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Dermatofibrosarcoma protuberans (DFSP) is an aggressive PDGFB-dependent cutaneous sarcoma characterized by infiltrative growth and frequent local recurrences. Some DFSP progress to a higher-grade fibrosarcomatous form, with rapid growth and increased risk of metastasis. Imatinib provides clinical benefit in approximately 50% of patients with unresectable or metastatic DFSP. However, efficacious medical therapies have not been developed for imatinib-resistant DFSP. We established a model of imatinib-resistant DFSP and evaluated CDK4/6 inhibition as a genomically credentialed targeted therapy. DFSP105, an imatinib-resistant human cell line, was established from a fibrosarcomatous DFSP (FS-DFSP), and was studied by SNP arrays and sequencing to identify targetable genomic alterations. Findings were validated in vitro and in vivo, and confirmed in a series including 12 DFSP and 6 FS-DFSP. SNP analysis of DFSP105 revealed a homozygous deletion encompassing CDKN2A and CDKN2B. The resultant p16 loss implicated CDK4/6 as a potential therapeutic target in DFSP. We further demonstrated CDKN2A homozygous deletion in 1 of 12 conventional DFSP and 2 of 6 FS-DFSP, whereas p16 expression was lost in 4 of 18 DFSP. In vitro treatment of DFSP105 with two structurally distinct selective CDK4/6 inhibitors, PD-0332991 and LEE011, led to inhibition of RB1 phosphorylation and inhibition of proliferation (GI50 160 nmol/L and 276 nmol/L, respectively). In vivo treatment of DFSP105 with PD-0332991 (150 mg/kg) inhibited xenograft growth in mice, in comparison with imatinib-treated or -untreated tumors. In conclusion, CDKN2A deletion can contribute to DFSP progression. CDK4/6 inhibition is a preclinically effective treatment against p16-negative, imatinib-resistant FS-DFSP, and should be evaluated as a therapeutic strategy in patients with unresectable or metastatic imatinib-resistant DFSP. Mol Cancer Ther; 14(6); 1346–53. ©2015 AACR.

Introduction: We previously demonstrated that dystrophin inactivation results from genomic DMD 5-end deletions in advanced GIST and leiomyosarcoma, fostering migration, invasion, anchorage independence, and invadopodia formation. Hence, dystrophin inactivation, the most common cause of muscular dystrophy, is also a tumorigenic mechanism in sarcoma. CDKN2A/CDKN2B inactivation is an early event in peripheral nerve sheath tumor (PNST) progression, associated with progression from neurofibroma (NF) to atypical NF. PRC2 inactivation is a later event during progression of some MPNST. Additional biologic markers are needed to distinguish NF from low-grade MPNST. In the present study, we identify and characterize dystrophin dysregulation at an early phase in PNST progression. Methods: Dystrophin immunohistochemistry (IHC) was performed in FFPE sections using the DYS1 monoclonal antibody. DYS1 and other dystrophin monoclonals were used in immunoblotting (IB) studies of snap-frozen PNST. Genomic analyses were performed by targeted gene capture and sequencing (Haloplex custom panel of 812 cancer-associated genes) and Cytoscan HD SNPs. Dystrophin restorations were performed in MPNST cultures using Dp116 (puromycin resistance) and Dp427 (neomycin resistance), either singly or in combination. Results: Dystrophin IHC against TMAs containing common subtypes of benign and malignant soft-tissue tumors showed that most soft-tissue tumor subtypes lacked detectable dystrophin expression whereas 18 of 19 schwannomas expressed dystrophin strongly and diffusely, at levels comparable to that in non-neoplastic skeletal muscle. Dystrophin was also expressed strongly in the neoplastic Schwann-cell component of neurofibromas (NF), whereas dystrophin expression was absent or weak in each of 17 MPNST. IB studies in a separate series of snap-frozen specimens showed that the dystrophin expressed strongly in schwannomas (classic histology = 4; cellular = 6) was coexpression of 427kDa (so-called myogenic) and 116kDa Schwann-cell specific isoforms. IB studies in snap-frozen MPNST showed loss of dystrophin 116kDa and 427kDa expression in 16 cases, retained expression in 1 case, and retained expression of 427kDa only, in 1 case. Genomic surveys demonstrated intragenic DMD deletions or inactivating mutations in 4 of 11 MPNST. Each of 3 NF transitioning to MPNST had strong dystrophin expression in the NF component but not in the associated MPNST. Conjoined restoration of 116kDa and 427kDa dystrophin isoforms (but not restoration of the 116kDa isoform, on its own) inhibited MPNST migration. IHC and IB studies (in progress) suggest that PNST dystrophin inactivation occurs concurrent with or subsequent to CDKN2A/CDKN2B inactivation during transition from NF to low-grade MPNST, and prior to PRC2 inactivation. Conclusions: We show that Dp116 and Dp427 dystrophin isoforms have tumor-suppressor properties in PNST. Extinction of these isoforms occurs in most MPNST, and in laboratory models is associated with increased cell migration. Dystrophin inactivation coincides with transition from NF to low-grade MPNST; therefore, dystrophin IHC might be useful in diagnostic distinction between atypical NF and MPNST. Citation Format: Inga-Marie Schaefer*, Armelle Dufresne*, Nacef Bahri, Marije A. J. de Rooij, Stacy M. Yanofsky, Yuexiang Wang, Chandrajit P. Raut, Laurence H. Baker, Adrian Marino-Enriquez, Matt van de Rijn, Jonathan A. Fletcher. Dystrophin is a tumor suppressor in peripheral nerve sheath tumors [abstract]. In: Proceedings of the AACR Conference on Advances in Sarcomas: From Basic Science to Clinical Translation; May 16-19, 2017; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2018;24(2_Suppl):Abstract nr B16.