NPM/ALK fusion mRNA expression in Hodgkin and Reed-Sternberg cells is rare but does occur: Results from single-cell cDNA analysis

Background: The translocation t(2;5)(p23;q35) leads to the fusion of the nucleophosmin gene (NPM) on chromosome 5q35 to the recently described receptor kinase ALK on 2p23. It is characteristic of a subgroup of CD30+ large-cell anaplastic non-Hodgkin's lymphoma (ALCL). Since some cases of Hodgkin's disease (HD) and ALCL share common features, a common pathogenesis has been proposed in a report of the expression of NPM/ALK fusion mRNA in 11/13 Hodgkin's lymphomas. Patients and methods: We approached this question by micromanipulatory isolation of single Hodgkin and Reed-Sternberg (H-RS) cells and subsequent RT-PCR amplification of NPM/ ALK fusion cDNA from these single cells. Results: Specificity of cell selection was shown by the HD-specific pattern of EBV-gene expression in single H-RS cells. In 4 out of 7 cases, NPM/ALK fusion cDNA was detected in the RNA from whole lymph node tissue. In 2 out of 9 cases, NPM/ ALK fusion sequences were amplified from single H-RS cells, albeit in a very low frequency (< 5%). Conclusions: These data indicate that NPM/ALK fusion transcripts do not play an early role in the pathogenesis of HD. Whether the rare expression of NPM/ALK is the result of clonal heterogeneity or an indication for clonal evolution and progression toward ALCL can only be answered by the repeated analysis of indicator cases during the course of the disease.