Analysis of candidate gene co-amplification with MYCN in neuroblastoma

Abstract Previous studies have revealed that the MYC N gene spans approximately 7kb, while the amplicon has been estimated to be 100kb to 1Mb long [1–3]. This implies that several other genes may be present on the MYC N amplicon. Such co-amplified genes could contribute to the malignant phenotype and might provide an explanation for why not all patients with MYC N amplification have a poor outcome. We investigated 7 neuroblastoma cell lines and 167 primary tumours for the co-amplification of candidate genes known to be present near the MYC N locus: ornithine decarboxylase, ribonucleotide reductase, syndecan-1 and a DEAD box protein gene, DDX1 . We also investigated further the pG21 expressed sequence previously reported to be co-amplified with MYC N. No co-amplification with the first 3 genes was found in any of the cell lines or tumour samples. DDX1 was found to be amplified along with MYC N in 4/6 (67%) cell lines and 18/38 (47%) of the MYC N amplified tumours. No amplification of DDX1 , ODC1 , RRM2 or syndecan-1 was found in the absence of MYC N amplification. DDX1 co-amplification was observed to occur mainly in stage 4 or 4S patients. With the exclusion of those with 4S disease, patients with DDX1 co-amplification had a significantly shorter mean (±SE) disease-free interval (4.1±1.4, n =8 months) compared with patients with MYC N amplification alone (19.6±4.5, n =17) ( P =0.04, Welch's unpaired t -test). The pG21 sequence was identical to part of the DDX1 gene. These observations indicate that there is at least 1 other gene co-amplified with MYC N in a proportion of cases and that those patients with DDX1 co-amplification tend to relapse more quickly. It also implies that the MYC N amplicon is of varied size and/or position relative to the MYC N gene.