Localization of regulatory elements controlling human MYCN expression

Expression of the cellular oncogene MYCN is restricted to few cell lineages and is highest during development both in mouse and in humans. In pursuit of elucidating the mechanisms underlying MYCN regulation we introduced a human MYCN clone (pNb-9) with approximately 2.5 kbp 5'- and 6 kbp 3'-flanking genomic sequences into different murine and human cell lines as well as into mice. In all cases we found a correlation between the expression of the exogenous and the endogenous MYCN. Among cell lines, only those expressing the endogenous gene also expressed the transfected gene. In the transgenic mice transcripts of the transgene were present in proportion to the transcripts of the endogenous MYCN gene with the highest level in the brain. Therefore, the genetic information necessary for regulated expression of MYCN appears to be contained in pNb-9. To localize the DNA-regions responsible for regulated expression, we generated MYCN/CAT hybrid genes with different portions of the putative MYCN promoter region linked to the reporter gene. Transient transfections into various murine and human cell lines identified three DNA regions apparently involved in the regulation of expression. One region about 200 bp upstream of the transcriptional start site is responsible for a basal level of MYCN expression. A second region located about 800 bp further upstream appears to be involved in cell type-specific gene activation. The third regulatory element is located at the 3' end of the first exon and/or in the first intron and may mediate tissue-specific down regulation of gene expression.