Academic literature on the topic 'Pathway FA/BRCA2'

ABSTRACT Fanconi anemia (FA) is an autosomal recessive cancer susceptibility syndrome with at least 11 complementation groups (A, B, C, D1, D2, E, F, G, I, J, and L), and eight FA genes have been cloned. The FANCD1 gene is identical to the breast cancer susceptibility gene, BRCA2. The FA proteins cooperate in a common pathway, but the function of BRCA2/FANCD1 in this pathway remains unknown. Here we show that monoubiquitination of FANCD2, which is activated by DNA damage, is required for targeting of FANCD2 to chromatin, where it interacts with BRCA2. FANCD2-Ub then promotes BRCA2 loading into a chromatin complex. FANCD2−/− cells are deficient in the assembly of DNA damage-inducible BRCA2 foci and in chromatin loading of BRCA2. Functional complementation with the FANCD2 cDNA restores BRCA2 foci and its chromatin loading following DNA damage. BRCA2−/− cells expressing a carboxy-terminal truncated BRCA2 protein form IR-inducible BRCA2 and FANCD2 foci, but these foci fail to colocalize. Functional complementation of these cells with wild-type BRCA2 restores the interaction of BRCA2 and FANCD2. The C terminus of BRCA2 is therefore required for the functional interaction of BRCA2 and FANCD2 in chromatin. Taken together, our results demonstrate that monoubiquitination of FANCD2, which is regulated by the FA pathway, promotes BRCA2 loading into chromatin complexes. These complexes appear to be required for normal homology-directed DNA repair.

Abstract The FA/BRCA pathway is involved in DNA damage repair and its importance in oncogenesis has only recently been implicated. Briefly, 8 FA/BRCA pathway family members facilitate the monoubiquitination of FANCD2. Upon monoubiquitination, FANCD2 translocates to the DNA repair foci where it interacts with other proteins to initiate DNA repair. Previously, we reported that the FA/BRCA pathway is upregulated in multiple myeloma cell lines selected for resistance to melphalan (Chen, et al, Blood 2005). Further, reducing FANCF in the melphalan resistant 8226/LR5 myeloma cell line partially reversed resistance, whereas overexpressing FANCF in the drug sensitive 8226/S myeloma line conferred resistance to melphalan. Others have reported, and we have also verified, that bortezomib enhances melphalan response in myeloma cells; however, the mechanism of enhanced melphalan activity in combination with bortezomib has not been reported. Based on our observation that the FA/BRCA pathway confers melphalan resistance, we hypothesized that bortezomib enhances melphalan response by targeting FA/BRCA DNA damage repair pathway genes. To investigate this hypothesis, we first analyzed FA/BRCA gene expression in 8226/S and 8226/LR5 cells treated with bortezomib, using a customized microfluidic card (to detect BRCA1, BRCA2, FANCA, FANCC, FANCD2, FANCE, FANCF, FANCG, FANCL, RAD51 and RAD51C) and q-PCR. Interestingly, we found that low dose (5nM) bortezomib decreased many FA/BRCA pathway genes as early as 2 hours, with maximal decreases seen at 24 hours. Specifically, 1.5- to 2.5-fold decreases in FANCA, FANCC, FANCD2, FANCE and RAD51C were seen 24 hours post bortezomib exposure. Moreover, pre-treatment of myeloma cells with low dose bortezomib followed by melphalan treatment revealed a greater than 2-fold reduction in FANCD2 gene expression levels. We also found that melphalan treatment alone enhanced FANCD2 protein expression and activation (monoubiquitination), whereas the combination treatment of bortezomib followed by melphalan decreased activation and overall expression of FANCD2 protein. Taken together, these results suggest that bortezomib enhances melphalan response in myeloma by targeting the FA/BRCA pathway. Further understanding of the role of the FA/BRCA pathway in determining melphalan response may allow for more customized and effective treatment of myeloma.

Fanconi anemia (FA) is a complex genetic disorder characterized by bone marrow failure (BMF), congenital defects, inability to repair DNA interstrand cross-links (ICLs), and cancer predisposition. FA presents two seemingly opposite characteristics: ( a) massive cell death of the hematopoietic stem and progenitor cell (HSPC) compartment due to extensive genomic instability, leading to BMF, and ( b) uncontrolled cell proliferation leading to FA-associated malignancies. The canonical function of the FA proteins is to collaborate with several other DNA repair proteins to eliminate clastogenic (chromosome-breaking) effects of DNA ICLs. Recent discoveries reveal that the FA pathway functions in a critical tumor-suppressor network to preserve genomic integrity by stabilizing replication forks, mitigating replication stress, and regulating cytokinesis. Homozygous germline mutations (biallelic) in 22 FANC genes cause FA, whereas heterozygous germline mutations in some of the FANC genes (monoallelic), such as BRCA1 and BRCA2, do not cause FA but significantly increase cancer susceptibility sporadically in the general population. In this review, we discuss our current understanding of the functions of the FA pathway in the maintenance of genomic stability, and we present an overview of the prevalence and clinical relevance of somatic mutations in FA genes.

