Ras oncogene-independent activation of RALB signaling is a targetable mechanism of escape from NRAS(V12) oncogene addiction in acute myeloid leukemia

Somatic mutations that lead to constitutive activation of NRAS and KRAS proto-oncogenes are among the most common in human cancer and frequently occur in acute myeloid leukemia (AML). An inducible NRAS(V12)-driven AML mouse model has established a critical role for continued NRAS(V12) expression in leukemia maintenance. In this model genetic suppression of NRAS(V12) expression results in rapid leukemia remission, but some mice undergo spontaneous relapse with NRAS(V12)-independent (NRI) AMLs providing an opportunity to identify mechanisms that bypass the requirement for Ras oncogene activity and drive leukemia relapse. We found that relapsed NRI AMLs are devoid of NRAS(V12) expression and signaling through the major oncogenic Ras effector pathways, phosphatidylinositol-3-kinase and mitogen-activated protein kinase, but express higher levels of an alternate Ras effector, Ralb, and exhibit NRI phosphorylation of the RALB effector TBK1, implicating RALB signaling in AML relapse. Functional studies confirmed that inhibiting CDK5-mediated RALB activation with a clinically relevant experimental drug, dinaciclib, led to potent RALB-dependent antileukemic effects in human AML cell lines, induced apoptosis in patient-derived AML samples in vitro and led to a 2-log reduction in the leukemic burden in patient-derived xenograft mice. Furthermore, dinaciclib potently suppressed the clonogenic potential of relapsed NRI AMLs in vitro and prevented the development of relapsed AML in vivo. Our findings demonstrate that Ras oncogene-independent activation of RALB signaling is a therapeutically targetable mechanism of escape from NRAS oncogene addiction in AML.

A Clinical Risk Score to Improve the Diagnosis of Tachycardia-Induced Cardiomyopathy in Childhood

Tachycardia-induced cardiomyopathy (TIC) is a treatable cause of heart failure in children, but there is little information as to which clinical variables best discriminate TIC from other forms of cardiomyopathy. TIC cases with dilated cardiomyopathy (DC) from 16 participating centers were identified and compared with controls with other forms of DC. Presenting clinical, echocardiographic, and electrocardiographic characteristics were collected. Heart rate (HR) percentile was defined as HR/median HR for age, and PR index as the PR/RR interval. P-wave morphology (PWM) was defined as possible sinus or nonsinus based on a predefined algorithm. Eighty TIC cases and 135 controls were identified. Cases demonstrated lower LV end-diastolic diameter and LV end-systolic diameter than DC controls (4.3 vs 6.5, p <0.001; 7.4 vs 10.9, p <0.001) and were less likely to receive inotropic medication at presentation (p <0.001 for both). Multivariable logistic regression identified HR percentile (OR 2.1 per 10% increase, CI 1.3 to 4.6; p = 0.014), PR index (OR 1.2, CI 1.1 to 1.4; p = 0.004), and nonsinus PWM (OR 104.9, CI 15.2 to 1,659.8; p <0.001) as predictive of TIC status. A risk score using HR percentile >130%, PR index >30%, and nonsinus PWM was associated with a sensitivity of 100% and specificity of 87% for the diagnosis of TIC. Model training and validation area under the curves were similar at 0.97 and 0.94, respectively. In conclusion, pediatric TIC may be accurately discriminated from other forms of DC using simple electrocardiographic parameters. This may allow for rapid diagnosis and early treatment of this condition.

Somatic and Germline TP53 Alterations in Second Malignant Neoplasms from Pediatric Cancer Survivors

Purpose: Second malignant neoplasms (SMNs) are severe late complications that occur in pediatric cancer survivors exposed to radiotherapy and other genotoxic treatments. To characterize the mutational landscape of treatment-induced sarcomas and to identify candidate SMN-predisposing variants, we analyzed germline and SMN samples from pediatric cancer survivors.Experimental Design: We performed whole-exome sequencing (WES) and RNA sequencing on radiation-induced sarcomas arising from two pediatric cancer survivors. To assess the frequency of germline TP53 variants in SMNs, Sanger sequencing was performed to analyze germline TP53 in 37 pediatric cancer survivors from the Childhood Cancer Survivor Study (CCSS) without any history of a familial cancer predisposition syndrome but known to have developed SMNs.Results: WES revealed TP53 mutations involving p53's DNA-binding domain in both index cases, one of which was also present in the germline. The germline and somatic TP53-mutant variants were enriched in the transcriptomes for both sarcomas. Analysis of TP53-coding exons in germline specimens from the CCSS survivor cohort identified a G215C variant encoding an R72P amino acid substitution in 6 patients and a synonymous SNP A639G in 4 others, resulting in 10 of 37 evaluable patients (27%) harboring a germline TP53 variant.Conclusions: Currently, germline TP53 is not routinely assessed in patients with pediatric cancer. These data support the concept that identifying germline TP53 variants at the time a primary cancer is diagnosed may identify patients at high risk for SMN development, who could benefit from modified therapeutic strategies and/or intensive posttreatment monitoring. Clin Cancer Res; 23(7); 1852-61. ©2016 AACR.

