Molecular genetics and Fanconi anaemia: new insights into old problems

Protective effect of acetyl-l-carnitine and α-lipoic acid against the acute toxicity of diepoxybutane to human lymphocytes

The biotransformation and oxidative stress may contribute to 1,2:3,4-diepoxybutane (DEB)-induced toxicity to human lymphocytes of Fanconi Anemia (FA) patients. Thus, the identification of putative inhibitors of bioactivation, as well as the determination of the protective role of oxidant defenses, on DEB-induced toxicity, can help to understand what is failing in FA cells. In the present work we studied the contribution of several biochemical pathways for DEB-induced acute toxicity in human lymphocyte suspensions, by using inhibitors of epoxide hydrolases, inhibitors of protective enzymes as glutathione S-transferase and catalase, the depletion of glutathione (GSH), and the inhibition of protein synthesis; and a variety of putative protective compounds, including antioxidants, and mitochondrial protective agents. The present study reports two novel findings: (i) it was clearly evidenced, for the first time, that the acute exposure of freshly isolated human lymphocytes to DEB results in severe GSH depletion and loss of ATP, followed by cell death; (ii) acetyl-l-carnitine elicits a significant protective effect on DEB induced toxicity, which was potentiated by α-lipoic acid. Collectively, these findings contribute to increase our knowledge of DEB-induce toxicity and will be very useful when applied in studies with lymphocytes from FA patients, in order to find out a protective agent against spontaneous and DEB-induced chromosome instability.

Cisplatin triggers apoptotic or nonapoptotic cell death in Fanconi anemia lymphoblasts in a concentration-dependent manner

Cells derived from Fanconi anemia (FA) patients are hypersensitive for cross-linking agents, such as cisplatin, that are potent inducers of programmed cell death (PCD). Here, we studied cisplatin hypersensitivity in FA in relation to the mechanism of PCD in lymphoblastoid cells representing FA groups A and C. In FA cells, a low concentration of cisplatin caused chromatin condensation, phosphatidylserine (PS) externalization, and the expression of an 18-kDa variant of Bax, all indicators of apoptotic cell death, and the latter suggesting the involvement of a mitochondrial route. However, procaspases-3, -8, and -9, and PARP were not cleaved, although small increases in caspase activity could be detected. At a high concentration of cisplatin, both FA and corrected cells showed a robust cleavage of procaspases and PARP. DNA fragmentation was clearly visible under high cisplatin conditions and to some extent at a low concentration in FA-A cells, but not in the FA-C cell line regardless of the presence of functional FANCC, suggesting an unknown deficiency in these cells. We conclude that hypersensitivity in FA cells is associated with a mixture of necrotic and apoptotic features that is best described as apoptotic-like cell death, and that a defective FA pathway does not interfere with the proper activation of caspase-mediated cell death.

Mitochondrial alterations in fanconi anemia fibroblasts following ultraviolet A or psoralen photoactivation

The genetic disease Fanconi anemia (FA), generally considered to be a DNA repair defect, has also been related to a deficiency in cellular defense against reactive oxygen species (ROS). Results show that mitochondrial matrix densification occurs rapidly and transiently in FA fibroblasts following 8-methoxypsoralen (8-MOP) photoreaction or ultraviolet A (320 to 380 nm) (UVA) irradiation. This effect is oxygen dependent because it is more important under 20 than under 5% oxygen tension. In contrast, in normal fibroblasts very little, if any, densification of mitochondrial matrix is induced by treatments even at the highest oxygen tension. The changes in matrix density in FA cells are accompanied by some modifications in transmembrane potential, linked to a Fenton-like reaction, and in mitochondrial cardiolipin content, differing from the responses of normal cells. These data are indicative of some sort of membrane damage induced by 8-MOP photoreaction and UVA irradiation, to which FA cells appear to be particularly sensitive.

Futile caspase-8 activation during the apoptotic cell death induced by DNA damaging agents in human B-lymphoblasts

Caspase-8 plays an essential role in apoptosis induced by Fas activation. Moreover, caspase-8 can be processed also in response to exposure to genotoxic agents. To decipher the role of caspase-8 in DNA damaging agent (DDA)-induced apoptosis as well as the pathway(s) leading to its activation in response to genotoxic stress, we investigated caspase-8 processing induced by ionizing radiation (IR) or mitomycin C (MMC) treatment in human B-lymphoblasts. Altogether, our observations establish that caspase-8 is actively processed in both receptor-mediated and DDA-induced cell death. However, while Fas-dependent apoptosis absolutely required caspase-8 activity, it is not necessary for completion of the apoptotic program induced by IR and MMC. Experiments performed to understand the molecular pathway(s) of the caspase-8 activation after DDA demonstrated that for both IR and MMC, the Fas/Fas-L interaction is dispensable. Data obtained from caspase inhibitors and from lymphoblasts carrying mutations in ATM and FANCC proteins, involved in DDA response, clearly showed that distinct mechanisms are responsible for caspase-8 activation by IR and MMC in B-lymphoblasts. IR-dependent processing of caspase-8 involves ATM, mitochondrial collapse, FANCC, and caspase-3 activation. Caspase-8 activation by MMC evokes the mitochondrial pathways involving FANCC but not ATM. Collectively, our data indicate that caspase-8 activation is essentially a bystander effect and not a major determinant of the behavior of DDA-exposed cells.

