Mismatch-repair deficient (MMR-D) malignancies include Lynch Syndrome (LS), which is secondary to germline mutations in one of the MMR genes, and the rare childhood-form of constitutional mismatch repair-deficiency (CMMR-D); caused by bi-allelic MMR gene mutations. A hallmark of LS-associated cancers is microsatellite instability (MSI), characterized by coding frameshift mutations (cFSM) in target genes. By contrast, tumors arising in CMMR-D patients are thought to display a somatic mutation pattern differing from LS. This study has the main goal to identify cFSM in MSI target genes relevant in CMMR-D and to compare the spectrum of common somatic mutations, including alterations in DNA polymerases POLE and D1 between LS and CMMR-D. CMMR-D-associated tumors harbored more somatic mutations compared to LS cases, especially in the TP53 gene and in POLE and POLD1, where novel mutations were additionally identified. Strikingly, MSI in classical mononucleotide markers BAT40 and CAT25 was frequent in CMMR-D cases. MSI-target gene analysis revealed mutations in CMMR-D-associated tumors, some of them known to be frequently hit in LS, such as RNaseT2, HT001, and TGFβR2. Our results imply a general role for these cFSM as potential new drivers of MMR-D tumorigenesis.

INTRODUCTION We report the first known cases of pediatric glioblastoma (GBM) with prior clinical NF1 diagnoses, one with concurrent germline Lynch syndrome (LS) and NF1, and the other with somatic NF1 mutation and germline constitutional mismatch repair deficiency (CMMRD). METHODS Two pediatric GBM cases with prior NF1 clinical diagnoses based on neurocutaneous criteria were reviewed. Next generation sequencing and immunohistochemical staining were used for somatic and germline NF1 and MMR gene mutation detection, and for MMR protein expression, respectively. RESULTS Sixteen year old male with prior NF1 clinical diagnosis had resection of right frontal GBM revealing somatic mutations of POLE and PMS2, but not NF1. His father had confirmed LS with MSH2 mutation and no neurocutaneous stigmata. Patient’s germline testing revealed both pathogenic MSH2 plus NF1 mutations confirming LS and NF1. Treatment consisted of chemoradiation with temozolomide followed by adjuvant temozolomide with stable disease at 8 cycles. Nineteen year old male with former NF1 clinical diagnosis had 2 GBMs, first in left midbrain biopsied revealing somatic PMS2 and NF1 mutations underwent radiation then 7 cycles of temozolomide, then new left frontal GBM underwent resection followed by radiation and 5 cycles of pembrolizumab stable at 5th cycle. CONCLUSION Children with NF1 stigmata and GBM can have concurrent NF1 and LS, or CMMRD with NF1 somatic mutations. Our patients tolerated alkylating agents, despite risk for secondary malignancies as upfront therapy and at recurrence checkpoint inhibitors. Upfront therapy in GBM with mismatch repair syndrome with checkpoint inhibitors should be studied.

