Leukemia in a trisomy 21 mosaic: specific involvement of the trisomic cells.

Acute monoblastic leukemia was diagnosed in a 32-month-old boy with mild signs of Down's syndrome. Chromosome analysis of cultured skin fibroblasts and peripheral blood lymphocytes disclosed a constitutional mosaicism (46,XY/47,XY,+21). At initial diagnosis of acute leukemia, additional chromosomal changes were found in bone marrow blasts that were consistent with the M5 subtype. The cytogenetic markers of the acute leukemia were restricted to the trisomic subset and disappeared during remission. These findings add further weight to the suggestion that the propensity of Down's syndrome patients to develop leukemia is directly related to their karyotype abnormality and that leukemia might be clonal in origin.