Background

Precise regulation of gene expression is essential for organisms to maintain life and respond to the environment. In the genomes of mammals, including humans, promoters and enhancers are key switches that regulate gene expression. Promoters are the sites where transcription begins. Enhancers, on the other hand, are regulatory regions that promote the expression of specific genes. Within cells, these two regions physically interact to induce gene expression, but it has been unclear how the physical distance between enhancers and promoters within the three-dimensional genome structure specifically affects the amount and temporal rhythm of gene expression.

Previous studies have largely focused on a binary model in which genes are activated when enhancers and promoters come together and repressed when they separate. Transcription has also been known to occur in the form of 'transcriptional bursts,' which are intermittent and explosive rather than continuous. However, due to technical limitations in tracking the transcription process in real-time at the single-cell level, the effect of changes in linear distance on the genome on the frequency and magnitude of transcriptional bursts, as well as cell-to-cell variability in gene expression (noise), has not been fully elucidated. This has limited the understanding of the mechanisms that control randomness in gene expression when cells differentiate or respond to external signals. Resolving this issue has been a long-standing challenge in the field.

Key Findings

The research team at the Friedrich Miescher Institute for Biomedical Research (FMI) solved this problem by combining real-time cell imaging technology with precise mathematical modeling. The team used mouse embryonic stem cells (mESCs) as a model. They constructed a scaffold system in which the same enhancer was placed at various linear genomic distances from the promoter for analysis. This allowed them to capture, in high resolution, how gene expression changes in real-time as the distance between the enhancer and promoter in mESCs varies on the genomic map.

The observed results deviated significantly from previous academic predictions. The process of generating ribonucleic acid (RNA) from the promoter was observed as 'burst clusters,' in which multiple bursts occurred in groups, rather than a single burst. As the distance between the enhancer and promoter on the genome decreased, the size or duration of the bursts themselves did not change. Instead, the frequency of burst cluster formation increased significantly.

Mathematical modeling analysis showed that the enhancer increases the probability of transitioning the promoter from its basal state to a 'burst cluster state' in which transcription actively occurs. When the enhancer is close to the promoter on the genome, burst clusters occur frequently and regularly throughout the cell population. This reduces cell-to-cell variation in gene expression, thereby enhancing the uniformity of gene expression. Conversely, as the distance between the enhancer increases, the frequency of burst clusters decreases sharply, and the random noise in the frequency of RNA synthesis increases in each cell.

Significance and Prospects

This study is considered to fundamentally redefine the operating principles of promoters and the mechanisms of transcriptional regulation in mammals. It demonstrates the physical principle that the frequency of burst clusters is finely controlled in an analog manner according to the distance between the enhancer and promoter (E-P), rather than simply turning genes on or off. It has revealed the fundamental mechanism that controls biological randomness when cells differentiate or respond to external stimuli.

Until now, the detailed pathways by which subtle changes in three-dimensional genome structure lead to genetic diseases or cancer have not been clearly elucidated. Applying this finding, it means that if the E-P distance changes abnormally due to genomic insertion or deletion, the frequency of transcriptional burst clusters may be disrupted, leading to cell-to-cell variation in gene expression noise. This provides new clues for elucidating the mechanisms of genetic diseases.

However, this study has the limitation that it was derived from artificially induced enhancer-promoter structures and simplified regulatory environments. In the actual chromatin environment, numerous regulatory factors and proteins act in a complex manner, so there is still a task of expanding the scope of the study to a complex natural genomic context. In the future, follow-up studies are expected to verify whether this control mechanism operates universally in various gene loci and in cells at actual developmental stages.