Anticipation and intergenerational repeat instability in spinocerebellar ataxia type 17.код для вставкиСкачать
Anticipation and Intergenerational Repeat Instability in Spinocerebellar Ataxia Type 17 Astrid Rasmussen, MD, PhD,1,2 Irene De Biase, MD, PhD,2 Marcela Fragoso-Benı́tez, MD,3 Marco Antonio Macı́as-Flores, MD,4 Petra Yescas, MSc,1 Adriana Ochoa, MSc,1 Tetsuo Ashizawa, MD,5 Marı́a Elisa Alonso, MD,1 and Sanjay I. Bidichandani, MBBS, PhD2,6 Spinocerebellar ataxia type 17 (SCA17) is caused by expansion of a CAG/CAA repeat in the TBP gene. Most pathogenic alleles are interrupted and are stably transmitted from parent to offspring without anticipation. We identified three SCA17 families with expansion of uninterrupted alleles, thus greatly increasing the number of known intergenerational transmissions of such alleles. We found that uninterrupted SCA17 alleles are unstable, associated with anticipation, and show a paternal expansion bias that increases with age. Even small increments in repeat length resulted in inordinate increases in anticipation. Anticipation was also associated with childhood presentation. Sequencing of all SCA17 alleles is required for effective genetic counseling. Ann Neurol 2007;61:607– 610 The autosomal dominant spinocerebellar ataxias (ADCAs) are a clinically and genetically heterogeneous group of disorders associated with mutations in 15 known genes.1 Ten of them are caused by expansion of unstable microsatellite repeats, most commonly the polyglutamineencoding CAG triplet repeat. Spinocerebellar ataxia type 17 (SCA17; MIM 607136) is caused by the ex- From the 1Department of Neurogenetics and Molecular Biology, Instituto Nacional de Neurologı́a y Neurocirugı́a Manuel Velasco Suárez, Mexico City, Mexico; 2Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, OK; 3Faculty of Medicine, Universidad La Salle, Mexico City, Mexico; 4Faculty of Medicine, Universidad Autónoma de Zacatecas, Zacatecas, Mexico; 5Department of Neurology, University of Texas Medical Branch, Galveston, TX; and 6 Department of Pediatrics, University of Oklahoma Health Sciences Center, Oklahoma City, OK. Received Dec 25, 2006, and in revised form Feb 16, 2007. Accepted for publication Feb 23, 2007. Published online May 1, 2007, in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/ana.21139 Address correspondence to Dr Bidichandani, Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, 975 NE 10th Street BRC 458, Oklahoma City, OK 73104. E-mail: firstname.lastname@example.org pansion of a polyglutamine-encoding CAG/CAA triplet repeat in the gene for TATA-binding protein (TBP),2,3 and is the cause of approximately 1.5% of ADCA. Clinically, the phenotype is complex, including ataxia, dementia, chorea, parkinsonism, and psychiatric symptoms. In contrast with the other ADCAs caused by CAG repeat expansion, anticipation is rare in SCA17.4 – 6 This has been attributed to the interrupted configuration of the CAG/CAA repeat alleles: 5⬘-(CAG)3(CAA)3(CAG)7-11 CAA CAG CAA (CAG)9-21 CAA CAG-3⬘, in which CAA also codes for glutamines but results in stabilization of the repeat tract during intergenerational transmission. Most expanded pathogenic alleles in SCA17 show this interrupted sequence composition, and expansions usually involve the second (3⬘) polymorphic CAG tract. In most parent-to-offspring transmissions, the expanded allele retains the original size and anticipation is therefore not observed. However, a total of 10 “uninterrupted” alleles have been reported to date that contain a shorter configuration of the repeat: 5⬘(CAG)3(CAA)3(CAG)⬎32 CAA CAG-3⬘. These alleles are unstable on parental transmission and may be associated with anticipation.7–10 However, the rarity of these alleles has precluded a systematic analysis of the intergenerational instability of such alleles. We identified 17 additional SCA17 patients belonging to 3 ADCA families, all of whom had expansions involving the uninterrupted configuration of TBP alleles. We were able to characterize five intergenerational transmissions in these pedigrees. In addition to the five previously described transmissions, it was possible for the first time to characterize the properties of intergenerational transmission of these SCA17 alleles. Our study confirms that these alleles are commonly seen in familial cases, frequently expand, and result in genetic anticipation. We also found that intergenerational expansions were significantly dependent on the sex and age of the transmitting parent. Comparatively small-length gains in the polyglutamine tract within TBP produced an