Spastic Ataxia, Charlevoix-Saguenay Type

A number sign (#) is used with this entry because spastic ataxia of the Charlevoix-Saguenay type (SACS, or ARSACS) is caused by homozygous or compound heterozygous mutation in the gene encoding the sacsin protein (SACS; 604490) on chromosome 13q12

For a discussion of genetic heterogeneity of spastic ataxia, see SPAX1 (108600).

Description

Autosomal recessive spastic ataxia of Charlevoix-Saguenay is a complex neurodegenerative disorder usually characterized by early childhood onset of cerebellar ataxia, pyramidal tract signs, and peripheral neuropathy. Most patients become wheelchair-bound; cognitive function is usually not affected. Some patients may have atypical features, such as later onset or initial presentation of peripheral neuropathy (summary by Baets et al., 2010).

Clinical Features

In French Canada, Bouchard et al. (1978) identified a distinctive form of early-onset spastic ataxia. They examined 42 patients from 24 sibships and knew of 24 other affected persons. None of the patients ever walked normally. The disease had a long course with little progression after age 20 years. The oldest patient was aged 52 years. Features include ataxia, dysarthria, spasticity, distal muscle wasting, nystagmus, defect in conjugate pursuit ocular movements, retinal striation (from prominent retinal nerves) obscuring the retinal blood vessels in places, and the frequent presence (57%) of mitral valve prolapse. The disorder bore some similarity to Troyer syndrome (275900). However, nystagmus and abnormal pursuit movements were not noted in Troyer syndrome. Inheritance was clearly autosomal recessive. Bouchard et al. (1978) suggested that the gene originated from a couple that lived in Quebec City about 1650 and was also ancestral to many cases of typical Friedreich ataxia (229300).

Bouchard et al. (1979) defined electromyographic differences from Friedreich ataxia. In ARSACS (an acronym suggested by Bouchard et al., 1979), more EMG signs of denervation were found and nerve conduction was slower. In the 2 conditions an identical and important abnormality of sensory nerve conduction was found. Bouchard (1985) knew of almost 200 patients with ARSACS and commented on 'the remarkable increased visibility of the retinal nerve fibers, which is characteristic of the disease.' Bouchard et al. (1979) pointed to greater incidence of EEG changes and lower IQ in ARSACS than in Friedreich ataxia. By CT scan and/or pneumoencephalography, Langelier et al. (1979) found in all 9 cases studied cerebellar atrophy limited in the main to the superior part of the vermis and anterior lobes.

Richter et al. (1999) commented on the clinical homogeneity of ARSACS with early-onset spastic ataxia, with prominent myelinated retinal nerve fibers as a particularly distinctive feature. More than 300 patients had been identified by their group; most of the families originated in the Charlevoix-Saguenay region of northeastern Quebec, where the carrier prevalence had been estimated to be 1/22.

Clinical Variability

El Euch-Fayache et al. (2003) reported 4 Tunisian families, 3 of which were consanguineous, with autosomal recessive ataxia showing linkage to the ARSACS locus. Mean age at onset was 4.5 years, and the clinical phenotype was homogeneous, with progressive cerebellar ataxia, a pyramidal syndrome with brisk knee reflexes and absent ankle reflexes, and a peripheral neuropathy. Several patients had pes cavus, hammertoes, and/or scoliosis. The authors commented on the phenotypic similarities to ARSACS, but noted that fundi with prominent retinal myelinated fibers were rarely encountered in their patients.

Criscuolo et al. (2004) and Grieco et al. (2004) reported 4 patients from Italy, 2 of whom were sibs, with ARSACS. All patients had typical signs and symptoms associated with the disorder, but retinal striation was either mild or not observed. Ogawa et al. (2004) reported 2 Japanese sibs with ARSACS who also had mild retinal striation. The combined findings of the 3 reports broadened the worldwide distribution of the disorder and suggested variability in severity of retinal striation among different ethnic groups.

Shimazaki et al. (2005) reported 2 Japanese brothers with ARSACS confirmed by genetic analysis (604490.0009). The phenotype was unique in that neither patient had spasticity or hyperreflexia, although both had extensor plantar responses, indicating pyramidal tract dysfunction. The authors hypothesized that the severe peripheral nerve degeneration found on sural nerve biopsy may have masked any spasticity. The younger brother had mildly decreased IQ scores.

