New Ref-1/APE1 targeted inhibitors demonstrating improved potency for clinical applications in multiple cancer types.

AP endonuclease-1/Redox factor-1 (APE1/Ref-1 or Ref-1) is a multifunctional protein that is overexpressed in most aggressive cancers and impacts various cancer cell signaling pathways. Ref-1's redox activity plays a significant role in activating transcription factors (TFs) such as NFκB, HIF1α, STAT3 and AP-1, which are crucial contributors to the development of tumors and metastatic growth. Therefore, development of potent, selective inhibitors to target Ref-1 redox function is an appealing approach for therapeutic intervention. A first-generation compound, APX3330 successfully completed phase I clinical trial in adults with progressing solid tumors with favorable response rate, pharmacokinetics (PK), and minimal toxicity. These positive results prompted us to develop more potent analogs of APX3330 to effectively target Ref-1 in solid tumors. In this study, we present structure-activity relationship (SAR) identification and validation of lead compounds that exhibit a greater potency and a similar or better safety profile to APX3330. In order to triage and characterize the most potent and on-target second-generation Ref-1 redox inhibitors, we assayed for PK, mouse and human S9 fraction metabolic stability, in silico ADMET properties, ligand-based WaterLOGSY NMR measurements, pharmacodynamic markers, cell viability in multiple cancer cell types, and two distinct 3-dimensional (3D) cell killing assays (Tumor-Microenvironment on a Chip and 3D spheroid). To characterize the effects of Ref-1 inhibition in vivo, global proteomics was used following treatment with the top four analogs. This study identified and characterized more potent inhibitors of Ref-1 redox function (that outperformed APX3330 by 5-10-fold) with PK studies demonstrating efficacious doses for translation to clinic.

Spatial Gene-Expression Profiling Unveils Immuno-oncogenic Programs of NF1-Associated Peripheral Nerve Sheath Tumor Progression.

PURPOSE: Plexiform neurofibromas (PNF) are benign peripheral nerve sheath tumors (PNST) associated with neurofibromatosis type 1 (NF1). Despite similar histologic appearance, these neoplasms exhibit diverse evolutionary trajectories, with a subset progressing to malignant peripheral nerve sheath tumor (MPNST), the leading cause of premature death in individuals with NF1. Malignant transformation of PNF often occurs through the development of atypical neurofibroma (ANF) precursor lesions characterized by distinct histopathologic features and CDKN2A copy-number loss. Although genomic studies have uncovered key driver events promoting tumor progression, the transcriptional changes preceding malignant transformation remain poorly defined. EXPERIMENTAL DESIGN: Here we resolve gene-expression profiles in PNST across the neurofibroma-to-MPNST continuum in NF1 patients and mouse models, revealing early molecular features associated with neurofibroma evolution and transformation. RESULTS: Our findings demonstrate that ANF exhibit enhanced signatures of antigen presentation and immune response, which are suppressed as malignant transformation ensues. MPNST further displayed deregulated survival and mitotic fidelity pathways, and targeting key mediators of these pathways, CENPF and BIRC5, disrupted the growth and viability of human MPNST cell lines and primary murine Nf1-Cdkn2a-mutant Schwann cell precursors. Finally, neurofibromas contiguous with MPNST manifested distinct alterations in core oncogenic and immune surveillance programs, suggesting that early molecular events driving disease progression may precede histopathologic evidence of malignancy. CONCLUSIONS: If validated prospectively in future studies, these signatures may serve as molecular diagnostic tools to augment conventional histopathologic diagnosis by identifying neurofibromas at high risk of undergoing malignant transformation, facilitating risk-adapted care.

Combined CDK4/6 and ERK1/2 Inhibition Enhances Antitumor Activity in NF1-Associated Plexiform Neurofibroma.

