Somatic inactivation of Nf1 in hematopoietic cells results in a progressive myeloproliferative disorder

Wiskott-Aldrich syndrome is an important differential diagnosis in male infants with juvenile myelomonocytic leukemialike features

A newborn presented with thrombocytopenia at birth and subsequently developed leukocytosis, monocytosis, and mild hepatomegaly. The bone marrow was normocellular with dysplasia and spontaneous granulocyte-monocyte colony formation was demonstrated. These findings fulfilled the diagnostic criteria of juvenile myelomonocytic leukemia. Then he developed atopic dermatitislike eczema, which led to the consideration of Wiskott-Aldrich syndrome (WAS). Lack of intracellular WASP expression and WASP gene mutation confirmed the diagnosis of WAS. After stem cell transplantation, he is alive in good condition with normal WASP expression. WAS should be considered as a differential diagnosis in male infants with juvenile myelomonocytic leukemialike features.

Aberrant cytokine signaling in leukemia

Abnormalities of cytokine and growth factor signaling pathways are characteristic of all forms of leukemia: lymphoid and myeloid, acute and chronic. In normal hematopoietic cells, cytokines provide the stimulus for proliferation, survival, self-renewal, differentiation and functional activation. In leukemic cells, these pathways are usurped to subserve critical parts of the malignant program. In this review, our current knowledge of leukemic cell cytokine signaling will be summarized, and some speculations on the significance and implications of these insights will be advanced. A better understanding of aberrant cytokine signaling in leukemia should provide additional targets for the rational therapy of these diseases.

Development of an allele-specific minimal residual disease assay for patients with juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia is an aggressive and frequently lethal myeloproliferative disorder of childhood. Somatic mutations in NRAS, KRAS, or PTPN11 occur in 60% of cases. Monitoring disease status is difficult because of the lack of characteristic leukemic blasts at diagnosis. We designed a fluorescently based, allele-specific polymerase chain reaction assay called TaqMAMA to detect the most common RAS or PTPN11 mutations. We analyzed peripheral blood and/or bone marrow of 25 patients for levels of mutant alleles over time. Analysis of pre-hematopoietic stem-cell transplantation, samples revealed a broad distribution of the quantity of the mutant alleles. After hematopoietic stem-cell transplantation, the level of the mutant allele rose rapidly in patients who relapsed and correlated well with falling donor chimerism. Simultaneously analyzed peripheral blood and bone marrow samples demonstrate that blood can be monitored for residual disease. Importantly, these assays provide a sensitive strategy to evaluate molecular responses to new therapeutic strategies.

What's new in the neuro-cardio-facial-cutaneous syndromes?

The RAS-MAPKinase pathway is a signal transduction cascade which has been studied extensively during the last decades for its role in human oncogenesis. Activation of this cascade is controlled by cycling of the RAS protein between an inactive and an active state and by phosphorylation of downstream proteins. The signalling cascade regulates cell proliferation, differentiation and survival. Disturbed RAS signalling in malignancies is caused by acquired somatic mutations in RAS genes or other components of this pathway. Recently, germline mutations in genes coding for different components of the RAS signalling cascade have been recognized as the cause of several phenotypically overlapping disorders, recently referred to as the neuro-cardio-facial-cutaneous syndromes. Neurofibromatosis type 1, Noonan, LEOPARD, Costello and cardiofaciocutaneous syndromes all present with variable degrees of psychomotor delay, congenital heart defects, facial dysmorphism, short stature, skin abnormalities and a predisposition for malignancy. These findings point to important roles for this evolutionary conserved pathway in oncogenesis, development, cognition and growth.

CONCLUSION

it has become obvious in recent years that the neuro-cardio-facial-cutaneous syndromes all share a common genetic and pathophysiologic basis. Dysregulation of the RAS-MAPKinase pathway is caused by germline mutations in genes involved in this pathway. Undoubtedly more genes causing related syndromes will be discovered in the near future since there are still a substantial number of genes in the pathway that are not yet associated with a known syndrome.

EWS/FLI-1 induces rapid onset of myeloid/erythroid leukemia in mice

EWS/FLI-1 is a chimeric oncogene generated by chromosomal translocation in Ewing tumors, a family of poorly differentiated pediatric tumors arising predominantly in bone but also in soft tissue. The fusion gene combines sequences encoding a strong transactivating domain from the EWS protein with the DNA binding domain of FLI-1, an ETS transcription factor. A related fusion, TLS/ERG, has been found in myeloid leukemia. To determine EWS/FLI-1 function in vivo, we engineered mice with Cre-inducible expression of EWS/FLI-1 from the ubiquitous Rosa26 locus. When crossed with Mx1-cre mice, Cre-mediated activation of EWS/FLI-1 resulted in the rapid development of myeloid/erythroid leukemia characterized by expansion of primitive mononuclear cells causing hepatomegaly, splenomegaly, severe anemia, and death. The disease could be transplanted serially into naïve recipients. Gene expression profiles of primary and transplanted animals were highly similar, suggesting that activation of EWS/FLI-1 was the primary event leading to disease in this model. The Cre-inducible EWS/FLI-1 mouse provides a novel model system to study the contribution of this oncogene to malignant disease in vivo.

