Clinical Information

Spinocerebellar Ataxia Type 1:

Spinocerebellar ataxia type 1 (SCA1) is characterized by progressive ataxia, dysarthria, eventual deterioration of bulbar functions, and ophthalmoplegia.

Onset typically occurs in the third to fourth decade of life. Most individuals present with difficulties in gait or slurred speech. SCA1 is caused by an expansion of the CAG (cytosine-adenine-guanine) trinucleotide repeat in the ATXN1 gene. This trinucleotide repeat is polymorphic in the general population, with the number of benign repeats ranging from 6 to 37. The pathogenicity of the repeat is dependent on the presence or absence of CAT (cytosine-adenine-thymine) trinucleotide repeats that interrupt the CAG repeats. Therefore, individuals with 36 to 37 uninterrupted CAG repeats are predisposed to having a child with an expanded allele. In affected individuals, the CAG expansions are greater than 38 uninterrupted CAG repeats or greater than 44 repeats, regardless of the presence or absence of CAT repeat interruptions. The presence of CAT repeats in an individual with 36 to 43 CAG repeats is considered normal and not disease-causing. In contrast, 38 CAG repeats without CAT repeats are of uncertain significance. There is a report of an individual with very last onset SCA1 with 38 CAG repeats. Reduced penetrance has been associated with 44 CAG repeats. As with other trinucleotide repeat disorders, large CAG expansions are associated with earlier onset and a more severe clinical course.

Spinocerebellar Ataxia Type 2:

Spinocerebellar ataxia type 2 (SCA2) is characterized by slowly progressive ataxia, dysarthria, and slow saccadic eye movements. The mean age of onset is in the fourth decade, but symptoms may appear from childhood to later adulthood. SCA2 is caused by an expansion of the CAG trinucleotide repeat in the ATXN2 gene. This trinucleotide repeat is polymorphic in the general population, with the number of benign repeats less than 32. However, 29 to 31 heterozygous repeats have been associated with an increased exponential risk for amyotrophic lateral sclerosis (ALS). Additionally, there has been a report of an individual homozygous for 31 repeats with late-onset cerebellar ataxia. In contrast, 27 repeats have been associated with a protective effect for ALS. In affected individuals, the CAG expansion is greater than 34 repeats, with the most common disease-causing alleles having 37 to 39 repeats. Larger CAG expansions are associated with an earlier age of onset but repeat length cannot predict age of onset or disease severity. A CAG expansion of 32 repeats is of unclear clinical significance. Repeats in the 33 to 34 range are associated with reduced penetrance.

Spinocerebellar Ataxia Type 3:

Spinocerebellar ataxia type 3 (SCA3), also known as Machado-Joseph disease, is characterized by progressive cerebellar ataxia and pyramidal signs. The age of onset is highly variable but most commonly occurs in the second to fifth decade of life. Individuals may present with gait problems, speech difficulties, clumsiness, or visual blurring. SCA3 is caused by an expansion of the CAG trinucleotide repeat in the ATXN3 gene. This trinucleotide repeat is polymorphic in the general population, with the number of benign repeats ranging from 12 to 44. In affected individuals, the CAG expansion ranges from 60 to 87 repeats. A loose correlation exists between repeat length and clinical phenotype. Individuals with 45 to 59 CAG repeats are predisposed to having a child with an expanded allele and may or may not have symptoms themselves. There have been reports of reduced penetrant and nonpenetrant alleles with repeats in this range.

Spinocerebellar Ataxia Type 6:

Spinocerebellar ataxia type 6 (SCA6) is characterized by adult-onset, slowly progressive cerebellar ataxia, dysarthria, and nystagmus. The mean age of onset is 43 to 52 years. Initial symptoms include unsteadiness, stumbling, and imbalance. SCA6 is caused by an expansion of the CAG trinucleotide repeat in the CACNA1A gene. This trinucleotide repeat is polymorphic in the general population, with the number of benign repeats less than 19. In affected individuals, the CAG expansion ranges from 20 to 33 repeats. Larger CAG expansions are associated with an earlier age of onset. A CAG expansion of 19 repeats is of unclear clinical significance. Individuals with 19 CAG repeats are predisposed to having a child with an expanded allele. Additionally, homozygous abnormal expansions have been reported in individuals with younger age of onset and a more severe phenotype.

Spinocerebellar Ataxia Type 7:

Spinocerebellar ataxia type 7 (SCA7) is characterized by progressive cerebellar ataxia, including dysarthria and dysphagia, and con-rod and retinal dystrophy. Onset ranges from infancy to the fifth or sixth decade of life. SCA7 is caused by an expansion of the CAG trinucleotide repeat in the ATXN7 gene. This trinucleotide repeat is polymorphic in the general population, with the number of benign repeats less than 19. In affected individuals, the CAG expansion is greater than 36 repeats. A CAG expansion of 19 to 27 repeats is of unclear clinical significance. Individuals with 28 to 33 repeats are predisposed to having a child with an expanded allele but are unlikely to have symptoms themselves. Thirty-four to 36 repeats are associated with reduced penetrance, and when symptoms do occur, they are more likely to be associated with later onset and a milder phenotype.