Chromodomain-helicase-DNA-binding domain proteins have been identified in a variety of organisms. This family of proteins, which consists of nine members, has been divided into three separate subfamilies: subfamily I, subfamily II, and subfamily III. All of the CHD proteins contain two tandem N-terminal chromodomains, a SWI/SNF-related ATPase domain, and a C-terminal DNA binding domain. The chromodomains facilitate binding to methylated lysine residues of histone proteins and confer interactions with specific regions of chromatin. The SWI/SNF-related ATPase domain utilizes the energy from ATP hydrolysis to modify chromatin structure. CHD1 is a euchromatic protein that associates with the promoters of active genes, and is required for the maintenance of open chromatin and pluripotency in embryonic stem cells. The two chromodomains of CHD1 facilitate its recruitment to active genes by binding to methyl-lysine 4 of histone H3, a mark associated with transcriptional activation. Yeast CHD1 is a component of the SAGA and SLIK histone acetyltransferase complexes, and is believed to link histone methylation with histone acetylation during transcriptional activation. The CHD2 protein is not well characterized; however, mouse knockout studies suggest important functions in development and tumor suppression. Homozygous CHD2 knockout mice exhibit delayed growth and perinatal lethality. Heterozygous knockout mice show increased mortality and gross organ abnormalities, in addition to increased extramedullary hematopoiesis and susceptibility to lymphomas. CHD2 mutant cells are defective in hematopoietic stem cell differentiation and exhibit aberrant DNA damage responses.
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