Spinocerebellar ataxia 8

Spinocerebellar ataxia 8 (SCA8) is more genetically complex than other SCAs.^[1] Typically, a genetic disorder is considered autosomal dominant if only one copy of the defective gene needs to be inherited in order to develop the disease, and autosomal recessive if two copies of the defective gene (one from each parent) are required to develop the disease. SCA8 is a dominant condition because only one copy of the causative gene needs to be inherited, but quite often, family histories give the impression that the condition is sporadic (appearing without a family history) or inherited in an autosomal recessive manner.^[1]

Genes are made up of substances known as nucleotides (identified by letters) linked together in chains. In SCA8, a change (mutation) in the ATXN8OS gene results in extra copies of a series of nucleotides identified by the letters CTG (this is called a trinucleotide repeat expansion). The ATXN8OS gene is genetically "unstable" which means that the number of repeats may expand or contract when the gene is passed on. Therefore, there are several potential outcomes for families in which the unstable gene is present: when a parent is unaffected, the number of CTG repeats can expand to cause the disease in a child; when a parent is affected, the number of repeats might contract (decrease) so that the child does not have the disease; an affected parent may have an affected child; or an unaffected parent may have an unaffected child. This unique aspect of SCA8 helps to explain why some family members can inherit the abnormal gene but not develop the disease, and why some family members develop the disease when neither parent is affected.^[1][2]

When an individual affected with spinocerebellar ataxia 8 (SCA8) has unaffected parents, molecular genetic testing of the parents can determine from which parent the unstable gene was transmitted. All affected individuals whose parents have been evaluated with molecular genetic testing have had one parent with an ATXN8OS repeat expansion. Expansions appearing for the first time in an affected individual (called a de novo mutation) have not been reported.^[2] These findings support the notion that SCA8 is inherited in an autosomal dominant manner.

When genetic testing of unaffected parent is not possible, individuals may not be able to determine from which parent the unstable gene was transmitted. However, genetic testing of other family members (such as aunts, uncles or cousins) may help to determine whether the gene was transmitted from an affected individual's mother or father. It has been observed that CTG repeats almost always expand when transmitted by the mother. In individuals with no apparent family history of SCA8, it is common for the mother to be asymptomatic and to have the repeat expansion with a shorter repeat length than her affected child. When the gene is transmitted by the father, the CTG repeats almost always contract in length, usually to a range that is less often associated with developing the disease.^[2] Another unusual feature of the SCA8 gene is that when the number of CTG expansions becomes too large, the gene no longer appears to cause disease. This means that an unaffected father may have a gene with a very large expansion, and it may once again contract when he passes it on to his children, causing the disease to appear in them.^[1]

It is recommended that individuals with specific questions about the inheritance of SCA8 and/or genetic testing for SCA8 speak with a genetics professional.

To find a medical professional who specializes in genetics, you can ask your doctor for a referral or you can search for one yourself. Online directories are provided by the American College of Medical Genetics and the National Society of Genetic Counselors. If you need additional help, contact a GARD Information Specialist. You can also learn more about genetic consultations from Genetics Home Reference.