Occurrence of myeloproliferative disorder in patients with Noonan syndrome

Case Report: Molecular autopsy underlie COVID-19-associated sudden, unexplained child mortality

Herein, we report a child with COVID-19 and seemingly no underlying disease, who died suddenly. The autopsy revealed severe anemia and thrombocytopenia, splenomegaly, hypercytokinemia, and a rare ectopic congenital coronary origin. Immunohistochemical analysis demonstrated that the patient had acute lymphoblastic leukemia of the B-cell precursor phenotype (BCP-ALL). The complex cardiac and hematological abnormalities suggested the presence of an underlying disease; therefore, we performed whole-exome sequencing (WES). WES revealed a leucine-zipper-like transcription regulator 1 (LZTR1) variant, indicating Noonan syndrome (NS). Therefore, we concluded that the patient had underlying NS along with coronary artery malformation and that COVID-19 infection may have triggered the sudden cardiac death due to increased cardiac load caused by high fever and dehydration. In addition, multiple organ failure due to hypercytokinemia probably contributed to the patient's death. This case would be of interest to pathologists and pediatricians because of the limited number of NS patients with LZTR1 variants; the complex combination of an LZTR1 variant, BCP-ALL, and COVID-19; and a rare pattern of the anomalous origin of the coronary artery. Thus, we highlight the significance of molecular autopsy and the application of WES with conventional diagnostic methods.

The molecular genetics of RASopathies: An update on novel disease genes and new disorders

Enhanced signaling through RAS and the mitogen-associated protein kinase (MAPK) cascade underlies the RASopathies, a family of clinically related disorders affecting development and growth. In RASopathies, increased RAS-MAPK signaling can result from the upregulated activity of various RAS GTPases, enhanced function of proteins positively controlling RAS function or favoring the efficient transmission of RAS signaling to downstream transducers, functional upregulation of RAS effectors belonging to the MAPK cascade, or inefficient signaling switch-off operated by feedback mechanisms acting at different levels. The massive effort in RASopathy gene discovery performed in the last 20 years has identified more than 20 genes implicated in these disorders. It has also facilitated the characterization of several molecular activating mechanisms that had remained unappreciated due to their minor impact in oncogenesis. Here, we provide an overview on the discoveries collected during the last 5 years that have delivered unexpected insights (e.g., Noonan syndrome as a recessive disease) and allowed to profile new RASopathies, novel disease genes and new molecular circuits contributing to the control of RAS-MAPK signaling.

Trametinib for Refractory Chylous Effusions and Systemic Complications in Children with Noonan Syndrome

OBJECTIVE

To evaluate the effect of the RAS-MAPK pathway inhibitor trametinib on medically refractory chylous effusions in 3 hospitalized patients with Noonan syndrome.

STUDY DESIGN

Pharmacologic MEK1/2 inhibition has been used to treat conditions associated with Noonan syndrome, given that activation of RAS-MAPK pathway variants leads to downstream MEK activation. We describe our experience with 3 patients with Noonan syndrome (owing to variants in 3 distinct genes) and refractory chylous effusions treated successfully with MEK inhibition. A monitoring protocol was established to standardize medication dosing and monitoring of outcome measures.

RESULTS

Subjects demonstrated improvement in lymphatic leak with additional findings of improved growth and normalization of cardiac and hematologic measurements. Trametinib was administered safely, with only moderate skin irritation in 1 subject.

CONCLUSIONS

Improvements in a variety of quantifiable measurements highlight the potential utility of MEK1/2 inhibition in patients with Noonan syndrome and life-threatening lymphatic disease. Larger, prospective studies are needed to confirm efficacy and assess long-term safety.

Simple and robust methylation test for risk stratification of patients with juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a rare myelodysplastic/myeloproliferative neoplasm that develops during infancy and early childhood. The array-based international consensus definition of DNA methylation has recently classified patients with JMML into the following 3 groups: high (HM), intermediate (IM), and low methylation (LM). To develop a simple and robust methylation clinical test, 137 patients with JMML were analyzed using the Digital Restriction Enzyme Analysis of Methylation (DREAM), which is a next-generation sequencing-based methylation analysis. Unsupervised consensus clustering of the discovery cohort (n = 99) using DREAM data identified HM (HM_DREAM; n = 35) and LM subgroups (LM_DREAM; n = 64). Of the 98 cases that could be compared with the international consensus classification, 90 HM (n = 30) and LM (n = 60) cases had 100% concordance with DREAM clustering results. Of the remaining 8 cases comprising the IM group, 4 were classified as belonging to the HM_DREAM group and 4 to the LM_DREAM group. A machine-learning classifier was successfully constructed using a support vector machine (SVM), which divided the validation cohort (n = 38) into HM (HM_SVM, n = 18) and LM (LM_SVM; n = 20) groups. Patients with the HM_SVM profile had a significantly poorer 5-year overall survival rate than those with the LM_SVM profile. In conclusion, we developed a robust methylation test using DREAM for patients with JMML. This simple and straightforward test can be easily incorporated into diagnosis to generate a methylation classification for patients so they can receive risk-adapted treatment in the context of future clinical trials.

Current Treatment of Juvenile Myelomonocytic Leukemia

Juvenile myelomonocytic leukemia (JMML) is a rare pediatric leukemia characterized by mutations in five canonical RAS pathway genes. The diagnosis is made by typical clinical and hematological findings associated with a compatible mutation. Although this is sufficient for clinical decision-making in most JMML cases, more in-depth analysis can include DNA methylation class and panel sequencing analysis for secondary mutations. NRAS-initiated JMML is heterogeneous and adequate management ranges from watchful waiting to allogeneic hematopoietic stem cell transplantation (HSCT). Upfront azacitidine in KRAS patients can achieve long-term remissions without HSCT; if HSCT is required, a less toxic preparative regimen is recommended. Germline CBL patients often experience spontaneous resolution of the leukemia or exhibit stable mixed chimerism after HSCT. JMML driven by PTPN11 or NF1 is often rapidly progressive, requires swift HSCT and may benefit from pretransplant therapy with azacitidine. Because graft-versus-leukemia alloimmunity is central to cure high risk patients, the immunosuppressive regimen should be discontinued early after HSCT.

