GATA2 hypomorphism induces chronic myelomonocytic leukemia in mice

The spectrum of GATA2 deficiency syndrome

Inherited or de novo germ line heterozygous mutations in the gene encoding the transcription factor GATA2 lead to its deficiency. This results in a constellation of clinical features including nontuberculous mycobacterial, bacterial, fungal, and human papillomavirus infections, lymphedema, pulmonary alveolar proteinosis, and myelodysplasia. The onset, or even the presence, of disease is highly variable, even in kindreds with the identical mutation in GATA2. The clinical manifestations result from the loss of a multilineage progenitor that gives rise to B lymphocytes, monocytes, natural killer cells, and dendritic cells, leading to cytopenias of these lineages and subsequent infections. The bone marrow failure is typically characterized by hypocellularity. Dysplasia may either be absent or subtle but typically evolves into multilineage dysplasia with prominent dysmegakaryopoiesis, followed in some instances by progression to myeloid malignancies, specifically myelodysplastic syndrome, acute myelogenous leukemia, and chronic myelomonocytic leukemia. The latter 3 malignancies often occur in the setting of monosomy 7, trisomy 8, and acquired mutations in ASXL1 or in STAG2. Importantly, myeloid malignancy may represent the primary presentation of disease without recognition of other syndromic features. Allogeneic hematopoietic stem cell transplantation (HSCT) results in reversal of the phenotype. There remain important unanswered questions in GATA2 deficiency, including the following: (1) Why do some family members remain asymptomatic despite harboring deleterious mutations in GATA2? (2) What are the genetic changes that lead to myeloid progression? (3) What causes the apparent genetic anticipation? (4) What is the role of preemptive HSCT?

GATA 2 Deficiency: Focus on Immune System Impairment

GATA2 deficiency is a disease with a broad spectrum of clinical presentation, ranging from lymphedema, deafness, pulmonary dysfunction to miscarriage and urogenital anomalies, but it is mainly recognized as an immune system and bone marrow disorder. It is caused by various heterozygous mutations in the GATA2 gene, encoding for a zinc finger transcription factor with a key role for the development and maintenance of a pool of hematopoietic stem cells; notably, most of these mutations arise de novo. Patients carrying a mutated allele usually develop a loss of some cell populations, such as B-cell, dendritic cell, natural killer cell, and monocytes, and are predisposed to disseminated human papilloma virus and mycobacterial infections. Also, these patients have a predisposition to myeloid neoplasms, including myelodysplastic syndromes, myeloproliferative neoplasms, chronic myelomonocytic leukaemia. The age of symptoms onset can vary greatly even also within the same family, ranging from early childhood to late adulthood; incidence increases by age and most frequently clinical presentation is between the second and third decade of life. Currently, haematopoietic stem cell transplantation represents the only curative treatment, restoring both the hematopoietic and immune system function.

Heterozygous variants in GATA2 contribute to DCML deficiency in mice by disrupting tandem protein binding

Accumulating lines of clinical evidence support the emerging hypothesis that loss-of-function mutations of GATA2 cause inherited hematopoietic diseases, including Emberger syndrome; dendritic cell, monocyte B and NK lymphoid (DCML) deficiency; and MonoMAC syndrome. Here, we show that mice heterozygous for an arginine-to-tryptophan substitution mutation in GATA2 (G2^R398W/+), which was found in a patient with DCML deficiency, substantially phenocopy human DCML deficiency. Mice heterozygous for the GATA2-null mutation (G2^-/+) do not show such phenotypes. The G2^R398W protein possesses a decreased DNA-binding affinity but obstructs the function of coexpressed wild-type GATA2 through specific cis-regulatory regions, which contain two GATA motifs in direct-repeat arrangements. In contrast, G2^R398W is innocuous in mice containing single GATA motifs. We conclude that the dominant-negative effect of mutant GATA2 on wild-type GATA2 through specific enhancer/silencer of GATA2 target genes perturbs the GATA2 transcriptional network, leading to the development of the DCML-like phenotype. The present mouse model provides an avenue for the understanding of molecular mechanisms underlying the pathogenesis of GATA2-related hematopoietic diseases.

Gata2 haploinsufficiency promotes proliferation and functional decline of hematopoietic stem cells with myeloid bias during aging

During aging, hematopoietic stem cell (HSC) function wanes with important biological and clinical implications for benign and malignant hematology, and other comorbidities, such as cardiovascular disease. However, the molecular mechanisms regulating HSC aging remain incompletely defined. GATA2 haploinsufficiency driven clinical syndromes initially result in primary immunodeficiencies and routinely evolve into hematologic malignancies on acquisition of further epigenetic mutations in both young and older patients. Using a conditional mouse model of Gata2 haploinsufficiency, we discover that during aging Gata2 promotes HSC proliferation, monocytosis, and loss of the common lymphoid progenitor. Aging of Gata2 haploinsufficient mice also offsets enhanced HSC apoptosis and decreased granulocyte-macrophage progenitor number normally observed in young Gata2 haploinsufficient mice. Transplantation of elderly Gata2 haploinsufficient HSCs impairs HSC function with evidence of myeloid bias. Our data demonstrate that Gata2 regulates HSC aging and suggest the mechanisms by which Gata2 mediated HSC aging has an impact on the evolution of malignancies in GATA2 haploinsufficiency syndromes.

