Trinucleotide Repeat Expansion Disorders – Study Notes
Trinucleotide Repeat Expansion Disorders – Comprehensive Study Notes
Trinucleotide repeats expansion disorders: overview
Definition: Disorders caused by unstable expansions within affected genes of DNA segments that consist of repeating units of three or more nucleotides in tandem. Common triplets include CAG, CGG, CTG, GAA.
Concept of polymorphism in normal alleles: All these genes have wild-type alleles that are polymorphic, meaning there is a variable number of repeat units in the normal population.
Pathogenic expansion: Repeats can increase beyond the normal polymorphic range, leading to abnormalities in gene expression and/or function.
Key terms:
Normal range of repeats
Premutation range
Disease range
Anticipation: successive generations show more severe disease and earlier onset due to expansion.
Biochemical mechanism most commonly proposed: slipped mispairing during DNA replication, which can cause expansions.
Tissue and timing of expansion:
Expansions occur in proliferating germ cells (e.g., spermatogonia during meiosis) and in non-proliferating somatic cells (e.g., neurons).
Depending on disease, expansions can occur during DNA replication and genome maintenance.
Major clinical note: These disorders are primarily neurological, though some have multisystem involvement (e.g., DM1, FRDA).
Classification of repeat disorders
Coding region expansions → toxic protein with polyglutamine tracts
Example: Huntington disease (HD).
Noncoding region expansions → affect RNA processing, transcription, or gene silencing
Examples: Fragile X syndrome, Myotonic dystrophy, Friedreich ataxia.
Anticipation phenomenon: severity increases and age of onset decreases in successive generations due to repeat expansion.
Mechanistic distinction:
Coding repeats (e.g., CAG) → translated into elongated polyglutamine tracts in the protein, causing toxic gain-of-function or aggregation.
Noncoding repeats (e.g., CGG, CTG, GAA) → primarily affect RNA function, processing, stability, or transcriptional silencing/epigenetics.
Commonly affected diseases (high-yield overview)
Diseases with coding CAG repeats (polyglutamine expansions)
Huntington disease (HD)
Several spinocerebellar ataxias (SCAs)
Diseases with noncoding repeats affecting RNA or gene expression
Fragile X syndrome (CGG repeats in FMR1, 5′ UTR)
Myotonic dystrophy types 1 and 2 (CTG repeats in DMPK; CCTG repeats in CNBP/ZNF9)
Friedreich ataxia (GAA repeats in FXN, intron 1; recessive)
Other examples with coding/noncoding expansions (brief): various SCAs and related ataxias with different repeat types (CAG, CTG, ATTCT, GAA, CGG).
Huntington disease (HD)
Genetics
Gene: HTT on chromosome 4p16.3
Expansion: CAG trinucleotide repeat in the coding region; repeats > 36 cause disease
Inheritance: Autosomal dominant
Clinical features
Progressive chorea, dystonia
Psychiatric disturbances and cognitive decline (executive dysfunction, memory, visuospatial skills)
Onset typically between 30–50 years
Neuropsychiatric: anxiety, irritability, depression, aggression, apathy, psychosis; obsessive-compulsive behaviors
Motor: chorea, rigidity, dystonia, akinesia, dysarthria, dysphagia
Neuropathology
Mutant HTT (mHTT) protein with expanded polyglutamine tract
Degeneration of medium spiny neurons in caudate nucleus and putamen
Larger repeats associated with earlier onset (anticipation)
Key note on anticipation and repeats
Anticipation is pronounced when paternally transmitted due to expansion dynamics in sperm (not exclusively, but with significant effect in HD)
Fragile X syndrome
Genetics
Gene: FMR1 on X chromosome; CGG repeat in the 5′ UTR
Inheritance: X-linked dominant (with variable penetrance depending on repeat size)
Repeat ranges (CGG)
Normal: approximately 6–54 repeats
Premutation: ~55–200 repeats
Full mutation (disease): >200 repeats
Pathophysiology
Hypermethylation of the FMR1 promoter → silencing of FMRP protein
Loss of FMRP leads to impaired synaptic plasticity and neural function
Clinical features
Males often more severely affected
Long face, large ears, macroorchidism after puberty
Behavioral features: autism spectrum traits, hyperactivity, anxiety
Additional notes
FMRP is an RNA-binding protein that regulates translation at synapses; its loss disrupts neuronal signaling
Myotonic dystrophy (DM) – focus on DM1; also DM2 overview
DM1 (Myotonic dystrophy type 1)
Gene: DMPK on chromosome 19q13.3
Repeat: CTG expansion in the gene; location is in the 3′ untranslated region (3′ UTR)
Inheritance: Autosomal dominant
Repeat ranges (CTG)
Normal: roughly 5–37 repeats
