Caution regarding the interpretation of homoallelism in polyglutamine multiplex assays: a recommendation for confirmatory testing of homozygous alleles.

Spinocerebellar ataxia type 7 (SCA7) is an inherited dominant neurodegenerative disease caused by the expansion of a CAG repeat within the ATXN7 gene. Standard molecular diagnostic testing for SCA7 involves amplification of the region surrounding the CAG repeat via end-labeled PCR and subsequent capillary electrophoresis. In addition, multiplex methods exist that may be used to test for multiple polyglutamine spinocerebellar ataxias in a single assay. Herein, we used a SCA7 singleplex method to screen 111 individuals for whom the multiplex method detected a single normal allele. A total of six retested individuals (5.4%) were shown to have a pathogenic expansion at the ATXN7 locus. An additional triplet-primed PCR method was used to test the same cohort, and revealed no further disease-causing alleles. This study demonstrates the importance of using complementary methods to rule out apparent homoallelism during molecular testing for polyglutamine diseases.