Inhibition of YAP/TAZ-driven TEAD activity prevents growth of NF2-null schwannoma and meningioma

Schwannoma tumours typically arise on the 8th cranial nerve and are mostly caused by loss of the tumour suppressor Merlin (NF2). There are no approved chemotherapies for these tumours and the surgical removal of the tumour carries a high risk of damage to the 8th or other close cranial nerve tissue. New treatments for schwannoma and other NF2-null tumours such as meningioma are urgently required. Using a combination of human primary tumour cells and mouse models of schwannoma, we have examined the role of the Hippo signalling pathway in driving tumour cell growth. Using both genetic ablation of the Hippo effectors YAP and TAZ as well as novel TEAD palmitoylation inhibitors, we show that Hippo signalling may be successfully targeted in vitro and in vivo to both block and, remarkably, regress schwannoma tumour growth. In particular, successful use of TEAD palmitoylation inhibitors in a pre-clinical mouse model of schwannoma points to their potential future clinical use. We also identify the cancer stem cell marker aldehyde dehydrogenase 1A1 (ALDH1A1) as a Hippo signalling target, driven by the TAZ protein in human and mouse NF2-null schwannoma cells, as well as in NF2-null meningioma cells, and examine the potential future role of this new target in halting schwannoma and meningioma tumour growth.

Human endogenous retrovirus type K promotes proliferation and confers sensitivity to anti-retroviral drugs in Merlin-negative schwannoma and meningioma

Deficiency of the tumour suppressor Merlin causes development of schwannoma, meningioma, and ependymoma tumours, which can occur spontaneously or in the hereditary disease neurofibromatosis type 2 (NF2). Merlin mutations are also relevant in a variety of other tumours. Surgery and radiotherapy are current first-line treatments; however, tumours frequently recur with limited treatment options. Here, we use human Merlin-negative schwannoma and meningioma primary cells to investigate the involvement of the endogenous retrovirus HERV-K in tumour development. HERV-K proteins previously implicated in tumorigenesis were overexpressed in schwannoma and all meningioma grades, and disease-associated CRL4DCAF1 and YAP/TEAD pathways were implicated in this overexpression. In normal Schwann cells, ectopic overexpression of HERV-K Env increased proliferation and upregulated expression of c-Jun and pERK1/2, which are key components of known tumorigenic pathways in schwannoma, JNK/c-Jun and RAS/RAF/MEK/ERK. Furthermore, FDA-approved retroviral protease inhibitors ritonavir, atazanavir, and lopinavir reduced proliferation of schwannoma and grade I meningioma cells. These results identify HERV-K as a critical regulator of progression in Merlin-deficient tumours and offer potential strategies for therapeutic intervention.

Activation of Raf signalling in NF2-null Schwann cells leads to sustained proliferation; an investigation of a new and inducible model for human schwannoma

Aims

The NF2 gene encodes the tumour suppressor Merlin, which is deleted in 100% of patients with the familial tumour predisposition syndrome neurofibromatosis type 2 but also in 70% of those who develop sporadic schwannomas. The Raf-TR mouse model uses a tamoxifen-inducible Raf-kinase/ oestrogen receptor fusion protein (Raf-TR) expressed in myelinating Schwann cells to mimic a nerve injury response in Schwann cell by activating Raf/MEK/ERK signalling in the absence of peripheral nerve injury.

We will assess whether Raf/MEK/ERK activation on an NF2 null background leads to tumourigenesis within the vestibular nerves and dorsal root ganglia (DRGs), two tumour sites identified in the Periostin-Cre mouse model in which schwannoma formation is spontaneous, with a view to generating an inducible NF2 null schwannoma mouse model.

Method

Mice with a Schwann cell specific loss of Merlin were crossed with mice carrying a tamoxifen-inducible Raf-TR gene to generate Raf-TR+/-; P0-Cre+/-; NF2fl/fl (Cre+) mice which were NF2 null and compared to Raf-TR+/-; P0-Cre-/-; NF2fl/fl (Cre-) littermate controls. Mice were injected with tamoxifen or vehicle for five consecutive days and their vestibular nerves and dorsal root ganglia (DRGs) were analysed at various timepoints . An EdU proliferation assay was used to quantify the proliferation in the vestibular ganglia, as well as the DRGs. Rates of proliferation were compared to Cre- age-matched littermate controls treated with tamoxifen or vehicle.

