OLIVOPONTOCEREBELLAR ATROPHY I
SPINOCEREBELLAR ATROPHY I
CEREBELLOPARENCHYMAL DISORDER I
OLIVOPONTOCEREBELLAR ATROPHY IV
MENZEL TYPE OPCA
OPCA IV
OPCA I
SCHUT-HAYMAKER TYPE OPCA
SCA1
CPD1
OPCA4
OPCA1
The autosomal dominant cerebellar degenerative disorders are generally referred to as 'spinocerebellar ataxias,' (SCAs) even though 'spinocerebellar' is a hybrid term, referring to both clinical signs and neuroanatomical regions (Margolis, 2003). Neuropathologists have defined SCAs as cerebellar ataxias ... The autosomal dominant cerebellar degenerative disorders are generally referred to as 'spinocerebellar ataxias,' (SCAs) even though 'spinocerebellar' is a hybrid term, referring to both clinical signs and neuroanatomical regions (Margolis, 2003). Neuropathologists have defined SCAs as cerebellar ataxias with variable involvement of the brainstem and spinal cord, and the clinical features of the disorders are caused by degeneration of the cerebellum and its afferent and efferent connections, which involve the brainstem and spinal cord (Schols et al., 2004; Taroni and DiDonato, 2004). Historically, Harding (1982) proposed a clinical classification for autosomal dominant cerebellar ataxias (ADCAs). ADCA I was characterized by cerebellar ataxia in combination with various associated neurologic features, such as ophthalmoplegia, pyramidal and extrapyramidal signs, peripheral neuropathy, and dementia, among others. ADCA II was characterized by the cerebellar ataxia, associated neurologic features, and the additional findings of macular and retinal degeneration. ADCA III was a pure form of late-onset cerebellar ataxia without additional features. SCA1, SCA2 (183090), and SCA3, or Machado-Joseph disease (109150), are considered to be forms of ADCA I. These 3 disorders are characterized at the molecular level by CAG repeat expansions on 6p24-p23, 12q24.1, and 14q32.1, respectively. SCA7 (607640), caused by a CAG repeat expansion in the ATXN7 gene (607640) on chromosome 3p13-p12, is a form of ADCA II. SCA5 (600224), SCA31 (117210), SCA6 (183086), and SCA11 (600432) are associated with phenotypes most suggestive of ADCA III. However, Schelhaas et al. (2000) noted that there is significant phenotypic overlap between different forms of SCA as well as significant phenotypic variability within each subtype. Classic reviews of olivopontocerebellar atrophies and of inherited ataxias in general include those of Konigsmark and Weiner (1970), who identified 5 types of olivopontocerebellar atrophy, Berciano (1982), Harding (1993), Schelhaas et al. (2000), and Margolis (2003).
Lucotte et al. (2001) demonstrated the feasibility of presymptomatic diagnosis in spinocerebellar ataxia-1. They studied a family in which the mean age of onset of the disorder was 38 years. Hitherto, presymptomatic testing for late-onset autosomal dominant disorders ... Lucotte et al. (2001) demonstrated the feasibility of presymptomatic diagnosis in spinocerebellar ataxia-1. They studied a family in which the mean age of onset of the disorder was 38 years. Hitherto, presymptomatic testing for late-onset autosomal dominant disorders had been largely confined to Huntington disease, which is a genetically homogeneous entity. The same protocol could be applied to dominantly inherited ataxias, with the additional requirement that the SCA type of the disorder must be determined in the family at risk.
Symptoms of SCA1 usually begin in the third or fourth decade of life, most often around age 30. In addition to cerebellar signs, there are upper motor neuron signs and extensor plantar responses. Involuntary choreiform movements may occur. ... Symptoms of SCA1 usually begin in the third or fourth decade of life, most often around age 30. In addition to cerebellar signs, there are upper motor neuron signs and extensor plantar responses. Involuntary choreiform movements may occur. Characteristic families with autosomal dominant spinocerebellar ataxia were reported by Menzel (1890), Waggoner et al. (1938), and Destunis (1944). Both the clinical and the pathologic pictures in the disorder described in a large kindred, known as Vandenberg, by Schut (1950) and by Schut and Haymaker (1951) were variable. Symptoms varied from those of spinocerebellar ataxia to spastic paraplegia. Identification as a form of OPCA was based on the presence of the major pathology in the inferior olivary nucleus and cerebellum with variable pontine involvement. The spinal cord showed variable loss of anterior motor horn cells and changes in the spinocerebellar tracts and posterior funiculus. Involvement of cranial nerves IX, X and XII was another distinguishing feature. Nino et al. (1980) reported a family in which the mean age of onset was 38.8 years. In addition to ataxia, affected persons showed lower bulbar palsies, hyperreflexia, scanning and explosive speech, incoordination, and, in some, slow motor-nerve conduction. Neuropathologic findings included atrophy of the cerebellum, pons and olives, degeneration of lower cranial nerve nuclei, and atrophy of the dorsal columns and spinocerebellar tracts. Deep tendon reflexes were increased and the Babinski sign was present. Pedersen (1980) reported an extensively affected Danish kindred. Clinical expression was highly variable so that different types of cerebellar ataxia had been diagnosed in individual members of the family. In at least 10, multiple sclerosis had been diagnosed. Robitaille et al. (1995) compared the neuropathologic features of SCA1 with those reported for SCA2 and SCA3. Unlike the findings in SCA1 and SCA3, brains in SCA1 show almost no neuronal loss from the pars compacta of the substantia nigra or in the locus ceruleus, whereas there is severe atrophy of the dentatorubral pathways. Both SCA1 and SCA2 show severe loss of Purkinje cell and degeneration of the olivocerebellar pathways, which is not seen in SCA3. All 3 disorders share severe atrophy of the nucleus pontis, sparing of the retina and optic nerve, and marked atrophy of Clarke columns and the spinocerebellar tracts. Argyrophilic glial inclusions have not been reported in any of these disorders. In 19 of 27 (70%) patients with confirmed SCA types 1, 2, 3, 6, or 7, van de Warrenburg et al. (2004) found electrophysiologic evidence of peripheral nerve involvement. Eight patients (30%) had findings compatible with a dying-back axonopathy, whereas 11 patients (40%) had findings consistent with a primary neuronopathy involving dorsal root ganglion and/or anterior horn cells; the 2 types were clinically almost indistinguishable. Four of 5 patients with SCA1 had a neuronopathy and 1 had a sensorimotor axonopathy.