188 Background: Germline testing for prostate cancer (PCA) is now central to treatment, screening, and hereditary cancer management. The Fanconi anemia (FA) pathway is a key DNA repair pathway involved in PCA biology and treatment. Prevalence of FA genes BRCA2, PALB2, and BRIP1 is well-described; however, multiple other FA genes are not routinely tested, with limited prevalence data. Here we report mutation prevalence of a spectrum of FA genes among men undergoing PCA multigene testing on the Evaluation and Management for Prostate Oncology, Wellness, and Risk (EMPOWeR) study. Methods: Eligibility includes any male with PCA or at-risk for PCA. Multigene testing includes 51 genes; FA pathway genes include BRCA2, PALB2, BRIP1, FANCA, FANCB, FANCC, FANCD2, FANCE, FANCF, FANCG, FANCI, FANCL, and FANCM. Multiple additional cancer risk genes were tested. Fisher’s exact tests were conducted to compare the prevalence of FA gene mutations between participants in the EMPOWeR study vs population prevalence reported in the literature. Statistical significance level of all tests was set a priori to 0.05. Results: The current cohort includes 235 participants. Characteristics are White (83.3%), Black (13.7%), PCA diagnosis (83.4%), mean age of PCA diagnosis 61.7 + 7.69 years, Gleason score >=7 (66.2%), and T3 or higher (29.4%). Genetic results were available for 179 participants. Overall, 11.1% of participants (n=20) had a pathogenic/likely pathogenic mutation identified. Among mutation carriers, 45% (n=9) had mutations in FA genes, including FANCA (n=3), BRCA2 (n=2), FANCM (n=1), FANCD2 (n=1), PALB2 (n=1), and BRIP1 (n=1). Table shows clinical characteristics of participants with mutations in FANCA, FANCM, and FANCD2. Further mutation spectrum included: CHEK2 (n=3), NBN (n=2), MUTYH (n=2), BRCA (n=1), ATM (n=1), HOXB13 (n=1), APC (n=1). Compared to population prevalence, FA mutation prevalence was significantly higher overall (5.0% vs. 0.6%, p = 0.010) and among mutation carriers (45% vs. 0.6%, p<0.001). Conclusions: While prevalence of FA genes BRCA2, PALB2, and BRIP1 is well-described, our study supports testing a broader range of FA genes given the prevalence rates, potential implications for clinical trials, targeted therapy, inherited syndromes, and reproductive implications.[Table: see text]

Abstract Abstract 2798 Poster Board II-774 Multiple Myeloma (MM) is an incurable malignancy of mature clonal B cells. The refractory nature of MM has long been attributed to the acquisition of drug resistance. Traditionally, mechanisms of drug resistance have been defined by acquired changes in the expression or function of specific gene products. To this end, we have recently demonstrated that selected resistance to the cytotoxic agent melphalan correlated with increased expression of components of the Fanconi Anemia (FA)/BRCA DNA repair pathway and a concomitant increase in repair of DNA interstrand cross-links (ICLs).(Hazlehurst et al Cancer Res 2003; Chen et al Blood 2005) Further, the exogenous expression of specific FANC components in RPMI 8226 cell lines enhanced ICL repair, favored the release from melphalan-induced S-phase delay, and rendered these cells partially resistant to melphalan treatment. Together, these results suggest a causal relationship between increased expression of FA DNA repair components, increased DNA repair, and acquired resistance to melphalan. Over the past decade a large body of evidence has emerged demonstrating that in addition to drug resistance mechanisms intrinsic to the cancer cell, there exist dynamic, de novo mechanisms coordinated by the tumor microenvironment resulting in an environment-mediated drug resistance (EM-DR). As such, we examined the potential role of the microenvironment in regulating the FA/BRCA DNA repair pathway. FA pathway protein expression was evaluated with anti-sera to FANCD1/BRCA2, FANCC, FANCD2, FANCI, FANCG and BRCA1 in drug sensitive RPMI 8226 cells and melphalan resistant 8226/LR5 cells in co-culture with the HS-5 bone marrow stromal cell line. With these preliminary results we present three novel findings. First, we demonstrate that expression of FA/BRCA pathway components is regulated by intracellular interactions in both MM cells and bone marrow stromal cells (BMSCs). Second, we show that the acquisition of drug resistance alters FANC protein expression profiles upon co-culture. Third, in the HS-5 BMSCs, mono-ubiquitinated FANCD2 is observed in the absence of detectable FANCG. In RPMI 8226 cells, Western blot analysis demonstrated an acute (within 30minutes) and prolonged (up to 48hours) time-dependent increase in expression of FANCD2/BRCA2, FANCC, FANCD2, and BRCA1 upon incubation with BMSCs relative to MM cells incubated alone. However, no appreciated increases in FANCI or FANCG were noted under the same conditions. Incubation of 8226/LR5s with HS-5 BMSCs demonstrated a slightly different up-regulation of FA/BRCA pathway protein