Multidetector Computed Tomography Assessment of Anatomical Ventricular Tachycardia Isthmuses in Repaired Tetralogy of Fallot

BACKGROUND

Tetralogy of Fallot (TOF) is associated with risk for sustained monomorphic ventricular tachycardia (VT). Preemptive electrophysiology study before transcatheter pulmonary valve placement is increasing, but the value of MDCT for anatomical VT isthmus assessment is unknown.

OBJECTIVES

The purpose of this study was to determine the impact of multidetector computed tomography (MDCT) in the evaluation of sustained monomorphic VT for repaired TOF.

METHODS

Consecutive pre-transcatheter pulmonary valve MDCT studies were identified, and anatomical isthmus dimensions were measured. For a subset of patients with preemptive electrophysiology study, MDCT features were compared with electroanatomical maps.

RESULTS

A total of 61 repaired TOFs with MDCT were identified (mean 35 ± 14 years, 58% men) with MDCT electroanatomical map pairs in 35 (57%). Calcification corresponding to patch material was present in 46 (75%) and was used to measure anatomical VT isthmuses. MDCT wall thickness correlated positively with number of ablation lesions and varied with functional isthmus properties (blocked isthmus 2.6 mm [Q1, Q3: 2.1, 4.0 mm], slow conduction 4.8 mm [Q1, Q3: 3.3, 6.0 mm], and normal conduction 5.6 mm [Q1, Q3: 3.9, 8.3 mm]; P < 0.001). A large conal branch was present in 6 (10%) and a major coronary anomaly was discovered in 3 (5%). Median ablation lesion distance was closer to the right vs the left coronary artery (10 mm vs 15 mm; P = 0.01) with lesion-to-coronary distance <5 mm in 3 patients.

CONCLUSIONS

MDCT identifies anatomical structures relevant to catheter ablation for repaired TOF. Wall thickness at commonly targeted anatomical VT isthmuses is associated with functional isthmus properties and increased thermal energy delivery.

Sinus rhythm QRS morphology reflects right ventricular activation and anatomical ventricular tachycardia isthmus conduction in repaired tetralogy of Fallot

BACKGROUND

Patients with repaired tetralogy of Fallot (TOF) are at risk for ventricular tachycardia (VT) related to well-described anatomical isthmuses.

OBJECTIVE

The purpose of this study was to explore QRS morphology as an indicator of anatomical isthmus conduction.

METHODS

Patients with repaired TOF and complete right bundle branch block referred for transcatheter pulmonary valve replacement (PVR) or presenting with sustained VT underwent comprehensive 3-dimensional mapping in sinus rhythm. Electrocardiographic characteristics were compared to right ventricular (RV) activation and anatomical isthmus conduction properties.

RESULTS

Twenty-two patients (19 pre-pulmonary valve replacement and 3 clinical VT) underwent comprehensive 3-dimensional mapping (median 39 years; interquartile range [IQR] 27-48 years; 12 [55%] male). Septal RV activation (median 40 ms; IQR 34-46 ms) corresponded to the nadir in lead V1 and free wall activation (median 71 ms; IQR 64-81 ms) to the transition point in the upstroke of the R' wave. Patients with isthmus block between the pulmonary annulus and the ventricular septal defect patch and between the ventricular septal defect patch and the tricuspid annulus (when present), were more likely to demonstrate lower amplitude R' waves in lead V1 (5.8 mV vs 9.4 mV; P = .005), QRS fragmentation in lead V1 (15 [94%] vs 2 [13%]; P < .001), and terminal S waves in lead aVF (15 [94%] vs 6 [40%]; P < .001) than those with intact conduction. During catheter ablation, these QRS changes developed during isthmus block.

CONCLUSION

For patients with repaired TOF, the status of septal isthmus conduction was evident from sinus rhythm QRS morphology. Low-amplitude, fragmented R' waves in lead V1 and terminal S waves in the inferior leads were related to septal isthmus conduction abnormalities, providing a mechanistic link between RV activation and common electrocardiographic findings.

Conduction System Pacing Versus Conventional Cardiac Resynchronization Therapy in Congenital Heart Disease

BACKGROUND

Dyssynchrony-associated left ventricular systolic dysfunction is a major contributor to heart failure in congenital heart disease (CHD). Although conventional cardiac resynchronization therapy (CRT) has shown benefit, the comparative efficacy of cardiac conduction system pacing (CSP) is unknown.

