Brca1 Deficiency Causes Bone Marrow Failure and Spontaneous Hematologic Malignancies in Mice

Abstract

Background: Fanconi anemia (FA) is caused by mutations in one of seventeen genes that make up the FA DNA double strand break (DSB) repair pathway. Recently, two individuals with biallelic germline BRCA1 mutations, each consisting of one null and one hypomorphic mutation, were identified and noted to have features consistent with FA, including congenital anomalies and increased chromosomal breakage of lymphocytes on exposure to diepoxybutane (Domchek et al. Cancer Discov. 2013 Apr; (4):399-405; and Sawyer et al. Cancer Discov. Epub. 2014 Dec 3.), adding BRCA1 as the newest FA gene. However, neither patient developed bone marrow failure (BMF), making the bone marrow effects of BRCA1 deficiency still unclear. Methods: To test the hypothesis that Brca1 is also essential in hematopoiesis, we developed a conditional mouse model with Mx-1 Cre-mediated Brca1 deletion and examined the effects of Brca1 deficiency on hematopoiesis in this model. Results: At baseline, Brca1-/- mice have macrocytic anemia and leukopenia. Further, by 6 months of age, 30% and 50% of the Brca1-/- mice develop spontaneous BMF or hematologic malignancies (HM), respectively. Brca1-/- mice develop a diverse range of HM, including T-cell lymphomas and acute myeloid leukemias, suggesting a defect in an early hematopoietic progenitor population. Methylcellulose colony forming assays also demonstrate a defect in progenitor cell function with Brca1-/- bone marrow cells forming fewer colonies (44.4±31.9) than Brca1+/+ cells (200.3±30.5, p=0.004) at baseline, and show FA-like hypersensitivity to the DNA cross-linking agent, Mitomycin C (MMC) (mean colony survival % at 10 nM MMC 40% versus 82% and at 50 nM 1% vs 56%). Spectral karyotyping of bone marrow cells from mice that developed BMF demonstrated chromatid exchanges and breaks. Similarly, multiple chromosomal translocations were seen in the myeloid leukemia cells, implicating genomic instability in the pathogenesis of these disorders. Conclusions: Taken together, our results show that loss of Brca1 in murine bone marrow causes hematopoietic defects and MMC sensitivity similar to that seen in humans with FA, providing strong evidence that Brca1 is critical for normal hematopoiesis and that Brca1 is a bona fide FA gene. This novel mouse model provides the opportunity to gain functional insight into the key stage(s) of hematopoiesis that require Brca1 and the effects of Brca1 haploinsufficiency, as seen in humans, on hematopoiesis. Further, as nearly all of the single gene FA mouse models to-date have failed to recapitulate the bone marrow phenotype of human FA, this model will be critical for deepening our understanding of the pathogenesis of FA. Disclosures No relevant conflicts of interest to declare.

MCM8- and MCM9 Deficiencies Cause Lifelong Increased Hematopoietic DNA Damage Driving p53-Dependent Myeloid Tumors

“…One mouse model displays MDS features after several generations of genetically generated telomere defects (Colla et al, 2015), and another model with an inducible BRCA1 knockout (Vasanthakumar et al, 2016) develops various hematopoietic disorders. This indicates that specific types of DNA damage can cause myeloid tumors in mice.…”

FouNdER mutations confer risk for leukemias

“…BRCA1/2 are known within our field as Fanconi anemia-like genes, whose function is essential in the bone marrow. [4][5][6] Other cancer syndromes arise from defects in nucleotide excision repair (NER), 7 including xeroderma pigmentosum (XP), which consists of 8 autosomal recessive forms (XP-A, XP-B, XP-C, XP-D, XP-E, XP-F, XP-G, and XP-V), each having a mutation in a component of the NER complex (XPA, ERCC3, XPC, ERCC2, DDB2, ERCC4, ERCC5, and POLH, respectively). People with XP have an extreme sensitivity to UV light, experiencing severe sunburns with minutes of exposure, dry skin (xeroderma), freckling (pigmentosum), hearing loss, poor coordination, loss of intellectual function, seizures, and development of squamous cell carcinomas and melanomas often as early as 10 years old in sun-exposed areas.…”