Impaired Spindle Assembly Checkpoint In Vivo Promotes MDS/AML in a Novel Mouse Model of Fanconi Anemia

The Fanconi anemia (FA/BRCA) signaling network prevents bone marrow failure and cancer by protecting genomic integrity. Biallelic germline mutations within this gene network result in Fanconi anemia, an inherited bone marrow failure syndrome characterized by genomic instability and a predisposition to bone marrow failure, myelodysplasia and cancer, particularly acute myelogenous leukemia (AML). Heterozygous inborn mutations in the BRCA branch of FA network increase risk of breast and ovarian cancers as well as other tumors, and somatic mutations of FA/BRCA genes occur in malignancies in non-Fanconi patients. Thus, disruption of FA/BRCA signaling promotes malignancies in the inherited genetic syndromes and in the general population.

The FA/BRCA network functions as a genome gatekeeper throughout the cell cycle. In interphase, the FA/BRCA network provides a crucial line of defense against mutagenesis by coordinating DNA damage response to a variety of genotoxic insults, from endogenous aldehydes to replication errors and mutagen exposure. Less is known about the role of the FA/BRCA pathway during mitosis. However, FA signaling has recently been implicated in multiple aspects of cell division, including the spindle assembly checkpoint (SAC) that ensures high-fidelity chromosome segregation at metaphase to anaphase transition; cytokinesis; centrosome maintenance and repair of ultrafine anaphase bridges. Although chromosomal instability due to mitotic errors is a hallmark of cancer, the in vivo contribution of abnormal mitosis to malignant transformation of FA-deficient hematopoietic cells remains unknown.

To determine whether error-prone chromosome segregation upon loss of FA signaling contributes to abnormal hematopoiesis and cancer, we generated a novel murine FA model by genetically weakening the SAC in the FA-deficient background. The resulting mice were viable and born at expected Mendelian ratios, but exhibited increased baseline in vivo chromosomal instability evidenced by elevated red blood cell micronucleation, increased frequency of chromosome missegregation and DNA breakage in microscopy-based cytome assays, and augmented bone marrow karyotype instability. Importantly, unlike FA or SAC control animals, the FA-SAC mice were prone to premature death due to the development of myelodysplasia and AML at young age, recapitulating disease manifestations of human Fanconi anemia.

This study provides the in vivo evidence supporting the essential role of compromised chromosome segregation in the development of myelodysplasia and acute leukemia due to impaired FA signaling. Our observations provide novel insights into complex mechanisms of genomic instability and carcinogenesis due to FA deficiency. Impaired mitosis is a well-established therapeutic target, and our independent ex vivo experiments using FA patient-derived primary cells show that exposure to antimitotic chemotherapeutics is synthetic lethal with loss of the FA network. Thus, our findings may have implications for future precision strategies against FA-deficient, chromosomally unstable hematopoietic cancers. The FA-SAC mouse model offers a preclinical platform to systematically test this hypothesis in vivo.