Huntington's disease: diagnosis by amplification of the CAG repeat

To develop and validate a test for the diagnosis of Huntington's disease by the direct detection, and sizing of, expanded CAG triplet repeats within the Huntington gene of affected individuals.

A polymerase chain reaction-based test which specifically amplifies the CAG repeat was developed using an ultra heat stable polymerase and [alpha 35S] delta ATP incorporation. Amplified alleles were separated on DNA sequencing gels and sized by comparison with a known sequence.

Analysis of 10 affected individuals showed abnormal alleles with repeat numbers ranging from 40 to 61. In one case, that had been diagnostically uncertain because there was no family history of Huntington's disease, the demonstration of an expanded allele confirmed the diagnosis. Analysis of this patient's elderly and unaffected father indicated that he had an allele at the extreme end of the normal range. In a second case with atypical neurological features the diagnosis was also established by the demonstration of an expanded allele.

This assay allows improved diagnosis of Huntington's disease including completely accurate presymptomatic and antenatal diagnosis. Easy access to this test has implications for clinical practice but acceptable guidelines for its use will need to be developed.