MUTATIONS IN CALCIUM CHANNELS CAUSING VERTIGO AND ATAXIA

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The experiments outlined in this proposal seek to investigate how mutations in CACNL1A4, which encodes a brain calcium channel subunit, lead to symptoms of episodic vertigo and ataxia in familial hemiplegic migraine (FHM), episodic ataxia type 2 (EA-2), and spinocerebellar ataxia type 6 (SCA6). Advances in molecular genetic techniques have lead to the determination of responsible mutant genes in numerous neurodegenerative conditions. Yet, the physiologic basis of disease often remains unclear because the function of the mutant gene product is not known. With the recent identification of mutations in CACNL1A4 involving a brain calcium channel known to be important in neuronal function, the three allelic disorders including FHM, EA-2, and SCA6 provide a model to elucidate the genotype-phenotype relationship. Molecular techniques will be applied to screen for mutations in over 30 unrelated patients in our data base who present with symptoms possibly caused by mutations in CACNL1A4. Novel point mutations have already been identified in two of these families, while small CAG repeat expansions were found in three other families. Site-directed mutagenesis combined with patch clamp electrophysiologic techniques will be applied to study the functional consequences of heterologously expressed mutated calcium channel subunit. Gain- or change- of- function, haploinsufficiency, dominant negative effect, and neurotoxic effect have all been proposed for the mutations in CACNL1A4. Electrophysiologic and biochemical techniques will be used to address these different possible mechanisms. The basis of therapeutic response to acetazolamide will also be studied. Elucidating the molecular physiological basis of phenotypic heterogeneity not only will contribute substantially to our understanding of a protein important for brain function but also may help develop rational therapy. The further relevance of this work to the more common basilar migraine and Meniere's syndrome is emphasized by the overlapping symptoms of vertigo and ataxia in the group of patients that we propose to study.