Functional Analysis of BORIS, a Novel DNA Binding Protein

Abstract

Abstract : Very recently, a new gene has been mapped to the 20q13.2 region that is commonly amplified in cancers of the breast and other tissues. This gene, BORIS (Brother of the Regulator of Imprinted Sites), is a paralogue of the gene encoding CTCF, a multifunctional DNA binding protein that utilizes different sets of zinc fingers to mediate distinct functions in regulation of gene expression. These functions include context-dependent promoter repression or activation, creation of modular hormone-responsive gene silencers, and formation of enhancer blocking elements (insulators). Several lines of evidence suggest that ubiquitously expressed CTCF is a critical determinant of cell growth regulation. Unlike CTCF, which is expressed ubiquitously, the expression of BORIS is normally restricted to specific cells in testes (the only cells where CTCF is not expressed), where it may play a role in reprogramming the methylation pattern of male germ line DNA. BORIS encodes an 11-zinc finger domain functionally equivalent to that in CTCF, while being completely divergent at the amino-and carboxy-termini-domains that have recently been shown to be critical for CTCF's insulator function. Based on the finding that BORIS maps to the 20q13.2 region frequently altered in many malignancies, various cancer cell lines were screened for BORIS expression. BORIS transcripts were found in substantial proportions of a wide variety of tumor cell types (Loukinov et al., manuscript in prep.). Analyses of 17 breast tumor cell lines and of 148 randomly selected primary breast cancer samples (obtained from A. Lindblom, Karolinska Hospital, Sweden) without CTCF mutations (A. Lindblom, unpublished) demonstrated frequencies of BORIS expression of 80 and 88%, respectively. In contrast, using an extremely sensitive 2 step multiplex RT-PCR method, the same team was unable to detect BORIS mRNA in any normal mammary tissues, or other normal somatic tissues (data not shown).