IMiDs uniquely synergize with TKIs to upregulate apoptosis of Philadelphia chromosome-positive acute lymphoblastic leukemia cells expressing a dominant-negative IKZF1 isoform

The long-term prognosis of Philadelphia chromosome-positive acute lymphoblastic leukemia (Ph + ALL) is still unsatisfactory even after the emergence of tyrosine kinase inhibitors (TKIs) against chimeric BCR-ABL, and this is associated with the high incidence of genetic alterations of Ikaros family zinc finger 1 (IKZF1), most frequently the hemi-allelic loss of exons 4–7 expressing a dominant-negative isoform Ik6. We found that lenalidomide (LEN), a representative of immunomodulatory drugs (IMiDs), which have been long used for the treatment of multiple myeloma, specifically induced accumulation of Ik6 with the disappearance of functional isoforms within 24 h (i.e., abrupt and complete shut-down of the IKZF1 activity) in Ik6-positive Ph+ALL cells in a neddylation-dependent manner. The functional IKZF3 isoforms expression was also abruptly and markedly downregulated. The LEN treatment specifically suppressed proliferation of Ik6-positive-Ph+ALL cells by inducing cell cycle arrest via downregulation of cyclins D3 and E and CDK2, and of importance, markedly upregulated their apoptosis in synergy with the TKI imatinib (IM). Apoptosis of IM-resistant Ph+ALL cells with T315I mutation of BCR-ABL was also upregulated by LEN in the presence of the newly developed TKI ponatinib. Analyses of flow cytometry, western blot, and oligonucleotide array revealed that apoptosis was caspase-/p53-dependent and associated with upregulation of pro-apoptotic Bax/Bim, enhanced dephosphorylation of BCR-ABL/Akt, and downregulation of oncogenic helicase genes HILLS, CDC6, and MCMs4 and 8. Further, the synergism of LEN with IM was clearly documented as a significant prolongation of survival in the xenograft mice model. Because this synergism was further potentiated in vitro by dexamethasone, a key drug for ALL treatment, the strategy of repositioning IMiDs for the treatment of Ik6-positive Ph+ALL patients certainly shed new light on an outpatient-based treatment option for achieving their long-term durable remission and higher QOL, particularly for those who are not tolerable to intensified therapeutic approaches.

Genetic complexity of chronic myelomonocytic leukemia

In recent years CMML has received increased attention as the most commonly observed MDS/MPN overlap syndrome. Renewed interest has occurred in part due to widespread adoption of next-generation sequencing panels that help render the diagnosis in the absence of morphologic dysplasia. Although most CMML patients exhibit somatic mutations in epigenetic modifiers, spliceosome components, transcription factors and signal transduction genes, it is increasingly clear that a small subset harbors an inherited predisposition to CMML and other myeloid neoplasms. More intriguing is the fact that the mutational spectrum observed in CMML is found in other types of myeloid leukemias, begging the question of how similar genetic backgrounds can lead to such divergent clinical phenotypes. In this review we present a contemporary snapshot of the genetic complexity inherent to CMML, explore the relationship between genotype-phenotype and present a stepwise model of CMML pathogenesis and progression.

GATA2 +9.5 enhancer: from principles of hematopoiesis to genetic diagnosis in precision medicine

PURPOSE OF REVIEW

By establishing mechanisms that deliver oxygen to sustain cells and tissues, fight life-threatening pathogens and harness the immune system to eradicate cancer cells, hematopoietic stem and progenitor cells (HSPCs) are vital in health and disease. The cell biological framework for HSPC generation has been rigorously developed, yet recent single-cell transcriptomic analyses have unveiled permutations of the hematopoietic hierarchy that differ considerably from the traditional roadmap. Deploying mutants that disrupt specific steps in hematopoiesis constitutes a powerful strategy for deconvoluting the complex cell biology. It is striking that a single transcription factor, GATA2, is so crucial for HSPC generation and function, and therefore it is instructive to consider mechanisms governing GATA2 expression and activity. The present review focuses on an essential GATA2 enhancer (+9.5) and how +9.5 mutants inform basic and clinical/translational science.

RECENT FINDINGS

+9.5 is essential for HSPC generation and function during development and hematopoietic regeneration. Human +9.5 mutations cause immunodeficiency, myelodysplastic syndrome, and acute myeloid leukemia. Qualitatively and quantitatively distinct contributions of +9.5 cis-regulatory elements confer context-dependent enhancer activity. The discovery of +9.5 and its mutant alleles spawned fundamental insights into hematopoiesis, and given its role to suppress blood disease emergence, clinical centers test for mutations in this sequence to diagnose the cause of enigmatic cytopenias.

SUMMARY

Multidisciplinary approaches to discover and understand cis-regulatory elements governing expression of key regulators of hematopoiesis unveil biological and mechanistic insights that provide the logic for innovating clinical applications.