expression with addition of increased FANCI expression and no increase in FANCD2 or FANCC expression. We also examined FANC protein expression in the HS-5 cells. Interestingly, in the BMSCs significant differences were noted in the FANC expression profiles. Co-culture of RPMI 8226 cells with HS-5 cells demonstrated only modest elevations in FANCD2; however, co-culture with drug-resistant 8226/LR5s resulted in increased levels of FANCD2, FANCI and BRCA1. These data indicate that different tumor cells may alternately influence FA/BRCA-mediated DNA repair and potentially drug resistance in juxtaposed bone marrow stroma. Curiously, we also observed mono-ubiquitinated FANCD2 in the absence of any detectable levels of FANCG protein under co-culture conditions. As the FA/BRCA DNA repair pathway has been associated with cell cycle progression, we evaluated cell cycle kinetics under the co-culture conditions. The results of BrdU analysis demonstrated that the observed changes in FA/BRCA protein expression in MM and BMSC could not be fully explained by cell-cycle distribution. Therefore, within this report we demonstrate for the first time that microenvironmental interactions can modulate the FA/BRCA DNA repair pathway in MM and BMSCs. These results suggest that the FA/BRCA DNA damage repair pathway may be an important modulatory component of EM-DR. Importantly, the potential de novo drug resistance likely involves both the MM tumor cell and adjacent stromal cells. Current and future studies will attempt to examine a causal relationship between increased FANC expression and melphalan (and other drug) resistance seen in co-culture conditions, as well as to identify specific signaling molecules and mechanisms controlling the enhanced expression in both cell models. Disclosures: No relevant conflicts of interest to declare.

Abstract Altered DNA repair mechanisms are responsible for survival of leukemia stem cells (LSCs) and/or leukemia progenitor cells (LPCs) accumulating numerous lethal DNA double-strand breaks (DSBs). DSBs resulting from stalled/broken replication forks in proliferating cells are primarily repaired by RAD51-mediated homologous recombination repair (HR), which depends on BRCA1-PALB2-BRCA2-RAD51 paralogs (BRCA pathway), while RAD52 pathway serves as redundant back-up mechanism. Enhanced self-renewal of LSCs and high proliferation rate of LPCs commit them to HR. It has been reported that inhibition of RAD52 either by the knockout, specific shRNA, or a small peptide aptamer induced synthetic lethality in BRCA pathway-deficient tumor cell lines and primary leukemia cells. Yet pharmacological inhibition of RAD52, which binds single-stranded DNA (ssDNA) and lacks enzymatic activity, has not been demonstrated. Here, we applied high-throughput screening and structure-based selection followed by biochemical assays and computer modeling to identify three leading compounds: (1) 20264, (2) RU-0084339, and (3) D-I03. Compound 20264 appeared to interact with the hotspot in RAD52 DNA binding domain 1 to interfere with ssDNA binding. RU-0084339 is a major allosteric inhibitor of RAD52 ssDNA binding domain which disassembles undecamer ring structure of RAD52. D-I03 abrogated RAD52-mediated ssDNA annealing and ssDNA pairing. RAD52 small molecule inhibitor (RAD52smi) reduced recruitment of RAD52 to DNA damage-induced nuclear foci and suppressed RAD52-mediated DNA double-strand break (DSB) repair activity in cells with negligible effects on other DSB repair pathways. Importantly, RAD52smi selectively eliminated cancer cell lines carrying BRCA1/2 inactivating mutations. Since inactivating mutations in BRCA pathway are rare in leukemias, individual BRCA pathway-deficient leukemias were identified by Gene Expression and Mutation Analysis (GEMA). Gene Expression approach applied microarrays, qRT-PCR, and/or flow cytometry to identify individual leukemias displaying downregulation of at least one gene in BRCA pathway. On the other hand, Gene Mutation strategy detected individual leukemias expressing an oncogene causing downregulation of BRCA pathway gene(s) (e.g., BCR-ABL1, MLL-AF9, AML1-ETO - mediated downregulation of BRCA1 and/or BRCA2) and harboring inactivating mutations in BRCA pathway (e.g., BRCA2 = FANCD1, and other FA genes). BRCA-deficient cells from individual patients indentified by GEMA were selectively sensitive to RAD52smi alone or in combination with already approved cytotoxic drugs. RAD52 is a promising new target because it is "druggable" by small molecule inhibitors. Moreover, inhibition of RAD52 by genetic knockout and small peptide aptamer did not exert any major negative effects in normal cells and tissues. Altogether, this work provided foundation for precision medicine guided synthetic lethality in BRCA-deficient leukemias exerted by small molecule inhibitors targeting novel mechanism - RAD52 dependent DSB repair. Disclosures No relevant conflicts of interest to declare.

