Abstracts

Several ion channel mutations have been identified in rare families with Mendelian inheritance of epilepsy. Large numbers of epilepsy patients have since been screened for mutations in ion channel genes, with little success. The cause of epilepsy in the majority of patients remains unknown. This study aims to identify regulators of neuronal ion channel gene expression and to test the hypothesis that epilepsy can be caused by mutations in these transcriptional regulators.
Quantitative trait locus (QTL) mapping has the potential to be applied to the analysis of the variation of gene expression. This is achieved by treating each gene on a microarray chip as a quantitative trait, and associating levels of gene expression with genotyping data. C57BL/6 and DBA/2 mice are known to differ in their susceptibility to seizures, therefore some of the differences in gene expression between these two strains will be related to seizures. Recombinant inbred (RI) strains derived from a C57BL/6J female and a DBA/2J male are members of the BXD set. By studying alterations in transcription in the BXD mapping panel, we were able to systematically map genes that modulate ion channel gene expression. This method is called [ldquo]transciptome-QTL[rdquo] mapping. All microarray and QTL mapping results are publicly available at WebQTL (webqtl.roswellpark.org).
We used transcriptome-QTL mapping to identify chromosomal regions that control the expression of selected sodium channels, potassium channels, GABA receptors and acetylcholine receptors. Transcription factors were selected as obvious candidates for the regulation of gene expression, however the QTLs detected may represent genes that are several steps upstream in the regulatory pathway. It was of interest that the LIM homeobox proteins (Lhx) featured predominantly. We first checked the abundance of Lhx genes to determine if they were so common that one would be expected on every chromosome, and found that there are only 8 known Lhx genes. Lhx1, Lhx2, Lhx3, and Lhx5 are all very close to the QTLs that control variation in expression levels of Gabra1, Kcnq2, Chrnb2, and Scn1b. Lhx4 and Lhx 9 are within 20cM of Gabrg2.
Homeobox genes have previously been associated with epilepsy and it has been suggested that molecules controlling cell fate decisions are also operative during seizure-induced neurogenesis and plasticity. Lhx proteins play an essential role in cell differentiation and development of both neuronal and non neuronal tissue. Therefore the variation in ion channel expression may be due to alterations in cell types or cell numbers. It is also possible that different members of the Lhx gene family directly control the expression of different ion channel proteins. Misexpression of a Lhx gene in epidermal cells of ascidian larva led to ectopic expression of a voltage-gated sodium channel that is normally expressed in the CNS therefore Lhx proteins may control membrane excitability by regulating ion channel gene expression.