DDRE-32. SETD2 HISTONE METHYLTRANSFERASE MUTATION STATUS PREDICTS TREATMENT RESPONSE IN GLIOBLASTOMA: STRATEGIES TO OVERCOME CHEMORESISTANCE

A major challenge in GBM treatment is tumor resistance to radiation and chemotherapy. A hallmark of GBM is the frequent mutation of epigenetic modifiers resulting in alteration of epigenetic signaling pathways. However, the effect of epigenetic signaling on chemotherapy response in GBM remains unknown. SETD2 is a histone methyl transferase that facilitates H3K36 tri-methylation. Here, we unveil the role of SETD2 mutations that frequently occur in GBM in tumor resistance to temozolomide chemotherapy. Targeted sequencing of the SETD2 gene in GBM tumor samples revealed that SETD2 mutations are associated with reduced overall survival in patients with methylated MGMT (methyl-guanine methyl transferase) promotor who received temozolomide. Consequently, we demonstrate that loss of SETD2 results in reduced H3K36me3 levels and a profound temozolomide resistance in GBM cells. MGMT-deficient tumors can acquire chemoresistance due to disrupted mismatch repair (MMR), a DNA repair pathway that converts primary temozolomide-induced DNA lesions into toxic DNA double-strand breaks. Strikingly, we found that SETD2 loss abrogates the expression of the MMR factor MSH6 indicating that chemoresistance in SETD2-deficient cells us due to disrupted MMR. Mechanistically, we show that SETD2 regulates MMR by promoting transcription of the MSH6 gene in GBM. Epigenetic modifiers have specific antagonists capable of reversing chromatin alterations induced by these modifiers. This provides a unique opportunity to restore chemotherapy response in SETD2-mutant GBM by targeting the antagonists of SETD2. We demonstrate that combined targeting of H3K36me3-specific histone de-methylases KDM4A and NO66 restores H3K36me3 levels along with MSH6 expression and sensitivity to temozolomide in SETD2-deficient GBM cells. Thus, our findings establish SETD2 mutation as a novel molecular marker predictive of chemotherapy response in GBM and provide a framework for a novel approach to overcome chemotherapy resistance in this malignant brain tumor by targeting an epigenetic pathway.

A Truncated NRIP1 Mutant Amplifies Microsatellite Instability of Colorectal Cancer by Regulating MSH2/MSH6 Expression, and Is a Prognostic Marker of Stage III Tumors

Microsatellite instability (MSI) is related to the alteration of mismatch repair (MMR) genes and plays a key role in colorectal cancer (CRC) pathogenesis. We previously reported that the transcription factor Nuclear Receptor Interacting Protein 1 (NRIP1) is involved in sporadic intestinal tumorigenesis. The aim of this study was to decipher its role in MSI CRC. By using different mouse models and engineered cell lines, we demonstrated that NRIP1 increased MSH2 and MSH6 MMR gene transcription and mRNA/protein levels. In human CRC cells, NRIP1 expression was associated with decreased MSI and the hypermutator phenotype, and with resistance to chemotherapy drugs. Using a cohort of 194 CRC patients, we detected in 22% of the cases a MSI-induced frameshift mutation in the NRIP1 coding sequence. This genetic alteration generates a truncated protein with a dominant negative activity that increased human CRC cell proliferation and impaired the regulation of MSH2 and MSH6 gene expression. Moreover, the NRIP1 mutant correlated with a decreased overall survival of patients with advanced CRC, especially when MLH1-deficient. By decreasing the expression of MSH2 and MSH6 gene expression, the NRIP1 variant may amplify MLH1-dependent CRC progression and behave as a new prognostic marker of advanced MSI CRC.

