Bone marrow failure as a risk factor for clonal evolution: prospects for leukemia prevention

An evolutionary approach to clonally complex hematologic disorders

Emerging clonal complexity has brought into question the way in which we perceive and, in turn, treat disorders of the hematopoietic system. Former models of cell-intrinsic clonal dominance driven by acquisition of driver genes in a stereotypic sequence are often insufficient in explaining observations such as clonal hematopoiesis, and new paradigms are in order. Here, we review the evidence both within the hematologic malignancy field and also borrow from perspectives rooted in evolutionary biology to reframe pathogenesis of hematologic disorders as dynamic processes involving complex interplays of genetic and non-genetic subclones and the tissue microenvironment in which they reside.

Expression, Regulation and Function of microRNA as Important Players in the Transition of MDS to Secondary AML and Their Cross Talk to RNA-Binding Proteins

Myelodysplastic syndromes (MDS), heterogeneous diseases of hematopoietic stem cells, exhibit a significant risk of progression to secondary acute myeloid leukemia (sAML) that are typically accompanied by MDS-related changes and therefore significantly differ to de novo acute myeloid leukemia (AML). Within these disorders, the spectrum of cytogenetic alterations and oncogenic mutations, the extent of a predisposing defective osteohematopoietic niche, and the irregularity of the tumor microenvironment is highly diverse. However, the exact underlying pathophysiological mechanisms resulting in hematopoietic failure in patients with MDS and sAML remain elusive. There is recent evidence that the post-transcriptional control of gene expression mediated by microRNAs (miRNAs), long noncoding RNAs, and/or RNA-binding proteins (RBPs) are key components in the pathogenic events of both diseases. In addition, an interplay between RBPs and miRNAs has been postulated in MDS and sAML. Although a plethora of miRNAs is aberrantly expressed in MDS and sAML, their expression pattern significantly depends on the cell type and on the molecular make-up of the sample, including chromosomal alterations and single nucleotide polymorphisms, which also reflects their role in disease progression and prediction. Decreased expression levels of miRNAs or RBPs preventing the maturation or inhibiting translation of genes involved in pathogenesis of both diseases were found. Therefore, this review will summarize the current knowledge regarding the heterogeneity of expression, function, and clinical relevance of miRNAs, its link to molecular abnormalities in MDS and sAML with specific focus on the interplay with RBPs, and the current treatment options. This information might improve the use of miRNAs and/or RBPs as prognostic markers and therapeutic targets for both malignancies.

Runx1 negatively regulates inflammatory cytokine production by neutrophils in response to Toll-like receptor signaling

RUNX1 is frequently mutated in myeloid and lymphoid malignancies. It has been shown to negatively regulate Toll-like receptor 4 (TLR4) signaling through nuclear factor κB (NF-κB) in lung epithelial cells. Here we show that RUNX1 regulates TLR1/2 and TLR4 signaling and inflammatory cytokine production by neutrophils. Hematopoietic-specific RUNX1 loss increased the production of proinflammatory mediators, including tumor necrosis factor-α (TNF-α), by bone marrow neutrophils in response to TLR1/2 and TLR4 agonists. Hematopoietic RUNX1 loss also resulted in profound damage to the lung parenchyma following inhalation of the TLR4 ligand lipopolysaccharide (LPS). However, neutrophils with neutrophil-specific RUNX1 loss lacked the inflammatory phenotype caused by pan-hematopoietic RUNX1 loss, indicating that dysregulated TLR4 signaling is not due to loss of RUNX1 in neutrophils per se. Rather, single-cell RNA sequencing indicates the dysregulation originates in a neutrophil precursor. Enhanced inflammatory cytokine production by neutrophils following pan-hematopoietic RUNX1 loss correlated with increased degradation of the inhibitor of NF-κB signaling, and RUNX1-deficient neutrophils displayed broad transcriptional upregulation of many of the core components of the TLR4 signaling pathway. Hence, early, pan-hematopoietic RUNX1 loss de-represses an innate immune signaling transcriptional program that is maintained in terminally differentiated neutrophils, resulting in their hyperinflammatory state. We hypothesize that inflammatory cytokine production by neutrophils may contribute to leukemia associated with inherited RUNX1 mutations.

