Biology subjects

Vassiliou, G.

Publications and source records attributed to Vassiliou, G..

6 recordsLinked to original sources

bioRxiv2025

Delayed Trp53 activation protects Dnmt3a-mutant hematopoietic stem cells from inflammatory attrition.

Hematopoietic stem cells (HSCs) accumulate somatic mutations over time, some conferring a fitness advantage that can lead to clonal hematopoiesis (CH). Mutations in DNMT3A, particularly at hotspot R882, are the most prevalent in CH and carry an increased risk of acute myeloid leukemia (AML). Although DNMT3A R882 mutations are linked to global DNA hypomethylation, the mechanisms underlying their selective advantage remain unclear. Here, we show that Dnmt3a-R882H mutant HSCs exhibit resilience under inflammatory and genotoxic stress. During IL-1{beta}-induced emergency granulopoiesis, Dnmt3a R882H/+ HSCs uncouple increased proliferation from stem cell exhaustion. In contrast, wild-type HSCs rapidly progress to terminal differentiation. We link this phenotype to a delayed activation of the p53-p21-DREAM axis, that allows mutant HSCs to avoid attrition, despite increased replication stress. Similarly, mutant HSCs exhibit delayed Trp53 activation following irradiation, but eventually recover a physiological Trp53 response. Analysis of patient data reveals shared phenotypic features between DNMT3A and monoallelic TP53 mutations in CH and myeloid neoplasms, highlighting potential functional similarities. Collectively, these findings suggest that the expansion of DNMT3A-mutant clones is affected by impaired TP53 signaling, which confers resilience against stressors. Therapeutic strategies targeting inflammatory pathways or the p53-p21-DREAM axis may reduce DNMT3A-CH expansion and/or progression and its associated risks.

molecular biology↗

bioRxiv2024

Maintenance of haematopoietic stem cells by JAK inhibition and increased tyrosine-unphosphorylated STAT5

Normal and malignant hematopoietic stem cells (HSCs) are controlled by extracellular cues including cytokine signalling through the JAK/STAT pathway. Here, we show that STAT5-deficient HSCs exhibit an unusual phenotype: while reduced multi-lineage repopulation and reduced self-renewal are commonly associated with overproliferation and exhaustion, they are instead associated with reduced cell-cycle progression and increased differentiation in STAT5-deficient HSCs. Mechanistic studies show that unphosphorylated-STAT5 (uSTAT5) contributes to this phenotype by constraining HSC differentiation, promoting HSC maintenance and upregulating transcriptional programs associated with stemness. The JAK1/2 inhibitor ruxolitinib increases levels of uSTAT5, constrains differentiation and proliferation of murine HSCs, promotes their maintenance and upregulates transcriptional programs associated with stemness. Ruxolitinib also enhances clonogenicity of normal human HSPCs, CALR-mutant murine HSCs and HSPCs from patients with myelofibrosis. Our results therefore reveal a previously unrecognized role for uSTAT5 in controlling HSC function, highlight JAK inhibition as a strategy for enhancing HSC function and provide insights into the failure of JAK inhibitors to eradicate myeloproliferative neoplasms.

cell biology↗

bioRxiv2022

HOXA9 forms a repressive complex with nuclear matrix-associated protein SAFB to maintain acute myeloid leukemia

HOXA9 is commonly upregulated in acute myeloid leukemia (AML), where it confers poor prognosis. Characterising the protein interactome of endogenous HOXA9 in human AML, we identified a chromatin complex of HOXA9 with the nuclear matrix attachment protein-SAFB. SAFB perturbation phenocopied HOXA9 knockout to decrease AML proliferation, increase differentiation and apoptosis in vitro and prolonged survival in vivo. Integrated genomic, transcriptomic and proteomic analyses further demonstrated that the HOXA9-SAFB-chromatin complex associates with NuRD and HP1{gamma} to repress the expression of factors associated with differentiation and apoptosis, including NOTCH, CEBP{delta}, S100A8, and CDKN1A. Chemical or genetic perturbation of NuRD and HP1{gamma} catalytic activity also triggered differentiation, apoptosis and the induction of these tumor-suppressive genes. Importantly, this mechanism is operative in other HOXA9-dependent AML genotypes. This mechanistic insight demonstrates active HOXA9-dependent differentiation block as a potent mechanism of disease maintenance in AML, that may be amenable to therapeutic intervention via therapies targeting the HOXA9/SAFB interface and/or NuRD and HP1{gamma} activity. Key Points- Identification of the endogenous human HOXA9 protein interactome in AML - HOXA9 forms a repressive complex with S/MAR binding protein (SAFB) that is critical for the maintenance of AML by facilitating proliferation and preventing differentiation and cell death. - The HOXA9/SAFB (H9SB) complex represses gene expression via recruitment of NuRD and HP1{gamma}.

