COAMPLIFICATION AND CONCOMITANT HIGH-LEVELS OF EXPRESSION OF A DEAD BOX GENE WITH MYCN IN HUMAN NEUROBLASTOMA

MYCN gene amplification is strongly correlated with poor prognosis in neuroblastoma (NB), the second most common solid pediatric tumor. Howe ver, increased MYCN expression seen in tumors that lack MYCN amplifica tion does not correlate with aggressive clinical behavior. Whereas the MYCN gene spans only 7 kb, the MYCN amplicon has been shown to range in size from 350 kb to more than 1 Mb. Given the large size of the amp licon, it is possible that additional genes are co-amplified in NBs wh ose expression may contribute to the aggressive phenotype associated w ith MYCN-amplified tumors. We isolated a cDNA clone from a human NE li brary that is identical to DDXI, a gene recently reported to be prefer entially expressed in two retinoblastoma cell lines that also express high levels of MYCN. DDXI belongs to a family of genes that encode DEA D (Asp-Glu-Ala-Asp) box proteins, putative ATP-dependent RNA helicases implicated in a number of cellular processes involving alterations of RNA secondary structure. We examined the frequency of DDXI amplificat ion in 15 NB cell lines, 1 neuroepithelioma cell line, and 122 NE tumo rs by Southern blot analyses, and we found that 7 of 10 MYCN-amplified cell lines and 27 of 40 (68%) MYCN-amplified tumors also harbored mul tiple copies of the DDXI gene. Amplification of DDXI was associated wi th high levels of DDXI mRNA expression in the NE cell lines and tumors as examined by Northern analysis. Neither DDXI gene amplification nor enhanced expression was observed in tumors or cell lines that lacked MYCN amplification. Because RNA helicases play important roles in both post-transcriptional and translational gene regulation, high levels o f DDXI expression consequent to genomic amplification may contribute t o the malignant phenotype of a subset of NBs. (C) 1995 Wiley-Liss, Inc .