Contribution of DNA Sequence and CAG Size to Mutation Frequencies of Intermediate Alleles for Huntington Disease: Evidence from Single Sperm Analyses

Abstract

New mutations for Huntington disease (HD) arise from intermediate alleles (IAs) with between 29 and 35 CAG repeats that expand on transmission through the paternal germline to 36 CAGs or greater. Using single sperm analysis, we have assessed CAG mutation frequencies for four IAs in families with sporadic HD (IANM) and IAs ascertained from the general population (IAGP) by analyzing 1161 single sperm from three persons. We show that IANM are more unstable than IAGP with identical size and sequence. Furthermore, comparison of different sized IAs and IAs with different sequences between the CAG and the adjacent CCG tracts indicates that DNA sequence is a major influence on CAG stability. These studies provide 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 in this family with sporadic HD, the likelihood for siblings to inherit a recurrent mutation > or = 36 CAG is approximately 10%. For IAGP of a similar size, the risk of inheriting an expanded allele of > or = 36 CAG through the paternal germline is approximately 6%. These risk estimates are higher than previously reported and provide additional information for counselling in these families. Further studies on persons with IAs will be needed to determine whether these results can be generalized to other families.

American College of Medical Genetics and Genomics Standards and Guidelines for Clinical Genetics Laboratories, 2014 edition: technical standards and guidelines for Huntington disease

“…[27][28][29][30][31] In the second category, rare A-to-G substitutions in the intervening 12-bp segment between the CAG and CCG tracts (CAG CAG CCG CCG) can result in increased meiotic instability of the tract as well as a miscalculation of ACMG StAndArdS And GuidelineS uninterrupted CAG repeat length based on the conventional calculation formula. 12,30 Southern blot. If the method routinely used by the laboratory is not validated to detect large CAG expansions, then Southern blotting should be used for the identification of expansions associated with juvenile-onset HD in cases of apparently homozygous normal genotypes.…”

“…6,11 The likelihood that transmission of an allele in this range will expand into an HD allele is dependent on several factors, including the sex of the transmitting individual, the size of the allele, the molecular configuration of the region surrounding the CAG repeat, and its haplotype. 11,12 This risk may be as high as 6-10% for paternal alleles carrying a CAG repeat of 35.…”

“…7 However, in rare cases, alleles in this range have been found in elderly asymptomatic individuals. 12,13 Although the limited number of documented cases has precluded the development of empirical penetrance risks for alleles in this range, some estimates of risk can be ascertained from an examination of individual case reports. 13,14 HD alleles with full penetrance.…”

REVIEW-ARTICLE Intermediate alleles of Huntington’s disease HTT gene in different populations worldwide: a systematic review

“…However, for both cases, changes in the allele size during transmission were small, and there was a sporadic risk of inheriting HD expanded alleles. These sporadic cases of HD (4.5%) came from individuals harboring IAs who had family history of HD, not being reported in offspring of those harboring IAs from the general population (Squitieri et al, 1994;Goldberg et al, 1995;Chong et al, 1997;Warby et al, 2009;Semaka et al, 2013a,b). One explanation for this should probably be their haplogroup.…”

Technical Standards and Guidelines for Huntington Disease Testing

“…2,6 The likelihood that transmission of an allele in this range will expand into an HD allele is dependent upon several factors, including sex of the transmitting individual, the size of the allele, the molecular configuration of the region surrounding the CAG repeat, and its haplotype. 6,7 This risk may be as high as 6% to 10% for paternal alleles carrying a CAG repeat of 35. 7 HD4.1.3 HD alleles with reduced penetrance: HD alleles with reduced penetrance are defined as alleles with 36 to 39 CAG repeats.…”