ISOCHROMOSOMES IN ACUTE LYMPHOBLASTIC-LEUKEMIA - I(21Q) IS A SIGNIFICANT FINDING

The incidence, type, and clonality of isochromosomes at diagnosis were investigated in acute lymphoblastic leukaemia (ALL). An isochromosome was detected in 50/1,035 (4.8%) of successfully karyotyped patients, 41/919 children (4.5%) and 9/116 adults (7.8%), who were diagnosed wit hin a 5 year period. Isochromosomes of 21q with breakpoints in the sho rt arm at p11 or in the long arm at q10 or q22 were identified in 15 p atients (1.4%) associated with B-lineage immunophenotype, a white bloo d cell count (WBC) of <10x10(9)/litre, and pseudo- or low hyperdiploid y. Isochromosomes of 17q and 7q occurred in 13 (1.3%) and 9 (0.9%) pat ients, respectively, and were associated with high hyperdiploidy. Isoc hromosomes of 9q and 6p occurred in 6 (0.6%) and 5 (0.5%) patients, re spectively, whereas i(Xp), i(1q), and i(8q) occurred in 1 patient each . The isochromosome occurred as the sole abnormality in 4 patients [3 with i(21 q) and 1 with i(7q)] and in the stemline, but with other chr omosomal changes, in 35 patients. It was confined to a clonally evolve d sideline in 11 patients. Isochromosomes occurred with established ab normalities in 7 patients: with t(1;19)-i(7q)/i(9q)/i(7q) and i(9q) ea ch in 1 patient; with t(4;11)-i(7q)/i(17q) in 1 and 2 patients, respec tively; and with t(9;22)-i(9q) in 1 patient. This study indicates that isochromosome formation can be an early chromosomal change and sugges ts that i(21q) occurs more frequently at diagnosis than has been previ ously suspected. (C) 1996 Wiley-Liss, Inc.