Preleukemic myelodysplastic syndromes (MDS): pathogenetical considerations based on retrospective clinicomorphological sequential studies.

Analysis of myelodysplastic syndromes (MDS) in 77 patients terminating in "overt" leukemia revealed sequential changes of marrow morphology with respect to cellularity and involvement of lineages, indicating that different "forms" of MDS represent in reality different phases. Three main phases were observed, which occurred in the following non-reversible sequence: (a) hypocellular dysplasia (H), (b) hypercellular dysplasia with predominance of erythropoiesis (E), and (c) with predominance of myelo(mono)poiesis (MM). Published studies suggest that these phases represent different manifestation stages of the stem cell lesion leading to MDS. Manifestation may probably be promoted by factors causing marrow aplasia. Transition to "overt" leukemia in several cases occurred locally, not throughout the whole marrow. In some patients a mature myelo(mono)-cytic cell population showed the capacity for widespread tissue infiltration, characteristic of leukemic cells. This argues against the theory that "overt" leukemia is the result of continuous dedifferentiation in MDS, but favours the concept of multiple initiating "events" leading to evolution of a leukemic subclone within a myelodysplastic clone. A connection between leukemia differentiation and MDS phase, from which leukemia developed, was also observed indicating that the risk of different stem cell subpopulations to become the target of the leukemia initiating "event" varies during the course of myelodysplasia.