Signal Transduction in Ribosome Biogenesis: A Recipe to Avoid Disaster

Energetically speaking, ribosome biogenesis is by far the most costly process of the cell and, therefore, must be highly regulated in order to avoid unnecessary energy expenditure. Not only must ribosomal RNA (rRNA) synthesis, ribosomal protein (RP) transcription, translation, and nuclear import, as well as ribosome assembly, be tightly controlled, these events must be coordinated with other cellular events, such as cell division and differentiation. In addition, ribosome biogenesis must respond rapidly to environmental cues mediated by internal and cell surface receptors, or stress (oxidative stress, DNA damage, amino acid depletion, etc.). This review examines some of the well-studied pathways known to control ribosome biogenesis (PI3K-AKT-mTOR, RB-p53, MYC) and how they may interact with some of the less well studied pathways (eIF2α kinase and RNA editing/splicing) in higher eukaryotes to regulate ribosome biogenesis, assembly, and protein translation in a dynamic manner.

Recent advances in understanding hematopoiesis in Fanconi Anemia

Fanconi anemia is an inherited disease characterized by genomic instability, hypersensitivity to DNA cross-linking agents, bone marrow failure, short stature, skeletal abnormalities, and a high relative risk of myeloid leukemia and epithelial malignancies. The 21 Fanconi anemia genes encode proteins involved in multiple nuclear biochemical pathways that effect DNA interstrand crosslink repair. In the past, bone marrow failure was attributed solely to the failure of stem cells to repair DNA. Recently, non-canonical functions of many of the Fanconi anemia proteins have been described, including modulating responses to oxidative stress, viral infection, and inflammation as well as facilitating mitophagic responses and enhancing signals that promote stem cell function and survival. Some of these functions take place in non-nuclear sites and do not depend on the DNA damage response functions of the proteins. Dysfunctions of the canonical and non-canonical pathways that drive stem cell exhaustion and neoplastic clonal selection are reviewed, and the potential therapeutic importance of fully investigating the scope and interdependences of the canonical and non-canonical pathways is emphasized.

Alcohol, stem cells and cancer

Dosage, gender, and genetic susceptibility to the effects of alcohol remained only partially elucidated. In this review, we summarize the current knowledge of the mechanisms underlying the role of alcohol in liver and gastrointestinal cancers. In addition, two recent pathways- DNA repair and TGF-β signaling which provide new insights into alcohol in the regulation of cancers and stem cells are also discussed here.

Acute Myeloid Leukemia With Myelodysplasia-Related Changes

OBJECTIVES

Acute myeloid leukemia with myelodysplasia-related changes (AML-MRC) is a heterogeneous disorder defined by morphologic, genetic, or clinical features. Genetic abnormalities associated with AML-MRC are often associated with adverse prognostic features, and many cases are preceded by a myelodysplastic syndrome (MDS) or a myelodysplastic/myeloproliferative neoplasm.

METHODS

Although the criteria of 20% or more blasts in blood or bone marrow and multilineage dysplasia affecting 50% or more of cells in two or more of the myeloid lineages seem straightforward for AML-MRC, identification of morphologic dysplasia among observers is not always consistent, and there is morphologic overlap with other leukemic disorders such as acute erythroleukemia.

RESULTS

Session 3 of the workshop cases displayed heterogeneity as expected within AML-MRC, yet several cases suggested that recently recognized entities may exist within this category, such as familial MDS/AML predisposition syndromes and rare cases of high-risk AML associated with the cryptic t(5;11)(q35;p15);NUP98-NSD1 that may masquerade as a del(5q). However, most cases of AML-MRC were usually associated with adverse genetic abnormalities, particularly -5/del(5q), -7/del(7q), and/or complex karyotypes.