Abstract Abstract 3372 Enhanced expression of the Fanconi Anemia (FA)/BRCA DNA repair pathway correlates with melphalan-resistance in multiple myeloma (MM) cell lines. Continued investigation demonstrated a bortezomib sensitive RelB/p50-mediated regulation of the FA/BRCA pathway contributed to the observed melphalan resistance.(Yarde et al 2009) The FA/BRCA pathway represents a co-dependent DNA damage response pathway involving thirteen loss of function complementation groups cloned from FA patients. The key functional event of this pathway is the interdependent mono-ubiquitination (Ub) of FANCD2 and FANCI (ID complex) by the E3 Ub-ligase activity of the FA core complex a multimer consisting of 8 FA (FANCA, B, C, E, F, G, L and M) and three non-FA proteins (FAAP100, FAAP24 and HES1). Formation of the core complex and mono-Ub of the ID complex appears to revolve around the flexible adapter protein FANCF. Nuclear localization of the core complex components requires binding of FANCA/G and FANCC/E subcomplexes to the C- terminal domain (CTD) and NTD domains of FANCF, respectively. This complex associates with FANCM:FAAP24 at sites of interstand crosslinks (ICL) via the FANCM-binding domain of FANCF, culminating in ID complex mono-Ub, recruitment of BRCA1, BRCA2/FANCD1, FANCJ and FANCN, and homologous recombination (HR) repair. Reduced function of this pathway has been associated with increased genomic instability, cancer susceptibility, and increased sensitivity to DNA cross-linking agents in FA. However, as predicted by the role of the FA/BRCA pathway in DNA repair, enhanced expression of the FA/BRCA pathway has been shown to play an important role in resistance to agents requiring HR for ICL repair. We next examined expression of this pathway in models of resistance to DNA damaging agents not predicted to utilize FA/BRCA activity. We screened 8226/Dox40 doxorubicin resistant and 8226/MR20 mitoxantrone resistant MM cell lines for expression of the 12 FA/BRCA pathway members with quantitative PCR (qPCR) using customized micro-fluidic cards. Interestingly, in these models of topoisomerase (topo) II inhibitor resistance qPCR demonstrated a 2.6 (p<0.05) and 1.7 (p<0.05) fold over expression of FANCF mRNA relative to drug sensitive RPMI8226 cells. Importantly, mRNA expression of other the eleven FA/BRCA pathway constituents was not increased relative to sensitive cells. To further characterize the relationship between FANCF and doxorubicin resistance, we examined mRNA and protein expression of FANCF in RMPI8226, 8226/Dox6 and 8226/Dox40 MM cell lines (representing progressive levels of doxorubicin resistance). FANCF qPCR demonstrated a 2 and 4.7 fold increased in mRNA expression in the 8226/Dox6 and 8226/Dox40 cell lines, respectively (p= 0.103 and p= 0.034) suggesting that increasing expression of FANCF correlated with increasing dox resistance. A similar doxorubicin resistance- dependent increase in FANCF protein was demonstrated by Western blot analysis of these cell lines. Consistent with mRNA results, FANCD2 or FANCG protein levels remained unchanged in the doxorubicin resistant versus sensitive cell lines These observations suggest that FANCF may contribute to topoII inhibitor-mediated DNA double strand break repair, a process that primarily thought to involve non-homologous end joining (NHEJ) independent of the FA/BRCA pathway. To determine if FANCF expression alone could facilitate doxorubicin resistance, pQCXIP-control or pQCXIP-FANCF constructs were expressed in RPMI8226 sensitive MM cells. MTT assays demonstrated a greater than 2 fold resistance to doxorubicin in FANCF over expressing cells at 48 and 96 hours (IC50: 1.33 ×10−6 and 5.3×10−9M) as compared to control cells (3.26×10−6 and 1.13×10−8M). Taken together, these results indicate that the flexible adaptor protein FANCF may participate in doxorubicin resistance independently of other FA/BRCA members. However, future studies will be needed to elucidate the nature of FANCF in doxorubicin resistance. Disclosures: No relevant conflicts of interest to declare.

We describe an infant female with a syndromic neurodevelopmental clinical phenotype and increased chromosome instability as cellular phenotype. Genotype characterization revealed heterozygous variants in genes directly or indirectly linked to DNA repair: a de novo X-linked HDAC8 pathogenic variant, a paternally inherited FANCG pathogenic variant and a maternally inherited BRCA2 variant of uncertain significance. The full spectrum of the phenotype cannot be explained by any of the heterozygous variants on their own; thus, a synergic contribution is proposed. Complementation studies showed that the FANCG gene from the Fanconi Anaemia/BRCA (FA/BRCA) DNA repair pathway was impaired, indicating that the variant in FANCG contributes to the cellular phenotype. The patient’s chromosome instability represents the first report where heterozygous variant(s) in the FA/BRCA pathway are implicated in the cellular phenotype. We propose that a multigenic contribution of heterozygous variants in HDAC8 and the FA/BRCA pathway might have a role in the phenotype of this neurodevelopmental disorder. The importance of these findings may have repercussion in the clinical management of other cases with a similar synergic contribution of heterozygous variants, allowing the establishment of new genotype–phenotype correlations and motivating the biochemical study of the underlying mechanisms.