Schols et al. (1997) compared clinical, electrophysiologic, and magnetic resonance imaging (MRI) findings to identify phenotypic characteristics of genetically defined SCA subtypes. Slow saccades, hyporeflexia, myoclonus, and action tremor suggested SCA2. SCA3 patients frequently developed diplopia, severe spasticity ... Schols et al. (1997) compared clinical, electrophysiologic, and magnetic resonance imaging (MRI) findings to identify phenotypic characteristics of genetically defined SCA subtypes. Slow saccades, hyporeflexia, myoclonus, and action tremor suggested SCA2. SCA3 patients frequently developed diplopia, severe spasticity or pronounced peripheral neuropathy, and impaired temperature discrimination, apart from ataxia. SCA6 presented with a predominantly cerebellar syndrome, and patients often had onset after 55 years of age. SCA1 was characterized by markedly prolonged peripheral and central motor conduction times in motor evoked potentials. MRI scans showed pontine and cerebellar atrophy in SCA1 and SCA2. In SCA3, enlargement of the fourth ventricle was the main sequel of atrophy. SCA6 presented with pure cerebellar atrophy on MRI. Overlap between the 4 SCA subtypes was broad, however. Among 65 patients with SCA1, SCA2, or SCA3, Burk et al. (1996) found reduced saccade velocity in 56%, 100%, and 30% of patients, respectively. MRI showed severe olivopontocerebellar atrophy in SCA2, similar but milder changes in SCA1, and very mild atrophy with sparing of the olives in SCA3. Careful examination of 3 major criteria of eye movements, saccade amplitude, saccade velocity, and presence of gaze-evoked nystagmus, permitted Rivaud-Pechoux et al. (1998) to assign over 90% of patients with SCA1, SCA2, or SCA3 to their genetically confirmed patient group. In SCA1, saccade amplitude was significantly increased, resulting in hypermetria. In SCA2, saccade velocity was markedly decreased. In SCA3, the most characteristic finding was the presence of gaze-evoked nystagmus. In an investigation of oculomotor function, Buttner et al. (1998) found that all 3 patients with SCA1, all 7 patients with SCA3, and all 5 patients with SCA6 had gaze-evoked nystagmus. Three of 5 patients with SCA2 did not have gaze-evoked nystagmus, perhaps because they could not generate corrective fast components. Rebound nystagmus occurred in all SCA3 patients, 33% of SCA1 patients, 40% of SCA6 patients, and none of SCA2. Spontaneous downbeat nystagmus only occurred in SCA6. Peak saccade velocity was decreased in 100% of patients with SCA2, 1 patient with SCA1, and no patients with SCA3 or SCA6. Saccade hypermetria was found in all types, but was most common in SCA3. Burk et al. (1999) found that gaze-evoked nystagmus was not associated with SCA2. However, severe saccade slowing was highly characteristic of SCA2. Saccade velocity in SCA3 was normal to mildly reduced. The gain in vestibuloocular reflex was significantly impaired in SCA3 and SCA1. Eye movement disorders of SCA1 overlapped with both SCA2 and SCA3. The reticulotegmental nucleus of the pons (RTTG), also known as the nucleus of Bechterew, is a precerebellar nucleus important in the premotor oculomotor circuits crucial for the accuracy of horizontal saccades and the generation of horizontal smooth pursuit. By postmortem examination, Rub et al. (2004) identified neuronal loss and astrogliosis in the RTTG in 1 of 2 SCA1 patients, 2 of 4 SCA2 patients, and 4 of 4 SCA3 patients that correlated with clinical findings of hypometric saccades and slowed and saccadic smooth pursuits. The 3 patients without these specific oculomotor findings had intact RTTG regions. The authors concluded that the neurodegeneration associated with SCA1, SCA2, and SCA3 affects premotor networks in addition to motor nuclei in a subset of patients. Using an analysis of covariance and multivariate models to examine symptom severity in 526 patients with SCA1, SCA2, SCA3, or SCA6, Schmitz-Hubsch et al. (2008) found that repeat length of the expanded allele, age at onset, and disease duration explained 60.4% of the ataxia score in SCA1, 45.4% in SCA2, 46.8% in SCA3. However, only age at onset and disease duration appeared to explain 33.7% of the score in SCA6. Similar findings were obtained for nonataxic symptoms. The study suggested that SCA1, SCA2, and SCA3 share a number of common biologic properties, whereas SCA6 is distinct in that its phenotype is more determined by age than by disease-related factors.
Banfi et al. (1994) determined that the CAG trinucleotide repeat identified by Orr et al. (1993) in SCA1 occurs in the ataxin-1 gene (601556.0001).
- Genetic Anticipation
Chung et al. (1993) found that 63% ... Banfi et al. (1994) determined that the CAG trinucleotide repeat identified by Orr et al. (1993) in SCA1 occurs in the ataxin-1 gene (601556.0001). - Genetic Anticipation Chung et al. (1993) found that 63% of paternal transmissions show an increase in repeat number, whereas 69% of maternal transmissions show no change or a decrease in repeat number. Sequence analysis showed that 98% of unexpanded alleles had an interrupted repeat configuration, whereas a contiguous repeat (CAG)n was found in expanded alleles. This indicated that the repeat instability in ATXN1 is more complex than a simple variation in repeat number and that the loss of an interruption predisposes the ATXN1 (CAG)n to expansion. Matilla et al. (1993) studied the expansion of the ATXN1 gene CAG repeat in a large family in which spinocerebellar ataxia showed the phenomenon of anticipation. There were 41 affected members with no juvenile cases of SCA1, the mean age of onset being 36 years. The family also showed the phenomenon of parental male bias; i.e., the age of onset was younger and the duration of illness before death was shorter in the members of the family who inherited the disorder from the father. In this large Spanish kindred, Matilla et al. (1993) found 9 clinically unaffected persons between ages 18 and 40 years who had expansions of the CAG repeat within the pathogenetic range. In 22 other genetically 'at risk' individuals, they found that the number of CAG repeats in the ATXN1 gene was within the normal range. Ranum et al. (1994) examined the frequency and variability of the ATXN1 repeat expansion in 87 kindreds with diverse ethnic backgrounds and dominantly inherited ataxia. All 9 families for which linkage to the ATXN1 region of 6p had previously been established showed repeat expansion, while 3 of the remaining 78 showed a similar abnormality. For 113 patients from the families with repeat expansion, inverse correlations between CAG repeat size and both age at onset and disease duration were observed. Repeat size accounted for 66% of the variation in age at onset in these patients. After correction for repeat size, interfamilial differences in age at onset remained significant, suggesting that additional genetic factors affect the expression of the ATXN1 gene product. Jodice et al. (1994) found trinucleotide repeat expansion in 64 subjects from 19 families: 57 patients with