Fanconi's anaemia: correlation of genetic complementation group with psoralen/UVA response

The rhythms of histones in regeneration: The epigenetic modifications determined by clock genes

The evolutionary establishment of an internal biological clock is a primordial event tightly associated with a 24-h period. Changes in the circadian rhythm can affect cellular functions, including proliferation, DNA repair and redox state. Even isolated organs, tissues and cells can maintain an autonomous circadian rhythm. These cell-autonomous molecular mechanisms are driven by intracellular clock genes, such as BMAL1. Little is known about the role of core clock genes and epigenetic modifications in the skin. Our focus was to identify BMAL1-driven epigenetic modifications associated with gene transcription by mapping the acetylation landscape of histones in epithelial cells responding to injury. We explored the role of BMAL1 in epidermal wound and tissue regeneration using a loss-of-function approach in vivo. We worked with BMAL1 knockout mice and a contraction-resistance wound healing protocol, determining the histone modifications using specific antibodies to detect the acetylation levels of histones H3 and H4. We found significant differences in the acetylation levels of histones in both homeostatic and injured skin with deregulated BMAL1. The intact skin displayed varied acetylation levels of histones H3 and H4, including hyperacetylation of H3 Lys 9 (H3K9). The most pronounced changes were observed at the repair site, with notable alterations in the acetylation pattern of histone H4. These findings reveal the importance of histone modifications in response to injury and indicate that modulation of BMAL1 and its associated epigenetic events could be therapeutically harnessed to improve skin regeneration.

Fanconi anemia, complementation group A, cells are defective in ability to produce incisions at sites of psoralen interstrand cross-links

The hypersensitivity of Fanconi anemia, complementation group A, (FA-A) cells to agents which produce DNA interstrand cross-links correlates with a defect in their ability to repair this type of damage. In order to more clearly elucidate this repair defect, chromatin-associated protein extracts from FA-A cells were examined for ability to endonucleolytically produce incisions in DNA at sites of interstrand cross-links. A defined 140 bp DNA substrate was constructed with a single site-specific monoadduct or interstrand cross-link produced by 4,5',8-trimethylpsoralen (TMP) plus long wavelength (UVA) light. Our results show that FA-A cells are defective in ability to produce dual incisions in DNA at sites of interstrand cross-links. Specifically, there is defective incision on the 3'- and 5'-sides of both the furan and pyrone sides of the cross-link. This defect is corrected in FA-A cells transduced with a retroviral vector expressing FANCA cDNA. At the site of a TMP monoadduct, FA-A cells can introduce incisions on both the 3'- and 5'-sides of the furan side monoadduct, but are defective in ability to produce these incisions on the pyrone side monoadduct. These studies also indicate that XPF is involved in production of the 5' incision by the normal extracts on these substrates. These results correlate with our previous work, which showed that FA-A cells are mainly defective in ability to repair psoralen interstrand cross-links with a lesser defect in ability to repair psoralen monoadducts. This defect in endonucleolytic incision at sites of TMP interstrand cross-links could be related to reduced levels of non-erythroid alpha spectrin (alphaSpIISigma*) in the extracts from FA-A cells. alphaSpIISigma* could act as a scaffold to align proteins involved in cross-link repair and enhance their interactions; a deficiency in alphaSpIISigma* could thus lead to reduced efficiency of repair and the decreased levels of incisions we observe at sites of interstrand cross-links in FA-A cells.