Disease: typically 50–1000+ repeats (some reports higher)
Pathophysiology: expanded CTG repeats in RNA form a toxic RNA; RNA foci interfere with multiple RNA-binding proteins and splicing, leading to multi-systemic manifestations
Clinical features: myotonia (delayed muscle relaxation), progressive muscle weakness, cataracts, cardiac conduction defects, endocrine issues (diabetes, hypogonadism), CNS involvement
Anticipation: present due to increasing CTG repeats across generations
DM2 (Proximal myotonic dystrophy, DM2 = proximal myotonic dystrophy type 2)
Gene: CNBP/ZNF9 on chromosome 3q21
Repeat: CCTG repeats in an intron
Inheritance: Autosomal dominant
Typical repeat length: on the order of hundreds to thousands of repeats (variable common range ~75–1000+)
Clinical features: similar myotonia and multi-systemic features, often with a different age of onset and pattern compared with DM1
Friedreich ataxia (FRDA)
Genetics
Gene: FXN on chromosome 9q21.11
Repeat: GAA repeats in intron 1
Inheritance: Autosomal recessive
Pathophysiology
GAA expansion leads to transcriptional silencing and reduced Frataxin protein
Frataxin is essential for mitochondrial iron-sulfur cluster formation; deficiency causes mitochondrial dysfunction and oxidative stress
Result: impaired energy production and increased cellular damage, especially in nervous system, heart, and pancreas
Clinical features
Progressive gait and limb ataxia; dysarthria; areflexia; loss of vibration sense
Hypertrophic cardiomyopathy, scoliosis, diabetes mellitus
Other signs: pes cavus, foot deformities, scoliosis, pes planus
Notes on disease mechanism and progression
Symptoms typically worsen over time; multiple organ systems affected due to energy failure and oxidative stress
Spinocerebellar Ataxias (SCAs)
General features
Diverse group with multiple etiologies, most commonly CAG expansions in coding regions (polyglutamine diseases)
Inheritance: Autosomal dominant
Pathology: expanded polyglutamine proteins cause Purkinje cell degeneration with widespread cerebellar and brainstem atrophy
Coding vs noncoding expansions in SCAs
Polyglutamine expansion SCAs (coding CAG repeats): SCA1, SCA2, SCA3 (Machado-Joseph), SCA6, SCA7, SCA17
Mechanism: toxic gain-of-function via polyglutamine tracts
Noncoding repeat expansion SCAs: SCA8 (CTG), SCA10 (ATTCT)
Mechanism: toxic RNA effects
Conventional mutations (not repeat expansions): SCA5 (β-III spectrin), SCA13 (KCNC3), SCA14 (PRKCG)
Mechanism: loss-of-function or channelopathies
Examples and notes from the reference table
SCA1 (ATXN1): CAG expansion; ataxia with pyramidal signs; AD; key concept: polyglutamine gain-of-function
SCA2 (ATXN2): CAG expansion; slow saccades; AD
SCA3 (ATXN3): CAG expansion; ataxia, dystonia, neuropathy; most common worldwide
SCA6 (CACNA1A): CAG expansion; pure cerebellar ataxia; late onset
SCA7 (ATXN7): CAG expansion; ataxia with retinal degeneration (vision loss)
SCA8 (ATXN8OS): CTG expansion; ataxia, dysarthria; RNA-mediated mechanism
SCA10 (ATXN10): ATTCT expansion; ataxia with seizures
SCA17 (TBP): CAG/CAA expansion; ataxia, dementia, seizures
SCA5 (SPTBN2): missense mutation; slowly progressive ataxia; transcription factor defect or structural protein
SCA13 (KCNC3): missense mutation; ataxia and seizures; channelopathy
Clinical vignettes and exam-style questions (practice prompts)
Huntington disease question (HD characteristic):
Statement options included: anticipation presence, premutation ranges, non-coding location, etc. Correct characteristic is that HD exhibits anticipation, often with paternal transmission driving expansion; the CAG repeat expansion in HTT is in the coding region, leading to polyglutamine expansion. Consider: Anticipation is common in HD and linked to repeat expansion dynamics in meiosis.
For a patient with a CAG expansion in HTT and progressive chorea: the mutation is best described as a polyglutamine tract expansion in a coding region.
Fragile X syndrome genetics question: CGG repeats in FMR1 cause hypermethylation and gene silencing; location is in the 5′ UTR; disease mechanism involves loss of FMRP, an RNA-binding protein that regulates translation at synapses.
Myotonic dystrophy question: DM1 features CTG repeats in DMPK; location is in the 3′ UTR; the pathogenic mechanism is toxic RNA with RNA foci causing mis-splicing and multi-systemic symptoms; anticipation is observed.
Friedreich ataxia question: GAA repeats in FXN intron 1 cause transcriptional silencing and reduced Frataxin; pathology involves mitochondrial dysfunction and energy failure; autosomal recessive inheritance.
