel loci and genes involved in epilepsy and other human complex disorders.
Analysis of chromosome substitution strains, Ch10 F2 populations, and ISCS created to date provides strong evidence for a seizure susceptibility locus on distal mouse Ch10 between markers rs13480781 and rs13480832. Further congenic strain creation is underway, and will help improve the resolution of our mapping and narrow the list of potential candidate genes.