title: Homozygosity for CAG mutation in Huntington disease is associated with a more severe clinical course.
abstract: Huntington disease is caused by a dominantly transmitted CAG repeat expansion mutation that is believed to confer a toxic gain of function on the mutant protein. Huntington disease patients with two mutant alleles are very rare. In other poly(CAG) diseases such as the dominant ataxias, inheritance of two mutant alleles causes a phenotype more severe than in heterozygotes. In this multicentre study, we sought differences in the disease features between eight homozygotes and 75 heterozygotes for the Huntington disease mutation. We identified subjects homozygous for the Huntington disease mutation by DNA testing and compared their clinical features (age at onset, symptom presentation, disease severity and disease progression) with those of a group of heterozygotes, who were assessed longitudinally. The age at onset of symptoms in the homozygote cases was within the range expected for heterozygotes with the same CAG repeat lengths, whereas homozygotes had a more severe clinical course. The observation of a more rapid decline in motor, cognitive and behavioural symptoms in homozygotes was consistent with the extent of neurodegeneration as available at imaging in three patients, and at the post-mortem neuropathological report in one case. Our analysis suggests that although homozygosity for the Huntington disease mutation does not lower the age at onset of symptoms, it affects the phenotype and the rate of disease progression. These data, once confirmed in a larger series of patients, point to the possibility that the mechanisms underlying age at onset and disease progression in Huntington disease may differ.
authors.affiliation: Neurogenetics Unit, IRCCS INM Neuromed, Pozzilli (IS), Italy. neurogen@neuromed.it
authors.forename: Ferdinando
authors.initials: F
authors.lastname: Squitieri
authors.forename: Cinzia
authors.initials: C
authors.lastname: Gellera
authors.forename: Milena
authors.initials: M
authors.lastname: Cannella
authors.forename: Caterina
authors.initials: C
authors.lastname: Mariotti
authors.forename: Giuliana
authors.initials: G
authors.lastname: Cislaghi
authors.forename: David C
authors.initials: DC
authors.lastname: Rubinsztein
authors.forename: Elisabeth W
authors.initials: EW
authors.lastname: Almqvist
authors.forename: David
authors.initials: D
authors.lastname: Turner
authors.forename: Anne-Catherine
authors.initials: AC
authors.lastname: Bachoud-Lévi
authors.forename: Sheila A
authors.initials: SA
authors.lastname: Simpson
authors.forename: Martin
authors.initials: M
authors.lastname: Delatycki
authors.forename: Vittorio
authors.initials: V
authors.lastname: Maglione
authors.forename: Michael R
authors.initials: MR
authors.lastname: Hayden
authors.forename: Stefano Di
authors.initials: SD
authors.lastname: Donato
mesh_terms: D000293:Adolescent; D000328:Adult; D017668:Age of Onset; D000368:Aged; D015331:Cohort Studies; D018450:Disease Progression; D005260:Female; D006579:Heterozygote; D006720:Homozygote; D006801:Humans; D006816:Huntington Disease / Q000000981:diagnostic imaging / Q000235:genetics*; D008297:Male; D008875:Middle Aged; D009154:Mutation / Q000235:genetics*; D014057:Tomography, X-Ray Computed