OBJECTIVES

To compare the clinical outcomes of CSP vs conventional CRT in CHD with biventricular, systemic left ventricular anatomy.

METHODS

Retrospective CSP data from 7 centers were compared with propensity score-matched conventional CRT control subjects. Outcomes were lead performance, change in left ventricular ejection fraction (LVEF), and QRS duration at 12 months.

RESULTS

A total of 65 CSP cases were identified (mean age 37 ± 21 years, 46% men). The most common CHDs were tetralogy of Fallot (n = 12 [19%]) and ventricular septal defect (n = 12 [19%]). CSP was achieved after a mean of 2.5 ± 1.6 attempts per procedure (38 patients with left bundle branch pacing, 17 with HBP, 10 with left ventricular septal myocardial). Left bundle branch area pacing [LBBAP] vs HBP was associated with a smaller increase in pacing threshold (Δ pacing threshold 0.2 V vs 0.8 V; P = 0.05) and similar sensing parameters at follow-up. For 25 CSP cases and control subjects with baseline left ventricular systolic dysfunction, improvement in LVEF was non-inferior (Δ LVEF 9.0% vs 6.0%; P = 0.3; 95% confidence limits: -2.9% to 10.0%) and narrowing of QRS duration was more pronounced for CSP (Δ QRS duration 35 ms vs 14 ms; P = 0.04). Complications were similar (3 [12%] CSP, 4 [16%] conventional CRT; P = 1.00).

CONCLUSIONS

CSP can be reliably achieved in biventricular, systemic left ventricular CHD patients with similar improvement in LVEF and greater QRS narrowing for CSP vs conventional CRT at 1 year. Among CSP patients, pacing electrical parameters were superior for LBBAP vs HBP.

Fecal Calprotectin Is a Predictor of Need for Rescue Therapy in Hospitalized Severe Colitis

BACKGROUND

Up to one-third of patients hospitalized for acute severe colitis secondary to inflammatory bowel diseases (IBD) do not adequately respond to intravenous steroids. There is an unmet need to identify a useful predictor for rescue treatment in this cohort of patients.

AIMS

The aim of this study was to assess the predictive efficacy of fecal calprotectin in identifying the need for medical or surgical therapy in patients with acute severe colitis.

METHODS

We conducted a multicenter retrospective cohort study including patients with ulcerative colitis (UC) who were hospitalized for severe exacerbation of colitis. The primary outcome was the need for in-hospital medical or surgical rescue therapy. Univariate and multivariate logistic regression was performed to identify predictors of rescue therapy.

RESULTS

Our study included 147 patients with UC. One-third (33%) required rescue therapy, and 13% underwent colectomy. Patients requiring rescue therapy had significantly higher fecal calprotectin (mean 1748 mcg/g vs 1353 mcg/g, P = .02) compared with those who did not. A fecal calprotectin >800 mcg/g independently predicted the need for inpatient medical rescue therapy (odds ratio, 2.61; 95% CI, 1.12-6.12). An admission calprotectin >800 mcg/g independently predicted surgery within 3 months (odds ratio, 2.88; 95% CI, 1.01-8.17).

CONCLUSIONS

Fecal calprotectin levels may serve as a useful noninvasive predictor of medical and surgical risk in individuals with UC presenting with acute severe colitis. This approach can facilitate earlier therapeutic interventions and improve outcomes.

Epicardial atrial pacing after the extracardiac Fontan operation: Feasibility of an entirely transvenous approach

This series describes an innovative technique for pacing in patients with sinus node dysfunction after extracardiac Fontan surgery. This transpulmonary approach to the left atrial epi-myocardium has been successfully applied to three patients at two centers and resulted in excellent acute and midterm pacing characteristics without known complications. The principal advantage of this procedure in comparison to prior iterations is the absence of pacing material within the pulmonary venous atrium, so that future systemic thromboembolism risk is minimized. The transpulmonary approach for permanent atrial pacing offers a novel solution to the unique challenges for patients after extracardiac Fontan operation.

Electrophysiological characteristics of atrial tachycardia recurrence: Relevance to catheter ablation strategies in adults with congenital heart disease

BACKGROUND

Catheter ablation outcomes for adults with congenital heart disease (ACHDs) are described, but recurrence mechanisms remain largely unknown.

OBJECTIVE

The purpose of this study was to identify the electrophysiological characteristics of atrial tachycardia (AT) recurrence in ACHD.

