BRCA1 and FancJ cooperatively promote interstrand crosslinker induced centrosome amplification through the activation of polo-like kinase 1

FANCM promotes PARP inhibitor resistance by minimizing ssDNA gap formation and counteracting resection inhibition

Poly(ADP-ribose) polymerase inhibitors (PARPis) exhibit remarkable anticancer activity in tumors with homologous recombination (HR) gene mutations. However, the role of other DNA repair proteins in PARPi-induced lethality remains elusive. Here, we reveal that FANCM promotes PARPi resistance independent of the core Fanconi anemia (FA) complex. FANCM-depleted cells retain HR proficiency, acting independently of BRCA1 in response to PARPis. FANCM depletion leads to increased DNA damage in the second S phase after PARPi exposure, driven by elevated single-strand DNA (ssDNA) gap formation behind replication forks in the first S phase. These gaps arise from both 53BP1- and primase and DNA directed polymerase (PRIMPOL)-dependent mechanisms. Notably, FANCM-depleted cells also exhibit reduced resection of collapsed forks, while 53BP1 deletion restores resection and mitigates PARPi sensitivity. Our results suggest that FANCM counteracts 53BP1 to repair PARPi-induced DNA damage. Furthermore, FANCM depletion leads to increased chromatin bridges and micronuclei formation after PARPi treatment, elucidating the mechanism underlying extensive cell death in FANCM-depleted cells.

Aurora A polyubiquitinates the BRCA1-interacting protein OLA1 to promote centrosome maturation

The BRCA1-interacting protein Obg-like ATPase 1 (OLA1) functions in centriole duplication. In this study, we show the role of the mitotic kinase Aurora A in the reduction of centrosomal OLA1. Aurora A binds to and polyubiquitinates OLA1, targeting it for proteasomal degradation. NIMA-related kinase 2 (NEK2) phosphorylates the T124 residue of OLA1, increases binding of OLA1 to Aurora A and OLA1 polyubiquitination by Aurora A, and reduces centrosomal OLA1 in G2 phase. The kinase activity of Aurora A suppresses OLA1 polyubiquitination. The decrease in centrosomal OLA1 caused by Aurora A-mediated polyubiquitination promotes the recruitment of pericentriolar material proteins in G2 phase. The E3 ligase activity of Aurora A is critical for centrosome amplification induced by its overexpression. The results suggest a dual function of Aurora A as an E3 ubiquitin ligase and a kinase in the regulation of centrosomal OLA1, which is essential for proper centrosome maturation in G2 phase.

BRCA1 transports the DNA damage signal for CDDP-induced centrosome amplification through the centrosomal Aurora A

Breast cancer gene 1 (BRCA1) plays roles in DNA repair and centrosome regulation and is involved in DNA damage-induced centrosome amplification (DDICA). Here, the centrosomal localization of BRCA1 and the kinases involved in centrosome duplication were analyzed in each cell cycle phase after treatment with DNA crosslinker cisplatin (CDDP). CDDP treatment increased the centrosomal localization of BRCA1 in early S-G2 phase. BRCA1 contributed to the increased centrosomal localization of Aurora A in S phase and that of phosphorylated Polo-like kinase 1 (PLK1) in late S phase after CDDP treatment, resulting in centriole disengagement and overduplication. The increased centrosomal localization of BRCA1 and Aurora A induced by CDDP treatment involved the nuclear export of BRCA1 and BRCA1 phosphorylation by ataxia telangiectasia mutated (ATM). Patient-derived variants and mutations at phosphorylated residues of BRCA1 suppressed the interaction between BRCA1 and Aurora A, as well as the CDDP-induced increase in the centrosomal localization of BRCA1 and Aurora A. These results suggest that CDDP induces the phosphorylation of BRCA1 by ATM in the nucleus and its transport to the cytoplasm, thereby promoting the centrosomal localization Aurora A, which phosphorylates PLK1. The function of BRCA1 in the translocation of the DNA damage signal from the nucleus to the centrosome to induce centrosome amplification after CDDP treatment might support its role as a tumor suppressor.

