Monosomy 7 in multilineage and acute lymphoblastic leukaemia.

Monosomy 7 as the sole cytogenetic abnormality was detected in five of 310 consecutive adult patients with acute leukaemia who were characterized by morphological, immunophenotypic and cytogenetic analyses. Morphologically, blast cells were myelomonocytic (FAB M4) in three and lymphoid (FAB L2) in two patients. By immunophenotyping, two M4 patients expressed terminal transferase (TdT) in 15-90% of myelomonoblasts (patients 3 and 1, respectively), and in the third M4 patient (no. 2), a 10% TdT+ component was present distinct from the bulk of myelomonoblasts. In one L2 patient (no. 4), the blast cells had an undifferentiated phenotype only expressing TdT and HLA-DR but lacking specific lymphoid and myeloid antigens, and patient 5 was typed as CD10+ ALL. Two patients had developed leukaemia following radiotherapy and/or chemotherapy for multiple myeloma or breast cancer. In two patients, induction chemotherapy induced a lineage switch in the immunophenotype without change in karyotype. These observations support the concept that monosomy 7 leukaemia results from the transformation of a multipotential stem cell.