A subtle deletion of 12p by routine cytogenetics is found to be a translocation to 21q by fluorescence in situ hybridization: t(12;21)(p13;q22)

A 4-year-old boy with acute lymphoblastic leukemia (ALL) whose leukemic cells contained at diagnosis a del(6q) had a different clone at relapse, characterized by on abnormal short arm of chromosome 12(12p). Fluorescence in situ hybridization (FISH) analysis using a cosmid probe from the 12p12-23 region indicated a translocation of the 12p to another unknown chromosome of ''G-group'' size. Further analysis by dual chromosome painting confirmed the presence of a translocation, identified as t(12;21)(p13;q22). Previously reported cases showed this translocation at diagnosis of ALL, whereas this is the first case showing the specific t(12;21)(p13;q22) at relapse. This case study illustrates the value of FISH in resolving subtle recurrent chromosomal rearrangements that escape detection by routine cytogenetic analysis. Subsequent molecular evaluation of the patient's leukemic cells with the t(12;21) demonstrated a TEL/AML1 fusion protein.