METHODS

ACHD AT procedures over a 10-year period were explored for AT or atrial fibrillation (AF) recurrence.

RESULTS

At 299 procedures in 250 ACHD (mean age 39 ± 15 years; 52% male), 464 ATs (360 intra-atrial reentrant tachycardia, 104 focal AT; median 2 [IQR 1-3] ATs per procedure) were targeted. Complete (n = 256 [86%]) or partial (n = 37 [12%]) success was achieved in 98% of procedures. Over a median of 3.0 (IQR 1.4-5.3) years of follow-up, 67 patients (27%) developed AT/AF recurrence after the index procedure. Recurrent vs index tachycardias were more often focal AT (38% vs 19%; P < .001), demonstrated longer cycle length (325 ms vs 280 ms; P = .003), required isoproterenol (50% vs 32%; P = .03), and involved the pulmonary venous atrium (PVA)/septum (53% vs 27%; P < .001). AF history (hazard ratio [HR] 2.0; interquartile range [IQR] 1.2-3.4; P = .01), incomplete success (HR 3.6; IQR 2.1-6.4; P < .001), and PVA substrate (HR 2.1; IQR 1.2-3.5; P = .006) were independently associated with AT/AF recurrence. After complete index procedure success and no AF history, 5-year actuarial freedom from AT/AF and AT alone were 77% and 80%.

CONCLUSION

After catheter ablation in ACHD, repeat ATs were frequently focal, requiring isoproterenol administration, or involved intra-atrial reentrant tachycardia within the PVA or atrial septum. Negative factors were partial success, index PVA substrate, and remote history of AF. These data support aggressive pharmacological provocation to eliminate all inducible tachycardias and coexisting PVA substrates at index procedures for ACHD.

Multicenter Outcomes of Catheter Ablation for Atrioventricular Reciprocating Tachycardia Mediated by Twin Atrioventricular Nodes

OBJECTIVES

This study sought to describe the electrophysiologic properties and catheter ablation outcomes for T-AVRT.

BACKGROUND

Although catheter ablation for atrioventricular (AV) reciprocating tachycardia via twin AV nodes (T-AVRT) is an established entity, there are few data on the electrophysiological properties and outcomes of this procedure.

METHODS

An international, multicenter study was conducted to collect retrospective procedural and outcomes data for catheter ablation of T-AVRT.

RESULTS

Fifty-nine patients with T-AVRT were identified (median age at procedure, 8 years [interquartile range, 4.4-17.0 years]; 49% male). Of these, 55 (93%) were diagnosed with heterotaxy syndrome (right atrial isomerism in 39, left atrial isomerism in 8, and indeterminate in 8). Twenty-three (39%) had undergone Fontan operation (12 extracardiac, 11 lateral tunnel). After the Fontan operation, atrial access was conduit or baffle puncture in 15 (65%), fenestration in 5 (22%), and retrograde in 3 (13%). Acute success was achieved in 43 (91%) of 47 attempts (targeting an anterior node in 23 and posterior node in 24). There was no high-grade AV block or change in QRS duration. Over a median of 3.8 years, there were 3 recurrences. Of 7 patients with failed index procedure or recurrent T-AVRT, 6 (86%) were associated with anatomical hurdles such as prior Fontan or catheter course through an interrupted inferior vena cava-to-azygous vein continuation (P = 0.11).

CONCLUSIONS

T-AVRT can be targeted successfully with low risk for recurrence. Complications were rare in this population. Anatomical challenges were common among patients with reduced short and long-term efficacy, representing opportunities for improvement in procedural timing and planning.

Mechanisms and outcomes of catheter ablation for biatrial tachycardia in adults with congenital heart disease

BACKGROUND

Biatrial tachycardia (BiAT) is a rare form of macroreentry not previously characterized in adults with congenital heart disease (ACHD).

OBJECTIVE

The purpose of this study was to determine the prevalence, mechanisms, and outcomes of catheter ablation for BiAT in ACHD.

METHODS

All ACHD undergoing catheter ablation for macroreentrant atrial tachycardia over a 10-year period were evaluated for evidence of BiAT. Patients were categorized as prior Senning, Fontan, or other biventricular operation. A novel biatrial global activation histogram (GAH) analysis was used to demonstrate the presence of interatrial connections (IACs).

