Brca1 deficiency causes bone marrow failure and spontaneous hematologic malignancies in mice

Abstract

• Brca1 deficiency causes Fanconi anemia-like cytopenias, mitomycin C hypersensitivity, and spontaneous bone marrow failure.• Brca1 is critical for the maintenance of normal hematopoietic progenitor function and genomic stability in the bone marrow.BRCA1 is critical for maintenance of genomic stability and interacts directly with several proteins that regulate hematopoietic stem cell function and are part of the Fanconi anemia (FA) double-strand break DNA repair pathway. The effects of complete BRCA1 deficiency on bone marrow (BM) function are unknown. To test the hypothesis that Brca1 is essential in hematopoiesis, we developed a conditional mouse model with Mx1-Cre-mediated Brca1 deletion. Mice lacking Brca1 in the BM have baseline cytopenias and develop spontaneous bone marrow failure or diverse hematologic malignancies by 6 months of age. Brca1 2/2 BM cells have a reduced capacity to form hematopoietic colonies in vitro and to reconstitute hematopoiesis in irradiated recipients, consistent with a hematopoietic progenitor functional defect. Brca1 2/2 BM cells also show FA-like hypersensitivity to the DNA crosslinking agent mitomycin C, and karyotypes feature genomic instability. Taken together, our results show that loss of Brca1 in murine BM causes hematopoietic defects similar to those seen in people with FA, which provides strong evidence that Brca1 is critical for normal hematopoiesis and that Brca1 is a bona fide FA-like gene. (Blood. 2016;127(3):310-313) IntroductionFanconi anemia (FA) is an inherited bone marrow failure (BMF) syndrome characterized by hypersensitivity to DNA crosslinking agents, congenital anomalies, BMF, and an increased risk of developing leukemia and solid tumors. 1 FA is caused by mutations in one of 17 genes that make up the FA DNA repair pathway. 2-4 BRCA1 binds directly to several FA proteins, all of which are essential for normal hematopoiesis, but the effects of BRCA1 deficiency on hematopoiesis are unknown. Several lines of evidence suggest that BRCA1 may also be an important regulator of hematopoiesis. First, BRCA1 is highly expressed in hematopoietic tissues, 5,6 whereas its expression is lost in myeloid leukemias. 7,8 Second, overexpression of Brca1 in the bone marrow (BM) disrupts stem cell quiescence and differentiation. 9 Finally, the first person ever reported to have biallelic BRCA1 mutations experienced unusually severe myelosuppression after exposure to the DNA crosslinking agent carboplatin, 10 which provides direct evidence that BRCA1 is important in human hematopoiesis.To test whether BRCA1 has a critical role in hematopoiesis, we created a conditional Mx1-Cre-induced Brca1 deficiency mouse model. We demonstrated that mice with homozygous deficiency of Brca1 in the BM have FA-like cytopenias, DNA crosslinking agent hypersensitivity, and susceptibility to BMF and hematologic malignancies (HMs). Furthermore, Brca1 2/2 cells have a hematopoietic precursor functional defect characterized by a reduced capacity to form hematopoietic colonies in vitro and to reconstitut...

Distinct Brca1 Mutations Differentially Reduce Hematopoietic Stem Cell Function

“…A fraction (27%; 7/26) of the Vav1-iCre;Brca1 F22-24/F22-24 mice that survived beyond 3 months of age did become moribund (hunched, immobile, cold) prior to death and developed lymphocyte infiltrated splenomegaly (Figure S2a). Unlike the prior report that found p53 mutations in some spleens that were Brca1 null (Vasanthakumar et al, 2016), when we used RNA-seq to analyze for mutations in expressed genes, only wild type p53 was found in these enlarged spleens. However, consistent with the same report, T-cell infiltration was present based on a decreased B-cell specific gene expression and an increased T-cell gene expression pattern in the enlarged spleens (Figure S2b).…”

“…However, consistent with the same report, T-cell infiltration was present based on a decreased B-cell specific gene expression and an increased T-cell gene expression pattern in the enlarged spleens (Figure S2b). These data suggest that deletion of Brca1 from mouse hematopoietic cells has the potential to promote the development of hematopoietic malignancies from surviving progenitors (Vasanthakumar et al, 2016). …”

“…Transplantation of Brca1 -deficient bone marrow cells into irradiated mice was associated with lower blood cell counts in recipient mice and a trend toward lower levels of donor cell reconstitution 10–15 days after transplantation. However, this study did not detect a significant reduction in HSC frequency and the consequences for the long-term reconstituting capacity of bone marrow cells was not assessed (Vasanthakumar et al, 2016). Therefore, it has not yet been tested whether Brca1 is required for HSC function or whether heterozygosity for BRCA1 mutations affects recovery after chemotherapy in humans or in mice.…”

“…Conditional deletion of Brca1 from breast epithelium in mice leads to the development of breast cancer, but only when combined with p53 deficiency (Drost and Jonkers, 2009; McCarthy et al, 2007). Two recent studies conditionally deleted Brca1 from hematopoietic cells (Santos et al, 2014; Vasanthakumar et al, 2016). One showed that leukemia cells transformed by MLL-AF9 exhibited reduced proliferation and increased differentiation in the absence of Brca1 (Santos et al, 2014).…”

Defects in homologous recombination repair behind the human diseases: FA and HBOC

“…It will be highly interesting to clarify why these patients do not (or tend not to) develop hematopoietic stem cell failure because patients belonging to FA-D1/N/ J/O/R/S all seem to have a similar pathophysiology due to HR deficiency. Of note, a recent study demonstrated that BRCA1 deficiency, specifically in mouse bone marrow, causes hematopoietic defects (Vasanthakumar et al 2016). …”