inordinate effect on the resulting phenotypic severity in subsequent generations. Although SCA17 is not normally considered in pediatric practice, more than a third of our patients showed onset of disease in childhood. Patients and Methods We identified a total of 19 patients with SCA17. All patients were Mexican mestizos, and whereas 17 belonged to 3 ADCA pedigrees, 2 were sporadic cases. Clinical information was available for all 19 patients, and DNA samples were available from both sporadic and 10 of the familial patients (3 each from Pedigrees I and U, and 4 from Pedigree Z) (Fig 1). Informed consent and genetic testing were performed as per guidelines of the institutional review board at Instituto Nacional de Neurologı́a y Neurocirugı́a. Polymerase chain reaction and direct sequencing was performed to identify the Rasmussen et al: Anticipation in SCA17 607 Fig 1. Clinical information of spinocerebellar ataxia type 17 (SCA17) pedigrees segregating uninterrupted expanded alleles. NT ⫽ not tested (DNA not available); NA ⫽ not applicable. Asterisk denotes patient died of unrelated cause. precise length and repeat configuration of the CAG/CAA alleles in the TBP gene as described previously.3 Information of age at onset of disease, time until dependence on wheelchair, and age at confinement to bed and/or death was obtained by direct interrogation of the primary caregiver and at least one other family member. Analysis of anticipation and intergenerational repeat instability was conducted using a combination of our newly identified five parent-tooffspring transmissions and the five previously reported in the literature.7–10 Results and Discussion By direct sequencing, we analyzed the precise structure of the expanded CAG/CAA repeats in all 12 patients from whom DNA samples were available. Surprisingly, all familial cases (n ⫽ 10) were found to carry the “uninterrupted” configuration of CAG/CAA alleles, and both sporadic cases showed the more common “interrupted” configuration. The uninterrupted alleles are uncommon, accounting for only 10 of 90 previously reported SCA17 patients.7–10 The length distribution of the 10 newly identified uninterrupted SCA17 alleles was similar to the 10 that have been previously reported by others (mean ⫽ 54.6, median ⫽ 54.5 [range, 51– 61] vs mean ⫽ 54.2, median ⫽ 53 triplets [range, 49 – 66]; p ⫽ 0.58, Mann–Whitney U test). The age of onset in our patients with uninterrupted alleles ranged from 11 to 48 years (mean ⫽ 28.3, median ⫽ 28.5 years). A highly significant correlation was noted between repeat 608 Annals of Neurology Vol 61 No 6 June 2007 length and the age of onset for our 10 patients (r ⫽ ⫺0.8; p ⫽ 0.003). Including our patients with the 10 previously reported patients with uninterrupted expanded alleles, the age of onset (mean ⫽ 31.25, median ⫽ 34 years [range, 3–76]) showed a highly significant correlation with the length of the expanded allele (mean ⫽ 54.4, median ⫽ 53 triplets [range, 49 – 66]; r ⫽ ⫺0.94; p ⫽ 10⫺9) (Fig 2A). The clinical features were consistent with SCA17: ataxia, dementia, chorea, parkinsonism, and psychiatric symptoms (see Fig 1). None of our patients had a history of seizures, which may be present in 20 to 25% of SCA17 patients. Four patients had mild sensorimotor axonal neuropathy, mainly in lower limbs, and two abnormal somatosensory-evoked potentials. Magnetic resonance imaging showed prominent cerebellar atrophy with variable degrees of brainstem and cerebral atrophy. The basal ganglia were normal, a consideration in ruling out multiple system atrophy. A systematic analysis was conducted for intergenerational differences in parameters of disease severity. We compared age of onset, number of years until loss of independent ambulation (determined as the age at which wheelchair use became necessary), and number of years until confinement to bed. All three parameters of disease severity showed significant increases on intergenerational transmission (Table). To study germline instability of uninterrupted SCA17 Fig 2. (A) Correlation of age of onset with the length (in triplets) of the uninterrupted expanded spinocerebellar ataxia type 17 (SCA17) allele. (B) Correlation of genetic anticipation with age of the parent transmitting the uninterrupted expanded SCA17 allele. alleles and whether this correlates with anticipation, we analyzed five of our intergenerational transmissions (see Fig 1) and five that have been reported previously in the literature. Six transmissions occurred via the paternal germline and four were maternal transmissions. The uninterrupted expanded