What is the inheritance pattern and clinical presentation of ATN1-related epilepsy (autosomal‑dominant CAG repeat expansion)?

ATN1-Related Epilepsy: Inheritance Pattern and Clinical Manifestations

ATN1-related epilepsy, also known as dentatorubral-pallidoluysian atrophy (DRPLA), follows an autosomal dominant inheritance pattern with CAG repeat expansion in the ATN1 gene, presenting most commonly with seizures, ataxia, cognitive impairment, and movement disorders—with larger CAG expansions (≥63 repeats) causing earlier onset and more severe phenotypes, particularly when paternally inherited.

Inheritance Pattern

Autosomal Dominant with Anticipation

• Inheritance mode: Autosomal dominant with variable penetrance based on CAG repeat size 1
• Pathogenic threshold: ≥48 CAG repeats cause disease 2
• Meiotic instability: The CAG repeat is unstable during transmission, particularly through paternal inheritance
  • Paternal transmission shows 2.47 times higher odds of epilepsy development compared to maternal transmission 3
  • Anticipation is prominent—offspring typically have earlier onset and more severe symptoms than parents 4, 5

CAG Repeat Size Categories

The ATN1 gene shows similar CAG repeat dynamics to other trinucleotide repeat disorders 1:

• Normal alleles: <48 CAG repeats
• Pathogenic expansions: ≥48 CAG repeats (typically 53-81 in reported cases) 6, 2
• Juvenile-onset threshold: ≥63 CAG repeats typically cause onset before age 21 with prominent epilepsy 5

Critical caveat: The largest CAG expansion reported in Chinese patients was 81 repeats, associated with very early onset and severe phenotype 2. Possible incomplete penetrance has been observed in some families 2.

Clinical Manifestations

Age-Dependent Phenotypes

Juvenile-onset (<20 years):

• Predominantly presents with epilepsy (46.7% as initial symptom) 6
• Mean age at onset: 16.9 years in epilepsy patients 3
• Larger CAG expansions (mean 66.7 repeats) 3

Adult-onset (>20 years):

• More likely to present with gait instability/ataxia (42.2% as initial symptom) 6
• Mean age at onset: 45.5 years in non-epilepsy patients 3
• Smaller CAG expansions (mean 59.2 repeats) 3

Core Clinical Features

Epilepsy manifestations (64.4% of all patients) 6:

• Focal seizures: Most common (40.0-76.5% of epilepsy cases) 3
• Myoclonic seizures: Frequently observed, though giant somatosensory-evoked potentials (typical of progressive myoclonic epilepsy) are rare 3
• Generalized tonic-clonic seizures: Common presentation 3

Neurological features (frequently co-occurring):

• Gait instability/ataxia: 88.9% 6
• Cognitive impairment: 73.3% 6, 4
• Speech impairment: 66.7% 6
• Involuntary movements/chorea: 48.9% 6

Psychiatric manifestations:

• Psychosis can develop, particularly in juvenile-onset cases 4
• Novel phenotypes including insomnia progressing to stupor have been reported 2

Electroencephalographic Findings

• Slow bursts: 38.0% 3
• Photoparoxysmal responses: 36.6% 3
• Interictal epileptiform discharges: 77.5% 3
• Important distinction: Giant somatosensory-evoked potentials were observed in only 2 patients and absent in 27, differentiating DRPLA from classic progressive myoclonic epilepsy 3

Neuroimaging

• Cerebellar atrophy (most common finding) 6, 4
• White matter involvement 6
• Brainstem atrophy 6
• Temporal pattern: Initial MRI may show minimal atrophy, with prominent atrophy developing on follow-up imaging 4

Genotype-Phenotype Correlation

Strong inverse correlation exists between CAG repeat size and age of onset 6, 5:

• This correlation is particularly robust in seizure-onset patients 6
• Less clear correlation in gait-instability onset subgroup 6
• CAG repeat size positively correlates with symptom severity 5

Clinical threshold: When CAG repeats ≥63, onset typically occurs before age 21 with obvious clinical symptoms 5

Diagnostic Considerations

When evaluating suspected ATN1-related epilepsy, the ACMG guidelines for CAG repeat disorders recommend 1:

• Testing indication: Consider in patients with unexplained combinations of seizures, ataxia, cognitive impairment, and compatible family history 6
• Alternative diagnosis context: DRPLA shows considerable clinical overlap with Huntington disease; ATN1 testing should be considered in HD-negative patients with compatible phenotype 1
• Genetic counseling: Mandatory for all positive results, with testing available for at-risk family members 1

Common pitfall: The phenotypic heterogeneity—even within the same family—can complicate diagnosis 5. Some family members with pathogenic expansions (50-54 repeats) may show minimal or no clinical symptoms, while others with larger expansions (63-75 repeats) have severe early-onset disease 5.