Relationship of p215BRCA1 to tyrosine kinase signaling pathways and the cell cycle in normal and transformed cells

BRCA1 Gene Expression in Breast Cancer: A Correlative Study between Real-time RT-PCR and Immunohistochemistry

Breast cancer is a major cause of cancer-related mortality in women. There are major discrepancies concerning the usefulness of various antibodies in detecting breast cancer susceptibility gene 1 (BRCA1) protein and its subcellular localization. The aim of the present study was to determine the specificity and sensitivity of immunohistochemistry (IHC) as a screening method for demonstrating BRCA1 expression. BRCA1 gene expression in archival paraffin-embedded breast cancer tissues was studied simultaneously at the protein and mRNA levels, and the two findings were compared. Forty-eight archival paraffin-embedded breast cancer tissues were studied for BRCA1 gene expression at protein level by IHC using four different antibodies against different BRCA1 epitopes and at mRNA level using real-time RT-PCR. BRCA1 mRNA expression was reduced or absent in 79% of the samples, and this finding correlated significantly with loss of BRCA1 protein expression in 83% of breast cancer tissues using one BRCA1 antibody studied (AB-1, against N-terminus epitope). The specificity of this antibody was 91.3%, and its sensitivity was 66.6%. There was no significant correlation between BRCA1 mRNA and protein expression as demonstrated by the remaining three antibodies. Antibody 8F7 had the highest sensitivity of 100%, but its specificity was 30.4% if mRNA levels were considered as the reference standard.

Reversion of the ErbB malignant phenotype and the DNA damage response

The ErbB or HER family is a group of membrane bound tyrosine kinase receptors that initiate signal transduction cascades, which are critical to a wide range of biological processes. When over-expressed or mutated, members of this kinase family form homomeric or heteromeric kinase assemblies that are involved in certain human malignancies. Targeted therapy evolved from studies showing that monoclonal antibodies to the ectodomain of ErbB2/neu would reverse the malignant phenotype. Unfortunately, tumors develop resistance to targeted therapies even when coupled with genotoxic insults such as radiation. Radiation treatment predominantly induces double strand DNA breaks, which, if not repaired, are potentially lethal to the cell. Some tumors are resistant to radiation treatment because they effectively repair double strand breaks. We and others have shown that even in the presence of ionizing radiation, active ErbB kinase signaling apparently enhances the repair process, such that transformed cells resist genotoxic signal induced cell death. We review here the current understanding of ErbB signaling and DNA double strand break repair. Some studies have identified a mechanism by which DNA damage is coordinated to assemblies of proteins that associate with SUN domain containing proteins. These assemblies represent a new target for therapy of resistant tumor cells.

MYC in breast tumor progression

Breast cancer is the second leading cause of cancer deaths and is the most frequently diagnosed cancer in women of industrialized nations. Breast cancer progression is a multistep process involving genetic and epigenetic alterations that drive normal breast cells into highly malignant derivatives with metastatic potential. MYC is a proto-oncogene whose protein product contains a basic helix-loop-helix domain. MYC functions as a transcription factor regulating up to 15% of all human genes. MYC is regulated at multiple levels, and the protein is a downstream effector of several signaling pathways. In breast cancer cells, MYC target genes are involved in cell growth, transformation, angiogenesis and cell-cycle control. BRCA1 is linked to transcriptional regulation through interaction with MYC. Although the relationship between amplification and overexpression is not clearly delineated, MYC amplification is significantly correlated with aggressive tumor phenotypes and poor clinical outcomes. MYC amplification is emerging as an important predictor of response to HER2-targeted therapies and its role in BRCA1-associated breast cancer makes it an important target in basal-like/triple-negative breast cancers.