Results

In the Periostin-Cre NF2 null schwannoma model, tumours form spontaneously in the DRGs and vestibular ganglia. In our new model, we see a clear increase in proliferation at 21 d post-injection in the NF2 null (Cre+) tamoxifen-treated mice compared to control (Cre-) tamoxifen-treated controls in both DRGs and vestibular ganglia. Cre- tamoxifen-treated mice do not show increased proliferation compared to Cre- vehicle controls. Taken together, this shows that activation of the Raf/MEK/ERK pathway in Schwann cells only causes a sustained proliferation response on an NF2 null background in the DRGs and vestibular ganglia. We are assessing later timepoints to further characterise tumour development in these mice.

Conclusion

Combining the Raf-TR mouse model to create a demyelinating phenotype with an NF2 null background leads to vastly increased rates of proliferation at the sites of schwannoma tumourigenesis within the peripheral nervous system: the DRGs and the vestibular ganglia. The high proliferation in the vestibular ganglia in particular is similar to the development of vestibular schwannomas in patients with Neurofibromatosis type 2. The new mouse model used in this study shows potential to be very useful as an inducible schwannoma tumour model, in which we can study the early events of tumour formation.

Use of a new mouse schwannoma tumour model to monitor changes in peripheral nerve morphology in Merlin null Schwann cells

Aims

Our lab is interested in signals that trigger schwannoma tumour formation and we have previously shown that peripheral nerve injury triggers tumour formation in nerves with Schwann cell-specific loss of the Merlin (NF2) tumour suppressor. The Ras/Raf/MAPK/ERK pathway activity in myelinating Schwann cells is involved in nerve regeneration, causing demyelination and recruitment of inflammatory cells in areas of nerve damage, as well as dedifferentiation of myelinating Schwann cells into a repair-competent state. We have used a mouse model expressing a tamoxifen-inducible Raf-Kinase estrogen receptor fusion protein (Raf-TR) in myelinating Schwann cells of the PNS in either a control wild-type Merlin or Merlin-null background. This allows us to determine the effects of an injury-like signal in Schwann cells and its role in generating schwannoma tumour development. We present here a detailed analysis of the proliferation of Schwann cells within the nerve and morphological changes in PNS structure following Raf-TR activation.

Method

The P0-promotor driving the Raf-TR transgene is active in myelinating Schwann cells but inactive in the non-myelinating population, allowing specific targeting of the myelinating Schwann cell population. In addition to the Raf-TR gene, the mice exhibit a separate P0-promotor controlled Cre floxed NF2 gene which undergoes Cre-mediated recombinase at embryonic day 13.5 causing NF2 knockout in all developing Schwann cells. Mice aged between 4-6 weeks received intraperitoneal injections of either 2mg Tamoxifen or oil vehicle for 5 consecutive days and were then studied at either 10 or 21 days post-first injection. The peripheral nervous system of the mice was studied with fluorescent immuno-histochemistry staining, semithin sections and transmission electron microscopy (TEM) on sciatic nerves and dorsal root ganglia (DRG).

Results

Activation of the Ras/Raf/MAPK/ERK pathway in NF2 null Schwann cells led to higher rates of proliferation within sciatic nerves at 10d post-tamoxifen injections. At both 10d and 21d Raf-TR+ NF2-null mice sciatic nerve fascicles were visibly larger with significantly more cell bodies present than controls, however at 21d the rate of proliferation had reduced. In the DRG, proliferation was higher in Raf-TR+ NF2-null mice compared to controls, with proliferation remaining high at 21 days. Quantitative imaging of peripheral nerve semi-thins analysed to date showed no significant difference in the number of myelin rings present in the fascicles between different genotypes. Additionally, dual immuno-histochemistry staining with Myelin Basic Protein and EdU, markers for myelin and proliferation respectively, appeared to show proliferation in the non-myelinating Schwann cell population. Results from staining with other cell markers will also be presented, as well as a detailed analysis of nerve structure using TEM.

Conclusion

While developmental myelination of Merlin-null Schwann cells appears largely normal, the reaction of Merlin-null Schwann cells in the nerve to an injury signal (activation of the Raf-TR) is remarkably different from those of control nerves. The high levels of proliferation in Merlin-null Schwann cells may be indicative of a higher tumorigenesis potential. While the proliferation of Merlin-null cells does reduce over time in the sciatic nerve, further experiments are now testing whether there may be ongoing tumour growth at other locations in the nervous system that are associated with NF2 tumours in human patients.

