Transcriptional Regulation of the SCL Locus: Identification of an Enhancer That Targets Primitive Erythroid Cells.

The stem cell leukaemia (SCL) gene encodes a bHLH protein that is essential for the formation of all haematopoietic lineages. In addition, maintenance of SCL expression is required for normal differentiation along the erythroid and megakaryocytic lineages, whereas failure to downregulate SCL transcription during T-cell differentiation is associated with T-cell ALL. Current evidence therefore demonstrates that appropriate transcriptional regulation is essential for the biological functions of SCL and this focuses attention on the mechanisms whereby transcription of SCL itself is initiated and maintained.

We have previously used biochemical, comparative genomic and transgenic assays to identify 5 distinct enhancers which target different subdomains of the normal SCL expression pattern. However these enhancers do not explain how erythroid expression of SCL is achieved and we have postulated the existence of an additional erythroid enhancer. It is also unclear whether the known SCL enhancers regulate neighbouring genes within the SCL locus. We have therefore quantitated the transcripts from SCL and its neighbouring genes in a large panel of human and murine haematopoietic cell types. Our results reveal a striking and unexpected co-expression of SCL and its downstream neighbour MAP17 (r=0.8; n=31). We demonstrate the existence of appropriately spliced low abundance SCL-MAP17 fusion transcripts suggesting that co-expression reflects transcriptional read-through rather than a shared enhancer. A systematic survey of histone H3 and H4 acetylation throughout the SCL locus was also performed in both cell lines and primary haematopoietic cells. A peak of acetylation downstream of MAP17 (and 40 kb downstream of SCL exon 1a) was found to correlate with expression of SCL but not other neighbouring genes. This region contains peaks of homology in 4-way genomic sequence comparisons (human/dog/mouse/rat) and functions as an erythroid-restricted enhancer in vitro. Morever, in transgenic mice this enhancer directs b-galactosidase expression to the vast majority of circulating primitive erythroblasts but not to fetal liver definitive erythroblasts.