Novel mutations in the sacsin gene in ataxia patients from Maritime Canada

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Abstract

We ascertained two families in Eastern Canada segregating a form of ataxia consistent with a recessive mode of inheritance. We performed a whole genome scan using dense SNP genotyping, and despite an absence of shared homozygosity in the families we defined linkage to a small region on chromosome 13. Direct DNA resequencing was employed to screen biologically relevant candidate genes in the interval, and two presumptive pathogenic mutations were found in the gene encoding sacsin. One variant is an obligate truncating mutation, the second is a missense variant in a highly conserved residue. Unexpectedly, one family was homozygous for the missense mutation, the other compound heterozygous for the two mutations. Our results expand the genotype phenotype correlation of mutations in the sacsin gene, and highlight the challenge of diagnosing genetically heterogeneous disorders on primarily clinical grounds. We demonstrate that whole genome genotyping on a modest scale can be productive in research, and potentially in a clinical context.

Introduction

Inherited ataxias represent a large family of genetic disorders with considerable heterogeneity [1], [2]. As a result, the molecular characterization of new patients and families with putative genetic ataxia remains challenging. Among ataxias already genetically characterized, there are significant overlaps in clinical phenotype (spinocerebellar ataxias include more than ten identified genes). Individual genes may mutate to cause subtly or severely different clinical presentations. Moreover, patients may be ascertained at various points in their lifetime, making studies of disease progression for diagnostic purposes problematic.

Ultimately, despite the best efforts of clinical investigators to classify and subclassify such well ascertained conditions, molecular characterization increasingly provides a critical diagnostic criterion. Such characterization often involves sequencing of genes previously reported to mutate to a given clinical condition; however in some situations where the total number of known genes for a condition is prohibitive, or if some aspects of clinical presentation are atypical, or if a causal mutation is atypical and difficult to identify by coding exon resequencing, full genome analysis may be a more efficient technical approach. In the current report, we present a molecular genetic analysis of two families from Maritime Canada, ascertained for ataxia. Although genetic studies performed in the purely clinical context suggested that the underlying molecular basis of the condition might be novel in these families, we have now shown that they result from mutations in the sacsin gene (HGNC symbol SACS). Our experience documents the comparable efficiency of whole genome analysis versus candidate gene approaches for molecular diagnosis when families are available, and expands the genotype phenotype correlation for mutations in sacsin.

Section snippets

Clinical ascertainment and consent

Patients were identified in the course of routine clinical ascertainment and treatment of movement disorders in the neurology clinic at the IWK Health Centre. Approval for the research study was obtained from the IWK research ethics board. All sampled family members provided informed consent to participate in the study. DNA was obtained from blood samples using routine extraction methods. All procedures were in accordance with ethical and methodological standards for human experimentation.

Genotyping and analysis

Whole

Clinical ascertainment

Among our collection of patients and families with various types of ataxia, two families were noted with a similar particular clinical presentation (Fig. 1, Table 1). The patients were all seen as adults. All affected patients had ataxia, dysarthria and pes cavus. All exhibited nystagmus, although this was not a prominent finding. Four of the six patients had a Babinski sign. Family 1 patients had hyperreflexia and more severe spasticity than those in family 2, however these examinations were

Discussion

We report the identification of two novel mutations in the sacsin gene in Acadian Canadian families segregating an autosomal recessive form of ataxia. Although numerous mutations have been reported in sacsin, recently reviewed [14], it was not an immediately obvious candidate gene due to clinical differences in presentation from the most common form of mutated sacsin, clinically termed ARSACS [15]. Moreover there are many different monogenic syndromes molecularly characterized to date in the

Acknowledgements

We are grateful to the family members who generously contributed their time and materials for this research. The following agencies provided funding for this project: Genome Canada, Genome Atlantic, Nova Scotia Health Research Foundation, Nova Scotia Research and Innovation Trust, IWK Health Centre Foundation, and Capital Health Research Fund.

References (28)

  • F. Ruschendorf et al.

    ALOHOMORA: a tool for linkage analysis using 10 K SNP array data

    Bioinformatics

    (2005 May 1)
  • R.C. Edgar

    MUSCLE: multiple sequence alignment with high accuracy and high throughput

    Nucleic Acids Res

    (2004)
  • K.B. Nicholas et al.

    Genedoc: analysis and visualization of genetic variation

    Embnet News

    (1997)
  • A. Durr et al.

    Clinical and genetic abnormalities in patients with Friedreich's ataxia

    N Engl J Med

    (1996 Oct 17)
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