Biology subjects

Toufektchan, E.

Publications and source records attributed to Toufektchan, E..

6 recordsLinked to original sources

bioRxiv2026

Squamous-state excursions activate APOBEC3A in cancer

The cytidine deaminase APOBEC3A is a major endogenous mutagen in human cancer, yet is rarely captured in bulk tumor RNA or protein profiles despite the prominent mutational scars it leaves in cancer genomes. The origin of these episodic mutational bursts has remained unclear, with prevailing models emphasizing sustained inflammatory signaling. Here, we show that APOBEC3A is induced in a rare subpopulation of cancer cells engaging a transient squamous differentiation program, linking APOBEC3A mutagenesis to lineage-state plasticity rather than persistent inflammatory signaling. Across breast and lung cancer cell lines and patient tumors, keratinocyte differentiation markers, including the stress keratins KRT6A and KRT16, are the strongest correlates of endogenous APOBEC3A expression. These days-long squamous-state excursions explain how APOBEC3A can leave durable mutational scars while remaining largely invisible to bulk tumor RNA and protein profiling. APOBEC3A catalytic activity reinforces selected components of this program through uracil excision and JNK-AP-1 signaling. The squamous differentiation transcription factor ZNF750 promotes APOBEC3A induction during squamous-state engagement in breast and lung cancer models. In established human squamous tumors, however, ZNF750 loss-of-function is associated with elevated APOBEC3A expression and APOBEC mutagenesis, revealing lineage-context-dependent regulation. These findings identify a transient differentiation state as a mutagenic intermediate, coupling cell-state plasticity to cancer genome evolution.

cancer biology↗

bioRxiv2025

ER-tethering directs TREX1 penetration of a BAF-dependent barrier at micronuclei

Micronuclei are membrane-encapsulated nuclear aberrations that form following chromosome segregation errors. Micronuclear membrane collapse permits access of the pattern recognition receptor cGAS and its antagonist, the TREX1 exonuclease. TREX1 carboxy-terminal domain mediated endoplasmic reticulum tethering association is essential for invasion into ruptured micronuclei, however the mechanisms underlying this dependency are unknown. Here, we identify barrier-to-autointegration nuclear assembly factor 1 (BAF) as a key regulator of TREX1 activity at micronuclei. BAF accumulates on micronuclei following membrane collapse and augments TREX1 recruitment in a manner that depends on BAF interactions with membrane-associated LEM-domain proteins. Despite delayed entry, TREX1 exhibits enhanced micronuclear DNA degradation and independence from ER-tethering in BAF-deficient cells. In accordance, recombinant BAF protein inhibits TREX1-mediated DNA degradation in vitro in a manner that depends on BAF DNA-binding. BAF similarly outcompetes cGAS for micronuclear DNA interaction and reduces cGAS activation at micronuclei. These findings reveal, a BAF-dependent protective barrier to diffusive entry of DNA binding proteins at ruptured micronuclei, explaining the requirement of TREX1 ER-tethering for micronuclear localization and suppression of productive cGAS DNA substrate interactions that activate innate immune responses in chromosomally unstable cells.

molecular biology↗

bioRxiv2025

Mitotic transcription ensures ecDNA inheritance through chromosomal tethering

Extrachromosomal DNA (ecDNA) are circular DNA bodies that play critical roles in tumor progression and treatment resistance by amplifying oncogenes across a wide range of cancer types. ecDNA lack centromeres and are thus not constrained by typical Mendelian segregation, enabling their unequal accumulation within daughter cells and associated increases in copy number. Despite intrinsic links to their oncogenic potential, the fidelity and mechanisms of ecDNA inheritance are poorly understood. Here, we show that ecDNA are protected against cytosolic mis-segregation through mitotic clustering and by tethering to the telomeric and subtelomeric regions of mitotic chromosomes. ecDNA-chromosome tethering depends on BRD4 transcriptional co-activation and mitotic transcription of the long non-coding RNA PVT1, which is co-amplified with MYC in colorectal and prostate cancer cell lines. Disruption of ecDNA-chromosome tethering through BRD4 inhibition, PVT1 depletion, or inhibiting mitotic transcription results in cytosolic mis-segregation, ecDNA reintegration, and the formation of homogeneously staining regions (HSRs). We propose that nuclear inheritance of ecDNA is facilitated by an RNA-mediated physical tether that links ecDNA to mitotic chromosomes and thus protects against cytosolic mis-segregation and chromosomal integration.

