Chromosomal aberrations in childhood acute lymphoblastic leukemia: diagnostic and prognostic implications

ABSTRACT

Cytogenetic analysis in ALL is often hampered by poor chromosome morphology, few malignant metaphases, undetectable chromosomal rearrangements due to regions of a similar size and banding pattern and sometimes only normal metaphases derived from normal cells are found after cell culture. Structural as well as numerical aberrations may therefore remain undetected using conventional G-banding. The application of modem molecular cytogenetic techniques including a broad set of fluorescence in situ hybridization [FISH] has greatly improved the detection rate of genetic changes in ALL. The present study was designed to estimate the incidences of different genetic subgroups in childhood ALL with abnormalities involving BCR/ABL, MLL, TEL/AML1 rearrangements, and p16 deletions using FISH technique and conventional cytogenetic analysis. We tried to demonstrate the usefulness of FISH technique. This study was conducted on BM and/or BP from 48 patients with childhood ALL. Their age range from 2-13 years mean age was 6.7 years. Patients were followed-up for 18 months [range 14-28 months]. Morphological, cytochemical, immunophenotyping, cytogenetic and FISH analysis were performed for every patient. FISH was performed with probes for BCR/ABL, MLL, TEL/AML1 rearrangements, and p16 deletions for each case of childhood ALL. Numerical and/or structural aberrations were identified in 52.1% of all cases by conventional G-banding alone. Numerical and/or structural aberrations were identified in 75% of all cases by the combination of conventional G-banding and interphase FISH. Gene rearrangements were disclosed by FISH in 11 [47.8%] of 23 patients who showed a normal banded karyotype or no mitotic cell in G-banding. The most common gene rearrangement was p16 deletion [21.27%] and the incidences of others were 15.9% for TEL/AML1, 12.1% for MLL, and 5% for BCR/ABL rearrangement. p16 homozygous deletions were observed in six cases [12.7%] and hemizygous deletions in four cases [8.5%]. One case had both in two different cell populations. p16 deletions were significantly more common among T-lineage ALL [T-ALL] patients than among precursor-B ALL patients. TEL/AML1 translocations were found in seven [7/44] [15.9%]. Three out of the seven cases show culture failure and none of the remaining cases showed t[12;21] in G-banding analysis. All those seven patients were pre-B cell lineage according to standard immunophenotyping. One patient showed the loss of one AML1 signal in addition to the TEL/AML1 fusion. MLL rearrangements [11q23 abnormalities] was detected in 5/41 [12.1%] by combined conventional cytogenetic analysis and by FISH. Two different types of MLL gene rearrangements were observed in FISH analysis; translation and deletion. One had split signal of the MLL gene caused by a translation between chromosome 6 and 11 t[6;11], detected by conventional cytogenetics. Amplification of MLL gene was observed in one case [2.27%]. Four of five cases with MLL translocations showed no chromosome abnormality involving 11q23 in G-banding analysis. All cases with MLL gene rearrangement were pre-B cell lineage according to standard immunophenotyping. BCR/ABL rearrangement t[9;22][q34;q11] was detected by conventional cytogenetic and by FISH in one case. Another one displayed BCR/ABL1 fusion signal by FISH only. FISH can overcome some limitations of conventional cytogenetic and molecular-genetic analyses and due to high sensitivity specific chromosomal aberrations in mitoses and/or interphase nuclei can be detected. FISH analysis using DNA probes specific for p16 deletion, TEL/AML1, MLL, and BCR/ABL gene rearrangements is a powerful tool for leukemia diagnosis and risk stratification and it should be used as a routine procedure for all patients with newly diagnosed ALL as well as for monitoring of treatment effect in children with ALL