SCA1 and 7 subjects predicted, by haplotype analysis, to carry the mutation. Comparison with a large set of normal chromosomes showed 2 distinct distributions with a much wider variation among expanded chromosomes. The sex of the transmitting parent played a major role in the size distribution of expanded alleles, those with more than 54 repeats being transmitted by affected fathers exclusively. Alleles with 46 to 54 repeats were transmitted by affected fathers and mothers in equal proportions. On the other hand, the sex ratio of offspring receiving either more than 54 or less than 54 repeats approached the expected 50:50. If a steady-state distribution of repeat numbers is assumed to persist through the generations, this raises the question as to why affected females transmitting alleles with more than 54 repeats are lacking, while females receiving more than 54 repeats exist. This may be explained, at least in part, by reduced biologic fitness. Detailed clinical follow-up of a subset of patients by Jodice et al. (1994) demonstrated significant relationships between increasing repeat number on expanded chromosomes and earlier age at onset, faster progression of the disease, and earlier age at death. Koefoed et al. (1998) performed single sperm analysis of (CAG)n stretches in SCA1 patients and asymptomatic carriers. A pronounced variation in the size of the expanded allele was found in sperm cells and in peripheral blood leukocytes, with a higher degree of instability in sperm cells, where an allele with 50 repeat units was contracted in 11.8%, further expanded in 63.5%, and unchanged in 24.6% of the single sperm analyzed. They also found a low instability of the normal alleles; the normal alleles from the individuals carrying a CAG repeat expansion was significantly more unstable than the normal alleles from control individuals (P less than 0.001), indicating an interallelic interaction between the expanded and the normal alleles. Matsuyama et al. (1999) studied 17 patients with SCA1. In one of these patients the expanded ATXN1 allele was interrupted by a CAT trinucleotide. The total number of CAG repeats was 58, predicting an age at onset of 22.0 years, in contrast to the actual age at onset of 50 years. In addition, brainstem atrophy was mild compared to that of a patient with 52 CAG repeats. Sequence analysis showed the repeat portion of the ATXN1 allele contained 45 uninterrupted CAG repeats with 2 interspersed CAT repeats in the subsequent 12 trinucleotides. Matsuyama et al. (1999) concluded that the age at onset of SCA1 is not determined by the total number of CAG repeats, but rather by the total number of uninterrupted CAG repeats. Zuhlke et al. (2002) performed genotype-phenotype correlation in intermediate alleles from 36 to 43 CAG repeats in the ATXN1 gene with respect to the presence of interrupting CAT trinucleotides. Alleles with 36 to 38 triplets were present in individuals with ataxia but without additional characteristic features of SCA1. SCA1 phenotypes were found for patients with 41 and 43 triplets. The 39 triplet allele missing CAT interruptions was associated with symptoms characteristic for SCA1 in 4 patients, whereas the interrupted allele with 39 triplets did not cause characteristic SCA1 features in 1 individual. These findings suggested a change from normal to pathologic alleles at 39 triplets depending on the presence of CAT interruptions in the CAG repeat. Stable inheritance of the uninterrupted 39 triplet allele was observed in 1 familial case of SCA1. Van de Warrenburg et al. (2005) applied statistical analysis to examine the relationship between age at onset and number of expanded triplet repeats from a Dutch-French cohort of 802 patients with SCA1 (138 patients), SCA2 (166 patients), SCA3 (342 patients), SCA6 (53 patients), and SCA7 (103 patients). The size of the expanded repeat explained 66 to 75% of the variance in age at onset for SCA1, SCA2, and SCA7, but less than 50% for SCA3 and SCA6. The relation between age at onset and CAG repeat was similar for all groups except for SCA2, suggesting that the polyglutamine repeat in the ataxin-2 protein exerts its pathologic effect in a different way. A contribution of the nonexpanded allele to age at onset was observed for only SCA1 and SCA6. Van de Warrenburg et al. (2005) acknowledged that their results were purely mathematical, but suggested that they reflected biologic variations among the diseases.
Giunti et al. (1994) examined members of 73 families who were affected with a variety of autosomal dominant late-onset cerebellar ataxias for the trinucleotide repeat expansion associated with the SCA1 locus. The mutation was found in 19 of ... Giunti et al. (1994) examined members of 73 families who were affected with a variety of autosomal dominant late-onset cerebellar ataxias for the trinucleotide repeat expansion associated with the SCA1 locus. The mutation was found in 19 of 38 kindreds with the SCA1 phenotype. However, it was not found in any of 8 families with olivopontocerebellar atrophy with maculopathy (164500), or in 24 kindreds with pure adult-onset cerebellar ataxia (SCA31; 117210), or in 12 patients with sporadic degenerative ataxia. The patients with the expansion were Italian, British, Malaysian, Bangladeshi, and Jamaican. Ranum et al. (1995) made use of the fact that the genes involved in 2 forms of autosomal dominant ataxia, that for Machado-Joseph disease (109150) and that for SCA1, have been isolated to assess the frequency of trinucleotide repeat expansions among individuals diagnosed with ataxia. They collected and analyzed DNA from individuals with both disorders. In both cases, the genes responsible for the disorder were found to have an expansion of an unstable CAG trinucleotide repeat. These individuals represented 311 families with adult-onset ataxia of unknown etiology, of which 149 families had dominantly inherited ataxia. Ranum et al. (1995) found that of these, 3% had SCA1 trinucleotide repeat expansions, whereas 21% were positive for the MJD trinucleotide expansion. For the 57 patients with MJD trinucleotide repeat expansions, strong inverse correlation between CAG repeat size and age at onset was observed (r = -0.838). Among the MJD patients, the normal and affected ranges of CAG repeat size were 14 to 40 and 68 to 82 repeats, respectively. For SCA1, the normal and affected ranges were much closer, namely 19 to 38 and 40 to 81 CAG repeats, respectively. In a nationwide survey of Japanese patients, Hirayama et al. (1994) found an estimated prevalence of the various forms of spinocerebellar degeneration to be 4.53 per 100,000. Of these, 12.6% were thought to have the Menzel type of spinocerebellar atrophy (SCA1). However, it was not clear how they distinguished this disorder from the other forms of OPCA. In Japan, Suzuki et al. (1995) found that all affected and presymptomatic individuals in 12 pedigrees with SCA1 (determined by haplotype per segregation analyses) carried an abnormally expanded allele with a range of 39 to 63 repeat units. This