CONTRIBUTION OF DNA-SEQUENCE AND CAG SIZE TO MUTATION FREQUENCIES OF INTERMEDIATE ALLELES FOR HUNTINGTON DISEASE - EVIDENCE FROM SINGLE SPERM ANALYSES

New mutations for Huntington disease (HD) arise from intermediate alle les (IAs) with between 29 and 35 CAG repeats that expand on transmissi on through the paternal germline to 36 CAGs or greater. Using single s perm analysis, we have assessed CAG mutation frequencies for four IAs in families with sporadic HD (IA(NM)) and IAs ascertained from the gen eral population (IA(GP)) by analyzing 1161 single sperm from three per sons. We show that IA(NM) are more unstable than IA(GP) with identical size and sequence. Furthermore, comparison of different sized IAs and IAs with different sequences between the CAG and the adjacent CCG tra cts indicates that DNA sequence is a major influence on CAG stability. These studies provide estimates of the likelihood of expansion of IA( NM) and IA(GP) to greater than or equal to 36 CAG repeats for these in dividuals. For an IA with a CAG of 35 in this family with sporadic HD, the likelihood for siblings to inherit a recurrent mutation greater t han or equal to 36 CAG is similar to 10%. For IA(GP) Of a similar size , the risk of inheriting an expanded allele of greater than or equal t o 36 CAG through the paternal germline is similar to 6%. These risk es timates are higher than previously reported and provide additional inf ormation for counselling in these families. Further studies on persons with IAs will be needed to determine whether these results can be gen eralized to other families.