RESULTS

Among 263 ACHD, BiAT was identified at 11 procedures in 10 patients (4.2%; median age 35 years; 30% male). The congenital category was Fontan in 6, Senning in 3, and biventricular in 2. Diagnosis of BiAT was associated with ablation era and mapping technology (P <.001) and could be confirmed with a novel GAH mapping approach for normally septated atrial connections. Catheter ablation targeted an IAC in 5 cases (Bjork Fontan and biventricular operations), a posterior isthmus in 3 (Senning operation), and the cavotricuspid isthmus or equivalent in 3 (lateral tunnel [LT] Fontan). Recurrence was isolated to ablation to sites at the expected location of the Bachmann bundle, and durable success could be achieved after repeat ablation.

CONCLUSION

BiAT occurs in approximately 4% of ACHD but is likely underrecognized. BiAT could be targeted at an IAC for normally septated atria and at a conventional critical isthmus after Senning and LT Fontan operations.

Cardiac implantable device outcomes and lead survival in adult congenital heart disease

BACKGROUND

Long-term outcomes of cardiac implantable electronic devices (CIEDs) are ill-defined in adult congenital heart disease (ACHD).

OBJECTIVE

To assess outcomes of transvenous (TV) and epicardial (EPI) CIEDs in ACHD.

METHODS

A retrospective review of CIEDs implanted in patients >18 yrs. followed at the Ahmanson/UCLA ACHD Center was performed. Patients were grouped by implant approach. Primary outcomes included time to CIED dysfunction, lead dysfunction and unplanned CIED reintervention.

RESULTS

Over a 27-year period, 283 CIEDs (208 TV, 75 EPI) were implanted in 260 ACHD patients. Dysfunction developed in 77 CIEDs (50 TV, 27 EPI) for which 62 underwent unplanned reintervention (47 TV, 15 EPI). Time to CIED dysfunction and unplanned reintervention did not differ by implant approach; however lead dysfunction was greater for EPI vs TV (HR 2.0, 95% CI 1.2-3.2, p = 0.01). Independent predictors of lead failure included cyanosis (HR 2.6, 95% CI 1.1-6.3; p = 0.03), implant indication other than bradycardia (HR 3.3, 95% CI 1.6-6.5; p < 0.01), right-sided Maze operation (HR 2.5, 95% CI 1.3-5.0; p = 0.01), and unipolar lead design (HR 4.5, 95% CI 1.8-11.5; p < 0.01). Importantly, EPI vs TV approach was not associated with lead dysfunction after adjusting for baseline covariates (HR 0.6, 95% CI 0.6-4.3; p = 0.3).

CONCLUSION

Overall CIED system dysfunction and reinterventions are similar, whereas lead dysfunction is greater among EPI than TV devices. Patient and procedural differences, rather than EPI vs TV implant approach alone, appear to drive CIED lead outcomes in the ACHD population.

Genotype Predicts Outcomes in Fetuses and Neonates With Severe Congenital Long QT Syndrome

OBJECTIVES

This study sought to determine the relationship between long QT syndrome (LQTS) subtype (LTQ1, LTQ2, LTQ3) and postnatal cardiac events (CEs).

BACKGROUND

LQTS presenting with 2:1 atrioventricular block or torsades de pointes in the fetus and/or neonate has been associated with risk for major CEs, but overall outcomes and predictors remain unknown.

METHODS

A retrospective study involving 25 international centers evaluated the course of fetuses/newborns diagnosed with congenital LQTS and either 2:1 atrioventricular block or torsades de pointes. The primary outcomes were age at first CE after dismissal from the newborn hospitalization and death and/or cardiac transplantation during follow-up. CE was defined as aborted cardiac arrest, appropriate shock from implantable cardioverter-defibrillator, or sudden cardiac death.

RESULTS

A total of 84 fetuses and/or neonates were identified with LQTS (12 as LQT1, 35 as LQT2, 37 as LQT3). Median gestational age at delivery was 37 weeks (interquartile range: 35 to 39 weeks) and age at hospital discharge was 3 weeks (interquartile range: 2 to 5 weeks). Fetal demise occurred in 2 and pre-discharge death in 1. Over a median of 5.2 years, there were 1 LQT1, 3 LQT2, and 23 LQT3 CEs (13 aborted cardiac arrests, 5 sudden cardiac deaths, and 9 appropriate shocks). One patient with LQT1 and 11 patients with LQT3 died or received cardiac transplant during follow-up. The only multivariate predictor of post-discharge CEs was LQT3 status (LQT3 vs. LQT2: hazard ratio: 8.4; 95% confidence interval: 2.6 to 38.9; p < 0.001), and LQT3, relative to LQT2, genotype predicted death and/or cardiac transplant (p < 0.001).

