Placental transmogrification of the lung (PTL), or pulmonary placental transmogrification (PPT), is a rare placentoid bullous lesion [1] that generally occurs in young or middle-aged individuals. It was first described by McChesney [2]; however, the cause of the disease remains unknown [3]. To date less than 40 adult patients and only 3 pediatric patients have been reported in the literature [4], [5], [6]. The PTL is so named because of the superficial resemblance of the lesion to placental villi, being composed of cystic spaces filled with papillary structures [1]. PTL is diagnosed based on microscopic observation of immature placental villous structures. PTL can be asymptomatic or cause recurrent pneumothorax, bronchopneumonia, and respiratory distress syndrome. Radiological findings of PTL can be confused with those of bullous lung diseases, cystic airway malformations, pneumonia, and bronchogenic cysts [7]. Thoracic CT findings indicate severe lung pathologies, such as fibrotic bands, septal thickening, emphysematous areas, honeycombing, and air trapping. Such findings are often seen in patients with severe lung conditions, including advanced fibrosis, chronic obstructive pulmonary disease (COPD), or interstitial lung diseases [8]. Furthermore, these findings are typically concerning not only for PTL patients, but also for those with other serious lung diseases. PTL exhibits a male predominance and commonly affects only 1 lung lobe [9].

Our group previously described a 13-year-old male patient with no respiratory complaints, but a history of pneumonia. PTL was diagnosed via a wedge lung biopsy [4]. Examination for a hydrocele (swelling of the scrotum) showed the patient also had a borderline testicular tumor that was determined to be Mullerian type borderline epithelial neoplasm, which is analogous to ovarian serous borderline tumors. In addition, any germline variants have not been detected in our whole-exome sequencing analyses that could explain the pathogenesis of PTL.

The key takeaway is that PTL, although rare and not fully understood, is a benign disease with a prognosis that is generally good in the absence of additional malignancies [1], [8]. Surgical removal of the affected lung tissue (lobectomy or pneumonectomy) is generally the curative treatment. Surgical excision of PTL tissue is recommended due to its malignant potential [8]. Although PTL is thought to be related to lymphatic malformation of the lung on an emphysematous background, its etiology is not fully known. Due to the limited number of published cases, functional studies have yet to be conducted. The present study aimed to determine the molecular signature of PTL tissue, which we think is an important step towards understanding the pathogenesis of the disease.