THE DER(21)T(12-21) CHROMOSOME IS ALWAYS FORMED IN A 12-21 TRANSLOCATION ASSOCIATED WITH CHILDHOOD ACUTE LYMPHOBLASTIC-LEUKEMIA

We studied 116 patients (93 children and 23 adults) with acute lymphob lastic leukaemia (ALL) using fluorescence in situ hybridization (FISH) with the yeast artificial chromosome (YAC) clone, 964c10, which inclu des the recently described ETS-like gene, TEL, on 12p13. FISH revealed that nine of the patients had a t(12;21), which had not been previous ly detected, The nine patients were all children, seven boys and two g irls, aged 1-10 years (median 3 years), had an early B immunophenotype , and achieved complete remission, although two of them experienced ha ematological relapse. In addition to the t(12;21), FISH also revealed that three of the nine had a del(12p) in the other homolog of chromoso me 12 or in the der(12) chromosome itself, and that two others had 12p translocations in the other chromosome 12 homolog. Although chromosom al rearrangements associated with the t(12;21) were heterogenous and c omplex, fusion of the sequences from chromosomes 12 and 21 on the der( 21)t(12;21) chromosomes was consistent, suggesting that the TEL-AML1 g ene fusion on the der(21) chromosome may be critical in leukaemogenesi s and that FISH or reverse transcriptase-polymerase chain reaction (RT -PCR) targeted to the chimaeric sequences on the der(21) will be most useful in detecting the t(12:21) or following a patient with the t(12; 21), which is one of the most frequent chromosomal rearrangements in b oth Caucasian and Asian childhood ALL.