Extreme Expansion Detection in Spinocerebellar Ataxia Type 2 and Type 7

The autosomal dominant cerebellar ataxias are each defined by progressive ataxia and variable association with other clinical findings (1 ). Numerous spinocerebellar ataxia (SCA) loci have been identified and several of the SCA genes have expansion of a CAG-repeat as the underlying mutation (2 ,3 ). Assays that determine the CAG-repeat length in SCA1, SCA2, SCA3, SCA6 , and SCA7 are used for both diagnostic and predictive testing purposes. Molecular genetic testing is necessary to establish a diagnosis of the SCAs that do not have unique clinical features. It is also a valuable tool in confirming a clinical diagnosis of SCAs that have characteristic clinical findings (such as retinal dystrophy in SCA7) (2 ,3 ). Assays typically utilize polymerase chain reaction (PCR) amplification of the repeat region, separation of PCR products by gel electrophoresis or capillary electrophoresis, and visualization of products by incorporation of radioactivity or dye into PCR products or staining with dye after product separation (4 –6 ).