Oncogenic KrasG12D Activation in the Nonhematopoietic Bone Marrow Microenvironment Causes Myelodysplastic Syndrome in Mice

The bone marrow microenvironment (BMME) is key player in regulation and maintenance of hematopoiesis. Oncogenic RAS mutations, causing constitutive activation of multiple tumor-promoting pathways, are frequently found in human cancer. So far in hematologic malignancies, RAS mutations have only been reported to occur in hematopoietic cells. In this study, we investigated the effect of oncogenic Kras expression in the BMME in a chimeric mouse model. We observed that an activating mutation of Kras in the nonhematopoietic system leads to a phenotype resembling myelodysplastic syndrome (MDS) characterized by peripheral cytopenia, marked dysplasia within the myeloid lineage as well as impaired proliferation and differentiation capacity of hematopoietic stem and progenitor cells. The phenotypic changes could be reverted when the BM was re-isolated and transferred into healthy recipients, indicating that the Kras^G12D -activation in the nonhematopoietic BMME was essential for the MDS phenotype. Gene expression analysis of sorted nonhematopoietic BM niche cells from Kras^G12D mice revealed upregulation of multiple inflammation-related genes including IL1-superfamily members (Il1α, Il1β, Il1f9) and the NLPR3 inflammasome. Thus, pro-inflammatory IL1-signaling in the BMME may contribute to MDS development. Our findings show that a single genetic change in the nonhematopoietic BMME can cause an MDS phenotype. Oncogenic Kras activation leads to pro-inflammatory signaling in the BMME which impairs HSPCs function. IMPLICATIONS: These findings may help to identify new therapeutic targets for MDS.

FAS and RAS related Apoptosis defects: From autoimmunity to leukemia

The human adaptive immune system recognizes almost all the pathogens that we encounter and all the tumor antigens that may arise during our lifetime. Primary immunodeficiencies affecting lymphocyte development or function therefore lead to severe infections and tumor susceptibility. Furthermore, the fact that autoimmunity is a frequent feature of primary immunodeficiencies reveals a third function of the adaptive immune system: its self-regulation. Indeed, the generation of a broad repertoire of antigen receptors (via a unique strategy of random somatic rearrangements of gene segments in T cell and B cell receptor loci) inevitably creates receptors with specificity for self-antigens and thus leads to the presence of autoreactive lymphocytes. There are many different mechanisms for controlling the emergence or action of autoreactive lymphocytes, including clonal deletion in the primary lymphoid organs, receptor editing, anergy, suppression of effector lymphocytes by regulatory lymphocytes, and programmed cell death. Here, we review the genetic defects affecting lymphocyte apoptosis and that are associated with lymphoproliferation and autoimmunity, together with the role of somatic mutations and their potential involvement in more common autoimmune diseases.

Juvenile myelomonocytic leukemia: who's the driver at the wheel?

Juvenile myelomonocytic leukemia (JMML) is a unique clonal hematopoietic disorder of early childhood. It is classified as an overlap myeloproliferative/myelodysplastic neoplasm by the World Health Organization and shares some features with chronic myelomonocytic leukemia in adults. JMML pathobiology is characterized by constitutive activation of the Ras signal transduction pathway. About 90% of patients harbor molecular alterations in 1 of 5 genes (PTPN11, NRAS, KRAS, NF1, or CBL), which define genetically and clinically distinct subtypes. Three of these subtypes, PTPN11-, NRAS-, and KRAS-mutated JMML, are characterized by heterozygous somatic gain-of-function mutations in nonsyndromic children, whereas 2 subtypes, JMML in neurofibromatosis type 1 and JMML in children with CBL syndrome, are defined by germline Ras disease and acquired biallelic inactivation of the respective genes in hematopoietic cells. The clinical course of the disease varies widely and can in part be predicted by age, level of hemoglobin F, and platelet count. The majority of children require allogeneic hematopoietic stem cell transplantation for long-term leukemia-free survival, but the disease will eventually resolve spontaneously in ∼15% of patients, rendering the prospective identification of these cases a clinical necessity. Most recently, genome-wide DNA methylation profiles identified distinct methylation signatures correlating with clinical and genetic features and highly predictive for outcome. Understanding the genomic and epigenomic basis of JMML will not only greatly improve precise decision making but also be fundamental for drug development and future collaborative trials.

JMML genomics and decisions

Juvenile myelomonocytic leukemia (JMML) is a unique clonal hematopoietic disorder of early childhood characterized by hyperactivation of the RAS signal transduction pathway. Approximately 90% of patients harbor molecular alteration in 1 of 5 genes (PTPN11, NRAS, KRAS, NF1, CBL), which define genetically and clinically distinct JMML subtypes. Three subtypes, PTPN11- , NRAS-, and KRAS-mutated JMML, are characterized by heterozygous somatic gain-of-function mutations in non syndromic children, while two subtypes, JMML in neurofibromatosis type 1 and in JMML in children with CBL syndrome, are characterized by germ line RAS disease and acquired biallelic inactivation of the respective tumor suppressor genes in hematopoietic cells. In addition to the initiating RAS pathway lesion, secondary genetic alterations within and outside of the RAS pathway are detected in about half the patients. Most recently, genome-wide DNA methylation profiles identified distinct methylation signatures correlating with clinical and genetic features and highly predictive of outcome. JMML is a stem cell disorder, and most JMML patients require allogeneic stem cell transplantation for long-term survival. However, spontaneous disease regression is noted in the majority of children with CBL-mutated JMML and in some NRAS-mutated cases. In the absence of 1 of the 5 canonical RAS pathway alteration, rare mutations in other RAS genes and non-JMML myeloproliferative disorders need to be excluded. Understanding the genetic basis of myeloproliferative disorders in early childhood will greatly improve clinical decision making.

Co-occurrence of hypertrophic cardiomyopathy and juvenile myelomonocytic leukemia in a neonate with Noonan syndrome, leading to premature death

We report a case of a neonate with Noonan syndrome presenting with concurrent hypertrophic cardiomyopathy and juvenile myelomonocytic leukemia, which resulted in premature death. Cases with Noonan syndrome diagnosed during the neonatal period might not necessarily show mild clinical course, and premature death is a possible outcome to be considered.

Genome-wide DNA methylation is predictive of outcome in juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML) is a myeloproliferative disorder of childhood caused by mutations in the Ras pathway. Outcomes in JMML vary markedly from spontaneous resolution to rapid relapse after hematopoietic stem cell transplantation. Here, we hypothesized that DNA methylation patterns would help predict disease outcome and therefore performed genome-wide DNA methylation profiling in a cohort of 39 patients. Unsupervised hierarchical clustering identifies three clusters of patients. Importantly, these clusters differ significantly in terms of 4-year event-free survival, with the lowest methylation cluster having the highest rates of survival. These findings were validated in an independent cohort of 40 patients. Notably, all but one of 14 patients experiencing spontaneous resolution cluster together and closer to 22 healthy controls than to other JMML cases. Thus, we show that DNA methylation patterns in JMML are predictive of outcome and can identify the patients most likely to experience spontaneous resolution.