Genetic disorders associated with cancer predisposition and genomic instability

Genomic instability in its broadest sense is a feature of virtually all neoplastic cells. In addition to the mutations and/or gene amplifications that appear to be a prerequisite for the acquisition of a neoplastic phenotype, human cancers exhibit other "markers" of genomic instability--in particular, a high degree of aneuploidy. Indeed, many studies have shown that aneuploidy is an almost invariant feature of cancer cells, and it has been argued by some that the emergence of aneuploid cells is a necessary step during tumorigenesis. The functional link between genomic instability and cancer is strengthened by the existence of several "genetic instability" disorders of humans that are associated with a moderate to severe increase in the incidence of cancers. These disorders include ataxia telangiectasia, Bloom's syndrome, Fanconi anemia, xeroderma pigmentosum, and Nijmegen breakage syndrome, all of which are very rare and are inherited in a recessive manner. Analysis of the cells from such cancer-prone individuals is clearly a potentially fruitful approach for delineating the genetic basis for instability in the genome. It is assumed that by identifying the underlying cause of genetic instability in these disorders, one can derive valuable information not only about the basis of particular genetic diseases, but also about the underlying causes of genomic instability in sporadic cancers in the general population. In this article, we review the clinical and cellular properties of genetic instability disorders associated with cancer predisposition. In particular, we focus on the rapid advances made in our understanding of these disorders that have derived from the cloning of the genes mutated in each case. Because in many instances the affected genes have analogs in lower eukaryotic species, we shall discuss how studies in yeasts in particular have proved valuable in our understanding of human diseases and predisposition to cancer.

DNA repair and chromatin structure in genetic diseases

Interaction of DNA repair proteins with damaged DNA in eukaryotic cells is influenced by the packaging of DNA into chromatin. The basic repeating unit of chromatin, the nucleosome, plays an important role in regulating accessibility of repair proteins to sites of damage in DNA. There are a number of different pathways fundamental to the DNA repair process. Elucidation of the proteins involved in these pathways and the mechanisms they utilize for interacting with damaged nucleosomal and nonnucleosomal DNA has been aided by studies of genetic diseases where there are defects in the DNA repair process. Two of these diseases are xeroderma pigmentosum (XP) and Fanconi anemia (FA). Cells from patients with these disorders are similar in that they have defects in the initial steps of the repair process. However, there are a number of important differences in the nature of these defects. One of these is in the ability of repair proteins from XP and FA cells to interact with damaged nucleosomal DNA. In XP complementation group A (XPA) cells, for example, endonucleases present in a chromatin-associated protein complex involved in the initial steps in the repair process are defective in their ability to incise damaged nucleosomal DNA, but, like the normal complexes, can incise damaged naked DNA. In contrast, in FA complementation group A (FA-A) cells, these complexes are equally deficient in their ability to incise damaged naked and similarly damaged nucleosomal DNA. This ability to interact with damaged nucleosomal DNA correlates with the mechanism of action these endonucleases use for locating sites of damage. Whereas the FA-A and normal endonucleases act by a processive mechanism of action, the XPA endonucleases locate sites of damage distributively. Thus the mechanism of action utilized by a DNA repair enzyme may be of critical importance in its ability to interact with damaged nucleosomal DNA.

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.

Correction of the DNA repair defect in Fanconi anemia complementation groups A and D cells

We have previously isolated from Fanconi anemia, complementation groups A (FA-A) and D (FA-D) cells, a DNA endonuclease complex which is defective in its ability to incise DNA containing interstrand cross-links produced by psoralen plus UVA light. The repair capabilities of the FA complexes, compared with those of the corresponding normal complex, have now been examined using two types of complementation analysis. First, introduction of the normal complex, by electroporation, into 8-methoxypsoralen (8-MOP) plus UVA treated FA-A and FA-D cells resulted in correction of their repair defect, determined by measuring repair-related unscheduled DNA synthesis (UDS). The FA-A and FA-D complexes could similarly complement the repair defect in each others' cells, but not in their own. Second, mixing the normal with the FA-A and FA-D complexes, or the FA-A with the FA-D complex, in a cell-free system resulted in correction of the defect in ability of these FA complexes to incise damaged DNA. These results indicate that the normal complex contains the proteins needed to correct the DNA repair defect in FA-A and FA-D cells and that the FA-A and FA-D complexes contain the protein needed to complement the repair defect in each other.