Two effects of GATA2 enhancer repositioning by 3q chromosomal rearrangements

Chromosomal inversion and translocation between 3q21 and 3q26 [inv (3)(q21.3q26.2) and t(3;3)(q21.3;q26.2), respectively] give rise to acute myeloid leukemia (AML) and myelodysplastic syndrome (MDS), which have poor prognoses. The chromosomal rearrangements reposition a GATA2 distal hematopoietic enhancer from the original 3q21 locus to the EVI1 (also known as MECOM) locus on 3q26. Therefore, the GATA2 enhancer from one of two GATA2 alleles drives EVI1 gene expression in hematopoietic stem and progenitor cells, which promotes the accumulation of abnormal progenitors and induces leukemogenesis. On the other hand, one allele of the GATA2 gene loses its enhancer, which results in reduced GATA2 expression. The GATA2 gene encodes a transcription factor critical for the generation and maintenance of hematopoietic stem and progenitor cells. GATA2 haploinsufficiency has been known to cause immunodeficiency and myeloid leukemia. Notably, reduced GATA2 expression suppresses the differentiation but promotes the proliferation of EVI1-expressing leukemic cells, which accelerates EVI1-driven leukemogenesis. A series of studies have shown that the GATA2 enhancer repositioning caused by the chromosomal rearrangements between 3q21 and 3q26 provokes misexpression of both the EVI1 and GATA2 genes and that these two effects coordinately elicit high-risk leukemia.

Quantitative and qualitative impairments in GATA2 and myeloid neoplasms

GATA2 is a key transcription factor critical for hematopoietic cell development. During the past decade, it became clear that heterozygous germline mutations in the GATA2 gene cause bone marrow failure and primary immunodeficiency syndrome, conditions that lead to a predisposition toward myeloid neoplasms, such as myelodysplastic syndrome, acute myeloid leukemia, and chronic myelomonocytic leukemia. Somatic mutations of the GATA2 gene are also involved in the pathogenesis of myeloid malignancies. Cases with GATA2 gene mutations are divided into two groups, resulting in either a quantitative deficiency or a qualitative defect in the GATA2 protein depending on the mutation position and type. In the former case, GATA2 mRNA expression from the mutant allele is markedly reduced or completely abrogated, and reduced GATA2 protein expression is involved in the pathogenesis. In the latter case, almost equal amounts of structurally abnormal and wildtype GATA2 proteins are predicted to be present and contribute to the pathogenesis. The development of mouse models of these human GATA2-related diseases has been undertaken, which naturally develop myeloid neoplasms.

Rapid progression to AML in a patient with germline GATA2 mutation and acquired NRAS Q61K mutation

GATA2 deficiency syndrome is caused by autosomal dominant, heterozygous germline mutations with widespread effects on immune, pulmonary and vascular systems. Patients commonly develop hematological abnormalities including bone marrow failure, myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML). We present a patient with GATA2 mutation and MDS who progressed to AML over four months. Whole exome and targeted deep sequencing identified a new p.Q61K NRAS mutation in the bone marrow at the time of AML development. Rapid development of AML is possible in the setting of germline GATA2 mutation despite stable MDS, supporting close monitoring and consideration of early allogeneic transplantation.

GATA2 hypomorphism induces chronic myelomonocytic leukemia in mice

The transcription factor GATA2 regulates normal hematopoiesis, particularly in‐ stem cell maintenance and myeloid differentiation. Various heteroallelic GATA2 gene mutations are associated with a variety of hematological neoplasms, including myelodysplastic syndromes and leukemias. Here, we report that impaired GATA2 expression induces myelodysplastic and myeloproliferative neoplasm development in elderly animals, and this neoplasm resembles chronic myelomonocytic leukemia in humans. GATA2 hypomorphic mutant (G2^fGN/fGN) mice that were generated by the germline insertion of a neocassette into the Gata2 gene locus avoided the early embryonic lethality observed in Gata2‐null mice. However, adult G2^fGN/fGN mice suffered from exacerbated leukocytosis concomitant with progressive anemia and thrombocytopenia and eventually developed massive granulomonocytosis accompanied by trilineage dysplasia. The reconstitution activity of G2^fGN/fGN mouse stem cells was impaired. Furthermore, G2^fGN/fGN progenitors showed myeloid lineage‐biased proliferation and differentiation. Myeloid progenitor accumulation started at a younger age in G2^fGN/fGN mice and appeared to worsen with age. G2^fGN/fGN mice showed increased expression of transcripts encoding cytokine receptors, such as macrophage colony‐stimulating factor receptor and interleukin‐6 receptor, in granulocyte‐monocyte progenitors. This increased expression could be correlated with the hypersensitive granulomonocytic proliferation reaction when the mice were exposed to lipopolysaccharide. Taken together, these observations indicate that GATA2 hypomorphism leads to a hyperreactive defense response to infections, and this reaction is attributed to a unique intrinsic cell defect in the regulation of myeloid expansion that increases the risk of hematological neoplasm transformation.