Translocation t(12;21)(p13;q22), associated with favorable prognosis, is one of the most frequent structural chromosomal aberrations found in pediatric B-lineage acute lymphoblastic leukemia (ALL). This aberration leads to the fusion of exon 5 of Tel gene with exons 2 or 3 of Aml1 gene and results in long and short transcripts of Tel-Aml1. The short variant is differed from long form by 39 base pairs, that reflects the loss of the exon 2 of Aml1 gene during rearrangement or in the process of alternative splicing.1
It is known that various splice-variants/transcripts of fusion genes can be associated with different clinical features. For example, leukemic clone expressing Bcr-Ablp210 is more responsive to Gleevec therapy comparing with Bcr-Ablp190 positive cells.2 There are data concerning differences in prognosis of patients with various transcripts of Pml-Rara, expressed in acute promyelocytic leukemia.3 Biological or clinical significance of various Tel-Aml1 transcripts is still unknown. Therefore, we evaluated expression of Tel-Aml1 transcript variants and their relationship to laboratory features of childhood ALL.
A total of 172 patients with B-lineage ALL (age 0.8–18.0 years) were included into this study. Diagnosis of ALL was verified according to FAB-classification. Immunological diagnosis was defined using scoring system devised by the European Group for the Immunological Classification of Leukemia (EGIL). Cytogenetic study was conducted using standard G-banding on slides with cultured leukemic cells. Expression of Tel-Aml1 was evaluated using nested RT-PCR.4
The obtained data were analyzed using Statistica 6.0 software. Hypothesis of equation of two means was confirmed by criteria of Mann–Whitney U-test or χ2. Results considered as significant at the value P<0.05. The study was approved by local Ethical Committee.
Expression of Tel-Aml1 was found in 20.3% of children with B-lineage ALL (35/172). In 91.4% (32/35) cases one of Tel-Aml1 variants prevailed, in two patients subtype of fusion gene was not determined and in one patient equal levels of both forms were found. The ratio of bone marrow samples from ALL patients with long and short variants was 3:1 (24/8). On the basis of transcript size differences, two subgroups of Tel-Aml1-positive ALL (Tel-Aml1pos) were selected and possible associations of transcript variants to immunophenotypic and cytogenetic markers of leukemic cells were studied.
As it is shown in Table 1, presence of long transcript was associated with the lower frequency of CD20 expression (B-lineage marker of later differentiation stages) and higher frequency of CD34 (marker of early stages of hematopoietic differentiation) and myelomarkers CD13 and CD33 compared to cells expressing short transcript. Leukemic cells with long transcript were characterized also by a higher frequency of expression of CD13 and/or CD33 myeloid markers. In both subgroups significant negative correlation was found between CD20 and CD34 (r=−0.49, P<0.05 for long transcript and r=−0.89, P<0.05 for group with short one). On the whole, in Tel-Aml1pos group CD20 expression correlated negatively to expression of CD34 (r=−0.65, P<0.05), CD13 (r=−0.53, P<0.05) and CD33 (r=−0.41, P<0.05), while expression of CD34 showed positive correlation with detection of CD13 (r=0.57, P<0.05) and CD33 (r=0.54; P<0.05). Similar relationship between CD20 and CD34 was found in Tel-Aml1neg group but it had been weaker (r=−0.2; P<0.05). Thus, we found that leukemic blasts with long transcript were the less differentiated (low level of CD20 and high one of CD34) and more frequently expressed myelomarkers. As differences in expression of immunophenotypic markers between clones with various transcripts were not absolute and both transcripts could be detected in leukemic cells obtained from the same patient, it should be investigated whether different transcripts are expressed in the same or in immunophenotypically diverse subpopulations of Tel-Aml1pos leukemic clone.
Metaphases were available for cytogenetic analysis in 62.5% (5/8) patients with short transcript and in 41.7% (10/24) with long one (Table 2). In all, 50% (5/10) of patients with long transcript had deletions of various locuses of the long arm of chromosome 6 and abnormalities of the second chromosome 12 were detected in 40% (4/10). Del(12)(p12) was found in two of four such cases. One patient (1/5) with short transcript had abnormality of the long arm of chromosome 6. There was no difference in frequency of aberrations with participation of chromosomes 6 and 12 between patients expressing various transcripts . Tel-Aml1pos patients characterized by significantly higher frequency of abnormalities involving long arm of chromosome 6 comparing with Tel-Aml1neg.
The long transcript was associated with a more immature precursor B phenotype whereas the short transcript represented a more mature precursor B phenotype. Further studies should address the significance of additional genetic abnormalities in chromosomes 6 and 12 to the immature phenotype of blasts with a long transcript.
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This project was funded by the project #B-736 of the International Science and Technology Center.
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Kustanovich, A., Belevtsev, M., Stasevich, I. et al. Expression of Tel-Aml1 transcript variants in pediatric acute lymphoblastic leukemia. Leukemia 20, 165–167 (2006). https://doi.org/10.1038/sj.leu.2404028