Rb regulates interactions between hematopoietic stem cells and their bone marrow microenvironment

Hematopoiesis is maintained by stem cells (HSCs) that undergo fate decisions by integrating intrinsic and extrinsic signals, with the latter derived from the bone marrow (BM) microenvironment. Cell-cycle regulation can modulate stem cell fate, but it is unknown whether this represents an intrinsic or extrinsic effector of fate decisions. We have investigated the role of the retinoblastoma protein (RB), a central regulator of the cell cycle, in hematopoiesis. Widespread inactivation of RB in the murine hematopoietic system resulted in profound myeloproliferation. HSCs were lost from the BM due to mobilization to extramedullary sites and differentiation. This phenotype was not intrinsic to HSCs, but, rather, was the consequence of an RB-dependent interaction between myeloid-derived cells and the microenvironment. These findings demonstrate that myeloproliferation may result from perturbed interactions between hematopoietic cells and the niche. Therefore, RB extrinsically regulates HSCs by maintaining the capacity of the BM to support normal hematopoiesis and HSCs.

A microenvironment-induced myeloproliferative syndrome caused by retinoic acid receptor gamma deficiency

Myeloproliferative syndromes (MPS) are a heterogeneous subclass of nonlymphoid hematopoietic neoplasms which are considered to be intrinsic to hematopoietic cells. The causes of MPS are largely unknown. Here, we demonstrate that mice deficient for retinoic acid receptor gamma (RARgamma), develop MPS induced solely by the RARgamma-deficient microenvironment. RARgamma(-/-) mice had significantly increased granulocyte/macrophage progenitors and granulocytes in bone marrow (BM), peripheral blood, and spleen. The MPS phenotype continued for the lifespan of the mice and was more pronounced in older mice. Unexpectedly, transplant studies revealed this disease was not intrinsic to the hematopoietic cells. BM from wild-type mice transplanted into mice with an RARgamma(-/-) microenvironment rapidly developed the MPS, which was partially caused by significantly elevated TNFalpha in RARgamma(-/-) mice. These data show that loss of RARgamma results in a nonhematopoietic cell-intrinsic MPS, revealing the capability of the microenvironment to be the sole cause of hematopoietic disorders.

Hyperactive Ras in developmental disorders and cancer

Ras genes are the most common targets for somatic gain-of-function mutations in human cancer. Recently, germline mutations that affect components of the Ras-Raf-mitogen-activated and extracellular-signal regulated kinase kinase (MEK)-extracellular signal-regulated kinase (ERK) pathway were shown to cause several developmental disorders, including Noonan, Costello and cardio-facio-cutaneous syndromes. Many of these mutant alleles encode proteins with aberrant biochemical and functional properties. Here we will discuss the implications of germline mutations in the Ras-Raf-MEK-ERK pathway for understanding normal developmental processes and cancer pathogenesis.

Deletion of Nf1 in neurons induces increased axon collateral branching after dorsal root injury

Ras-mediated signaling pathways participate in multiple aspects of neural development and function. For example, Ras signaling lies downstream of neurotrophic factors and Trk family receptor tyrosine kinases to regulate neuronal survival and morphological differentiation, including axon extension and target innervation. Neurofibromin, the protein encoded by the tumor suppressor gene Nf1, is a negative regulator of Ras [Ras-GAP (GTPase-activating protein)], and we previously demonstrated that Nf1 null embryonic sensory and sympathetic neurons can survive and differentiate independent of neurotrophin support. In this report, we demonstrate that Nf1 loss in adult sensory neurons enhances their intrinsic capacity for neurite outgrowth and collateral branching in vitro and in vivo after dorsal root injury. In contrast to the permanent sensory deficits observed in control mice after dorsal rhizotomy, neuron-specific Nf1 mutant mice spontaneously recover proprioceptive function. This phenomenon appears to be mediated both by a cell-autonomous capacity of spared Nf1-/- DRG neurons for increased axonal sprouting, and by non-cell-autonomous contribution from Nf1-/- neurons in the denervated spinal cord.

Abnormal hematopoiesis in Gab2 mutant mice

Gab2 is an important adapter molecule for cytokine signaling. Despite its major role in signaling by receptors associated with hematopoiesis, the role of Gab2 in hematopoiesis has not been addressed. We report that despite normal numbers of peripheral blood cells, bone marrow cells, and c-Kit(+)Lin(-)Sca-1(+) (KLS) cells, Gab2-deficient hematopoietic cells are deficient in cytokine responsiveness. Significant reductions in the number of colony-forming units in culture (CFU-C) in the presence of limiting cytokine concentrations were observed, and these defects could be completely corrected by retroviral complementation. In earlier hematopoiesis, Gab2-deficient KLS cells isolated in vitro responded poorly to hematopoietic growth factors, resulting in an up to 11-fold reduction in response to a cocktail of stem cell factor, flt3 ligand, and thrombopoietin. Gab2-deficient c-Kit(+)Lin(-) cells also demonstrate impaired activation of extracellular signal-regulated kinase (ERK) and S6 in response to IL-3, which supports defects in activating the phosphatidylinositol-3 kinase (PI-3K) and mitogen-associated protein kinase (MAPK) signaling cascades. Associated with the early defects in cytokine response, competitive transplantation of Gab2(-/-) bone marrow cells resulted in defective long-term multilineage repopulation. Therefore, we demonstrate that Gab2 adapter function is intrinsically required for hematopoietic cell response to early-acting cytokines, resulting in defective hematopoiesis in Gab2-deficient mice.