CONCLUSIONS

In this large multicenter study, fetuses and/or neonates with LQT3 but not those with LQT1 or LQT2 presenting with severe arrhythmias were at high risk of not only frequent, but lethal CEs.

Abstract IA31: Genetic approaches for testing candidate drug targets for Ras-induced tumorigenesis in vivo

Despite remarkable progress over the past few years, developing chemical inhibitors that directly and selectively target mutant Ras proteins remains a fundamental challenge. Given this, inhibitors of proteins that function upstream or downstream of oncogenic Ras are a promising alternative therapeutic strategy for RAS mutant tumors. However, the complexity of Ras signaling networks and adaptive mechanisms that result in pathway “rewiring” complicates identifying genes and proteins that are essential for the growth and survival of these cancers. Genetically engineered mice that express mutant Ras alleles from the endogenous locus are a robust system for uncovering pathway dependencies and potential therapeutic vulnerabilities. We have generated novel strains of mice to address two specific questions. First, we collaborated with Kevin Haigis to generate a “second site” C181S substitution within a conditional NrasLSL-G12D mutant allele (Nat Genet 2008;40:600) to ask if palmitoylation is essential for myeloid transformation in vivo. We intercrossed mice carrying this mutation with the Mx1-Cre strain and induced Cre recombinase expression in the hematopoietic compartment by injecting compound mutant pups with polyI:polyC. As previously reported, homozygous NrasLSL-G12D mice uniformly developed an aggressive myeloproliferative disorder (MPD) characterized by markedly elevated blood leukocyte counts, splenomegaly, and death by age 9 months. By contrast, homozygous NrasLSL-G12D,C181S mice remained healthy with no evidence of hematologic disease despite constitutively elevated levels of RasGTP in bone marrow cells that were similar to those observed in NrasLSL-G12D mice. Whereas normal NRas and N-RasG12D proteins are predominantly in the plasma membrane, N-RasG12D, C181S was largely mis-localized in the cytosol. We also generated Mx1-Cre, NrasG12D,C181S/G12D compound heterozygous mutant mice to investigate potential interactions between oncogenic NrasG12D alleles with and without the C181S substitution. Over half of these mice developed aggressive hematologic malignancies, albeit with prolonged latency compared to homozygous NrasG12D mice. Importantly, genetic analysis of diseased tissues revealed reduced frequency or complete absence of the NrasG12D,C181S allele due to secondary somatic genetic events, which indicates strong selective pressure to overcome the growth-suppressive properties of unpalmitoylated N-Ras. Together, these studies validate the palmitoylation/depalmitoylation cycle as a therapeutic target in NRAS mutant hematologic malignancies and establish a novel model system for addressing this question in other cancers. In another ongoing project, we collaborated with Pedro Perez-Macera and David Tuveson to modify a LSL-KrasG12D knock-in allele to introduce a second-site amino acid Y64G substitution that severely impairs binding to PI3 kinase. Surprisingly, mice expressing KrasG12D,Y64G in the germline are viable and phenotypically normal, although they are born at a lower than expected Mendelian frequency. Whereas Mx1-Cre, KrasG12D/+ mice uniformly develop aggressive MPD and die by 4 months of age, KrasG12D,Y64G animals have normal blood counts at 12-18 months of age. However, 100% of KrasG12D,Y64G mice develop lung lesions by 1 year of age, and ultimately succumb from lung tumors with a median survival of 496 days. The majority of these lesions are classified pathologically as atypical lymphoid proliferation or papillary adenomas, with a few mice developing adenocarcinomas. These studies demonstrating that expressing a Y64G amino acid substitution in the context of oncogenic KrasG12D rescues embryonic lethality, abrogates myeloid disease, and attenuates lung tumorigenesis are generally consistent with and also extend elegant studies of Pi3kca mutant mice from the Downward lab (Cell 2007;129:957; Cancer Cell 2013;24:617). Beyond the bone marrow and lung, KrasG12D,Y64G mice are a potent genetic tool for dissecting the role of aberrant PI3 kinase signaling in lung, pancreatic, colon, and other tissues characterized by tumors driven by somatic KRAS mutations, and these data have implications for treating human cancers with KRAS mutations.