Abstract Fanconi anemia is a multi-gene cancer susceptibility and bone marrow failure syndrome. In the current model, at least eight proteins (FANCA, -B-C,-E, -F, -G, -L, -M) are part of a nuclear complex that is required for the S phase and DNA-damage dependent monoubiquitination of FANCD2. This event is thought to functionally link the FA complex proteins to major breast cancer susceptibility proteins BRCA1, BRCA2 (FANCD1), and the BRCA1-associated helicase Brip1(FANCJ). An understanding of the function of the FA protein network is incomplete not only because some FA proteins are still unidentified, but also because the functions of individual proteins may be interdependent and are difficult to assess out of context with the entire FA network. We recently developed a cell-free system to evaluate the function of the Fanconi/BRCA pathway proteins in an S phase context in Xenopus egg extracts (Sobeck, et al. 2006). Egg extracts are naturally cell-cycle synchronized and mimic the complex interplay of proteins that support cellular DNA replication and regulated DNA damage checkpoint activation. Intricate protein interactions can be assayed in egg extracts, even without knowing each of the components if there is a measurable endpoint. We tested the hypothesis that the mobility shift of FANCD2 could be used as an endpoint in cell-free assays to determine FA pathway function. We found that an antibody specific for the Xenopus FANCD2 protein detected a single band of the expected size in western blots of proteins separated by SDS-PAGE from unstimulated egg extracts. Addition of DNA substrates to extracts resulted in the appearance of a slower mobility form of FANCD2, consistent with the monoubiquitinated FANCD2-L isoform observed in human cells following DNA damage. We measured inhibition or stimulation of xFANCD2-L in the presence of a series of candidate compounds. We found compounds that inhibit FANCD2-L, including curcumin, which was also identified in a cell-based assay as an inhibitor of FANCD2-L (Chirnomas, et al., 2006). Thus, this cell-free assay successfully mirrors the outcome obtained with a small molecule inhibitor of the FA/BRCA pathway in cell-based assays. This new approach is an improvement relative to cell-based screens because the extracts are fully synchronized, which maximizes the sensitivity of detection of S-phase events. Moreover, cell-free screens are rapid, inexpensive and well suited for semi- or high-throughput methods to identify small molecules that modulate the FA/BRCA DNA-damage response pathway.

The proteins from the Fanconi Anemia (FA) pathway of DNA repair maintain DNA replication fork integrity by preventing the unscheduled degradation of nascent DNA at regions of stalled replication forks. Here, we ask if the bacterial pathogen H. pylori exploits the fork stabilisation machinery to generate double stand breaks (DSBs) and genomic instability. Specifically, we study if the H. pylori virulence factor CagA generates host genomic DSBs through replication fork destabilisation and collapse. An inducible gastric cancer model was used to examine global CagA-dependent transcriptomic and proteomic alterations, using RNA sequencing and SILAC-based mass spectrometry, respectively. The transcriptional alterations were confirmed in gastric cancer cell lines infected with H. pylori. Functional analysis was performed using chromatin fractionation, pulsed-field gel electrophoresis (PFGE), and single molecule DNA replication/repair fiber assays. We found a core set of 31 DNA repair factors including the FA genes FANCI, FANCD2, BRCA1, and BRCA2 that were downregulated following CagA expression. H. pylori infection of gastric cancer cell lines showed downregulation of the aforementioned FA genes in a CagA-dependent manner. Consistent with FA pathway downregulation, chromatin purification studies revealed impaired levels of Rad51 but higher recruitment of the nuclease MRE11 on the chromatin of CagA-expressing cells, suggesting impaired fork protection. In line with the above data, fibre assays revealed higher fork degradation, lower fork speed, daughter strands gap accumulation, and impaired re-start of replication forks in the presence of CagA, indicating compromised genome stability. By downregulating the expression of key DNA repair genes such as FANCI, FANCD2, BRCA1, and BRCA2, H. pylori CagA compromises host replication fork stability and induces DNA DSBs through fork collapse. These data unveil an intriguing example of a bacterial virulence factor that induces genomic instability by interfering with the host replication fork stabilisation machinery.