The role of Shp2 (PTPN11) in cancer

Tyrosyl phosphorylation, which is controlled by protein-tyrosine kinases (PTKs) and protein-tyrosine phosphatases (PTPs), regulates numerous cellular processes. Altered expression and/or mutations in PTKs are linked to many forms of cancer, yet until recently little was known about the roles of PTPs in normal cells or in cancer. Earlier work established that a member of the PTP superfamily, PTEN, is an important tumor suppressor gene. We now know that at least one other PTP, the SH2 domain-containing phosphatase Shp2, is a bona fide oncogene that is mutated in several types of leukemia and hyperactivated by other mechanisms in some solid tumors. Understanding how Shp2 and other PTPs contribute to oncogenesis should provide new insights into pathogenesis and might suggest new targets for anti-neoplastic drugs.

The GAP-related domain of neurofibromin attenuates proliferation and downregulates N- and K-Ras activation in Nf1-negative AML cells

Inactivation of the NF1 tumor suppressor causes myeloproliferative diseases. NF1 encodes a GTPase activating protein (GAP) for Ras. Myeloid cells with loss of NF1 have high levels of Ras-GTP, functionally equivalent to the effects of RAS oncogenes. We investigated the effects of the NF1 GAP-related domain (GRD) in proliferation, apoptosis and Ras-GTP levels in Nf1-negative acute myeloid leukemia (AML) cells. In AML cells, with cooperating mutations, the expression of the neurofibromin GRD causes significant reductions of N- and K-Ras-GTP levels, which is not incompatible with AML cell survival, but which is strongly selected against due to suppression of proliferation.

An unexpected new role of mutant Ras: perturbation of human embryonic development

The Ras signaling pathway controls important cellular responses to growth factors, and somatic mutations in RAS genes and other components of the Ras pathway, such as PTPN11 (encoding the protein-tyrosine phosphatase SHP-2) and BRAF, are found in human malignancies. Ras proteins are guanosine nucleotide-binding proteins that cycle between active guanosine triphosphate (GTP)-bound and inactive guanosine diphosphate (GDP)-bound conformations. Neoplasia-associated Ras mutations frequently affect amino acids G12, G13, or Q61 and decrease the intrinsic guanosine triphosphatase (GTPase) activity by ten- to twentyfold. The GTPase activity is crucial for Ras inactivation by hydrolysis and release of a phosphate group from Ras·GTP to produce Ras·GDP. We and others have recently discovered germline mutations in the KRAS gene in individuals diagnosed with Noonan and cardio–facio–cutaneous (CFC) syndrome, two clinically overlapping disorders characterized by short stature, distinct facial anomalies, heart defects, and other abnormalities. Noonan syndrome-associated mutations V14I and T58I K-Ras activate Ras but have milder biochemical effects than somatic mutations encountered in cancers, offering an explanation why these K-Ras lesions are tolerated during embryonic development. Together with recent findings of BRAF, MEK1, and MEK2 mutations in CFC syndrome and HRAS mutations in Costello syndrome, another clinically related disorder, it has now become clear that Noonan-like features (short stature, relative macrocephaly, facial anomalies, learning difficulties) that are found in these three related disorders are a result of constitutive activation of the Ras–Raf–extracellular signal-regulated and mitogen-activated protein kinase pathway.

K-RasG12D expression induces hyperproliferation and aberrant signaling in primary hematopoietic stem/progenitor cells

Defining how cancer-associated mutations perturb signaling networks in stem/progenitor populations that are integral to tumor formation and maintenance is a fundamental problem with biologic and clinical implications. Point mutations in RAS genes contribute to many cancers, including myeloid malignancies. We investigated the effects of an oncogenic Kras(G12D) allele on phosphorylated signaling molecules in primary c-kit(+) lin(-/low) hematopoietic stem/progenitor cells. Comparison of wild-type and Kras(G12D) c-kit(+) lin(-/low) cells shows that K-Ras(G12D) expression causes hyperproliferation in vivo and results in abnormal levels of phosphorylated STAT5, ERK, and S6 under basal and stimulated conditions. Whereas Kras(G12D) cells demonstrate hyperactive signaling after exposure to granulocyte-macrophage colony-stimulating factor, we unexpectedly observe a paradoxical attenuation of ERK and S6 phosphorylation in response to stem cell factor. These studies provide direct biochemical evidence that cancer stem/progenitor cells remodel signaling networks in response to oncogenic stress and demonstrate that multi-parameter flow cytometry can be used to monitor the effects of targeted therapeutics in vivo. This strategy has broad implications for defining the architecture of signaling networks in primary cancer cells and for implementing stem cell-targeted interventions.

