New discovery reveals how cohesin complex alters gene expression

Cohesin is a protein that forms a ring-shaped complex which wraps and alters the DNA molecule shape. It moves through the DNA and creates specific loops in the genetic material which determine the architecture of the genome and gene expression. Some mutations in the genes of the cohesion complex are responsible for rare diseases (cohesinopathies), such as the Cornelia de Lange syndrome (SCdL) or Roberts syndrome, which affect several organs and cause malformations during development.

However, deciphering how cohesins work, how they are located in specific areas of the genome and finding their role in the DNA control activity is still a scientific challenge in molecular biology. Now, the journal Nucleic Acids Research is dedicating its cover to a study that opens a new view on how to understand how the cohesion complex can couple to the chromatin structure and alter the expression of the genes that cause cohesinopathies.

The study involves the participation of teams led by Professor Eva Estébanez-Perpiñá, from the UB's Faculty of Biology and the Institute of Biomedicine (IBUB) — with head offices at the Barcelona Science Park (PCB) —, and the experts Gordon L. Hager, from the National Institutes of Health (NIH) in Bethesda (United States), and Frank Dequiedt, from the University of Liege (Belgium).

The protein that forms loops in the human genome

The protein that makes the folding of the genome into DNA loops possible is made up of four subunits. "To date, researchers have described 25 proteins that regulate these subunits and their biological function", notes Professor Estébanez-Perpiñá, who leads a team built within the research group from the UB Chair on Rare Diseases, at the UB's Faculty of Biology and led by Professor Marisol Montolio.

"In human cells, two distinct cohesin isoforms are found and differentiate into the subunit known as STAG. Therefore, STAG-1 cohesin and STAG-2 cohesin have been described. These isoforms differ in the composition of the SMC1, SMC3 and SCC1/RAD21 subunits", notes the researcher, member of the Department of Molecular Biochemistry and Biomedicine at the Faculty of Biology.

Previous studies had described how the NIPBL protein (cohesion loading factor) protein, bound to the MAU2 protein, enables cohesion to bind to specific points on the DNA known as gene enhancers. These genomic regions are DNA sequences where binding to transcription factors, such as members of the nuclear receptor superfamily, takes place.

Now, this paper reveals how the NIPBL protein interacts with both the MAU2 protein and the glucocorticoid receptor (GR), a transcription factor of the nuclear receptor superfamily essential for cellular functions.

"This NIPBL-MAU2-GR ternary complex modulates the transcription, since it facilitates the interaction of the glucocorticoid receptor (GR) with NIPBL and MAU2, which is the cohesion-loading factor. When the GR interacts with these two proteins, it alters the structure of chromatin and affects the process of gene expression", note Alba Jiménez-Panizo and Andrea Alegre-Martí (IBUB), Juan de la Cierva researchers and co-authors of the paper with an outstanding participation in the study.

Thus, the NIPBL-MAU2-GR ternary complex becomes key to regulating the expression of genes under the control of the glucocorticoid receptor.

As part of the study, the team has used several state-of-the-art techniques for microscopic visualization of real-time molecular complexes that bind to chromatin. Complementary biochemical and biophysical techniques have also been applied to analyse the complex from different structural and cellular perspectives.

Cornelia de Lange syndrome

The new study will help improve our understanding of Cornelia de Lange syndrome, which is caused by mutations in the NIPBL, SMC1A, HDAC8, RAD21 and SMC3 genes. "These mutations affect key subunits of both cohesin and the proteins that regulate its organization and function. Understanding the molecular mechanisms by which the complexes do not form correctly and cause the chromosomes not to be organized in a functional way is key to understanding the disease", the researchers explain.

The mechanism described in this article suggests that other nuclear receptors may interact with NIPBL similarly. In this context, the team will continue to explore the functional status of the glucocorticoid receptor (GR) and the molecular biology of the complexes it forms in the cell, in addition to studying the molecular mechanisms underlying diseases such as asthma and other autoimmune pathologies.