Trinucleotide Repeat Disorders

• Repeat expansion and genetic instability: Normal individuals carry a stable number of tandem repeats (typically 5-37 copies depending on the specific locus). When repeat number exceeds a critical threshold—often called the "full mutation" range—the repeats become increasingly unstable during DNA replication. This instability is mediated by slippage of DNA polymerase during replication, particularly during meiosis, where large expansions commonly occur. The propensity for expansion is influenced by repeat sequence composition, flanking DNA sequences, and epigenetic factors. Once expanded, the repeats are prone to further expansion (rarely contraction) in subsequent generations, explaining the phenomenon of genetic anticipation wherein offspring of affected parents often inherit larger repeats and manifest earlier-onset, more severe disease. The intergenerational expansion is particularly pronounced in paternal transmission for CAG repeat disorders (Huntington, SBMA) and maternal transmission for CGG repeats (fragile X).

• Toxic RNA and protein aggregation mechanisms: Expanded repeats are transcribed into toxic RNA harboring hundreds of abnormal repeat sequences. For CAG repeat disorders (Huntington, SBMA, SCA), the expanded mRNA is translated into proteins containing long polyglutamine tracts, which form pathological protein aggregates and inclusions within neurons. These polyglutamine proteins sequester transcription factors (particularly CREB-binding protein and other histone acetyltransferases), leading to transcriptional dysfunction and altered histone acetylation patterns. For CGG repeats (fragile X), the hyperexpanded repeats trigger FMR1 gene silencing through DNA hypermethylation at the promoter, leading to loss of fragile X mental retardation protein (FMRP), a crucial regulator of synaptic plasticity and dendritic spine development. For CTG repeats (myotonic dystrophy), expanded CUG-containing mRNAs sequester the MBNL1 protein (muscleblind-like 1), disrupting normal RNA splicing of muscle-specific transcripts and causing characteristic splicing defects. For GAA repeats (Friedreich ataxia), the mechanism involves both transcriptional silencing of the frataxin gene and formation of unusual triplex DNA structures that block transcription.

• Cellular dysfunction and neurodegeneration: The toxic mechanisms converge on several cellular pathways leading to neuronal dysfunction. Polyglutamine aggregates impair the ubiquitin-proteasome system and autophagy, leading to accumulation of damaged proteins and organelles. Mitochondrial dysfunction occurs through direct effects of mutant proteins and impaired mitochondrial biogenesis, reducing ATP production and increasing oxidative stress. Excitotoxicity results from altered glutamate signaling and calcium homeostasis. RNA sequestration causes widespread splicing abnormalities affecting hundreds of transcripts, disrupting neuronal function broadly. These mechanisms preferentially affect specific neuronal populations due to differences in protein expression, mitochondrial reserve capacity, and intrinsic vulnerability to proteotoxic stress. For example, medium spiny neurons in the striatum are selectively vulnerable in Huntington disease, while Purkinje cells and dentate nuclei neurons are affected in spinocerebellar ataxias, and motor neurons are particularly vulnerable in spinal and bulbar muscular atrophy.

• Repeat length-phenotype correlation: A critical principle in trinucleotide repeat disorders is the correlation between repeat expansion size and disease severity and age of onset. Generally, longer repeats confer earlier symptom onset and more aggressive disease progression. This correlation is explained by increased formation of toxic aggregates or more profound gene silencing with larger repeat numbers. However, this correlation is imperfect (incomplete penetrance in some ranges) due to modifying genetic factors, epigenetic variation, and cell-type-specific threshold effects. For Huntington disease, repeats of 36-39 CAGs show reduced penetrance and later onset, while >60 repeats typically cause juvenile-onset disease with rapid progression.

• Epigenetic modifications and chromatin remodeling: Beyond the primary repeat expansion effect, chromatin remodeling occurs at and around the trinucleotide repeat loci. In fragile X syndrome, CGG repeat expansion leads to heterochromatin formation and DNA methylation, progressively silencing the FMR1 gene. In Friedreich ataxia, similar epigenetic silencing of the frataxin gene occurs. These epigenetic changes can be dynamic and influenced by environmental factors, contributing to phenotypic variability even among individuals with similar repeat lengths.

