LAUSR

Introduction: DICER1 is a ribonuclease III enzyme that processes pre-microRNAs into mature 20-23 nucleotide microRNAs required for their normal function. Heterozygous germline DICER1 loss-of-function mutations are associated with a cancer predisposition syndrome in which affected individuals present at a young age with rare neoplasms including pleuropulmonary blastoma (PPB), pineoblastoma, embryonal rhabdomyosarcoma (ERMS), Sertoli-Leydig cell tumors (SLCT) and thyroid hyperplasia. These tumors have secondary somatic missense mutations in the remaining DICER1 allele in exons 24 and 25 encoding the RNAse IIIb domain. Contribution of DICER1 somatic mutations to carcinogenesis in the adult population is largely unknown. Methods: Caris Life Sciences (Phoenix, AZ) database of over 25,000 human cancers containing complete gene sequences of 592 genes (NGS) was searched for mutations in DICER1 gene. Histologic (HE age range was from 3 to 79 years of age. The most common primary site was in the endometrium/uterus (15), followed by ovary (8), lung (3), cervix, brain, kidney, retroperitoneum and thyroid (1 each). Majority of endometrial malignancies were observed in adult women (N=15; age 17-79 y. o.), of whom 7 presented with mixed epithelial-mesenchymal type malignancies (carcino- and adeno-sarcomas). Most common (7) ovarian malignancy was SLCT (with and without sarcomatoid elements). One male patient presented with epithelioid glioblastoma (48 y. o.). Three lung cancers seen in adults (39 and two 74 y. o.) were PPB, adenocarcinoma NOS and mucinous adenocarcinoma, respectively. Kidney neproblastoma and cervical ERMS with heterologous elements were observed in 3 and 50 y. o. females, respectively. Overall, DICER1 mutated tumors exhibited low mutational load with the exception of one endometrial adenocarcinoma associated with POLE/MSH6 mutations. Conclusion: Biallelic somatic DICER1 mutations can be found in tumors of adult patients and are frequently associated with mixed-type malignancies. Sarcomatoid tumor elements were detected not just in endometrial malignancies, but in 2 SLCTs, cervical and thyroid malignancies. These findings are consistent with a recently reported role of DICER1 in epithelial-to-mesenchymal transition (EMT) in experimental cancers, and tumors with such histology should prompt investigations into DICER1 status. Citation Format: Zoran Gatalica, Michelle Ellis, Jeff Swensen. DICER1 mutations are associated with diverse and frequently histologically mixed-type malignancies in young and adult patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4605.