An interrupted 34-CAG repeat SCA-2 allele in patients with sporadic spinocerebellar ataxia.

In spinocerebellar ataxia type 2 (SCA-2), a difference of three CAG repeats distinguishes normal alleles (14 to 31 repeats) from pathogenic alleles (34 to 57 repeats). All sequenced pathogenic alleles have a pure CAG repeat structure, whereas interrupted repeats have been seen exclusively in normal alleles. The authors present two patients with sporadic SCA with an interrupted 34-CAG repeat allele, (CAG)24(CAA)(CAG)9, who showed a phenotype compatible with SCA-2. The interrupted allele coding for a 34 pure polyglutamine tract may cause the SCA phenotype.