Beta common receptor inactivation attenuates myeloproliferative disease in Nf1 mutant mice

Neurofibromatosis type 1 (NF1) syndrome is caused by germline mutations in the NF1 tumor suppressor, which encodes neurofibromin, a GTPase activating protein for Ras. Children with NF1 are predisposed to juvenile myelomonocytic leukemia (JMML) and lethally irradiated mice given transplants with homozygous Nf1 mutant (Nf1-/-) hematopoietic stem cells develop a fatal myeloproliferative disorder (MPD) that models JMML. We investigated the requirement for signaling through the GM-CSF receptor to initiate and sustain this MPD by generating Nf1 mutant hematopoietic cells lacking the common beta chain (Beta c) of the GM-CSF receptor. Mice reconstituted with Nf1-/-, beta c-/- stem cells did not develop evidence of MPD despite the presence of increased number of immature hematopoietic progenitors in the bone marrow. Interestingly, when the Mx1-Cre transgene was used to inactivate a conditional Nf1 mutant allele in hematopoietic cells, concomitant loss of beta c-/- reduced the severity of the MPD, but did not abrogate it. Whereas inhibiting GM-CSF signaling may be of therapeutic benefit in JMML, our data also demonstrate aberrant proliferation of Nf1-/-myeloid progenitors that is independent of signaling through the GM-CSF receptor.

PTPN11 is the first identified proto-oncogene that encodes a tyrosine phosphatase

Elucidation of the molecular mechanisms underlying carcinogenesis has benefited tremendously from the identification and characterization of oncogenes and tumor suppressor genes. One new advance in this field is the identification of PTPN11 as the first proto-oncogene that encodes a cytoplasmic tyrosine phosphatase with 2 Src-homology 2 (SH2) domains (Shp2). This tyrosine phosphatase was previously shown to play an essential role in normal hematopoiesis. More recently, somatic missense PTPN11 gain-of-function mutations have been detected in leukemias and rarely in solid tumors, and have been found to induce aberrant hyperactivation of the Ras-Erk pathway. This progress represents another milestone in the leukemia/cancer research field and provides a fresh view on the molecular mechanisms underlying cell transformation.

Rce1 deficiency accelerates the development of K-RAS-induced myeloproliferative disease

The RAS proteins undergo farnesylation of a carboxyl-terminal cysteine (the "C" of the carboxyl-terminal CaaX motif). After farnesylation, the 3 amino acids downstream from the farnesyl cysteine (the -aaX of the CaaX motif) are released by RAS-converting enzyme 1 (RCE1). We previously showed that inactivation of Rce1 in mouse fibroblasts mislocalizes RAS proteins away from the plasma membrane and inhibits RAS transformation. Therefore, we hypothesized that the inactivation of Rce1 might inhibit RAS transformation in vivo. To test this hypothesis, we used Cre/loxP recombination techniques to simultaneously inactivate Rce1 and activate a latent oncogenic K-RAS allele in hematopoietic cells in mice. Normally, activation of the oncogenic K-RAS allele in hematopoietic cells leads to rapidly progressing and lethal myeloproliferative disease. Contrary to our hypothesis, the inactivation of Rce1 actually increased peripheral leukocytosis, increased the release of immature hematopoietic cells into the circulation and the infiltration of cells into liver and spleen, and caused mice to die more rapidly. Moreover, in the absence of Rce1, splenocytes and bone marrow cells expressing oncogenic K-RAS yielded more and larger colonies when grown in methylcellulose. We conclude that the inactivation of Rce1 worsens the myeloproliferative disease caused by oncogenic K-RAS.

Disclosure of candidate genes in acute myeloid leukemia with complex karyotypes using microarray-based molecular characterization

PURPOSE

To identify novel genomic regions of interest in acute myeloid leukemia (AML) with complex karyotypes, we applied comparative genomic hybridization to microarrays (array-CGH), allowing high-resolution genome-wide screening of genomic imbalances.

PATIENTS AND METHODS

Sixty AML cases with complex karyotypes were analyzed using array-CGH; parallel analysis of gene expression was performed in a subset of cases.

RESULTS

Genomic losses were found more frequently than gains. The most frequent losses affected 5q (77%), 17p (55%), and 7q (45%), and the most frequent genomic gains 11q (40%) and 8q (38%). Critical segments could be delineated to genomic fragments of only 0.8 to a few megabase-pairs of DNA. In lost/gained regions, gene expression profiling detected a gene dosage effect with significant lower/higher average gene expression levels across the genes located in the respective regions. Furthermore, high-level DNA amplifications were identified in several regions: 11q23.3-q24.1 (n = 7), 21q22 (n = 6), 11q23.3 (n = 5), 13q12 (n = 3), 8q24 (n = 3), 9p24 (n = 2), 12p13 (n = 2), and 20q11 (n = 2). Parallel analysis of gene expression in critical amplicons displayed overexpressed candidate genes (eg, C8FW and MYC in 8q24).

CONCLUSION

In conclusion, a large spectrum of genomic imbalances, including novel recurring changes in AML with complex karyotypes, was identified using array-CGH. In addition, the combined analysis of array-CGH data with gene expression profiles allowed the detection of candidate genes involved in the pathogenesis of AML.

Somatic activation of a conditional KrasG12D allele causes ineffective erythropoiesis in vivo

Somatic activation of a conditional targeted Kras(G12D) allele induces a fatal myeloproliferative disease in mice that closely models juvenile and chronic myelomonocytic leukemia. These mice consistently develop severe and progressive anemia despite adequate numbers of clonogenic erythroid progenitors in the bone marrow and expanded splenic hematopoiesis. Ineffective erythropoiesis is characterized by impaired differentiation. These results demonstrate that endogenous levels of oncogenic Ras have cell lineage-specific effects and support efforts to modulate Ras signaling for therapy of anemia in patients with myelodysplastic syndromes and myeloproliferative disorders.

