429

ASCL1

ASH1|HASH1|MASH1|bHLHa46;achaete-scute complex homolog 1 (Drosophila);ASCL1;achaete-scute complex homolog 1 (Drosophila)

ASH-1|achaete scute protein|achaete-scute complex-like 1|achaete-scute homolog 1|class A basic helix-loop-helix protein 46

12q23.2

protein-coding

Neuroblastoma is a childhood tumor originating from cells of the developing sympathetic nervous system. The disease exhibits a remarkable phenotypic diversity reflected in the outcome, ranging from spontaneous regression to fataldisease. Mammalian achaete-scute homologue 1 (MASH-1 or HASH-1 in humans), a basic helix-loop-helix transcription (bHLH) factor, is transiently expressed in migrating sympatho-adrenal precursor cells. This gene, which is essential for proper development of the sympathetic nervous system, is expressed in a majorityof primary neuroblastomas and neuroblastoma cell lines indicating an embryonal origin of the tumor. One important negative regulator of MASH-1 expression is the bHLH factor hairy and Enhancer of split homolog-1 (HES-1), which in turn is under positive control of the Notch signaling cascade. When neuroblastoma cells are induced to differentiate, as indicated by neuronal morphology and upregulation of neuronal marker genes, the HASH-1 expression is rapidly downregulated with a concomitant, transient upregulation of HES-1. Furthermore, a constitutively active form of Notch-1 inhibits induced differentiation of neuroblastoma cells. In this review, the role of the Notch-signaling cascade in neuroblastoma, with focus on the bHLH factors HASH-1 and HES-1, will be discussed.

Neuroblastoma is one of the most common solid tumors in childhood. With the aim of developing a targeting vector for neuroblastoma, we cloned and characterized an enhancer in the 5'-flanking regions of the MASH1 gene by a random-trap methodfrom a 36 kb cosmid DNA. The enhancer-containing clone was identified by the expression of GFP when transfected into neuroblastoma cell lines. The enhancer-luciferase activity is higher in neuroblastoma cell lines, IMR32, BE2 and SH-SY5Y, compared with those in non-neuroblastoma cell lines, U1242 glioma, N417 small cell lung cancer and EOMA hemangioma. The core enhancer was determined within a 0.2 kb fragment, yielding three- to fourfold higher activity than that of the MASH1 promoter alone in IMR32 and BE2. This area possesses GATA- and CREB-binding sites, as well as the E-box. EMSA on this area demonstrated that CREB/ATF could bind the DNA. Chromatin immunoprecipitation assay revealed that N-myc, CREB, and co-activators CBP and PCAF, but not HDAC1, are bound to the core enhancer at the same time as the co-activators and N-myc bind to the promoter. This supports the idea that the commonly overexpressed genes HASH1 and N-myc areregulated in concert, confirming their importance as prognostic markers or targets for therapy.