Biology subjects

Ansari, A. Z.

Publications and source records attributed to Ansari, A. Z..

4 recordsLinked to original sources

bioRxiv2026

BRD4 recruitment desilences transcription without erasure or depletion of repressive chromatin

How genes are desilenced within mesoscale repressive chromatin is a crucial yet poorly understood phenomenon. Prevailing models posit that methylation of lysine-9 of histone H3 (H3K9me3) engages heterochromatin protein 1 (HP1) to drive chromatin compaction and transcriptional silencing. The erasure of this repressive mark and its replacement with acetyl/acyl groups recruits positive factors such as BRD4/BET to elicit gene transcription. We report that in Friedreichs ataxia, a synthetic gene regulator (SynGR1) drives transcription across repressive chromatin without removal or replacement of H3K9me3 or HP1. By selectively recruiting BRD4/BET into repressive GAA-repeats in frataxin (FXN), SynGR1 creates a paradoxical state where gene transcription and repressive chromatin co-exist. Contrary to convention, we find that BRD4 readily partitions into phase separated HP1 condensates in vitro and into HP1 puncta in patient-derived cells, thus presenting a mechanistic explanation for desilencing transcription without the dispersal of mesoscale repressive chromatin. Epigenetic drugs that gate sequential steps in transcription, synergistically stimulate FXN expression while concomitantly increasing, rather than eliminating, repressive H3K9me3 and HP1 levels. More broadly, this study highlights the dynamic nature of repressive chromatin and the context-dependence of epigenetic marks in regulating gene expression.

cell biology↗

bioRxiv2024

Reconciling competing models on the roles of condensates and soluble complexes in transcription factor function

Phase separation explains the exquisite spatial and temporal regulation of many biological processes, but the role of transcription factor-mediated condensates in gene regulation is contentious, requiring head-to-head comparison of competing models. Here, we focused on the prototypical yeast transcription factor Gcn4 and assessed two models for gene transcription activation, i.e., mediated via soluble complexes or transcriptional condensates. Both models rely on the ability of transcription factors and coactivators to engage in multivalent interactions. Unexpectedly, we found that propensity to form homotypic Gcn4 condensates does not correlate well with transcriptional activity. Contrary to prevailing models, binding to DNA suppresses Gcn4 phase separation. Notably, the ability of Gcn4 to form soluble complexes with coactivator subunit Med15 closely mirrored the propensity to recruit Med15 into condensates, indicating that these properties are intertwined and cautioning against interpretation of mutational data without head-to-head comparisons. However, Gcn4 variants with the highest affinity for Med15 do not function as well as expected and instead have activities that reflect their abilities to phase separate with Med15. These variants therefore indeed form cellular condensates, and those attenuate activity. Our results show that transcription factors can function as soluble complexes as well as condensates, reconciling two seemingly opposing models, and have implications for other phase-separating systems. Highlights- Homotypic phase separation propensities of Gcn4 variants do not predict in vivo activities well. - DNA binding leads to solubilization of GCN4 condensates, which is reversed by Med15 association. - The abilities to co-phase separate and form soluble complexes with Med15 are highly intertwined. - Variants with high affinities for Med15 form condensates that attenuate function.

biophysics↗

bioRxiv2022

Linking Dynamic DNA Secondary Structures to Genome Instability

Genomic double-stranded DNA (dsDNA) becomes single-stranded (ssDNA) during replication, transcription, and DNA repair. ssDNA is therefore believed to be transient, occurring in only a fraction of the genome at a given time, and variable amongst a population of cells. These transiently formed ssDNA segments can also adopt alternative, dynamic DNA conformations, such as cruciform DNA, triplexes, quadruplexes and others. To determine whether there are stable and conserved regions of ssDNA, we utilized our previously developed method S1-END-seq 1 to convert ssDNA to DNA double strand breaks (DSBs), which are then processed for high-throughput sequencing. This approach revealed two predominant dynamic DNA structures: cruciform DNA formed by expanded (TA)n repeats that accumulated uniquely in microsatellite unstable human cancer cell lines, and DNA triplexes (H-DNA) formed by homopurine/homopyrimidine (hPu/hPy) mirror repeats common across a variety of human cell lines. Triplex-forming repeats accumulated during replication, blocked DNA synthesis and were hotspots of somatic mutation. In contrast, pathologically expanded (hPu/hPy) repeats in Friedreichs ataxia patient cells formed a replication-independent and transcription-inducible DNA secondary structure. Our results identify dynamic DNA secondary structures in vivo that contribute to elevated genome instability.

molecular biology↗

bioRxiv2020

Single position substitution of hairpin pyrrole-imidazole polyamides imparts distinct DNA-binding profiles across the human genome

Regulating desired loci in the genome with sequence-specific DNA-binding molecules is a major goal for the development of precision medicine. Pyrrole-imidazole (Py-Im) polyamides are synthetic molecules that can be rationally designed to target specific DNA sequences to both disrupt and recruit transcriptional machinery. While in vitro binding has been extensively studied, in vivo effects are often difficult to predict using current models of DNA binding. Determining the impact of genomic architecture and the local chromatin landscape on polyamide-DNA sequence specificity remains an unresolved question that impedes their effective deployment in vivo. In this report we identified polyamide-DNA interaction sites across the entire genome, by covalently crosslinking and capturing these events in the nuclei of human LNCaP cells. This method, termed COSMIC-seq, confirms the ability of hairpin-polyamides, with similar architectures but differing at a single ring position, to retain in vitro specificities and display distinct genome-wide binding profiles. These results underpin the development of Py-Im polyamides as DNA-targeting molecules that mediate their regulatory or remedial functions at desired genomic loci.

biochemistry↗