• CAG repeat expansion (Huntington disease, SBMA, Spinocerebellar ataxias): These represent autosomal dominant disorders with the exception of SBMA, which is X-linked recessive. Risk factors include family history of affected relatives, larger paternal transmissions (due to greater meiotic instability in spermatogenesis), and anticipation in offspring of affected parents. Huntington disease has near-complete penetrance for repeats >40 CAGs, while repeats of 36-39 show reduced penetrance. Affected individuals have a 50% chance of transmitting the disease allele to each offspring.

• CGG repeat expansion (Fragile X syndrome, FXTAS): Fragile X syndrome is the leading inherited cause of intellectual disability, affecting approximately 1 in 4,000 males and 1 in 6,000-8,000 females. Risk increases with maternal transmission due to preferential meiotic expansion in oogenesis and subject to X-inactivation in heterozygous females. Carriers with intermediate (55-200 repeats) or full mutation (>200 repeats) show maternal transmission risk; males with full mutations are severely affected, while heterozygous females show variable phenotypes based on X-inactivation patterns. Fragile X-associated tremor/ataxia syndrome (FXTAS) occurs in older male carriers and female carriers with full mutations.

• CTG repeat expansion (Myotonic dystrophy type 1): Myotonic dystrophy type 1 (DM1) represents one of the most common muscular dystrophies in adults (1 in 2,500-8,000). Risk is inherited in autosomal dominant fashion with maternal transmissions causing particularly large expansions, leading to congenital myotonic dystrophy in neonates with severe manifestations. Paternal transmissions tend to cause smaller expansions associated with later-onset disease.

• GAA repeat expansion (Friedreich ataxia): Friedreich ataxia is autosomal recessive, requiring biallelic expansion of GAA repeats (both chromosomes affected). It is the most common inherited ataxia in populations of European descent (1 in 20,000-50,000). Unlike other trinucleotide repeat disorders, Friedreich ataxia typically does not show genetic anticipation because it requires two expanded alleles inherited from carrier parents who themselves are asymptomatic or mildly affected.

• Other repeat expansions: Include FMR2 (FRAXE, CGG repeats), SCA variants (multiple CAG-repeat loci), and others. Each has specific inheritance patterns, prevalence, and phenotypes.

• Genetic modifiers and protective factors: Genetic background influences disease severity and age of onset beyond the primary repeat expansion. Specific genetic polymorphisms in DNA repair genes, mitochondrial function genes, and inflammatory pathway genes modify disease expression. Environmental factors such as stress, exercise, sleep deprivation, and medications may influence symptom manifestation and disease progression.

• Huntington disease cardinal features: Typically presents in the 4th-5th decade with a triad of movement disorder, cognitive decline, and psychiatric symptoms. Chorea (involuntary, irregular, flowing movements) is the hallmark motor sign, beginning subtly as fidgeting or difficulty with fine motor tasks and progressing to generalized involuntary movements. Bradykinesia and rigidity may accompany or precede chorea. Cognitive decline manifests as impaired executive function, memory deficits, slowed processing speed, and progressive dementia. Psychiatric manifestations include depression (often preceding motor symptoms), irritability, aggression, apathy, anxiety, and obsessive-compulsive behaviors. Juvenile-onset Huntington disease (onset before age 20, typically with >60 CAGs) presents with rigidity (Westphal variant) rather than chorea and more rapid cognitive deterioration. Sleep disturbances, weight loss despite adequate caloric intake, and sexual dysfunction are common.

• Myotonic dystrophy clinical presentation: DM1 typically presents with myotonia—prolonged muscle stiffness and delayed muscle relaxation after contraction—which is often the first symptom, accompanied by progressive proximal muscle weakness. Characteristic facial appearance includes ptosis, facial weakness, and a transverse smile. Myotonia can be demonstrated clinically by percussion of muscles (percussion myotonia) or by asking patients to make a tight fist and then rapidly open their hands (delayed hand opening). Cataracts develop early and are often the first sign in younger patients; posterior subcapsular cataracts are pathognomonic. Congenital myotonic dystrophy (maternal transmission with very large expansions) presents in neonates with severe generalized hypotonia, feeding difficulties, respiratory compromise, and intellectual disability. Adult-onset DM1 shows multisystem involvement: cardiac arrhythmias and conduction abnormalities (first-degree AV block, atrial fibrillation), gastrointestinal dysmotility, hypogonadism, insulin resistance and diabetes, cognitive dysfunction (particularly executive dysfunction), and hypersomnia.

