Severson, Eric A.

749 Background: Solid Pseudopapillary Neoplasms (SPNs) of the pancreas are rare accounting for 1-2% of all pancreatic tumors. Previous studies had shown that a pathogenic mutation of the CTNNB1 gene is present in over 90% of the SPN tumors with very limited information available on the specific molecular changes present in SPN. Here we report the results of next generation genetic testing of a large series of SPN tumors from the Foundation Medicine database. Methods: Foundation Medicine database from 07/2012 to 04/2019 was reviewed. A total of 31 cases of SPN tumors were identified out of 12,892 cases of pancreatic cancers. Information collected included demographic information on the patients as well as tumor analysis for next generation genetic sequencing of 315 genes associated with cancer with FoundationOne or 324 genes associated with FoundationOneCDx. Microsatellite stability status and tumor mutation burden were determined. Results: Twenty nine out of 31 cases (93%) had a CTNNB1 mutation. Seventy percent of the CTNNB1 mutations were on position 32 (14 cases, 41%) or in position 37 (9 cases, 29%). Other sites of CTNNB1 mutations included position 34 (3 cases, 9.6%), position 2 (2 cases, 6.4%) and position 38 (1 case, 3.2%). Two cases did not have a CTNNB1 mutation, one had a CDKN2A mutation and the other had no detectable mutations. Most cases had additional mutations aside from the CTNNB1, the most common were TP53 (3 cases, 9.6%) and LRP1B (2 cases, 6.4%). Other accompanying mutations were seen just once. Twenty-five percent of these cases had actionable gene mutations, each found in one case including: MSH2, BRCA2, ATM, XRCC3, ATRX, PTEN, ESR1, CDKN2, and PIK3CA. Conclusions: Next generation genetic testing of SPN tumors is of clinical benefit since it identifies actionable mutations in 29% of the cases.

NOTCH1 is frequently mutated in adenoid cystic carcinoma (ACC). To test the idea that immunohistochemical (IHC) staining can identify ACCs with NOTCH1 mutations, we performed IHC for activated NOTCH1 (NICD1) in 197 cases diagnosed as ACC from 173 patients. NICD1 staining was positive in 194 cases (98%) in 2 major patterns: subset positivity, which correlated with tubular/cribriform histology; and diffuse positivity, which correlated with a solid histology. To determine the relationship between NICD1 staining and NOTCH1 mutational status, targeted exome sequencing data were obtained on 14 diffusely NICD1-positive ACC specimens from 11 patients and 15 subset NICD1-positive ACC specimens from 15 patients. This revealed NOTCH1 gain-of-function mutations in 11 of 14 diffusely NICD1-positive ACC specimens, whereas all subset-positive tumors had wild-type NOTCH1 alleles. Notably, tumors with diffuse NICD1 positivity were associated with significantly worse outcomes (P=0.003). To determine whether NOTCH1 activation is unique among tumors included in the differential diagnosis with ACC, we performed NICD1 IHC on a cohort of diverse salivary gland and head and neck tumors. High fractions of each of these tumor types were positive for NICD1 in a subset of cells, particularly in basaloid squamous cell carcinomas; however, sequencing of basaloid squamous cell carcinomas failed to identify NOTCH1 mutations. These findings indicate that diffuse NICD1 positivity in ACC correlates with solid growth pattern, the presence of NOTCH1 gain-of-function mutations, and unfavorable outcome, and suggest that staining for NICD1 can be helpful in distinguishing ACC with solid growth patterns from other salivary gland and head and neck tumors.

6087 Background: NTRK1-3 fusions, identified in pediatric radiation-associated thyroid cancers, are low frequency oncogenic drivers that can trigger constitutive activation of TrK1-3 tyrosine kinase domains with responsively heightened downstream signaling through RAS-MAPK, PI3K-AKT, and/or PLCγ-PKC. NTRK point mutations are rare and of largely uninvestigated oncogenic potential, but some appear oncogenic. Methods: Advanced thyroid cancer (TC) patients (pts) requiring systemic therapy at Mayo Clinic sites underwent tumor genetic interrogation using Foundation One analyses with the goal of informing salvage therapeutic strategies. In this IRB-approved study, data from pts with TCs bearing NTRK point mutations were retrospectively analyzed, as this occurrence was unexpected but potentially of prognostic and/or therapeutic relevance. Results: Five of 55 pts (9%) with advanced TCs subject to Foundation One tumor interrogation had NTRK1-3 point mutations (allele frequency, AF, 15-49%); none had fusions. All 5 tumors had microsatellite stability; 4/5 had low tumor mutation burden (2-5 mutations/megabase). NTRK mutations in 3 anaplastic TC (ATC) pts (15% prevalence) were: NTRK3 I749M (AF 15%; overall survival, OS, 15 mos), NTRK1 R744H (AF not available; OS 9.5 mos), and NTRK1 R583H (AF 20.1%; OS > 4.9 mos); NTRK2 S167Y (AF 49%; OS > 51 mos) or NTRK1 R6W (AF 42%; OS > 60 mos) were found in one papillary (PTC) and one Hürthle cell (HCC) TC pt respectively (summary table below). Conclusions: NTRK point mutations, but not fusions, were unexpectedly identified in 9% of assessed advanced TCs - but remarkably in 15% of ATCs. Mutated NTRKs, along with oncogenic p53 and TERT, may contribute to TC progression, especially in ATC, consequent to downstream activation of MAPK and PI3K-AKT pathways. We posit that NTRK point mutations may have therapeutic implications for treatment of refractory disease. [Table: see text]