Expandable Tandem Repeat Above the Instability Threshold
trigger
The shared substrate is a germline tandem repeat expanded (or in the premutation range) above a length/instability threshold, at which the repeat forms slipped-strand and hairpin secondary structures that become substrates for aberrant repair. This applies across the repeat classes of the STR expansion diseases — CAG (Huntington disease, the polyglutamine SCAs, DRPLA, SBMA), CGG (FMR1), CTG (DMPK), and GAA (FXN) — with the specific expandable locus substituted by the conforming disorder.
Downstream
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Aberrant Mismatch-Repair Processing of the Repeat
The slipped/hairpin repeat structure formed above the instability threshold is engaged by the MMR machinery.
Show evidence (2 references)
"Fragile X-associated disorders are Repeat Expansion Diseases that result from expansion of a CGG/CCG-repeat in the FMR1 gene."
Establishes the expandable-repeat substrate for the CGG class, generalizing the trigger beyond the CAG/polyglutamine disorders.
"The Huntington's disease gene (HTT) CAG repeat mutation undergoes somatic expansion that correlates with pathogenesis. Modifiers of somatic expansion may therefore provide routes for therapies targeting the underlying mutation, an approach that is likely applicable to other trinucleotide repeat diseases."
Establishes the CAG expandable repeat and the explicit pan-repeat generalization that grounds the module's cross-disease scope.
Aberrant Mismatch-Repair Processing of the Repeat
amplifier
The MutSβ complex (MSH2-MSH3) recognizes the slipped/hairpin repeat structure and, instead of faithfully repairing it, licenses incremental expansion; the MutLγ complex (MLH1-MLH3) acts downstream of MutSβ to effect the expansion, while FAN1 (an interstrand-cross-link-repair nuclease) and EXO1 oppose it and are protective. This aberrant, gain-of-toxic-outcome use of a normally protective repair pathway is the disorder-agnostic molecular driver of the expansion; human genetic-modifier studies converge on these same MMR factors.
Downstream
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Somatic Repeat Expansion in Post-Mitotic Cells
Aberrant MMR processing drives repeated rounds of incremental expansion in somatic tissue.
Show evidence (3 references)
"MSH3, the MSH2-binding partner in the MutSβ complex, is required for 98% of germ line expansions and all somatic expansions in this model."
Demonstrates that the MutSβ (MSH2-MSH3) complex is required for all somatic repeat expansions in a CGG-repeat model, the core molecular driver.
"MLH1/MLH3 (MutLγ), a protein complex that can act downstream of MutSβ in MMR, is also required for all germ line and somatic expansions."
Establishes the MutLγ (MLH1-MLH3) effector complex acting downstream of MutSβ, and (with the EXO1-protective finding of the same study) the opposing/protective arm of the machinery.
"Validation of leads including the mismatch repair protein MSH3, and interstrand cross-link repair protein FAN1, suggest the mechanism is driven by somatic CAG instability, which is supported by the protective effect of CAA substitutions in the CAG tract."
Human genetic-modifier evidence across HD and the spinocerebellar ataxias implicates MSH3 (driver) and FAN1 (protective) and the CAA-interruption stabilizing effect, corroborating the mouse mechanism in human disease.
Somatic Repeat Expansion in Post-Mitotic Cells
central effector
Because the aberrant MMR-driven expansion continues throughout life and is not limited to replication, the repeat keeps expanding in specific post-mitotic somatic cells — most consequentially neurons — generating a mosaic of progressively larger alleles far exceeding the inherited germline length. This ongoing, cell-type-biased somatic expansion (not the static germline allele) is the rate-limiting driver of pathology and is the key conformance target: conforming disorder entries substitute their own expanding repeat and vulnerable cell population (striatal medium spiny neurons in Huntington disease; cerebellar and brainstem neurons in SCA3).
Downstream
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Cell-Type-Selective Toxicity Threshold Crossing
When somatic expansion in a vulnerable cell exceeds its toxicity threshold, the cell tips into the disease-specific downstream consequence.
Show evidence (2 references)
"genome-wide association studies have identified genetic modifiers, mostly DNA repair genes, that significantly influence disease onset and progression. These findings point to somatic CAG repeat expansions in affected tissues as a key pathological mechanism."
Human GWAS evidence establishes somatic repeat expansion in affected tissue, driven by DNA-repair modifiers, as the key pathological mechanism.
"Mlh1 is essential for somatic CAG expansions and that it is an enhancer of nuclear huntingtin accumulation in striatal neurons."
Directly links MMR-driven somatic expansion to the downstream disease protein burden in the vulnerable neuronal population, defining the seam to the toxicity route.
Cell-Type-Selective Toxicity Threshold Crossing
effector
When cumulative somatic expansion in a vulnerable cell pushes the repeat past that cell's toxicity threshold, the cell tips into the disorder-specific downstream consequence — translated polyglutamine proteotoxicity for CAG, repeat-RNA gain-of-toxicity for CTG, or transcriptional silencing for CGG and GAA. Because the expansion rate is cell-type-specific, this threshold crossing is the event that explains the selective regional vulnerability of a ubiquitously expressed gene and is the seam handing off to the disorder's toxicity module.
Downstream
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Repeat-Length-Dependent Onset, Progression, and Anticipation
The aggregate of cell-level threshold crossings determines the organism-level timing and severity of disease.
Show evidence (2 references)
"Striatal pallidal neurons (SPNs) with 150-500 + CAG repeats seem to lose positive and then negative features of neuronal identity, de-repress senescence/apoptosis genes, ultimately leading to cell death."
Shows that crossing a high somatic-expansion threshold in vulnerable striatal neurons drives loss of neuronal identity and cell death, the cellular tipping point into the disease phenotype.
"Mlh1 is essential for somatic CAG expansions and that it is an enhancer of nuclear huntingtin accumulation in striatal neurons."
The same MMR-driven somatic expansion enhances the toxic disease-protein burden specifically in the vulnerable striatal neurons, linking expansion to the cell-type-selective toxicity threshold.
Repeat-Length-Dependent Onset, Progression, and Anticipation
consequence
The organism-scale readout of the molecular engine: somatic-expansion burden sets the age of onset — earlier than the inherited germline allele alone predicts — and the rate of progression, while germline instability with parent-of-origin bias produces intergenerational anticipation. These are the defining clinical hallmarks of the repeat expansion diseases (mirrored by the STRchive locus tags somatic_instability, anticipation, and length_affects_onset), here derived mechanistically from MMR-driven instability rather than merely correlated with germline repeat length.
Show evidence (2 references)
"Somatic expansion of the CAG repeat length, beyond the inherited length, has been associated with hastening the onset of symptoms compared to that predicted from the germline length."
Establishes that somatic expansion beyond the germline length hastens onset, the organism-level consequence of the instability engine.
"genome-wide association studies have identified genetic modifiers, mostly DNA repair genes, that significantly influence disease onset and progression."
DNA-repair genetic modifiers influence both onset and progression, confirming the instability engine as the determinant of the clinical time course.