repeat size inversely correlated with the age of onset. However, contrary to previous reports, the size of the repeat did not correlate with gender of the transmitting parent. CAG triplet repeat instability on paternal transmission was not observed. Wakisaka et al. (1995) determined the haplotype cosegregating with SCA1 in 12 Japanese pedigrees. Although the alleles of the ATXN1 haplotype varied from pedigree to pedigree depending on the distance from the SCA1 locus, the affected and presymptomatic subjects carried the same alleles at 2 loci, D6S288 and D6S274. All the families with SCA1 had migrated from either the Miyagi or Yamagata Prefectures, neighboring areas in the Tokohu District, the northern part of Honshu, which is the main island of Japan. The findings suggested to the authors that SCA1 in the Japanese, at least those residing in Hokkaido, derived from a single common ancestry. Goldfarb et al. (1996) studied 78 SCA1 patients from a large Siberian kindred which included 1,484 individuals, 225 of whom are known to be affected and 656 of whom were at risk. Normal alleles had 25 to 37 trinucleotide repeats, whereas expanded alleles contained 40 to 55 repeats. The disease was not fully penetrant inasmuch as there was one 66-year-old woman with 44 CAG repeats who was asymptomatic. Of her 7 children, 4 were affected, including a homozygous daughter and another child with 44 repeats. Two symptomatic individuals who had expansions on both chromosomes demonstrated clinical manifestations that corresponded to the size of the larger allele. In Catalonia, Genis et al. (1995) found a large kindred traced to a common ancestor born in 1735 that segregated spinocerebellar ataxia-1. Affected individuals all had 1 allele with between 41 in 59 repeats, whereas asymptomatic individuals for the most part fell in the range of 6 to 39 repeats. Two asymptomatic individuals, an 18-year-old female and a 25-year-old male, had 41 repeats. Klockgether et al. (1994) analyzed DNA from 19 German families with autosomal dominant cerebellar ataxia and 61 unrelated individuals with idiopathic cerebellar ataxia with a mean age of onset of 53.6 years. Heterozygosity for the ATXN1 triplet repeat expansion was diagnosed in 5 out of 19 of the autosomal dominant kindreds. In contrast, none of the 61 cases of idiopathic adult-onset cerebellar ataxia showed this expansion. This suggested that SCA1 is not a significant cause of idiopathic cerebellar ataxia in Germany. Studying 77 German families with autosomal dominant cerebellar ataxia of SCA types 1, 2, 3, and 6, Schols et al. (1997) found that the SCA1 mutation accounted for 9%, SCA2 for 10%, SCA3 for 42%, and SCA6 for 22%. There was no family history of ataxia in 7 of 27 SCA6 patients. Age at onset correlated inversely with repeat length in all subtypes. Yet the average effect of 1 CAG unit on age of onset was different for each SCA subtype. Riess et al. (1997) found that in both SCA1 and SCA3 patients in German families there was distortion of the mendelian 1:1 segregation of the disease. They noted that mutations in the ataxin-1 gene are responsible for autosomal dominant spinocerebellar ataxia in about 10% of all families, whereas SCA3 is the most common cause in Germany, accounting for up to 50% of cases. Ramesar et al. (1997) investigated 14 South African kindreds and 22 sporadic individuals with SCA for expanded ATXN1 (601556.0001) and ATXN3 (607047.0001) repeats. The authors stated that, in the present study, ATXN1 mutations accounted for 43% of known ataxia families in the Western Cape region. They found that expanded ATXN1 and CAG repeats cosegregated with the disorder in 6 of the families, 5 of mixed ancestry and 1 Caucasian, and were also observed in a sporadic case from the indigenous Black African population. The use of the microsatellite markers D6S260, D6S89, and D6S274 provided evidence that the expanded ATXN1 repeats segregated with 3 distinct haplotypes in the 6 families. None of the families nor the sporadic individuals showed expansion of the MJD repeat. Among 202 Japanese and 177 Caucasian families with autosomal dominant SCA, Takano et al. (1998) found that the prevalence of SCA1 was significantly higher in the Caucasian population (15%) compared to the Japanese population (3%). This corresponded to higher frequencies of large normal ATXN1 CAG repeat alleles (greater than 30 repeats) in Caucasian controls compared to Japanese controls. The findings suggested that large normal alleles contribute to the generation of expanded alleles that lead to dominant SCA. In Spain, Pujana et al. (1999) performed molecular analysis on 87 unrelated familial and 60 sporadic cases of spinocerebellar ataxia of autosomal dominant type. For the familial cases of ADCA, 6% were SCA1, 15% were SCA2, 15% were SCA3, 1% represented SCA6, 3% were SCA7, and, in 1%, the diagnosis was DRPLA (125370), an extremely rare mutation in Caucasoid populations. About 58% of ADCA cases remained genetically unclassified. All the SCA1 cases belonged to the same geographic area and shared a common haplotype for the SCA1 mutation. The expanded alleles ranged from 41 to 59 repeats for SCA1, 35 to 46 for SCA2, 67 to 77 for SCA3, and 38 to 113 for SCA7. The 1 SCA6 case had 25 repeats and the 1 DRPLA case had 63 repeats. The highest CAG repeat variation in meiotic transmission of expanded alleles was detected in SCA7, this being an expansion of 67 units in one paternal transmission, giving rise to a 113 CAG repeat allele in a patient who died at 3 years of age. Meiotic transmissions showed a tendency to more frequent paternal transmission of expanded alleles in SCA1 and maternal in SCA7. All SCA1 and SCA2 expanded alleles analyzed consisted of pure CAG repeats, whereas normal alleles were interrupted by 1 to 2 CAT trinucleotides in SCA1, except for 3 alleles of 6, 14, and 21 CAG repeats, and by 1 to 3 CAA trinucleotides in SCA2. The failure to find SCA or DRPLA mutations in the 60 sporadic cases of spinocerebellar ataxia is consistent with the lack of evidence of de novo mutations noted by Andrew et al. (1997). Pareyson et al. (1999) evaluated 73 Italian families with type I ADCA. SCA1 was the most common genotype, accounting for 41% of cases (30 families); SCA2 was slightly less frequent (29%, 21 families), and the remaining families were negative for the SCA1, SCA2, and SCA3 mutations. Among the positively genotyped families, SCA1 was found most frequently in families from northern Italy (50%), while SCA2 was the most common mutation in families from the southern part of the country (56%). Slow saccades and decreased deep tendon reflexes were observed significantly more frequently in SCA2 patients, while increased deep tendon reflexes and nystagmus were more common in SCA1. Storey et al. (2000) examined the frequency of mutations for SCA types 1, 2, 3, 6, and 7 in southeastern Australia. Of 63 pedigrees or individuals with positive tests, 30% had SCA1, 15% had SCA2, 22% had SCA3, 30% had SCA6, and 3% had SCA7. Ethnic origin was of importance in determining