Hyperphosphorylation of the BARD1 Tumor Suppressor in Mitotic Cells

Although the BRCA1 tumor suppressor has been implicated in a number of cellular processes, it plays an especially important role in the DNA damage response as a regulator of cell cycle checkpoints and DNA repair pathways. In vivo, BRCA1 exists as a heterodimer with the BARD1 protein, and many of its biological functions are mediated by the BRCA1-BARD1 complex. Here, we show that BARD1 is phosphorylated in a cell cycle-dependent manner and that the hyperphosphorylated forms of BARD1 predominate during M phase. By mobility shift analysis and mass spectrometry, we have identified seven sites of mitotic phosphorylation within BARD1. All sites exist within either an SP or TP sequence, and two sites resemble the consensus motif recognized by cyclin-dependent kinases. To examine the functional consequences of BARD1 phosphorylation, we used a gene targeting knock-in approach to generate isogenic cell lines that express either wild-type or mutant forms of the BARD1 polypeptide. Analysis of these lines in clonogenic survival assays revealed that cells bearing phosphorylation site mutations are hypersensitive to mitomycin C, a genotoxic agent that induces interstrand DNA cross-links. These results implicate BARD1 phosphorylation in the cellular response to DNA damage.

BRCA1 expression in canine mammary dysplasias and tumours: relationship with prognostic variables

BRCA1 is a nuclear phosphoprotein that participates in the regulation of the cell cycle. The role of the BRCA1 gene in canine mammary tissue and mammary tumours has not been studied. The present study examined immunohistochemically the expression and intracellular distribution of BRCA1 protein in two normal, seven dysplastic and 44 neoplastic canine mammary glands and its relationship with clinical and pathological variables and other prognostic parameters. Strong nuclear immunolabelling of BRCA1 protein was observed in the epithelial cells of the normal mammary glands and mammary dysplasias. The majority of benign tumours, and more especially of malignant tumours, showed a significant reduction in the nuclear expression of BRCA1 protein and an increase in cytoplasmic expression. Loss of BRCA1 expression was associated with high proliferation marker Ki-67 and ER-alpha negative tumours. The reduction and aberrant distribution of BRCA1 in canine mammary tumours were significantly associated with malignant characteristics. The results may indicate that BRCA1 has a role in the malignant behaviour of these tumours.

Highlight: BRCA1 and BRCA2 proteins in breast cancer

Heterozygous carriers of loss-of-function germline mutations in the BRCA1 or BRCA2 breast cancer susceptibility genes have a predisposition to breast and ovarian cancer. Multiple functions have been ascribed to the products of these genes, linking them to pathways that inhibit progression to neoplasia. Various investigators have assigned roles for these tumor suppressor gene products in the cell functions of genome repair, transcription, and growth control. There is emerging evidence that BRCA1 may participate in ubiquitin E3 ligase activity. BRCA1 and BRCA2 have each been implicated in chromatin remodeling dynamics via protein partnering. Ubiquitin ligase and chromatin remodeling activities need not be mutually exclusive and both may function in DNA repair, transcriptional regulation, or cell cycle control. Here we highlight certain recent findings and currently unanswered questions regarding BRCA1 and BRCA2 in breast cancer.

Constitutive association of BRCA1 and c-Abl and its ATM-dependent disruption after irradiation

BRCA1 plays an important role in mechanisms of response to double-strand breaks, participating in genome surveillance, DNA repair, and cell cycle checkpoint arrests. Here, we identify a constitutive BRCA1-c-Abl complex and provide evidence for a direct interaction between the PXXP motif in the C terminus of BRCA1 and the SH3 domain of c-Abl. Following exposure to ionizing radiation (IR), the BRCA1-c-Abl complex is disrupted in an ATM-dependent manner, which correlates temporally with ATM-dependent phosphorylation of BRCA1 and ATM-dependent enhancement of the tyrosine kinase activity of c-Abl. The BRCA1-c-Abl interaction is affected by radiation-induced modification to both BRCA1 and c-Abl. We show that the C terminus of BRCA1 is phosphorylated by c-Abl in vitro. In vivo, BRCA1 is phosphorylated at tyrosine residues in an ATM-dependent, radiation-dependent manner. Tyrosine phosphorylation of BRCA1, however, is not required for the disruption of the BRCA1-c-Abl complex. BRCA1-mutated cells exhibit constitutively high c-Abl kinase activity that is not further increased on exposure to IR. We suggest a model in which BRCA1 acts in concert with ATM to regulate c-Abl tyrosine kinase activity.