Muona et al. (2015) reported 2 unrelated patients with ARSACS, confirmed by genetic analysis, who were ascertained from a cohort of 84 patients with progressive myoclonic epilepsy who underwent exome sequencing. One patient had ataxia in early infancy, delayed motor development, intellectual disability, and absence seizures at age 3 years. She developed progressive myoclonic epilepsy with action myoclonus at age 14 years. The other patient had onset of myoclonus at age 13 years, seizures at age 15, mild learning disability, and progressive ataxia. She was wheelchair-bound at age 22 years. Muona et al. (2015) noted that progressive myoclonic epilepsy had not previously been described in ARSACS. Both patients were compound heterozygous for missense mutations; functional studies of the variants were not performed.

Armour et al. (2016) reevaluated 2 male twins who were originally reported by Fitzsimmons and Guilbert (1987) as having early-onset slowly progressive spastic paraplegia, dysarthria, and low-normal intellectual capacity. In addition, both patients had skeletal abnormalities of the hands and feet: brachydactyly, cone-shaped epiphyses, and an abnormal metaphyseal-phalangeal pattern profile. Fitzsimmons and Guilbert (1987) concluded that the patients had a novel syndrome, which was later designated 'Fitzsimmons-Guilbert syndrome;' however, exome sequencing performed by Armour et al. (2016) found that the patients were compound heterozygous for mutations in the SACS gene, resulting in the correct diagnosis of autosomal recessive spastic ataxia-6. In addition, the patients carried a heterozygous truncating mutation in the TRPS1 gene (604386), consistent with a diagnosis of type I trichorhinophalangeal syndrome (190350), a rare disorder associated with brachydactyly. Thus, the patients had 2 different genetic diseases that explained the unusual phenotype.

Mapping

Consistent with the hypothesis of a founder effect (see POPULATION GENETICS), Richter et al. (1999) observed excess shared homozygosity at 13q11 among patients in a genomewide scan of 12 families. Analysis of 19 pedigrees demonstrated very tight linkage between the ARSACS locus and an intragenic polymorphism of the gamma-sarcoglycan gene (SGCG; 608896), which maps to 13q12, but genomic DNA sequence analysis of all 8 exons of SGCG revealed no disease-causing mutation. On the basis of haplotypes composed of 7 marker loci that span 11.1 cM, the most likely position of the ARSACS locus was 0.42 cM distal to the SGCG polymorphism. Two groups of ARSACS-associated haplotypes were identified: a large group that carried a common SGCG allele and a small group that carried a rare SGCG allele. The haplotype groups did not appear to be closely related. Therefore, although chromosomes within each haplotype group may harbor a single ARSACS mutation identical by descent, the 2 mutations could have independent origins.

In a large Tunisian family with autosomal recessive cerebellar ataxia associated with a pyramidal syndrome and peripheral neuropathy, Mrissa et al. (2000) demonstrated linkage to chromosome 13q11-q12, the same locus as ARSACS.

Molecular Genetics

Engert et al. (2000) identified 2 mutations in the SACS gene (604490.0001, 604490.0002), which resides on chromosome 13q11, in ARSACS families that lead to protein truncation. The 2 different mutations corresponded to the 2 different haplotypes previously identified.

In 4 Tunisian families with autosomal recessive ataxia phenotypically similar to ARSACS, 3 of which were consanguineous, El Euch-Fayache et al. (2003) identified 4 mutations in the SACS gene (604490.0003-604490.0006).

Criscuolo et al. (2004) and Ogawa et al. (2004) identified mutations in the SACS gene in ARSACS patients from southern Italy and Japan, respectively (see 604490.0007 and 604490.0008).

Breckpot et al. (2008) reported a Belgian patient with ARSACS who was found to be compound heterozygous for a point mutation in the SACS gene and a de novo 1.54-Mb microdeletion on chromosome 13q12.12 involving 6 genes, including the SACS gene. The microdeletion was detected using array comparative genomic hybridization, and was postulated to result from nonallelic homologous recombination. The patient had typical clinical features of ARSACS with the addition of moderate perceptive hearing loss.

