Let-7 restrains an epigenetic circuit in AT2 cells to prevent fibrogenic intermediates in pulmonary fibrosis

MicroRNA-mediated post-transcriptional regulation of lung alveolar type 2 (AT2) and AT1 cell differentiation remains understudied. Here, we demonstrate that the let-7 miRNA family plays a homeostatic role in AT2 quiescence by preventing the uncontrolled accumulation of AT2 transitional cells and promoting AT1 differentiation. Using mouse and organoid models, we show that genetic ablation of let-7a1/let-7f1/let-7d cluster (let-7afd) in AT2 cells prevents AT1 differentiation and leads to KRT8 transitional cell accumulation in progressive pulmonary fibrosis. Integration of AGO2-eCLIP with RNA-sequencing identified direct let-7 targets within an oncogene feed-forward regulatory network, including BACH1/EZH2/MYC, which drives an aberrant fibrotic cascade. Additional CUT&RUN-sequencing analyses revealed that let-7afd loss disrupts histone acetylation and methylation, driving epigenetic reprogramming and altered gene transcription in profibrotic AT2 cells. This study identifies let-7 as a central hub linking unchecked oncogenic signaling to impaired AT2 cell plasticity and fibrogenesis. Loss of let-7, a key microRNA, causes lung alveolar stem cells to grow abnormally and dieprematurely, leading to scarring and lung damage. Here the study links let-7 loss to cancer-like signals and epigenomic gene changes, revealing new treatment paths for lung fibrosis.