Novel genetic characteristics of multifocal micronodular pneumocyte hyperplasia (MMPH): a case report with frequent BRAF mutations analyzed by next-generation sequencing supporting benign behaviors of MMPH.

A woman in her 30s, who was clinically diagnosed with tuberous sclerosis complex, underwent lung transplantation due to lymphangioleiomyomatosis with concomitant multifocal micronodular pneumocyte hyperplasia (MMPH). Histologically, MMPH lesions demonstrated variety in histology; some showed homogenous cells with mild nuclear atypia and elastic fibers proliferation, and the others showed enlarged nuclei without elastic fibers. Because the natural history of MMPH is not well characterized, we used next-generation sequencing to perform a comprehensive genetic analysis for the MMPH lesions to explore their malignant potential. Regardless of their histological variety, three of four lesions had BRAF missense mutations, especially the types frequently detected in atypical adenomatous hyperplasia that is considered to be benign rather than a precursor of adenocarcinoma. None of them had major driver mutations of lung adenocarcinoma, except for BRAF mutations. In conclusion, our study of the lesions from this patient indicated the benign characteristic of MMPH.