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De novo Huntington disease caused by 26–44 CAG repeat expansion on a low-risk haplotype

Huntington disease (HD, OMIM #143100) is a dominantly inherited neurodegenerative disorder due to a CAG repeat expansion in the HTT gene, encoding a polyglutamine tract in the N-terminal part of the huntingtin protein. Most cases are inherited from an affected parent, but in about 10% of cases the condition appears to be de novo.1 De novo or sporadic cases are usually due to CAG repeat expansion of intermediate alleles. Intermediate alleles have 27–35 CAG repeats, and the higher the number of repeats, the higher the risk for expansion into disease range, usually upon paternal transmission.2 In most cases, the change in repeat size is minor, and gradual increases into the disease range over several generations is the basis of new genetic mutations and stable disease prevalence. So far, the largest single-step expansions reported were from 27 to 383 and from 35 to 582 CAG repeats. It has recently been shown that intermediate alleles and disease alleles share the same haplotypes, which is expected if intermediate alleles are the main source of new mutation cases. The high-risk haplotypes are called A1 and A2, and are both prevalent among Caucasians but rare in other ethnic groups.4