Adenosine nucleotide modulates the physical interaction between hMSH2 and BRCA1

We have identified the physical interaction between the Breast Cancer susceptibility gene product BRCA1 and the Hereditary Non-Polyposis Colorectal Cancer (HNPCC) and DNA mismatch repair (MMR) gene product hMSH2, both in vitro and in vivo. The BRCA1-hMSH2 association involved several well-defined regions of both proteins which include the adenosine nucleotide binding domain of hMSH2. Moreover, the interaction of BRCA1 with purified hMSH2-hMSH6 appears to be modulated by adenosine nucleotide much like G protein downstream interaction/signaling is modulated by guanosine nucleotide. BARD1, another BRCA1-interacting protein, was also found to interact with hMSH2. In addition, BRCA1 was found to associate with both hMSH3 and hMSH6, the heterodimeric partners of hMSH2. These observations implicate BRCA1/BARD1 as downstream effectors of the adenosine nucleotide-activated hMSH2-hMSH6 signaling complex, and suggest a global role for BRCA1 in DNA damage processing. The functional interaction between BRCA1 and hMSH2 may provide a partial explanation for the background of gynecological and colorectal cancer in both HNPCC and BRCA1 kindreds, respectively.

BRCA1 and cell signaling

The breast cancer and ovarian cancer susceptibility gene BRCA1 encodes a nucleoprotein whose mutations or aberrant expression is associated with both inherited and sporadic cancers. Studies over the last 6 years have suggested that BRCA1 may function as a scaffold in the assembly of a multi-protein complex, which plays a role in gene transcription, DNA damage repair, and transcription-coupled DNA damage repair. In this review, we discuss the implications drawn from the studies of BRCA1-interacting proteins and the cellular signaling pathways that may be involved in controlling the functions of BRCA1.

VCP, a weak ATPase involved in multiple cellular events, interacts physically with BRCA1 in the nucleus of living cells

BRCA1, a breast/ovarian cancer susceptibility gene, undergoes mutations in as many as 50% of familial breast tumors. Recent studies indicate that BRCA1 may be involved in DNA damage repair. Here, we demonstrate that the BRCA1 protein physically associates with valosin-containing protein (VCP), a member of the ATPases associated with a variety of cellular activities (AAA) superfamily. In vitro studies revealed that VCP, via its N- terminal region, binds to amino acid residues 303-625 in the BRCA1 protein. Although found predominantly in the cytoplasm and, less abundantly, in the nucleus, VCP can be translocated from the nucleus after stimulation with epidermal growth factor. Collectively, our results suggest that VCP, by binding to BRCA1, participates in the DNA damage-repair function as an ATP transporter, possibly facilitating the transcription-coupled repair.

BRCA1 is phosphorylated at serine 1497 in vivo at a cyclin-dependent kinase 2 phosphorylation site

BRCA1 is a cell cycle-regulated nuclear protein that is phosphorylated mainly on serine and to a lesser extent on threonine residues. Changes in phosphorylation occur in response to cell cycle progression and DNA damage. Specifically, BRCA1 undergoes hyperphosphorylation during late G1 and S phases of the cell cycle. Here we report that BRCA1 is phosphorylated in vivo at serine 1497 (S1497), which is part of a cyclin-dependent kinase (CDK) consensus site. S1497 can be phosphorylated in vitro by CDK2-cyclin A or E. BRCA1 coimmunoprecipitates with an endogenous serine-threonine protein kinase activity that phosphorylates S1497 in vitro. This cellular kinase activity is sensitive to transfection of a dominant negative form of CDK2 as well as the application of the CDK inhibitors p21 and butyrolactone I but not p16. Furthermore, BRCA1 coimmunoprecipitates with CDK2 and cyclin A. These results suggest that the endogenous kinase activity is composed of CDK2-cyclin complexes, at least in part, concordant with the G1/S-specific increase in BRCA1 phosphorylation.

