BRIT1 regulates early DNA damage response, chromosomal integrity, and cancer

Abstract

BRIT1, initially identified as an hTERT repressor, has additional functions at DNA damage checkpoints. Here, we demonstrate that BRIT1 formed nuclear foci minutes after irradiation. The foci of BRIT1 colocalized with 53BP1, MDC1, NBS1, ATM, RPA, and ATR. BRIT1 was required for activation of these elements, indicating that BRIT1 is a proximal factor in the DNA damage response pathway. Depletion of BRIT1 increased the accumulation of chromosomal aberrations. In addition, decreased levels of BRIT1 were detected in several types of human cancer, with BRIT1 expression being inversely correlated with genomic instability and metastasis. These results identify BRIT1 as a crucial DNA damage regulator in the ATM/ATR pathways and suggest that it functions as a tumor suppressor gene.

Figure 1

BRIT1 forms IRIF immediately after IR and is required for NBS1, 53BP1, MDC1, and p-ATM IRIF formation A: U2OS cells were analyzed for IRIF formation at the indicated times after irradiation (10 Gy).B: U2OS cells were transfected with control (luciferase) or BRIT1 siRNA#1 twice as described; 48 hr after the second transfection, cells were treated or not treated with 10 Gy of ionizing radiation, and 1 hr later they were fixed and stained with antibodies to BRIT1 and 53BP1. Cells were then washed and stained with rhodamine- or FITC-conjugated antibodies; nuclei were visualized by DAPI staining. The 53BP1 foci were abolished in cells in which BRIT1 was completely knocked down; the cells with incomplete knockdown of BRIT1 did show 53BP1 foci (bottom panel). C: U2OS cells were transfected and treated as described in B except that the antibodies used were to BRIT1 and MDC1 (top) or BRIT1 and p-ATM (bottom). D: U2OS cells were transfected and treated as described for B except that the antibodies used were to BRIT1 and NBS1. Scale bars, 20 μm.

Figure 2

BRIT1 is required for UV-induced formation of ATR, RPA, and p-Rad17 foci U2OS cells were transfected with control (luciferase) or BRIT1 siRNA twice; at 48 hr after the second transfection, cells were treated or not treated with 50 J/m ² of UV radiation. One hour after irradiation, cells were fixed and costained with antibodies to BRIT1 and ATR (A), RPA34 (B), p-RPA34 (C, left panel), and p-Rad17 (C, right panel). Cells were then washed and stained with rhodamine- or FITC-conjugated antibodies. Nuclei were visualized by DAPI staining. Scale bars, 20 μm.

Figure 3

BRIT1 is required for the association of MDC1, 53BP1, p-NBS1, RPA, and p-Rad17 with chromatin and for the UV-induced phosphorylation of RPA and Rad17 A: U2OS cells were transfected twice with luciferase or with two different BRIT1 siRNAs; at 48 hr after the second transfection, cells were treated or not treated with 10 Gy of ionizing radiation. Two hours after irradiation, chromatin-enriched sediments were subjected to Western blot analysis and probed with antibodies against BRIT1, MDC1, 53BP1, p-NBS1, NBS1, and ORC2. B: U2OS cells were transfected as described for A and treated with 50 J/m²UV radiation; 2 hr later, chromatin-enriched fractions were subjected to Western blotting and probed with antibodies against BRIT1, RPA70, p-RPA34, RPA34, p-Rad17, Rad17, and ORC2.C: U2OS cells were treated as described above except that cells were exposed to 30 J/m² of UV; 8 hr after irradiation, whole-cell lysates were collected and subjected to Western blot analysis and probed with antibodies against BRIT1, p-RPA34, RPA34, p-Rad17, and actin.

Figure 4

BRIT1 deficiency leads to chromosomal aberrations HMEC cells were mock transfected or transfected with luciferase or one of three different BRIT1 siRNAs twice; 72 hr later, cells were analyzed by metaphase spreads. The top panels show examples of abnormal chromosomes, and the bottom panel summarizes percentages of cells containing chromosomal aberrations.

