Immunological probing of induction and repair of 8-methoxypsoralen photoadducts in DNA from Fanconi anemia and normal human fibroblasts: quantitative analysis by electron microscopy

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.

Damage-resistant DNA synthesis in Fanconi anemia cells treated with a DNA cross-linking agent

Fanconi anemia (FA) is a recessive disorder associated with diverse congenital anomalies, progressive bone marrow failure, and a marked predisposition to develop cancer. At the cellular level, FA is characterized by a prolonged G(2) phase in proliferating cells and a marked hypersensitivity to both the cytotoxic and the clastogenic effects of agents which produce DNA interstrand cross-links. Treatment with these agents leads to even further prolongation of the G(2) phase in FA cells. We now show that FA cells, from four different complementation groups, fail to decrease their rates of replicative DNA synthesis, as do normal cells, following treatment with a DNA cross-linking agent. This may be responsible for the prolongation of the G2 phase seen in these cells, and suggests that the fundamental defect in response of FA cells to DNA cross-linking agents may be in the S phase, rather than the G(2) phase, of the cell cycle.

Is Fanconi anemia caused by a defect in the processing of DNA damage?

Fanconi anemia (FA) is an autosomal genetic disease characterized by a complex array of developmental disorders, a high predisposition to bone marrow failure and to acute myelogenous leukemia. The chromosomal instability and the hypersensitivity to DNA cross-linking agents led to its classification with the DNA repair disorders. This review aimed at establishing whether it is still appropriate to consider 1/approximately FA within a DNA repair framework taking into account the recently discovered genetic heterogeneity characteristics of the defect (eight complementation groups). We discuss the possibility that the FA proteins interact to form a complex which may control different functions, including the processing of specific DNA lesions. Such a complex may act as a sensor to initiate protective systems as well as transcription of specific genes specifying, among others proteins, growth factors. Such steps may be organized as a linear cascade or more likely under the form of a web network.

6,4,4'-Trimethylangelicin photoadduct immunodetection in DNA: induction and repair in Fanconi's anemia and normal human fibroblasts

6,4,4'-Trimethylangelicin (TMA)-photoinduced monoadducts (MAs) were detected and quantified on DNA of normal human and Fanconi's anemia (FA) fibroblasts (complementation groups A and D) by immuno-electron microscopy. TMA-modified DNA was extracted from the cells just after photoreaction, or after a subsequent 24 h repair period, for analysis of the MA processing capabilities of the different cell lines. Unmodified DNA was extracted from the control cells in parallel. The immunoreaction with antibody 7E3 was performed on single-stranded DNA fragments obtained by heat-formamide denaturation. On single-stranded DNA fragments scanned in the electron microscope, IgG-labeled MA sites appeared as isolated or clustered IgG molecules, which were not homogeneously distributed. The isolated IgG and the different clusters (doublets, triplets or near-neighbors (within a distance of 250 nucleotides)) were measured separately for induction frequency and removal. Few interstrand cross-links (CLs) were present on X-shape DNA fragments. At time zero, the distribution patterns of TMA-photoinduced IgG-labeled MA sites and CLs, and their amount per 10(6) nucleotides, were similar in the three cell lines. After the 24 h repair period, FA cells from two different genetic complementation groups demonstrated impaired incision-excision repair capabilities for both MAs (singlets or clusters) and CLs when compared with normal cells. In each cell line, the relative proportions of TMA-induced lesions remaining at time 24 h were similar to those initially induced. This implies analogous processing kinetics towards the TMA-photoinduced clusters of MAs and CLs in a given cell line.

Molecular spectrum of mutations induced at the HPRT locus by a cross-linking agent in human cell lines with different repair capacities

Molecular characterization of mutations photoinduced by a cross-linking agent, 4,5',8-trimethylpsoralen (Me3Pso), in normal human lymphoblasts was conducted in parallel with lymphoblasts derived from Fanconi anemia patients. Such cells have been previously described to be impaired in repair of psoralen photolesions. The endogenous HPRT locus was used as a target gene. The treatment of cells with Me3Pso in combination with 365 nm irradiation leads to the formation of interstrand cross-links, and specific monoadducts. Our analysis revealed that the mutagenic processing of Me3Pso photoadducts in normal human cells results essentially in base substitutions (84%). These are localized to sequences shown previously to be favored for the formation of Me3Pso monoadducts. The mutagenic processing of the same lesions in Fanconi anemia cells results in fewer base substitutions (22%), with deletions (66%) being the predominant class of mutation. In contrast to prokaryotic systems, frameshifts are poorly represented among Me3Pso induced mutations in human cells. In spite of important differences between the kinds of mutations observed in the two cell lines, our analysis reveals similarities in the type of base substitutions and their sequence distribution. In both normal and Fanconi anemia cell lines mutations, mostly targeted on thymine residues, are preferentially located on the non-transcribed strand.