Juvenile myelomonocytic leukemia (JMML) is an aggressive disease with limited options for treatment. Here, the authors utilize DNA methylation based subgroups in JMML to predict clinical outcome.

Down syndrome, RASopathies, and other rare syndromes

In this article we discuss the occurrence of myeloid neoplasms in patients with a range of syndromes that are due to germline defects of the RAS signaling pathway and in patients with trisomy 21. Both RAS mutations and trisomy 21 are common somatic events contributing to leukemogenis. Thus, the increased leukemia risk observed in children affected by these conditions is biologically highly plausible. Children with myeloid neoplasms in the context of these syndromes require different treatments than children with sporadic myeloid neoplasms and provide an opportunity to study the role of trisomy 21 and RAS signaling during leukemogenesis and development.

Noonan syndrome-causing genes: Molecular update and an assessment of the mutation rate

Noonan syndrome is a common autosomal dominant disorder characterized by short stature, congenital heart disease and facial dysmorphia with an incidence of 1/1000 to 2500 live births. Up to now, several genes have been proven to be involved in the disturbance of the transduction signal through the RAS-MAP Kinase pathway and the manifestation of Noonan syndrome. The first gene described was PTPN11, followed by SOS1, RAF1, KRAS, BRAF, NRAS, MAP2K1, and RIT1, and recently SOS2, LZTR1, and A2ML1, among others. Progressively, the physiopathology and molecular etiology of most signs of Noonan syndrome have been demonstrated, and inheritance patterns as well as genetic counseling have been established. In this review, we summarize the data concerning clinical features frequently observed in Noonan syndrome, and then, we describe the molecular etiology as well as the physiopathology of most Noonan syndrome-causing genes. In the second part of this review, we assess the mutational rate of Noonan syndrome-causing genes reported up to now in most screening studies. This review should give clinicians as well as geneticists a full view of the molecular aspects of Noonan syndrome and the authentic prevalence of the mutational events of its causing-genes. It will also facilitate laying the groundwork for future molecular diagnosis research, and the development of novel treatment strategies.

Progress and Prospects in Pediatric Leukemia

Pediatric leukemia is the single most common malignancy affecting children, representing up to 30% of all pediatric cancers. Dramatic improvements in survival for acute lymphoblastic leukemia (ALL) have taken place over the past 4 decades with outcomes approaching 90% in the latest studies. However, progress has been slower for myeloid leukemia and certain subgroups like infant ALL, adolescent/young adult ALL, and relapsed ALL. Recent advances include recognition of molecularly defined subgroups, which has ushered in precision medicine approaches. We discuss the current understanding of the biology of the various childhood leukemias, recent advances in research, and future challenges in this field.

An International MDS/MPN Working Group's perspective and recommendations on molecular pathogenesis, diagnosis and clinical characterization of myelodysplastic/myeloproliferative neoplasms

In the 2008 WHO classification, chronic myeloid malignancies that share both myelodysplastic and myeloproliferative features define the myelodysplastic/myeloproliferative group, which includes chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, atypical chronic myeloid leukemia, refractory anemia with ring sideroblasts and thrombocytosis, and myelodysplastic/myeloproliferative unclassified. With the notable exception of refractory anemia with ring sideroblasts and thrombocytosis, there is much overlap among the various subtypes at the molecular and clinical levels, and a better definition of these entities, an understanding of their biology and an identification of subtype-specific molecular or cellular markers are needed. To address some of these challenges, a panel comprised of laboratory and clinical experts in myelodysplastic/myeloproliferative was established, and four independent academic MDS/MPN workshops were held on: 9(th) March 2013, in Miami, Florida, USA; 6(th) December 2013, in New Orleans, Louisiana, USA; 13(th) June 2014 in Milan, Italy; and 5(th) December 2014 in San Francisco, USA. During these meetings, the current understanding of these malignancies and matters of biology, diagnosis and management were discussed. This perspective and the recommendations on molecular pathogenesis, diagnosis and clinical characterization for adult onset myelodysplastic/myeloproliferative is the result of a collaborative project endorsed and supported by the MDS Foundation.

Acute lymphoblastic leukemia in the context of RASopathies

Noonan syndrome is associated with a range of malignancies including acute lymphoblastic leukemia (ALL). However, little information is available regarding the frequency, natural history, characteristics and prognosis of ALL in Noonan syndrome or RASopathies in general. Cross-referencing data from a large prospective cohort of 1176 patients having a molecularly confirmed RASopathy with data from the French childhood cancer registry allowed us to identify ALL in 6 (0.5%) patients including 4/778 (0.5%) with a germline PTPN11 mutation and 2/94 (2.1%) with a germline SOS1 mutation. None of the patients of our series with CFC syndrome (with germline BRAF or MAP2K1/MAP2K2 mutation - n = 121) or Costello syndrome (with HRAS mutation - n = 35) had an ALL. A total of 19 Noonan-ALL were gathered by adding our patients to those of the International Berlin-Munster-Frankfurt (I-BFM) study group and previously reported patients. Strikingly, all Noonan-associated ALL were B-cell precursor ALL, and high hyperdiploidy with more than 50 chromosomes was found in the leukemia cells of 13/17 (76%) patients with available genetics data. Our data suggest that children with Noonan syndrome are at higher risk to develop ALL. Like what is observed for somatic PTPN11 mutations, NS is preferentially associated with the development of hyperdiploid ALL that will usually respond well to chemotherapy. However, Noonan syndrome patients seem to have a propensity to develop post therapy myelodysplasia that can eventually be fatal. Hence, one should be particularly cautious when treating these patients.

