Co-Existence of Various Sub-Clones in TEL-AML1 Positive Acute Lymphoblastic Leukemia in Association with Sub-Microscopic Deletion of AML1.

The TEL/AML1 fusion gene is the most common gene rearrangement in pediatric acute lymphoblastic leukemia (ALL). Although considered to be a low risk leukemia it has a 20% risk of late relapse. The coexistence of different sub clones at diagnosis, based on polymerase chain reaction (PCR) studies of Ig/TCR gene rearrangement, was recently reported in this subtype of ALL. Their different response to chemotherapy may explain the emergence of certain sub clones at relapse, and may serve as a marker for minimal residual disease follow-up.

Several chromosomal rearrangements such as t(9;22), t(8;21), inv(16) and rearrangements of the MLL gene are frequently associated with submicroscopic deletions and some of them have prognostic significance. Such deletions were not reported in t(12;21) positive ALL.

Bone marrow cells from 76 pediatric patients with ALL at diagnosis were analyzed for the presence of the TEL/AML1 fusion gene by interphase fluorescence in situ hybridization (FISH). We used a new system of combined analysis enabling a very large-scale study of the cells of interest with regard to morphology, FISH and immunophenotyping. Fourteen patients were positive for the translocation. Four of them had several sub clones associated with various combinations of additional chromosomal abnormalities. The most striking was an atypical and unexpected hybridization pattern consistent with a submicroscopic deletion of the 5′ region of the AML1 breakpoint (intron2) not previously reported. We describe the use of a larger probe for AML1 (AML1/ETO) to exclude the possibility of insertion of TEL into the AML1 region without breakage and to reduce the false positivity due to optical fusion. This may enable a better monitoring of minimal residual disease in cases with submicroscopic deletion. All patients had some sub-clones with TEL deletion. Other abnormalities included trisomy and tetrasomy 21 as well as double TEL-AML1 fusion.

The analysis of numerous sub-clones at presentation in these patients suggests clonal evolution at an early stage of the disease. These sub-clones may have different sensitivities to chemotherapy, and some of them may reappear at relapse. The frequency of AML1 deletion in t(12;21) in addition to other chromosomal abnormalities, is unknown. The involvement of these findings in the generation of leukemic sub clones, their prognostic significance and role in minimal residual disease follow-up deserves further studies in a large number of patients and a longer follow-up.