molecular biology↗

bioRxiv2022

TREX1 restricts CRISPR-Cas9 genome editing in human cells

CRISPR-Cas mediated homology-directed repair (HDR) can flexibly introduce desired mutations at targeted sites in a genome. But achieving high HDR efficiencies is a major hurdle in many cellular contexts. Moreover, cells from patients with mutations in DNA repair factors can exhibit low CRISPR-Cas-mediated HDR, complicating genome editing as a potential treatment. We used genome-wide screening in Fanconi anemia (FA) patient lymphoblastic cell lines to uncover suppressors of CRISPR-Cas mediated HDR. Surprisingly, we found that a single exonuclease called TREX1 is an important determinant of HDR efficiency when single-stranded templates are used as a repair template. TREX1 expression acts as a biomarker for CRISPR-Cas mediated HDR, such that cell lines expressing high levels of TREX1 have poor HDR that can be rescued by TREX1 removal. CRISPR-Cas mediated HDR can also be rescued by using single-stranded DNA templates that are chemically protected in a manner consistent with TREX1s exonucleolytic activity. Overall, our data provide a mechanistic explanation for why some cells are easier to edit than others and suggest a route to increase CRISPR-Cas mediated HDR in TREX-expressing context.

molecular biology↗

bioRxiv2022

Epigenetic dysregulation from chromosomal transit in micronuclei

Chromosomal instability (CIN) and epigenetic alterations are characteristics of advanced and metastatic cancers [1-4], yet whether they are mechanistically linked is unknown. Here we show that missegregation of mitotic chromosomes, their sequestration in micronuclei [5, 6], and subsequent micronuclear envelope rupture [7] profoundly disrupt normal histone post-translational modifications (PTMs), a phenomenon conserved across humans and mice as well as cancer and non-transformed cells. Some of the changes to histone PTMs occur due to micronuclear envelope rupture whereas others are inherited from mitotic abnormalities prior to micronucleus formation. Using orthogonal techniques, we show that micronuclei exhibit extensive differences in chromatin accessibility with a strong positional bias between promoters and distal or intergenic regions. Finally, we show that inducing CIN engenders widespread epigenetic dysregulation and that chromosomes which transit in micronuclei experience durable abnormalities in their accessibility long after they have been reincorporated into the primary nucleus. Thus, in addition to genomic copy number alterations, CIN can serve as a vehicle for epigenetic reprogramming and heterogeneity in cancer.

cancer biology↗

bioRxiv2020

ER-directed TREX1 limits cGAS recognition of micronuclei

Chromosomal instability in cancer results in the formation of nuclear aberrations termed micronuclei. Spontaneous loss of micronuclear envelope integrity exposes DNA to the cytoplasm, leading to chromosome fragmentation and innate immune activation. Despite connections to cancer genome evolution and anti-tumor immunity, the mechanisms underlying damage and immune sensing of micronuclear DNA are poorly understood. Here, we use a novel method for the purification of micronuclei and live-cell imaging to show that the ER-associated nuclease TREX1 inhibits cGAS sensing of micronuclei by stably associating with and degrading micronuclear DNA upon micronuclear envelope rupture. We identify a TREX1 mutation, previously associated with autoimmune disease, that untethers TREX1 from the ER, disrupts TREX1 localization to micronuclei, alleviates micronuclear DNA damage, and enhances cGAS recognition of micronuclei. Together, these results establish ER-directed resection of micronuclear DNA by TREX1 as a critical regulator of cytosolic DNA sensing in chromosomally unstable cells and provide a mechanistic basis for the importance of TREX1 ER-tethering in preventing autoimmunity.

molecular biology↗