Name: SKY-0515 in Huntington's disease: Pathophysiology
Published: 2026-09-16T18:00:43-04:00

Mutant huntingtin: toxic gain of function

“In mutation carriers, huntingtin is produced with abnormally long polyglutamine sequences that confer toxic gains of function and predispose the protein to fragmentation, resulting in neuronal dysfunction and death.” (opens the source at this quote in a new tab)

“The expanded polyQ tract confers a toxic gain of function, resulting in the dysregulation of vital cellular processes, which ultimately leads to protein aggregation and cell death” (opens the source at this quote in a new tab)

“At the cellular level mutant huntingtin results in neuronal dysfunction and death through a number of mechanisms, including disruption of proteostasis, transcription and mitochondrial function and direct toxicity of the mutant protein. Early macroscopic changes are seen in the striatum with involvement of the cortex as the disease progresses.” (opens the source at this quote in a new tab)

Striatal degeneration: neuropathological grading

“The earliest changes were seen in the medial paraventricular portions of the caudate nucleus (CN), in the tail of the CN, and in the dorsal part of the putamen. Counts of neurons in the CN reveal that 50% are lost in grade 1 and that 95% are lost in grade 4” (opens the source at this quote in a new tab)

Genetic modifiers of onset: DNA repair genes identified by GWAS

“Association at MLH1 and pathway analysis of the full GWA results support a role for DNA handling and repair mechanisms in altering the course of HD.” (opens the source at this quote in a new tab)

“The timing of onset shows no significant association with HTT cis-eQTLs but is influenced, sometimes in a sex-specific manner, by polymorphic variation at multiple DNA maintenance genes, suggesting that the special onset-determining property of the uninterrupted CAG repeat is a propensity for length instability that leads to its somatic expansion.” (opens the source at this quote in a new tab)
“Four genes at 3 loci were significant after Bonferroni correction: FAN1 (p = 1.9E-22), MSH3 (1.9E-8), PMS1 (6.1E-7), and ASNSD1 (5.3E-6) with later onset associated with increased FAN1, PMS1, and ASNSD1 expression and decreased MSH3 expression.” (opens the source at this quote in a new tab)

“Repeatedly implicated MMR genes include MSH3, MLH1, PMS1, PMS2, and LIG1” (opens the source at this quote in a new tab)

Genetic modifiers of progression: MSH3 and PMS1

“The lead single nucleotide polymorphism (SNP) in TRACK-HD (rs557874766) was genome-wide significant in the meta-analysis” (opens the source at this quote in a new tab)
“encodes an aminoacid change (Pro67Ala) in MSH3” (opens the source at this quote in a new tab)
“Knockout of Msh3 reduces somatic expansion in Huntington's disease mouse models, suggesting this mechanism as an area for future therapeutic investigation.” (opens the source at this quote in a new tab)

“Genome-wide association studies (GWASs) of Huntington disease (HD) have identified six DNA maintenance gene loci (among others) as modifiers and implicated a two step-mechanism of pathogenesis: somatic instability of the causative HTT CAG repeat with subsequent triggering of neuronal damage.” (opens the source at this quote in a new tab)
“In addition, we identified additional modifier effects at the PMS1 and PMS2 loci and implicated a potential second locus on chromosome 7.” (opens the source at this quote in a new tab)

Somatic CAG repeat expansion in striatal projection neurons

“We found that the HTT CAG repeat expands somatically from 40-45 to 100-500+ CAGs in SPNs. Somatic expansion from 40 to 150 CAGs had no apparent cell-autonomous effect, but SPNs with 150-500+ CAGs lost positive and then negative features of neuronal identity, de-repressed senescence/apoptosis genes, and were lost.” (opens the source at this quote in a new tab)
“We conclude that in HD, at any one time, most neurons have an innocuous but unstable HTT gene, and that HD pathogenesis is a DNA process for almost all of a neuron’s life.” (opens the source at this quote in a new tab)

“Somatic expansion within the striatum correlates strongly with earlier disease onset” (opens the source at this quote in a new tab)
“In this model, somatic instability acts as a catalyst for expansion, while a subsequent toxic gain-of-function mechanism drives cell death once the repeat length crosses a critical threshold.” (opens the source at this quote in a new tab)

PMS1 as a modifier of somatic CAG repeat expansion

“Homozygous but not heterozygous inactivation of PMS1 also reduces CAG repeat expansion, supporting PMS1 as a genetic modifier of HD and a potential target for therapeutic intervention.” (opens the source at this quote in a new tab)

Multiple pathogenic mechanisms

“Mutant HTT (mHTT) disrupts transcription, interferes with immune and mitochondrial function, and is aberrantly modified post-translationally. Evidence suggests that the mHTT RNA is toxic, and at the DNA level, somatic CAG repeat expansion in vulnerable cells influences the disease course.” (opens the source at this quote in a new tab)