The Mutational Landscape of Myeloid Leukaemia in Down Syndrome

Children with Down syndrome (DS) are particularly prone to haematopoietic disorders. Paediatric myeloid malignancies in DS occur at an unusually high frequency and generally follow a well-defined stepwise clinical evolution. First, the acquisition of mutations in the GATA1 transcription factor gives rise to a transient myeloproliferative disorder (TMD) in DS newborns. While this condition spontaneously resolves in most cases, some clones can acquire additional mutations, which trigger myeloid leukaemia of Down syndrome (ML-DS). These secondary mutations are predominantly found in chromatin and epigenetic regulators—such as cohesin, CTCF or EZH2—and in signalling mediators of the JAK/STAT and RAS pathways. Most of them are also found in non-DS myeloid malignancies, albeit at extremely different frequencies. Intriguingly, mutations in proteins involved in the three-dimensional organization of the genome are found in nearly 50% of cases. How the resulting mutant proteins cooperate with trisomy 21 and mutant GATA1 to promote ML-DS is not fully understood. In this review, we summarize and discuss current knowledge about the sequential acquisition of genomic alterations in ML-DS.

Functional profiling of single CRISPR/Cas9-edited human long-term hematopoietic stem cells

In the human hematopoietic system, rare self-renewing multipotent long-term hematopoietic stem cells (LT-HSCs) are responsible for the lifelong production of mature blood cells and are the rational target for clinical regenerative therapies. However, the heterogeneity in the hematopoietic stem cell compartment and variable outcomes of CRISPR/Cas9 editing make functional interrogation of rare LT-HSCs challenging. Here, we report high efficiency LT-HSC editing at single-cell resolution using electroporation of modified synthetic gRNAs and Cas9 protein. Targeted short isoform expression of the GATA1 transcription factor elicit distinct differentiation and proliferation effects in single highly purified LT-HSC when analyzed with functional in vitro differentiation and long-term repopulation xenotransplantation assays. Our method represents a blueprint for systematic genetic analysis of complex tissue hierarchies at single-cell resolution.

Mutations in the zebrafish hmgcs1 gene reveal a novel function for isoprenoids during red blood cell development

Erythropoiesis is the process by which new red blood cells (RBCs) are formed and defects in this process can lead to anemia or thalassemia. The GATA1 transcription factor is an established mediator of RBC development. However, the upstream mechanisms that regulate the expression of GATA1 are not completely characterized. Cholesterol is 1 potential upstream mediator of GATA1 expression because previously published studies suggest that defects in cholesterol synthesis disrupt RBC differentiation. Here we characterize RBC development in a zebrafish harboring a single missense mutation in the hmgcs1 gene (Vu57 allele). hmgcs1 encodes the first enzyme in the cholesterol synthesis pathway and mutation of hmgcs1 inhibits cholesterol synthesis. We analyzed the number of RBCs in hmgcs1 mutants and their wild-type siblings. Mutation of hmgcs1 resulted in a decrease in the number of mature RBCs, which coincides with reduced gata1a expression. We combined these experiments with pharmacological inhibition and confirmed that cholesterol and isoprenoid synthesis are essential for RBC differentiation, but that gata1a expression is isoprenoid dependent. Collectively, our results reveal 2 novel upstream regulators of RBC development and suggest that appropriate cholesterol homeostasis is critical for primitive erythropoiesis.

Functional Profiling of Single CRISPR/Cas9-Edited Human Long-Term Hematopoietic Stem Cells

In the human hematopoietic system, rare self-renewing multi-potent long-term hematopoietic stem cells (LT-HSCs) are responsible for the lifelong production of mature blood cells and are the rational target for clinical regenerative therapies. However, the heterogeneity in the hematopoietic stem cell compartment and variable outcomes of CRISPR/Cas9 editing make functional interrogation of rare LT-HSCs challenging. Here, we report high efficiency LT-HSC editing at single cell resolution using electroporation of modified synthetic gRNAs and Cas9 protein. Targeted short isoform expression of the GATA1 transcription factor elicited distinct differentiation and proliferation effects in single LT-HSC when analyzed with functional in vitro differentiation and long-term repopulation xenotransplantation assays. Our method represents a blueprint for systematic genetic analysis of complex tissue hierarchies at single cell level.

Mutations in the zebrafishhmgcs1gene reveal a novel function for isoprenoids during red blood cell development

Erythropoiesis is the process by which new red blood cells (RBCs) are formed and defects in this process can lead to anemia or thalassemia. The GATA1 transcription factor is an established mediator of RBC development. However, the upstream mechanisms that regulate the expression ofGATA1are not completely characterized. Cholesterol is one potential upstream mediator ofGATA1expression because previously published studies suggest that defects in cholesterol synthesis disrupt RBC differentiation. Here we characterize RBC development in a zebrafish harboring a single missense mutation in thehmgcs1gene (Vu57 allele).hmgcs1encodes the first enzyme in the cholesterol synthesis pathway and mutation ofhmgcs1inhibits cholesterol synthesis. We analyzed the number of RBCs inhmgcs1mutants and their wildtype siblings. Mutation ofhmgcs1resulted in a decrease in the number of mature RBCs, which coincides with reducedgata1aexpression. We combined these experiments with pharmacological inhibition and confirmed that cholesterol and isoprenoid synthesis are essential for RBC differentiation, but thatgata1aexpression is isoprenoid dependent. Collectively, our results reveal two novel upstream regulators of RBC development and suggest that appropriate cholesterol homeostasis is critical for primitive erythropoiesis.Key PointsThe products of the cholesterol synthesis pathway regulate red blood cell development during primitive erythropoiesis.Isoprenoids regulate erythropoiesis by modulating the expression of the GATA1 transcription factor.