2012/2013
L’anemia di Fanconi (FA) è una malattia genetica rara caratterizzata da malformazioni congenite, pancitopenia, predisposizione al cancro e aumentata sensibilità ad agenti, quali diepossibutano e mitomicina C, che formano legami tra i due filamenti di DNA. La FA è causata da almeno 16 geni che costituiscono, insieme ad altri componenti, un pathaway di riparazione del DNA. L’eterogeneità è uno dei principali motivi che complica la diagnosi molecolare della FA. E’ pertanto necessario un processo a più livelli che implica lo screening di molti esoni o, in alternativa, l’allestimento di linee cellulari e l’analisi di complementazione per la caratterizzazione del gene candidato. Gli scopi di questa tesi pertanto sono diretti a: • Ridurre i tempi per l’identificazione del gene mutato sostituendo l’analisi di complementazione con quella di espressione delle proteine FA basandosi sul presupposto che prodotti mutati siano rapidamente degradati; • Caratterizzare dal punto di vista molecolare gli effetti delle varianti identificate dall’analisi di sequenza. Per quanto riguarda il primo obiettivo, ci siamo focalizzati sullo studio della proteina FANCA in 44 linee cellulari linfoblastoidi appartenenti ai diversi gruppi di complementazione. E’ emerso che, fatta eccezione per FA-G, l’espressione di FANCA non è alterata da mutazioni nei geni FANCB, FANCC e FANCD2. Per quanto riguarda i pazienti con mutazioni in FANCA, invece, abbiamo osservato una correlazione tra il tipo di mutazione e il livello di espressione della proteina che può quelli essere paragonabile a quella dei controlli nel caso di mutazioni missenso o ampie delezioni in frame. In accordo con l’ipotesi invece, in presenza di mutazioni nonsenso e frameshift in entrambi gli alleli del gene, non si ha produzione di proteina. Sulla base di questi dati possiamo concludere che l’analisi di FANCA non è soddisfacente per assegnare ai pazienti il corrispondente gruppo di complementazione. Tuttavia, da questo studio è emersa l’ipotesi di un’associazione tra l’espressione stabile delle proteine FANCA mutate e un fenotipo meno grave nei pazienti. I dati preliminari dimostrano che queste proteine non sono traslocate nel nucleo e che quindi un’eventuale attività residua non sia da attribuire al processo di riparazione del DNA. Un potenziale ruolo andrebbe forse indagato a livello citoplasmatico dove, come sta emergendo dalla letteratura, almeno FANCG e FANCC, svolgono una funzione all’interno del mitocondrio tale da giustificare l’elevato grado di stress ossidativo delle cellule FA. Per il secondo obiettivo, lo studio dei casi arruolati nell'ambito dell'AIEOP (Associazione Italiana Ematologia Oncologia Pediatrica) ha consentito l'identificazione delle mutazioni in 100 famiglie. Dall’analisi dei dati emerge che la maggior parte delle mutazioni colpisce il gene FANCA (85%), seguito da FANCG (9%), FANCC (3%), FANCD2 (2%) e FANCB (1%). In assenza del dato di complementazione e/o in presenza di varianti alle quali non è sempre possibile attribuire un chiaro effetto patogenetico, sono state eseguite ulteriori indagini. Si citano a titolo di esempio la caratterizzazione delle ampie delezioni intrageniche mediante MLPA, l’analisi bioinformatica e a livello di RNA delle alterazioni di splicing che, qualora in frame, sono state ulteriormente confermate anche a livello proteico e, infine, lo studio bioinformatico di patogenicità delle sostituzioni aminoacidiche. La formulazione di un algoritmo efficace e rapido per la diagnosi molecolare della FA, nonché la chiara definizione del significato patogenetico delle varianti identificate, è molto importante per corretta presa in carico del paziente e della famiglia sia per l’identificazione dei portatori che per la diagnosi prenatale.
XXVI Ciclo
1984

The Fanconi Anemia (FA)/BRCA DNA damage repair pathway plays a critical role in the cellular response to stress induced by DNA alkylating agents and greatly influences drug response in cancer treatment. We recently reported that FA/BRCA DNA damage repair pathway genes are overexpressed and causative for resistance in multiple myeloma (MM) cell lines selected for resistance to melphalan. We hypothesized that the FA/BRCA DNA damage repair pathway mediates response and resistance to chemotherapeutic agents used to treat multiple myeloma and other cancers, and targeting this pathway is vital to overcoming drug resistance. In this dissertation, we show that FA/BRCA pathway genes are collectively overexpressed in MM, prostate, and ovarian cancer cell lines selected for resistance to melphalan and cisplatin, respectively. Interestingly, cells selected for resistance to topoisomerase II inhibitors selectively overexpress only FANCF. We also show that FA/BRCA pathway expression can be inhibited by the proteasome inhibitor bortezomib. FA/BRCA pathway mRNA expression was inhibited by bortezomib in myeloma cell lines and patient samples. FANCD2 gene and protein expression are downregulated by bortezomib, and remain attenuated in the face of melphalan treatment. Melphalan-induced FANCD2 foci formation was also inhibited by bortezomib, and this drug enhanced melphalan-induced DNA damage, likely via inhibition of FA-mediated DNA damage repair. Next, we analyzed regulation of the FA/BRCA pathway. We demonstrate that NF-kappaB, specifically the Re1B/p50 subunits, transcriptionally regulates members of the FA/BRCA pathway, and inhibition of these subunits by siRNA, BMS-345541, and bortezomib reduces FA/BRCA pathway expression. Furthermore, knocking down Re1B and p50 simultaneously attenuates FANCD2 protein expression and results in diminished DNA repair and enhanced sensitivity to melphalan. Importantly, melphalan resistance was restored when FANCD2 was re-expressed in these cells. We also show that bortezomib regulates FANCD2 protein expression directly, by inhibiting FANCD2 synthesis. Finally, we demonstrate that low-dose bortezomib arrests cells in G0/G1 and also overcomes the S-phase arrest induced by melphalan, likely via inhibition of ATR. Overall, our findings provide evidence for targeting the FA/BRCA pathway, either directly or indirectly, via inhibition of NF-kappaB or ATR, to enhance chemotherapeutic response and reverse drug resistance in multiple myeloma and other cancers.

