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 alleles(IAs) with between 29 and 35 CAG repeats that expand on transmissionthrough the paternal germline to 36 CAGs or greater. Using single spermanalysis, we have assessed CAG mutation frequencies for four IAs infamilies with sporadic HD (IANM) and IAs ascertained from the generalpopulation (IAGP) by analyzing 1161 single sperm from three persons. Weshow that IANM are more unstable than IAGP with identical size andsequence. Furthermore, comparison of different sized IAs and IAs withdifferent sequences between the CAG and the adjacent CCG tracts indicatesthat DNA sequence is a major influence on CAG stability. These studiesprovide estimates of the likelihood of expansion of IANM and IAGP to >or = 36 CAG repeats for these individuals. For an IA with a CAG of 35 inthis family with sporadic HD, the likelihood for siblings to inherit arecurrent mutation > or = 36 CAG is approximately 10%. For IAGP of asimilar size, the risk of inheriting an expanded allele of > or = 36 CAGthrough the paternal germline is approximately 6%. These risk estimates arehigher than previously reported and provide additional information forcounselling in these families. Further studies on persons with IAs will beneeded to determine whether these results can be generalized to otherfamilies.