SCA type: 4 of 9 SCA2 index cases were of Italian origin, and 4 of 14 SCA3 index cases were of Chinese origin. Zhou et al. (2001) performed molecular analysis of 109 patients in 75 Chinese families with autosomal dominant SCA and 16 patients with sporadic SCA or spastic paraplegia. SCA type 1 was found in 5 families (7%), and all patients with the SCA1 phenotype were heterozygous for alleles with CAG repeat numbers ranging from 51 to 64 (control groups, 26-35). There was a significant negative correlation between age of disease onset and number of CAG repeat units. SCA3/MJD was found in 26 families, SCA2 in 9 families, SCA6 in 2 families, and SCA7 in 2 families. The combined frequency of SCA1, SCA2, and SCA3/MJD was 53%. None of the 16 sporadic cases was positive for the mutations tested, and no patients were positive for SCA8 (608768), SCA12, or DRPLA. Clinically, the authors noted that SCA3/MJD tended to manifest more frequently with ophthalmoparesis, eyelid retraction, facial myokymia, ataxia, spasticity, and amyotrophy. The frequency of single CAT interruptions in the ATXN1 gene was higher in the Siberian Sakha control group, which also had a higher prevalence of SCA1 than the Chinese population, suggesting that a substitution of CAT for CAG may be the initial event contributing to the generation of expanded alleles. Of 253 unrelated Korean patients with progressive cerebellar ataxia, Lee et al. (2003) identified 52 (20.6%) with expanded CAG repeats. The most frequent SCA type was SCA2 (33%), followed by SCA3 (29%), SCA6 (19%), SCA1 (12%), and SCA7 (8%). There were characteristic clinical features, such as hypotonia and optic atrophy for SCA1, hyporeflexia for SCA2, nystagmus, bulging eye, and dystonia for SCA3, and macular degeneration for SCA7. Mittal et al. (2005) found SCA1 in 37 (22%) of 167 Indian families with ADCA. The frequency of SCA1 in the south Indian population was twice (33%) that of the north Indian population (16%). The nonaffected repeat length ranged from 21 to 39 triplets. Haplotype analysis identified an ancestral C-4-C haplotype (dbSNP rs1476464, D6S288, and dbSNP rs2075974) that was mostly present in the affected individuals, suggesting that this background might have been predisposed for repeat expansion. This haplotype, when present in the nonaffected chromosomes, had multiple interruptions in the repeat tract, which the authors hypothesized would provide genetic stability. However, in disease chromosomes, this haplotype showed large normal (greater than 30 repeats) expansions and was associated with the expanded chromosomes in about 44% of SCA1 families. Among 113 Japanese families from the island of Hokkaido with autosomal dominant SCA, Basri et al. (2007) found that SCA6 was the most common form of the disorder, identified in 35 (31%) families. Thirty (27%) families had SCA3, 11 (10%) had SCA1, 5 (4%) had SCA2, 5 (4%) had DRPLA, 10 (9%) had 16q22-linked SCA, and 1 (1%) had SCA14 (605361). The specific disorder could not be identified in 16 (14%) families.
The phenotypic manifestations of spinocerebellar ataxia type 1 (SCA1) are not specific; thus, the diagnosis of SCA1 rests on molecular genetic testing. ...
Diagnosis
Clinical DiagnosisThe phenotypic manifestations of spinocerebellar ataxia type 1 (SCA1) are not specific; thus, the diagnosis of SCA1 rests on molecular genetic testing. Molecular Genetic TestingGene. ATXN1 is the only gene in which mutations are known to cause SCA1. Expansion of the CAG repeat in ATXN1 is the mutational mechanism in all families with SCA1 examined to date [Matilla et al 1993, Orr et al 1993, Jodice et al 1994, Orr & Zoghbi 2001]. The European Molecular Genetics Quality Network (EMQN) has recently published best practice guidelines for the genetic testing of the spinocerebellar ataxias including SCA1 [Sequeiros et al 2010a, Sequeiros et al 2010b]. See ; updated information is available at www.scabase.eu.Allele sizes Normal alleles. 6-44 CAG repeats [Quan et al 1995, Servadio et al 1995, Goldfarb et al 1996]. Pathogenicity of alleles in the 36 to 44 range depends on the presence or absence of CAT trinucleotide repeats that interrupt the CAG repeats (see following Note). Alleles in the 36 to 44 CAG repeat range are considered normal if they have CAT interruptions; if they do not, they may be in the mutable normal (36-38 CAG repeats) or pathogenic (39-44 CAG repeats) range. Note: (1) Alleles of fewer than 21 CAG repeats are not associated with the SCA1 phenotype. (2) Alleles with 21-35 CAG repeats are normal alleles and have not been associated with the SCA1 phenotype. These normal alleles have been found to have CAT trinucleotide repeat interruption(s) and are considered non-mutable. However, if these are uninterrupted alleles, the stability will need to be determined [Chung et al 1993]. (3) Distinguishing normal interrupted alleles from mutable normal uninterrupted alleles in the 36-38 CAG repeat range as well as from pathogenic uninterrupted alleles in the 39-44 CAG repeat range requires additional evaluation by SfaNI restriction analysis [Chung et al 1993]. A modified method (‘‘dual-fluorescence labeled PCR-restriction fragment length analysis’’) for directly detecting the uninterrupted CAG repeat stretch has been reported [Lin et al 2008].Mutable normal (intermediate) alleles. 36-38 CAG repeats without CAT interruptions. Mutable normal alleles have not been associated with symptoms, but can expand into the abnormal range on transmission to offspring. Note: Distinguishing normal CAT-interrupted alleles from mutable normal uninterrupted alleles in the 36-38 CAG repeat range and pathogenic alleles in the 39-44 CAG repeat range requires additional evaluation by SfaNI restriction analysis [Chung et al 1993] or sequencing. Reduced penetrance alleles. A woman with 44 CAG repeats with CAT repeat interruptions had an affected father but was herself asymptomatic at age 66 years [Goldfarb et al 1996]; thus, she may be an example of reduced penetrance. Full penetrance alleles. Alleles with more than 39 CAG repeats [Orr et al 1993, Quan et al 1995, Goldfarb et al 1996, Sequeiros et al 2010b]. An allele with 39 CAG repeats without the CAT repeat interruptions has the lowest number of repeats to be associated with symptoms [Zuhlke et al 2002]. However, 39-44 CAG repeat alleles have to be uninterrupted by CAT repeats to be considered abnormal and likely to be associated with symptoms. There is an inverse correlation between the size of the expansion and the age at onset. Complex alleles may occur; one individual has been reported with symptomatic SCA1 with a 58 CAG-repeat sequence interrupted by two CAT repeats [Matsuyama et al 1999]; however, this person had an uninterrupted 45 CAT repeat stretch. Note: Distinguishing normal CAT-interrupted alleles from pathogenic uninterrupted alleles in the 39-44 repeat range requires additional evaluation by SfaNI restriction analysis [Chung et al 1993] or sequencing.Clinical testing Targeted mutation analysis by direct amplification of the ATXN1 CAG repeat region identifies more than 99% of individuals with a disease-causing ATXN1 mutation. In some cases of infantile-onset SCA, direct amplification of the ATXN1 CAG repeat may not detect large repeat lengths in the hundreds. Southern blot analysis, long-range PCR, or CAG-triplet repeat primed PCR analysis can be used to quantitate the CAG repeat number when infantile-onset SCA1 is suspected.Table 1. Summary of Molecular Genetic Testing Used in SCA1View in own windowGeneTest MethodMutation DetectedMutation Detection Frequency by Test Method 1 Test AvailabilityATXN1Targeted mutation analysis (PCR)