Immunolocalization of BRCA1 protein in normal breast tissue and sporadic invasive ductal carcinomas: a correlation with other biological parameters

AIM

BRCA1, a nuclear phosphoprotein, normally functions as a negative regulator of the cell cycle and may be an active inhibitor of neoplastic progression. Mutation of the BRCA1 gene has been demonstrated in 80% of familial breast cancer. Decreased mRNA levels or aberrant subcellular locations of BRCA1 have been identified in breast cancer lines and in sporadic cases of breast cancer tissues. The expression of BRCA1 in large series of variously differentiated breast carcinomas with correlation with other biological parameters has not been clarified.

METHODS AND RESULTS

The BRCA1 expression in normal breast tissue (n = 15) and in sporadic cases of invasive ductal carcinoma (n=108) was determined using immunohistochemistry. BRCA1 expression was correlated with other prognostic parameters including p53, c-erbB-2, bcl-2, oestrogen receptor (ER), histological grade, tumour size, axillary lymph node status and age. BRCA1 was exclusively (100%) localized in the nuclei of normal ductal and lobular epithelia. However, this nuclear expression pattern was variable in breast carcinoma (76.8%). Loss of nuclear BRCA1 expression (22 of 108 cases, 20.4%) correlated well with high histological grade (P<0.025) and bcl-2-negative tumours (P<0.05) and frequently in ER-negative tumours.

CONCLUSION

BRCA1 nuclear expression could be considered to represent the normal or physiological phenotype. Complete loss of BRCA1 nuclear expression in breast cancer and its correlation with other poor prognostic markers suggest that BRCA1 expression may play an important role in the pathogenesis and prognosis of sporadic breast carcinoma. Altered BRCA1 phenotype may therefore provide an additional prognostic parameter for breast cancer.

BRCA1 and BRCA2 proteins: roles in health and disease

Between 5% and 10% of all breast cancer is hereditary, with patients having a strong family history of the disease. The remaining 90-95% of cases are classed as sporadic. Within the inherited group, 80-90% of cases are the result of germline mutations affecting two recently identified genes: BRCA1 and BRCA2. Since the sequencing of these genes, considerable research on the genetics of the mutation carriers has been performed, with less attention having been focused on the BRCA1 and BRCA2 proteins themselves. The structure and function of the protein products thus continues to hold mystery and might be the key to the full understanding of this complex disease.

Localization of BRCA1 protein at the cellular level

Based on its amino acid sequence and the existence of three nuclear localization signal (NLS)3 regions, BRCA1 is likely to be a cell cycle-dependent nuclear protein, regulated by cyclin-dependent kinases (cdk) and associated with nuclear proteins such as Rad51 and BARD1, involved in transcription regulation and participating in DNA replication checkpoints. However, many authors have also described a cytoplasmic expression pattern. Moreover, BRCA1 was present not only in a dot like pattern in the nucleus but also associated with a channel-like system of cytoplasm and endoplasmic reticulum invaginating into the nucleus. BRCA1 expression patterns can also be influenced by alternative splice variants and by cell cycle-dependent expression level and localization. Further ultrastructural and confocal studies using C-terminal antibodies, that do not react with C-terminal truncated form of BRCA1 should shed new light upon the exact localization of BRCA1.

Expression of BRCA1 and BRCA2 in normal and neoplastic cells

Current evidence strongly supports a role for the breast cancer susceptibility genes, BRCA1 and BRCA2, in both normal development and carcinogenesis. Valuable clues regarding the function of these genes have been garnered through studies of their patterns of expression. A central feature of the in vivo pattern of BRCA1 and BRCA2 expression is that each of these putative tumor suppressor genes is expressed at maximal levels in rapidly proliferating cells. This feature is consistent with in vitro observations that BRCA1 and BRCA2 are expressed in a cell cycle-dependent manner. This feature is also well illustrated during mammary gland development wherein the expression of BRCA1 and BRCA2 is induced in rapidly proliferating cellular compartments undergoing differentiation, such as terminal end buds during puberty and developing alveoli during pregnancy. Strikingly, the spatial and temporal patterns of BRCA1 and BRCA2 expression are virtually indistinguishable during embryonic development and in multiple adult tissues despite the fact that these genes are unrelated. These observations have contributed to the emerging hypothesis that these genes function in similar regulatory pathways.