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Original Contribution
January 28, 2004

Penetrance of the Fragile X–Associated Tremor/Ataxia Syndrome in a Premutation Carrier Population

Author Affiliations

Author Affiliations: MIND Institute (Drs Jacquemont, R. Hagerman, and Herman, Mr Jardini, and Mss Gane and Harris) and Departments of Pediatrics (Dr R. Hagerman) and Neurology (Dr Zhang), University of California Davis Medical Center, Sacramento; Departments of Neurology (Drs Leehey and Hall) and Medicine (Dr Grigsby), University of Colorado Health Sciences Center, Denver; Department of Mathematics and Statistics, San Diego State University (Dr Levine), San Diego, Calif; Departments of Radiology (Dr Brunberg), Pathology (Dr Greco), and Biological Chemistry (Drs Tassone and P. Hagerman), University of California, Davis, School of Medicine, Davis; and Department of Pediatrics, Rush-Presbyterian-St Luke's Medical Center, Chicago, Ill (Dr Berry-Kravis).

JAMA. 2004;291(4):460-469. doi:10.1001/jama.291.4.460

Context Premutation expansions (55-200 CGG repeats) of the fragile X mental retardation 1 (FMR1) gene are frequent in the general population, with estimated prevalences of 1 per 259 females and 1 per 813 males. Several articles have recently described the presence of late-onset neurological symptoms in male carriers of premutation (FMR1) alleles. The main clinical features described in this newly identified syndrome are cerebellar ataxia and intention tremor. Additional documented symptoms include short-term memory loss, executive functional deficits, cognitive decline, parkinsonism, peripheral neuropathy, lower-limb proximal muscle weakness, and autonomic dysfunction.

Objective To study the penetrance of the fragile X–associated tremor/ataxia syndrome (FXTAS) among premutation carriers.

Design, Setting, and Participants Family-based study of 192 individuals (premutation carriers and controls) whose families belong to the Northern or Southern California Fragile X Associations. Data were collected (March 2002-April 2003) through a survey and a standardized neurological examination, which was videotaped and subsequently scored in a blinded fashion.

Main Outcome Measures Penetrance of intention tremor and ataxia among adult carriers (aged ≥50 years) of premutation expansions of the FMR1 gene.

Results Data from the survey of 192 individuals demonstrated an age-related penetrance of the combination of reported intention tremor and gait ataxia in male carriers (17%, 38%, 47%, and 75% [lower-bound estimates] for participants aged 50-59, 60-69, 70-79, and ≥80 years, respectively). The male carrier group had an age-adjusted 13-fold increased risk (95% confidence interval, 3.9-25.4; P = .003) of combined intention tremor and gait ataxia when compared with male controls. The clinical examination data from 93 individuals demonstrated that male carriers experienced more difficulties on each of 3 standardized neurological rating scales compared with controls (P<.05). Female carrier scores were also higher than those of female controls (P<.05) on 2 of the 3 neurological rating scales, but no participant was identified with probable or definite FXTAS.

Conclusions The study demonstrates that older male carriers of premutation alleles of the FMR1 gene are at high risk of developing FXTAS. Since male premutation carriers are relatively common in the general population, older men with ataxia and intention tremor should be screened for the FMR1 mutation, especially if these signs are accompanied by parkinsonism, autonomic dysfunction, or cognitive decline, regardless of family history.