SCA question: many SCAs are polyglutamine disorders due to CAG repeats in coding regions; some SCAs involve noncoding repeats with RNA gain-of-function toxicity (e.g., SCA8, SCA10).
Fragile X premutation risks: individuals with 55–200 CGG repeats are at risk for expansion in offspring and FXTAS/FXPOI phenomena; diagnostic and counseling implications are significant.
DM2 DM1 contrast: DM2 involves CNBP/ZNF9 gene with CCTG repeats; DM2 is autosomal dominant; DM1 is CTG repeats in DMPK; both show anticipation but via different repeat types.
Genetic counseling, penetrance, and practical implications
Penetrance and expressivity
Repeat length strongly correlates with disease risk and severity; larger repeats generally translate to earlier onset and more severe disease (anticipation).
Counseling considerations
Discuss potential for anticipation, generational risk, and reproductive options
Consider testing for adult relatives; the right not to know genetic status is a valid ethical consideration in counseling contexts
Testing and management implications
Genetic testing informs prognosis, family planning, and surveillance for multisystemic manifestations (e.g., cardiac monitoring in DM1/DM2, diabetes in FRDA, etc.)
Key concepts and connections to foundational principles
Molecular genetics concepts
Repeated DNA sequences can be unstable and expand during replication and genome maintenance
Epigenetic silencing via DNA methylation can result from repeat expansion (e.g., FMR1 promoter hypermethylation in Fragile X)
RNA toxicity vs. protein toxicity: Noncoding repeats often cause disease via RNA-mediated mechanisms; coding repeats induce toxic polypeptide generation (polyglutamine expansions)
Pathophysiology and clinical correlations
Polyglutamine expansion diseases show neurodegeneration with motor and cognitive symptoms; relate to selective vulnerability of neurons (e.g., medium spiny neurons in HD, Purkinje cells in SCAs)
Energy metabolism and mitochondrial dysfunction are central in FRDA due to Frataxin deficiency
Real-world relevance
Anticipation affects family planning and prognosis discussions
Multisystem involvement in DM1/FRDA requires multidisciplinary care (cardiac, endocrine, respiratory, orthopedic, neuromuscular)
Notable numerical references and ranges (LaTeX-formatted where helpful)
General repeat ranges (illustrative):
Normal range:
Premutation:
Disease range:
Anticipation: onset age across generations satisfies
Specific disease repeat counts (examples from the deck):
Spinal and bulbar muscular atrophy (SBMA, AR gene): normal repeats; disease repeats
Fragile X syndrome (FMR1): normal ; premutation ; full mutation > repeats
Myotonic dystrophy type 1 (DMPK): normal ; disease 9 ext{–} 3737 ext{–} 1216 ext{–} 29200 ext{–} 9007 ext{–} 2549 ext{–} 75$$
Fragile X-related CGG dynamics, etc.
Summary of key mechanisms by repeat type
Coding region repeats (e.g., CAG in HD, many SCAs): produce elongated polyglutamine tracts in proteins → toxic protein gain-of-function and aggregation.
Noncoding repeats affecting RNA processing (e.g., CGG in FMR1, CTG in DMPK, CTG/NBG family in DM2): cause RNA-mediated toxicity, mis-splicing, transcriptional dysregulation, and/or epigenetic silencing.
GAA repeats in FRDA: cause transcriptional silencing of FXN via heterochromatin formation; downstream mitochondrial dysfunction due to Frataxin deficiency.
Quick-reference disease map (one-line per disease for recall)
Huntington disease: HTT, CAG coding expansion → polyglutamine toxicity; autosomal dominant; chorea with psychiatric and cognitive features; caudate/putamen degeneration.
Fragile X syndrome: FMR1, CGG in 5′ UTR → promoter hypermethylation and gene silencing; X-linked dominant; intellectual disability, facial features, macroorchidism.
Myotonic dystrophy type 1: DMPK, CTG in 3′ UTR → RNA toxicity and mis-splicing; autosomal dominant; myotonia, weakness, cataracts, cardiac conduction defects.
Myotonic dystrophy type 2: CNBP/ZNF9, CCTG repeats in intron → RNA pathology; autosomal dominant; overlapping but distinct clinical features from DM1.
Friedreich ataxia: FXN, GAA in intron 1 → transcriptional silencing; autosomal recessive; ataxia, cardiomyopathy, diabetes, scoliosis.
Spinocerebellar ataxias: multiple disorders with coding (CAG) or noncoding repeats; autosomal dominant; ataxia with varying additional features (retinal degeneration, seizures, neuropathy, pyramidal signs).
Bibliography (for further reading)
Chapter 5, Lynn B. Jorde, John C. Carey, Michael Bamshad. Medical Genetics, 6th Edition. Elsevier, 2019.