Clinical Experience With the Subcutaneous Implantable Cardioverter–Defibrillator in Adults With Congenital Heart Disease

Background—

Sudden cardiac death is a major contributor to mortality for adults with congenital heart disease. The subcutaneous implantable cardioverter–defibrillator (ICD) has emerged as a novel tool for prevention of sudden cardiac death, but clinical performance data for adults with congenital heart disease are limited.

Methods and Results—

A retrospective study involving 7 centers over a 5-year period beginning in 2011 was performed. Twenty-one patients (median 33.9 years) were identified. The most common diagnosis was single ventricle physiology (52%), 9 palliated by Fontan operation and 2 by aortopulmonary shunts: d -transposition of the great arteries after Mustard/Senning (n=2), tetralogy of Fallot (n=2), aortic valve disease (n=2), and other biventricular surgery (n=4). A prior cardiac device had been implanted in 7 (33%). The ICD indication was primary prevention in 67% and secondary in 33% patients. The most common reason for subcutaneous ICD placement was limited transvenous access for ventricular lead placement (n=10) followed by intracardiac right-to-left shunt (n=5). Ventricular arrhythmia was induced in 17 (81%) and was converted with ≤80 Joules in all. There was one implant complication related to infection, not requiring device removal. Over a median follow-up of 14 months, 4 patients (21%) received inappropriate and 1 (5%) patient received appropriate shocks. There was one arrhythmic death related to asystole in a single ventricle patient.

Conclusions—

Subcutaneous ICD implantation is feasible for adults with congenital heart disease patients. Most candidates have single ventricle heart disease and limited transvenous options for ICD placement. Despite variable anatomy, this study demonstrates successful conversion of induced ventricular arrhythmia and reasonable rhythm discrimination during follow-up.

Stat5 is critical for the development and maintenance of myeloproliferative neoplasm initiated by Nf1 deficiency

Juvenile myelomonocytic leukemia is a rare myeloproliferative neoplasm characterized by hyperactive RAS signaling. Neurofibromin1 (encoded by the NF1 gene) is a negative regulator of RAS activation. Patients with neurofibromatosis type 1 harbor loss-of-function mutations in NF1 and have a 200- to 500-fold increased risk of juvenile myelomonocytic leukemia. Leukemia cells from patients with juvenile myelomonocytic leukemia display hypersensitivity to certain cytokines, such as granulocyte-macrophage colony-stimulating factor. The granulocyte-macrophage colony-stimulating factor receptor utilizes pre-associated JAK2 to initiate signals after ligand binding. JAK2 subsequently activates STAT5, among other downstream effectors. Although STAT5 is gaining recognition as an important mediator of growth factor signaling in myeloid leukemias, the contribution of STAT5 to the development of hyperactive RAS-initiated myeloproliferative disease has not been well described. In this study, we investigated the consequence of STAT5 attenuation via genetic and pharmacological approaches in Nf1-deficient murine models of juvenile myelomonocytic leukemia. We found that homozygous Stat5 deficiency extended the lifespan of Nf1-deficient mice and eliminated the development of myeloproliferative neoplasm associated with Nf1 gene loss. Likewise, we found that JAK inhibition with ruxolitinib attenuated myeloproliferative neoplasm in Nf1-deficient mice. Finally, we found that primary cells from a patient with KRAS-mutant juvenile myelomonocytic leukemia displayed reduced colony formation in response to JAK2 inhibition. Our findings establish a central role for STAT5 activation in the pathogenesis of juvenile myelomonocytic leukemia and suggest that targeting this pathway may be of clinical utility in these patients.