Figure 5

BRIT1 deficiencies in human cancer A: Array-based CGH was used to test 87 specimens of human serous ovarian cancer with bacterial artificial chromosome probes. Log base two logarithmic ratios of test to reference intensity were calculated for the probes across the region containing BRIT1. More than half of the samples arrayed showed a reduction in copy number, whereas approximately 30% showed a loss of at least one copy of the region across chromosome 8 encompassing BRIT1. B: The losses of BRIT1 gene copy in the ovarian cancer CGH database were compared with the gain or loss of at least one copy number change using different probes to other chromosome regions. The specimens that exhibited more BRIT1 gene loss correlated with the overall copy changes of various genes. C: Comparing BRIT1 RNA transcript levels in benign ovarian tissue specimens and ovarian tumor specimens demonstrated a statistically significant reduction in BRIT1 RNA levels in the tumors as compared with the mean of RNA levels of the benign specimens. D: CGH of 54 breast cancer cell lines showed that a striking majority showed large decreases in copy number, with loss of at least one copy, for the region on chromosome 8 containing BRIT1. E: Expression of BRIT1 RNA in human normal (untransformed) breast cell lines or breast cancer cell lines were analyzed by quantitative real-time RT-PCR. F: Left: total cell lysates were obtained from the cultured cell lines, subjected to Western blot analysis, and probed with antibodies against BRIT1 or actin. Right: the indicated cell lines were treated with 10 Gy of ionizing radiation. One hour after irradiation, cells were fixed and costained with antibodies to BRIT1 or phospho-ATM antibody. Scale bar, 20 μm. Note: the phospho-ATM foci appeared in a very small fraction of BT20 or MCF7 cells that showed detectable BRIT1 expression. G: Prostate cancer specimens were analyzed by immunohistochemical staining and probed with BRIT1-specific antibody. The brownish color indicates BRIT1 expression in the regions of normal tissue, benign prostate hypertrophy (BPH), or prostate cancer from the same patient specimen. Scale bar, 50 μm. H: BRIT1 RNA transcript levels were tested for correlation with time to metastasis of breast cancer in a database maintained by Stanford University. Normal to high BRIT1 levels were associated with longer time to develop metastasis and an overall reduction in the incidence of metastasis.

Figure 6

The deleted BRIT1 gene identified from a breast cancer patient fails to activate foci formation for MDC1, p-RPA, and p-ATM A: Exon 10 deletion of the BRIT1 gene (BR7) was identified in a breast patient sample by sequence analysis (coding exons shown in gray). The premature termination (indicated by red x, bottom schematic) results in a protein lacking two C terminus BRCT domains. B: U2OS cells were transfected with FLAG-V, FLAG-BRIT1, or FLAG-BR7 constructs. Four hours after the transfection, cells were mock transfected or transfected with control luciferase or BRIT1 siRNA#1. Forty-eight hours after the siRNA transfection, cells were treated with IR (10 Gy). One hour after irradiation, cells were fixed and stained with the MDC1 antibody. Cells were then washed and subsequently stained with rhodamine-antibody. Nuclei were visualized by DAPI staining (left). Scale bars, 20 μm. The lysates from the cells analyzed above were subjected to Western blot analysis and probed with BRIT1 antibody (right) to confirm the expression of the exogenous FLAG-BRIT1 and FLAG-BR7 protein. C: MCF7 cells were transfected with FLAG-V, FLAG-BRIT1, or FLAG-BR7 constructs. Forty-eight hours after transfection, cells were treated with either UV (50 J/m²) or IR (10 Gy). One hour after irradiation, cells were fixed and stained with the p-RPA and p-ATM antibodies, respectively. Cells were then washed and subsequently stained with rhodamine- or FITC-conjugated antibodies. Nuclei were visualized by DAPI staining. The lysates from the cells analyzed above were subjected to Western blot analysis and probed with BRIT1 antibody (right) to confirm the expression of the exogenous FLAG-BRIT1 and FLAG-BR7 protein.

Figure 7

A model of BRIT1 functions in checkpoint signaling, chromosomal integrity, and cancer suppression In addition to regulating BRCA1 and Chk1 expression, BRIT1 is also required for the function of DNA damage sensors or mediators in both the ATM and ATR pathways; BRIT1 is also required for the maintenance of chromosomal integrity and, potentially, for tumor suppression.