Effect of hydroxyurea and normal plasma on DNA synthesis in lymphocytes from Fanconi anemia patients

Fanconi anemia (FA) is characterized at the cellular level by a high frequency of spontaneous chromosomal aberrations; crosslinking agents cause an abnormal increase in the frequency of chromosomal damage, and semiconservative DNA synthesis is severely inhibited. Deoxyribonucleotides are needed in both semiconservative and repair DNA synthesis. To investigate the involvement of deoxyribonucleotide pools in the inhibition of DNA synthesis in FA, we evaluated the effect on FA lymphocytes of hydroxyurea (HU), an inhibitor of ribonucleotide reductase which is known to alter the intracellular levels of deoxyribonucleotides. To achieve this goal, lymphocyte cultures of 4 FA patients and 4 normal individuals were used. Cultures were treated with HU and/or mitomycin C and normal human plasma. All cultures were processed to detect the number of DNA synthesizing nuclei by autoradiography. Scoring of 2000 nuclei for each kind of culture every 6 h in the last 24 h of incubation showed that, in long incubation periods, DNA synthesis in FA is largely inhibited by HU and this hypersensitivity may be partially decreased by addition of normal human plasma. It is known that recovery from damage induced by HU involves several enzymes such as flavin oxido-reductase, superoxide dismutase and catalase which are involved in the production or scavenging of O2 radicals; FA cells are deficient in the detoxification of oxygen and this could explain the response of FA cells to HU.

Molecular analysis of Fanconi anaemia

The autosomal recessive genetic disease, Fanconi anaemia, is perceived as another manifestation of defective cellular DNA repair, just as in the autosomal recessive disease Xeroderma pigmentosum. The biochemistry and cellular biology of Xeroderma pigmentosum have been convincingly elucidated, but the same has not been true for Fanconi anaemia. In this review we consider the pleiotropic nature of Fanconi anaemia, its clinical and cellular variability and its genetic heterogeneity. We take into account the wealth of experimental findings available and offer a novel hypothesis involving feedback control of DNA replication during S phase of the cell cycle to explain the basic defect in the disease.

Identification of a HeLa mRNA fraction which corrects the mitomycin C sensitivity of irs1 cells

The hamster cell mutant irs1 is defective in its response to DNA lesions caused by a variety of mutagens, particularly cross-linking agents. These cells have been assigned to complementation group 2 of X-ray-sensitive mutants and the mutated gene is called XRCC2(X-ray repair cross complementing). We have identified, by microinjection, a human mRNA fraction which can transiently correct the sensitivity of these cells to cross-linking agents. This fraction contains mRNAs of 3.5 kb (+/- 0.25) including, therefore, the transcript of the XRCC2 gene.

Irreversible repression of DNA synthesis in Fanconi anemia cells is alleviated by the product of a novel cyclin-related gene

Primary fibroblasts from patients with the genetic disease Fanconi anemia, which are hypersensitive to cross-linking agents, were used to screen a cDNA library for sequences involved in their abnormal cellular response to a cross-linking challenge. By using library partition and microinjection of in vitro-transcribed RNA, a cDNA clone, pSPHAR (S-phase response), which is able to correct the permanent repression of semiconservative DNA synthesis rates characteristic of these cells, was isolated. Wild-type SPHAR mRNA is expressed in all fibroblasts so far analyzed, including those of Fanconi anemia patients. Correction of the abnormal response in these cells appears therefore to be due to overexpression after cDNA transfer rather than to genetic complementation. The cDNA contains an open reading frame coding for a polypeptide of 7.5 kDa. Rabbit antiserum directed against a SPHAR peptide detects a protein of 7.9 kDa in Western blots (immunoblots) of whole-cell extracts from proliferating, but not resting, fibroblasts. The deduced amino acid sequence of SPHAR contains a motif found in the cyclins, and it is proposed that SPHAR acts within the injected cell by interfering with the cyclin-controlled maintenance of S phase. In agreement with this proposal, normal cells transfected with an antisense SPHAR expression vector have a significantly reduced rate of DNA synthesis during S phase and a prolonged G2 phase, reflecting the need for postreplicative DNA processing before entry into mitosis.