Activation of MAPK/ERK signalling in Merlin-null Schwann cells leads to increased and sustained immune cell infiltration in the peripheral nervous system

Aims

Previous work has shown that increased numbers of macrophages are associated with more rapid schwannoma tumour growth and we are interested in signals that control entry of macrophages and other immune cells into these tumours. Activation of the Raf-kinase domain and the Raf/MEK/ERK pathway within Schwann cells has been observed to induce an inflammatory response in peripheral nerves in the absence of injury. Activation of an inducible Raf-kinase transgene in Schwann cells allows modelling of acute demyelination of peripheral nerves without nerve injury. This Raf-oestrogen receptor fusion protein (Raf-TR) is activated by the oestrogen analogue Tamoxifen and so allows targeted, controlled activation of the Raf/MEK/ERK pathway within the Schwann cells.

Here, in order to understand drivers of tumour formation, we assess the effect of MAPK activation in Merlin-null Schwann cells upon immune cell infiltration within the PNS.

Method

RafTR-P0CRE-NF2fl/fl mice of 4-6 weeks age were injected daily (IP) with 2mg of 4-hydroxy-tamoxifen or vehicle (corn oil) control for 5 consecutive days. RafTR was activated on either a Merlin (NF2) wild-type (NF2 fl/fl, P0-CRE-) or NF2 null (NF2 fl/fl, P0-CRE+) background and effects on immune cell infiltration studied in each condition.

Immunofluorescence was performed in the dorsal root ganglia (DRGs) and sciatic nerves of mice to identify various immune cell infiltrates at various timepoints. These will include neutrophils, mast cells, T-Cells and macrophages using the cell markers Csf3r, C-kit, CD3 and IBA1 respectively.

Results

At 21 days post treatment, a significantly increased infiltration of macrophages within the sciatic nerve and dorsal root ganglia was observed in mice treated with Tamoxifen when compared to vehicle controls. Loss of NF2 led to a massive increase in the number of macrophages recruited to peripheral nerves in tamoxifen-treated mice compared to Cre- mice and Cre+ treated with vehicle alone. Further assessment of other immune cell infiltration including neutrophils, mast cells and T cells are ongoing.

Conclusion

Raf/MEK/ERK signalling, in the absence of tumour suppressor Merlin, significantly increases the infiltration of inflammatory cells such as macrophages into peripheral nerves even in the absence of injury. As this effect is enhanced in NF2 null mice, this suggests that Merlin plays an important role in inhibiting the inflammatory response in peripheral nerves. It also suggests that Merlin could be involved in maintaining the blood nerve barrier (BNB), as in its absence the greater influx of immune cells into the nerves and DRGs suggests a more complete loss of BNB function than just activation of the Raf/MEK/ERK cascade alone.

HuR/ELAVL1 drives malignant peripheral nerve sheath tumor growth and metastasis

Cancer cells can develop a strong addiction to discrete molecular regulators, which control the aberrant gene expression programs that drive and maintain the cancer phenotype. Here, we report the identification of the RNA-binding protein HuR/ELAVL1 as a central oncogenic driver for malignant peripheral nerve sheath tumors (MPNSTs), which are highly aggressive sarcomas that originate from cells of the Schwann cell lineage. HuR was found to be highly elevated and bound to a multitude of cancer-associated transcripts in human MPNST samples. Accordingly, genetic and pharmacological inhibition of HuR had potent cytostatic and cytotoxic effects on tumor growth, and strongly suppressed metastatic capacity in vivo. Importantly, we linked the profound tumorigenic function of HuR to its ability to simultaneously regulate multiple essential oncogenic pathways in MPNST cells, including the Wnt/β-catenin, YAP/TAZ, RB/E2F, and BET pathways, which converge on key transcriptional networks. Given the exceptional dependency of MPNST cells on HuR for survival, proliferation, and dissemination, we propose that HuR represents a promising therapeutic target for MPNST treatment.

