Abstract 4934: Mosaic RNase IIIb domain <i>DICER1</i> mutations in children with multiple primary tumors

Abstract The DICER1 syndrome or pleuropulmonary blastoma (PPB) familial tumor and dysplasia syndrome (PPB FTDS) (OMIM #601200) is caused by heterozygous germ-line mutations in the microRNA maturation pathway gene, DICER1. Several rare phenotypes constitute the syndrome including PPB, cystic nephroma (CN), ovarian Sertoli-Leydig cell tumors (SLCT), intra-ocular medulloepithelioma, nasal chondromesenchymal hamartoma (NCMH), pineoblastoma, pituitary blastoma, multinodular goitre (MNG) and other rare childhood sarcomas and dysplasias. Highly characteristic second somatic mutations have been identified in DICER1-associated tumors, affecting amino acid residues central to the catalytic activity of the RNase IIIb domain. We describe four children with multiple primary tumours associated with the DICER1 syndrome. Sanger sequencing of constitutional DNA obtained from peripheral blood lymphocytes and/or saliva revealed no likely deleterious germ-line DICER1 mutations. We subsequently sequenced the region encoding the DICER1 RNase IIIa and RNase IIIb domains in gDNA extracted from the tumor samples, and noted the presence of the same RNase IIIb missense mutation in multiple tumors from each patient (Patient A: c.5437G>C; Patient B: c.5125G>A; Patient C: c.5439G>C; Patient D: c.5425G>A). We performed targeted capture followed by deep sequencing on DNA extracted from both normal and tumor tissue, which revealed the presence of the respective RNase IIIb mutations in a low percentage of sequencing reads (0.2 - 13%) in constitutional DNA from three of the four patients (Patients A, B and C). The relative abundance of the allele harboring the DICER1 RNase IIIb mutation was significantly higher in the tumors compared to normal tissue from the surrounding organ and/or distant sites. Taken together, these findings indicate a mosaic origin of the DICER1 RNase IIIb missense mutations. The mosaic origin of Patient D's mutation remains to be unequivocally established. We further hypothesized that, in the setting of a mosaic DICER1 RNase IIIb mutations, we might discover second somatic mutations outside of the RNase IIIb domain which initiate two-hit tumorigenesis as seen in most DICER1-related tumors. Sequencing data identified individually distinct second somatic, likely-deleterious DICER1 mutations in Patient A's left ovary SLCT and sinonasal inflammatory polyp, in Patient C's NCMH, and in Patient D's Type II PPB which arose in a pre-existing lung cyst. Each of these second somatic mutations are predicted to prematurely truncate the DICER1 protein. We demonstrate that mosaic DICER1 RNase IIIb missense mutations are an occasional and important genetic cause of the DICER1 syndrome in patients presenting with multiple primary tumors associated with the syndrome, and that for tumor initiation, they appear to be accompanied by second somatic truncating non-RNase IIIb DICER1 mutations. Citation Format: Leanne de Kock, Barbara Rivera Polo, Mona Wu, Evan Weber, Claudio Sandoval, Saskia M. J. Hopman, J. Hans M. Merks, Annet van Hagen, D. A. Plager, Nelly Sabbaghian, Nancy Hamel, Dorothée Bouron-Dal Soglio, John R. Priest, William D. Foulkes. Mosaic RNase IIIb domain DICER1 mutations in children with multiple primary tumors. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4934. doi:10.1158/1538-7445.AM2015-4934

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