1546 Background: Lynch syndrome (LS) is characterized by deficiency in DNA mismatch repair (MMR). Several models can predict the probability of MMR gene mutations, but all were derived from populations in Northern Europe and America. We aimed to characterize MMR mutations and to evaluate the performance of 4 models (Leiden, MMRpredict, MMRpro, and PREMM1,2,6) in an ethnically diverse high-risk population. Methods: Mutation and pedigree data from 387 distinct probands tested for germline MMR mutations at a single center between 2003-2012 were analyzed. Mutation testing was triggered by suggestive tumor microsatellite results and/or clinical/family history. Race/ethnicity was self-reported: 84 (22%) were ethnically-diverse (African American, 19; Hispanic, 29; Asian/Arabic, 34; and other, 2) and 303 (78%) were white. Area under the receiver operating curve (AUC) for all 4 models was analyzed by logistic regression. Results: Pathogenic mutations were detected in 152 patients (39%; MLH1 in 37, MSH2/EPCAM in 84, MSH6 in 21, and PMS2 in 10). The detection rate was significantly lower in ethnically-diverse patients when compared to white patients (30 vs. 42%; p=0.04). The mutation frequency of the major MMR genes MLH1 and MSH2 differed, although the overall mutation distribution did not reach statistical significance (MLH1, 40 vs. 22%; MSH2: 36 vs. 59%; MSH6: 20 vs. 13%; and PMS2: 4 vs. 7%; p=0.10 vs. white). The Leiden, MMRpredict, MMRpro, and PREMM1,2,6 models trended toward inferior discrimination for the ethnically diverse patients, with AUCs of 0.63, 0.64, 0.63, and 0.66 respectively (vs. AUCs of 0.71, 0.69, 0.72, and 0.76 for white patients), but the difference was not statistically significant(p=0.11, 0.17, 0.09, and 0.08, respectively). Adjusting for family size did not significantly alter results. Conclusions: In ethnically-diverse patients, the overall detection rate for pathogenic MMR mutations is significantly lower, with associated differences in mutation frequencies and in predictive model performance. We caution against relying solely on predictive scores, as high-risk ethnically-diverse patients without an identified mutation may deserve to be followed as clinical LS patients.

Purpose: National guidelines recommend universal germline genetic testing (GT) for patients with Pancreatic Ductal Adenocarcinoma (PDAC), but rates of testing remain low. Given the aggressiveness of PDAC, the window of opportunity for GT is short and often overshadowed by treatment initiation and other clinical milestones. Thus, there is an unmet need for a model that streamlines GT and makes it available to a wider audience in a rapid fashion. Moreover, in pandemic times, video-based alternatives for medical care are increasingly relevant. Methods: We implemented a novel care delivery model in which a seven-minute educational video describing the benefits, risks, and implications of GT was shown to PDAC patients. The video was shown in lieu of an initial consult with a genetic counselor. Only patients who had not undergone GT or previously met with a genetic counselor were included. After watching the video, patients could elect to pursue GT and get tested on-side or remotely (at home). Genetic counselors disclosed results and provided post-test counseling by phone. Clinical and germline data were collected through medical records on a cohort of PDAC patients seen at the Gastrointestinal Center-MD Anderson during a 2-year enrollment period (May 2019-July 2021), which included the COVID-19 pandemic period. Results: A total of 286 PDAC patients watched the educational video. From 175 patients that watched the video pre-pandemic, 12 declined testing, whereas in the post-pandemic period, none of the 111 patients declined testing (6.9% vs 0%; p<0.004). We excluded data from 29 patients who elected to undergo GT but declined to participate in the registry. From the 241 patients with successfully collected samples, 21 patients (8.7%) had a pathogenic variant (PV), 38 patients (15.8%) had a Variant of Uncertain Significance (VUS), and 182 patients (75.5%) tested negative. The pathogenic variants detected included: BRCA2 (most frequent), ATM, BRCA1, CDKN2A, PALB2 and APC. Conclusions: GT can have tremendously beneficial effects, such as qualifying for targeted treatment options and facilitating cancer prevention in probands’ at-risk family members. Comparing uptake of GT pre- versus post-pandemic suggests that patients were more willing to trust information from a video platform, likely due to the global effect of living in a “virtual“ society as a result of the pandemic. We suggest an approach in which every PDAC patient is shown a genetics educational video and given the choice to undergo GT and post-result counseling, greatly reducing the burden on genetic counselors. We report here the feasibility of implementing video-based germline testing in PDAC patients which resulted in unexpectedly high uptake levels, particularly post-pandemic. Further investigations are needed to explore the feasibility of a fully remote GT model in diverse populations to assess additional barriers to universal GT. Citation Format: Saumya Kasliwal, Seyda Baydogan, Devon Harrison, Mark Hurd, Maureen Mork, Anirban Maitra, Florencia McAllister. Video-based germline testing for individuals with pancreatic ductal adenocarcinoma: Influence of COVID-19 pandemic [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 2244.

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