We present the case of a term neonate with respiratory distress, found to have a type I PPB and a germline VUS in DICER1 predicted to affect splicing. DICER1 is a member of the ribonuclease II family and is involved in the generation of microRNAs that modulate posttranscriptional gene expression. It is involved in many processes, including lung development, maintenance of stem cells, cell cycle progression, and tumorigenesis.22 Individuals with pathogenic variants in DICER1 are at increased risk of certain malignancies including PPBs, cystic nephroma, thyroid nodules, ovarian Sertoli-Leydig Cell tumor, thoracic embryonal rhabdomyosarcoma, nasal chondromesenchymal hamartoma, genitourinary sarcomas, neuroendocrine tumors, pineoblastoma, pituitary blastoma, Wilms tumor, multinodular goiter, and several other sarcomas.7,23 Approximately 70% of all patients with PPB harbor germline mutations in DICER1 that can be identified with standard clinical testing.6–8 Conversely, penetrance is relatively low, meaning many patients with pathogenic DICER1 variants do not develop a malignancy during their lifetime. Recent estimates suggest that 5.3% of non-proband DICER1 carriers develop a malignancy by the age of 10, increasing to 19% by age 50.9 Screening recommendations include surveillance for the broad range of associated tumors, with particular attention to the chest, abdomen/pelvis, and thyroid.7 For patients with a diagnosed malignancy, additional surveillance is required.
Sequencing of DICER1 in the patient demonstrated a germline single nucleotide variant that did not change the amnio acid but was predicted by computational tools to alter splicing which was classified as a VUS. Sequencing of tumor tissue identified a known pathogenic variant in the RNase IIIb domain in addition to the germline VUS, likely representing a second hit. Clinical RNA-sequencing confirmed that the variant did indeed result in a 29-amino acid in-frame deletion in a minority of transcripts, potentially sufficient for tumorigenesis; however, because the clinical laboratory felt overall expression of DICER1 was unchanged the result was classified as indeterminate. Review of the raw RNA sequencing data obtained from the clinical testing lab revealed that 3/61 transcripts contained the c.2523A > G variant and at least 6 aberrant transcripts could be identified which were not included in the count of 61 transcripts at the c.2523 position. Thus, while interpreted as indeterminate by the clinical lab, the fact that only 5% of transcripts contained the c.2523A > G variant suggests that it may in fact result in reduced expression of DICER1. This highlights a challenge with clinical interpretation of RNA-seq data and the benefit of analyzing original sequencing data in uncertain cases.
PPBs can be categorized into three groups, which are thought to represent the spectrum of progression: type I, or purely cystic PPB; type II, or cystic/solid PPB; and type III, or purely solid PPB. An additional subtype, type Ir (type I-regressed), represents a group of type I PPBs that no longer demonstrate malignant cells.15 These subtypes correlate with the probability of recurrence and mortality, with type I/Ir PPB having the best prognosis and overall survival estimated at > 90%.16 While patients with type II and III PPB are typically treated with surgery and adjuvant chemotherapy, those with type I or Ir PPB can often be cured with surgery alone. However, after definitive management, almost 10% of patients with type I/Ir will progress to type II/III PPB, subsequently conferring a poorer prognosis.6,16 Recurrence or progression can occur as late as 6 years after initial diagnosis,6 necessitating long-term surveillance.
There is no current consensus regarding optimal management of PPB.2,6 Timing of surgery and chemotherapy is dependent upon the type and size of the PPB. Typically, pulmonary wedge resection is favored for type I PPB.2 If the tumor is central, hilar, or multifocal in location, more extensive resection—such as pulmonary lobectomy—may be required. A similar approach is taken for type Ir PPB. Types I and Ir PPB have not been reported to metastasize and can be cured if complete resection with wide negative margins is achieved, although recurrence remains a concern.2 For type II and type III tumors, treatment with both surgical resection and systemic chemotherapy is critical.2 The anatomy and size of the tumor drives the surgical approach for these lesions.8 Surgical resection for type II and III PPB tumors includes a variety of approaches including wedge resection, lobectomy, or pneumonectomy. If lobectomy is required for type II or type III PPB, the pleural surfaces are resected en bloc with the primary tumor and the affected pulmonary lobe; this is most commonly performed as an open procedure.2;17–19 Type II and type III tumors larger than 10 cm may be exceptionally difficult to resect in entirety with wide negative margins.2 If the type II or type III tumor is large with extensive spread to the pleural surface, the only method of achieving local control may be an extrapulmonary pneumonectomy,2;17–19 which requires extensive resection of not only the plural surfaces but also the pericardium, phrenic nerve, and diaphragm, as well as division of the pulmonary hilar vessels.2 This procedure requires pericardial defect reconstruction and diaphragmatic reconstruction with fenestrated patches,2 and has a high rate of complications, including post-pneumonectomy syndrome. Post-pneumonectomy syndrome is caused my mediastinal shift with severe compression or kinking of the mainstem bronchus, trachea, and/or great vessels. 2;17–19 In rare cases, PPB tumors have demonstrated endobronchial or pulmonary vascular extension;1 echography and bronchoscopy should be performed in individuals with advanced PPB to aid with operative planning.2
The documented possibility of recurrence following surgical resection provides some rationale to consider treating all patients with type I PPB with adjuvant chemotherapy; however, the data are controversial and there is no current standard of care.20 Current best practice recommendations include an individualized approach and tumor board discussion. In the setting of a microscopically complete resection, adjuvant chemotherapy may be considered to prevent relapse or progression. Likewise, in the case of a resection with microscopic residue it may be reasonable to consider either repeat surgery to achieve complete resection or initiate systemic chemotherapy. Finally, in cases with suspected tumor spillage, decisions should be made on an individual basis, based on patient characteristics and surgical course. Chemotherapy selection has been modeled after rhabdomyosarcoma regimens, including VAC (vincristine, cyclophosphamide, actinomycin-D), VA (vincristine, actinomycin-D), IVA (ifosfamide, vincristine, actinomycin-D), and VAIA (ifosfamide, vincristine, actinomycin-D, doxorubicin).20,21
In summary, we present the case of a term neonate with a congenital type 1 PPB after presenting with respiratory distress, found to have a germline DICER1 VUS with altered transcriptional activity by RNA-seq. PPB is a rare mesenchymal malignancy of the lung which may be mistaken for a CPAM due to similarities to PPB in both imaging and pathology. A diagnosis of PPB should always raise suspicion for DICER1 syndrome, and testing for germline mutations is warranted for patients, and, if positive, their first-degree relatives. DICER1 syndrome puts individuals at higher risk for other malignancies and a tumor surveillance protocol may be initiated. RNA sequencing is an emerging modality that may clarify VUSs identified by germline testing, but challenges with interpretation and varying practices among clinical laboratories may warrant reanalysis of original sequencing data to fully understand the impact of the genomic change under investigation.