Acute megakaryoblastic leukemia in an infant with t(1;22)(p13;q13) and complex secondary chromosomal aberrations including hyperdiploidy. D. Wei¹, K.H. Ramesh¹, D.T. Walsh¹, C. Johnson¹, V.R. Pulijaal¹, H. Ratech¹, E.A. Kolb², L.A. Cannizzaro¹. 1) Dept Pathology, Montefiore Medical Ctr. Bronx, NY 10467; 2) Dept Pediatrics, Albert Einstein Col Med. Children's Hospital at Montefiore Bronx, NY 10467.
   Acute megakaryoblastic leukemia (AMKL) is an uncommon acute leukemia. It is classified as acute myeloid leukemia-M7 (AML-M7) in the FAB system, and as AMKL in the WHO system. AMKL occurs at all ages. The t(1;22)(p13;q13), characteristic of AMKL, is restricted to infants and children. It has been reported as the sole chromosomal aberration (CA) in about 60% of AMKL cases below 6 months of age and above the age of 6 months, the t(1;22) is often associated with complex secondary CA including hyperdiploidy. In both cases, the prognosis appears to be poor. We report a 2 years old infant with a history of anemia, thrombocytopenia and peripheral blasts. Bone marrow (BM) biopsy showed normocellular marrow, diffusely replaced by blasts and essentially no normal hematopoiesis except for a few scattered megakaryocytes. Immunohistochemical stains revealed blasts that expressed CD117+, CD15-, CD34-, CD68-, bcl-2- (protein). Flow cytometry revealed CD7+, CD13+, CD33+, CD34-, CD38+, CD64+, CD117+, MPO-, CD56-, TdT-, HLA-DR-, and 5% CD61+. The differential diagnoses included acute promyelocytic leukemia, acute monoblastic leukemia and AMKL. Cytogenetic analysis of BM showed a 57,XY,+Y,+1,der(1)t(1;22)(p13;q13)x2,+2,+add(4)(q31),+6,der(7)t(1;7)(p36;q36),+8, +9,+10,+13,+19,+21,-22,+der(22)t(1;22)(p13;q13)t(1;?)(p36;?)[17]/46,XY[3] karyotype. The t(1;22) was questionable by G-banding, as the der(22) had evolved with additional unidentifiable changes. FISH analyses however, confirmed the der(1) and the der(22) chromosomes to be products of the t(1;22). The final cytogenetic results were strongly in favor of AMKL in this infant. The diagnosis of AMKL, particularly in infants and children is difficult to establish in spite of all the hematological parameters pointing to acute leukemia. Therefore, chromosome analyses and FISH tests are highly significant in the final diagnosis of AMKL.