CONCLUSIONS

Whole-genome sequencing and array studies may identify genetic abnormalities, such as those affecting TP53, which may provide prognostic information.

Missing Cells: Pathophysiology, Diagnosis, and Management of (Pan)Cytopenia in Childhood

Peripheral blood cytopenia in children can be due to a variety of acquired or inherited diseases. Genetic disorders affecting a single hematopoietic lineage are frequently characterized by typical bone marrow findings, such as lack of progenitors or maturation arrest in congenital neutropenia or a lack of megakaryocytes in congenital amegakaryocytic thrombocytopenia, whereas antibody-mediated diseases such as autoimmune neutropenia are associated with a rather unremarkable bone marrow morphology. By contrast, pancytopenia is frequently associated with a hypocellular bone marrow, and the differential diagnosis includes acquired aplastic anemia, myelodysplastic syndrome, inherited bone marrow failure syndromes such as Fanconi anemia and dyskeratosis congenita, and a variety of immunological disorders including hemophagocytic lymphohistiocytosis. Thorough bone marrow analysis is of special importance for the diagnostic work-up of most patients. Cellularity, cellular composition, and dysplastic signs are the cornerstones of the differential diagnosis. Pancytopenia in the presence of a normo- or hypercellular marrow with dysplastic changes may indicate myelodysplastic syndrome. More challenging for the hematologist is the evaluation of the hypocellular bone marrow. Although aplastic anemia and hypocellular refractory cytopenia of childhood (RCC) can reliably be differentiated on a morphological level, the overlapping pathophysiology remains a significant challenge for the choice of the therapeutic strategy. Furthermore, inherited bone marrow failure syndromes are usually associated with the morphological picture of RCC, and the recognition of these entities is essential as they often present a multisystem disease requiring different diagnostic and therapeutic approaches. This paper gives an overview over the different disease entities presenting with (pan)cytopenia, their pathophysiology, characteristic bone marrow findings, and therapeutic approaches.

Molecular characteristics of a pancreatic adenocarcinoma associated with Shwachman-Diamond syndrome

BACKGROUND

Shwachman-Diamond syndrome (SDS) is characterized by hypoplasia of the bone marrow and exocrine pancreas and a high risk of leukemia. It is unknown whether solid tumors are part of the disease phenotype.

PROCEDURE

We performed copy number alterations using Affymetrix human SNP 6.0 array. Furthermore, we did direct sequencing of pancreatic cancer-related genes and immunohistochemical expression of selective proteins.

RESULTS

Among 41 patients with SDS who enrolled on the registry, we identified one male patient with a solid tumor: moderately differentiated pancreatic ductal adenocarcinoma. The tumor harbored 41 copy number alterations (CNAs) and had no regions of loss of heterozygosity (LOH). None of these CNAs were exclusive to the tumor. One copy of the tumor suppressor genes CTNNA3 and LGALS9C was lost in both the peripheral blood and tumor. Direct sequencing of TP53, KRAS, and NRAS revealed no mutations. Immunohistochemical staining for cyclin D1, E-cadherin, p53 MLH1 and MSH2 and β-catenin, was similar to that seen in non-hereditary pancreatic cancer.

CONCLUSIONS

Our case raises the possibility that solid tumors are associated with SDS, thereby broadening the clinical phenotype of the disease. The relatively young age at cancer diagnosis and the specific involvement of the pancreas make the possibility of an association with SDS likely. Similar to leukemia in SDS, the pancreatic cancer developed in hypoplastic tissues. This observation and the relative genomic stability of the tumor strengthen the hypothesis of improved adaptation of malignant clones among a population of disadvantaged cells as a mechanism for tumor expansion in SDS.