• Fragile X syndrome manifestations: Intellectual disability is the primary feature, typically moderate in males with full mutations (IQ 20-55) and variable in heterozygous females. Physical features include macrocephaly, long face, large ears, and macro-orchidism in males. Behavioral manifestations include autism spectrum features, attention-deficit/hyperactivity disorder, social anxiety, and stereotyped behaviors (hand flapping, hand biting). Female carriers may show mild cognitive impairment, learning disabilities, or premutation-associated features. Fragile X-associated tremor/ataxia syndrome (FXTAS) occurs in older carriers (typically males >50 years) with premutation repeats (55-200 CGGs) and presents with progressive cerebellar ataxia, intention tremor, parkinsonism, cognitive decline, and autonomic dysfunction.

• Friedreich ataxia presentation: Typically presents between ages 10-25 with progressive ataxia of gait and limbs, dysarthria, and dysdiadochokinesia. Early features include absent deep tendon reflexes (distinctive for ataxia) despite evidence of pyramidal tract involvement (Babinski sign). Scoliosis is nearly universal and may be the first clinical sign. Cardiac involvement includes hypertrophic cardiomyopathy (present in >90% of patients), leading to arrhythmias, heart failure, and sudden cardiac death. Diabetes mellitus develops in approximately one-third of patients. Hearing loss is common. Visual symptoms from optic nerve atrophy occur in some patients. Cognitive dysfunction is generally absent or mild, distinguishing Friedreich ataxia from other progressive neurological disorders.

• Spinal and bulbar muscular atrophy (Kennedy syndrome): X-linked disorder presenting in mid-adulthood with proximal muscle weakness (particularly bulbar and lower extremity), fasciculations, and cramps. Gynecomastia and reduced fertility are distinctive features related to androgen receptor dysfunction. Disease is much milder than other trinucleotide repeat disorders, with slow progression and preserved life expectancy.

• Spinocerebellar ataxias (various CAG repeats): Present with progressive cerebellar ataxia, oculomotor dysfunction (saccadic intrusions, nystagmus), pyramidal or extrapyramidal signs depending on the specific SCA subtype, and cognitive decline. Age of onset varies from childhood to late adulthood depending on repeat length.

• Genetic testing by DNA sequencing or PCR: The gold standard for diagnosis is trinucleotide repeat expansion testing using specialized molecular techniques. Standard PCR amplification may fail for very large repeats (>1,000 bp), requiring long-range PCR or other advanced methods. Southern blot analysis can detect very large expansions and determine exact repeat size when PCR fails. For Huntington disease, CAG repeat counting shows: <26 repeats (normal/no disease risk), 26-35 repeats (intermediate, reduced penetrance), 36-39 repeats (reduced penetrance), ≥40 repeats (full penetrance). For fragile X syndrome, CGG repeat expansion testing distinguishes: <55 repeats (normal), 55-200 repeats (premutation, at-risk carriers), >200 repeats (full mutation). For myotonic dystrophy type 1, CTG repeat counting correlates with disease severity: 5-37 repeats (normal), 38-49 repeats (permutation, may show minimal symptoms), 50-149 repeats (classic DM1), 150-2,000+ repeats (congenital DM1). Sensitivity and specificity of molecular testing approach 100% when properly performed, making this the definitive diagnostic test.

• Clinical diagnostic criteria: Diagnosis is supported by characteristic clinical presentation combined with family history and genetic testing. For Huntington disease, diagnostic criteria include onset of progressive chorea in adulthood and either family history or genetic confirmation of CAG expansion >39. For Friedreich ataxia, diagnostic criteria include progressive ataxia of gait, absent lower extremity reflexes, and genetic confirmation of GAA repeat expansion (or characteristic findings on nerve conduction studies and imaging in suspected cases pending genetic confirmation).

