Doctoral Dissertations

Date of Award

12-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Biomedical Sciences

Major Professor

Lisa Stubbs

Committee Members

Michael Mucenski, Mary Ann Handel, Dabney Johnson

Abstract

With the exception of stroke, epilepsy is the most prevalent human neurological disorder, affecting approximately 4.5 million people in the United States. Epilepsy is an altered state of consciousness that occurs when the brain cells electrochemically fire and discharge in an erratic and abnormal manner. A significant fraction of childhood epilepsies involves a genetic component. Petit mal epilepsy, one of the four major types of epilepsy, begins in childhood and often remits at adolescence. This disorder is characterized by epileptic absences, which are a clouding or loss of consciousness lasting for 2-15 seconds, accompanied by a generalized 3 second spike-wave epileptic discharge.

The mouse mutant, tottering (tg), is considered one of the best animal models for this type of epilepsy because of its similar early age of onset and absence seizures. In addition, tottering homozygotes show degenerative changes of cerebellar cells, a defect which has also been implicated in human and mouse epilepsies. Using a combination of comparative mapping strategies in conjunction with a candidate gene approach, the gene responsible for tg, Cacnlla4, has been isolated, sequenced, characterized, and mapped to a defined region in mouse chromosome 8 and human chromosome 19p13.1. Concurrent with our research, other researchers have identified mutations in Cacnlla4 to be responsible for the leaner allele of tottering, as well as three allelic human chromosome 19 disorders; Familial Hemiplegic Migraine (MHP), Episodic Ataxia Type 2 (EA2), and Spinocerebellar Ataxia 6 (SCA6). Taken together, these findings open the door for future research leading to a greater understanding of the biological basis, and better treatment for epilepsy, migraine, and ataxia.

This dissertation research project was designed towards the (1) comparative mapping of human chromosome 19p and (2) isolation and characterization of the gene responsible for the epileptic mouse mutant, tottering. This dissertation illustrates the power of utilizing information from a well characterized physical map of a human chromosome in conjunction with a high resolution mouse genetic map to identify candidate genes for disorders in both species. In order to present the data in a logical order, the manuscript has been divided into four parts. Part I provides data, in the form of three separate publications, describing the construction of a high resolution genetic map of the central region of mouse chromosome 8. The detailed genetic map provided the critical framework necessary for cloning the tottering gene. Part II describes the isolation and characterization of the tottering gene, Cacnlla4, which encodes for the α1A subunit of a voltage-gated calcium channel. While investigating the temporal embryonic expression of the tottering gene, alternative transcripts were detected in E10.5 wild type mouse embryos. Preliminary data regarding the isolation and mapping of a gene possibly responsible for the observed minor embryonic transcript is discussed in Part III. Part IV provides background information about what is currently known about the structure, function, and modulation of voltage-gated calcium channels. In addition, several hypotheses on the nature of the tottering phenotype are proposed combining basic knowledge of voltage-gated calcium channels and existing biochemical, pharmacological, and molecular data of tottering mutants.

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