Background

Genome-wide association studies (GWAS) have identified thousands of genetic variants associated with major autoimmune diseases, such as rheumatoid arthritis and inflammatory bowel disease, over the past decades. More than 90% of variants that increase disease susceptibility are located in non-coding regions that do not directly change protein amino acids. Most researchers have presumed that these variants affect enhancers or promoters that regulate the expression of causal genes.

The problem is that it is difficult to determine which target genes a specific variant physically contacts using only 1D sequence information. This is because it is common for variants and genes to be separated by hundreds of thousands of base pairs (bp) on a linear genome map. Existing 3D chromatin structure analysis technologies, such as Hi-C, lacked precision, with resolution remaining at the several kilobase (kb) level. It was nearly impossible to isolate and observe subtle folding changes that appeared only on the allele carrying the mutation among the two alleles derived from maternal and paternal lineages. Consequently, the task of identifying causal variants that actually trigger inflammatory responses among the numerous risk variants discovered statistically has faced a long period of stagnation.

Key Findings

By combining high-resolution chromatin 3D structure capture technology with allele-specific analysis methods, researchers examined the genomes of immune cells from inflammatory disease patients at the sub-kilobase level. By precisely comparing chromatin loop formation between the allele harboring a single nucleotide polymorphism (SNP) and the wild-type allele, they confirmed that specific disease variants create new binding sites for CTCF, a protein that maintains chromatin structure.

The newly created neo-CTCF sites produced abnormal chromatin loops that did not previously exist. This essentially resulted in a structural reorganization where boundaries that originally functioned as insulators collapsed, allowing the promoters of distant pro-inflammatory genes and active enhancers to come into direct contact. When researchers corrected the variant sequence in immune cell lines using CRISPR technology, the formation of abnormal loops disappeared, and the expression levels of excessively secreted inflammatory cytokines were also normalized to baseline levels.

While previous studies focused only on changes in transcription factor binding affinity due to variants, this achievement clearly demonstrated the molecular mechanism by which a single non-coding nucleotide variant reshapes the 3D structural insulation boundary to trigger disease.

Significance and Outlook

This study shifts the functional interpretation of GWAS-discovered variants from a 1D sequence-based approach to a high-resolution 3D genomic topology dimension. This is because it elucidated the molecular causal chain by which non-coding variants alter spatial genomic topology at the single-nucleotide level and trigger autoimmune cascades.

In the future, it is highly likely to become established as a standard analytical framework for identifying causal variants and mapping target genes across all complex genetic diseases, not just chronic inflammatory diseases. However, the high cost and complex computational requirements of high-resolution chromatin 3D structure analysis are prerequisite challenges to be solved for application to large-scale patient groups. Accumulation of additional data must also follow to fully reflect the diversity of chromatin loops, which vary dynamically by tissue and cell differentiation stage.