Residency · Residency · Neurology

Trinucleotide Repeat Disorders Beyond Huntington

Introduction

Trinucleotide repeat expansion disorders are a distinct class of genetic neurological diseases caused by the abnormal amplification of short tandem DNA repeats within or near specific genes. While Huntington disease is the most widely recognized, this family includes a diverse group of conditions affecting different neural systems. Understanding the shared molecular mechanisms, genetic features such as anticipation and incomplete penetrance, and the unique clinical presentations of each disorder is essential for diagnosis and genetic counseling.

Shared Molecular Mechanisms

These disorders share several important features. Dynamic mutations arise because repeat lengths are unstable during DNA replication and meiosis, tending to expand across generations. Anticipation refers to the phenomenon whereby expansion of the repeat across generations leads to earlier onset and more severe disease in successive generations. Parent-of-origin effects are significant: paternal transmission often produces larger expansions (especially in Huntington disease and SCA7), while maternal transmission predominates in the congenital form of myotonic dystrophy type 1. Each disorder has defined normal, intermediate (premutation), and pathogenic repeat ranges reflecting a threshold effect. The pathogenic mechanisms vary by disorder and include toxic gain-of-function through polyglutamine aggregation in coding CAG repeat disorders, loss of gene expression as seen in fragile X and Friedreich ataxia, and RNA toxicity as occurs in myotonic dystrophy, SCA8, FXTAS, and C9orf72.

Polyglutamine (PolyQ) Disorders

Spinocerebellar Ataxias (SCAs)

SCA1 (ATXN1)

SCA1 is caused by a CAG repeat in the ataxin-1 gene, with 39 or more repeats being pathogenic. It presents as progressive cerebellar ataxia with pyramidal signs, bulbar dysfunction, and peripheral neuropathy. It has an earlier onset and faster progression compared to SCA2 and SCA3, and MRI shows marked brainstem and cerebellar atrophy.

SCA2 (ATXN2)

SCA2 is caused by a CAG repeat in the ataxin-2 gene, with 33 or more repeats being pathogenic. The presentation includes cerebellar ataxia with characteristically slow saccadic eye movements, peripheral neuropathy, and hyporeflexia. Intermediate-length expansions (27-33 repeats) are associated with increased risk of ALS. MRI demonstrates prominent olivopontocerebellar atrophy.

SCA3 / Machado-Joseph Disease (ATXN3)

SCA3 is the most common SCA worldwide, caused by a CAG repeat in the ataxin-3 gene with 60 or more repeats being pathogenic. The phenotype is highly variable and includes cerebellar ataxia, ophthalmoplegia, pyramidal and extrapyramidal signs, peripheral neuropathy, and bulging eyes with facial fasciculations. Dystonia may be prominent, especially in younger-onset cases.

SCA6 (CACNA1A)

SCA6 involves a small CAG repeat expansion (20 or more repeats) in the alpha-1A voltage-dependent calcium channel gene. It presents as pure cerebellar ataxia with very slow progression, typically with onset in the 40s-60s. Notably, it is allelic with episodic ataxia type 2 (caused by point mutations) and familial hemiplegic migraine type 1. Because the repeat sizes are small, they are relatively stable and anticipation is minimal.

SCA7 (ATXN7)

SCA7 is caused by a CAG repeat in the ataxin-7 gene, with 37 or more repeats being pathogenic. Its distinguishing feature is progressive visual loss due to cone-rod retinal dystrophy in addition to cerebellar ataxia. SCA7 shows the most pronounced anticipation of all SCAs, especially with paternal transmission, and infantile-onset forms have been described.

Spinal and Bulbar Muscular Atrophy (Kennedy Disease)

Kennedy disease is X-linked, caused by a CAG expansion in the androgen receptor (AR) gene with 38 or more repeats being pathogenic. It affects males, while females are asymptomatic carriers. The condition presents as a lower motor neuron syndrome with proximal limb weakness, bulbar weakness (dysarthria, dysphagia), facial fasciculations, and tongue atrophy. Androgen insensitivity features are characteristic and include gynecomastia, testicular atrophy, reduced fertility, and diabetes. Sensory neuropathy on NCS distinguishes it from ALS. CK is elevated, and EMG shows chronic denervation. Progression is slow, and lifespan is often near normal.

Dentatorubral-Pallidoluysian Atrophy (DRPLA)

DRPLA is caused by a CAG repeat in the ATN1 gene, with 48 or more repeats being pathogenic. It is more common in Japanese populations. The clinical presentation varies by age of onset: younger-onset cases feature myoclonus epilepsy, ataxia, and dementia, while adult-onset cases present with ataxia, choreoathetosis, and dementia.

Non-PolyQ Trinucleotide Repeat Disorders

Myotonic Dystrophy Type 1 (DM1)

DM1 is caused by a CTG repeat expansion in the 3' UTR of the DMPK gene, with 50 or more repeats being pathogenic. It is the most common adult-onset muscular dystrophy and follows autosomal dominant inheritance. The pathogenic mechanism involves RNA toxicity: expanded CUG RNA forms hairpin structures that sequester splicing regulators (MBNL proteins), causing widespread splicing dysregulation.

