Imputing causality and clonal dynamics from single-cell transcriptomics in paroxysmal nocturnal hemoglobinuria

Paroxysmal nocturnal hemoglobinuria (PNH) originates from hematopoietic stem cells (HSCs) harboring somatic mutations in the phosphatidylinositol glycan class A (PIGA) gene. Clonal expansion of PIGA-mutated cells occurs uniquely in the setting of bone marrow (BM) failure, but specific pathophysiologic mechanisms remain unclear. We performed single-cell RNA sequencing (scRNA-seq) of BM cells from patients with large (> 50%) and small (10-50%) PNH cell fractions. In patients with large PNH cell fractions, phenotypically normal hematopoietic stem and progenitor cells (HSPCs) upregulated immune response and apoptosis pathways and downregulated cell-cycling pathways compared with PNH-type HSPCs. BM effector cells upregulated immune response pathways, and cell-cell communication between effector cells and normal HSPCs was greater than in controls. In contrast, in patients with small PNH cell fractions, transcriptional changes in normal HSPCs were reversed: downregulation of immune response pathways and upregulation of the cell-cycling pathways. Notably, transcriptional differences associated with PNH cell fractions were primarily in normal HSCs, whereas PNH-type HSCs showed similar transcriptional profiles between patients with large and small PNH cell fractions. These results implicate immunological negative selection against normal HSCs in PNH. Error-corrected DNA sequencing of patients' blood samples identified multiple PIGA mutations in each patient, consistent with strong selection for the resulting phenotype.