Mutations in RIT1 cause Noonan syndrome with possible juvenile myelomonocytic leukemia but are not involved in acute lymphoblastic leukemia

Noonan syndrome is a heterogeneous autosomal dominant disorder caused by mutations in at least eight genes involved in the RAS/MAPK signaling pathway. Recently, RIT1 (Ras-like without CAAX 1) has been shown to be involved in the pathogenesis of some patients. We report a series of 44 patients from 30 pedigrees (including nine multiplex families) with mutations in RIT1. These patients display a typical Noonan gestalt and facial phenotype. Among the probands, 8.7% showed postnatal growth retardation, 90% had congenital heart defects, 36% had hypertrophic cardiomyopathy (a lower incidence compared with previous report), 50% displayed speech delay and 52% had learning difficulties, but only 22% required special education. None had major skin anomalies. One child died perinatally of juvenile myelomonocytic leukemia. Compared with the canonical Noonan phenotype linked to PTPN11 mutations, patients with RIT1 mutations appear to be less severely growth retarded and more frequently affected by cardiomyopathy. Based on our experience, we estimate that RIT1 could be the cause of 5% of Noonan syndrome patients. Because mutations found constitutionally in Noonan syndrome are also found in several tumors in adulthood, we evaluated the potential contribution of RIT1 to leukemogenesis in Noonan syndrome. We screened 192 pediatric cases of acute lymphoblastic leukemias (96 B-ALL and 96 T-ALL) and 110 cases of juvenile myelomonocytic leukemias (JMML), but detected no variation in these tumoral samples, suggesting that Noonan patients with germline RIT1 mutations are not at high risk to developing JMML or ALL, and that RIT1 has at most a marginal role in these sporadic malignancies.

Clinical management of myelodysplastic syndrome/myeloproliferative neoplasm overlap syndromes

The myelodysplastic/myeloproliferative neoplasms (MDS/MPNs) are a unique group of hematologic malignancies characterized by concomitant myelodysplastic and myeloproliferative features. According to the 2008 WHO classification, the category includes atypical chronic myeloid leukemia (aCML), chronic myelomonocytic leukemia (CMML), juvenile myelomonocytic leukemia (JMML), MDS/MPN-unclassifiable (MDS/MPN-U), and the provisional entity refractory anemia with ring sideroblasts and thrombocytosis (RARS-T). Although diagnosis currently remains based on clinicopathologic features, the incorporation of next-generation platforms has allowed for the recent molecular characterization of these diseases which has revealed unique and complex mutational profiles that support their distinct biology and is anticipated to soon play an integral role in diagnosis, prognostication, and treatment. Future goals of research should include the development of disease-modifying therapies, and further genetic understanding of the category will likely form the foundation of these efforts.

Shp2 and Pten have antagonistic roles in myeloproliferation but cooperate to promote erythropoiesis in mammals

Previous data suggested a negative role of phosphatase and tensin homolog (Pten) and a positive function of SH2-containing tyrosine phosphatase (Shp2)/Ptpn11 in myelopoiesis and leukemogenesis. Herein we demonstrate that ablating Shp2 indeed suppressed the myeloproliferative effect of Pten loss, indicating directly opposing functions between pathways regulated by these two enzymes. Surprisingly, the Shp2 and Pten double-knockout mice suffered lethal anemia, a phenotype that reveals previously unappreciated cooperative roles of Pten and Shp2 in erythropoiesis. The lethal anemia was caused collectively by skewed progenitor differentiation and shortened erythrocyte lifespan. Consistently, treatment of Pten-deficient mice with a specific Shp2 inhibitor suppressed myeloproliferative neoplasm while causing anemia. These results identify concerted actions of Pten and Shp2 in promoting erythropoiesis, while acting antagonistically in myeloproliferative neoplasm development. This study illustrates cell type-specific signal cross-talk in blood cell lineages, and will guide better design of pharmaceuticals for leukemia and other types of cancer in the era of precision medicine.

RAS diseases in children

RAS genes encode a family of 21 kDa proteins that are an essential hub for a number of survival, proliferation, differentiation and senescence pathways. Signaling of the RAS-GTPases through the RAF-MEK-ERK pathway, the first identified mitogen-associated protein kinase (MAPK) cascade is essential in development. A group of genetic syndromes, named "RASopathies", had been identified which are caused by heterozygosity for germline mutations in genes that encode protein components of the RAS/MAPK pathway. Several of these clinically overlapping disorders, including Noonan syndrome, Noonan-like CBL syndrome, Costello syndrome, cardio-facio-cutaneous (CFC) syndrome, neurofibromatosis type I, and Legius syndrome, predispose to cancer and abnormal myelopoiesis in infancy. This review focuses on juvenile myelomonocytic leukemia (JMML), a malignancy of early childhood characterized by initiating germline and/or somatic mutations in five genes of the RAS/MAPK pathway: PTPN11, CBL, NF-1, KRAS and NRAS. Natural courses of these five subtypes differ, although hematopoietic stem cell transplantation remains the only curative therapy option for most children with JMML. With whole-exome sequencing studies revealing few secondary lesions it will be crucial to better understand the RAS/MAPK signaling network with its crosstalks and feed-back loops to carefully design early clinical trials with novel pharmacological agents in this still puzzling leukemia.

Peripheral blood cells from children with RASopathies show enhanced spontaneous colonies growth in vitro and hyperactive RAS signaling

Germline mutations in genes coding for molecules involved in the RAS/RAF/MEK/ERK pathway are the hallmarks of a newly classified family of autosomal dominant syndromes termed RASopathies. Myeloproliferative disorders (MPDs), in particular, juvenile myelomonocytic leukemia, can lead to potentially severe complications in children with Noonan syndrome (NS). We studied 27 children with NS or other RASopathies and 35 age-matched children as control subjects. Peripheral blood (PB) cells from these patients were studied for in vitro colony-forming units (CFUs) activity, as well as for intracellular phosphosignaling. Higher spontaneous growth of both burst-forming units-erythroid (BFU-E) and CFU-granulocyte/macrophage (CFU-GM) colonies from RAS-mutated patients were observed as compared with control subjects. We also observed a significantly higher amount of GM-colony-stimulating factor-induced p-ERK in children with RASopathies. Our findings demonstrate for the first time that PB cells isolated from children suffering from NS or other RASopathies without MPD display enhanced BFU-E and CFU-GM colony formation in vitro. The biological significance of these findings clearly awaits further studies. Collectively, our data provide a basis for further investigating of only partially characterized hematological alterations present in children suffering from RASopathies, and may provide new markers for progression toward malignant MPD in these patients.