PURPOSE: Plexiform neurofibromas (PNF) are peripheral nerve sheath tumors that cause significant morbidity in persons with neurofibromatosis type 1 (NF1), yet treatment options remain limited. To identify novel therapeutic targets for PNF, we applied an integrated multi-omic approach to quantitatively profile kinome enrichment in a mouse model that has predicted therapeutic responses in clinical trials for NF1-associated PNF with high fidelity. EXPERIMENTAL DESIGN: Utilizing RNA sequencing combined with chemical proteomic profiling of the functionally enriched kinome using multiplexed inhibitor beads coupled with mass spectrometry, we identified molecular signatures predictive of response to CDK4/6 and RAS/MAPK pathway inhibition in PNF. Informed by these results, we evaluated the efficacy of the CDK4/6 inhibitor, abemaciclib, and the ERK1/2 inhibitor, LY3214996, alone and in combination in reducing PNF tumor burden in Nf1flox/flox;PostnCre mice. RESULTS: Converging signatures of CDK4/6 and RAS/MAPK pathway activation were identified within the transcriptome and kinome that were conserved in both murine and human PNF. We observed robust additivity of the CDK4/6 inhibitor, abemaciclib, in combination with the ERK1/2 inhibitor, LY3214996, in murine and human NF1(Nf1) mutant Schwann cells. Consistent with these findings, the combination of abemaciclib (CDK4/6i) and LY3214996 (ERK1/2i) synergized to suppress molecular signatures of MAPK activation and exhibited enhanced antitumor activity in Nf1flox/flox;PostnCre mice in vivo. CONCLUSIONS: These findings provide rationale for the clinical translation of CDK4/6 inhibitors alone and in combination with therapies targeting the RAS/MAPK pathway for the treatment of PNF and other peripheral nerve sheath tumors in persons with NF1.

Multicenter, prospective, phase II study of maintenance bevacizumab for children and adults with NF2-related schwannomatosis and progressive vestibular schwannoma.

BACKGROUND: Prospective data on maintenance therapy with bevacizumab for persons with NF2-related schwannomatosis (NF2-SWN) is lacking. In this prospective multicenter phase II study, we evaluated the efficacy, safety, and tolerability of bevacizumab for maintenance therapy in children and adults with NF2-SWN and hearing loss due to vestibular schwannomas (VS). METHODS: Following induction therapy, participants received bevacizumab 5 mg/kg every 3 weeks for 18 months. Participants were monitored for changes in hearing, tumor size, and quality of life (QOL), and for adverse events. Hearing loss was defined as a statistically significant decline in word recognition score (WRS) or pure-tone average compared to the study baseline; tumor growth was defined as >20% increase in volume compared to baseline. RESULTS: Twenty participants with NF2-SWN (median age 23.5 years; range, 12.5-62.5 years) with hearing loss in the target ear (median WRS 70%, range 2%-94%) received maintenance bevacizumab. Freedom from hearing loss in the target ear was 95% after 48 weeks, 89% after 72 weeks, and 70% after 98 weeks. Freedom from tumor growth in the target VS was 94% after 48 weeks, 89% after 72 weeks, and 89% after 98 weeks. NF2-related QOL remained stable for 98 weeks whereas tinnitus-related distress decreased. Maintenance bevacizumab was well tolerated, with 3 participants (15%) discontinuing treatment due to adverse events. CONCLUSIONS: Maintenance bevacizumab (5 mg/kg every 3 weeks) is associated with high rates of hearing and tumor stability during 18 months of follow-up. No new unexpected adverse events related to bevacizumab were identified in this population.

SIK2 kinase synthetic lethality is driven by spindle assembly defects in FANCA-deficient cells.