Citation Format: Kevin M. Shannon. Genetic approaches for testing candidate drug targets for Ras-induced tumorigenesis in vivo [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr IA31.

Permanent conduction system pacing for congenitally corrected transposition of the great arteries: A Pediatric and Congenital Electrophysiology Society (PACES)/International Society for Adult Congenital Heart Disease (ISACHD) Collaborative Study

BACKGROUND

Congenitally corrected transposition of the great arteries (CCTGA) is associated with spontaneous atrioventricular block and pacing-induced cardiomyopathy. Conduction system pacing is a potential alternative to conventional cardiac resynchronization therapy (CRT).

OBJECTIVE

The purpose of this study was to determine the outcomes of conduction system pacing for CCTGA.

METHODS

Retrospective data were collected from 10 international centers.

RESULTS

His bundle (HBP) or left bundle branch pacing (LBBP) was attempted in 15 CCTGA patients (median age 23 years; 87% male). Previous surgery had been performed in 8 and chronic ventricular pacing in 7. Conduction system pacing (11 HBP, 2 LBBP 2; nonselective in 10, selective in 3) was acutely successful in 13 (86%) without complication. In 9 cases, electroanatomic mapping was available and identified the distal His bundle and proximal left bundle branches within the morphologic left ventricle below the pulmonary valve separate from the mitral annulus. Median implant HV interval was 42 ms (interquartile range [IQR] 35-48), R wave 6 mV (IQR 5-18), and threshold 0.5 V (IQR 0.5-1.2) at median 0.5 ms. QRSd was unchanged compared to junctional escape rhythm (124 vs 110 ms; P = .17) and decreased significantly compared to baseline ventricular pacing (112 vs 164 ms; P <.01). At a median of 8 months, all patients were alive without significant change in pacing threshold or lead dysfunction. New York Heart Association functional class improved in 5 patients.

CONCLUSION

Permanent conduction system pacing is feasible in CCTGA by either HBP or proximal LBBP. Narrow paced QRS and stable lead thresholds were observed at intermediate follow-up. Unique anatomic characteristics may favor this approach over conventional CRT.

Glucocorticoids paradoxically facilitate steroid resistance in T cell acute lymphoblastic leukemias and thymocytes

Glucocorticoids (GCs) are a central component of therapy for patients with T cell acute lymphoblastic leukemia (T-ALL), and although resistance to GCs is a strong negative prognostic indicator in T-ALL, the mechanisms of GC resistance remain poorly understood. Using diagnostic samples from patients enrolled in the frontline Children's Oncology Group (COG) T-ALL clinical trial AALL1231, we demonstrated that one-third of primary T-ALLs were resistant to GCs when cells were cultured in the presence of IL-7, a cytokine that is critical for normal T cell function and that plays a well-established role in leukemogenesis. We demonstrated that in these T-ALLs and in distinct populations of normal developing thymocytes, GCs paradoxically induced their own resistance by promoting upregulation of IL-7 receptor (IL-7R) expression. In the presence of IL-7, this augmented downstream signal transduction, resulting in increased STAT5 transcriptional output and upregulation of the prosurvival protein BCL-2. Taken together, we showed that IL-7 mediates an intrinsic and physiologic mechanism of GC resistance in normal thymocyte development that is retained during leukemogenesis in a subset of T-ALLs and is reversible with targeted inhibition of the IL-7R/JAK/STAT5/BCL-2 axis.

CRLF2 rearrangement in Ph-like acute lymphoblastic leukemia predicts relative glucocorticoid resistance that is overcome with MEK or Akt inhibition

Philadelphia chromosome-like (Ph-like) acute lymphoblastic leukemia (ALL) is a genetically heterogeneous subtype of B-cell ALL characterized by chromosomal rearrangements and mutations that result in aberrant cytokine receptor and kinase signaling. In particular, chromosomal rearrangements resulting in the overexpression of cytokine receptor-like factor 2 (CRLF2) occur in 50% of Ph-like ALL cases. CRLF2 overexpression is associated with particularly poor clinical outcomes, though the molecular basis for this is currently unknown. Glucocorticoids (GCs) are integral to the treatment of ALL and GC resistance at diagnosis is an important negative prognostic factor. Given the importance of GCs in ALL therapy and the poor outcomes for patients with CRLF2 overexpression, we hypothesized that the aberrant signal transduction associated with CRLF2 overexpression might mediate intrinsic GC insensitivity. To test this hypothesis, we exposed Ph-like ALL cells from patient-derived xenografts to GCs and found that CRLF2 rearranged (CRLF2_R) leukemias uniformly demonstrated reduced GC sensitivity in vitro. Furthermore, targeted inhibition of signal transduction with the MEK inhibitor trametinib and the Akt inhibitor MK2206, but not the JAK inhibitor ruxolitinib, was sufficient to augment GC sensitivity. These data suggest that suboptimal GC responses may in part underlie the poor clinical outcomes for patients with CRLF2 overexpression and provide rationale for combination therapy involving GCs and signal transduction inhibitors as a means of enhancing GC efficacy.

