HUNTINGTONS-DISEASE - DIAGNOSIS BY AMPLIFICATION OF THE CAG REPEAT

Aim. To develop and validate a test for the diagnosis of Huntington's disease by the direct detection, and sizing of, expanded CAG triplet r epeats within the Huntington gene of affected individuals. Methods. A polymerase chain reaction-based test which specifically amplifies the CAG repeat was developed using an ultra heat stable polymerase and [al pha S-35] partial-derivative ATP incorporation. Amplified alleles were separated on DNA sequencing gels and sized by comparison with a known sequence. Results. Analysis of 10 affected individuals showed abnorma l 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 conf irmed the diagnosis. Analysis of this patient's elderly and unaffected father indicated that he had a allele at the extreme end of the norma l range. In a second case with atypical neurological features the diag nosis was also established by the demonstration of an expanded allele. Conclusions. This assay allows improved diagnosis of Huntington's dis ease including completely accurate presymptomatic and antenatal diagno sis. Easy access to this test has implications for clinical practice b ut acceptable guidelines for its use will need to be developed.