Cytogenetics in Fanconi Anemia: The Importance of Follow-Up and the Search for New Biomarkers of Genomic Instability

Fanconi Anemia (FA) is a disease characterized by genomic instability, increased sensitivity to DNA cross-linking agents, and the presence of clonal chromosomal abnormalities. This genomic instability can compromise the bone marrow (BM) and confer a high cancer risk to the patients, particularly in the development of Myelodysplastic Syndrome (MDS) and Acute Myeloid Leukemia (AML). The diagnosis of FA patients is complex and cannot be based only on clinical features at presentation. The gold standard diagnostic assay for these patients is cytogenetic analysis, revealing chromosomal breaks induced by DNA cross-linking agents. Clonal chromosome abnormalities, such as the ones involving chromosomes 1q, 3q, and 7, are also common features in FA patients and are associated with progressive BM failure and/or a pre-leukemia condition. In this review, we discuss the cytogenetic methods and their application in diagnosis, stratification of the patients into distinct prognostic groups, and the clinical follow-up of FA patients. These methods have been invaluable for the understanding of FA pathogenesis and identifying novel disease biomarkers. Additional evidence is required to determine the association of these biomarkers with prognosis and cancer risk, and their potential as druggable targets for FA therapy.

Moonlighting at the Poles: Non-Canonical Functions of Centrosomes

Centrosomes are best known as the microtubule organizing centers (MTOCs) of eukaryotic cells. In addition to their classic role in chromosome segregation, centrosomes play diverse roles unrelated to their MTOC activity during cell proliferation and quiescence. Metazoan centrosomes and their functional doppelgängers from lower eukaryotes, the spindle pole bodies (SPBs), act as important structural platforms that orchestrate signaling events essential for cell cycle progression, cellular responses to DNA damage, sensory reception and cell homeostasis. Here, we provide a critical overview of the unconventional and often overlooked roles of centrosomes/SPBs in the life cycle of eukaryotic cells.

Roles of RACK1 in centrosome regulation and carcinogenesis

Receptor for activated C kinase 1 (RACK1) regulates various cellular functions and signaling pathways by interacting with different proteins. Recently, we showed that RACK1 interacts with breast cancer gene 1 (BRCA1), which regulates centrosome duplication. RACK1 localizes to centrosomes and spindle poles and is involved in the proper centrosomal localization of BRCA1. The interaction between RACK1 and BRCA1 is critical for the regulation of centrosome number. In addition, RACK1 contributes to centriole duplication by regulating polo-like kinase 1 (PLK1) activity in S phase. RACK1 binds directly to PLK1 and Aurora A, promoting the phosphorylation of PLK1 and activating the Aurora A/PLK1 signaling axis. Overexpression of RACK1 causes centrosome amplification, especially in mammary gland epithelial cells, inducing overactivation of PLK1 followed by premature centriole disengagement and centriole re-duplication. Other proteins, including hypoxia-inducible factor α, von Hippel-Lindau protein, heat-shock protein 90, β-catenin, and glycogen synthase kinase-3β, interact with RACK1 and play roles in centrosome regulation. In this review, we focus on the roles and underlying molecular mechanisms of RACK1 in centrosome regulation mediated by its interaction with different proteins and the modulation of their functions.

Dysregulation of the centrosome induced by BRCA1 deficiency contributes to tissue-specific carcinogenesis

Alterations in breast cancer gene 1 (BRCA1), a tumor suppressor gene, increase the risk of breast and ovarian cancers. BRCA1 forms a heterodimer with BRCA1-associated RING domain protein 1 (BARD1) and functions in multiple cellular processes, including DNA repair and centrosome regulation. BRCA1 acts as a tumor suppressor by promoting homologous recombination (HR) repair, and alterations in BRCA1 cause HR deficiency, not only in breast and ovarian tissues but also in other tissues. The molecular mechanisms underlying BRCA1 alteration-induced carcinogenesis remain unclear. Centrosomes are the major microtubule-organizing centers and function in bipolar spindle formation. The regulation of centrosome number is critical for chromosome segregation in mitosis, which maintains genomic stability. BRCA1/BARD1 function in centrosome regulation together with Obg-like ATPase (OLA1) and receptor for activating protein C kinase 1 (RACK1). Cancer-derived variants of BRCA1, BARD1, OLA1, and RACK1 do not interact, and aberrant expression of these proteins results in abnormal centrosome duplication in mammary-derived cells, and rarely in other cell types. RACK1 is involved in centriole duplication in the S phase by promoting polo-like kinase 1 activation by Aurora A, which is critical for centrosome duplication. Centriole number is higher in cells derived from mammary tissues compared with in those derived from other tissues, suggesting that tissue-specific centrosome characterization may shed light on the tissue specificity of BRCA1-associated carcinogenesis. Here, we explored the role of the BRCA1-containing complex in centrosome regulation and the effect of its deficiency on tissue-specific carcinogenesis.

