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Revisiting pleuropulmonary blastoma and atypical choroid plexus papilloma in a young child: DICER1 syndrome or not?

To the editor: In 2016, Liu et al. reported a child who developed Type I pleuropulmonary blastoma (PPB) at age 1 year and later developed atypical choroid plexus papilloma (aCPP).1 A germline pathogenic variant (PV) in DICER1 was not identified. Because PPB is the sentinel disease of DICER1 syndrome, a childhood tumor predisposition syndromewhose phenotypic range continues to evolve, we set out to confirm absence of a germline PV and explore alternative abnormalities in DICER1 to explain the tumors in this child. A Fluidigm array covering the entire DICER1 coding sequence and exon boundarieswas performed inDNAderived fromperipheral blood mononuclear cells (PBMC) from the patient and parents, and multiplex ligation-dependent probe amplification to investigate large gene rearrangements was performed.2 Both assays gave negative results. Sanger sequencing of PPB tumor tissue revealed two mutations in DICER1: a hotspot mutation in exon 25 (c.5438A >G, p.[E1813G]) and a frameshift mutation in exon 15 (c.2392_2393insA, p.(T798Nfs*33)). Investigating the possible relationship between the patient's aCPP and the PPB, we performed Sanger sequencing of the patient's aCPP; no DICER1 variants were found. Because mosaicism is described in patients with DICER1 syndrome,3,4 we performed digital droplet polymerase chain reaction in PBMC-derived DNA, aCPP, PPB, and normal lung from the patient. Six blood DNAs from healthy individuals were used as controls. The absence of the mutations in the patient's aCPP confirms that mosaicism for aDICER1 PV as a cause of the two tumors is ruled out (Figures 1A and 1B) and (SupplementaryMaterial S1). To determine the phase of the two mutations found and to investigate whether any full-length DICER1 protein is likely to be present in the lung tumor, we performed a cloning experiment in the cDNA from the patient's PPB (methods described in Supplementary Material S1). Twenty-three of the 24 clones had only the c.5438A > G, p.E1813G variant, likely because the c.2392_2393insA, p.T798Nfs*33 allele is subject to nonsense-mediated decay. We conclude that the two variants are in trans (Figure 1C). To investigate other plausible genetic causes, whole exome sequencing (WES) was done on gDNA from the patient, his parents, and aCPP tumor tissue.We analyzedWESdata focusing on 114 known cancer susceptibility genes5 (our analysis did not include copy number variants; methods are described in Supplementary Material S1). The only PV found by WES was c.263dupT, p.(I88*fs) in SBDS, present in both the patient and his father. However, SBDS is not a plausible cause of the two phenotypes.6 The cause of the patient's aCPP remains enigmatic. We conclude that: (a) the PPB results from somatically acquired, tumor-restricted mutations in DICER1, a mechanism previously reported,4 and (b) the patient's aCPP is genetically unrelated to his PPB. Based on current understandings of DICER1 disease, we believe that the child has no heritable defect in DICER1 and no generalized susceptibility to DICER1 phenotypes. The child does not have DICER1 syndrome. Further, aCPP is not a phenotype of DICER1 syndrome. The child's family appears not to be at increased cancer risk. When traditionalmolecularmechanismsofDICER1disease arenotpresent, intensive investigations can be of great benefit to the patient and family.