AMPLIFICATION OF A DEAD BOX GENE (DDXI) WITH THE MYCN GENE IN NEUROBLASTOMAS AS A RESULT OF COSEGREGATION OF SEQUENCES FLANKING THE MYCN LOCUS

A DEAD box gene (DDX1) characterized by a motif with a putative RNA he licase was found at elevated levels, with multiple copies, in a neurob lastoma and in some retinoblastoma cell lines in which the MYCN gene w as amplified. The present study was aimed at determining whether ampli fication of the DDX1 gene is critical for human neuroblastomas exhibit ing MYCN gene amplification. Extended DNA panels of tumors and cell li nes revealed amplification of the DDX1 gene in approximately half of t he specimens exhibiting MYCN gene amplification, which is in good agre ement with a finding reported recently. Because its profile was simila r to that of the cDNA marker G21 and another flanking DNA marker, clon e 8, both of which localize outside the core of the amplicon of the MY CN gene, we noted that we could localize the DDX1 gene in relation to the MYCN gene. Utilizing pulsed-field gel electrophoresis according to a method based on the combinatorial alignment of multiple single dige sts and a 5.5-megabase map surrounding the MYCN locus, we mapped the D DX1 gene within a 100 kb region about 400 kb upstream from the MYCN ge ne, where G21 is localized. Further hybridization experiments with bot h genes, complete sequencing of G21, and its comparison with that of t he DDX1 gene eventually confirmed that the DDX1 gene is identical to G 21. G21 is a cDNA clone isolated by differential screening of a librar y from a neuroblastoma cell line, IMR-32, but its function has not yet been identified. Coamplification of the DDX1 gene with the MYCN gene is a consequence of the segregation of continuous DNA stretches spanni ng both loci during the amplification process. (C) 1996 Wiley-Liss, In c.