Interlineage switch from myeloid to lymphatic malignancies is a rare phenomenon. Progression from a BCR-ABL negative chronic myeloproliferative disorder (CMPD) to acute lymphoblastic leukemia (ALL) has been reported in very few cases. We describe the case of a 62-year-old man who developed precursor T-cell (pre-T) ALL 18 months after the diagnosis of an unclassifiable chronic myeloproliferative syndrome (CMPD, U), which had been treated with hydroxyurea (HU) over 12 months. The transformation from CMPD to pre-T-ALL was accompanied by clonal evolution from normal to a high hyperdiploid karyotype with an i(17q): 52,XY,+X,+2,+13,+14,+15,i(17)(q10),+19. Diverse pathways should be considered in this transformation process: although therapeutic induction of ALL is extremely rare and no MLL/11q23 rearrangement was detected by chromosome banding analyses and interphase fluorescence in situ hybridization (FISH), T-lineage ALL might have been caused by HU therapy for the CMPD. Chance coincidence of both disorders seems improbable, given the short interval from the diagnosis of the CMPD to the development of the pre-T-ALL, but nonetheless must be considered. A third explanation might be provided by a spontaneous interlineage switch, which would give further support to the theory that the CMPDs are disorders of a pluripotent stem cell. Interlineage switch might result from an aberrant differentiation of the malignant clone or from selection within a mixed population.