The Fanconi anemia (FA) pathway safeguards genomic stability through cell cycle regulation and DNA damage repair. The canonical tumor suppressive role of FA proteins in the repair of DNA damage during interphase is well established, but their function in mitosis is incompletely understood. Here, we performed a kinome-wide synthetic lethality screen in FANCA-/- fibroblasts, which revealed multiple mitotic kinases as necessary for survival of FANCA-deficient cells. Among these kinases, we identified the depletion of the centrosome kinase SIK2 as synthetic lethal upon loss of FANCA. We found that FANCA colocalizes with SIK2 at multiple mitotic structures and regulates the activity of SIK2 at centrosomes. Furthermore, we found that loss of FANCA exacerbates cell cycle defects induced by pharmacological inhibition of SIK2, including impaired G2-M transition, delayed mitotic progression, and cytokinesis failure. In addition, we showed that inhibition of SIK2 abrogates nocodazole-induced prometaphase arrest, suggesting a novel role for SIK2 in the spindle assembly checkpoint. Together, these findings demonstrate that FANCA-deficient cells are dependent upon SIK2 for survival, supporting a preclinical rationale for targeting of SIK2 in FA-disrupted cancers.

Mitotic Errors Promote Genomic Instability and Leukemia in a Novel Mouse Model of Fanconi Anemia.

Fanconi anemia (FA) is a disease of genomic instability and cancer. In addition to DNA damage repair, FA pathway proteins are now known to be critical for maintaining faithful chromosome segregation during mitosis. While impaired DNA damage repair has been studied extensively in FA-associated carcinogenesis in vivo, the oncogenic contribution of mitotic abnormalities secondary to FA pathway deficiency remains incompletely understood. To examine the role of mitotic dysregulation in FA pathway deficient malignancies, we genetically exacerbated the baseline mitotic defect in Fancc-/- mice by introducing heterozygosity of the key spindle assembly checkpoint regulator Mad2. Fancc-/-;Mad2+/- mice were viable, but died from acute myeloid leukemia (AML), thus recapitulating the high risk of myeloid malignancies in FA patients better than Fancc-/-mice. We utilized hematopoietic stem cell transplantation to propagate Fancc-/-; Mad2+/- AML in irradiated healthy mice to model FANCC-deficient AMLs arising in the non-FA population. Compared to cells from Fancc-/- mice, those from Fancc-/-;Mad2+/- mice demonstrated an increase in mitotic errors but equivalent DNA cross-linker hypersensitivity, indicating that the cancer phenotype of Fancc-/-;Mad2+/- mice results from error-prone cell division and not exacerbation of the DNA damage repair defect. We found that FANCC enhances targeting of endogenous MAD2 to prometaphase kinetochores, suggesting a mechanism for how FANCC-dependent regulation of the spindle assembly checkpoint prevents chromosome mis-segregation. Whole-exome sequencing revealed similarities between human FA-associated myelodysplastic syndrome (MDS)/AML and the AML that developed in Fancc-/-; Mad2+/- mice. Together, these data illuminate the role of mitotic dysregulation in FA-pathway deficient malignancies in vivo, show how FANCC adjusts the spindle assembly checkpoint rheostat by regulating MAD2 kinetochore targeting in cell cycle-dependent manner, and establish two new mouse models for preclinical studies of AML.

Brigatinib causes tumor shrinkage in both NF2-deficient meningioma and schwannoma through inhibition of multiple tyrosine kinases but not ALK.

Neurofibromatosis Type 2 (NF2) is an autosomal dominant genetic syndrome caused by mutations in the NF2 tumor suppressor gene resulting in multiple schwannomas and meningiomas. There are no FDA approved therapies for these tumors and their relentless progression results in high rates of morbidity and mortality. Through a combination of high throughput screens, preclinical in vivo modeling, and evaluation of the kinome en masse, we identified actionable drug targets and efficacious experimental therapeutics for the treatment of NF2 related schwannomas and meningiomas. These efforts identified brigatinib (ALUNBRIG®), an FDA-approved inhibitor of multiple tyrosine kinases including ALK, to be a potent inhibitor of tumor growth in established NF2 deficient xenograft meningiomas and a genetically engineered murine model of spontaneous NF2 schwannomas. Surprisingly, neither meningioma nor schwannoma cells express ALK. Instead, we demonstrate that brigatinib inhibited multiple tyrosine kinases, including EphA2, Fer and focal adhesion kinase 1 (FAK1). These data demonstrate the power of the de novo unbiased approach for drug discovery and represents a major step forward in the advancement of therapeutics for the treatment of NF2 related malignancies.