• Neuroimaging findings: MRI may support diagnosis and assess disease burden. Huntington disease shows striatal atrophy (particularly caudate) with corresponding ventricular enlargement, correlating with disease stage and progression rate. Friedreich ataxia demonstrates spinal cord atrophy and absence of intrinsic spinal cord signal abnormalities (unlike demyelinating disease). Fragile X syndrome may show enlarged lateral ventricles and hypoplasia of the cerebellar vermis. Myotonic dystrophy shows selective muscle atrophy (particularly tibialis anterior and sternoclavicular region) on MRI with fatty infiltration. Brain MRI in spinocerebellar ataxias shows cerebellar atrophy.

• Neurophysiologic testing: Electromyography (EMG) and nerve conduction studies are helpful in specific disorders. In myotonic dystrophy, EMG demonstrates characteristic repetitive discharges ("dive-bomber" appearance on audio output) that are pathognomonic for myotonia. In Friedreich ataxia, nerve conduction studies typically show axonal sensory neuropathy (reduced sensory amplitudes with relatively preserved motor conduction, initially). In SBMA, EMG shows denervation and fasciculations typical of motor neuron disease with male gender and later age of onset distinguishing this from ALS.

• Ophthalmologic examination: Important for fragile X syndrome (retinal abnormalities), myotonic dystrophy (posterior subcapsular cataracts, retinal degeneration), Friedreich ataxia (optic nerve atrophy), and Kearns-Sayre syndrome (pigmentary retinopathy). Slit-lamp examination is particularly valuable for detecting early cataracts in myotonic dystrophy.

• Cardiac evaluation: Essential for Friedreich ataxia (echocardiography to assess for hypertrophic cardi

Immediate stabilisation

• Acute psychiatric risk in Huntington disease (HD): active suicidal ideation is a medical emergency requiring safety assessment and possible hospitalisation before any chorea drug is started.
• Bradyarrhythmia/syncope in myotonic dystrophy type 1 (DM1) or heart failure in Friedreich ataxia (FA): telemetry, ECG, and echocardiography first; treat per ACC/AHA/HRS bradycardia and ACC/AHA heart failure guidelines.

First-line disease-specific therapy

• HD chorea — VMAT2 inhibitors (tetrabenazine, deutetrabenazine, valbenazine): deplete presynaptic dopamine in the striatum, reducing chorea. The AAN guideline on pharmacologic treatment of HD chorea supports tetrabenazine as an effective option; deutetrabenazine and valbenazine are FDA-approved with better tolerability profiles.
• DM1 myotonia — sodium-channel blockers (mexiletine): reduces repetitive muscle-fibre discharges; use only after cardiac conduction has been characterised.
• FA — Nrf2 activator omaveloxolone: FDA-approved to slow neurologic progression by augmenting antioxidant/mitochondrial gene transcription in a frataxin-deficient cell.
• Fragile X syndrome — behavioural and educational intervention first (AAP health-supervision guidance), with SSRIs for anxiety, stimulants for ADHD, and atypical antipsychotics (aripiprazole, risperidone) for irritability/aggression.

Escalation and second-line

• Atypical antipsychotics (olanzapine, risperidone) for HD chorea when psychosis, irritability, or weight loss coexist — dopamine blockade treats both domains.
• SSRIs for HD depression/obsessionality; benzodiazepines for anxiety-driven chorea exacerbation.

Definitive/procedural

• Permanent pacemaker or ICD in DM1 with high-grade AV block or ventricular arrhythmia; scoliosis correction in FA; PEG tube for dysphagia; cataract extraction in DM1.

Contraindicated/cautions

• Tetrabenazine in untreated depression or suicidality, and with MAOIs (FDA labeling).
• Dopaminergic agents worsen chorea (levodopa is reserved for the rigid Westphal juvenile variant).
• Depolarising neuromuscular blockers and opioid/sedative sensitivity in DM1 anaesthesia.
• Predictive testing of asymptomatic minors is discouraged by ACMG/Huntington Study Group–style counseling standards.