Pediatric Bone Marrow Transplantation: Intensive Care Management

Bone marrow transplantation (BMT) has assumed in creasing importance in the treatment of bone marrow failure, hematopoietic malignancies, congenital immuno deficiencies, and solid tumors. Children undergoing BMT are at high risk for infection and hemorrhage dur ing the period of aplasia. In addition, life-threatening complications of circulatory, pulmonary, gastrointesti nal, hepatic, and renal function are common and fre quently require intensive supportive care. This review provides an overview of pediatric BMT that focuses on management problems relevant to intensive care. Thor ough pretransplantation assessment of underlying organ dysfunction is mandatory before undertaking BMT. The complications associated with preconditioning regi mens that use total body irradiation and high doses of ablative chemotherapy are described. Finally, problems involving individual organs are discussed by systems. The challenge of improving the results of BMT in the treatment of childhood malignant and hematopoietic disorders depends, in large part, on successful preven tive measures and good management of complications that occur immediately before and within the first 100 days after transplantation. As BMT is indicated for treat ment of an increasing number of diseases, more patients will require the care of intensivists familiar with trans plantation-related complications.

Abstract IA20: Targeting hyperactive Ras signaling in acute leukemia

Ras proteins are molecular switches that cycle between active guanosine triphosphate (GTP) and inactive guanosine diphosphate (GDP)-bound conformations (Ras-GTP and Ras-GDP). Guanine nucleotide exchange factors (GNEFs) stimulate nucleotide exchange on Ras, resulting in increased Ras-GTP levels. Upon GTP binding, the switch I and switch II domains of Ras undergo a conformational change that allows them to interact productively with downstream effectors including Raf, phosphatidylinositol 3-kinase (PI3K), and Ral-GDS to activate kinase signaling cascades that regulate cell proliferation, differentiation, and survival. Mutational, biochemical, and cell biologic studies of human cancers and experiments in mouse models strongly implicate deregulated signaling through the PI3K/Akt/mTOR and Raf/MEK/ERK cascades in cancer initiation and maintenance. Ras-GTP is hydrolyzed to Ras-GDP through an intrinsic GTPase activity. This slow “off” reaction is greatly augmented by GTPase activating proteins (GAPs). Thus, the competing activities of GNEFs and GAPs regulate Ras-GTP levels in vivo. Neurofibromin, the protein encoded by NF1, and p120 GAP are the predominant Ras GAPs in mammalian cells. Codons 12, 13, and 61 of RAS genes are the most common targets of dominant mutations in human cancer, with NRAS codon 12 mutations predominating in AML. Substitutions in these residues result in constitutively elevated levels of Ras-GTP due to reduced intrinsic GTP hydrolysis and resistance to GAPs. RAS and NF1 mutations occur in pediatric patients with a spectrum of hematologic malignancies including juvenile myelomonocytic leukemia (JMML), acute myeloid leukemia (AML), and acute lymphoblastic leukemia (ALL).

Because oncogenic Ras proteins are exceedingly challenging biochemical targets, most recent drug discovery efforts have focused on inhibiting downstream signaling molecules such as Raf, MEK, and Akt with many compounds. We have engineered and exploited strains of mice that recapitulate endogenous expression of cancer-associated RAS alleles or NF1 inactivation to model human acute leukemias and to conduct biologic and preclinical studies. In particular, we have deployed the MOL4070LTR retrovirus as an insertional mutagen to generate genetically heterogeneous transplantable acute leuekmias characterized by hyperactive Ras signaling. This system provides a novel in vivo forward genetic strategy for introducing cooperating mutations and for generating clonal heterogeneity. Leukemia cells can be manipulated ex vivo and transplanted into congenic recipients. This experimental flexibility provides an opportunity to establish cohorts of mice that are engrafted with the same primary cancers for conducting preclinical testing and for investigating mechanisms of intrinsic and acquired drug resistance.

I will discuss biologic and preclinical therapeutic studies in which we treated primary Nras, Kras, and Nf1 mutant AMLs characterized by hyperactive Ras signaling with targeted agents in vivo, isolated drug resistant clones at relapse, and compared drug sensitive and resistant clones to discover genes that modulate resistance. This latter strategy represents a potent and unbiased strategy to address mechanisms of resistance to kinase inhibitors, which has emerged as a major clinical problem in cancer therapeutics, and for investigating therapeutic strategies that might be effective in human cancers in which oncogenic RAS is a driver mutation. Our results in these robust AML models support testing drug combinations that include Raf/MEK/ERK pathway inhibitors and other targeted and conventional agents.

Citation Format: Kevin M. Shannon. Targeting hyperactive Ras signaling in acute leukemia. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Pediatric Cancer Research: From Mechanisms and Models to Treatment and Survivorship; 2015 Nov 9-12; Fort Lauderdale, FL. Philadelphia (PA): AACR; Cancer Res 2016;76(5 Suppl):Abstract nr IA20.