Juvenile myelomonocytic leukemia

Juvenile myelomonocytic leukemia (JMML), a rare myeloid malignancy that occurs in young children, is considered a clonal disease originating in pluripotent stem cells of the hematopoietic system. The pathogenesis of JMML involves disruption of signal transduction through the RAS pathway, with resultant selective hypersensitivity of JMML cells to granulocyte-macrophage colony-stimulating factor. Progress has been made in understanding aspects of the molecular basis of JMML. How these molecular mechanisms may lead to targeted therapeutics and improved outcomes remains to be elucidated. Allogeneic hematopoietic stem cell transplant is the only curative option for children with JMML, and it is fraught with frequent relapse and significant toxicity.

K-RasV14I recapitulates Noonan syndrome in mice

Noonan syndrome (NS) is an autosomal dominant genetic disorder characterized by short stature, craniofacial dysmorphism, and congenital heart defects. NS also is associated with a risk for developing myeloproliferative disorders (MPD), including juvenile myelomonocytic leukemia (JMML). Mutations responsible for NS occur in at least 11 different loci including KRAS. Here we describe a mouse model for NS induced by K-Ras(V14I), a recurrent KRAS mutation in NS patients. K-Ras(V14I)-mutant mice displayed multiple NS-associated developmental defects such as growth delay, craniofacial dysmorphia, cardiac defects, and hematologic abnormalities including a severe form of MPD that resembles human JMML. Homozygous animals had perinatal lethality whose penetrance varied with genetic background. Exposure of pregnant mothers to a MEK inhibitor rescued perinatal lethality and prevented craniofacial dysmorphia and cardiac defects. However, Mek inhibition was not sufficient to correct these defects when mice were treated after weaning. Interestingly, Mek inhibition did not correct the neoplastic MPD characteristic of these mutant mice, regardless of the timing at which the mice were treated, thus suggesting that MPD is driven by additional signaling pathways. These genetically engineered K-Ras(V14I)-mutant mice offer an experimental tool for studying the molecular mechanisms underlying the clinical manifestations of NS. Perhaps more importantly, they should be useful as a preclinical model to test new therapies aimed at preventing or ameliorating those deficits associated with this syndrome.

Bedside to bench in juvenile myelomonocytic leukemia: insights into leukemogenesis from a rare pediatric leukemia

Juvenile myelomonocytic leukemia (JMML) is a typically aggressive myeloid neoplasm of childhood that is clinically characterized by overproduction of monocytic cells that can infiltrate organs, including the spleen, liver, gastrointestinal tract, and lung. JMML is categorized as an overlap myelodysplastic syndrome/myeloproliferative neoplasm (MDS/MPN) by the World Health Organization and also shares some clinical and molecular features with chronic myelomonocytic leukemia, a similar disease in adults. Although the current standard of care for patients with JMML relies on allogeneic hematopoietic stem cell transplant, relapse is the most frequent cause of treatment failure. Tremendous progress has been made in defining the genomic landscape of JMML. Insights from cancer predisposition syndromes have led to the discovery of nearly 90% of driver mutations in JMML, all of which thus far converge on the Ras signaling pathway. This has improved our ability to accurately diagnose patients, develop molecular markers to measure disease burden, and choose therapeutic agents to test in clinical trials. This review emphasizes recent advances in the field, including mapping of the genomic and epigenome landscape, insights from new and existing disease models, targeted therapeutics, and future directions.

Juvenile myelomonocytic leukaemia and Noonan syndrome

BACKGROUND

Infants with Noonan syndrome (NS) are predisposed to developing juvenile myelomonocytic leukaemia (JMML) or JMML-like myeloproliferative disorders (MPD). Whereas sporadic JMML is known to be aggressive, JMML occurring in patients with NS is often considered as benign and transitory. However, little information is available regarding the occurrence and characteristics of JMML in NS.

METHODS AND RESULTS

Within a large prospective cohort of 641 patients with a germline PTPN11 mutation, we identified MPD features in 36 (5.6%) patients, including 20 patients (3%) who fully met the consensus diagnostic criteria for JMML. Sixty percent of the latter (12/20) had severe neonatal manifestations, and 10/20 died in the first month of life. Almost all (11/12) patients with severe neonatal JMML were males. Two females who survived MPD/JMML subsequently developed another malignancy during childhood. Although the risk of developing MPD/JMML could not be fully predicted by the underlying PTPN11 mutation, some germline PTPN11 mutations were preferentially associated with myeloproliferation: 10/48 patients with NS (20.8%) with a mutation in codon Asp61 developed MPD/JMML in infancy. Patients with a p.Thr73Ile mutation also had more chances of developing MPD/JMML but with a milder clinical course. SNP array and whole exome sequencing in paired tumoral and constitutional samples identified no second acquired somatic mutation to explain the occurrence of myeloproliferation.

CONCLUSIONS

JMML represents the first cause of death in PTPN11-associated NS. Few patients have been reported so far, suggesting that JMML may sometimes be overlooked due to early death, comorbidities or lack of confirmatory tests.

Structural insights into Noonan/LEOPARD syndrome-related mutants of protein-tyrosine phosphatase SHP2 (PTPN11)

Background

The ubiquitous non-receptor protein tyrosine phosphatase SHP2 (encoded by PTPN11) plays a key role in RAS/ERK signaling downstream of most, if not all growth factors, cytokines and integrins, although its major substrates remain controversial. Mutations in PTPN11 lead to several distinct human diseases. Germ-line PTPN11 mutations cause about 50% of Noonan Syndrome (NS), which is among the most common autosomal dominant disorders. LEOPARD Syndrome (LS) is an acronym for its major syndromic manifestations: multiple Lentigines, Electrocardiographic abnormalities, Ocular hypertelorism, Pulmonary stenosis, Abnormalities of genitalia, Retardation of growth, and sensorineural Deafness. Frequently, LS patients have hypertrophic cardiomyopathy, and they might also have an increased risk of neuroblastoma (NS) and acute myeloid leukemia (AML). Consistent with the distinct pathogenesis of NS and LS, different types of PTPN11 mutations cause these disorders.

Results

Although multiple studies have reported the biochemical and biological consequences of NS- and LS-associated PTPN11 mutations, their structural consequences have not been analyzed fully. Here we report the crystal structures of WT SHP2 and five NS/LS-associated SHP2 mutants. These findings enable direct structural comparisons of the local conformational changes caused by each mutation.

Conclusions

Our structural analysis agrees with, and provides additional mechanistic insight into, the previously reported catalytic properties of these mutants. The results of our research provide new information regarding the structure-function relationship of this medically important target, and should serve as a solid foundation for structure-based drug discovery programs.

