The Fanconi anemia complementation group C gene (FAC) suppresses transformation of mutant fibroblasts by the SV40 virus

Fanconi anemia neuroinflammatory syndrome: brain lesions and neurologic injury in Fanconi anemia

ABSTRACT

Fanconi anemia (FA) is a complex inherited bone marrow failure syndrome characterized by chromosomal instability and defective DNA repair, causing sensitivity to DNA interstrand crosslinking agents. Our understanding of the full adult phenotype of the disease continues to evolve, because most patients with FA died of marrow failure in the first decade of life before more recent advances in allogeneic hematopoietic cell transplantation. Herein, we report a previously undescribed, clinically concerning, progressive neurologic syndrome in patients with FA. Nine nonimmunosuppressed pediatric patients and young adults with FA presented with acute and chronic neurological signs and symptoms associated with distinct neuroradiological findings. Symptoms included, but were not limited to, limb weakness, papilledema, gait abnormalities, headaches, dysphagia, visual changes, and seizures. Brain imaging demonstrated a characteristic radiographic appearance of numerous cerebral and cerebellar lesions with associated calcifications and often a dominant ring-enhancing lesion. Tissue from the dominant brain lesions in 4 patients showed nonspecific atypical glial proliferation, and a small number of polyomavirus-infected microglial cells were identified by immunohistochemistry in 2 patients. Numerous interventions were pursued across this cohort, in general with no improvement. Overall, these patients demonstrated significant progressive neurologic decline. This cohort highlights the importance of recognizing FA neuroinflammatory syndrome, which is distinct from malignancy, and warrants careful ongoing evaluation by clinicians.

Fanconi anemia patients are more susceptible to infection with tumor virus SV40

Fanconi anemia (FA) is a recessive DNA repair disease characterized by a high predisposition to developing neoplasms. DNA tumor polyomavirus simian virus 40 (SV40) transforms FA fibroblasts at high efficiency suggesting that FA patients could be highly susceptible to SV40 infection. To test this hypothesis, the large tumor (LT) antigen of SV40, BKV, JCV and Merkel Cell (MC) polyomaviruses were tested in blood samples from 89 FA patients and from 82 of their parents. Two control groups consisting of 47 no-FA patients affected by other genetic bone marrow failure diseases and 91 healthy subjects were also evaluated. Although JCV, BKV and MC were not found in any of the FA samples, the prevalence and viral load of SV40 were higher in FA patients (25%; mean viral load: 1.1×10² copies/10⁵cells) as compared with healthy individuals (4.3%; mean viral load: 0.8×10¹ copies/10⁵cells) and genetic controls (0%) (p<0.005). A marked age-dependent frequency of SV40 was found in FA with respect to healthy subjects suggesting that, although acquired early in life, the virus can widespread more easily in specific groups of population. From the analysis of family pedigrees, 60% of the parents of SV40-positive probands were positive for the virus compared to 2% of the parents of the SV40-negative probands (p<0.005). It is worthy of note that the relative frequency of SV40-positive relatives detected in this study was the highest ever reported, showing that asymptomatic FA carriers are also more susceptible to SV40. In conclusion, we favor the hypothesis that SV40 spread could be facilitated by individuals who are genetically more susceptible to infection, such as FA patients. The increased susceptibility to SV40 infection seems to be associated with a specific defect of the immune system which supports a potential interplay of SV40 with an underlying genetic alteration that increases the risk of malignancies.

Upregulated ATM gene expression and activated DNA crosslink-induced damage response checkpoint in Fanconi anemia: implications for carcinogenesis

Fanconi anemia (FA) predisposes to hematopoietic failure, birth defects, leukemia, and squamous cell carcinoma of the head and neck (HNSCC) and cervix. The FA/BRCA pathway includes 8 members of a core complex and 5 downstream gene products closely linked with BRCA1 or BRCA2. Precancerous lesions are believed to trigger the DNA damage response (DDR), and we focused on the DDR in FA and its putative role as a checkpoint barrier to cancer. In primary fibroblasts with mutations in the core complex FANCA protein, we discovered that basal expression and phosphorylation of ATM (ataxia telangiectasia mutated) and p53 induced by irradiation (IR) or mitomycin C (MMC) were upregulated. This heightened response appeared to be due to increased basal levels of ATM in cultured FANCA-mutant cells, highlighting the new observation that ATM can be regulated at the transcriptional level in addition to its well-established activation by autophosphorylation. Functional analysis of this response using gamma-H2AX foci as markers of DNA double-stranded breaks (DSBs) demonstrated abnormal persistence of only MMC- and not IR-induced foci. Thus, we describe a processing defect that leads to general DDR upregulation but specific persistence of DNA crosslinker-induced damage response foci. Underscoring the significance of these findings, we found resistance to DNA crosslinker-induced cell cycle arrest and apoptosis in a TP53-mutant, patient-derived HNSCC cell line, whereas a lymphoblastoid cell line derived from this same individual was not mutated at TP53 and retained DNA crosslinker sensitivity. Our results suggest that cancer in FA may arise from selection for cells that escape from a chronically activated DDR checkpoint.

