Forced expression of MSR repeat transcripts above a threshold limit breaks heterochromatin organisation

Area of Science:

• Epigenetics
• Molecular Biology
• Genomics

Background:

• Mouse heterochromatin contains transcriptionally competent major satellite repeat (MSR) sequences.
• MSR RNA is hypothesized to play a role in maintaining heterochromatin integrity.

Purpose of the Study:

• To investigate the functional role of MSR RNA in heterochromatin organization.
• To establish a system for modulating MSR transcription and its effects on heterochromatin.

Main Methods:

• Development of an inducible dCas9-effector system in mouse embryonic fibroblasts (MEFs).
• Targeting MSR sequences with dCas9-Repressor or dCas9-Activator to modulate transcription.
• Analysis of heterochromatin organization, MSR transcript levels, and cell viability.

Main Results:

• A deregulation threshold of >300-fold for MSR transcript levels was identified.
• Exceeding this threshold disrupts heterochromatin structural organization and chromosome segregation.
• MEF cells with elevated MSR RNA above the threshold exhibit inviability and irreversible defects.

Conclusions:

• Restricting MSR RNA output is essential for maintaining heterochromatin integrity.
• Heterochromatin organization is governed by the transcriptional chromatin state and MSR RNA.
• MSR transcript levels may indicate physiological or pathological conditions.