The CINs of Polo-Like Kinase 1 in Cancer

Simple Summary

Many alterations specific to cancer cells have been investigated as targets for targeted therapies. Chromosomal instability is a characteristic of nearly all cancers that can limit response to targeted therapies by ensuring the tumor population is not genetically homogenous. Polo-like Kinase 1 (PLK1) is often up regulated in cancers and it regulates chromosomal instability extensively. PLK1 has been the subject of much pre-clinical and clinical studies, but thus far, PLK1 inhibitors have not shown significant improvement in cancer patients. We discuss the numerous roles and interactions of PLK1 in regulating chromosomal instability, and how these may provide an avenue for identifying targets for targeted therapies. As selective inhibitors of PLK1 showed limited clinical success, we also highlight how genetic interactions of PLK1 may be exploited to tackle these challenges.

Abstract

Polo-like kinase 1 (PLK1) is overexpressed near ubiquitously across all cancer types and dysregulation of this enzyme is closely tied to increased chromosomal instability and tumor heterogeneity. PLK1 is a mitotic kinase with a critical role in maintaining chromosomal integrity through its function in processes ranging from the mitotic checkpoint, centrosome biogenesis, bipolar spindle formation, chromosome segregation, DNA replication licensing, DNA damage repair, and cytokinesis. The relation between dysregulated PLK1 and chromosomal instability (CIN) makes it an attractive target for cancer therapy. However, clinical trials with PLK1 inhibitors as cancer drugs have generally displayed poor responses or adverse side-effects. This is in part because targeting CIN regulators, including PLK1, can elevate CIN to lethal levels in normal cells, affecting normal physiology. Nevertheless, aiming at related genetic interactions, such as synthetic dosage lethal (SDL) interactions of PLK1 instead of PLK1 itself, can help to avoid the detrimental side effects associated with increased levels of CIN. Since PLK1 overexpression contributes to tumor heterogeneity, targeting SDL interactions may also provide an effective strategy to suppressing this malignant phenotype in a personalized fashion.

RACK1 regulates centriole duplication through promoting the activation of polo-like kinase 1 by Aurora A

Breast cancer gene 1 (BRCA1) contributes to the regulation of centrosome number. We previously identified receptor for activated C kinase 1 (RACK1) as a BRCA1-interacting partner. RACK1, a scaffold protein that interacts with multiple proteins through its seven WD40 domains, directly binds to BRCA1 and localizes to centrosomes. RACK1 knockdown suppresses centriole duplication, whereas RACK1 overexpression causes centriole overduplication in a subset of mammary gland-derived cells. In this study, we showed that RACK1 binds directly to polo-like kinase 1 (PLK1) and Aurora A, and promotes the Aurora A-PLK1 interaction. RACK1 knockdown decreased phosphorylated PLK1 (p-PLK1) levels and the centrosomal localization of Aurora A and p-PLK1 in S phase, whereas RACK1 overexpression increased p-PLK1 level and the centrosomal localization of Aurora A and p-PLK1 in interphase, resulting in an increase of cells with abnormal centriole disengagement. Overexpression of cancer-derived RACK1 variants failed to enhance the Aurora A-PLK1 interaction, PLK1 phosphorylation and the centrosomal localization of p-PLK1. These results suggest that RACK1 functions as a scaffold protein that promotes the activation of PLK1 by Aurora A in order to promote centriole duplication.This article has an associated First Person interview with the first author of the paper.