Harnessing preclinical mouse models to inform human clinical cancer trials

The urgent need for better cancer treatments has stimulated interest in employing small-animal models to evaluate potential drug therapies. Robust mouse models of many human cancers have been generated using sophisticated technologies for engineering germ-line mutations. As we enter into an age of targeted therapeutics, these strains provide novel platforms for validating new anticancer drugs, assessing therapeutic index, identifying surrogate markers of tumor progression, and defining epigenetic and environmental influences on tumorigenesis.

Inherited predispositions and hyperactive Ras in myeloid leukemogenesis

Identifying the molecular basis for inherited cancer predispositions reveals genes that when mutated, play a critical role in the earliest stages of tumorigenesis. Although rare, inherited predispositions to myeloid leukemias have led to a greater understanding of pathways important for myeloid proliferation and maturation. In particular, elucidating why children with neurofibromatosis type 1 (NF1) and Noonan syndrome (NS) are predisposed to juvenile myelomonocytic leukemia (JMML) has uncovered a critical role of hyperactive Ras signaling in normal myeloid growth and leukemogenesis. Here, we review studies of human samples and experiments performed in genetically engineered strains of mice investigating the molecular and biochemical basis of aberrant growth in JMML. These strains model human disease features and provide an opportunity to investigate novel therapeutic strategies that may ultimately cure JMML and other myeloid malignancies characterized by hyperactive Ras.

Expression of endogenous oncogenic V600EB-raf induces proliferation and developmental defects in mice and transformation of primary fibroblasts

Mutations of the human B-RAF gene are detected in approximately 8% of cancer samples, primarily in cutaneous melanomas (70%). The most common mutation (90%) is a valine-to-glutamic acid mutation at residue 600 (V600E; formerly V599E according to previous nomenclature). Using a Cre/Lox approach, we have generated a conditional knock-in allele of (V600E)B-raf in mice. We show that widespread expression of (V600E)B-Raf cannot be tolerated in embryonic development, with embryos dying approximately 7.5 dpc. Directed expression of mutant (V600E)B-Raf to somatic tissues using the IFN-inducible Mx1-Cre mouse strain induces a proliferative disorder and bone marrow failure with evidence of nonlymphoid neoplasia of the histiocytic type leading to death within 4 weeks of age. However, expression of mutant B-Raf does not alter the proliferation profile of all somatic tissues. In primary mouse embryonic fibroblasts, expression of endogenous (V600E)B-Raf induces morphologic transformation, increased cell proliferation, and loss of contact inhibition. Thus, (V600E)B-Raf is able to induce several hallmarks of transformation in some primary mouse cells without evidence for the involvement of a cooperating oncogene or tumor suppressor gene.

Hyperactivation of the RAS signaling pathway in myelodysplastic syndrome with AML1/RUNX1 point mutations

AML1/RUNX1 mutations have been reported frequently in myelodysplastic syndrome (MDS) patients, especially those diagnosed with refractory anemia with excess blast (RAEB), RAEB in transformation (RAEBt), or AML following MDS (these categories are defined as MDS/AML). Although AML1 mutations are suspected to play a pivotal role in the development of MDS/AML, acquisition of additional genetic alterations is also necessary. We analyzed gene alterations in MDS/AML patients with AML1 mutations, comparing them to alterations in those without an AML1 mutation. AML1 mutations were significantly associated with -7/7q-, whereas MDS/AML patients without AML1 mutations showed a high frequency of -5/5q- and a complex karyotype. Patients with AML1 mutations showed more mutations of their FLT3, N-RAS, PTPN11, and NF1 genes, resulting in a significantly higher mutation frequency for receptor tyrosine kinase (RTK)-RAS signaling pathways in AML1-mutated MDS/AML patients compared to AML1-wild-type MDS/AML patients (38% versus 6.3%, P < 0.0001). Conversely, p53 mutations were detected only in patients without AML1 mutations. Furthermore, blast cells of the AML1-mutated patients expressing surface c-KIT, and SHP-2 mutants contributed to prolonged and enhanced extracellular signal-regulated kinase activation following stem cell factor stimulation. Our results suggest that MDS/AML arising from AML1/RUNX1 mutations has a significant association with -7/7q- alteration, and frequently involves RTK-RAS signaling pathway activation.

Chromosomal change during 6-mercaptopurine (6-MP) therapy in juvenile myelomonocytic leukemia: the growth of a 6-MP-refractory clone that already exists at onset

Among 11 JMML children, two had an abnormal karyotype, and nine had a normal karyotype at onset. In one patient with trisomy 8 and four patients with a normal karyotype, a new clone with an aberrant karyotype emerged 1-14 months after 6-mercaptopurine (6-MP) therapy as shown by G-banding analyses. Fluorescence in situ hybridization disclosed that an abnormal clone existed in approximately 3-6% of bone marrow cells at onset or before 6-MP therapy in all the four cases examined, and increased to approximately 12-90% during the treatment. In culture with granulocyte-macrophage colony-stimulating factor, cytogenetically abnormal clones that proliferated during 6-MP therapy possessed significantly less sensitivity to the antimetabolite, compared with cells that decreased in numbers after the therapy. A PTPN11 mutation was detected in all of granulocyte-macrophage colonies irrespective of karyotypic aberration in one patient, whereas approximately 80% of erythroid colonies and 20% of mixed colonies possessed neither a PTPN11 mutation nor chromosomal abnormalities. The appearance of chromosomal aberrations shown by G-banding during 6-MP therapy in some JMML cases may result, in part, from the growth of a 6-MP-refractory clone that already exists at onset. It is possible that treatment with 6-MP promotes progression of the disease.