In the context of this thesis, I investigated the molecular causes and functional consequences of genetic instability using a human inherited disease, Fanconi anemia. FA patients display a highly variable clinical phenotype, including congenital abnormalities, progressive bone marrow failure and a high cancer risk. The FA cellular phenotype is characterized by spontaneous and inducible chromosomal instability, and a typical S/G2 phase arrest after exposure to DNA-damaging agents. So far, 13 genes have been identified, whose biallelic (or, in the case of X-linked FANCB, hemizygous) mutations cause this multisystem disorder. The FA proteins interact in a multiprotein network, instrumental and essential in the cellular response to DNA damage. A more comprehensive summary of Fanconi anemia and its myriad clinical, cellular and molecular manifestations is provided in the introduction section of this thesis. The results of my experimental work are presented as published papers and manuscripts ready to be submitted. In the first publication, I investigated the connection between FA genes and bladder tumors. The question I tried to answer was whether a disruption of the FA/BRCA pathway may be a frequent and possibly causal event in bladder cancer, explaining the hypersensitivity of these cells to DNA-crosslinking agents. On the basis of my experimental data I arrived at the conclusion that disruption of the FA/BRCA pathway might be detrimental rather than advantageous for the majority tumor types by rendering them vulnerable towards DNA damaging agents and oxidative stress. The second publication deals with the gene coding for the core complex protein FANCE and tries to answer the question why FANCE is so rarely affected among FA-patients. The conclusion from these studies is that like FANCF, FANCE functions as a probable adaptor protein with a high tolerance towards amino acid substitutions which would explain the relative rareness of FA-E patients. I have also investigated the FANCL gene whose product functions as the catalytic subunit of the E3 ligase. The third publication addresses this issue by providing the first comprehensive description of genetic alterations and phenotypic manifestations in a series of three FA-L patients. The results of my study show that genetic alterations of FANCL are compatible with survival, these alterations may include large deletions such as so far common only in the FANCA gene, FA-L phenotypes can be mild to severe, and FANCL belongs to the group of FA genes that may undergo somatic reversion. The central protein of the FA/BRCA network, FANCD2, is the subject of the fourth publication presented in this thesis. Most importantly, we were able to show that there are no biallelic null mutations in FANCD2. Correspondingly, residual protein of both FANCD2-isotypes (FANCD2-S and FANCD2-L) was present in all available patient cell lines. This suggests that complete abrogation of the FANCD2 protein cannot be tolerated and causes early embryonic lethality. There are at least three FA proteins that are not required for the posttranslational modification of FANCD2. One of these proteins is the 5’-3’ helicase BRIP1 (BRCA1-interacting protein 1), a protein that interacts directly with the breast cancer susceptibility protein BRCA1. I participated in the identification of BRIP1 as the FA protein FANCJ. This discovery is described in the fifth publication of this thesis. The newly discovered protein BRIP1/FANCJ seems to act as one of the mediators of genomic maintenance downstream of FANCD2. Another protein identified downstream of FANCD2 is PALB2. PALB2 was originally discovered as “partner and localizer of BRCA2”. In a candidate gene approach we tested patients with early childhood cancers but without mutations in BRCA2 for mutations in PALB2 (publication 6). PALB2 was identified as a novel FA gene and designated FANCN. FA-N patients are very severely affected. The last publication included in my thesis describes the identification of the FA gene FANCI as the second monoubiquitinated member of the FA/BRCA pathway (publication 7). We identified biallelic mutations in KIAA1794 in four FA patients, thus proving the genuine FA-nature of this candidate sequence. The general discussion provides a synopsis of the results and conclusions of my work with the state of art of FA research