Postoperative and short-term atrial tachyarrhythmia burdens after transcatheter vs surgical pulmonary valve replacement among congenital heart disease patients

OBJECTIVE

We examined the atrial tachyarrhythmia (AT) burden among patients with congenital heart disease (CHD) following transcatheter (TC-) or surgical (S-) pulmonary valve replacement (PVR).

DESIGN/SETTING

This was a retrospective observational study of patients who underwent PVR from 2010 to 2016 at UCLA Medical Center.

PATIENTS

Patients of all ages who had prior surgical repair for CHD were included. Patients with a history of congenitally corrected transposition of the great arteries, underwent a hybrid PVR procedure, or had permanent atrial fibrillation (AF) without a concomitant ablation were excluded.

OUTCOME MEASURES

The primary outcome was a time-to-event analysis of sustained AT. Sustained ATs were defined as focal AT, intra-atrial reentrant tachycardia/atrial flutter, or AF lasting at least 30 seconds or terminating with cardioversion or antitachycardia pacing.

RESULTS

Two hundred ninety-seven patients (TC-PVR, n = 168 and S-PVR, n = 129) were included. During a median follow-up of 1.2 years, nine events occurred in TC-PVR group (5%) vs 23 events in S-PVR group (18%). In the propensity adjusted models, the following factors were associated with significant risk of AT after PVR: history of AT, age at valve implantation, severe right atrial enlargement, and S-PVR. In the secondary analysis, TC-PVR was associated with lower adjusted risk of AT events in the postoperative epoch (first 30 days), adjusted IRR 0.31 (0.14-0.97), P = .03, but similar risk in the short-term epoch, adjusted IRR 0.64 (0.14-2.94), P = .57.

CONCLUSION

There was an increased risk of AT in the first 30 days following S-PVR compared to TC-PVR. Additional factors associated with risk of AT events after PVR were a history of AT, age at valve implantation, and severe right atrial enlargement.

Monomorphic Ventricular Arrhythmias in Athletes

Ventricular arrhythmias are challenging to manage in athletes with concern for an elevated risk of sudden cardiac death (SCD) during sports competition. Monomorphic ventricular arrhythmias (MMVA), while often benign in athletes with a structurally normal heart, are also associated with a unique subset of idiopathic and malignant substrates that must be clearly defined. A comprehensive evaluation for structural and/or electrical heart disease is required in order to exclude cardiac conditions that increase risk of SCD with exercise, such as hypertrophic cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy. Unique issues for physicians who manage this population include navigating athletes through the decision of whether they can safely continue their chosen sport. In the absence of structural heart disease, therapies such as radiofrequency catheter ablation are very effective for certain arrhythmias and may allow for return to competitive sports participation. In this comprehensive review, we summarise the recommendations for evaluating and managing athletes with MMVA.

A minimally invasive hybrid approach for cardiac resynchronization of the systemic right ventricle

BACKGROUND

Patients with systemic right ventricle (RV) often develop progressive heart failure and may benefit from cardiac resynchronization therapy (CRT); however, the optimal strategy for CRT has not been defined.

METHODS

A retrospective review of all the patients with systemic RV failure undergoing a hybrid transcatheter-surgical approach was performed. Procedural technique and outcomes are reported.

RESULTS

Six patients underwent detailed electroanatomical mapping of the systemic RV followed by a new hybrid approach targeting latest endocardial activation, which was followed by focused epicardial mapping. The exact site of latest endocardial activation was variable but localized to the basolateral RV in all cases. Sites of latest activation tended to be more superior during contralateral ventricular pacing versus intact atrioventricular conduction (P = 0.06). Latest endocardial activation at the targeted site occurred at 157 ms (interquartile range [IQR] = 120-181 ms) and corresponding epicardial activation at 174 ms (IQR = 140-198 ms), after the onset of the QRS complex. Following the hybrid CRT, the QRS duration decreased from a median of 193 to 147 ms and the fractional area of change increased from a median of 15.5% to 30% (P < 0.001). Patients were discharged to home after a median of 4 days. Of the three patients who were initially referred for transplant evaluation, two (66%) of them no longer met the criteria following CRT.