Characterization of Cardiac Troponin Elevation in the Setting of Pediatric Supraventricular Tachycardia

Cardiac troponin (cTn) is currently considered the gold standard biomarker for detection of myocardial necrosis. Patients with supraventricular tachycardia (SVT) often present with symptoms resulting in cTn assessment; however, there are no data on the results of such testing in childhood. We hypothesized that cTn elevation would be common in the pediatric SVT population and would portend a benign prognosis. A retrospective review of all pediatric patients (≤21 years) presenting with SVT was performed. Clinical and electrocardiographic variables from the emergency department (ED) presentation were reviewed and clinical outcomes during subsequent follow-up assessed. Of 128 patients seen in the ED for SVT, cTn was assessed in 48 (38 %). Of patients with cTn assessment, 14 (29 %) patients demonstrated cTn elevation. Univariate predictors of cTn elevation included presentation with respiratory or gastrointestinal symptoms (50 vs 12 % and 42 vs 9 %; p = 0.008 and p = 0.01, respectively), lower mean arterial blood pressure (73 vs 85 mm Hg, p = 0.009), higher age-adjusted tachycardia rate (z score 9.3 vs 7.2, p < 0.001), and longer tachycardia duration (4.2 vs 1.0 h, p = 0.02). Multivariate logistic regression confirmed the association of age-adjusted tachycardia rate (odds ratio [OR] 3.8 per heart rate z score, confidence interval [CI] 1.9-11.8, p = 0.003) and duration (OR 1.5 per hour, CI 1.1-2.5, p = 0.03). Clinical outcome was excellent with no adverse sequelae during a median of 2.9 years of follow-up. Cardiac Tn elevation is common in the pediatric population presenting with SVT. Episode severity, characterized by respiratory or gastrointestinal symptoms, lower mean blood pressure, and increased tachycardia rate and duration are predictive. Clinical follow-up is favorable.

Abstract B01: Mechanisms of treatment resistance following Ras targeted therapy in acute myeloid leukemia

Ras proto-oncogenes and numerous effectors of Ras signaling are recurrently mutated in acute myeloid leukemia (AML). While Ras represents an attractive therapeutic target, attempts at targeting oncogenic Ras directly have not been successful. To evaluate potential mechanisms of resistance to Ras targeted treatment in AML, our group leveraged a robust genetically-engineered mouse model in which AML is driven by expression of a tetracycline-repressible, constitutively active NRAS oncogene, tre-NRAS(V12), and a Mll-AF9 “knock-in” leukemogenic fusion gene (tre-NRAS(V12) & Mll-AF9 or tNM AML). The leukemia cells in this model are “addicted” to NRAS(V12), and mimicking Ras targeted treatment by doxycycline (Dox)-mediated suppression of the tre-NRAS(V12) oncogene expression results in rapid eradication of AML cells in vitro and in vivo. Furthermore, prolonged suppression of NRAS(V12) expression in NRAS(V12)-dependent (NRD) AML leads to the spontaneous development of NRAS(V12)-independent (NRI), “relapsed/refractory”, AML in about half of Dox treated mice. To elucidate the mechanism(s) that lead to the development of “relapsed/refractory” NRI AML following suppression of NRAS(V12) we performed transcriptome-wide next generation RNA-sequencing and comprehensive flow cytometric analysis of cancer signaling pathways comparing the parental NRD AML and two NRI AMLs. We confirmed that the NRAS(V12) oncogene is not aberrantly re-expressed. Furthermore, endogenous Ras gene expression is not up-regulated, and there is no evidence of reactivation of canonical Ras effector signaling pathways in either of the NRI AMLs by phospho-flow cytometry. Our preliminary analysis of cancer signaling pathways and transcriptome-wide RNA sequencing have identified candidate mediators of NRAS(V12)-independent AML growth and survival including the Myb proto-oncogene and anti-apoptotic Bcl2 that are enriched in both NRI AMLs relative to NRD AML at both the transcript and protein level. Furthermore, Myc protein is enriched in both NRI AMLs compared to NRD AML. We are currently performing a more comprehensive analysis of our next-generation RNA sequencing data to refine our list of candidate genes, and investigating the potential functional roles of Bcl2, Myb, and Myc in the development of NRI AML. In our initial functional studies, inhibition of Bcl2 activity suppresses NRI AML leukemic colony formation in vitro, and we are currently evaluating the ability of enforced expression of Bcl2, Myb, or Myc in NRD AML cells to render them NRAS(V12)-independent. To further investigate the translational potential of our findings, we are also evaluating the anti-leukemic effect of clinically relevant inhibitors of Bcl2 with inhibitors of canonical Ras effector pathways (RAF-MEK-ERK and PI3K-AKT-mTOR) in preclinical AML models. In this way we hope to gain a better understanding of mechanisms of treatment resistance to Ras targeted therapies, and thereby provide a foundation for the rational development of novel targeted treatment approaches for AML.

