A human CAGinSTEM platform for decoding HTT repeats' somatic instability links CAG interruption to HD pathology in...

Area of Science:

• Genetics
• Neuroscience
• Molecular Biology

Background:

• Somatic CAG repeat instability in the Huntingtin (HTT) gene is a critical factor in Huntington's disease (HD) pathogenesis.
• Understanding the genetic modifiers of this instability is crucial for developing therapeutic strategies.
• Previous research has focused on various factors, but the precise role of cis genetic elements within the HTT repeat itself requires further elucidation.

Purpose of the Study:

• To investigate the impact of cis genetic elements, specifically interruptions within the CAG repeat of the HTT gene, on somatic CAG instability.
• To develop and utilize a novel human stem cell platform for modeling Huntington's disease and analyzing HTT repeat dynamics.
• To identify specific DNA repeat compositions that can modulate or abolish CAG instability in human neurons.

Main Methods:

• Development of the CAGinSTEM platform using CRISPR-engineered human stem cells with varying CAG repeat lengths and clinical haplotypes.
• Third-generation sequencing was employed for precise analysis of CAG repeat length and composition.
• Manipulation of cis genetic elements, particularly CAA interruptions within the CAG repeat, to assess their effect on instability.

Main Results:

• Interruptions within the CAG repeat, especially involving the penultimate CAA, significantly modulate HTT gene instability.
• The loss or duplication of the penultimate CAA sequence was linked to altered instability.
• Four internal CAA interruptions were found to completely abolish CAG instability, reversing HD-associated cellular phenotypes like altered striatal fate and nuclear disorganization.

Conclusions:

• The composition of the HTT DNA repeat directly influences CAG instability, with cis modifiers playing a critical role.
• Specific interruption patterns, such as multiple CAA repeats, can serve as potent suppressors of HTT repeat instability.
• The CAGinSTEM platform provides a powerful in vitro model for studying HTT repeat instability and exploring therapeutic interventions for Huntington's disease.