Scientific and clinical opportunities for modeling blood disorders with embryonic stem cells

Our considerable wealth of data concerning hematologic processes has come despite difficulties working with stem and progenitor cells in vitro and their propensity to differentiate. Key methodologies that have sought to overcome such limitations include transgenic/knock-out animals and in vitro studies using murine embryonic stem cells, because both permit investigation of the formation of hematopoietic tissue from nonhematopoietic precursors. Although there have been many successful studies in model animals for understanding hematopoietic-cell development, differences between lower vertebrates and humans have left gaps in our understanding. Clearly, human-specific strategies to study the onset of hematopoiesis, particularly the earliest events leading to the specification of both normal and abnormal hematopoietic tissue, could bring an investigational renaissance. The recent availability of human embryonic stem (hES) cells suggests that such a system is now at hand. This review highlights the potential of hES cells to model human hematologic processes in vitro with an emphasis on disease targets.

Targeted therapy: for kids, too

Innovative hypothesis-driven clinical trials have achieved major successes over the past several decades in treating children and adolescents with cancer. DNA-damaging cytotoxic agents have cured children with cancer. While the mission is not yet accomplished, chemotherapy has been validated. None of these drugs were designed specifically for a pediatric disease. Continued progress will require new strategies. Now being tested for adult cancers, these strategies include gene therapy, immunotherapy, cancer prevention, and signal transduction inhibitor (STI) therapy. Of these, the most promising is STI therapy, also known as molecular therapeutics or targeted therapy. For this therapy to succeed, components of signal transduction (i.e., candidate drug targets) must be identified, the targets relevant to cancers, and the drugs available for trial. Because STI therapy is biologically driven and because therapy will be tailored depending on the molecular profile of a specific patient's tumor, clinical pediatric oncologists will need to acquire greater understanding of signaling pathways and their therapeutic relevance. With examples drawn from pediatric oncology, the critical steps in the pre-clinical development of targeted therapy are reviewed here.

Genetics of Myeloid Malignancies: Pathogenetic and Clinical Implications

Myeloid malignancies are clonal disorders that are characterized by acquired somatic mutation in hematopoietic progenitors. Recent advances in our understanding of the genetic basis of myeloid malignancies have provided important insights into the pathogenesis of acute myeloid leukemia (AML) and myeloproliferative diseases (MPD) and have led to the development of novel therapeutic approaches. In this review, we describe our current state of understanding of the genetic basis of AML and MPD, with a specific focus on pathogenetic and therapeutic significance. Specific examples discussed include RAS mutations, KIT mutations, FLT3 mutations, and core binding factor rearrangements in AML, and JAK2 mutations in polycythemia vera, essential thrombocytosis, and chronic idiopathic myelofibrosis.

Transcriptional repression of the Neurofibromatosis-1 tumor suppressor by the t(8;21) fusion protein

Von Recklinghausen's disease is a relatively common familial genetic disorder characterized by inactivating mutations of the Neurofibromatosis-1 (NF1) gene that predisposes these patients to malignancies, including an increased risk for juvenile myelomonocytic leukemia. However, NF1 mutations are not common in acute myeloid leukemia (AML). Given that the RUNX1 transcription factor is the most common target for chromosomal translocations in acute leukemia, we asked if NF1 might be regulated by RUNX1. In reporter assays, RUNX1 activated the NF1 promoter and cooperated with C/EBPalpha and ETS2 to activate the NF1 promoter over 80-fold. Moreover, the t(8;21) fusion protein RUNX1-MTG8 (R/M), which represses RUNX1-regulated genes, actively repressed the NF1 promoter. R/M associated with the NF1 promoter in vivo and repressed endogenous NF1 gene expression. In addition, similar to loss of NF1, R/M expression enhanced the sensitivity of primary myeloid progenitor cells to granulocyte-macrophage colony-stimulating factor. Our results indicate that the NF1 tumor suppressor gene is a direct transcriptional target of RUNX1 and the t(8;21) fusion protein, suggesting that suppression of NF1 expression contributes to the molecular pathogenesis of AML.

Dido gene expression alterations are implicated in the induction of hematological myeloid neoplasms

The myelodysplastic/myeloproliferative diseases (MDS/MPDs) are a heterogeneous group of myeloid neoplasms that share characteristics with chronic myeloproliferative diseases and myelodysplastic syndromes. The broad spectrum of clinical manifestations makes MDS/MPDs extremely difficult to diagnose and treat, with a median survival time of 1-5 years. No single gene defect has been firmly associated with MDS/MPDs, and no animal models have been developed for these diseases. The association of deletions on chromosome 20q with myeloid malignancies suggests the presence of unidentified tumor suppressor genes in this region. Here we show that the recently identified death inducer-obliterator (Dido) gene gives rise to at least 3 polypeptides (Dido1, Dido2, and Dido3) through alternative splicing, and we map the human gene to the long arm of chromosome 20. We found that targeting of murine Dido caused a transplantable disease whose symptoms and signs suggested MDS/MPDs. Furthermore, 100% of human MDS/MPD patients analyzed showed Dido expression abnormalities, which we also found in other myeloid but not lymphoid neoplasms or in healthy donors. Our findings suggest that Dido might be one of the tumor suppressor genes at chromosome 20q and that the Dido-targeted mouse may be a suitable model for studying MDS/MPD diseases and testing new approaches to their diagnosis and treatment.

