Novel compounds that synergize with aminoglycoside G418 or eRF3 degraders for translational readthrough of nonsense mutant TP53 and PTEN

The TP53 and PTEN tumour suppressor genes are inactivated by nonsense mutations in a significant fraction of human tumours. TP53 nonsense mutatant tumours account for approximately one million new cancer cases per year worldwide. We have screened chemical libraries with the aim of identifying compounds that induce translational readthrough and expression of full-length p53 protein in cells with nonsense mutation in this gene. Here we describe two novel compounds with readthrough activity, either alone or in combination with other known readthrough-promoting substances. Both compounds induced levels of full-length p53 in cells carrying R213X nonsense mutant TP53. Compound C47 showed synergy with the aminoglycoside antibiotic and known readthrough inducer G418, whereas compound C61 synergized with eukaryotic release factor 3 (eRF3) degraders CC-885 and CC-90009. C47 alone showed potent induction of full-length PTEN protein in cells with different PTEN nonsense mutations. These results may facilitate further development of novel targeted cancer therapy by pharmacological induction of translational readthrough.

TMOD-25. LATENT SOX9-POSITIVE CELLS BEHIND MYC-DRIVEN MEDULLOBLASTOMA RELAPSE

Tumor recurrence developing from therapy resistance, immune escape and metastasis is the leading cause of death in medulloblastoma, the most frequent malignant pediatric brain tumor. Amplification of MYC genes is the most common genetic alteration in Group 3 and Group 4 subgroups that constitute two thirds of medulloblastoma. SOX9 is a transcription factor present in stem cells in the normal brain but is limited to rare, quiescent cells in medulloblastoma patients with MYC gene amplifications. By studying paired primary-recurrent patient samples and patient-derived xenografts we here identified significant accumulation of SOX9-positive cells in Group 3 and Group 4 relapses. To follow relapse at the single cell level we developed an inducible dual Tet model of MYC-driven MB, where MYC was re-directed from the treatment-sensitive bulk cells to resistant, dormant SOX9-positive cells by doxycycline. In this model, distant recurrent tumors and spinal metastases developed. SOX9 promoted immune escape, DNA repair suppression and was essential for recurrence. Tumor cell dormancy was non-hierarchical, migratory and depended on MYC suppression by SOX9 to promote relapse. By using computational modeling and treatment we also showed how doxorubicin and MGMT inhibitors were specifically targeting recurrent cells that could be of potential use in future treatments for patients affected by these fatal relapses.

MEDU-26. LATENT SOX9-POSITIVE CELLS RESPONSIBLE FOR MYC-DRIVEN MEDULLOBLASTOMA RECURRENCE

Tumor recurrence is the leading cause of death among children with medulloblastoma, the most common type of malignant pediatric brain tumors. The mechanisms behind medulloblastoma recurrence are not fully understood. We previously showed that the transcription factor SOX9 promotes cisplatin treatment resistance in medulloblastoma. Here we show that SOX9 levels correlate with poor prognosis in Group 3 tumors. By studying paired primary-recurrent medulloblastoma samples and patient-derived xenograft (PDX) models we further identified rare SOX9-positive slow-cycling, therapy-resistant tumor cells that accumulate in relapses and in leptomenigeal metastases of Group 3 and Group 4 patients. By using an inducible Tet-OFF transgenic (GTML) mouse model for malignant MYCN-driven Group 3 tumors we identified rare SOX9-positive, quiescent brain tumor cells that are more resistant to cisplatin. Dox treatment normally cures GTML transgenic animals that developed aggressive medulloblastoma by turning MYCN off. However, when crossing the Tet-OFF GTML model with a Tet-ON rtTA-Sox9 model we can redirect MYCN expression to the Sox9 promoter ultimately driving brain tumor recurrence from rare SOX9-positive cells with 100% penetrance. In this novel animal model, recurrent tumors were actively disseminating from the hindbrain to the spinal cord and into the forebrain similar to distant relapses found in patients. By overexpressing SOX9 in human Group 3 tumor cells, MYC was directly inhibited and cell proliferation was decreased. PDX models of Group 3 tumors further showed increased levels of SOX9-positivity and less proliferative cells in metastatic compartments. Expression profiling revealed that recurrences were more inflammatory, metastatic, immune evasive and showed elevated MGMT methyltransferase levels which depleted recurrent cells and sensitized them for chemotherapy when using the MGMT inhibitor lomeguatrib. To summarize, our data clarify important and complex mechanisms by which latent medulloblastoma cells fail to respond to standard therapy and generate relapses.

Abstract 2688: A forward genetics screen of murine brain tumors identifies novel candidate genes involved in gliomagenesis

Glioma is the most frequent malignant brain tumor in adults. Platelet-derived growth factor (PDGF) signaling is commonly activated in glioma. We have used a retrovirus-driven PDGFB-induced murine glioma model that causes tumors that closely resemble human gliomas of various grades. Knowing that retroviruses have a capacity to induce insertional mutagenesis, we have employed whole genome sequencing to identify potential genes that, together with PDGFB, drive glioma development.