Repair of DNA interstrand cross-links

DNA interstrand cross-links (ICLs) are very toxic to dividing cells, because they induce mutations, chromosomal rearrangements and cell death. Inducers of ICLs are important drugs in cancer treatment. We discuss the main properties of several classes of ICL agents and the types of damage they induce. The current insights in ICL repair in bacteria, yeast and mammalian cells are reviewed. An intriguing aspect of ICLs is that a number of multi-step DNA repair pathways including nucleotide excision repair, homologous recombination and post-replication/translesion repair all impinge on their repair. Furthermore, the breast cancer-associated proteins Brca1 and Brca2, the Fanconi anemia-associated FANC proteins, and cell cycle checkpoint proteins are involved in regulating the cellular response to ICLs. We depict several models that describe possible pathways for the repair or replicational bypass of ICLs.

[Fanconi's anemia and molecular biology research]

Fanconi's anemia is a rare autosomal recessive disease characterized by congenital abnormalities, a progressive pancytopenia and a predisposition to cancer. The diagnosis is based on an abnormal increase of spontaneous chromosome breakage, more specifically on a clear-cut increase of chromosome breakage in the presence of bifunctional alkylating agents. Eight complementation groups (A to H) have been defined, and the genes corresponding to four of these groups have been cloned (FANCA, FANCC, FANCF and FANCG). The function of the proteins encoded by the genes of Fanconi's anemia remains unknown. Numerous studies indicate that different cellular processes are probably involved, including DNA repair pathways, apoptosis, cell cycle regulation and oxygen metabolism. Nevertheless, the exact cellular and molecular mechanisms implicated in Fanconi's anemia remain a challenge for fundamental research. The treatment of Fanconi's anemia is also the subject of intense research, bearing principally upon bone marrow transplantation, which is successful in the case of HLA-identical sibling donors, and gene therapy, which is still at a preliminary stage on the clinical level.

Practicing pediatric pathology without a microscope

This article highlights changes in the field of pediatric pathology that have resulted from technical advances in prenatal diagnostics, immunohistochemistry, cytogenetics, and molecular genetics. The relatively new and growing need for specialized training in fetal pathology is used as an example. Comprehensive evaluation of human fetuses has become a requisite skill for many diagnostic pathologists, in part because contemporary prenatal diagnostic techniques have shifted the demographics of many congenital conditions from spontaneous term delivery to mid-gestation termination of pregnancy. The information provided by the pathologist has a tremendous impact for families and clinicians as they consider recurrence risks in future pregnancies. As most specimens from therapeutic terminations have gross dysmorphology, which may or may not constitute a recognizable pattern of human malformation, their analysis requires additional skills and methods that were traditionally the domain other specialists (e.g., medical geneticists). The pathologist must learn to identify syndromes, to be aware of their underlying etiology and pathogenesis, and to utilize advanced cytogenetic methods (e.g., fluorescence in situ hybridization), flow cytometry, or specific mutational analysis when appropriate. At a minimum, important anatomic details must be well documented and appropriate tissue samples should be obtained and stored to facilitate more specific diagnostic testing in the future.

Fanconi anemia lymphocytes: effect of DL-alpha-tocopherol (Vitamin E) on chromatid breaks and on G2 repair efficiency

The high frequency of chromosomal breaks in Fanconi anemia (FA) lymphocytes has been related to the increased oxidative damage shown by these cells. The effect of 100 microM DL-alpha-tocopherol (Vitamin E) on the level of chromosomal damage in mitosis was studied in lymphocytes from five FA patients and from age matched controls, both under basal conditions and when G2 repair was prevented by 2.5 mM caffeine (G2 unrepaired damage). In addition, the effect of this antioxidant on G2 duration and the efficiency of G2 repair was also evaluated in the sample. alpha-Tocopherol (AT) decreased the frequency of chromosomal damage (under basal and inhibited G2 repair conditions) and the duration of G2 in FA cells. This antioxidant protective effect, expressed as the decrease in chromatid breaks, was greater in FA cells (50.8%) than in controls (25%). The efficiency of the G2 repair process (G2 R rate) defined as the ratio between the percentage of chromatid breaks repaired in G2 and the duration of this cell cycle phase was lesser in FA cells (10.6) than in controls (22.6). AT treatment slightly increased this G2 R rate, both in FA cells and controls. These results suggest that an increased oxidative damage and a lower G2 repair rate may be simultaneously involved in the high frequency of chromatid damage detected in FA cells.

