Loss of 5‐hydroxymethylcytosine (5hmC) has been associated with mutations of the ten–eleven translocation (TET) enzymes in several types of cancer. However, tumors with wild‐type TET genes can also display low 5hmC levels, suggesting that other mechanisms involved in gene regulation might be implicated in the decline of this epigenetic mark. Here we show that DNA hypermethylation and loss of DNA hydroxymethylation, as well as a marked reduction of activating histone marks in the TET3 gene, impair TET3 expression and lead to a genome‐wide reduction in 5hmC levels in glioma samples and cancer cell lines. Epigenetic drugs increased expression of TET3 in glioblastoma cells and ectopic overexpression of TET3 impaired in vitro cell growth and markedly reduced tumor formation in immunodeficient mice models. TET3 overexpression partially restored the genome‐wide patterns of 5hmC characteristic of control brain samples in glioblastoma cell lines, while elevated TET3 mRNA levels were correlated with better prognosis in glioma samples. Our results suggest that epigenetic repression of TET3 might promote glioblastoma tumorigenesis through the genome‐wide alteration of 5hmC.

Background: GATA1 is an essential transcription factor for both polyploidization and megakaryocyte (MK) differentiation. The polyploidization defect observed in GATA1 variant carriers is not well understood.Objective: To extensively phenotype two pedigrees displaying different variants in the GATA1 gene and determine if GATA1 controls MYH10 expression levels, a key modulator of MK polyploidization.Method: A total of 146 unrelated propositi with constitutional thrombocytopenia were screened on a multigene panel. We described the genotype-phenotype correlation in GATA1 variant carriers and investigated the effect of these novel variants on MYH10 transcription using luciferase constructs. Results:The clinical profile associated with the p.L268M variant localized in the C terminal zinc finger was unusual in that the patient displayed bleeding and severe platelet aggregation defects without early-onset thrombocytopenia. p.N206I localized in the N terminal zinc finger was associated, on the other hand, with severe thrombocytopenia (15G/L) in early life. High MYH10 levels were evidenced in platelets of GATA1 variant carriers. Analysis of MKs anti-GATA1 chromatin immunoprecipitation-sequencing data revealed two GATA1 binding sites, located in the 3′ untranslated region and in intron 8 of the MYH10 gene. Luciferase reporter assays showed their respective role in the regulation of MYH10 gene expression. Both GATA1 variants significantly alter intron 8 driven MYH10 transcription. Conclusion:The discovery of an association between MYH10 and GATA1 is a novel one. Overall, this study suggests that impaired MYH10 silencing via an intronic regulatory element is the most likely cause of GATA1-related polyploidization defect.

Expansion of human megakaryoblasts from peripheral blood-derived CD34+ cells is commonly used to characterize inherited or acquired thrombocytopenia and evaluate defects in megakaryocyte (MK) differentiation, MK maturation and proplatelet formation. We applied single-cell RNA sequencing to understand local gene expression changes during megakaryopoiesis (days 6 and 11 of differentiation) in peripheral CD34+ cells from healthy controls and patients with ETV6-related thrombocytopenia. Analysis of gene expression and regulon activity revealed distinct clusters partitioned into seven major cell stages: hematopoietic stem/progenitor cells (HSPC), common-myeloid progenitors (CMP), MK-primed CMP, granulocyte-monocyte progenitors, megakaryocyte-erythroid progenitors (MEP), MK progenitor /mature MK (MKP/MK) and platelets. We observed a subpopulation of MEP that arose directly from HSPC, deviating from the canonical MK differentiation pathway. ETV6 deficiency was characterized by an increase in HSPC, a decrease in MKP/MK, and a lack of platelets. ETV6 deficiency also led to the development of aberrant MEP and MKP/MK cell populations. Genes involved in mitochondrial and DNA repair pathways were downregulated, while genes involved in translation pathways were upregulated. Analysis of patient samples and hematopoietic cell lines transduced with an ETV6 variant revealed increased translation in MK. Ribosomal protein small 6 (RPS6) levels in MK, platelets and peripheral blood mononuclear cells was consistent with the translation findings. Our results provide a framework to understand peripheral CD34+ cell-derived megakaryocytic cultures. Our observations also shed light on ETV6-variant pathology and reveal potential targets for diagnostic and therapeutic purposes.