Recent innovations in tissue-specific gene modifications in the mouse

Annotating the functions of individual genes in in vivo contexts has become the primary task of mouse genetics in the post-genome era. In addition to conventional approaches using transgenic technologies and gene targeting, the recent development of conditional gene modification techniques has opened novel opportunities for elucidating gene function at the level of the whole mouse to individual tissues or cell types. Tissue-specific gene modifications in the mouse have been made possible using site-specific DNA recombinases and conditional alleles. Recent innovations in this basic technology have facilitated new types of experiments, revealing novel insights into mammalian embryology. In this review, we focus on these recent innovations and new technical issues that impact the success of these conditional gene modification approaches.

Neurofibromatosis type 1 - a model for nervous system tumour formation?

Neurofibromatosis type 1 (NF1) is a common genetic condition in which affected individuals develop benign and malignant nervous system tumours. Genetically engineered mouse (GEM) models of these NF1-associated nervous system tumours recapitulate several of the unique clinical aspects of the disease. Moreover, these Nf1 GEM models allow for a direct examination of the earliest stages of tumour evolution, including the contributions that Nf1(+/-) cellular elements and cooperating genetic changes make to facilitate the transition from the pre-neoplastic to the neoplastic state and, in some cases, to promote malignant progression.

Functional analysis of leukemia-associated PTPN11 mutations in primary hematopoietic cells

PTPN11 encodes the protein tyrosine phosphatase SHP-2, which relays signals from growth factor receptors to Ras and other effectors. Germline PTPN11 mutations underlie about 50% of Noonan syndrome (NS), a developmental disorder that is associated with an elevated risk of juvenile myelomonocytic leukemia (JMML). Somatic PTPN11 mutations were recently identified in about 35% of patients with JMML; these mutations introduce amino acid substitutions that are largely distinct from those found in NS. We assessed the functional consequences of leukemia-associated PTPN11 mutations in murine hematopoietic cells. Expressing an E76K SHP-2 protein induced a hypersensitive pattern of granulocyte-macrophage colony-forming unit (CFU-GM) colony growth in response to granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin 3 (IL-3) that was dependent on SHP-2 catalytic activity. E76K SHP-2 expression also enhanced the growth of immature progenitor cells with high replating potential, perturbed erythroid growth, and impaired normal differentiation in liquid cultures. In addition, leukemia-associated SHP-2 mutations conferred a stronger phenotype than a germline mutation found in patients with NS. Mutant SHP-2 proteins induce aberrant growth in multiple hematopoietic compartments, which supports a primary role of hyperactive Ras in the pathogenesis of JMML.

Chronic myelomonocytic leukemia: lost in classification?

Chronic myelomonocytic leukemia (CMML) comprises a spectrum of disease variably considered as a myelodysplastic (MDS) and/or myeloproliferative (MPD) disorder. Now classified by the WHO within a separate nosological group from MDS or MPD, the reality is that there is a dynamic of evolution through increasing monocyte counts in one-third of patients. The principal clinical difference between CMML and other MPD is the presence of ineffective hematopoiesis, manifesting as more frequent anemia and thrombocytopenia in CMML. A fundamental biological characteristic shared with MPD is progenitor hypersensitivity to growth factors, but the pathways mediating this likely differ, as does the lineage specificity. Activation of the STAT pathway in MPD contrasts with frequent RAS pathway activation in CMML. Therapy of CMML is unsatisfactory, with the median age dictating that supportive care and control of myeloproliferation remains the mainstay for the majority. Intensive chemotherapy alone is of little benefit, and stem cell transplantation is the only curative modality in the small number of eligible patients, although outcome remains suboptimal. A deeper understanding of the biological basis of CMML may lead to targeted therapy analogous to the evolving management of MPD best exemplified for chronic myeloid leukemia.

Prognostic, therapeutic, and mechanistic implications of a mouse model of leukemia evoked by Shp2 (PTPN11) mutations

The SH2-containing tyrosine phosphatase Shp2 (PTPN11) is required for growth factor and cytokine signaling. Germline Shp2 mutations cause Noonan Syndrome (NS), which is associated with increased risk of juvenile myelomonocytic leukemia (JMML). Somatic Shp2 mutations occur in sporadic JMML and other leukemias. We found that Shp2 mutants associated with sporadic leukemias transform murine bone marrow cells, whereas NS mutants are less potent in this assay. Transformation requires multiple domains within Shp2 and the Shp2 binding protein Gab2, and is associated with hyperactivation of the Erk, Akt, and Stat5 pathways. Mutant Shp2-transduced BM causes a fatal JMML-like disorder or, less commonly, lymphoproliferation. Shp2 mutants also cause myeloproliferation in Drosophila. Mek or Tor inhibitors potently inhibit transformation, suggesting new approaches to JMML therapy.