Abstract B43: Requirement for PI3 kinase interaction in K-RasG12D-driven leukemogenesis

Background: Somatic RAS mutations occur in approximately 30% of human cancers. Oncogenic Ras proteins activate multiple downstream effectors including Raf, PI3 kinase (PI3K), and Ral-GDS. The tyrosine 64 residue, which is located in the Switch II effector binding domain of Ras, is an important contact for binding PI3K catalytic subunits. To investigate the role of PI3K binding in hematopoeitic transformation in vivo, we created a “second site” Lox-STOP-Lox (LSL- KrasG12D/Y64G) knock in allele.

Methods: LSL-KrasG12D and LSL-KRasG12D/Y64G mice on a 129/Sv background were crossed with C57BL/6 Mx1-Cre mice. These mice were injected with polyIpolyC (pIpC) at 21 days of age to excise the LSL casette, which was confirmed by QPCR. Colony growth potential of primary bone marrow cells was assessed by sacrificing six to ten week old Mx1-Cre;LSL-KrasG12D, Mx1-Cre;LSL-KrasG12D/Y64G, and wild type mice, isolating bone marrow, and plating 50,000 cells in methylcellulose media containing 0, 0.01, 0.1, 1, or 10 ng/ml GM-CSF. Activation of Ras effector pathways was assessed in macrophages derived from primary bone marrow cells harvested from 6-10 week old mice and grown in the presence of M-CSF.

Results: As previously described, inducing oncogenic KrasG12D expression in the hematopoietic compartment of Mx1-Cre;KrasG12D mice results in death by three months of age from an aggressive myeloproliferative neoplasm (MPN) that accurately models chronic and juvenile myelomonocytic leukemias. MPN development is greatly attenuated in Mx1-Cre;LSL-KrasG12D/Y64G mice, which develop anemia beginning at six months of age, but do not display elevated blood leukocyte counts and have &gt;90% survival at eight months. Bone marrow cells from Mx1-Cre;LSL-KrasG12D mice form myeloid progenitor colonies in methylcellulose without added cytokines and show profound hypersensitivity to GM-CSF. By contrast, G12D/Y64G doubly mutant progenitors require GM-CSF for colony formation, and display modestly elevated cytokine sensitivity compared to wild type progenitor cells. Whereas cultured Mx1-Cre;LSL-KrasG12D and Mx1-Cre;LSL-KrasG12D/Y64G macrophages contain markedly elevated levels of active Ras-GTP, they exhibit different patterns of effector pathway activation. In particular, basal levels of phosphorylated ERK and Akt (pERK and pAkt) are unexpectedly higher in Mx1-Cre;LSL-KRasG12D/Y64G macrophages, implicating the PI3K pathway in feedback responses to oncogenic Ras output.

Conclusions: Our initial data support an important role of efficient PI3K activation in KrasG12D-driven leukemia and uncover unexpected complexity in cellular responses to Ras-GTP. We are continuing to monitor blood counts in aging Mx1-Cre;LSL-KrasG12D/Y64G mice and are analyzing effects of this mutation on eythroid differentiaion and on hematopoieitc stem and progentior cell (HSPC) populations. In addition, this novel conditional mutant allele will be a valuable genetic tool for assessing the importance of oncogenic-Ras-mediated PI3K signaling in other cancers with frequent somatic KRAS mutations including lung, pancreatic, and colon.

Transcaval Puncture for Access to the Pulmonary Venous Atrium After the Extracardiac Total Cavopulmonary Connection Operation

Background—

Patients with surgically palliated total cavopulmonary connection are at risk for recurrent atrial arrhythmia requiring catheter ablation. Transcatheter procedures for those with extracardiac conduits (extracardiac-total cavopulmonary connection) are perhaps the most challenging because of exclusion of the venous circulation from the arrhythmia substrate. Puncture through the inferior vena cava to the pulmonary venous atrium may be an effective route for access in these patients.

Methods and Results—

The pediatric and adult congenital surgical databases were explored for patients with extracardiac-total cavopulmonary connection and postoperative computed tomography imaging to assess for the presence of clinically relevant (>3 mm) apposition between the inferior vena cava and pulmonary venous atrium (cavoatrial overlap). The degree of overlap between the structures was measured by 2 blinded reviewers. Patients were stratified by surgical repair in childhood versus adult congenital heart disease. Thirty-seven patients were identified, with cavoatrial overlap observed in 9 (36%) of pediatric and 1 (9%) of adult congenital heart disease–repaired patients. Time elapsed after surgery was associated with cavoatrial overlap in the pediatric cohort ( P =0.034) and was identified in all pediatric patients with computed tomography imaging ≥8 years after surgery. Three patients underwent successful transcaval puncture during the study period without complication.