PAK1 inhibition reduces tumor size and extends the lifespan of mice in a genetically engineered mouse model of Neurofibromatosis Type 2 (NF2).

Neurofibromatosis Type II (NF2) is an autosomal dominant cancer predisposition syndrome in which germline haploinsufficiency at the NF2 gene confers a greatly increased propensity for tumor development arising from tissues of neural crest derived origin. NF2 encodes the tumor suppressor, Merlin, and its biochemical function is incompletely understood. One well-established function of Merlin is as a negative regulator of group A serine/threonine p21-activated kinases (PAKs). In these studies we explore the role of PAK1 and its closely related paralog, PAK2, both pharmacologically and genetically, in Merlin-deficient Schwann cells and in a genetically engineered mouse model (GEMM) that develops spontaneous vestibular and spinal schwannomas. We demonstrate that PAK1 and PAK2 are both hyper activated in Merlin-deficient murine schwannomas. In preclinical trials, a pan Group A PAK inhibitor, FRAX-1036, transiently reduced PAK1 and PAK2 phosphorylation in vitro, but had insignificant efficacy in vivo. NVS-PAK1-1, a PAK1 selective inhibitor, had a greater but still minimal effect on our GEMM phenotype. However, genetic ablation of Pak1 but not Pak2 reduced tumor formation in our NF2 GEMM. Moreover, germline genetic deletion of Pak1 was well tolerated, while conditional deletion of Pak2 in Schwann cells resulted in significant morbidity and mortality. These data support the further development of PAK1-specific small molecule inhibitors and the therapeutic targeting of PAK1 in vestibular schwannomas and argue against PAK1 and PAK2 existing as functionally redundant protein isoforms in Schwann cells.

Irradiation of Nf1 mutant mouse models of spinal plexiform neurofibromas drives pathologic progression and decreases survival.

BACKGROUND: Genetically susceptible individuals can develop malignancies after irradiation of normal tissues. In the context of therapeutic irradiation, it is not known whether irradiating benign neoplasms in susceptible individuals promotes neoplastic transformation and worse clinical outcomes. Individuals with Neurofibromatosis 1 (NF1) are susceptible to both radiation-induced second malignancies and spontaneous progression of plexiform neurofibromas (PNs) to malignant peripheral nerve sheath tumors (MPNSTs). The role of radiotherapy in the treatment of benign neoplasms such as PNs is unclear. METHODS: To test whether radiotherapy promotes neoplastic progression of PNs and reduces overall survival, we administered spinal irradiation (SI) to conditional knockout mouse models of NF1-associated PNs in 2 germline contexts: Nf1 fllfl ; PostnCre + and Nf1 fl/- ; PostnCre + . Both genotypes develop extensive Nf1 null spinal PNs, modeling PNs in NF1 patients. A total of 101 mice were randomized to 0 Gy, 15 Gy (3 Gy × 5), or 30 Gy (3 Gy × 10) of spine-focused, fractionated SI and aged until signs of illness. RESULTS: SI decreased survival in both Nf1 fllfl mice and Nf1 fl/- mice, with the worst overall survival occurring in Nf1 fl/- mice receiving 30 Gy. SI was also associated with increasing worrisome histologic features along the PN-MPNST continuum in PNs irradiated to higher radiation doses. CONCLUSIONS: This preclinical study provides experimental evidence that irradiation of pre-existing PNs reduces survival and may shift PNs to higher grade neoplasms.

Exploring transcriptional regulators Ref-1 and STAT3 as therapeutic targets in malignant peripheral nerve sheath tumours.

