Planning genetic studies on primary adult-onset dystonia: Sample size estimates based on examination of first-degree relatives

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Abstract

Primary adult-onset dystonia is thought to be partly genetic, but families large enough for a genome wide search are difficult to find. We examined the first-degree relatives of 76 primary adult-onset dystonia patients to assess the feasibility of model-free nonparametric methods that allow either screening of candidate loci (case–control design, transmission disequilibrium test [TDT], and sibling-TDT [S-TDT]) or identification of novel genes (affected sib-pair [ASP] method). Among the examined relatives, 1/34 parents, 13/149 siblings and 10/125 offspring were affected by adult-onset dystonia. The predicted sample sizes to detect a gene conferring an Odds ratio of 3.0 were 99 for case–control and TDT methodology, 148 for S-TDT, and 107 to 173 for an ASP study assuming three major loci. Based on our family structure, TDT, S-TDT, and ASP methods would required screening of about 220, 700, and 580 to 939 probands respectively. Analysing subpopulations with different types of dystonia, TDT required fewer probands with cervical/hand dystonia, S-TDT needed fewer probands with cranial dystonia. These sample size estimates suggest that the S-TDT may be feasible, whereas collection of cases for both TDT and ASP approaches would represent a major collaborative challenge.

Introduction

Primary adult-onset dystonia, the most common form of dystonia [1], is assumed to be partly genetic [2], [3]. Nevertheless, families large enough to warrant linkage analysis are difficult to find owing to the late onset and the reduced penetrance of the disease [2], [3]. Studies of a few large mendelian families have identified the DYT1 gene in a family with hand dystonia an, respectively [2], [3], [4], [5]]. These findings however could not be replicated in other families (suggesting genetic heterogeneity) as well as in apparently sporadic series [2], [3]. Likewise, candidate gene studies based on unrelated case–control sets gave unreliable results [6], [7]. Hence the gene(s) that possibly lend risk to commonly occurring adult-onset dystonia are not known.

As an alternative to linkage analysis in large families, model-free non-parametric methods can either screen candidate loci or identify novel genes in complex diseases without requiring knowledge of the inheritance pattern [8], [9], [10]. Tools for screening candidate loci include association studies such as conventional case–control design, transmission disequilibrium test (TDT), and sibling-transmission disequilibrium test (S-TDT). The conventional case–control design relies on unrelated case–control sets. As the frequency of genetic polymorphisms can differ in populations with different ancestral origin, spurious associations can arise from mismatching of control subjects. TDT and S-TDT overcome this problem because cases are matched with internal family controls (living parents in TDT, unaffected siblings in S-TDT). TDT is based on the assumption that if one parent is heterozygous for an allele, then this will be inherited 50% of the time to an affected offspring. If the mutation is associated with the disease, frequency of transmission would be greater than expected. S-TDT is robust to mismatching but incomplete penetrance of alleles among unaffected siblings makes this a less powerful approach. Tools for identifying novel loci include the affected sib-pair (ASP) method which is based on the hypothesis that sibs concordant for a disease should show greater than expected concordance for marker alleles that are closely linked to the disease locus. The ease with which the various methods identify disease loci depends on the degree of disease clustering in families and the availability of the appropriate family type (i.e. trios comprising of an affected individual and both parents for TDT, unaffected siblings for the S-TDT, and affected sibling pairs for the ASP method). Whereas these genetic tools have been tested in other complex diseases including multiple sclerosis [12], stroke [13], Parkinson's disease [14], and Alzheimer disease [15], no study has explored their applicability in adult-onset dystonia.

A major problem in applying these techniques to adult-onset dystonia is that it is still unclear whether genetic differences underlie the variable clinical expression of adult-onset dystonia. The differences in prevalence, age of onset, sex distribution, and tendency to spread among cranial, cervical and hand presentation of adult-onset dystonia [1], [16] and the report of families characterized by the same dystonia type [2] support distinct genetic entities. On the other hand, the coexistence of different focal dystonias in the same individual as the result of spread [16], the intrafamilial variability of clinical expression observed in the majority of reported families [2], [17], [18], and the recently reported linkage of the same locus (DYT7) to two families with different phenotype [4], [5] suggest that the same susceptibility genetic factor(s) may contribute to a spectrum of clinical manifestations as observed in DYT1 and Dopa-responsive dystonia [2], [3]. Under the hypothesis of a common genetic basis, it is possible that other specific genetic and/or environmental factors contribute to the variability of clinical expression.

We set up a family study based on the largest sample of families of patients with primary adult-onset dystonia systematically investigated so far and used familial information to assess the feasibility of the aforementioned approaches. We presented sample size estimates for adult-onset dystonia overall and for subpopulations with cranial, cervical, and hand dystonia.

Section snippets

Subjects

Probands were identified over an 18-month period among consecutive outpatients attending the movement disorders clinic of the Department of Neurological and Psychiatric Sciences of the University of Bari and living at no more than 3 hours' travelling distance from the center. The study was approved by the local ethical committee. Inclusion criteria were: a diagnosis of adult-onset dystonia according to published criteria [16]; age of onset > 20 years; duration of disease > 1 year; no other

Probands

During the study period, 98 patients met eligibility criteria but 22 (22.5%) refused to participate in the study. The reasons of the refusal are illustrated in Fig. 1. The two groups did not differ for demographic and clinical features (Table 1). Among participants, mean age of onset (± S.D.) was significantly higher in the BSP group than in focal cervical dystonia (CD) and focal hand dystonia groups (57.1 ± 12 vs. 42.2 ± 7 vs. 43.4 ± 6.8 years; one-way ANOVA: F = 13.1; p < 0.0001). Women predominated

Discussion

The information from the largest series of families of patients with primary adult-onset dystonia systematically examined so far confirm that families large enough to warrant linkage analysis are difficult to find and suggest that, among candidate gene approaches, the TDT may be impractical for exploring the overall genetic susceptibility to the condition due to the limited availability of trios with both parents alive. Owing to the large number of patients with unaffected sibs alive, the S-TDT

Acknowledgements

This work was supported by Grant No. GGP05165 funded by the Comitato Promotore Telethon, Italy.

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