The Effects of Genetic and Epigenetic Alterations of BARD1 on the Development of Non-Breast and Non-Gynecological Cancers

Breast Cancer 1 (BRCA1) gene is a well-characterized tumor suppressor gene, mutations of which are primarily found in women with breast and ovarian cancers. BRCA1-associated RING domain 1 (BARD1) gene has also been identified as an important tumor suppressor gene in breast, ovarian, and uterine cancers. Underscoring the functional significance of the BRCA1 and BARD1 interactions, prevalent mutations in the BRCA1 gene are found in its RING domain, through which it binds the RING domain of BARD1. BARD1-BRCA1 heterodimer plays a crucial role in a variety of DNA damage response (DDR) pathways, including DNA damage checkpoint and homologous recombination (HR). However, many mutations in both BARD1 and BRCA1 also exist in other domains that significantly affect their biological functions. Intriguingly, recent genome-wide studies have identified various single nucleotide polymorphisms (SNPs), genetic alterations, and epigenetic modifications in or near the BARD1 gene that manifested profound effects on tumorigenesis in a variety of non-breast and non-gynecological cancers. In this review, we will briefly discuss the molecular functions of BARD1, including its BRCA1-dependent as well as BRCA1-independent functions. We will then focus on evaluating the common BARD1 related SNPs as well as genetic and epigenetic changes that occur in the non-BRCA1-dominant cancers, including neuroblastoma, lung, and gastrointestinal cancers. Furthermore, the pro- and anti-tumorigenic functions of different SNPs and BARD1 variants will also be discussed.

Fanconi Anemia Pathway: Mechanisms of Breast Cancer Predisposition Development and Potential Therapeutic Targets

The maintenance of genomic stability is crucial for species survival, and its failure is closely associated with tumorigenesis. The Fanconi anemia (FA) pathway, involving 22 identified genes, plays a central role in repairing DNA interstrand cross-links. Importantly, a germline defect in any of these genes can cause Fanconi's anemia, a heterogeneous genetic disorder, characterized by congenital growth abnormalities, bone marrow failure, and predisposition to cancer. On the other hand, the breast cancer susceptibility genes, BRCA1 and BRCA2, also known as FANCS and FANCD1, respectively, are involved in the FA pathway; hence, researchers have studied the association between the FA pathway and cancer predisposition. Here, we mainly focused on and systematically reviewed the clinical and mechanistic implications of the predisposition of individuals with abnormalities in the FA pathway to cancer, especially breast cancer.

Centrosome amplification: a suspect in breast cancer and racial disparities

The multifaceted involvement of centrosome amplification (CA) in tumorigenesis is coming into focus following years of meticulous experimentation, which have elucidated the powerful abilities of CA to promote cellular invasion, disrupt stem cell division, drive chromosomal instability (CIN) and perturb tissue architecture, activities that can accelerate tumor progression. Integration of the extant in vitro, in vivo and clinical data suggests that in some tissues CA may be a tumor-initiating event, in others a consequential 'hit' in multistep tumorigenesis, and in some others, non-tumorigenic. However, in vivo data are limited and primarily focus on PLK4 (which has CA-independent mechanisms by which it promotes aggressive cellular phenotypes). In vitro breast cancer models suggest that CA can promote tumorigenesis in breast cancer cells in the setting of p53 loss or mutation, which can both trigger CA and promote cellular tolerance to its tendency to slow proliferation and induce aneuploidy. It is thus our perspective that CA is likely an early hit in multistep breast tumorigenesis that may sometimes be lost to preserve aggressive karyotypes acquired through centrosome clustering-mediated CIN, both numerical and structural. We also envision that the robust link between p53 and CA may underlie, to a considerable degree, racial health disparity in breast cancer outcomes. This question is clinically significant because, if it is true, then analysis of centrosomal profiles and administration of centrosome declustering drugs could prove highly efficacious in risk stratifying breast cancers and treating African American (AA) women with breast cancer.

Depletion of tumor suppressor Kank1 induces centrosomal amplification via hyperactivation of RhoA

Chromosome instability, frequently found in cancer cells, is caused by a deficiency in cell division, including centrosomal amplification and cytokinesis failure, and can result in abnormal chromosome content or aneuploidy. The small GTPase pathways have been implicated as important processes in cell division. We found that knockdown of a tumor suppressor protein Kank1 increases the number of cells with a micronucleus or bi-/multi-nuclei, which was likely caused by centrosomal amplification. Kank1 interacts with Daam1, known to bind to and activate a small GTPase, RhoA, in actin assembly. Knockdown of Kank1 or overexpression of Daam1, respectively, hyperactivates RhoA, potentially leading to the modulation of the activity of Aurora-A, a key regulator of centrosomal functions, eventually resulting in centrosomal amplification. Kank1 is also associated with contractile ring formation in collaboration with RhoA, and its deficiency results in the interruption of normal daughter cell separation, generating multinucleate cells. Such abnormal segregation of chromosomes may cause further chromosomal instability and abnormal gene functions, leading to tumorigenesis. Thus, Kank1 plays a crucial role in regulating the activity of RhoA through retrieving excess Daam1 and balancing the activities of RhoA and its effectors.