Conclusions—

Puncture through a region of overlap between the inferior vena cava and pulmonary venous atrium is feasible. Cavoatrial overlap is present in a substantial proportion of patients undergoing extracardiac-total cavopulmonary connection in childhood and is associated with a longer time elapsed since surgery.

Abstract B15: NRASG12V oncogene mediates self-renewal in a murine model of acute myelogenous leukemia

Mutant RAS oncoproteins activate signaling molecules that drive oncogenesis in multiple human tumors including acute myelogenous leukemia (AML). However, the specific functions of these pathways in AML are unclear, thwarting the rational application of targeted therapeutics. To elucidate the downstream functions of activated NRAS in AML, we employed a murine model that harbors Mll-AF9 and a tetracycline repressible, activated NRAS (NRASG12V). We performed gene expression microarray and RNA sequencing of our AML cells in the presence and absence of NRASG12V. By employing computational approaches to explore NRASG12V-responsive genes in our model, we found that NRASG12V enforced the leukemia self-renewal gene expression signature and was required to maintain an MLL-AF9 and Myb-dependent leukemia self-renewal gene expression program. In functional assays, NRASG12V was required for leukemia self-renewal independently of its effects on growth and survival. We used CyTOF (mass cytometry) for a multiplexed analysis of RAS-dependent signaling intermediates, and found that Mac-1Low cells, which harbor leukemia stem cells, were preferentially sensitive to NRASG12V withdrawal. Using RAS-pathway inhibitors, we found NRASG12V maintained leukemia self-renewal through mTor and Mek pathway activation, implicating these pathways as potential targets for cancer stem cell-specific therapies. Together, these experimental results define a RAS oncogene-driven function that is critical for leukemia maintenance and represents a novel mechanism of oncogene addiction. Recent work has shown that NRASG12V has bimodal effects in hematopoietic stem cells (Li et al. Nature 2013). To understand the mechanism of these bimodal effects, we have performed single cell RNA sequencing on our AML model. We expect that these analyses will reveal the cell-type specific NRASG12V –mediated mechanisms of leukemia self renewal.

Citation Format: Zohar Sachs, Rebecca S. LaRue, Hanh T. Nguyen, Karen Sachs, Nurul Azyan Mohd Hassan, Ernesto Diaz-Flores, Susan K. Rathe, Aaron L. Sarver, Sean C. Bendall, Ngoc A. Ha, Miechaleen D. Diers, Garry P. Nolan, Kevin M. Shannon, David A. Largaespada. NRASG12V oncogene mediates self-renewal in a murine model of acute myelogenous leukemia. [abstract]. In: Proceedings of the AACR Special Conference on RAS Oncogenes: From Biology to Therapy; Feb 24-27, 2014; Lake Buena Vista, FL. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(12 Suppl):Abstract nr B15. doi: 10.1158/1557-3125.RASONC14-B15

MLL3 is a haploinsufficient 7q tumor suppressor in acute myeloid leukemia

Recurring deletions of chromosome 7 and 7q [-7/del(7q)] occur in myelodysplastic syndromes and acute myeloid leukemia (AML) and are associated with poor prognosis. However, the identity of functionally relevant tumor suppressors on 7q remains unclear. Using RNAi and CRISPR/Cas9 approaches, we show that an ∼50% reduction in gene dosage of the mixed lineage leukemia 3 (MLL3) gene, located on 7q36.1, cooperates with other events occurring in -7/del(7q) AMLs to promote leukemogenesis. Mll3 suppression impairs the differentiation of HSPC. Interestingly, Mll3-suppressed leukemias, like human -7/del(7q) AMLs, are refractory to conventional chemotherapy but sensitive to the BET inhibitor JQ1. Thus, our mouse model functionally validates MLL3 as a haploinsufficient 7q tumor suppressor and suggests a therapeutic option for this aggressive disease.

Predictors of myocardial recovery in pediatric tachycardia-induced cardiomyopathy

BACKGROUND

Tachycardia-induced cardiomyopathy (TIC) carries significant risk of morbidity and mortality, although full recovery is possible. Little is known about the myocardial recovery pattern.

OBJECTIVE

The purpose of this study was to determine the time course and predictors of myocardial recovery in pediatric TIC.