≤ ~100 CAG repeats >99%Clinical Targeted mutation analysis (Southern blot, long-range PCR, CAG-primed PCR)> ~100 CAG repeats <1% 21. The ability of the test method used to detect a mutation that is present in the indicated gene2. Typically observed in individuals with infantile or childhood onset in families with SCA1Information on specific allelic variants may be available in Molecular Genetics (see Table A. Genes and Databases and/or Pathologic allelic variants).Testing StrategyTo confirm/establish the diagnosis in a proband requires molecular genetic testing to identify the ATXN1 CAG repeat expansion. Predictive testing for at-risk asymptomatic adult family members requires prior confirmation of the diagnosis in the family by molecular genetic testing. Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mutation in the family.Genetically Related (Allelic) DisordersNo other phenotypes are known to be caused by mutations in ATXN1.
Spinocerebellar ataxia type 1 (SCA1) is characterized by ataxia, dysarthria, and eventual deterioration of bulbar functions [Klockgether et al 1998, Filla et al 2000]. Onset is typically in the third or fourth decade, although early onset in childhood has been documented [Currier et al 1972, Zoghbi et al 1988, Schöls et al 1997]. In adult-onset SCA1, the duration of illness from onset to death ranges from ten to 30 years; individuals with juvenile-onset disease (whose symptoms appear before age 13 years) show more rapid progression and more severe disease, and die before age 16 years [Zoghbi et al 1988]. ...
Natural History
Spinocerebellar ataxia type 1 (SCA1) is characterized by ataxia, dysarthria, and eventual deterioration of bulbar functions [Klockgether et al 1998, Filla et al 2000]. Onset is typically in the third or fourth decade, although early onset in childhood has been documented [Currier et al 1972, Zoghbi et al 1988, Schöls et al 1997]. In adult-onset SCA1, the duration of illness from onset to death ranges from ten to 30 years; individuals with juvenile-onset disease (whose symptoms appear before age 13 years) show more rapid progression and more severe disease, and die before age 16 years [Zoghbi et al 1988]. In the last few years, large-scale natural history studies of some of the common SCAs (including SCA1) using validated neurologic rating scales and timed measures of motor function have been in progress in many countries. The annual increase in the scale for assessment and rating of ataxia (SARA) score for SCA1, SCA2, SCA3, and SCA6 combined in a one-year follow-up study was 1.38 ± 0.37; the SARA score quantifies various aspects of appendicular and limb ataxia; a score of 40 indicates maximum dysfunction [Schmitz-Hübsch et al 2010]. The majority of affected individuals initially present with difficulties in gait; slurred speech is also common. They may first notice problems of balance in going down stairs or making sudden turns; athletic individuals may notice difficulties at an earlier stage of disease in the course of activities that require a high degree of control, such as skiing or dancing. Affected individuals may display brisk deep tendon reflexes, hypermetric saccades, and nystagmus in the early stages of disease [Genis et al 1995]. Mild dysphagia, indicated by choking on food and drink, may also occur early in the disease. As the disease progresses the saccadic velocity slows and an up-gaze palsy develops. Nystagmus often disappears with evolving saccadic abnormalities.As the ataxia worsens, other cerebellar signs such as dysmetria, dysdiadochokinesia, and hypotonia become apparent.Optic nerve atrophy and variable degrees of ophthalmoparesis may be detected in some individuals.Muscle atrophy, decreased or absent deep tendon reflexes, and loss of proprioception or vibration sense may occur in the middle or late stages of the disease [van de Warrenburg et al 2004].Individuals may experience mild decline in memory and in verbal and nonverbal intelligence; the degree of cognitive impairment correlates with severity of disease. Executive dysfunction may also occur [Burk et al 2001, Burk et al 2003].Extrapyramidal signs tend to take the form of chorea and dystonia and occur in advanced disease [Wu et al 2004].Bulbar dysfunction (atrophy of facial and masticatory muscles, perioral fasciculations, and severe dysphagia leading to frequent aspiration) become prominent in the final stages of the disease [Shiojiri et al 1999]. Affected individuals eventually develop respiratory failure, which is the main cause of death. Juvenile-onset SCA1 is characterized by severe brain stem dysfunction in addition to the cerebellar symptoms. The brain stem dysfunction occurs rapidly, leading to death within four to eight years of symptom onset.Electrophysiologic studies. A sensory-predominant polyneuropathy can be documented in a significant number of persons with SCA1 by nerve conduction studies [Abele et al 1997, Kubis et al 1999, Pareyson et al 1999]. Visual evoked potentials and motor evoked potentials following transcranial magnetic stimulation are abnormal in most individuals with SCA1 [Abele et al 1997]. Neuroimaging. Computed tomography (CT) and magnetic resonance imaging (MRI) of the brain reveal pontocerebellar atrophy [Döhlinger et al 2008]. More sophisticated quantitative techniques such as voxel-based morphometry show volume loss in cerebellum and brain stem involving both gray and white matter [Guerrini et al 2004, Ginestroni et al 2008, Goel et al 2011]. Neuropathology. Neuropathologic studies reveal atrophy of cerebellum and brain stem [Schut & Haymaker 1951, Robitaille et al 1997]. In the cerebellum, the Purkinje cells are severely depleted and the vermis may be maximally affected; the flocculonodular lobe is relatively spared [Robitaille et al 1997]. There is some loss of dentate neurons, some of which may show “grumose” degeneration [Yamada et al 2008]. Granule cells are moderately lost and torpedos may be seen [Genis et al 1995]. Calbindin immuncytochemistry reveals reduced dendritic arbors [Genis et al 1995]. Brain stem shows loss of basis pontis neurons and olivary neurons. There is loss of afferent fibers in middle and inferior cerebellar peduncles leading to loss of myelin stain reactivity, as well as neuronal loss in the oculomotor nuclei and the ninth and tenth cranial nerve nuclei. The spinal cord shows loss of anterior horn cells and neurons from the Clarke’s column and there is loss of fibers in the posterior column.