Bone marrow failure in Fanconi anemia is triggered by an exacerbated p53/p21 DNA damage response that impairs hematopoietic stem and progenitor cells

Fanconi anemia (FA) is an inherited DNA repair deficiency syndrome. FA patients undergo progressive bone marrow failure (BMF) during childhood, which frequently requires allogeneic hematopoietic stem cell transplantation. The pathogenesis of this BMF has been elusive to date. Here we found that FA patients exhibit a profound defect in hematopoietic stem and progenitor cells (HSPCs) that is present before the onset of clinical BMF. In response to replicative stress and unresolved DNA damage, p53 is hyperactivated in FA cells and triggers a late p21(Cdkn1a)-dependent G0/G1 cell-cycle arrest. Knockdown of p53 rescued the HSPC defects observed in several in vitro and in vivo models, including human FA or FA-like cells. Taken together, our results identify an exacerbated p53/p21 "physiological" response to cellular stress and DNA damage accumulation as a central mechanism for progressive HSPC elimination in FA patients, and have implications for clinical care.

Fanconi Anemia

Fanconi anemia (FA) is the most frequent inherited cause of BM failure (BMF). Fifteen FANC genes have been identified to date, the most prevalent being FANCA, FANCC, FANCG, and FANCD2. In addition to classical presentations with progressive BMF during childhood and a positive chromosome breakage test in the blood, atypical clinical and/or biological situations can be seen in which a FA diagnosis has to be confirmed or eliminated. For this, a range of biological tools have been developed, including analysis of skin fibroblasts. FA patients experience a strong selective pressure in the BM that predisposes to clonal evolution and to the emergence in their teens or young adulthood of myelodysplasia syndrome (MDS) and/or acute myeloid leukemia (AML) with a specific pattern of somatic chromosomal lesions. The cellular mechanisms underlying (1) the hematopoietic defect which leads to progressive BMF and (2) somatic clonal evolutions in this background, are still largely elusive. Elucidation of these mechanisms at the molecular and cellular levels should be useful to understand the physiopathology of the disease and to adapt the follow-up and treatment of FA patients. This may also ultimately benefit older, non-FA patients with aplastic anemia, MDS/AML for whom FA represents a model genetic condition.

Tumor necrosis factor restricts hematopoietic stem cell activity in mice: involvement of two distinct receptors

Both Tnfrsf1a and Tnfrsf1b are needed for the suppressive effect of TNF on murine HSC activity in vitro and in vivo.

Whereas maintenance of hematopoietic stem cells (HSCs) is a requisite for life, uncontrolled expansion of HSCs might enhance the propensity for leukemic transformation. Accordingly, HSC numbers are tightly regulated. The identification of physical cellular HSC niches has underscored the importance of extrinsic regulators of HSC homeostasis. However, whereas extrinsic positive regulators of HSCs have been identified, opposing extrinsic repressors of HSC expansion in vivo have yet to be described. Like many other acute and chronic inflammatory diseases, bone marrow (BM) failure syndromes are associated with tumor necrosis factor-α (TNF) overexpression. However, the in vivo relevance of TNF in the regulation of HSCs has remained unclear. Of considerable relevance for normal hematopoiesis and in particular BM failure syndromes, we herein demonstrate that TNF is a cell-extrinsic and potent endogenous suppressor of normal HSC activity in vivo in mice. These effects of TNF involve two distinct TNF receptors.

S-phase fraction as a useful marker for prognosis and therapeutic response in patients with aplastic anemia

BACKGROUND

The functional definition of aplastic anemia (AA) is the failure of hematopoietic stem cells to proliferate. The aim of the present study was to analyze the S-phase fraction (SPF) (proliferative activity) in patients with AA at diagnosis to explore its relationship with disease characteristics and its value in discriminating among patients with different prognoses. We also investigated whether the SPF value influenced the response to immunosuppressive therapy in AA patients.

PATIENTS AND METHODS

The analysis of SPF at the time of diagnosis was carried out by flow cytometry on peripheral blood samples from 53 consecutive patients with AA and 30 age- and sex-matched controls. All patients were given cyclosporine and followed up periodically to determine response to therapy.