Abstract 6384: TEAD inhibition reduces tumor proliferation in Merlin null meningioma and schwannoma

Introduction:

Meningiomas are the most common intracranial tumor and arise from the arachnoidal cell layer of the meninges. Schwannomas develop from Schwann cells which provide myelin and trophic support in the peripheral nerves. The tumor suppressor Merlin is deleted in approximately 50-60% of meningiomas and 70% of schwannomas. Loss of Merlin results in dysregulation of the Hippo pathway and consequent aberrant activation of the transcriptional coactivators YAP and TAZ. Nuclear localisation of YAP and TAZ has been shown to drive tumor phenotypes in many cancers. The TEAD family of transcription factors are major targets of YAP and TAZ-dependent signaling. TEAD family members undergo autopalmitoylation and this modification is required for transcriptional activation by YAP or TAZ. Vivace therapeutics have identified small molecule inhibitors of autopalmitoylation that block the interaction of TEAD family members with YAP and TAZ. This study seeks to establish the efficacy of small molecule inhibitors of TEAD activity in meningioma and schwannoma tumor cells.

Experimental Procedures:

Meningioma cell lines, primary human meningioma and schwannoma cells, as well as a schwannoma mouse model (PerisotinCRE /NF2 fl/fl) were treated with Vivace inhibitors to assess their effects upon tumor cell proliferation and TEAD driven transcription.

Results:

TEAD inhibition significantly reduced the proliferation of the BenMen-1 meningioma cell line, as well as in primary human meningioma cells. In schwannoma, inhibitors reduced proliferation and also reduced the expression of CTGF, a target of TEAD activation in both primary human meningioma and schwannoma cells. These novel inhibitors are also currently being tested in the Periostin CRE-NF2fl/fl schwannoma mouse model to evaluate their efficacy in vivo.

Conclusions:

The use of these novel compounds to inhibit TEAD function shows great potential as an effective treatment for reducing the rate of cell proliferation in meningioma and schwannoma, two clinically important tumor types.

Citation Format: Liyam Laraba, Julio Grimm de Guibert, Tracy T. Tang, Leonard Post, David B. Parkinson. TEAD inhibition reduces tumor proliferation in Merlin null meningioma and schwannoma [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6384.

Abstract 5957: Roles for the cancer stem cell marker Aldh1A1 in Merlin null nervous system tumors

Our lab is interested in the development and growth of schwannoma and meningioma tumors of the nervous system. Approximately 70% of these tumors arise due to loss of the tumor suppressor Merlin. In a mouse model of schwannoma (P0-CRE NF2fl/fl), we observed increased mRNA levels of the cancer stem cell marker Aldehyde dehydrogenase 1A1 (Aldh1A1) in Merlin-null Schwann cells. Immunostaining showed elevated levels of Aldh1A1 protein in this mouse model and in another mouse model (Postn-CRE NF2fl/fl), in which schwannoma tumors arise spontaneously within the vestibulocochlear nerve and dorsal root ganglia of the nervous system. Similarly, staining of human schwannoma samples also showed a strong elevation of Aldh1A1 protein in all tumor cells. Parallel analysis of meningioma tumor samples, in which Merlin loss also occurs, also showed a strong increase of Aldh1A1 protein levels. Western blotting enabled identification of the mechanism by which Merlin loss regulates Aldh1A1. Merlin/TAZ double knockout, but not Merlin/YAP double knockout, sciatic nerves show reduced Aldh1A1 expression. This implicates Hippo signalling through the TAZ effector as the mechanism of Aldh1A1 upregulation both in vitro and in vivo in both tumor types. Chemical inhibition with Aldh1a1-specific inhibitor (NCT-505) or shRNA knockdown of Aldh1A1 in Merlin-null schwannoma and meningioma cells in culture attenuated tumor cell growth, proving Aldh1A1 has a role in tumor cell proliferation; studies using Aldh1A1 null mice to test this in vivo are ongoing. A major target of Aldh1A1 signalling is the regulation of retinoic acid (RA) signalling. We are examining expression and activation of components of RA signalling in both schwannoma and meningioma tumor cells to identify whether this may be of therapeutic use in the future for these two tumor types. In conclusion, the TAZ-dependent upregulation of Aldh1A1 in Merlin-null schwannoma and meningioma tumor cells has a functional role in driving cell proliferation in these two clinically important tumor types. Further work will seek to identify the mechanism by which Aldh1A1 activity contributes to tumor growth.

Citation Format: Lily Hillson, Liyam Laraba, Aditya G. Shivane, Philip Edwards, Ganesha Bantukallu, Anton Simeonov, Natalia J. Martinez, Shyh-Ming Yang, David B. Parkinson. Roles for the cancer stem cell marker Aldh1A1 in Merlin null nervous system tumors [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 5957.