Compositions and methods for spinocerebellar ataxia

1. A method of diagnosing spinocerebellar ataxia 13 (SCA13) or predicting the risk of developing SCA13 in a human comprising:

a) analyzing a nucleic acid sample obtained from the human for the presence or absence of a mutation in the transmembrane domain of the KCNC3 gene; and

b) determining the presence or absence of a missense mutation in the nucleotide sequence of a transmembrane domain of the KCNC3 gene in the sample, wherein the missense mutation in the nucleotide sequence of the transmembrane domain results in a change in the output characteristics of fast spiking cerebellar neurons, whereby determining the presence of the missense mutation in the nucleotide sequence of the transmembrane domain is indicative that the individual has or is at risk of developing SCA13.

2. The method of claim 1 , wherein the transmembrane domain is selected from the group consisting of the S4 and S5 transmembrane domain.

3. The method of claim 2 , wherein the transmembrane domain is the S4 transmembrane domain, and wherein the nucleotide sequence of wild type S4transmembrane domain encodes a wild type polypeptide comprising amino acid sequence SEQ ID NO: 20.

4. The method of claim 2 , wherein the transmembrane domain is the S5 transmembrane domain, and wherein the nucleotide sequence of wild type S5transmembrane domain encodes a wild type polypeptide-comprising amino acid sequence SEQ ID NO: 21.

5. The method of claim 1 , wherein the mutation is detected by dot blot hybridization.

6. The method of claim 1 , wherein the mutation is detected by southern blot hybridization.

7. The method of claim 1 , wherein the nucleic acid sample is amplified and wherein said mutation is detected by sequencing said amplification product.

8. The method of claim 1 , wherein the mutation causes an amino acid substitution in SEQ ID NO: 17 selected from the group consisting of R420H and F448L.

9. The method of claim 1 , wherein the human has one or more of the following phenotypes selected from the group consisting of: epilepsy, mental retardation, cerebellar ataxia, extrapyramidal signs, dysarthria, oculomotor abnormalities, motor neuron signs, cognitive decline, autonomic dysfunction, sensory deficits and psychiatric manifestations.

10. The method of claim 1 , wherein the mutation causes an amino acid substitution of R423H.