SINGLE SPERM ANALYSIS OF THE TRINUCLEOTIDE REPEATS IN THE HUNTINGTONS-DISEASE GENE - QUANTIFICATION OF THE MUTATION FREQUENCY-SPECTRUM

The CAG triplet repeat region of the Huntington's disease gene was amp lified in 923 single sperm from three affected and two normal individu als. Average-size alleles (15-18 repeats) showed only three contractio n mutations among 475 sperm (0.6%). A 30 repeat normal allele showed a n 11% mutation frequency. The mutation frequency of a 36 repeat interm ediate allele was 53% with 8% of all gametes having expansions which b rought the allele size into the HD disease range (greater than or equa l to 38 repeats). Disease alleles (38-51 repeats) showed a very high m utation frequency (92-99%). As repeat number increased there was a mar ked elevation in the frequency of expansions, in the mean number of re peats added per expansion and the size of the largest observed expansi on. Contraction frequencies also appeared to increase. with allele siz e but decreased as repeat number exceeded 36. Our sperm typing data ar e bf a discrete nature rather than consisting of smears of PCR product from pooled sperm. This allowed the observed mutation frequency spect ra to be compared to the distribution calculated using discrete stocha stic models based on current molecular ideas of the expansion process. An excellent fit was found when the model specified that a random num ber of repeats are added during the progression of the polymerase thro ugh the repeated region.