Background Malignant peripheral nerve sheath tumors (MPNSTs) are aggressive sarcomas with complex molecular and genetic alterations. Powerful tumor suppressors CDKN2A and TP53 are commonly disrupted along with NF1, a gene that encodes a negative regulator of Ras. Many additional factors have been implicated in MPNST pathogenesis. A greater understanding of critical drivers of MPNSTs is needed to guide more informed targeted therapies for patients. RABL6A is a newly identified driver of MPNST cell survival and proliferation whose in vivo role in the disease is unknown. Methods Using CRISPR-Cas9 targeting of Nf1+Cdkn2a or Nf1+Tp53 in the mouse sciatic nerve to form de novo MPNSTs, we investigated the biological significance of RABL6A in MPNST development. Terminal tumors were evaluated by western blot, qRT-PCR, and immunohistochemistry. Results Mice lacking Rabl6 displayed slower tumor progression and extended survival relative to wildtype animals in both genetic contexts. YAP oncogenic activity was selectively downregulated in Rabl6-null, Nf1+Cdkn2a lesions whereas loss of RABL6A caused upregulation of the CDK inhibitor, p27, in all tumors. Paradoxically, both models displayed elevated Myc protein and Ki67 staining in terminal tumors lacking RABL6A. In Nf1+p53 tumors, cellular atypia and polyploidy were evident and increased by RABL6A loss. Conclusions These findings demonstrate RABL6A is required for optimal progression of NF1 mutant MPNSTs in vivo in both Cdkn2a and p53 inactivated settings. However, sustained RABL6A loss may provide selective pressure for unwanted alterations, including increased Myc, cellular atypia and polyploidy, that ultimately promote a hyper-proliferative tumor phenotype akin to drug resistant lesions.

Purpose: Malignant peripheral nerve sheath tumors (MPNSTs) are lethal, Ras-driven sarcomas that lack effective therapies. We investigated effects of targeting CDK4/6, MEK, and/or programmed death-ligand 1 (PD-L1) in preclinical MPNST models. Experimental Design: Patient-matched MPNSTs and precursor lesions were examined by FISH, RNAseq, IHC, and Connectivity-Map analyses. Antitumor activity of CDK4/6 and MEK inhibitors was measured in MPNST cell lines, patient-derived xenografts (PDXs), and de novo mouse MPNSTs, with the latter used to determine anti-PD-L1 response. Results: Patient tumor analyses identified CDK4/6 and MEK as actionable targets for MPNST therapy. Low-dose combinations of CDK4/6 and MEK inhibitors synergistically reactivated the retinoblastoma (RB1) tumor suppressor, induced cell death, and decreased clonogenic survival of MPNST cells. In immune-deficient mice, dual CDK4/6-MEK inhibition slowed tumor growth in 4 of 5 MPNST PDXs. In immunocompetent mice, combination therapy of de novo MPNSTs caused tumor regression, delayed resistant tumor outgrowth, and improved survival relative to monotherapies. Drug-sensitive tumors that regressed contained plasma cells and increased cytotoxic T cells, whereas drug-resistant tumors adopted an immunosuppressive microenvironment with elevated MHC II-low macrophages and increased tumor cell PD-L1 expression. Excitingly, CDK4/6-MEK inhibition sensitized MPNSTs to anti-PD-L1 immune checkpoint blockade (ICB) with some mice showing complete tumor regression. Conclusions: CDK4/6-MEK inhibition induces a novel plasma cell-associated immune response and extended antitumor activity in MPNSTs, which dramatically enhances anti-PD-L1 therapy. These preclinical findings provide strong rationale for clinical translation of CDK4/6-MEK-ICB targeted therapies in MPNST as they may yield sustained antitumor responses and improved patient outcomes.

<p>Supplementary Table S5. Abbreviated list of top kinase inhibitory compounds predicted to be effective against MPNSTs from C-Map analyses of the MPNST transcriptome from NF1 patients.</p>

<p>Supplementary Figure S3. Tumor growth kinetics of individual de novo MPNSTs during therapy</p>

Malignant peripheral nerve sheath tumors (MPNSTs) are deadly, Ras-driven sarcomas that lack effective therapies. Many tumors are unresectable and toxic chemotherapies are ineffectual. The sensitivity of MPNSTs to immune checkpoint blockade (ICB) therapies is not known, although sarcomas are generally unresponsive. Patient tumor analyses comparing MPNSTs to benign precursors identified Ras effectors, MEK and CDK4/6 kinases, as rational targets for therapy. We tested the efficacy of dual MEK-CDK4/6 inhibition versus single drug therapy using preclinical MPNST models. In MPNST cells, low-dose drug combinations synergistically reactivated the retinoblastoma (RB1) tumor suppressor, induced cell death and decreased clonogenic survival. In de novo MPNSTs in immune competent mice, combination therapy caused tumors to shrink, substantially delayed resistant tumor outgrowth and improved survival relative to monotherapies. Tumor regression was associated with an immune activation gene expression profile featuring plasma cell infiltration, an event not previously observed with MEK-CDK4/6 inhibitor therapy. In other human tumor types, intratumoral plasma cells prognose better overall survival and improved response to ICB therapies. Excitingly, MEK-CDK4/6 therapy sensitized de novo MPNSTs to anti-programmed death ligand 1 (PD-L1) therapy with the combination achieving complete tumor ablation and apparent cure in 10% of mice. These findings implicate a critical role for plasma cells in mediating MPNST regression in response to kinase inhibitor therapy and sensitizing tumors to PD-L1 targeting. This novel therapeutic combination is a promising option for MPNST therapy that could improve ICB efficacy in other solid tumors. Citation Format: Joshua Lingo, Jordan Kohlmeyer, Courtney Kaemmer, Amanda Scherer, Ellen Voigt, Michael Chimenti, Munir Tanas, Varun Monga, Ben Darbro, David Meyerholz, Rebecca Dodd, Dawn Quelle. MEK-CDK4/6 inhibition induces plasma cell tumor infiltration and sensitizes de novo MPNSTs to immune checkpoint blockade [abstract]. In: Proceedings of the AACR Special Conference: Tumor Immunology and Immunotherapy; 2022 Oct 21-24; Boston, MA. Philadelphia (PA): AACR; Cancer Immunol Res 2022;10(12 Suppl):Abstract nr A02.