Juvenile myelomonocytic leukemia: molecular pathogenesis informs current approaches to therapy and hematopoietic cell transplantation

Juvenile myelomonocytic leukemia (JMML) is a rare childhood leukemia that has historically been very difficult to confidently diagnose and treat. The majority of patients ultimately require allogeneic hematopoietic cell transplantation (HCT) for cure. Recent advances in the understanding of the pathogenesis of the disease now permit over 90% of patients to be molecularly characterized. Pre-HCT management of patients with JMML is currently symptom-driven. However, evaluation of potential high-risk clinical and molecular features will determine which patients could benefit from pre-HCT chemotherapy and/or local control of splenic disease. Furthermore, new techniques to quantify minimal residual disease burden will determine whether pre-HCT response to chemotherapy is beneficial for long-term disease-free survival. The optimal approach to HCT for JMML is unclear, with high relapse rates regardless of conditioning intensity. An ongoing clinical trial in the Children's Oncology Group will test if less toxic approaches can be equally effective, thereby shifting the focus to post-HCT immunomanipulation strategies to achieve long-term disease control. Finally, our unraveling of the molecular basis of JMML is beginning to identify possible targets for selective therapeutic interventions, either pre- or post-HCT, an approach which may ultimately provide the best opportunity to improve outcomes for this aggressive disease.

Functional evaluation of circulating hematopoietic progenitors in Noonan syndrome

Noonan syndrome (NS) is an autosomal dominant disorder, characterized by short stature, multiple dysmorphisms and congenital heart defects. A myeloproliferative disorder (NS/MPD), resembling juvenile myelomonocytic leukemia (JMML), is occasionally diagnosed in infants with NS. In the present study, we performed a functional evaluation of the circulating hematopoietic progenitors in a series of NS, NS/MPD and JMML patients. The different functional patterns were compared with the aim to identify a possible NS subgroup worthy of stringent hematological follow-up for an increased risk of MPD development. We studied 27 NS and 5 JMML patients fulfilling EWOG-MDS criteria. The more frequent molecular defects observed in NS were mutations in the PTPN11 and SOS genes. The absolute count of monocytes, circulating CD34⁺ hematopoietic progenitors, their apoptotic rate and the number of circulating CFU-GMs cultured in the presence of decreasing concentrations or in the absence of granulocyte-macrophage colony-stimulating factor (GM-CSF) were evaluated. All JMML patients showed monocytosis >1,000/μl. Ten out of the 27 NS patients showed monocytosis >1,000/μl, which included the 3 NS/MPD patients. In JMML patients, circulating CD34⁺ cells were significantly increased (median, 109.8/μl; range, 44–232) with a low rate of apoptosis (median, 2.1%; range, 0.4–12.1%), and circulating CFU-GMs were hyper-responsive to GM-CSF. NS/MPD patients showed the same flow cytometric pattern as the JMML patients (median, CD34⁺ cells/μl, 205.7; range, 58–1374; median apoptotic rate, 1.4%; range, 0.2–2.4%) and their circulating CFU-GMs were hyper-responsive to GM-CSF. These functional alterations appeared 10 months before the typical clinical manifestations in 1 NS/MPD patient. In NS, the CD34⁺ absolute cell count and circulating CFU-GMs showed a normal pattern (median CD34⁺ cells/μl, 4.9; range, 1.3–17.5), whereas the CD34⁺ cell apoptotic rate was significantly decreased in comparison with the controls (median, 8.6%; range, 0–27.7% vs. median, 17.6%; range, 2.8–49.6%), suggesting an increased CD34⁺ cell survival. The functional evaluation of circulating hematopoietic progenitors showed specific patterns in NS and NS/MPD. These tests are a reliable integrative tool that, together with clinical data and other hematological parameters, could help detect NS patients with a high risk for a myeloproliferative evolution.

Noonan syndrome

Noonan syndrome is a genetic multisystem disorder characterised by distinctive facial features, developmental delay, learning difficulties, short stature, congenital heart disease, renal anomalies, lymphatic malformations, and bleeding difficulties. Mutations that cause Noonan syndrome alter genes encoding proteins with roles in the RAS-MAPK pathway, leading to pathway dysregulation. Management guidelines have been developed. Several clinically relevant genotype-phenotype correlations aid risk assessment and patient management. Increased understanding of the pathophysiology of the disease could help development of pharmacogenetic treatments.

Immunologic consequences of chemotherapy for acute myeloid leukemia

There are few data characterizing the immunologic consequences of chemotherapy for acute myeloid leukemia (AML) and almost nothing is known about the effects of chemotherapy in a pediatric AML cohort. We identified T-cell subsets, B-cell subsets, and used Enzyme-linked immunosorbent spot analyses to define the function of T cells and B cells in 7 pediatric patients with AML on chemotherapy. The data show that the effects of chemotherapy disproportionately target the B cell and depletion of B cells is associated with impaired responses to the inactivated influenza vaccine. Diminished T-cell numbers were also observed although the magnitude of the effect was less than what was seen for B cells. Furthermore, measures of T-cell function were largely intact. We conclude that humoral immunity is significantly affected by chemotherapy for AML.

Advances in the genetics of high-risk childhood B-progenitor acute lymphoblastic leukemia and juvenile myelomonocytic leukemia: implications for therapy

Hematologic malignancies of childhood comprise the most common childhood cancers. These neoplasms derive from the pathologic clonal expansion of an abnormal cancer-initiating cell and span a diverse spectrum of phenotypes, including acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), myeloproliferative neoplasms (MPN), and myelodysplastic syndromes (MDS). Expansion of immature lymphoid or myeloid blasts with suppression of normal hematopoiesis is the hallmark of ALL and AML, whereas MPN is associated with proliferation of 1 or more lineages that retain the ability to differentiate, and MDS is characterized by abnormal hematopoiesis and cytopenias. The outcomes for children with the most common childhood cancer, B-progenitor ALL (B-ALL), in general, is quite favorable, in contrast to children affected by myeloid malignancies. The advent of highly sensitive genomic technologies reveals the remarkable genetic complexity of multiple subsets of high-risk B-progenitor ALL, in contrast to a somewhat simpler model of myeloid neoplasms, although a number of recently discovered alterations displayed by both types of malignancies may lead to common therapeutic approaches. This review outlines recent advances in our understanding of the genetic underpinnings of high-risk B-ALL and juvenile myelomonocytic leukemia, an overlap MPN/MDS found exclusively in children, and we also discuss novel therapeutic approaches that are currently being tested in clinical trials. Recent insights into the clonal heterogeneity of leukemic samples and the implications for diagnostic and therapeutic approaches are also discussed.

