An in vivo screen of noncoding loci reveals that is a gatekeeper of an Ikaros-dependent checkpoint during haematopoiesis.

Haematopoiesis relies on tightly controlled gene expression patterns as development proceeds through a series of progenitors. While the regulation of hematopoietic development has been well studied, the role of noncoding elements in this critical process is a developing field. In particular, the discovery of new regulators of lymphopoiesis could have important implications for our understanding of the adaptive immune system and disease. Here we elucidate how a noncoding element is capable of regulating a broadly expressed transcription factor, Ikaros, in a lymphoid lineage-specific manner, such that it imbues Ikaros with the ability to specify the lymphoid lineage over alternate fates. Deletion of the locus, which is proximal to Ikaros, led to a severe reduction in early lymphoid progenitors, exerting control over the earliest fate decisions during lymphoid lineage commitment. locus deletion led to alterations in Ikaros isoform expression and a significant reduction in Ikaros protein. The locus may function through direct DNA interaction as Hi-C analysis demonstrated an interaction between the two loci. Finally, we identify an Ikaros-regulated erythroid-lymphoid checkpoint that is governed by in a lymphoid-lineage-specific manner. appears to act as a gatekeeper of Ikaros's broad lineage-specifying functions, selectively stabilizing Ikaros activity in the lymphoid lineage and permitting diversion to the erythroid fate in its absence. These findings represent a key illustration of how a transcription factor with broad lineage expression must work in concert with noncoding elements to orchestrate hematopoietic lineage commitment.