Mutually exclusive expression of a helix‐loop‐helix gene and N‐myc in human neuroblastomas and in normal development.

We have isolated a novel human gene encoding a helix‐loop‐helix (HLH) protein by molecularly cloning chromosome 1p36‐specific CpG islands. The gene termed heir‐1 was localized to the neuroblastoma consensus deletion at 1p36.2‐p36.12. Its predicted protein is 95.8% identical to the mouse HLH462 protein and has clear homology to the mouse Id and Drosophila emc proteins. Heir‐1 does not encode a basic DNA binding domain as found in basic HLH proteins. The gene is expressed specifically at high abundance in adult lung, kidney and adrenal medulla, but not in adult brain. Despite prominent heir‐1 expression in adrenal medulla, which is a prime target for neuroblastomas, 10 out of 12 neuroblastoma‐derived cell lines revealed very low levels of heir‐1 mRNA. Low heir‐1 expression was generally found in tumor cell lines with N‐myc overexpression, whereas the two cell lines displaying high heir‐1 levels did not overexpress N‐myc. Mutually exclusive expression of both genes was also found by in situ hybridization in developing mouse tissues, particularly in the forebrain neuroectoderm. We conclude that heir‐1 expression is reduced specifically in the majority of neuroblastomas and suggest an inverse correlation between heir‐1 and N‐myc expression in neuroblastoma tumors and in embryonic development.