BACKGROUND: MPNST is a rare soft-tissue sarcoma that can arise from patients with NF1. Existing chemotherapeutic and targeted agents have been unsuccessful in MPNST treatment, and recent findings implicate STAT3 and HIF1-α in driving MPNST. The DNA-binding and transcriptional activity of both STAT3 and HIF1-α is regulated by Redox factor-1 (Ref-1) redox function. A first-generation Ref-1 inhibitor, APX3330, is being tested in cancer clinical trials and could be applied to MPNST. METHODS: We characterised Ref-1 and p-STAT3 expression in various MPNST models. Tumour growth, as well as biomarkers of apoptosis and signalling pathways, were measured by qPCR and western blot following treatment with inhibitors of Ref-1 or STAT3. RESULTS: MPNSTs from Nf1-Arfflox/floxPostnCre mice exhibit significantly increased positivity of p-STAT3 and Ref-1 expression when malignant transformation occurs. Inhibition of Ref-1 or STAT3 impairs MPNST growth in vitro and in vivo and induces apoptosis. Genes highly expressed in MPNST patients are downregulated following inhibition of Ref-1 or STAT3. Several biomarkers downstream of Ref-1 or STAT3 were also downregulated following Ref-1 or STAT3 inhibition. CONCLUSIONS: Our findings implicate a unique therapeutic approach to target important MPNST signalling nodes in sarcomas using new first-in-class small molecules for potential translation to the clinic.

Cabozantinib for neurofibromatosis type 1-related plexiform neurofibromas: a phase 2 trial.

Neurofibromatosis type 1 (NF1) plexiform neurofibromas (PNs) are progressive, multicellular neoplasms that cause morbidity and may transform to sarcoma. Treatment of Nf1fl/fl;Postn-Cre mice with cabozantinib, an inhibitor of multiple tyrosine kinases, caused a reduction in PN size and number and differential modulation of kinases in cell lineages that drive PN growth. Based on these findings, the Neurofibromatosis Clinical Trials Consortium conducted a phase II, open-label, nonrandomized Simon two-stage study to assess the safety, efficacy and biologic activity of cabozantinib in patients ≥16 years of age with NF1 and progressive or symptomatic, inoperable PN ( NCT02101736 ). The trial met its primary outcome, defined as ≥25% of patients achieving a partial response (PR, defined as ≥20% reduction in target lesion volume as assessed by magnetic resonance imaging (MRI)) after 12 cycles of therapy. Secondary outcomes included adverse events (AEs), patient-reported outcomes (PROs) assessing pain and quality of life (QOL), pharmacokinetics (PK) and the levels of circulating endothelial cells and cytokines. Eight of 19 evaluable (42%) trial participants achieved a PR. The median change in tumor volume was 15.2% (range, +2.2% to -36.9%), and no patients had disease progression while on treatment. Nine patients required dose reduction or discontinuation of therapy due to AEs; common AEs included gastrointestinal toxicity, hypothyroidism, fatigue and palmar plantar erythrodysesthesia. A total of 11 grade 3 AEs occurred in eight patients. Patients with PR had a significant reduction in tumor pain intensity and pain interference in daily life but no change in global QOL scores. These data indicate that cabozantinib is active in NF1-associated PN, resulting in tumor volume reduction and pain improvement.

Numbers of long-term hematopoietic stem cells from bone marrow of fanca and fancc knockout mice can be greatly enhanced by their collection and processing in physioxia conditions.

Fanconi anemia (FA) is associated with bone marrow failure. Bone marrow (BM) from patients with FA and fanca-/- and fancc-/- mice are deficient in hematopoietic stem (HSCs) and progenitor cells (HPCs). Decreased HSCs/HPCs compromise their use in human and mouse hematopoietic cell transplantation (HCT) and gene therapy to correct genetic defects causing FA. We reported increased collection of HSCs from mouse bone marrow and mobilized peripheral blood, and human cord blood of normal donors after collection/processing in low (3%) oxygen (physioxia). We assessed comparative contents of long-term (LT)-HSCs from BM of fanca-/- and fancc-/- when collected/processed at 3% O2, in order to negate effects of extra physiological shock stress (EPHOSS) induced by collection/processing in ambient air. Collection/processing of BM from fanca-/- and fancc-/- mice in physioxia demonstrated a ≥3-fold increase in LT-HSCs compared to that in ambient air. This was associated with decreased phenotypic multipotential progenitor cells and functional granulocyte macrophage, erythroid, and multi-potential progenitors, results similar to that for BM from normal donor mice. Increased collection of HSCs could have clinical applicability for gene therapy and HCT.

