Concomitant DDX1 and MYCN gain in neuroblastoma

DDX1, a gene mapping to the 2p24 region, has been observed to be co-amplified with MYCN in neuroblastoma. Co-amplification of the DDX1 gene is a consequence of the short physical distance between the two genes. Recently, it has been found that neuroblastoma cells can show a low increase in MYCN gene copy number, defined as MYCN gain. We studied 13 neuroblastomas with MYCN gain to evaluate the status of the DDX1 gene. We investigated DDX1/MYCN gain by double-colour FISH on interphase nuclei. All cases showed concomitant low extra copy number of DDX1 and MYCN. Heterogeneous distribution of nuclei displaying DDX1/MYCN gain was observed in almost all tumours, suggesting a clonal evolution of cells with DDX1/MYCN gain. This is the first report that shows DDX1 co-gained with MYCN in neuroblastoma and indicates that DDX1 over-representation is closely associated with an increase in MYCN copy number in neuroblastoma cells. Since DDX1 has already been found co-amplified with MYCN, DDX1 gain seems to be a further rearrangement due to the physical proximity of the two genes. Moreover, all patients with DDX1/MYCN gain show a good overall survival but a high frequency of adverse events.