Current management of juvenile myelomonocytic leukemia and the impact of RAS mutations

Juvenile myelomonocytic leukemia (JMML) is a rare clonal myelodysplastic/myeloproliferative disorder that affects young children. It is characterized by hypersensitivity of JMML cells to granulocyte-macrophage colony-stimulating factor (GM-CSF) in vitro. The pathogenesis of JMML seems to arise from constitutional activation of the GM-CSF/RAS (a GTPase) signaling pathway, a result of mutations in RAS, NF1, PTPN11, and CBL that interfere with downstream components of the pathway. Most patients with JMML usually experience an aggressive clinical course, and hematopoietic stem cell transplantation (HSCT) is currently the only curative treatment, although the high rates of relapses and graft failures are of great concern. In contrast, a certain proportion of patients experience a stable clinical course for a considerable period of time, and sometimes the disease even spontaneously resolves without any treatment. Recent studies have provided us with increased knowledge of genotype-phenotype correlations in JMML, and suggested that differences in clinical courses may reflect genetic status. Thus, genotype-based management is of current international interest, especially for JMML with RAS mutations. Cumulative evidence suggests that RAS mutations can be related to favorable clinical outcomes, and HSCT may not have to be a mandatory therapeutic option for a portion of patients with this mutation, although a consensus regarding genotype-based management has not yet been achieved. Further efforts toward identifying which patients who will do well without HSCT are required.

Recent advances in the pathogenesis and treatment of juvenile myelomonocytic leukaemia

Myeloid neoplasms derive from the pathological clonal expansion of an abnormal stem cell and span a diverse spectrum of phenotypes including acute myeloid leukaemia (AML), myeloproliferative neoplasms (MPN) and myelodysplastic syndromes (MDS). Expansion of myeloid blasts with suppression of normal haematopoiesis is the hallmark of AML, whereas MPN is associated with over-proliferation of one or more lineages that retain the capacity to differentiate, and MDS is characterized by cytopenias and aberrant differentiation. MPD and MDS can progress to AML, which is likely due to the acquisition of cooperative mutations. Juvenile myelomonocytic leukaemia (JMML) is an aggressive myeloid neoplasm of childhood that is clinically characterized by overproduction of monocytic cells that can infiltrate organs, including the spleen, liver, gastrointestinal tract, and lung. JMML is categorized as an overlap MPN/MDS by the World Health Organization and also shares some clinical and molecular features with chronic myelomonocytic leukaemia, a similar disease in adults. While the current standard of care for patients with JMML relies on allogeneic haematopoietic stem cell transplant (HSCT), relapse is the most frequent cause of treatment failure. This review outlines our understanding of the genetic underpinnings of JMML with a recent update on the discovery of novel CBL mutations, as well as a brief review on current therapeutic approaches.

Cancer in Noonan, Costello, cardiofaciocutaneous and LEOPARD syndromes

Noonan syndrome (NS), Costello syndrome (CS), cardiofaciocutaneous syndrome (CFCS), and LEOPARD syndrome (now also referred to as Noonan syndrome with multiple lentigines or NSML) are clinically overlapping dominant disorders that are caused by mutations in RAS signaling pathway genes. The spectrum of cancer susceptibility in this group of disorders has not been studied in detail. We identified more than 1900 cases of NS, CS, CFCS, or NSML reported in the literature between 1937 and 2010; 88 cancers were reported. The most common cancers reported in 1051 NS subjects were neuroblastoma (n = 8), acute lymphoblastic leukemia (n = 8), low grade glioma (n = 6), and rhabdomyosarcoma (n = 6). These associations are biologically plausible, given that somatic RAS pathway mutations are known to occur in these specific cancers. In addition, 40 childhood cases of myeloproliferative disease were described in individuals with NS, several of whom experienced a benign course of this hematologic condition. We confirmed the previously described association between CS and cancer in 268 reported individuals: 19 had rhabdomyosarcoma, 4 had bladder cancer, and 5 had neuroblastoma. By age 20, the cumulative incidence of cancer was approximately 4% for NS and 15% for CS; both syndromes had a cancer incidence peak in childhood. The cancers described in CFCS and NSML overlapped with those reported in NS and CS. Future epidemiologic studies will be required to confirm the described cancer spectrum and to estimate precise cancer risks.

Cancer in Noonan, Costello, cardiofaciocutaneous and LEOPARD syndromes

Noonan syndrome (NS), Costello syndrome (CS), cardiofaciocutaneous syndrome (CFCS), and LEOPARD syndrome (now also referred to as Noonan syndrome with multiple lentigines or NSML) are clinically overlapping dominant disorders that are caused by mutations in RAS signaling pathway genes. The spectrum of cancer susceptibility in this group of disorders has not been studied in detail. We identified more than 1900 cases of NS, CS, CFCS, or NSML reported in the literature between 1937 and 2010; 88 cancers were reported. The most common cancers reported in 1051 NS subjects were neuroblastoma (n = 8), acute lymphoblastic leukemia (n = 8), low grade glioma (n = 6), and rhabdomyosarcoma (n = 6). These associations are biologically plausible, given that somatic RAS pathway mutations are known to occur in these specific cancers. In addition, 40 childhood cases of myeloproliferative disease were described in individuals with NS, several of whom experienced a benign course of this hematologic condition. We confirmed the previously described association between CS and cancer in 268 reported individuals: 19 had rhabdomyosarcoma, 4 had bladder cancer, and 5 had neuroblastoma. By age 20, the cumulative incidence of cancer was approximately 4% for NS and 15% for CS; both syndromes had a cancer incidence peak in childhood. The cancers described in CFCS and NSML overlapped with those reported in NS and CS. Future epidemiologic studies will be required to confirm the described cancer spectrum and to estimate precise cancer risks.