SV40 large T-antigen disturbs the formation of nuclear DNA-repair foci containing MRE11

The accumulation of DNA repair proteins at the sites of DNA damage can be visualized in mutagenized cells at the single cell level as discrete nuclear foci by immunofluorescent staining. Formation of nuclear foci in irradiated human fibroblasts, as detected by antibodies directed against the DNA repair protein MRE11, is significantly disturbed by the presence of the viral oncogene, SV40 large T-antigen. The attenuation of foci formation was found in both T-antigen immortalized cells and in cells transiently expressing T-antigen, indicating that it is not attributable to secondary mutations but to T-antigen expression itself. ATM-mediated nibrin phosphorylation was not altered, thus the disturbance of MRE11 foci formation by T-antigen is independent of this event. The decrease in MRE11 foci was particularly pronounced in T-antigen immortalized cells from the Fanconi anaemia complementation group FA-D2. FA-D2 cells produce essentially no MRE11 DNA repair foci after ionizing irradiation and have a significantly increased cellular radiosensitivity at low radiation doses. The gene mutated in FA-D2 cells, FANCD2, codes for a protein which also locates to nuclear foci and may, therefore, be involved in MRE11 foci formation, at least in T-antigen immortalized cells. This finding possibly links Fanconi anaemia proteins to the frequently reported increased sensitivity of Fanconi anaemia cells to transformation by SV40. From a practical stand point these findings are particularly relevant to the many studies on DNA repair which exploit the advantages of SV40 immortalized cell lines. The interference of T-antigen with DNA repair processes, as demonstrated here, should be borne in mind when interpreting such studies.

Intracellular localization of the Fanconi anemia complementation group A protein

Mutations in the Fanconi anemia (FA) complementation group A (FANCA) gene leads to bone marrow failure, developmental abnormalities and cancer predisposition. To map the intracellular site of FANCA, we constructed a plasmid vector which linked in-frame the enhanced green fluorescent protein (EGFP cDNA) to the 5' end of the FANCA cDNA (pDAS-3). We studied the expression of pDAS-3 in the FANCA mutant fibroblast cell line (GM6914). MMC sensitivity of pDAS-3 transfected cells was comparable to wild-type fibroblasts. The resulting fluorescence pattern in the stable pDAS-3 cell line expressing the fusion protein was primarily nuclear. EGFP-selected cells (lacking FANCA) remain hypersensitive to MMC and maintained a cytoplasmic fluorescence pattern. Using deletion mutants of pDAS-3, a nuclear localization domain was identified at the amino terminus of the polypeptide. Western blot results of FANCA protein confirmed the presence of FANCA in nuclear fractions and FANCA protein levels did not vary during cell cycling. This nuclear trafficking of FANCA should guide future work in defining the function of this protein.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

DNA Cross-Linker–Induced G2/M Arrest in Group C Fanconi Anemia Lymphoblasts Reflects Normal Checkpoint Function

Cells from individuals with Fanconi anemia (FA) arrest excessively in the G2/M cell cycle compartment after exposure to low doses of DNA cross-linking agents. The relationship of this abnormality to the fundamental genetic defect in such cells is unknown, but many investigators have speculated that the various FA genes directly regulate cell cycle checkpoints. We tested the hypothesis that the protein encoded by the FA group C complementing gene (FAC) functions to control a cell cycle checkpoint and that cells from group C patients (FA[C]) have abnormalities of cell cycle regulation directly related to the genetic mutation. We found that retroviral transduction of FA(C) lymphoblasts with wild-type FAC cDNA resulted in normalization of the cell cycle response to low-dose mitomycin C (MMC). However, when DNA damage was quantified in terms of cytogenetic damage or cellular cytotoxicity, we found similar degrees of G2/M arrest in response to equitoxic amounts of MMC in FA(C) cells as well as in normal lymphoblasts. Similar results were obtained using isogenic pairs of uncorrected, FAC- or mock-corrected (neo only) FA(C) cell lines. To test the function of other checkpoints we examined the effects of hydroxyurea (HU) and ionizing radiation on cell cycle kinetics of FA(C) and normal lymphoblasts as well as with isogenic pairs of uncorrected, FAC-corrected, or mock-corrected FA(C) cell lines. In all cases the cell cycle response of FA(C) and normal lymphoblasts to these two agents were identical. Based on these studies we conclude that the aberrant G2/M arrest that typifies the response of FA(C) cells to low doses of cross-linking agents does not represent an abnormal cell cycle response but instead represents a normal cellular response to the excessive DNA damage that results in FA(C) cells following exposure to low doses of cross-linking agents.

Molecular biology of Fanconi anemia

Fanconi anemia (FA) is a rare, autosomal recessive disease characterized by multiple congenital abnormalities, bone marrow failure, and cancer susceptibility. Although traditionally described as a classic clinical syndrome, as more is discovered regarding its basic molecular and cell biology, FA is emerging as a true premalignant syndrome. Two of the genes of the five known complementation groups have been cloned, and work to understand their function is underway. Further understanding of these gene products has lent new ideas concerning modes of novel therapy, including gene therapy. The impact of molecular biology on our understanding of basic biology and the clinical care of FA patients is discussed.

Elevated homologous recombination activity in fanconi anemia fibroblasts

It is widely believed that Fanconi anemia cells possess a reduced ability to repair inter-strand DNA cross-links. While the mechanism through which inter-strand DNA cross-links are removed from mammalian chromosomes is unknown, these lesions are repaired via homologous recombination in lower eukaryotes and bacteria. Based on the hypothesis that a similar mechanism of DNA repair functions in mammalian somatic cells, we measured homologous recombination activity in diploid fibroblasts from healthy donors, and Fanconi anemia patients. Somewhat surprisingly, homologous recombination levels in nuclear protein extracts prepared from Fanconi anemia cells were nearly 100-fold higher than in extracts prepared from control cells. We observed a similar increase in the activity of a 100-kDa homologous DNA pairing protein in extracts from Fanconi anemia cells. Transfection studies confirmed that plasmid homologous recombination levels in intact Fanconi anemia cells were substantially elevated, compared with control cells. These results suggest that inappropriately elevated levels of homologous recombination activity may contribute to the genomic instability and cancer predisposition that characterize Fanconi anemia.