Schwannoma development is mediated by Hippo pathway dysregulation and modified by RAS/MAPK signaling.

Schwannomas are tumors of the Schwann cells that cause chronic pain, numbness, and potentially life-threatening impairment of vital organs. Despite the identification of causative genes, including NF2 (Merlin), INI1/SMARCB1, and LZTR1, the exact molecular mechanism of schwannoma development is still poorly understood. Several studies have identified Merlin as a key regulator of the Hippo, MAPK, and PI3K signaling pathways; however, definitive evidence demonstrating the importance of these pathways in schwannoma pathogenesis is absent. Here, we provide direct genetic evidence that dysregulation of the Hippo pathway in the Schwann cell lineage causes development of multiple schwannomas in mice. We found that canonical Hippo signaling through the effectors YAP/TAZ is required for schwannomagenesis and that MAPK signaling modifies schwannoma formation. Furthermore, cotargeting YAP/TAZ transcriptional activity and MAPK signaling demonstrated a synergistic therapeutic effect on schwannomas. Our new model provides a tractable platform to dissect the molecular mechanisms underpinning schwannoma formation and the role of combinatorial targeted therapy in schwannoma treatment.

Nf1-Mutant Tumors Undergo Transcriptome and Kinome Remodeling after Inhibition of either mTOR or MEK.

Loss of the tumor suppressor NF1 leads to activation of RAS effector pathways, which are therapeutically targeted by inhibition of mTOR (mTORi) or MEK (MEKi). However, therapeutic inhibition of RAS effectors leads to the development of drug resistance and ultimately disease progression. To investigate molecular signatures in the context of NF1 loss and subsequent acquired drug resistance, we analyzed the exomes, transcriptomes, and kinomes of Nf1-mutant mouse tumor cell lines and derivatives of these lines that acquired resistance to either MEKi or mTORi. Biochemical comparisons of this unique panel of tumor cells, all of which arose in Nf1+/- mice, indicate that loss of heterozygosity of Nf1 as an initial genetic event does not confer a common biochemical signature or response to kinase inhibition. Although acquired drug resistance by Nf1-mutant tumor cells was accompanied by altered kinomes and irreversibly altered transcriptomes, functionally in multiple Nf1-mutant tumor cell lines, MEKi resistance was a stable phenotype, in contrast to mTORi resistance, which was reversible. Collectively, these findings demonstrate that Nf1-mutant tumors represent a heterogeneous group biochemically and undergo broader remodeling of kinome activity and gene expression in response to targeted kinase inhibition.

A molecular basis for neurofibroma-associated skeletal manifestations in NF1.

PURPOSE: Plexiform neurofibromas (pNF) develop in children with neurofibromatosis type 1 (NF1) and can be associated with several skeletal comorbidities. Preclinical mouse studies revealed Nf1 deficiency in osteoprogenitor cells disrupts, in a MEK-dependent manner, pyrophosphate (PPi) homeostasis and skeletal mineralization. The etiology of NF-associated skeletal manifestations remains unknown. METHODS: We used mouse models of NF1 neurofibromas to assess bone mineralization of skeletal structures adjacent to tumors. Expression of genes involved in pyrophosphate homeostasis was assessed in mouse and human NF tumors and Schwann cell cultures. We used dual-energy X-ray absorptiometry (DXA) to assess tumor-associated changes in bone mineral density (BMD) in an individual with NF1 following treatment with the MEK inhibitor selumetinib. RESULTS: We detected increased nonmineralized bone surfaces adjacent to tumors in mouse models of NF1 neurofibromas. Expression of Enpp1, a PPi-generating ectophosphatase, and ANKH, a PPi transporter, was increased in mouse and human neurofibroma-derived tissues and Schwann cells, respectively. In one patient, tumor-associated reductions in BMD were partially rescued following therapy with selumetinib. CONCLUSION: Results indicate that NF-associated skeletal pathologies in NF1 are associated with dysregulated pyrophosphate homeostasis in adjacent NF tumors and suggest that treatment of NFs with MEK inhibitors may improve skeletal manifestations of the disease.

