LB46
BRCA1: Notch another target?
Caoimhe Nic An tSaoir, Niamh O'Brien, Paul Mullan
Queen's University Belfast, UK
Background
An estimated 5-10% of all breast cancers are hereditary and BRCA1, the
breast and ovarian cancer susceptibility gene, accounts for approximately 20%
of these cases. BRCA1 related breast cancers are poorly differentiated,
aggressive tumours which lack expression of ER-α. Evidence suggests that
BRCA1 operates as a mammary epithelial stem cell regulator. Microarray analysis
using a BRCA1 mutant HCC1937 EV and a BRCA1 wild-type cell line reveal Notch
receptors and ligands as putative BRCA1 targets. Notch signalling is a
classical stem cell regulator which has been shown to be deregulated in breast
cancer. γ-secretase inhibitors (GSI) which inhibit Notch signalling are
currently in use in clinical trials to treat breast cancer.
Results
Decreased BRCA1 expression, or re-expression of wild-type BRCA1 in a
deficient model alters transcription of Notch receptors (1, 2 and 3) and Notch
ligands (Jagged1 and Delta-1). Re-expression of wild-type BRCA1 in BRCA1 mutant
breast cancer cells recovered ER-α, Notch1 and Jagged-1 expression and
rendered cells responsive to exogenous Notch stimulation. BRCA1 depletion by
siRNA resulted in a direct decrease in Notch and Jagged-1 promoter activation.
BRCA1 and p63 co-localise to a 250bp region of the Jagged-1 internal enhancer
element. Loss of p63 impairs BRCA1 localisation to this region. Notch1 and
Jagged1 specific siRNA resulted in decreased luminal gene expression. Treatment
of cells with GSI reduced ER-α promoter activity and protein expression.
Treatment with DSL increased ER-α promoter activity.
Conclusion
BRCA1 regulates transcription of the stem cell regulator Jagged-1 in a p63
dependent manner. Functional Notch signalling is required for ER-α
expression implicating BRCA1 regulation of Notch in normal mammary gland
development. For the first time we demonstrate a direct relationship between
BRCA1, Notch signalling and expression of differentiation markers in mammary
epithelial cells. This evidence strengthens the hypothesis that BRCA1 operates
as a mammary stem cell regulator.