alleles were prone to further expansion; further expansion was noted in 9 of 10 transmissions (range, 1–13 repeats/generation; mean ⫽ 3.8; 95% confidence interval [CI], 2.11–5.49), and only one allele remained unchanged. Notwithstanding the small number of transmissions, it was noted that larger expansions occurred during paternal transmission (paternal: median ⫽ 4.5; maternal: median ⫽ 1.5; p ⫽ 0.04). Anticipation was seen in all intergenerational transmissions, with the age of onset being 1 to 35 years earlier in the subsequent generation (mean ⫽ 18.7 years; 95% CI, 13.4 –23.9). Anticipation correlated with parental age (r ⫽ 0.75; p ⫽ 0.007), suggestive of progressive agedependent instability of the uninterrupted repeat tract in the germline (see Fig 2B). A definite parental bias was noted with greater anticipation occurring via paternal transmission (range, 16 –35 years; mean ⫽ 26.83; 95% CI, 24 –29.7) compared with maternal transmission (range, 1–14 years; mean ⫽ 5.5; 95% CI, 3.9 –9), which was highly significant ( p ⫽ 0.009). The observed anticipation, measured as number of years per generation, is relatively high when compared with other diseases caused by expansion of CAG repeats. It is similar to SCA7,11 which is known to have high levels of anticipation (mean SCA17 vs SCA7: 18.7 vs 23.8 yr/generation). However, the high level of anticipation seen in SCA7 families is usually associated with correspondingly large intergenerational increases in repeat length (mean ⫽ 10 triplets). Comparatively, much smaller intergenerational changes in repeat length were required in SCA17 (mean ⫽ 3.8 triplets; 95% CI, 2.1– 5.5). Each triplet gained on intergenerational transmission has a greater impact for anticipation in SCA17 (mean ⫽ 5.69 yr/triplet gained) than in SCA7 (mean ⫽ 1.9 yr/triplet gained),12 indicating that the context of the polyglutamine tract within TBP may be more critical than in other polyglutamine-bearing proteins. This is perhaps not too surprising, given the widespread effects one might expect from changes in the numerous genes with which TATA-binding protein normally interacts. SCA17 is not normally considered in pediatric practice. However, 7 of our 17 ADCA patients (41%) had an age of onset of younger than 21 years. Each of the three pedigrees had at least one child with onset of younger than 21 years (see Fig 1). In addition, in one child (Patient U1), the onset preceded that of the parent and others in the parental generation. These data stress the importance of considering SCA17 in the differential diagnosis of pediatric ataxia patients. The detection of expanded alleles for the diagnosis of ADCAs is commonly performed by estimating sizes of the alleles by electrophoresis of amplified alleles. However, given the dramatic difference in the intergenerational in- Table. Anticipation in Spinocerebellar Ataxia Type 17 (SCA17) Pedigrees with Uninterrupted Alleles Characteristics Age of onset (range), yr Loss of independent ambulation (range), yr Confinement to bed (range), yr Generation I 40.6 (38.3–42.9) 12.5 (10.9–14.1) 15.3 (13.5–17.2) Generation II 20.4 p ⬍0.001 (16.5–24.2) 5.3 ⬍0.01 (4.6–5.9) 5.3 ⬍0.05 (5.1–5.6) Rasmussen et al: Anticipation in SCA17 609 stability of uninterrupted and interrupted SCA17 alleles, we recommend that all SCA17 alleles should be sequenced to provide effective genetic counseling. In conclusion, we have shown that intergenerational transmission of the uninterrupted mutant alleles in SCA17 almost always results in further expansion and genetic anticipation. Previously inconclusive observations regarding genetic anticipation in SCA17 were most likely due to the lack of distinction between interrupted and uninterrupted alleles. TBP is unusual in its susceptibility to small gains in polyglutamine tract length, resulting in high levels of genetic anticipation for relatively small increments in repeat length. This work was supported by Consejo Nacional de Ciencia y Tecnologı́a (SALUD-2003-C01-028, A.R.), the NIH (NINDSNS047596, S.I.B.; NS041547, T.A.), Muscular Dystrophy Association (MDA 3973, S.I.B.), Oklahoma Center for the Advancement of Sciences and Technology (HR05-009, S.I.B.), Friedreich Ataxia Research Alliance (S.I.B.), and postdoctoral fellowship from the National Ataxia Foundation (I.D.B.). References 1. Bird TD. Hereditary ataxia overview. In: GeneReviews: genetic disease online reviews at GeneTests-GeneClinics. Seattle, WA: University of Washington. Available at: http:// www.geneclinics.org. Accessed December 2006. 2. 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