Im Rahmen der vorliegenden Dissertation wurden molekulare Ursachen und funktionale Konsequenzen genetischer Instabilität am Beispiel der menschlichen Erbkrankheit Fanconi Anämie (FA) untersucht. FA Patienten zeigen einen sehr variablen klinischen Phänotyp, der in der Regel angeborene Fehlbildungen, progressives Knochenmarkversagen und ein hohes Risiko für Tumorerkrankungen beinhaltet. Der zelluläre Phänotyp der FA ist durch eine spontane und induzierbare chromosomale Instabilität und einen typischen S/G2-Phasen-Arrest nach Exposition mit DNA-schädigenden Agentien charakterisiert. Biallelische oder -im Fall des X-chromosomalen FANCB- hemizygote Mutationen, die zu dieser Erkrankung führen, wurden in bislang 13 Genen identifiziert. Die FA Proteine arbeiten in einem gemeinsamen Netzwerk und sind essentiell beteiligt an der zellulären Antwort auf DNA Schädigung. Eine umfassendere Übersicht über Fanconi Anämie und ihre vielfältigen klinischen, zellulären und molekularen Erscheinungsformen ist in der Einleitung dieser Dissertation gegeben. Die Ergebnisse meiner experimentellen Arbeiten sind in Form von publizierten Fachartikeln und fertigen Manuskripten dargestellt. In der ersten Publikation habe ich den Zusammenhang von FA Genen und Harnblasentumoren untersucht. Die Frage, die ich zu beantworten versucht habe, war, ob ein Defekt im FA/BRCA Weg eine mögliche Ursache für die Entstehung von Blasentumoren sein könnte. Aufgrund meiner experimentellen Daten bin ich zu dem Schluss gekommen, dass ein Defekt im FA/BRCA Weg für einen Tumor vermutlich eher schädlich als vorteilhaft ist, da so ein Defekt den Tumor gegenüber DNA-schädigenden Agentien und oxidativem Stress anfällig machen würde. Meine zweite Publikation befasst sich mit dem Kern-Komplex Protein FANCE und versucht die Frage zu beantworten, warum das FANCE Gen in so wenigen FA Patienten betroffen ist. Die Schlussfolgerung dieser Arbeit war, dass FANCE vermutlich genauso wie FANCF im Kern-Komplex die Rolle eines Adaptor-Proteins mit einer hohen Toleranz gegenüber Aminosäure-Austauschen innehat, was die relative Seltenheit von Patienten dieser Untergruppe erklären könnte. Ich habe weiterhin das FANCL Gen untersucht, dessen Produkt als katalytische Untereinheit der E3-Ligase fungiert. Die dritte Publikation in dieser Dissertation befasst sich mit diesem Thema und enthält eine umfassende Beschreibung von genetischen Veränderungen und phänotypischen Auswirkungen in einer Gruppe von 3 FA-L Patienten. Die Ergebnisse meiner Arbeit haben allerdings gezeigt, dass genetische Veränderungen in FANCL mit dem Leben vereinbar sind, dass diese Veränderungen sehr große Deletionen beinhalten können, was bisher nur für FANCA gezeigt werden konnte, dass FA-L Phänotypen von mild bis schwer betroffen reichen können und dass FANCL zu den Genen gehört, in denen somatische Reversionen stattfinden. Das Schlüsselprotein des FA/BRCA Netzwerks, FANCD2, ist das Thema der vierten Publikation in dieser Dissertation. Insbesondere konnten wir zeigen, dass es keine biallelischen Nullmutationen in FANCD2 zu geben scheint. Dementsprechend war Restprotein von beiden FANCD2-Isoformen, FANCD2-L und FANCD2-S, in allen verfügbaren Patienten-Zelllinien nachweisbar. Dies ließ vermuten, dass ein komplettes Fehlen des FANCD2 Proteins nicht tolerierbar ist und frühe embryonale Letalität verursacht. Es mindestens drei Proteine, die nicht für diese posttranslationale Modifikation benötigt werden. Eines dieser Proteine ist die 5’-3’ Helikase BRIP1 (BRCA1-interagierendes Protein 1), ein Protein, das direkt mit dem Brustkrebs-assoziierten Protein BRCA1 interagiert. Ich war an der Identifizierung von BRIP1 als FA Protein (FANCJ) beteiligt. Diese Entdeckung ist in der fünften Publikation meiner Dissertation beschrieben. Das neu entdeckte Protein BRIP1/FANCJ, das direkt mit BRCA1 interagiert, scheint als einer der Mediatoren zur Aufrechterhaltung genomischer Stabilität downstream von FANCD2 zu wirken. Ein weiteres Protein downstream von FANCD2 ist PALB2. PALB2 wurde ursprünglich als „Partner und Lokalisierer von BRCA2“ entdeckt. In einer Kandidatengen-Studie haben wir Patienten mit frühkindlichen Tumoren, aber ohne Mutationen in BRCA2, auf Mutationen in PALB2 untersucht (Publikation 6). Aufgrund unserer Ergebnisse haben wir PALB2 als ein neues FA Gen identifiziert und haben es FANCN genannt. Genauso wie FA-D1 Patienten sind FA-N Patienten sehr schwer betroffen. Die letzte Publikation meiner Dissertation beschreibt die Identifikation des FA Genes FANCI, dessen Produkt das zweite monoubiquitinierte Mitglied des FA/BRCA Weges darstellt (Publikation 7). Wir haben in vier Patienten biallelische Mutationen in KIAA1794 gefunden, und so zeigen können, dass KIAA1794 wirklich ein FA Gen ist. Die generelle Diskussion birgt eine Synopsis der Ergebnisse und Schlussfolgerungen meiner Forschung mit dem aktuellen Wissensstand über FA