CONCLUSIONS

Whereas latest endocardial activation for the systemic RV appears to localize to the basolateral region, the optimal lead position may be variable. An approach utilizing endocardial mapping followed by a limited surgical incision and confirmation of latest activation may result in minimally invasive surgery and a favorable acute CRT response.

AMPK/FIS1-Mediated Mitophagy Is Required for Self-Renewal of Human AML Stem Cells

Leukemia stem cells (LSCs) are thought to drive the genesis of acute myeloid leukemia (AML) as well as relapse following chemotherapy. Because of their unique biology, developing effective methods to eradicate LSCs has been a significant challenge. In the present study, we demonstrate that intrinsic overexpression of the mitochondrial dynamics regulator FIS1 mediates mitophagy activity that is essential for primitive AML cells. Depletion of FIS1 attenuates mitophagy and leads to inactivation of GSK3, myeloid differentiation, cell cycle arrest, and a profound loss of LSC self-renewal potential. Further, we report that the central metabolic stress regulator AMPK is also intrinsically activated in LSC populations and is upstream of FIS1. Inhibition of AMPK signaling recapitulates the biological effect of FIS1 loss. These data suggest a model in which LSCs co-opt AMPK/FIS1-mediated mitophagy as a means to maintain stem cell properties that may be otherwise compromised by the stresses induced by oncogenic transformation.

Transvenous pacemaker implantation after the bidirectional Glenn operation for patients with complex congenital disease

INTRODUCTION

The bidirectional Glenn operation for congenital heart disease produces anatomical constraints to conventional transvenous pacemaker implantation. An iliac approach, although not previously described in this population, is potentially a preferable alternative to a thoracotomy for epicardial pacing.

METHODS AND RESULTS

A single-center retrospective review was performed for all patients that underwent transvenous pacemaker implantation following the bidirectional Glenn operation with partial biventricular repair. Follow-up data, implant indications, and techniques were recorded. Five patients underwent a transvenous iliac approach (median age 26.9 years, interquartile range [IQR] 25.8-27.6). Pacing indications included AV block in 3 patients (2 requiring cardiac resychronization therapy) and sinus node dysfunction in 2. Implanted leads were atrial in 4 and ventricular in 3 (1 of the latter was placed in the coronary sinus). In two cases, transvenous leads were tunneled to a preexisting epicardial abdominal generator. Median follow-up was 4.1 years (range 1.0-16.7 years). One patient underwent device revision for lead position-related groin discomfort; a second patient developed atrial lead failure following a Maze operation and underwent lead replacement by the iliac approach. Patients were not routinely anticoagulated postprocedure given lead position in the subpulmonary circulation. At last follow-up, all patients were alive. One patient underwent heart transplantation 6 months after implant with only partial resolution of pacing-induced cardiomyopathy.

CONCLUSIONS

Trans-iliac pacemaker placement may be an effective alternative to surgery for patients requiring permanent pacing after the Glenn operation.

Neural Transplantation for Huntington's Disease: Experimental Rationale and Recommendations for Clinical Trials

Huntington's disease (HD) is a neurodegenerative disorder affecting motor function, personality, and cognition. This paper reviews the experimental data that demonstrate the potential for transplantation of fetal striatum and trophic factor secreting cells to serve as innovative treatment strategies for HD. Transplantation strategies have been effective in replacing lost neurons or preventing the degeneration of neurons destined to die in both rodent and nonhuman primate models of HD. In this regard, a logical series of investigations has proven that grafts of fetal striatum survive, reinnervate the host, and restore function impaired following excitotoxic lesions of the striatum. Furthermore, transplants of cells genetically modified to secrete trophic factors such as nerve growth factor protect striatal neurons from degeneration due to excitotoxicity or mitochondrial dysfunction. Given the disabling and progressive nature of HD, coupled with the absence of any meaningful medical therapy, it is reasonable to consider clinical trials of neural transplantation for this disease. Fetal striatal implants will most likely be the first transplant strategy attempted for HD. This paper describes the variable parameters we believe to be critical for consideration for the design of clinical trials using fetal striatal implants for the treatment of HD.

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.