Huntington disease

• Aspiration pneumonia: progressive bulbar chorea and dysphagia; signalled by wet voice, coughing with thin liquids, recurrent fevers. The leading cause of death and an emergency when hypoxaemia develops.
• Suicide and completed self-harm: frontostriatal disinhibition plus depression; a hopelessness/plan disclosure is an emergency.
• Cachexia: hypermetabolism from continuous involuntary movement — weight loss despite normal or increased intake.
• Falls and subdural haematoma: gait chorea plus impaired postural reflexes.

Treatment-related

• VMAT2-inhibitor depression/suicidality and parkinsonism: central monoamine depletion; new hypokinesia, masked facies, or new hopelessness signals it — an emergency if suicidal.
• Neuroleptic malignant syndrome / drug-induced parkinsonism with antipsychotics: fever, rigidity, elevated creatine kinase — emergency.
• QT prolongation with VMAT2 inhibitors and antipsychotics; ECG surveillance.

Myotonic dystrophy type 1

• Sudden cardiac death: fibrofatty replacement of the conduction system → progressive AV block and ventricular arrhythmia; signalled by widening PR/QRS on serial ECG or syncope — emergency.
• Respiratory failure: diaphragmatic weakness plus central hypoventilation; morning headache, hypersomnia, hypercapnia.
• Anaesthetic catastrophe: exaggerated response to sedatives/opioids and myotonic contracture with depolarising blockade — postoperative apnoea is an emergency.
• Cataracts, insulin resistance, GI dysmotility, congenital hypotonia in maternally transmitted disease.

Friedreich ataxia

• Cardiomyopathy: frataxin-deficient mitochondria in cardiomyocytes → hypertrophic then dilated phenotype with heart failure and atrial fibrillation; the usual cause of death. New dyspnoea or rapid AF is an emergency.
• Diabetes mellitus: beta-cell mitochondrial failure — manage per ADA Standards of Care.
• Scoliosis, sensorineural hearing loss, optic atrophy, wheelchair dependence.
• Omaveloxolone: transaminase elevation and BNP rise/fluid overload — monitor liver tests per labeling.

Fragile X spectrum

• Seizures, mitral valve prolapse/aortic root dilation from connective-tissue laxity, and in premutation carriers FXPOI (primary ovarian insufficiency) and FXTAS.

• Anticipation is the signature: earlier onset and worse disease in each generation because repeats expand in meiosis. Paternal transmission drives large CAG expansions (juvenile HD); maternal transmission drives the huge CTG and CGG expansions (congenital DM1, full-mutation fragile X).
• Friedreich ataxia is the odd one out: autosomal recessive, GAA, and classically without anticipation because both alleles come from asymptomatic carriers. The stem gives ataxia + absent deep tendon reflexes + Babinski sign + hypertrophic cardiomyopathy + scoliosis, often with diabetes.
• Best next step for suspected HD in a symptomatic adult: targeted CAG repeat sizing (PCR) after pretest genetic counseling — not MRI. Caudate atrophy with boxcar lateral ventricles supports but never establishes the diagnosis.
• The single most tested drug association: tetrabenazine (VMAT2 inhibitor) for HD chorea causes depression and suicidality and is contraindicated in untreated depression — this is the AAN/FDA-flagged trap. Do not choose a dopamine agonist; it worsens chorea.
• Juvenile HD is rigid, not choreic (Westphal variant, usually >60 repeats) — a common distractor when the stem describes a teenager with parkinsonism and seizures.
• DM1 buzzwords: delayed hand-grip relaxation, percussion myotonia, ptosis with a transverse smile, frontal balding, testicular atrophy, and posterior subcapsular cataracts. Molecular basis: CUG-repeat RNA sequesters MBNL1, a toxic RNA gain-of-function, not a missing protein.
• The DM1 fact that changes management: get a surveillance ECG — conduction block causes sudden death, and pacing, not myotonia control, is the life-saving intervention.
• Fragile X: macroorchidism, long face, large everted ears, intellectual disability, autism features; full mutation >200 CGG with promoter hypermethylation silencing FMR1. Premutation carriers are cognitively normal but develop FXTAS (males) and FXPOI (females) — the classic distractor is attributing FXTAS to the full mutation.