Identification of a novel transcriptional repressor element located in thefirst intron of the human BRCA1 gene

Loss or lowered expression of BRCA1 in non-familial breast cancer has been shown in several recent studies. Understanding how BRCA1 expression is regu lated should provide new insights into the role of BRCA1 in sporadic breast cancer. We have recently identified a critical 18-base pair (bp) DNA eleme nt within the minimal BRCA1 promoter whereupon the formation of a specific protein-DNA complex and transcription of BRCA1 is dependent. We now report a non tissue-specific transcriptional repressor activity, located more than 500 bp into the first intron of BRCA1. Progressive deletions from the 3'-e nd of intron I and reporter gene assays localized the repressor activity to an 83-bp region. Electrophoretic mobility shift assays with this 83 bp DNA and various sub-fragments of it showed binding of nuclear proteins to a 36 bp BstNI-BseRI fragment. Functional transcriptional repression by this 36 bp DNA could be conferred on a heterologous thymidine kinase promoter. Anal ysis of multiple reporter gene constructs containing the BRCA1 genomic regi on driving transcription in both directions suggests that the putative nega tive regulatory element functions to block transcription only in the BRCA1 direction, although the promoter is shared by the divergently transcribed N BR2 gene.