RELATIONAL MAPPING OF MYCN AND DDX1 IN BAND 2P24 AND ANALYSIS OF AMPLICON ARRAYS IN DOUBLE MINUTE CHROMOSOMES AND HOMOGENEOUSLY STAINING REGIONS BY USE OF FREE CHROMATIN FISH

MYCN amplification has been observed in diverse neuronal tumors includ ing neuroblastoma, retinoblastoma, and small cell carcinoma of the lun g, and has been correlated with a poor prognosis in advanced-stage neu roblastomas. Recent studies have shown a co-amplification of DDX1, a D EAD box gene, and MYCN in retinoblastoma and neuroblastoma. DDX1 has b een mapped to within a megabase of the MYCN gene in band 2p24. In the present study, the relational map of DDX1 and MYCN by fluorescence in situ hybridization (FISH) mapping to metaphase cells and extended free chromatin fibers indicated that DDX1 is telomeric to MYCN. Dual-color FISH analysis of amplicons within arrays of extended chromatin fibers was performed to examine the physical relationship of MYCN and DDX1 w ithin double minute chromosomes (dmins) and homogeneously staining reg ions (hsrs). No regular reiterated amplicon repeat unit was present in the hsrs, but detailed analysis of the configurations of DDX1 and MYC N within each array indicated that multiple rearrangements generated a complex hsr amplicon structure. Similarly, analysis of a cell line be aring dmins showed that a composite amplicon structure involving delet ions and/or duplications of MYCN and DDX1 is a feature of dmin formati on. These data are consistent with a molecular mechanism involving man y rearrangements during the evolution of gene amplification; resulting in complex amplicon structures with distinct changes in relative gene copy number and considerable variation in intragenic distances betwee n coamplified genes. (C) 1997 Wiley-Liss, Inc.