From UpToDate (Pathogenesis of paroxysmal nocturnal hemoglobinuria):
"PNH STEM CELL — PNH originates from an acquired genetic defect (mutation) in a multipotent hematopoietic stem cell, or in a hematopoietic progenitor cell that acquires stem cell properties and is able to survive, expand, and self-renew. PNH can arise de novo or in the setting of an underlying bone marrow disorder such as aplastic anemia (AA), myelodysplastic syndrome (MDS), or primary myelofibrosis (PMF) ."
- Basically since there's a mutated cell population you have an increased chance of abnormal cellular proliferation. And in this case there should be awareness that the presentation may be broad, with findings such as AA, MDS and/or PMF. PNH could be the result, or cause, of the broad presentation, including the finding of malignancy.
Also from UpToDate (Clinical manifestations and diagnosis of paroxysmal nocturnal hemoglobinuria):
"Progression to acute leukemia — Some patients with PNH develop acute leukemia [60-68]. The lifetime risk is 5 percent or less. The leukemia may evolve from either the PNH clone or a non-PNH clone. Acute myeloid leukemia (AML) is most common, especially acute erythroleukemia (M6 in the French American British [FAB] classification) [66-68]. Some reports have suggested that AML arose from an abnormal PNH clone [64,69,70]. Lymphoid leukemias have also been reported ."