The genetics of Fanconi's anaemia

Fanconi's anaemia (FA) is an inherited bone marrow failure syndrome characterized by considerable clinical and cellular heterogeneity. This has also been recently demonstrated at the genetic and molecular levels following cloning of four out of the seven FA genes. Although this now enables molecular diagnosis in the majority of patients, because of the considerable molecular heterogeneity, the diepoxybutane/mitomycin-C stress test based on the increased chromosomal instability seen in FA cells, compared to normal controls, remains the front-line diagnostic test. This FA cell hallmark has led to the suggestion that FA may represent a defect in DNA repair although the precise function of the cloned FA genes remains unknown. Recent data suggest that they function in a novel cell pathway which has an important role in maintaining chromosome stability. The advances in the genetics of FA have already had some impact on diagnosis--for example, identification of patients with somatic mosaicism who have atypical clinical presentations--but to date they have had little impact on treatment. However, new treatments may now follow; indeed, for a number of reasons, FA may be a good candidate for haemopoietic gene therapy.

DNA repair mechanisms and acute myeloblastic leukemia

DNA repair mechanisms play a vital role in maintaining genomic integrity. With the wealth of knowledge gained recently on these processes it is becoming clear that defects in repair proteins and proteins associated with the regulation of repair are connected to many different human diseases including cancer. This paper has aimed to review the four major DNA repair processes and in particular concentrate on their association with acute myeloblastic leukemia (AML).

Do Fanconi anemia genes control cell response to cross-linking agents by modulating cytochrome P-450 reductase activity?

The Fanconi anemia (FA) genes play an important role in maintaining chromosomal stability and the defense of mammalian cells against cross-linking agents, such as cisplatin and mitomycin C (MMC). Cells derived from FA patients display a characteristic hypersensitivity toward cross-linking agents. Despite great progression in our understanding of the mechanisms that protect cells against these potent anti-cancer drugs, the specific roles of FA gene products in these processes have not been delineated. Recent studies have shown that the FA group C gene product, FANCC, can bind to and regulate the activity of cytochrome P450-reductase (P450R), an enzyme involved in the bioactivation of MMC. In this mini-review, this finding is placed in the context of complex mechanisms involved in the bioreductive activation of MMC and the hypersensitivity of FA cells to MMC. Although it would be premature to attribute the FA phenotype wholly to an abnormal activation of MMC, the regulation of P450R by FANCC suggests a novel link between one or more FA gene products, the cellular oxidative state, and the response to chemotherapeutic agents. Copyright 2000 Harcourt Publishers Ltd.

Loss of the Fanconi anemia group C protein activity results in an inability to activate caspase-3 after ionizing radiation

Fanconi anemia (FA) is a human genetic disease featuring cancer predisposition, genetic instability and DNA damage hypersensitivity. Although abnormalities in DNA repair and cell cycle checkpoint have been proposed as the underlying defect in this syndrome, these hypotheses did not provide full explanations of the complex phenotype. Although not exclusive of such possibilities, alterations in the control of apoptosis might account for the pleiotropic phenotype of this syndrome. We and others have previously reported a deregulation of the apoptotic response to mitomycin C, suggesting that the products of the Fanconi anemia group C protein (FANCC) contribute to the regulation of apoptosis. To explore the functional importance of the apoptotic alterations in FA we analyzed biochemical steps of the execution phase of apoptosis stimulated by another DNA damaging agent, the gamma-ray using FA cell lines derived from complementation group C (FA-C) independent patients. It is shown that the poly(ADP-ribose) polymerase, a target of caspase-3, is not cleaved in FA-C after ionizing radiation (IR). Moreover, caspase-3 is not processed in its active form and, its activity is not increased by IR in FA-C cells compared to normal cells. Altogether, these results demonstrate that loss of the FANCC activity results in a deficiency of the IR-induced apoptosis which is due to an inability to activate caspase-3. Our work suggests that apoptosis signaling induced by mitomycin C and IR is subject to common regulation involving the FANCC protein.

A physical complex of the Fanconi anemia proteins FANCG/XRCC9 and FANCA

Fanconi anemia (FA) is a recessively inherited disease characterized at the cellular level by spontaneous chromosomal instability and specific hypersensitivity to cross-linking agents. FA is genetically heterogeneous, comprising at least eight complementation groups (A-H). We report that the protein encoded by the gene mutated in complementation group G (FANCG) localizes to the cytoplasm and nucleus of the cell and assembles in a molecular complex with the FANCA protein, both in vivo and in vitro. Endogenous FANCA/FANCG complex was detected in both non-FA cells and in FA cells from groups D and E. By contrast, no complex was detected in specific cell lines belonging to groups A and G, whereas reduced levels were found in cells from groups B, C, F, and H. Wild-type levels of FANCA/FANCG complex were restored upon correction of the cellular phenotype by transfection or cell fusion experiments, suggesting that this complex is of functional significance in the FA pathway. These results indicate that the cellular FA phenotype can be connected to three biochemical subtypes based on the levels of FANCA/FANCG complex. Disruption of the complex may provide an experimental strategy for chemosensitization of neoplastic cells.