METHODS

An international multicenter study of pediatric TIC was conducted. Children ≤18 years with incessant tachyarrhythmia, cardiac dysfunction (left ventricular ejection fraction [LVEF] <50%), and left ventricular (LV) dilation (left ventricular end-diastolic dimension [LVEDD] z-score ≥2) were included. Children with congenital heart disease or suspected primary cardiomyopathy were excluded. Primary end-points were time to LV systolic functional recovery (LVEF ≥55%) and normal LV size (LVEDD z-score <2).

RESULTS

Eighty-one children from 17 centers met inclusion criteria: median age 4.0 years (range 0.0-17.5 years) and baseline LVEF 28% (interquartile range 19-39). The most common arrhythmias were ectopic atrial tachycardia (59%), permanent junctional reciprocating tachycardia (23%), and ventricular tachycardia (7%). Thirteen required extracorporeal membrane oxygenation (n = 11) or ventricular assist device (n = 2) support. Median time to recovery was 51 days for LVEF and 71 days for LVEDD. Two (4%) underwent heart transplantation, and 1 died (1%). Multivariate predictors of LV systolic functional recovery were age (hazard ratio [HR] 0.61, P = .040), standardized tachycardia rate (HR 1.16, P = .015), mechanical circulatory support (HR 2.61, P = .044), and LVEF (HR 1.33 per 10% increase, p=0.005). For normalization of LV size, only baseline LVEDD (HR 0.86, P = .008) was predictive.

CONCLUSION

Pediatric TIC resolves in a predictable fashion. Factors associated with faster recovery include younger age, higher presenting heart rate, use of mechanical circulatory support, and higher LVEF, whereas only smaller baseline LV size predicts reverse remodeling. This knowledge may be useful for clinical evaluation and follow-up of affected children.

Characterization of Anatomic Ventricular Tachycardia Isthmus Pathology After Surgical Repair of Tetralogy of Fallot

Background—

Although catheter ablation has been used to target the critical isthmuses for re-entrant monomorphic ventricular tachycardia in tetralogy of Fallot, the anatomy and histology of these regions have not been fully characterized. Autopsy hearts with tetralogy of Fallot were evaluated to clarify the pathological substrate.

Methods and Results—

Twenty-seven hearts with the diagnosis of tetralogy of Fallot were examined. Anatomically defined isthmuses included (1A) ventriculotomy-to-tricuspid annulus, (1B) ventriculotomy-to-ventricular septal defect patch, (2) ventriculotomy-to-pulmonary annulus, (3) pulmonary annulus-to-ventricular septal defect patch, and (4) ventricular septal defect patch-to-tricuspid annulus. Length and wall thickness were measured for all specimens, and light microscopy was performed for those surviving surgery. For subjects ≥5 years at death, isthmuses 1A and 1B were present in 88%, isthmus 2 in 25%, isthmus 3 in 94%, and isthmus 4 in 13%. Isthmus 1A had the greatest dimensions (mean length, 3.9±1.08; thickness, 1.5±0.3 cm), isthmus 1B intermediate dimensions (mean length, 2.4±0.8; thickness, 1.1±0.4 cm), and isthmuses 2, 3, and 4 the smallest dimensions (mean length, 1.5±0.5, 1.4±0.8, and 0.6±0.4 cm; thickness, 0.5±0.2, 0.6±0.2, and 0.3±0.04 cm, respectively). Histological examination (n=7) revealed increased fibrosis in anatomic isthmuses relative to nonisthmus controls.

Conclusions—

Consistencies in isthmus dimensions and histology are found among patients with repaired tetralogy of Fallot. Isthmus 1A is associated with the largest morphological dimensions, whereas the nearby newly described isthmus 1B is significantly smaller. Of isthmuses with the smallest dimensions, isthmus 3 is the most common.

Gain of MYC underlies recurrent trisomy of the MYC chromosome in acute promyelocytic leukemia

The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.

Gain of chromosome 8 is the most common chromosomal gain in human acute myeloid leukemia (AML). It has been hypothesized that gain of the MYC protooncogene is of central importance in trisomy 8, but the experimental data to support this are limited and controversial. In a mouse model of promyelocytic leukemia in which the MRP8 promoter drives expression of the PML-RARA fusion gene in myeloid cells, a Myc allele is gained in approximately two-thirds of cases as a result of trisomy for mouse chromosome 15. We used this model to test the idea that MYC underlies acquisition of trisomy in AML. We used a retroviral vector to drive expression of wild-type, hypermorphic, or hypomorphic MYC in bone marrow that expressed the PML-RARA transgene. MYC retroviruses cooperated in myeloid leukemogenesis and suppressed gain of chromosome 15. When the PML-RARA transgene was expressed in a Myc haploinsufficient background, we observed selection for increased copies of the wild-type Myc allele concomitant with leukemic transformation. In addition, we found that human myeloid leukemias with trisomy 8 have increased MYC. These data show that gain of MYC can contribute to the pathogenic effect of the most common trisomy of human AML.