Localization of Fanconi anemia C protein to the cytoplasm of mammalian cells

Features of chromosomal aberrations, hypersensitivity to DNA crosslinking agents, and predisposition to malignancy have suggested a fundamental anomaly of DNA repair in Fanconi anemia. The function of the recently isolated FACC (Fanconi anemia group C complementing) gene for a subset of this disorder is not yet known. The notion that FACC plays a direct role in DNA repair would predict that the polypeptide should reside in the nucleus. In this study, a polyclonal antiserum raised against FACC was used to determine the subcellular location of the polypeptide. Immunofluorescence and subcellular fractionation studies of human cell lines as well as COS-7 cells transiently expressing human FACC showed that the protein was localized primarily to the cytoplasm under steady-state conditions, transit through the cell cycle, and exposure to crosslinking or cytotoxic agents. However, placement of a nuclear localization signal from the simian virus 40 large tumor antigen at the amino terminus of FACC directed the hybrid protein to the nuclei of transfected COS-7 cells. These observations suggest an indirect role for FACC in regulating DNA repair in this group of Fanconi anemia.

Fanconi anemia revisited: old ideas and new advances

This review summarizes both historical and more recent data on the clinical, cellular and genetic features of Fanconi anemia (FA), a rare autosomal recessive disorder. FA patients are characterized by pancytopenia, congenital malformations, growth delay and an increased susceptibility to the development of malignancies, particularly acute myelogenous leukemia. FA cells show chromosomal fragility, slow growth and increased sensitivity to DNA crosslinking agents. FA can be caused by defects in any one of at least four genes. Two general hypotheses have been proposed to explain the underlying defect: loss of a DNA repair function or of a step in the defense toward oxygen toxicity. After many attempts to clone the FA genes, the first one, that defective in group C, has been cloned by complementation of the increased sensitivity of FA(C) cells to mitomycin C and diepoxybutane. This gene (FACC) codes for a novel protein and is ubiquitously expressed. Mutations in various FA(C) patients that cause loss of function have been identified. The review concludes by suggesting directions for future research in FA.

Hypersensitivity to oxygen is a uniform and secondary defect in Fanconi anemia cells

Cells from patients with Fanconi anemia (FA) frequently show an increased sensitivity to DNA crosslinking agents such as mitomycin C (MMC). FA cells also show abnormal sensitivity to oxygen tension. In order to examine the correlation between the two cellular defects in FA, several FA fibroblast lines were tested for their sensitivity to MMC and oxygen by colony-formation frequency. The sensitivity to MMC in different FA lines varied in a broad range from normal level to extreme hypersensitivity, whereas all of the FA lines showed similar hypersensitivity to oxygen. When FA fibroblasts were transformed by SV40 large T-antigen, the hypersensitivity to oxygen was normalized while the MMC sensitivity still remained. These results suggest that the cellular sensitivity to oxygen is a secondary defect rather than a primary effect of mutations in FA. However, it is a more uniform phenotype than the MMC sensitivity, and therefore, it may be closely related to the common clinical symptoms of FA. Since 1% oxygen showed the highest colony-formation frequency for FA cells, establishment of FA primary fibroblasts was attempted at the low oxygen condition. FA fibroblast cells showed greatly enhanced growth and migration at 1% oxygen resulting in fast establishment of FA primary fibroblasts.

A damage-recognition protein which binds to DNA containing interstrand cross-links is absent or defective in Fanconi anemia, complementation group A, cells

A DNA binding protein with specificity for DNA containing interstrand cross-links induced by 4,5',8-trimethylpsoralen (TMP) plus long wavelength ultraviolet (UVA) light has been identified in normal human chromatin. Protein binding to DNA was determined using a gel mobility shift assay and an oligonucleotide containing a hot spot for formation of psoralen interstrand cross-links. Specificity of the damage-recognition protein for cross-links was demonstrated both by a positive correlation between level of cross-link formation in DNA and extent of protein binding and by effective competition by treated but not undamaged DNA for the binding protein. Chromatin protein extracts from cells from individuals with the genetic disorder, Fanconi anemia, complementation group A (FA-A), which have decreased ability to repair damage produced by TMP plus UVA light, failed to show any protein binding to TMP plus UVA treated DNA. We have previously shown that these chromatin protein extracts contain a DNA endonuclease complex, pI 4.6, which specifically recognizes and incises DNA containing interstrand cross-links and which in FA-A cells is defective in its ability to incise this damaged DNA (Lambert et al. (1992) Mutation Res., 273, 57-71). Together, these findings suggest that the DNA binding protein identified is involved in recognition and repair of DNA interstrand cross-links.

