Amplification of the ABL Gene in T-Cell Acute Lymphoblastic Leukemia (T-ALL).

In acute leukemias gene amplification occurs with incidence of 1–10%. On conventional cytogenetic (CC) studies it is usually seen either intrachromosomally as homogeneously staining regions (HSRs) or extrachromosomally as double minute chromosomes (dmins). Fluorescence in Situ Hybridization (FISH) has established that the genes most frequently amplified in ALL are AML1 and MLL. Recently, the Leukaemia Research Fund UK Cancer Cytogenetic Group has detected the amplification of the ABL gene in 5/210 childhood and in 3/70 adult T-ALL and has suggested that this genetic abnormality might identify patients with a generally poor event-free survival (EFS). The present study was aimed at determining the incidence and clinical significance of ABL amplification in a series of 31 consecutive adult T-ALL patients. All of them had been submitted to routine FISH screening for BCR/ABL and TEL/AML1 fusions and for MLL amplification. ABL amplification was detected by chance in two patients (6.4%). In one CC did not yield analysable metaphases and in the other it showed the following karyotype:

46,XX/46,XX,t(1;3)(p34;p21),del(6)(q21),del(7)(q32). FISH with a painting probe specific for chromosome 9 detected an occult trisomy in the patient without analysable mitosis and a normal pattern in the other one. In both patients the number of ABL signals varied from cell to cell and the observer was always unable to count them properly. FISH on mitotic cells showed that ABL additional copies were localized neither on chromosome 9 nor on any other chromosome and revealed that amplification was extrachromosomal in nature even if no dmins were visualized. Therefore, it was hypothesized that the amplified ABL sequences might be localized on submicroscopic extrachromosomal structures, the episomes. In order to check whether the ABL gene was really over-expressed we performed a quantitative RT PCR (Q RT PCR) assay using the β-2-microglobulin as reference gene and total RNA from a normal subject for calibration. Quantification was made using the DDCt method. By this way we found that on clinical diagnosis the two patients expressed the ABL gene nine and twelve times more than the control.

From a clinical point of view both patients were males. They had a high white blood cell count (31.8 and 21.8x10⁹/L); their blast cells exhibited a T-ALL immunophenotype and a L2 morphology; their lactic dehydrogenase level was elevated. One patient achieved a complete remission (CR) of fifteen month duration and relapsed while still on maintenance treatment, the other did not respond to chemotherapy. In the former patient ABL expression was normal in CR but increased again on disease recurrence. In conclusion our data show that i) FISH is absolutely required to identify a new subset of T-ALL patients characterized by ABL amplification, ii) the role of ABL amplification in T-ALL pathogenesis is still obscure, iii) large cooperative studies are required to better define the clinical outcome of these patients whose EFS seems to be poor, iiii) Q RT PCR might be used to quantify minimal residual disease.