Baets et al. (2010) identified homozygous or compound heterozygous mutations in the SACS gene in 11 (12.9%) of 85 index patients with phenotypes suggestive of ARSACS. Eighteen different mutations were identified, including 11 missense, 5 frameshift, 1 nonsense, and 1 in-frame deletion. A founder allele was identified in 4 unrelated Belgian families. Five patients had onset after age 20 years, including 1 with onset at age 40. In addition, some patients presented with predominant features of a peripheral neuropathy, although most eventually developed the classic signs of the disorder, such cerebellar ataxia and pyramidal signs. Only 1 of 17 patients had mild mental retardation, and 2 had reduced IQ. There were no clear genotype/phenotype correlations.

Heterogeneity

Bouhlal et al. (2008) reported an unusual, highly consanguineous Tunisian family in which 11 individuals had autosomal recessive ataxia caused by 3 distinct gene defects. Seven patients who also had low vitamin E levels were all homozygous for the common 744delA mutation in the TTPA gene (600415.0001), consistent with a diagnosis of AVED (277460). Two patients with normal vitamin E levels were homozygous for a mutation in the FXN gene (606829.0001), consistent with a diagnosis of FRDA (229300). The final 2 patients with normal vitamin E levels carried a mutation in the SACS gene (604490), consistent with a diagnosis of ARSACS. The clinical phenotype was relatively homogeneous, although the 2 patients with SACS mutations had hyperreflexia of the knee. One asymptomatic family member was compound heterozygous for the TTPA and FXN mutations. Bouhlal et al. (2008) emphasized the difficulty of genetic counseling in deeply consanguineous families.

Population Genetics

De Braekeleer et al. (1993) estimated that the incidence at birth of this spastic ataxia syndrome in French Canadians of the Saguenay-Lac-Saint-Jean (SLSJ) region was 1/1,932, giving a carrier frequency of 1/21, for the period 1941-1985. The mean inbreeding coefficient was twice higher and the mean kinship coefficient 3 times higher among affected families than among control families. In the SLSJ region, the birth places of the ARSACS individuals and their parents did not show a clustered distribution. The genealogy of the families suggested that the high incidence of ARSACS in SLSJ and Charlevoix is likely to be the result of a founder effect and that a unique mutation accounts for most, if not all, of the ARSACS cases known in these regions.

De Braekeleer and Gauthier (1996) calculated the inbreeding coefficient of 567 probands from the Saguenay-Lac-Saint-Jean region of northeastern Quebec who suffered from one of the autosomal recessive disorders that are unusually frequent there. At least 2 of them with spastic ataxia of the Charlevoix-Saguenay type and sensorimotor polyneuropathy with or without agenesis of the corpus callosum (218000) were found almost only in that population. The mean inbreeding coefficient of the group containing all 567 probands was 2.73 times higher than that of the matched controls. Parental consanguinity was found in 75 of 567 probands (13%), but only 5% were born to matings between spouses related as second-degree cousins or closer. No marriage between uncle and niece and only 2 marriages between first-degree cousins were identified in the disorder group. These results strongly suggested that the high incidence of the autosomal recessive disorders in that region of Quebec is the result of founder effect.

Vermeer et al. (2008) identified pathogenic mutations in the SACS gene in 16 (37%) of 43 Dutch probands with early-onset ataxia before age 25 years. Sixteen novel mutations were identified. The phenotype was homogeneous and similar to that reported for other patients with this disorder.

History

Hennekam (1994) reported a girl with slowly progressive difficulties in walking, dysarthria, growth retardation, brachydactyly, and cone-shaped epiphyses, and suggested the eponym Fitzsimmons syndrome because the phenotype was reminiscent of that reported by Fitzsimmons and Guilbert (1987). Lacassie et al. (1999) reported monozygotic female twins, aged 62 years, with mental retardation, dysarthria, progressive spastic paraplegia, and brachydactyly type E who had been institutionalized since age 33 years. Differences from the patients reported by Fitzsimmons and Guilbert (1987) included more severe mental retardation and a different metacarpal-phalangeal pattern profile, suggesting either an expanded phenotype of the Fitzsimmons-Guilbert syndrome or a different entity. In a follow-up, Armour et al. (2016) stated that the patient reported by Hennekam (1994) was lost to follow-up and the sisters reported by Lacassie et al. (1999) had both died. Thus, it was not possible to perform genetic studies of these 3 patients, as was done in the patients originally reported by Fitzsimmons and Guilbert (1987).