The clinical features are multisystemic. Myotonia, the delayed muscle relaxation after contraction, manifests as grip myotonia and percussion myotonia. Progressive weakness is distal greater than proximal, with facial weakness and jaw and temporal muscle wasting producing the characteristic "hatchet face" appearance. Cardiac conduction abnormalities, including first-degree AV block and bundle branch block, carry a risk of sudden cardiac death. Posterior subcapsular cataracts with an iridescent "Christmas tree" appearance are characteristic. Excessive daytime sleepiness, cognitive impairment, and apathy are common neurological features. Endocrine manifestations include insulin resistance, hypogonadism, and testicular atrophy. Gastrointestinal involvement includes dysphagia, gastroparesis, and constipation. Congenital DM1, caused by very large expansions of 1,000 or more repeats and transmitted by affected mothers, presents with severe hypotonia, respiratory failure, intellectual disability, and facial diplegia.

Myotonic Dystrophy Type 2 (DM2)

DM2 is caused by a CCTG repeat expansion in the CNBP gene, with 75 or more repeats being pathogenic. Unlike DM1, weakness is proximal greater than distal. Myotonia, cataracts, and cardiac conduction defects are similar to DM1 but generally milder. There is no congenital form.

Friedreich Ataxia (FRDA)

Friedreich ataxia is caused by a GAA repeat expansion in intron 1 of the frataxin (FXN) gene. It is autosomal recessive, with 66 or more repeats on both alleles being pathogenic (compound heterozygosity with a point mutation is rare). Loss of frataxin expression leads to mitochondrial iron accumulation and oxidative damage. Onset is typically before age 25, with most cases presenting before age 15.

The clinical presentation includes progressive gait and limb ataxia (both cerebellar and sensory), loss of deep tendon reflexes (areflexia) combined with extensor plantar responses -- a distinctive combination. Posterior column dysfunction causes loss of vibration and proprioception. Dysarthria develops over time. Hypertrophic cardiomyopathy is present in 60-75% of patients and is the leading cause of death. Scoliosis, pes cavus, and diabetes mellitus (10-30%) are additional features. Omaveloxolone (Skyclarys), an Nrf2 activator, was FDA approved in 2023 and modestly slows neurological progression.

Fragile X-Associated Tremor/Ataxia Syndrome (FXTAS)

FXTAS is caused by CGG repeats in the premutation range (55-200 repeats) of the FMR1 gene, distinct from full fragile X syndrome (greater than 200 repeats). It affects male premutation carriers over age 50, with females being less commonly affected. The presentation includes progressive intention tremor, cerebellar ataxia, parkinsonism, cognitive decline, and neuropathy. MRI shows bilateral middle cerebellar peduncle T2 hyperintensity (the MCP sign, present in 60% of cases). The mechanism involves RNA toxicity, which is distinct from the loss-of-function seen in full fragile X syndrome.

Genetic Testing and Counseling

Specific repeat-primed PCR or Southern blot assays are required for each disorder. Standard exome sequencing does not reliably detect repeat expansions, making disorder-specific testing essential. Predictive testing in at-risk individuals requires careful pre-test and post-test genetic counseling. Anticipation must be explained to families: children of affected individuals may have earlier onset and more severe disease. Prenatal diagnosis and preimplantation genetic testing are available.

Clinical Pearls

Anticipation is a hallmark of trinucleotide repeat disorders, and a family history of progressively earlier onset and increasing severity across generations is a strong clinical clue. Slow saccades distinguish SCA2 from other SCAs, retinal degeneration distinguishes SCA7, and pure cerebellar ataxia with late onset suggests SCA6. Myotonic dystrophy type 1 is a multisystem disease, and cardiac conduction monitoring (annual ECG, consider Holter) is mandatory because sudden cardiac death from arrhythmia is a leading cause of mortality. Friedreich ataxia uniquely combines areflexia with extensor plantar responses, and hypertrophic cardiomyopathy must be screened for and monitored regularly. Kennedy disease (SBMA) can mimic ALS, and the presence of gynecomastia, sensory neuropathy, and slow progression should prompt androgen receptor CAG repeat testing.

References

1. Durr A. Autosomal dominant cerebellar ataxias: polyglutamine expansions and beyond. Lancet Neurol. 2010;9(9):885-894.
2. Johnson NE, Butterfield RJ, Mayne K, et al. Population-based prevalence of myotonic dystrophy type 1 using genetic analysis of statewide blood screening program. Neurology. 2021;96(7):e1045-e1053.
3. Pandolfo M. Friedreich ataxia: the clinical picture. J Neurol. 2009;256(Suppl 1):3-8.
4. Paulson H. Repeat expansion diseases. Handb Clin Neurol. 2018;147:105-123.

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