cancer biology↗

bioRxiv2021

Cancer tolerance to chromosomal instability is driven by Stat1 inactivation in vivo

Chromosomal instability is a hallmark of cancer, but also an instigator of aneuploidy-induced stress, reducing cellular fitness. To better understand how cells with CIN adjust to aneuploidy and adopt a malignant fate in vivo, we performed a genome-wide mutagenesis screen in mice. We find that specifically aneuploid tumors inactivate Stat1 signaling in combination with increased Myc activity. By contrast, loss of p53 is common, but not enriched in CIN tumors. Validation in another tissue type confirmed that CIN promotes immune cell infiltration, which is alleviated by Stat1 loss combined with Myc activation, but not with p53 inactivation, or Myc activation alone. Importantly, we find that this mechanism is preserved in human aneuploid cancers. We conclude that aneuploid cancers inactivate Stat1 signaling to circumvent immune surveillance.

cancer biology↗

bioRxiv2021

Clonal dynamics of haematopoiesis across the human lifespan

Age-related change in human haematopoiesis causes reduced regenerative capacity1, cytopenias2, immune dysfunction3 and increased risk of blood cancer. The cellular alterations that underpin the abruptness of this functional decline after the age of 70 years remain elusive. We sequenced 3579 genomes from single-cell-derived colonies of haematopoietic stem cell/multipotent progenitors (HSC/MPPs) across 10 haematologically normal subjects aged 0-81 years. HSC/MPPs accumulated 17 mutations/year after birth and lost 30bp/year of telomere length. Haematopoiesis in adults aged <65 was massively polyclonal, with high indices of clonal diversity and a stable population of 20,000-200,000 HSC/MPPs contributing evenly to blood production. In contrast, haematopoiesis in individuals aged >75 showed profoundly decreased clonal diversity. In each elderly subject, 30-60% of haematopoiesis was accounted for by 12-18 independent clones, each contributing 1-34% of blood production. Most clones had begun their expansion before age 40, but only 22% had known driver mutations. Genome-wide selection analysis estimated that 1/34 to 1/12 non-synonymous mutations were drivers, occurring at a constant rate throughout life, affecting a wider pool of genes than identified in blood cancers. Loss of Y chromosome conferred selective benefits on HSC/MPPs in males. Simulations from a simple model of haematopoiesis, with constant HSC population size and constant acquisition of driver mutations conferring moderate fitness benefits, entirely explained the abrupt change in clonal structure in the elderly. Rapidly decreasing clonal diversity is a universal feature of haematopoiesis in aged humans, underpinned by pervasive positive selection acting on many more genes than currently identified.

genomics↗

bioRxiv2019

Genetic modification of primary human B cells generates translationally-relevant models of high-grade lymphoma

Sequencing studies of Diffuse Large B Cell Lymphoma (DLBCL) have identified hundreds of recurrently altered genes. However, it remains largely unknown whether and how these mutations may contribute to lymphomagenesis, either individually or in combination. Existing strategies to address this problem predominantly utilize cell lines, which are limited by their initial characteristics and subsequent adaptions to prolonged in vitro culture. Here, we describe a novel co-culture system that enables the ex vivo expansion and viral transduction of primary human germinal center B cells. The incorporation of CRISPR/Cas9 technology enables high-throughput functional interrogation of genes recurrently mutated in DLBCL. Using a backbone of BCL2 with either BCL6 or MYC we have identified co-operating oncogenes that promote growth and survival, or even full transformation into synthetically engineered models of DLBCL. The resulting tumors can be expanded and sequentially transplanted in vivo, providing a scalable platform to test putative cancer genes and for the creation of mutation-directed, bespoke lymphoma models.

cancer biology↗