The Neuroblastoma Amplified Gene, NAG: Genomic Structure and Characterisation of the 7.3 Kb Transcript Predominantly Expressed in Neuroblastoma

Amplification of the MYCN oncogene in neuroblastoma is associated with poor prognosis. The amplified unit of DNA can be up to 1 Mb in size and so could contain additional genes which affect tumour phenotype. The neuroblastoma amplified gene (NAG) gene was initially located 400 kb telomeric to MYCN at 2p24 and reported to be co-amplified in 5/8 (63%) cell lines and 9/13 (70%) tumours. The sequence of a 4.5 kb transcript was proposed from the analysis of overlapping cDNA clones. However, our Northern blot hybridisation experiments indicate that the main RNA species expressed in neuroblastoma is 7-8 kb in size. We describe for the first time the cloning and sequencing of the 7.3 kb transcript of the NAG gene together with its precise genomic location and full exon structure. The 5' end of the gene is located 30 kb telomeric to DDX1, with the two genes lying in opposite orientations. The 52 exons of the 7.3 kb transcript cover 420 kb of genomic DNA. In vitro translation studies confirmed the protein coding potential of the transcript. Co-amplification of the entire NAG gene with MYCN was found in 1/6 (17%) neuroblastoma cell lines and 10/50 (20%) primary tumours. Previous studies had measured co-amplification of only the 5' end of the gene, nearest to MYCN. In this study, co-amplification of the NAG gene was found to be significantly associated with low disease stage in MYCN-amplified tumours (P=0.0063).