The chromatin remodeling factor ISW-1 integrates organismal responses against nuclear and mitochondrial stress

Age-associated changes in chromatin structure have a major impact on organismal longevity. Despite being a central part of the ageing process, the organismal responses to the changes in chromatin organization remain unclear. Here we show that moderate disturbance of histone balance during C. elegans development alters histone levels and triggers a stress response associated with increased expression of cytosolic small heat-shock proteins. This stress response is dependent on the transcription factor, HSF-1, and the chromatin remodeling factor, ISW-1. In addition, we show that mitochondrial stress during developmental stages also modulates histone levels, thereby activating a cytosolic stress response similar to that caused by changes in histone balance. These data indicate that histone and mitochondrial perturbations are both monitored through chromatin remodeling and involve the activation of a cytosolic response that affects organismal longevity. HSF-1 and ISW-1 hence emerge as a central mediator of this multi-compartment proteostatic response regulating longevity.

Computational study of transcription factor binding sites

Romain Fernand Pietro Groux

Any living organism contains a whole set of instructions encoded as genes on the DNA. This set of instructions contains all the necessary information that the organism will ever need, from its development to a mature individual to environment specific responses. Since all these instructions are not needed at the same time, the gene expression needs to be regulated. Eukaryotic genomes are stored inside nuclei as chromatin. The chromatin is the association of DNA with dedicated storage proteins - the histones - and the necessary machinery to regulate and express genes (RNA polymerases or RNAPs). In the nuclei, histones are assembled into octamers around which are wrapped ~148bp of DNA. This structure is known as the nucleosome. The repetition of nucleosomes along the genome allows to drastically compact the genome, eventually allowing to fit it inside the nucleus. However, this comes at the cost of rendering the DNA sequence inaccessible to DNA readers, such as the RNAPs and transcription factors (TFs). TFs are a class of proteins that have the remarkable property of recognizing and binding specific DNA sequences. More striking, each TF can recognize a multitude of different - but similar - DNA sequences providing TFs with a wide sequence specificity range. Eventually, this allows the cell to recruit TFs at dedicated locations in the genome called regulatory elements (REs). The action of TFs at REs is crucial to gene expression. Indeed, TFs are involved in many processes such as the opening of the chromatin structure or the recruitment of RNAPs. However if TFs can influence the chromatin structure, the opposite is also true as histones impede TF binding on DNA. Thus the regulation of genes relies on a subtle and complex crosstalk between the chromatin and TFs. To better understand how TFs and chromatin interact together to regulate gene expression, I lead several projects prospecting TF binding specificity and the chromatin structure at REs in human. First, I used ENCODE next generation sequencing (NGS) data to explore how TF binding influences the nearby nucleosome organization and the propensity of some TFs to bind together. The results suggest that regular nucleosome arrays are found near all TFs. They also point out two special cases. When CTCF binds with the cohesin complex, it seems to drive the nucleosome organization, which is a unique feature among all TFs investigated. Additionally I present evidence supporting that EBF1 is a pioneer factor - a special class of TFs able to bind nucleosome. Secondly, I developed several clustering algorithms and software to partition genomic regions according to NGS data and/or on their DNA sequences. These methods allow to discover important trends, for instance different nucleosome architectures . I illustrated the usefulness of these methods for the study of chromatin accessibility data and the identification of REs. Thirdly, I participated to the assessment of SMiLE-seq, a new microfluidic device that generates TF specificity data. The creation of TF specificity models and their comparison with other publicly available models demonstrated the value of SMiLE-seq to study TF specificity. Finally, I participated in the development of a software that predicts TF binding sites. A careful benchmarking suggested that this software is - at the time of writing - the best available software in terms of speed while showing other performances similar to its competitors.

EPFL2020

The Gene-Regulatory Footprint of Aging Highlights Conserved Central Regulators

Johan Auwerx, Maroun Bou Sleiman, Richardus Houtkooper, Pooja Jha, Xu Wang

Many genes and pathways have been linked to aging, yet our understanding of underlying molecular mechanisms is still lacking. Here, we measure changes in the transcriptome, histone modifications, and DNA methylome in three metabolic tissues of adult and aged mice. Transcriptome and methylome changes dominate the liver aging footprint, whereas heart and muscle globally increase chromatin accessibility, especially in aging pathways. In mouse and human data from multiple tissues and regulatory layers, age-related transcription factor expression changes and binding site enrichment converge on putative aging modulators, including ZIC1, CXXC1, HMGA1, MECP2, SREBF1, SREBF2, ETS2, ZBTB7A, and ZNF518B. Using Mendelian randomization, we establish possible epidemiological links between expression of some of these transcription factors or their targets, including CXXC1, ZNF518B, and BBC3, and longevity. We conclude that conserved modulators are at the core of the molecular footprint of aging, and variation in tissue-specific expression of some may affect human longevity.

2020

Toward a mechanistic understanding of variable chromatin modules

Gerard Llimos Aubach

Our genome is a long sequence of DNA that contains all the information to be able to constitute a living organism like us, similarly to what the letters in a book do to create a story. This sequence, which is a stretch of molecules called nucleotides, is almost identical between organisms of the same species (>99.9% in humans), however it varies in a small percentage due to some nucleotides being different at specific positions of the genome. This variation in the DNA will therefore influence our traits and affect our susceptibility to a certain condition. Thus, their study is highly relevant in genetics and biomedicine since they allow not only to predict the predisposition to a disease, such as cancer, but also to understand the molecular and cellular mechanism behind a certain phenotype. The effect of a genetic variant on a phenotype is given by their capacity to modulate the expression of a gene, either by directly impacting its sequence and thus changing the protein, or by regulating its expression levels. Interestingly, more than 88% of the disease-associated genetic variants present in humans are located outside genes, on the so-called non-coding part of the genome. This poses a challenge to identify what gene the genetic variant affects and to understand how it does so. Genetic variants do not only influence gene expression but they also affect the epigenetics state of the DNA (i.e. DNA methylation, histone marks, transcription factor (TF) binding...) and its 3D conformation, consequently modifying the activity of gene regulatory elements like promoters and enhancers. Previously, it has been shown that some of these regulatory regions located in the same locus exhibit a high level of molecular coordination and interindividual variation, mostly affected by genetic variation or environmental effects. These regions were termed variable chromatin modules (VCMs) or cis-regulatory domains (CRDs) and are thought to be the manifestation of fine-grained regulatory units of the genome. Understanding how a VCM is formed and the molecular basis behind the cooperativity between regulatory elements is an outstanding question in the field. In this work, we aim at resolving part of this puzzle by mechanistically dissecting one VCM that is influenced by a non-coding germline variant in B cells. Using a set of tools such as genetic engineering, TF binding assays and chromosome conformation studies, we demonstrate that this variant creates a de novo binding site for the TF MEF2, which acts as a pioneering factor for dozens of other collaborating TFs. In turn, this actuates a cluster of enhancers spanning a region over 150 kilobases (kb) that regulates AXIN2 gene expression, and is associated with a global compaction of the chromatin. In addition, in line with the known tumor-suppressor properties of AXIN2, we found that this variant influences chronic lymphocytic leukemia (CLL) progression and predisposition. Together, the characterization of the AXIN2 VCM provides an unique example to the community of how a germline variant is able to switch on an entire genomic locus with high degree of cooperativity between regulatory elements, since it captures both the formation and transition behavior of a VCM. Furthermore, given the link with CLL, this focal variant could have translational potential by serving as a patient-stratifying or treatment diagnostic.