Probands. A strong correlation exists between the number of CAG repeats and severity of disease: the larger the CAG repeat, the earlier the onset and more severe the disease. However, the correlation is broad; only 50% to 70% of age at onset variance can be explained by CAG repeat size [Orr et al 1993, Schöls et al 1997, Stevanin et al 2000]. ...
Genotype-Phenotype Correlations
Probands. A strong correlation exists between the number of CAG repeats and severity of disease: the larger the CAG repeat, the earlier the onset and more severe the disease. However, the correlation is broad; only 50% to 70% of age at onset variance can be explained by CAG repeat size [Orr et al 1993, Schöls et al 1997, Stevanin et al 2000]. The largest expansions of the CAG repeat tract are found in individuals with infantile- or juvenile-onset SCA1, who typically experience more rapid disease progression and are most commonly the offspring of affected males. Some clinical signs (facio-lingual atrophy, dysphagia, skeletal muscle atrophy, and possibly ophthalmoparesis) are more common with larger repeat size, independent of disease duration. Affected individuals with more than 52 CAG repeats tend to become significantly disabled five years after the onset of disease.Individuals homozygous for two mutant ATXN1 alleles do not appear to develop disease that is more severe than what can be predicted by the larger of their two alleles. At-risk individuals. The age of onset, severity, specific symptoms, and progression of the disease are variable and cannot be predicted by family history or results of molecular genetic testing.
The inherited spinocerebellar ataxias (SCAs) are a heterogeneous group of neurologic disorders that defy easy differentiation on the basis of clinical criteria alone. Inter- and intrafamilial variability is too great to permit definitive classification without molecular genetic testing. See also Ataxia Overview. ...
Differential Diagnosis
The inherited spinocerebellar ataxias (SCAs) are a heterogeneous group of neurologic disorders that defy easy differentiation on the basis of clinical criteria alone. Inter- and intrafamilial variability is too great to permit definitive classification without molecular genetic testing. See also Ataxia Overview. SCA2 and SCA3 (Machado-Joseph Disease [MJD]) have age of onset and neurologic signs similar to those seen in SCA1, although their phenotypes tend to be more heterogeneous. Individuals with SCA2, for example, show earlier and more severe abnormalities of saccade velocity, loss of deep tendon reflexes, and polyneuropathy than do individuals with SCA1. Individuals with SCA3 may display prominent extrapyramidal signs (parkinsonism, pill-rolling tremor, bradykinetic-rigid syndromes) in the early stages of disease and sometimes exhibit little ataxia. Nystagmus, gaze palsy, and abnormal vestibulo-ocular reflexes can also occur earlier and with greater frequency in individuals with SCA3, but the eye movement disorder of SCA1 overlaps with SCA2 and SCA3 [Burk et al 1999]. Generalized areflexia can be seen in SCA2, SCA3, and SCA4, but is uncommon in SCA1. SCA17 and dentatorubral pallidoluysian atrophy (DRPLA) are other inherited ataxias caused by expanded CAG repeats; these disorders often exhibit a more florid phenotype with added extrapyramidal signs, cognitive decline, and myoclonus (DRPLA only). SCA5, SCA6, and SCA8 tend to progress more slowly than SCA1 and to show more purely cerebellar signs, with fewer symptoms that reflect widespread neuropathology.If an affected individual has visual loss related to a maculopathy, the most likely diagnosis is SCA7, which can be tested for first. Note that not all individuals with SCA7 have visual loss related to a maculopathy; however, history of visual loss in other affected family members may suggest a diagnosis of SCA7. Other more recently defined SCAs are related to point mutations or other types of repeat expansions. Overall, they are rarer, often having been described in a limited number of families. Recent reviews can be consulted for their differential diagnostic features [Durr 2010, Soong & Paulson 2007]. Friedreich ataxia is usually associated with childhood onset and depressed tendon reflexes. Inheritance is autosomal recessive.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).Adult-onset SCA1Early-onset SCA1
To establish the extent of disease in an individual diagnosed with spinocerebellar ataxia type 1 (SCA1), the following evaluations are recommended:...
Management
Evaluations Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with spinocerebellar ataxia type 1 (SCA1), the following evaluations are recommended:Medical historyNeurologic examination Molecular genetic testing Medical genetics consultationTreatment of ManifestationsManagement of individuals with SCA1 remains supportive as no known therapy to delay or halt the progression of the disease exists. Affected persons should be followed by a neurologist with consultation from physiatrists, physical and occupational therapists, and other specialists as needed. Certain manifestations directly or indirectly related to the disease such as spasticity, depression, and pain may require appropriate pharmacotherapy. Although neither exercise nor physical therapy has been shown to stem the progression of incoordination or muscle weakness, individuals should maintain activity. Canes and walkers help prevent falls. Modification of the home with such conveniences as grab bars, raised toilet seats, and ramps to accommodate motorized chairs may be necessary. Speech therapy and communication devices such as writing pads and computer-based devices may benefit those with dysarthria. Weighted eating utensils and dressing hooks help maintain a sense of independence. Weight control is important because obesity can exacerbate difficulties with ambulation and mobility. When dysphagia becomes troublesome, video esophagrams can identify the consistency of food least likely to trigger aspiration. Repeated aspiration or significant weight loss may also point to the need for a feeding device in some.Prevention of Primary ManifestationsSee Therapies Under Investigation.Prevention of Secondary ComplicationsVitamin supplements are recommended, particularly if caloric intake is reduced. SurveillanceNeurologic evaluation every three to six months is appropriate Agents/Circumstances to AvoidAffected individuals should avoid alcohol as well as medications known to be neurotoxic such as those that cause neuropathy (e.g., isoniazid, large-dose vitamin B6) or those associated with central nervous system toxicity (e.g., diphenylhydantoin).Evaluation of Relatives at RiskSee Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationLithium [Watase et al 2007] and insulin-like growth factor 1 [Vig et al 2006] have improved neurologic function in a mouse model of SCA1; no human trials with these have been done to date. Riluzole has been shown to provide some symptomatic relief of ataxia in a mixed group of patients including persons with SCA1; however, further investigation is needed [Ristori et al 2010].Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.OtherTremor-controlling drugs do not work well for cerebellar tremors.
Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED....
Molecular Genetics
Information in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Spinocerebellar Ataxia Type 1: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDATXN16p22.3
Ataxin-1ATXN1 homepage - Mendelian genesATXN1Data are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Spinocerebellar Ataxia Type 1 (View All in OMIM) View in own window 164400SPINOCEREBELLAR ATAXIA 1; SCA1 601556ATAXIN 1; ATXN1Normal allelic variants. ATXN1 spans an estimated 450 kb of DNA and consists of nine exons. The coding region is 2448 bp long. The 5' untranslated region is found in the first seven exons, and the region encoding the ataxin-1 protein is located within the large exons 8 and 9, which are 2079 and 7805 bp, respectively. Both the 5' untranslated and 3' untranslated region of the ATXN1 transcript are extremely long at 935 bp and 7000 bp, respectively. Normal alleles may contain six to 44 CAG repeats and are interrupted with one to three CAT trinucleotides. Pathologic allelic variants. Alleles of 39 or more uninterrupted CAG repeats are associated with disease. Somatic and meiotic instability has been observed for the ATXN1 CAG repeats, particularly in tissues that have higher mitotic potential, such as peripheral blood cells and sperm [Chong et al 1995]. The presence of the CAT interruption within the CAG repeat tract has demonstrated a stabilizing effect in somatic tissues. Comparative analysis of a large normal allele (39 repeats with CAT interruptions) with a small expanded allele (40 uninterrupted repeats) revealed that the interrupted allele was somatically stable, whereas the allele with an uninterrupted CAG tract was unstable [Chong et al 1995]. Normal gene product. The CAG repeat encodes a glutamine tract in ataxin-1, a nuclear protein of unknown function. The transcript expressed from ATXN1 is approximately 11 kb and is found in a wide variety of different cell and tissue types [Servadio et al 1995]. Normal ataxin-1 has 792 to 829 amino acids, depending on the number of CAG repeats that encode the polyglutamine tract within the protein. Ataxin-1 has been postulated to have several functions in the nucleus, including transcription regulation and RNA processing. Deletion of ATXN1 leads to mild impairment of spatial learning in mice. But no SCA1-like phenotypes were produced by complete deletion of ATXN1, arguing against a loss-of-function mechanism in SCA1 pathogenesis [Matilla et al 1998]. Abnormal gene product. In SCA1, as in several other polyglutamine diseases, the mutant protein accumulates in the nucleus into a single aggregate, often referred to as a nuclear inclusion (NI). It is believed that the expanded polyglutamine tract resulting from the CAG expansion results in misfolding of mutant ataxin-1 leading to insoluble aggregates. Because these NIs also accumulate, affecting portions of the cell's protein refolding and degradation machinery (chaperones, ubiquitin, and proteasomal subunits), it is thought that impaired protein clearance underlies the pathogenesis of SCA1 and related diseases. At least three lines of evidence support this hypothesis: Neuronal degeneration is accelerated when ubiquitination is impaired in SCA1 transgenic mice [Cummings et al 1999]. Overexpression of specific chaperones suppresses neurodegeneration in fly and mouse models of polyglutamine disorders [Fernandez-Funez et al 2000, Cummings et al 2001]. Polyglutamine-expanded ataxin-1 decreases the activity of the proteasome in cell culture [Park et al 2005]. Studies revealed that serine 776 in ataxin-1, which is phosphorylated by Akt kinase, mediates specific protein-protein interactions and is critical for pathogenicity of mutant ataxin-1 [Chen et al 2003, Emamian et al 2003]. Preventing phosphorylation at serine 776 by substituting alanine at this location reduces the toxicity of mutant ataxin-1. Genetic studies in Drosophila revealed that components of the PI3K-Akt signaling pathway are modifiers of ataxin-1-induced degeneration and that reduction of Akt activity subdues ataxin-1 toxicity.A review provides details of the many proteins that have been found to interact with ataxin-1, including many transcriptional co-regulators and proteins involved in RNA binding and metabolism [Matilla-Duenas et al 2008].One of the functionally important domains of ataxin-1 is the conserved AXH domain that is homologous to a portion of the high mobility group box transcription factor-binding protein 1 (HBP1). Many proteins appear to interact with ataxin-1 via the AXH domain including the ataxin-1 paralog BOAT and several transcriptional regulators such as SMRT, Drosophila SENS, the human homolog of the Drosophila repressor CIC (Capicua), and the ROR α-Tip60 complex [Zoghbi & Orr 2009]. It has been shown that duplication of ATXN1L, a paralog of ATXN1, suppresses SCA1 neuropathology by decreasing incorporation of mutant ataxin-1 into the native complex containing Capicua [Bowman et al 2007]. These studies suggest that SCA1 pathogenesis is mediated at least in part by modulating the normal activity of ataxin-1. Among ataxin 1-interacting proteins, the pathogenic importance of Tip60 [Gehrking et al 2011] and 14-3-3ε [Jafar-Nejad et al 2011] have been recently demonstrated in transgenic mice. Ataxin-1 also has several residues at which it is SUMOylated, suggesting its role in transcriptional regulation [Riley et al 2005]. Additionally, disrupting the nuclear localizing signal toward the N terminus prevents ataxin-1 entry into the nucleus and abolishes toxicity of mutant ataxin-1 [Klement et al 1998], suggesting that the pathogenic process is mainly localized to the nucleus. An analysis of the genomic expression profile in SCA1 transgenic mice showed consistently altered levels of mRNA from five genes forming a biologic cohort centered on glutamate signaling pathways in Purkinje cells [Serra et al 2004]. These findings identify this pathway as a target to investigate potential therapies in animal models. In addition, transcriptional dysregulation of calcium homeostasis genes also appears to be an early feature [Lin et al 2000].