Presentation on BRCA1- the first breast cancer susceptibility gene
-BRCA1 in Breast cancer
-most common cancer in women (1 in 8)
-10% of those breast cancers are hereditary
-of hereditary breast cancer, BRCA1 is the most common mutation
-BRCA1-mutated cancers tend to be more aggressive
-they occur at a younger age
-are basal phenotype (ER/PR/HER2 negative)
-are usually diagnosed as a high grade infiltrating ductal carcinoma
-individuals with a mutation in BRCA1 have a lifetime risk of breast cancer of up
to 85%
-BRCA1 in Ovarian cancer
-ovarian cancer is more rare, occurring 1 in 67 women
-but it is much more fatal, with a mortality rate of about 55% after 5 years
-in those women diagnosed with invasive ovarian cancer, 12% of them have a germline
BRCA1 mutation
-a mutation in BRCA1 indicates a 40% lifetime risk of developing ovarian cancer
*diagram: comparison of normal risk vs. risk with BRCA1 mutation
-Discovery of the Gene
-BRCA1 was first cloned using positional cloning by genetic linkage analysis in 1994
-it was mapped to 17q21
-The Gene
-80 kb of DNA and encodes a 7.8 kb transcript composed of 24 exons
-the gene consists of 1863 amino acids and is expressed in most proliferating cells
-RING finger domain on the N-terminus
-these domains are often involved in ubiquitination pathways
-2 nuclear localization motifs on exon 11
-so we know that BRCA1 is localized to the nucleus
-BRCT domain on the C-terminus
-conserved sequence for DNA repair and cell-cycle regulation
-transcription is induced in late G1 and remains elevated during S phase
*discuss the image
-BRCA1 in the organism
-it is a caretaker of genomic stability
-BRCA1 was found to be a tumor suppressor
-this was proven by knockout mice. Homozygous knockout is lethal for the mouse
embryo at approximately day 8 or 9 with severe growth deficit
-BRCA1 as a tumor suppressor is also supported by the fact that:
-loss of heterozygosity is needed for tumorigenesis
-loss of BRCA1 promotes genomic instability
-overexpression of BRCA1 leads to growth suppression
-Molecular Roles of BRCA1
-it acts in many different cellular complexes, which have a variety of functions
-here we will focus on 4: DNA repair, gene transcription regulation, cell cycle
checkpoint control, and ubiquitination
-Double strand break DNA repair
-can be caused from ionizing radiation like X-rays
-there are 2 types of DNA repair
-non homologous end joining
-homologous recombination
-Non homologous recombination
-the broken ends of the chromosome are simply brought together and rejoined
-a protein complex assembles at the site of damage and ligates the broken strands
-this is a very error prone method, however
-translocations and chromosomal rearrangements can occur
-Homologous recombination
-in homologous recombination, BRCA1 associates with RAD51
-these two proteins co-localize with BRCA2 to the site of DNA damage
-recall the steps of homologous recombination: invasion of the damaged strand
into the homologous DNA template. DNA synthesis using the homologous template. This forms
a Holliday junction, which is then resolved to reform the 2 functional chromosomes
-when functional BRCA1 proteins are not available, the cell must rely on the
error-prone method of non homologous end joining DNA repair. It appears that the resulting
chromosome instability is a crucial feature of carcinogenesis.
-Transcription Regulation
-it has been proven that BRCA1 associates with RNA polymerase, which suggests it is a
component of the cell’s transcriptional machinery
-further studies demonstrate that BRCA1 can act as either a co-repressor of transcription
or a co-activator of transcription, depending on which transcription factor it is associated with
-for example, one of its targets for co-activation is p21
-BRCA1 works with the transcription factor p53 to promote transcription of p21,
which is a cyclin-dependent kinase inhibitor that is important at the G1-S checkpoint
-This all suggests that BRCA1 is able to manipulate gene transcription in response to
DNA damage
-Cell cycle checkpoint
-BRCA1 also plays a role in cell cycle checkpoint control by regulating the cell cycle
(again) in response to DNA damage
-through this, it influences the G1-S, Intra-S, and the G2-M checkpoints
-for example, BRCA1 is responsible for the transcription of Cyclin B, which is needed
for entry into mitosis
-Ubiquitination
-BRCA1 is an E3 ligase, which is involved in the ubiquitination pathway we have
discussed in class
-BRCA1 exists as a heterodimer with another protein called BARD1
-the ring domain of BRCA1 binds to various E2s
-the details of BRCA1 ubiquitination are not completely understood. It is clear, however,
that BRCA1 has different effects on different substrates
-Recent reports also show that ubiquitination might be involved in the recruitment of
BRCA1 to DNA damage sites
-Tumor specificity model
-This next part was my favorite part of researching BRCA1. When I first discussed my
presentation with Professor Duronio, we talked about introducing “unanswered questions” such
as why a mutation in BRCA1 only causes breast and ovarian cancer. I was excited to learn that
just recently, a study was recently conducted that explains the mechanism by which mutations in
BRCA1 specifically cause breast cancer
-So this research came out of the Comprehensive Cancer Center at the University of
Michigan.
-First, it has been shown that most BRCA1 breast cancers exhibit the “basal-like”
or triple negative phenotype
-this means the tumor cells are ER/PR/and Her2 negative
-This phenotype strikingly resembles breast stem cells
-Essentially through this research at Michigan, they were able to prove that BRCA1 is
needed for the differentiation of breast stem cells
-so the loss of BRCA1 results in an increased amount of stem cells (which
isn’t good)
-and if you couple that with the functions of BRCA1 in DNA repair, cell
cycle regulation, etc- then you have an increased amount of genetically unstable breast stem cells
-and this is the perfect situation for further oncogenic events