Tissue-specific expression of human achaete-scute homologue-1 in neuroendocrine tumors: transcriptional regulation by dual inhibitory regions.

Malignancies with neuroendocrine (NE) features such as medullary thyroid cancer (MTC) and small cell lung cancer (SCLC) are prototypic neoplasms arising from peripheral endocrine cells. The mechanisms that regulate the NE phenotype in these tumors and their cellular precursors are not well understood. However, a basic helix-loop-helix transcription factor that is homologous to Drosophila neural fate determination proteins may have a central role. Human achaete-scute homologue-1 (hASH1), a human homologue of the Drosophila achaete-scute complex, is highly expressed in MTC, SCLC, and pheochromocytomas. To determine what mechanisms allow constitutive expression of hASH1 in NE tumors, we cloned human genomic DNA fragments containing the hASH1 gene and characterized its promoter region. We show that hASH1 expression is restricted to NE cell lines by a transcriptionally regulated mechanism. Dual promoters initiate hASH1 transcription, with the predominant site being an evolutionarily conserved initiator (INR) element. Transient transfection studies provide evidence for a generalized enhancer region that has high activity in all cell lines tested. Restriction of hASH1 expression to NE tumor cells depends on two tissue-specific repressor regions, present in the proximal and distal (> 13.5 kb) 5'-flanking region. Understanding the mechanisms of tissue-specific control of hASH1 gene expression provides a useful model to explore regulatory cascades influencing both normal nervous system development and the NE phenotype of tumors such as MTC and SCLC.