Long-term bone marrow culture in Fanconi's anaemia

Fanconi's anaemia (FA) is the most common of the constitutional aplastic anaemias; the mechanisms leading to aplasia in this disease are poorly understood. A number of mechanisms have been implicated in the pathogenesis of acquired aplastic anaemia (AA), including a stem cell defect, an immune reaction against haematopoietic cells or defective function of the marrow microenvironment. To investigate the pathophysiology of this disorder we have performed bone marrow colony forming unit-granulocyte macrophage (CFU-GM) assays and long-term bone marrow culture (LTC) in 22 cases of FA compared with 17 cases of acquired AA. Defective in vitro haematopoiesis was observed in all patients with FA, including several cases with normal peripheral blood counts. The mean CFU-GM value for the FA group was approximately 15 times lower than for the normal group. A correlation was seen between CFU-GM and the severity of neutropenia in FA. In LTC an adherent layer formed in all cases of FA; despite this fact CFU-GM were either not generated or rapidly fell to zero in all patients. LTC is a sensitive method for the detection of impaired granulopoiesis in FA and reveals defects in all patients with this disease.

Human genetic instability syndromes: single gene defects with increased risk of cancer

The experimental findings of the last 5 years are reviewed for the genetic instability syndromes: Xeroderma pigmentosum, Fanconi's anaemia, Ataxia telangiectasia and Bloom's syndrome. In these autosomal recessive genetic diseases, single gene defects lead to genetic instability, increased mutation rates and cancer. Deficiencies in the ability to effectively repair DNA lesions have been suggested for all of these syndromes. The status of characterization of these DNA repair defects is presented and the possible mechanisms of lesion fixation as mutation are discussed. The four known human genes whose mutation leads to inherited genetic instability are described.

Increased mutagen sensitivity in human cultured fibroblasts with constitutively high micronucleus levels

The induction of micronuclei (MN) by incubation with different mutagenic agents was tested in diploid fibroblast cultures obtained from 15 probands with constitutively high MN rates (15.75-77.25 MN/500 cells; average 36.27 +/- 17.60 MN) and 15 probands (controls) with low MN rates (4-13.75 MN/500 cells; average 8.97 +/- 2.73 MN). In order to find out whether fibroblast cultures of individuals with increased spontaneous MN levels exhibit an increased sensitivity to various agents with different genotoxic mechanisms, we studied the induction of MN in these cell cultures by ultraviolet (UV) irradiation, mitomycin C (MMC), N-methyl-N-nitro-N-nitrosoguanidine (MNNG) and benzo-(a)pyrene-diol-expoxid (BPDE). In addition, we tested aphidicolin (APC), a polymerase alpha inhibitor, which is a potent inducer of common fragile sites. Probands with spontaneously high MN showed an significantly increased sensitivity to UV (p less than or equal to 0.005), MMC (p less than or equal to 0.005), and BPDE (p less than or equal to 0.005). No significant differences were found for MNNG and APC as compared to controls.

Defective DNA endonuclease activities in Fanconi's anemia cells, complementation groups A and B