Gliomas were induced by RCAS virus injection into the brains of mice expressing the RCAS retroviral receptor from specific promoters. Genomic DNA from tumor cell lines was probed for retroviral tags and sequenced to identify genomic targets of the retrovirus. A streamlined analysis pipeline was developed for retrovirus integration detection and mapping to the reference mouse genome. Integration sites were analyzed and a common integration site (CIS) label was assigned to a gene, given that it was either tagged by a retrovirus more than once within a discovery set or found within the Retroviral Tagged Cancer Gene Database (RTCGD).

In a small discovery subset of 15 murine gliomas, we have identified 40 CIS, of which 37 were validated by Sanger sequencing. When compared with previously identified CIS in RTCGD, 5.5% of them were shared with our older screen, where we overexpressed PDGFB from another retrovirus in order to induce glioma. Less CIS genes were shared with other published tumor models induced by viruses driven by other cancer genes/viruses.

The majority of genes identified in our screen were tagged twice. However, Nfic, Cuecd1, Thra, Foxj1 and Nrxn1 were tagged three times, Ppfibp1 and Rhbg four times, and Mir29a/29b-1 seven times. As compared to control tumor lines, two top candidate genes, Mir29a and Ppfibp1, demonstrated significantly increased expression in tumor lines in were they were respectively tagged. Mir29a is often found downregulated in human tumors including gliomas, still high levels of Mir29a are sometimes found in certain aggressive cancers and in metastases.

Interestingly, we found that specific PDGFR inhibition negatively regulates Mir29a, indicating a possible role for PDGF signaling in Mir29a regulation. Ppfibp1 has not been extensively studied in cancer. However, Ppfibp1 seems to have a subgroup-specific expression in human glioblastoma, making it an interesting candidate for further analysis.

Here we present a new screening method that can be employed to identify genes involved in PDGFB-driven gliomagenesis. So far, we have identified 37 candidate genes by whole genome sequencing. Two of the most frequently tagged candidates, Mir29a and Ppfibp1 were upregulated as a consequence of retroviral mutagenesis. Their precise role in driving glioma formation in collaboration with PDGF is currently explored.

Citation Format: Matko Cancer, Holger Weishaupt, Gabriela Rosen, Ignas Bunikis, Yiwen Jiang, Smitha Sreedharan, Sara Bolin, Ulf Gyllensten, Oren J. Becher, Lene Uhrbom, Adam Ameur, Fredrik J. Swartling. A forward genetics screen of murine brain tumors identifies novel candidate genes involved in gliomagenesis. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 2688.

Temporal arteritis in polymyalgia rheumatica: immune complex deposits and the role of the leukocyte elastase in the pathogenesis

Immunohistochemical studies were performed on the temporal artery of 34 patients with clinically established polymyalgia rheumatica (PR) or temporal arteritis, 6 patients with vasculitis, and 25 patients with various diseases. The combined immunofluorescence and peroxidase-anti-Peroxidase Methode zeigte Immunoglobulin- und C3-Ablagerunin histologically affected and to some degree also in unaffected arteries of patients with PR and in all patients with temporal arteritis. The deposits were found both inter- and intracellularly, and contained IgA and to a lesser extend IgG, IgM, and C3. Linear deposits of leukocyte elastase were found along the fragmented internal lamina, and decaying polymorphonuclear (PMN) leukocytes surrounded by elastase-containing inclusions were found in the neighborhood of zones rich in elastic material. These findings suggest that immune complex deposition is a prominent feature of temporal arteritis and that the PMN elastase is probably involved in the destruction of elastic fibers. The combined immunohistochemical investigation appears to increase the diagnostic value of temporal artery biopsy.

Degradation in vivo of articular cartilage in rheumatoid arthritis by leucocyte elastase from polymorphonuclear leucocytes

Using a specific substrate, no leucocyte elastase activity could be detected in 55 synovial fluids, including 29 from patients with rheumatoid arthritis (RA). However, a high percentage of samples contained phagocytic inclusions of elastase, alpha 1-proteinase inhibitor (alpha 1-PI) and alpha 2-macroglobulin (alpha 2-MG) in both the polymorphonuclear (PMN) and mononuclear phagocytes. Immunofluorescence and indirect peroxidase-antiperoxidase staining of articular cartilage (ACA) from 52% of 21 patients with RA and one with juvenile RA (JRA) showed presence of elastase in the superficial layer of microscopically intact but proteoglycan depleted pannus-free ACA. In histologically altered pannus-free RA-ACA superficial elastase deposits were found in 24% of the cases. Adjacent ACA sections contained IgG, C3, alpha 1-PI and rarely alpha 2-MG. RA-ACA below or surrounded by pannus showed close contact with intact and decaying PMN in 62% and 48% of the cases, respectively. ACA specimens from patients with degenerative disease and systemic lupus were negative. These findings strongly suggest that PMN leucocyte elastase is operative in the degradation of RA-ACA and JRA-ACA, and that this activity is largely dependent upon the presence of entrapped immune complexes in such cartilage.