Lynch syndrome-associated lung cancer: pitfalls of an immunotherapy-based treatment strategy in an unusual tumor type

Lynch syndrome is a hereditary cancer predisposition syndrome caused by germline alterations in mismatch repair (MMR) genes leading to increased risk of colon cancer as well as other cancer types. Non-small cell lung cancer (NSCLC) is not among typical Lynch syndrome-associated tumors: pembrolizumab, an immune checkpoint inhibitor, is actually approved for the treatment of NSCLC patients and represents a promising treatment option for patients with advanced metastatic MMR-deficient cancer, regardless of tumor origin. This case report describes the clinical presentation and management of a 74-year-old female with a history of rectal adenocarcinoma and ovarian cancer, who has a documented frameshift pathogenic variant in the exon 8 of MSH6 gene and an intronic variant in the BRCA2 gene (classified as a variant of uncertain significance), affected by NSCLC with brain metastases. Despite these premises, the patient was treated with pembrolizumab and she did not benefit from this kind of treatment.

CTIM-30. EFFICACY OF PEMBROLIZUMAB IN PATIENTS WITH PITUITARY CARCINOMA: RESULTS FROM A PHASE II STUDY

Pituitary carcinoma is an aggressive tumor characterized by metastatic spread beyond the sellar region that leads to debilitating symptoms and poor survival. Pituitary carcinomas recur despite conventional multimodality treatments. Given the recent advances in the use of immune checkpoint inhibitors (CPIs) to treat various solid cancers, there is interest in exploring the role of immunotherapy for treating aggressive, refractory pituitary tumors. We treated four pituitary carcinoma patients with pembrolizumab as part of a phase II clinical trial (NCT02721732). Here, we present their clinical course and outcomes and correlate responses with available molecular data: hypermutation status, PD-L1 staining, tumor-infiltrating lymphocyte score, microsatellite status and tumor mutational burden. Patients 1 and 2, with heavily pretreated, refractory corticotroph pituitary carcinoma, had partial radiographic (60% and 32% per irRECIST, respectively) and hormonal responses. Patient 1’s response continues 42 months after initiation of pembrolizumab and his baseline tumor tissue obtained after treatment with temozolomide demonstrated a hypermutator phenotype with MSH2 and MSH6 gene mutations. Patient 2’s tumor was not sampled after exposure to temozolomide, but prior somatic mutational testing was negative. Patient 3 (non-functioning corticotroph tumor) had a best response of stable disease for four months. Patient 4 (prolactin-secreting carcinoma) had progressive disease. The latter two patients’ tumors did not demonstrate a hypermutator phenotype after treatment with temozolomide. PD-L1 staining was negative in all tumors. TIL score was 2 in Patients 1 and 4, negative in Patient 3 and not available in Patient 2. All patients tolerated the treatment well with mild adverse events. Our study generates the hypothesis that an alkylating agent-induced hypermutator phenotype may be an indicator of response to CPIs in pituitary carcinomas. The role of CPI in treating patients with pituitary carcinoma and mechanisms of hypermutation in this population require further study.

Association of single nucleotide polymorphisms rs63751445 of the MSH2 gene and rs863224614 of the MSH6 gene with susceptibility to breast cancer in samples from Northeast Brazil

Breast cancer (BC) is the cancer with the greatest epidemiological impact on the female population worldwide. The disease has a multifactorial etiology, with genetic implications that are not fully understood. In this context, genetic changes in the mismatch repair mechanism are notable for their potential relationship with BC, especially the single nucleotide polymorphisms (SNPs), which are the most common type of genetic variation. The aim of this study was to evaluate for the first time the influence of the SNPs rs63751445 (A>G) of the MSH2 gene and rs863224614 (T>G) of the MSH6 gene for susceptibility to CM. For that, 100 samples obtained by histopathological examination of patients from the Northeast region of Brazil were used. The methodology used was the Didesoxy Single Allele Specific PCR (DSASP) method. Statistical analysis was performed by comparison with the control population (population in Hardy-Weinberg equilibrium) using Pearson's Chi-square and Fischer's exact tests. It was concluded that these two SNPs may be associated with susceptibility to BC in the studied population.