Early administration of imatinib mesylate reduces plexiform neurofibroma tumor burden with durable results after drug discontinuation in a mouse model of neurofibromatosis type 1.

BACKGROUND: Neurofibromatosis type 1 (NF1) is a common genetic disorder characterized by plexiform neurofibromas (pNF), which are thought to be congenital tumors that arise in utero and enlarge throughout life. Genetic studies in murine models delineated an indispensable role for the stem cell factor (SCF)/c-kit pathway in pNF initiation and progression. A subsequent phase 2 clinical trial using imatinib mesylate to inhibit SCF/c-kit demonstrated tumor shrinkage in a subset of preexisting pNF; however, imatinib's role on preventing pNF development has yet to be explored. PROCEDURE: We evaluated the effect of imatinib dosed at 10-100 mg/kg/day for 12 weeks to one-month-old Nf1flox/flox ;PostnCre(+) mice, prior to onset of pNF formation. To determine durability of response, we then monitored for pNF growth at later time points, comparing imatinib- with vehicle-treated mice. We assessed gross and histopathological analysis of tumor burden. RESULTS: Imatinib administered preventatively led to a significant decrease in pNF number, even at doses as low as 10 mg/kg/day. Tumor development continued to be significantly inhibited after cessation of imatinib dosed at 50 and 100 mg/kg/day. In the cohort of treated mice that underwent prolonged follow-up, the size of residual tumors was significantly reduced as compared with age-matched littermates that received vehicle control. CONCLUSIONS: Early administration of imatinib inhibits pNF genesis in vivo, and effects are sustained after discontinuation of therapy. These findings may guide clinical use of imatinib in young NF1 patients prior to the substantial development of pNF.

Genetic disruption of the small GTPase RAC1 prevents plexiform neurofibroma formation in mice with neurofibromatosis type 1

Neurofibromatosis type 1 (NF1) is a common cancer predisposition syndrome caused by mutations in the NF1 tumor suppressor gene. NF1 encodes neurofibromin, a GTPase-activating protein for RAS proto-oncogene GTPase (RAS). Plexiform neurofibromas are a hallmark of NF1 and result from loss of heterozygosity of NF1 in Schwann cells, leading to constitutively activated p21RAS. Given the inability to target p21RAS directly, here we performed an shRNA library screen of all human kinases and Rho-GTPases in a patient-derived NF1-/- Schwann cell line to identify novel therapeutic targets to disrupt PN formation and progression. Rho family members, including Rac family small GTPase 1 (RAC1), were identified as candidates. Corroborating these findings, we observed that shRNA-mediated knockdown of RAC1 reduces cell proliferation and phosphorylation of extracellular signal-regulated kinase (ERK) in NF1-/- Schwann cells. Genetically engineered Nf1flox/flox;PostnCre+ mice, which develop multiple PNs, also exhibited increased RAC1-GTP and phospho-ERK levels compared with Nf1flox/flox;PostnCre- littermates. Notably, mice in which both Nf1 and Rac1 loci were disrupted (Nf1flox/floxRac1flox/flox;PostnCre+) were completely free of tumors and had normal phospho-ERK activity compared with Nf1flox/flox ;PostnCre+ mice. We conclude that the RAC1-GTPase is a key downstream node of RAS and that genetic disruption of the Rac1 allele completely prevents PN tumor formation in vivo in mice.