Modeling neurofibromatosis type 1 tumors in the mouse for therapeutic intervention

Von Recklinghausen's neurofibromatosis is a dominantly inherited cancer syndrome. Its gene encodes neurofibromin, a protein with ras GTPase-activating function (rasGAP) and, therefore, all NF1-associated pathology is thought to originate from selective deregulation of the ras pathway. We have constructed a variety of mouse models for NF1 that permit recapitulation of the most common tumors seen in patients. In addition, these mouse models offer insights into tumor origin and into paracrine interactions. Given the molecular and pathological fidelity of the mouse tumors to the human counterparts, it is hoped that these mouse strains will serve as effective tools for therapeutic discovery.

Glioma Formation in Neurofibromatosis 1 Reflects Preferential Activation of K-RAS in Astrocytes

Children with the tumor predisposition syndrome, neurofibromatosis 1 (NF1), develop optic pathway gliomas. The NF1 gene product, neurofibromin, functions as a negative regulator of RAS, such that NF1 inactivation results in RAS hyperactivation. Recent studies have highlighted the divergent biological and biochemical properties of the various RAS isoforms, which prompted us to examine the consequence of Nf1 inactivation in astrocytes on RAS isoform activation in vitro and in vivo. In this report, we show that only K-RAS is activated in Nf1−/− astrocytes and that activation of K-RAS, but not H-RAS, accounts for the proliferative advantage and abnormal actin cytoskeleton–mediated processes observed in Nf1−/− astrocytes in vitro. Moreover, dominant inhibitory K-RAS corrects these abnormalities in Nf1−/− astrocytes invitro. Lastly, we show that Nf1+/− mice with astrocyte-specific activated K-RAS expression in vivo develop optic pathway gliomas, similar to our previously reported Nf1+/− mice with astrocyte Nf1 inactivation. Collectively, our results show that K-RAS is the primary target for neurofibromin GTPase-activating protein activity in vitro and in vivo and that K-RAS activation in astrocytes recapitulates the biochemical, biological, and tumorigenic properties of neurofibromin loss.

Childhood and adolescent lymphoid and myeloid leukemia

Remarkable progress has been made in the past decade in the treatment and in the understanding of the biology of childhood lymphoid and myeloid leukemias. With contemporary improved risk assessment, chemotherapy, hematopoietic stem cell transplantation and supportive care, approximately 80% of children with newly diagnosed acute lymphoblastic leukemia and 50% of those with myeloid neoplasm can be cured to date. Current emphasis is placed not only on increased cure rate but also on improved quality of life. In Section I, Dr. Ching-Hon Pui describes certain clinical and biologic features that still have prognostic and therapeutic relevance in the context of contemporary treatment programs. He emphasizes that treatment failure in some patients is not due to intrinsic drug resistance of leukemic cells but is rather caused by suboptimal drug dosing due to host compliance, pharmacodynamics, and pharmacogenetics. Hence, measurement of minimal residual disease, which accounts for both the genetic (primary and secondary) features of leukemic lymphoblasts and pharmacogenomic variables of the host, is the most reliable prognostic indicator. Finally, he contends that with optimal risk-directed systemic and intrathecal therapy, cranial irradiation may be omitted in all patients, regardless of the presenting features. In Section II, Dr. Martin Schrappe performs detailed analyses of the prognostic impact of presenting age, leukocyte count, sex, immunophenotype, genetic abnormality, early treatment response, and in vitro drug sensitivity/resistance in childhood acute lymphoblastic leukemia, based on the large database of the Berlin-Frankfurt-Münster consortium. He also succinctly summarizes the important treatment components resulting in the improved outcome of children and young adolescents with this disease. He describes the treatment approach that led to the improved outcome of adolescent patients, a finding that may be applied to young adults in the second and third decade of life. Finally, he believes that treatment reduction under well-controlled clinical trials is feasible in a subgroup of patients with excellent early treatment response as evidenced by minimal residual disease measurement during induction and consolidation therapy. In Section III, Dr. Raul Ribeiro describes distinct morphologic and genetic subtypes of acute myeloid leukemia. The finding of essentially identical gene expression profiling by DNA microarray in certain specific genetic subtypes of childhood and adult acute myeloid leukemia suggests a shared leukemogenesis. He then describes the principles of treatment as well as the efficacy and toxicity of various forms of postremission therapy, emphasizing the need of tailoring therapy to both the disease and the age of the patient. Early results suggest that minimal residual disease measurement can also improve the risk assessment in acute myeloid leukemia, and that cranial irradiation can be omitted even in those with central-nervous-system leukemia at diagnosis. In Section IV, Dr. Charlotte Niemeyer describes a new classification of myelodysplastic and myeloproliferative diseases in childhood, which has greatly facilitated the diagnosis of myelodysplastic syndromes and juvenile myelomonocytic leukemia. The recent discovery of somatic mutations in PTPN11 has improved the understanding of the pathobiology and the diagnosis of juvenile myelomonocytic leukemia. Together with the findings of mutations in RAS and NF1 in the other patients, she suggests that pathological activation of RAS-dependent pathways plays a central role in the leukemogenesis of this disease. She then describes the various treatment approaches for both juvenile myelomonocytic leukemia and myelodysplastic syndromes in the US and Europe, emphasizing the differences between childhood and adult cases for the latter group of diseases. She also raises some controversial issues regarding treatment that will require well-controlled international clinical trials to address.