The corrected QT interval before and after heart transplantation

Heart donor candidates have severe neurologic injuries that have been associated with significant prolongation of the corrected QT (QTc) interval. Screening for an underlying abnormality of cardiac repolarization such as the long-QT syndrome thus becomes difficult. The aims of this study were to establish normal values and determine factors associated with prolongation of pre- and post-transplantation QTc intervals in a large cohort of heart transplantation donors and recipients. The medical records of 179 donors and 112 recipients were reviewed for historical, electrocardiographic, and neuroimaging data. After linear regression analysis, gunshot wounds were associated with the shortest mean pre-transplantation QTc interval of 447 +/- 51 ms (p = 0.016), whereas all other mechanisms of brain injury were associated with markedly prolonged QTc intervals. Overall, the mean QTc interval decreased from 467 +/- 58 to 446 +/- 47 ms (p <0.001), the mean QRS duration increased from 87 +/- 16 to 98 +/- 21 ms (p <0.001), and the mean QT dispersion did not change significantly after transplantation. The only factor associated with a prolonged QTc interval in the post-transplantation period was hypokalemia, with a mean QTc of 468 +/- 37 ms (p = 0.047). In conclusion, the mechanism of donor brain injury is associated with alterations in the pre-transplantation QTc interval, with the shortest intervals related to gunshot wounds. Fewer than 5% of the donor population was found to have QTc interval > or =580 ms. For those afflicted by gunshot wounds, <5% had QTc intervals > or =550 ms. This information can be used in pre-transplantation donor assessment, and post-transplantation management can be tailored to avoid the occurrence of ventricular arrhythmia.

Long-term survival after cardiac arrest in hypertrophic cardiomyopathy

BACKGROUND

Patients with hypertrophic cardiomyopathy (HCM) and aborted cardiac arrest are generally regarded as a high-risk subgroup susceptible to future major cardiac events and an unfavorable prognosis. However, outcome over extended time periods after major arrhythmic events is unresolved in such HCM patients.

OBJECTIVE

This study sought to more completely define the natural history of HCM.

METHODS

Of 916 HCM patients in the Minneapolis Heart Institute registry, 39 experienced either cardiac arrest (n = 21) or an appropriate shock from a prophylactically implanted cardioverter-defibrillator (ICD) (n = 18), and were assessed prospectively.

RESULTS

Age at initial arrhythmic event was 34 +/- 17 years (range 8 to 68; 67% <40 years). Of the 39 study patients, 32 (82%) survived after their initial cardiac event (for 9.4 +/- 7.6 years; up to 30 years), including 14 patients for >or=10 years (36%) and 4 patients >or=20 years (10%). Of the 32 survivors, 15 (47%) have not experienced subsequent events, and 17 (53%) had >or=1 additional cardiac arrest or appropriate ICD intervention. Annual HCM-related mortality was 1.4%, similar to general HCM populations, and 88% of patients were free of HCM-related death over the follow-up period. Survival from potentially lethal arrhythmias was associated with no or only mild heart failure symptoms in 29 of 32 patients (91%) at most recent evaluation.

CONCLUSION

In HCM, long-term survival up to 30 years may follow cardiac arrest with or without ICD intervention. Disabling heart failure symptoms were uncommon after these arrhythmic events, at last follow-up. These novel observations regarding the natural history of HCM underscore the unpredictability of the arrhythmogenic substrate, which may remain dormant over extended periods of time.

Single-cell profiling identifies aberrant STAT5 activation in myeloid malignancies with specific clinical and biologic correlates

Progress in understanding the molecular pathogenesis of human myeloproliferative disorders (MPDs) has led to guidelines incorporating genetic assays with histopathology during diagnosis. Advances in flow cytometry have made it possible to simultaneously measure cell type and signaling abnormalities arising as a consequence of genetic pathologies. Using flow cytometry, we observed a specific evoked STAT5 signaling signature in a subset of samples from patients suspected of having juvenile myelomonocytic leukemia (JMML), an aggressive MPD with a challenging clinical presentation during active disease. This signature was a specific feature involving JAK-STAT signaling, suggesting a critical role of this pathway in the biological mechanism of this disorder and indicating potential targets for future therapies.