[Case report of a cystic lung disease: From a rarity to the discovery of an unknown genetic variant].

Introduction: Cystic lung diseases are rare, with numerous differential diagnoses. Iconographic discovery consequently necessitates medical examinations in view of proposing an etiological orientation.

Case Report: A 57-year-old woman consulted in pulmonology following fortuitous detection of a cystic lung disease on an abdominal CT scan. Complementary medical examinations did not allow orientation towards a particular diagnosis. During a follow-up consultation, the patient informed her pulmonologist of the recent detection of a monoallelic variant of a FAT4 gene in one of her daughters, who was suffering from edema of the lower limbs secondary to a disease of the lymphatic system. As our patient had a similar history, she likewise received a genetic analysis. A monoallelic variant not described in the genetic databases was observed, and considered as a probable pathogenic variant (class 4/5 on the pathogenicity scale of genetic variants).

Conclusion: After analyzing the available literature data, we raise questions about a possible link between this variant of the FAT4 gene, chronic lymphedema and our patient's cystic lung disease.