A systematic comparison reveals substantial differences in chromosomal versus episomal encoding of enhancer activity

Area of Science:

• Genomics
• Molecular Biology
• Epigenetics

Background:

• Candidate enhancers are often identified using chromatin features but require functional validation.
• Reporter assays are standard for validation but typically use episomes, which may not reflect physiological chromatin states.
• Differences in cis-regulation between episomal and chromosomal contexts are poorly understood.

Purpose of the Study:

• To systematically compare the functional activities of candidate liver enhancers in both episomal and chromosomally integrated contexts.
• To develop and apply a novel lentivirus-based massively parallel reporter assay (lentiMPRA) for this comparison.
• To assess the predictability of enhancer activity using genomic annotations and sequence-based models in different contexts.

Main Methods:

• Development and application of a lentivirus-based massively parallel reporter assay (lentiMPRA).
• Direct comparison of 2236 candidate liver enhancer activities in episomal versus chromosomally integrated states.
• Correlation analysis with ENCODE chromatin annotations and development of predictive models.

Main Results:

• Functional activities of enhancers significantly differ between episomal and chromosomally integrated contexts.
• Chromosomally integrated assays yield more reproducible and predictable results.
• A combined model using chromatin annotations and sequence information achieved R² of 0.362 for predicting chromosomal assay results, outperforming models using either data type alone or predicting episomal assays.

Conclusions:

• Reporter assay context (episomal vs. chromosomal) critically impacts functional validation of enhancers.
• Chromosomal integration provides a more physiologically relevant and predictable system for enhancer characterization.
• These findings necessitate a re-evaluation of enhancer identification and validation strategies, favoring chromosomally based methods.