Temporal order of RNase IIIb and loss-of-function mutations during development determines phenotype in DICER1 syndrome: a unique variant of the two-hit tumor suppression model

Abstract

Pleuropulmonary blastoma (PPB) is the most frequent pediatric lung tumor and often the first indication of a pleiotropic cancer predisposition, <i>DICER1</i> syndrome, comprising a range of other individually rare, benign and malignant tumors of childhood and early adulthood. The genetics of <i>DICER1</i>-associated tumorigenesis are unusual in that tumors typically bear neomorphic missense mutations at one of five specific "hotspot" codons within the RNase IIIb domain of <i>DICER 1</i>, combined with complete loss of function (LOF) in the other allele. We analyzed a cohort of 124 PPB children for predisposing <i>DICER1</i> mutations and sought correlations with clinical phenotypes. Over 70% have inherited or <i>de novo</i> germline LOF mutations, most of which truncate the <i>DICER1</i> open reading frame. We identified a minority of patients who have no germline mutation, but are instead mosaic for predisposing <i>DICER1</i> mutations. Mosaicism for RNase IIIb domain hotspot mutations defines a special category of <i>DICER1</i> syndrome patients, clinically distinguished from those with germline or mosaic LOF mutations by earlier onsets and numerous discrete foci of neoplastic disease involving multiple syndromic organ sites. A final category of PBB patients lack predisposing germline or mosaic mutations and have sporadic (rather than syndromic) disease limited to a single PPB tumor bearing tumor-specific RNase IIIb and LOF mutations. We propose that acquisition of a neomorphic RNase IIIb domain mutation is the rate limiting event in <i>DICER1</i>-associated <i> </i>tumorigenesis, and that distinct clinical phenotypes associated with mutational categories reflect the temporal order in which LOF and RNase IIIb domain mutations are acquired during development.