RESULTS

Based on the median SPF, AA patients were divided into two groups: patients with SPF < 0.59% (n = 27) and patients with SPF > 0.59% (n = 26). An SPF > 0.59% was associated with advanced age (P = .02) and elevated serum LDH level (P = .01). Patients with an SPF > 0.59% also had a higher incidence of paroxysmal nocturnal hemoglobinuria and cytogenetic abnormalities. During a median follow-up of 18 months, 3.7% of patients with SPF < or = 0.59 and 11.5% of patients with SPF > 0.59% developed dysplasia and one patient with SPF > 0.59% converted into AML. A significantly higher (P = .018) overall response rate of 53.9% was found in patients with SPF > 0.59% versus 22.2% of patients with SPF < or = 0.59% at 6 months.

CONCLUSIONS

Independently of the peripheral blood count, the SPF at diagnosis may provide information on the expected response to immunosuppressive therapy and the propensity for disease to evolve into MDS/AML. Hence, SPF may serve as an early indicator for the evolution of MDS/AML in patients with AA and thus contribute to therapeutic decisions.

Functionally distinct hematopoietic stem cells modulate hematopoietic lineage potential during aging by a mechanism of clonal expansion

Aging of the hematopoietic stem cell compartment is believed to contribute to the onset of a variety of age-dependent blood cell pathophysiologies. Mechanistic drivers of hematopoietic stem cell (HSC) aging include DNA damage accumulation and induction of tumor suppressor pathways that combine to reduce the regenerative capacity of aged HSCs. Such mechanisms do not however account for the change in lymphoid and myeloid lineage potential characteristic of HSC aging, which is believed to be central to the decline of immune competence and predisposition to myelogenous diseases in the elderly. Here we have prospectively isolated functionally distinct HSC clonal subtypes, based on cell surface phenotype, bearing intrinsically different capacities to differentiate toward lymphoid and myeloid effector cells mediated by quantitative differences in lineage priming. Finally, we present data supporting a model in which clonal expansion of a class of intrinsically myeloid-biased HSCs with robust self-renewal potential is a central component of hematopoietic aging.

Congenital amegakaryocytic thrombocytopenia and thrombocytopenia with absent radii

Thrombocytopenia is a relatively common clinical problem in hospitalized neonates, and it is critical to distinguish infants who have rare congenital thrombocytopenias from those who have acquired disorders. Two well-described inherited thrombocytopenia syndromes that present in the newborn period are congenital amegakaryocytic thrombocytopenia (CAMT) and thrombocytopenia with absent radii (TAR). Although both are characterized by severe (< 50,000/microL) thrombocytopenia at birth, the molecular mechanisms underlying these disorders and their clinical presentations and courses are distinct. CAMT is an autosomal recessive disorder caused by mutations in the thrombopoietin (TPO) receptor c-Mpl. TAR is a syndrome of variable inheritance and unclear genetic etiology consisting of thrombocytopenia in association with bilateral absent radii and frequently additional congenital abnormalities. This article summarizes the current understanding of the pathophysiology and clinical course of CAMT and TAR.

Shwachman-Diamond syndrome: implications for understanding the molecular basis of leukaemia

Inherited bone marrow failure syndromes provide extremely useful genetic models for understanding leukaemogenesis because the initial genetic defect can be identified and the risk of leukaemia is very high. Shwachman-Diamond syndrome is one of the most common inherited bone marrow failure syndromes and an example of such a model. Here, I describe the malignant features of Shwachman-Diamond syndrome and discuss the potential molecular mechanisms that can lead to leukaemia.

Discovering early molecular determinants of leukemogenesis

Truncating mutations of the G-CSF receptor are found during disease course in nearly half of all patients with severe congenital neutropenia. In this issue of the JCI, Liu et al. demonstrate that these mutations confer a competitive clonal advantage upon HSCs in mice and that the advantage is conditional because it is observed only in the presence of the ligand G-CSF (see the related article beginning on page 946). Once activated, the mutant receptor requires the function of Stat5 in order to effect clonal expansion of this stem cell population. The results support the notion that early molecular steps in this and other neoplastic processes represent adaptations in which, through somatic mutations, "unfit" stem cells gain a measure of fitness by altering their relationships with their microenvironment.