Other inherited neurologic conditions

Huntington disease and HTT

Huntington disease can affect movement, thinking, behavior, and psychiatric health. It is caused by an expanded CAG repeat in the HTT gene.

The expansion is inherited in an autosomal dominant manner. Each child of a person with a disease-causing HTT expansion has a 50 percent chance of inheriting it. Testing directly measures the number of CAG repeats and can usually determine whether the expansion is in a disease-associated range.

Because HTT is a repeat expansion, ordinary exome sequencing or a sequence-only gene panel may not detect it. A dedicated expansion test is required.

Predictive testing in an adult without symptoms can be highly informative. Established protocols include genetic counseling, neurologic and psychological assessment, careful disclosure, and follow-up support. The result can have major implications for the person and their biological relatives.

Testing children for the usual adult-onset form is generally not recommended when no childhood medical benefit would result. Juvenile-onset symptoms create a different clinical situation that requires specialist evaluation.

Genetic forms of Parkinson disease

Most Parkinson disease is not explained by one highly penetrant inherited variant. A minority is monogenic, and additional variants change risk without guaranteeing disease.

Genes associated with autosomal dominant Parkinson disease include LRRK2, SNCA, and VPS35. Recessive forms can involve PRKN, PINK1, PARK7, and other genes, often with younger onset. Some GBA1 variants increase Parkinson risk and may also have implications for Gaucher disease, depending on whether one or both gene copies are affected.

These genes do not behave identically. Inheritance, penetrance, age of onset, associated features, and the meaning for relatives differ. Some people with a pathogenic variant never develop Parkinson disease, while others have a clearer single-gene cause.

A genetic evaluation may be particularly useful with very young onset, several affected relatives, a known family variant, features that are unusual for typical Parkinson disease, ancestry associated with specific founder variants, or interest in a gene-targeted study. Testing may also uncover carrier or family planning information that is separate from Parkinson risk.

Spinocerebellar ataxias and other hereditary ataxias

Ataxia describes impaired coordination. It is a clinical sign, not one disease. Ataxia can result from medications, alcohol, vitamin deficiency, autoimmune disease, stroke, tumor, infection, or another acquired condition. It can also be genetic.

Hereditary ataxias include many disorders with dominant, recessive, X-linked, or mitochondrial inheritance. The spinocerebellar ataxias, often shortened to SCA, are one group. SCA1, SCA2, SCA3, SCA6, SCA7, and several others are commonly caused by repeat expansions in different genes.

Some ataxias begin in childhood and others in adulthood. Additional features may include neuropathy, spasticity, seizures, vision loss, hearing loss, parkinsonism, cognitive change, or autonomic symptoms. The accompanying features, age of onset, ancestry, MRI findings, and family pattern help determine which tests are most useful.

Repeat expansions are a major reason a negative exome is not the end of an ataxia evaluation. Some expansions need targeted analysis or genome-based methods that the original test may not have included. Conversely, ordering a very broad panel before evaluating acquired and treatable causes can produce uncertain findings without answering the clinical question.

Why the testing method matters

Different conditions require different laboratory approaches:

• HTT requires repeat-expansion testing.
• Many SCAs require one or more dedicated expansion assays.
• C9orf72 requires targeted repeat analysis.
• Some Parkinson genes require both sequencing and deletion or duplication analysis.
• Exome or genome testing may help when many genes remain plausible, but neither method detects every expansion or structural change equally.

A report that says "negative genetic testing" is incomplete without the test name, genes, methods, and date. Testing technology and the interpretation of variants change over time.

Start with the neurologic pattern

The most efficient evaluation begins with the person's symptoms and examination. A movement-disorders or neurogenetics specialist may distinguish chorea, parkinsonism, ataxia, weakness, dystonia, and apraxia, which can sound similar in a family story but point toward different tests.

A three-generation history should record the first symptoms and age of onset, not only the final diagnosis. Bring copies of prior genetic reports whenever possible. The exact variant and laboratory interpretation are more useful than remembering that a relative "tested positive for a gene."

When possible, testing an affected relative first makes an unaffected relative's later result easier to interpret.