Przejściowy zespół mieloproliferacyjny – praca poglądowa

Children with Down syndrome (DS) are at increased risk of hematological disorders, in particular hematopoietic neoplastic diseases. A disorder characteristic for patients with DS is transient myeloproliferative disorder (TMD) occurring in about 30% of newborns with DS. The development of TMD occurs as a result of the cooperation of two genetic disorders – trisomy 21 (T21) and mutation of the GATA1 transcription factor gene. The clinical course of TMD varies from clinically silent to symptomatic with the occurrence of life-threatening complications. Most patients have spontaneous remission within 3 months without treatment. Patients who develop life-threatening complications may require cytostatic treatment. About 10-20% of patients who develop TMD during the neonatal period in the first 4 years of life will develop acute myeloid leukemia in Down syndrome (myeloid leukemia of Down syndrome – ML-DS). Due to the high risk of developing ML-DS, patients who have been diagnosed with TMD during the neonatal period should be under close hematological care up to 4 years of age.

TAF10 Interacts with GATA1 Transcription Factor and Controls Mouse Erythropoiesis

The ordered assembly of a functional preinitiation complex (PIC), composed of general transcription factors (GTFs) is a prerequisite for the transcription of protein coding genes by RNA polymerase II. TFIID, comprised of the TATA binding protein (TBP) and 13 TBP-associated factors (TAFs), is the GTF that is thought to recognize the promoter sequences allowing site-specific PIC assembly. Transcriptional cofactors, such as SAGA (Spt-Ada-Gcn5-acetyltransferase), are also necessary to have tightly regulated transcription initiation. However, a new era on the role of the GTFs and specifically on the role of TFIID in tissue specific and promoter specific transcriptional regulation has emerged in the light of novel findings regarding the differentiation programs of different cell types1. TAF10 is a subunit of both the TFIID and the SAGA co-activator HAT complexes2. The role of TAF10 is indispensable for early embryonic transcription and mouse development as knockout (KO) embryos die early in gestation between E3.5 and E5.5, around the stage when the supply of maternal protein becomes insufficient3. However, when analyzing TFIID stability and transcription it was noted that not all cells and tissues were equally affected by the loss of TAF10. The contribution of the two TAF10-containing complexes (TFIID, SAGA) to erythropoiesis remains elusive. Ablation of TAF10 specifically in erythroid cells by crossing the TAF10-Lox with the EpoR-Cre mouse led to a differentiation block at around E13.5 with erythroid progenitor cells accumulating at a higher percentage (26% in the KO embryos vs 16% in the WTs at E12.5) at the double positive stage KIT+CD71+ and giving rise to fewer mature TER119+ cells in the fetal liver. At E13.5 embryos were dead with almost no erythroid cells in the fetal liver. Gene expression analysis of the fetal liver cells of the embryos revealed down-regulation of GATA1 expression and its target genes, bh1&bmaj/min globins and KLF1 transcription factor while expression of other genes known to have a role in mouse hematopoiesis remained unaffected (MYB, GATA2, PU.1). In order to get insight to the role of TAF10 during erythropoiesis we analyzed the composition of both TAF10-containing complexes (TFIID and SAGA) by mass spectrometry. We found that their stoichiometry changes slightly but not fundamentally during erythroid differentiation and development (human fetal liver erythroid progenitors, human blood erythroid progenitors and mouse erythroid progenitor cells) and no major rearrangements were generated in the composition of the TFIID as it was reported in other cell differentiation programs (e.g. skeletal differentiation, hepatogenesis). Additionally, we found GATA1 transcription factor only in the fetal liver and not in the adult erythroid cells in the mass spectrometry data of TAF10 immunoprecipitations (IPs), an interaction that we confirmed by reciprocal IP of TAF10 and GATA1 in MEL and mouse fetal liver cells. Most importantly, we checked whether TAF10 binding is enriched on the GATA1 locus in human erythroid cells during the fetal and the adult stage in erythroid proerythroblasts and we found that there is enriched binding of TAF10 in the palindromic GATA1 site in the fetal stage. Our results support a developmental role for TAF10 in GATA1 regulated genes, including GATA1 itself, during erythroid differentiation emphasizing the crosstalk between the transcriptional machinery and activators in erythropoiesis. References 1. Goodrich JA, Tjian R (2010) Unexpected roles for core promoter recognition factors in cell-type-specific transcription and gene regulation. Nature reviews Genetics 11: 549-558 2 .Timmers HT, Tora L (2005) SAGA unveiled. Trends Biochem Sci 30: 7-10 3. Mohan WS, Jr., Scheer E, Wendling O, Metzger D, Tora L (2003) TAF10 (TAF(II)30) is necessary for TFIID stability and early embryogenesis in mice. Mol Cell Biol 23: 4307-4318 Disclosures No relevant conflicts of interest to declare.