Abstract 3088: Biallelic DICER1 mutations in sporadic pleuropulmonary blastoma

Pleuropulmonary blastoma (PPB) is an extremely rare and aggressive pediatric pulmonary malignancy. Recently, DICER1 mutations were reported to be heterozygous frameshift or nonsense mutations of germline origin, suggesting an important role of haploinsufficiency of DICER1. On the other hand, Dicer1 heterozygous deficient mice did not develop PPB, suggesting that DICER1 haploinsufficiency alone may not be sufficient for tumor development. Thus, to identify genetic alterations underlying the pathogenesis of PPB, we performed an integrated molecular study of PPB analyzed by whole-exome, RNA and micro RNA sequencing in conjunction with SNP array analyses. Whole-exome sequencing of 7 cases with sporadic PPB and targeted deep sequencing of DICER1 was performed in additional 5 cases. DICER1 mutations were found in 11/12 cases by whole-exome or deep sequencing, in which 6 cases carried compound heterozygous mutations. Two cases carried homozygous mutations, which were caused by UPD involving the 14q harboring the DICER1 locus. In total, biallelic DICER1 mutations were found in 8/11 cases with DICER1 mutations. Four cases were confirmed that compound heterozygotes of a germline nonsense/frameshift and a somatic missense mutation, two were homozygous for somatic, missense mutations. Conspicuously, all the 9 missense mutations were located within the RNase IIIb domain with a mutational hotspot at G1809, for which somatic origin was confirmed in 7 mutations. To assess the impact of the DICER1 hotspot mutation on RNAase IIIb activity, we performed miRNA sequencing in 3 cases with G1809R mutations. miRNA of fetal lung was used as a control. Comparing the ratio of miRNA or miRNA* read count to pre-miRNA read count, the tumor with G1809R mutant showed significantly low levels of the miRNA product, but the effects were less clear in the levels of the miRNA*, suggesting that G1809R mutants result in reduced RNase IIIb activity but retention of RNase IIIa activity. We also performed RNA sequencing in 5 cases, but apparent breakpoint cluster region was not detected. Since 17p LOH was found in 8/11 cases, we checked the TP53 mutation status in 12 cases by deep sequencing. We detected TP53 mutations in 5/12 cases, in which all 5 cases were accompanied by 17p LOH and leading to biallelic TP53 inactivation. In conclusion, biallelic DICER1 mutations were quite common in PPB, invariably accompanied by a somatic RNase IIIb domain mutation. A majority of cases had mutated DICER1 alleles in germline with or without an additional RNase IIIb domain mutation in the remaining allele. Given that the novel hotspot mutation, G1809R, revealed reduced RNase IIIb activity, alternative RNase IIIb activity could be involved in the tumorigenesis of PPB. Recurrent mutations were rare in PPB, except for frequent TP53 deletions/mutations. Our results provide a novel insight into the critical role of DICER1 mutations and importance of TP53 inactivation in the pathogenesis of PPB. Citation Format: Masafumi Seki, Kenichi Yoshida, Yuichi Shiraishi, Yusuke Sato, Teppei Shimamura, Riki Nishimura, Kenichi Chiba, Hiroko Tanaka, Keisuke Kato, Motohiro Kato, Ryoji Hanada, Yuko Nomura, Myoung-Ja Park, Toshiaki Ishida, Akira Oka, Satoru Miyano, Yasuhide Hayashi, Seishi Ogawa. Biallelic DICER1 mutations in sporadic pleuropulmonary blastoma. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 3088. doi:10.1158/1538-7445.AM2014-3088