Cells from patients with the inherited disorder, Fanconi's anemia (FA), were analyzed for endonucleases which recognize DNA interstrand cross-links and monoadducts produced by psoralen plus UVA irradiation. Two chromatin-associated DNA endonuclease activities, defective in their ability to incise DNA-containing adducts produced by psoralen plus UVA light, have been identified and isolated in nuclei of FA cells. In FA complementation group A (FA-A) cells, one endonuclease activity, pI 4.6, which recognizes psoralen intercalation and interstrand cross-links, has 25% of the activity of the normal human endonuclease, pI 4.6, on 8-methoxypsoralen (8-MOP) plus UVA-damaged DNA. In FA complementation group B (FA-B) cells, a second endonuclease activity, pI 7.6, which recognizes psoralen monoadducts, has 50% and 55% of the activity, respectively, of the corresponding normal endonuclease on 8-MOP or angelicin plus UVA-damaged DNA. Kinetic analysis reveals that both the FA-A endonuclease activity, pI 4.6, and the FA-B endonuclease activity, pI 7.6, have decreased affinity for psoralen plus UVA-damaged DNA. Both the normal and FA endonucleases showed approximately a 2.5-fold increase in activity on psoralen plus UVA-damaged reconstituted nucleosomal DNA compared to damaged non-nucleosomal DNA, indicating that interaction of these FA endonucleases with nucleosomal DNA is not impaired. These deficiencies in two nuclear DNA endonuclease activities from FA-A and FA-B cells correlate with decreased levels of unscheduled DNA synthesis (UDS), in response to 8-MOP or angelicin plus UVA irradiation, in these cells in culture.

Partial complementation of the Fanconi anemia defect upon transfection by heterologous DNA. Phenotypic dissociation of chromosomal and cellular hypersensitivity to DNA cross-linking agents

Transfectants obtained by mouse DNA-mediated gene transfer in Fanconi anemia (FA) primary fibroblasts from the genetic complementation groups A and B were examined for the frequencies of chromosomal aberrations and cytotoxicity following treatments by cross-linking agents. Cells from group A (FA 150), which is the most sensitive to such agents, are partially corrected for both the chromosomal and cellular hypersensitivity to 8-methoxypsoralen photoaddition. In contrast, after treatment with mitomycin C (MMC), only the chromosomal sensitivity is re-established to a near normal level. The opposite is true for FA group B cells (FA 145), i.e. cell survival to MMC is partially corrected, whereas the frequency of MMC-induced chromosomal aberration remains close to that of the untransfected cells. The partial phenotypic correction of the two end points examined is interpreted as indicating either a gene dosage effect or the necessity of introducing more than one gene type in order to achieve complete recovery of a normal phenotype. The phenotypic dissociation between the clastogenic and cellular hypersensitivity to cross-linking agents may offer the opportunity of isolating separately the responsible gene(s) by conventional rescue techniques.

Correction of the ultraviolet light induced DNA-repair defect in xeroderma pigmentosum cells by electroporation of a normal human endonuclease

Cells from patients with xeroderma pigmentosum, complementation group A (XPA), are known to be defective in repair of pyrimidine dimers and other forms of damage produced by 254-nm ultraviolet (UVC) radiation. We have isolated a DNA endonuclease, pI 7.6, from the chromatin of normal human lymphoblastoid cells which recognizes damage produced by UVC light, and have introduced this endonuclease into UVC-irradiated XPA cells in culture to determine whether it can restore their markedly deficient DNA repair-related unscheduled DNA synthesis (UDS). Introduction of the normal endonuclease, which recognizes predominantly pyrimidine dimers, but not the corresponding XPA endonuclease into UVC-irradiated XPA cells restored their levels of UDS to approximately 80% of normal values. Electroporation of both the normal and the XPA endonuclease into normal human cells increases UDS in normal cells to higher than normal values. These results indicate that the normal endonuclease can restore UDS in UVC-irradiated XPA cells. They also indicate that XPA cells have an endonuclease capable of increasing the efficiency of repair of UVC damage in normal cells.

Identification of a HeLa mRNA fraction which can correct the DNA-repair defect in Fanconi anaemia fibroblasts

Injection of Fanconi anaemia (complementation group A) fibroblasts with HeLa mRNA is shown to correct their abnormal response to a psoralen cross-linking challenge, namely permanent repression of DNA synthesis. Injection of gradient-fractionated mRNA led to identification of a single fraction, containing mRNA of approximately 650 bases, which is responsible for this effect. This finding suggests that Fanconi anaemia (group A) cells are deficient in a small protein, up to 20 kDa in size, which is involved in the cellular response to DNA interstrand cross-links.