Important molecular genetic markers of colorectal cancer

Colorectal cancer (CRC) ranks third in the incidences of cancer morbidity and mortality worldwide. CRC is rather heterogeneous with regard to molecular genetic characteristics and pathogenic pathways. A wide spectrum of biomarkers is used for molecular subtype determination, prognosis, and estimation of sensitivity to different drugs in practice. These biomarkers can include germline and somatic mutations, chromosomal aberrations, genomic abnormalities, gene expression alterations at mRNA or protein level and changes in DNA methylation status. In the present review we discuss the most important and well-studied CRC biomarkers, and their potential clinical significance and current approaches to molecular classification of colorectal tumors.

FANCA safeguards interphase and mitosis during hematopoiesis in vivo

The Fanconi anemia (FA/BRCA) signaling network controls multiple genome-housekeeping checkpoints, from interphase DNA repair to mitosis. The in vivo role of abnormal cell division in FA remains unknown. Here, we quantified the origins of genomic instability in FA patients and mice in vivo and ex vivo. We found that both mitotic errors and interphase DNA damage significantly contribute to genomic instability during FA-deficient hematopoiesis and in nonhematopoietic human and murine FA primary cells. Super-resolution microscopy coupled with functional assays revealed that FANCA shuttles to the pericentriolar material to regulate spindle assembly at mitotic entry. Loss of FA signaling rendered cells hypersensitive to spindle chemotherapeutics and allowed escape from the chemotherapy-induced spindle assembly checkpoint. In support of these findings, direct comparison of DNA crosslinking and anti-mitotic chemotherapeutics in primary FANCA-/- cells revealed genomic instability originating through divergent cell cycle checkpoint aberrations. Our data indicate that FA/BRCA signaling functions as an in vivo gatekeeper of genomic integrity throughout interphase and mitosis, which may have implications for future targeted therapies in FA and FA-deficient cancers.

FancJ (Brip1) loss-of-function allele results in spermatogonial cell depletion during embryogenesis and altered processing of crossover sites during meiotic prophase I in mice

Fancj, the gene associated with Fanconi anemia (FA) Complementation Group J, encodes a DNA helicase involved in homologous recombination repair and the cellular response to replication stress. FANCJ functions in part through its interaction with key DNA repair proteins, including MutL homolog-1 (MLH1), Breast Cancer Associated gene-1 (BRCA1), and Bloom syndrome helicase (BLM). All three of these proteins are involved in a variety of events that ensure genome stability, including the events of DNA double strand break (DSB) repair during prophase I of meiosis. Meiotic DSBs are repaired through homologous recombination resulting in non-crossovers (NCO) or crossovers (CO). The frequency and placement of COs are stringently regulated to ensure that each chromosome receives at least one CO event, and that longer chromosomes receive at least one additional CO, thus facilitating the accurate segregation of homologous chromosomes at the first meiotic division. In the present study, we investigated the role of Fancj during prophase I using a gene trap mutant allele. Fancj (GT/GT) mutants are fertile, but their testes are very much smaller than wild-type littermates, predominantly as a result of impeded spermatogonial proliferation and mildly increased apoptosis during testis development in the fetus. This defect in spermatogonial proliferation is consistent with mutations in other FA genes. During prophase I, early events of synapsis and DSB induction/repair appear mostly normal in Fancj (GT/GT) males, and the FANCJ-interacting protein BRCA1 assembles normally on meiotic chromosome cores. However, MLH1 focus frequency is increased in Fancj (GT/GT) males, indicative of increased DSB repair via CO, and is concomitant with increased chiasmata at diakinesis. This increase in COs in the absence of FANCJ is associated with increased localization of BLM helicase protein, indicating that BLM may facilitate the increased rate of crossing over in Fancj (GT/GT) males. Taken together, these results demonstrate a critical role for FANCJ in spermatogenesis at two stages: firstly in the proliferative activity that gives rise to the full complement of testicular spermatogonia and secondly in the establishment of appropriate CO numbers during prophase I.