The NPM-ALK and the ATIC-ALK fusion genes can be detected in non-neoplastic cells

Anaplastic large cell lymphoma (ALCL) is frequently associated with the t(2 ;5)(p23;q35) translocation. It creates a NPM-ALK fusion gene, fusing the an aplastic lymphoma kinase (ALK) gene (2p23) and the nucleo-phosmin (NPM) gen e (5q35). Other rearrangements involving the ALK gene have recently been sh own to be associated with ALCL, among which the ATIC-ALK rearrangement resu lting from the inv(2)(p23q35) translocation is probably the most recurrent. The aims of the present study were to investigate the presence of NPM-ALK and ATIC-ALK fusion genes in ALCL, using a real-time 5 ' exonuclease-based reverse-transcription polymerase chain reaction (RT-PCR). This sensitive te chnique was also applied to investigate whether both fusion genes might be detected in Hodgkin's disease cases and in reactive lymphoid tissue. Result s of the RT-PCR were compared to ALI( immunostaining, cytogenetics, and flu orescence in situ hybridization (FISH) results. RT-PCR detected the NPM-ALK and ATIC-ALK fusions at high levels in 8 and 3 of a total of 13 ALK-positi ve ATCL cases. One ALK-positive ALCL case was negative for both fusion gene s analyzed but revealed a new ALK-related translocation t(2;17)(p23;q25) by cytogenetic and FISH analysis. In addition, of the eight ALK-positive ALCL cases that were strongly positive for the NPM-ALK fusion, three cases also showed the presence of the ATIC-ALK fusion, although at much lower levels. Similarly, out of the three strongly positive ATIC-ALK cases, one case was positive for the NPM-ALK fusion, at low levels. Finally, the NPM-ALK and t he ATIC-ALK fusions were detected, at equally low levels, respectively in 1 3 and 5 ALK-negative ALCL cases, in II and 5 Hodgkin's disease cases and in 20 and 1 non-neoplastic lymphoid tissues. The distinction between the high - and low-level detection was confirmed by relative Quantitative RT-PCR for a representative number of cases. Of interest is the fact that the high-le vel detection coincided with the presence of ALK gene rearrangement detecte d by cytogenetics and FISH and may reflect a central role of the transcript in the oncogenic mechanism of ALK-positive ALCL. Low-level detection is no t supported by cytogenetics and FISH, presumably due to the presence of the transcripts in only a small minority of normal cells not detectable by the se techniques. Our findings demonstrate that NPM-ALK and ATIC-ALK fusion tr anscripts may be detected in conditions other than ALK-positive ALCL includ ing reactive lymphoid tissues, although at low levels, suggesting the prese nce of the transcripts in normal (bystander) cells. Moreover, they suggest that the ALK gene rearrangement by itself might be insufficient to induce t umor formation. They further question the validity of quantitative real-tim e RT-PCR for monitoring minimal residual disease in ALCL. Finally, the newl y identified translocation t(2;17)(p23;q25) can be added to the list of ALK gene rearrangements occurring in ALK-positive ALCL.