Cancer risk in patients with Noonan syndrome carrying a PTPN11 mutation

Noonan syndrome (NS) is characterized by short stature, facial dysmorphisms and congenital heart defects. PTPN11 mutations are the most common cause of NS. Patients with NS have a predisposition for leukemia and certain solid tumors. Data on the incidence of malignancies in NS are lacking. Our objective was to estimate the cancer risk and spectrum in patients with NS carrying a PTPN11 mutation. In addition, we have investigated whether specific PTPN11 mutations result in an increased malignancy risk. We have performed a cohort study among 297 Dutch NS patients with a PTPN11 mutation (mean age 18 years). The cancer histories were collected from the referral forms for DNA diagnostics, and by consulting the Dutch national registry of pathology and the Netherlands Cancer Registry. The reported frequencies of cancer among NS patients were compared with the expected frequencies using population-based incidence rates. In total, 12 patients with NS developed a malignancy, providing a cumulative risk for developing cancer of 23% (95% confidence interval (CI), 8–38%) up to age 55 years, which represents a 3.5-fold (95% CI, 2.0–5.9) increased risk compared with that in the general population. Hematological malignancies occurred most frequently. Two malignancies, not previously observed in NS, were found: a malignant mastocytosis and malignant epithelioid angiosarcoma. No correlation was found between specific PTPN11 mutations and cancer occurrence. In conclusion, this study provides first evidence of an increased risk of cancer in patients with NS and a PTPN11 mutation, compared with that in the general population. Our data do not warrant specific cancer surveillance.

Clinical and hematologic findings in Noonan syndrome patients with PTPN11 gene mutations

Reports on Noonan syndrome (NS) have documented multiple types of coagulation defects and bleeding diathesis, and a wide range of clinical presentations. Early studies suggested that a large proportion of NS patients have coagulation defects, whereas more recent reports indicate low rates of coagulopathy. The aim of this study was to evaluate phenotypic characteristics, PTPN11 gene mutations, and hematological and coagulation parameters in 30 clinically diagnosed cases of NS. One of the NS patients had a history of easy bruising; however, his hematological and coagulation tests were normal. None of the other patients had clinical coagulation problems. In the NS group, values for platelet count, activity of factors XI, XII, and protein C were significantly lower than the corresponding means for the control group. However, the results of coagulation tests in the NS group were diagnostically inconclusive and only one patient had clinical signs of coagulopathy. Interestingly, two NS patients had low protein C activity. One of these children had an A1517C mutation and transient myelodysplasia. The other patient had a C1528G mutation in exon 13 that has not been reported previously. Neither of these individuals experienced a thrombotic event or any complication during approximately 3 years of follow-up. For all patients clinically diagnosed with NS, a thorough history of coagulation issues should be taken and first-line coagulation testing should be done to evaluate for bleeding diathesis. However, if these assessments reveal nothing abnormal, complications related to coagulation are unlikely and extensive testing is unnecessary.

Disorders of dysregulated signal traffic through the RAS-MAPK pathway: phenotypic spectrum and molecular mechanisms

RAS GTPases control a major signaling network implicated in several cellular functions, including cell fate determination, proliferation, survival, differentiation, migration, and senescence. Within this network, signal flow through the RAF-MEK-ERK pathway-the first identified mitogen-associated protein kinase (MAPK) cascade-mediates early and late developmental processes controlling morphology determination, organogenesis, synaptic plasticity, and growth. Signaling through the RAS-MAPK cascade is tightly controlled; and its enhanced activation represents a well-known event in oncogenesis. Unexpectedly, in the past few years, inherited dysregulation of this pathway has been recognized as the cause underlying a group of clinically related disorders sharing facial dysmorphism, cardiac defects, reduced postnatal growth, ectodermal anomalies, variable cognitive deficits, and susceptibility to certain malignancies as major features. These disorders are caused by heterozygosity for mutations in genes encoding RAS proteins, regulators of RAS function, modulators of RAS interaction with effectors, or downstream signal transducers. Here, we provide an overview of the phenotypic spectrum associated with germline mutations perturbing RAS-MAPK signaling, the unpredicted molecular mechanisms converging toward the dysregulation of this signaling cascade, and major genotype-phenotype correlations.

Germline CBL mutations cause developmental abnormalities and predispose to juvenile myelomonocytic leukemia

CBL encodes a member of the Cbl family of proteins, which functions as an E3 ubiquitin ligase. We describe a dominant developmental disorder resulting from germline missense CBL mutations, which is characterized by impaired growth, developmental delay, cryptorchidism and a predisposition to juvenile myelomonocytic leukemia (JMML). Some individuals experienced spontaneous regression of their JMML but developed vasculitis later in life. Importantly, JMML specimens from affected children show loss of the normal CBL allele through acquired isodisomy. Consistent with these genetic data, the common p.371Y>H altered Cbl protein induces cytokine-independent growth and constitutive phosphorylation of ERK, AKT and S6 only in hematopoietic cells in which normal Cbl expression is reduced by RNA interference. We conclude that germline CBL mutations have developmental, tumorigenic and functional consequences that resemble disorders that are caused by hyperactive Ras/Raf/MEK/ERK signaling and include neurofibromatosis type 1, Noonan syndrome, Costello syndrome, cardiofaciocutaneous syndrome and Legius syndrome.

Phosphatase-dependent and -independent functions of Shp2 in neural crest cells underlie LEOPARD syndrome pathogenesis

The tyrosine phosphatase SHP2 (PTPN11) regulates cellular proliferation, survival, migration, and differentiation during development. Germline mutations in PTPN11 cause Noonan and LEOPARD syndromes, which have overlapping clinical features. Paradoxically, Noonan syndrome mutations increase SHP2 phosphatase activity, while LEOPARD syndrome mutants are catalytically impaired, raising the possibility that SHP2 has phosphatase-independent roles. By comparing shp2-deficient zebrafish embryos with those injected with mRNA encoding LEOPARD syndrome point mutations, we identify a phosphatase- and Erk-dependent role for Shp2 in neural crest specification and migration. We also identify an unexpected phosphatase- and Erk-independent function, mediated through its SH2 domains, which is evolutionarily conserved and prevents p53-mediated apoptosis in the brain and neural crest. Our results indicate that previously enigmatic aspects of LEOPARD syndrome pathogenesis can be explained by the combined effects of loss of Shp2 catalytic function and retention of an SH2 domain-mediated role that is essential for neural crest cell survival.

Acute myeloid leukemia

Acute myeloid leukemia (AML) is a heterogeneous group of leukemias that result from clonal transformation of hematopoietic precursors through the acquisition of chromosomal rearrangements and multiple gene mutations. As a result of highly collaborative clinical research by pediatric cooperative cancer groups worldwide, disease-free survival has improved significantly during the past 3 decades. Further improvements in outcomes of children who have AML probably will reflect continued progress in understanding the biology of AML and the concomitant development of new molecularly targeted agents for use in combination with conventional chemotherapy drugs.