The DNA crosslink-induced S-phase checkpoint depends on ATR-CHK1 and ATR-NBS1-FANCD2 pathways

Detection of somatic mosaicism and classification of Fanconi anemia patients by analysis of the FA/BRCA pathway

Fanconi anemia (FA) is characterized by congenital abnormalities, bone marrow failure, chromosome fragility, and cancer susceptibility. Eight FA-associated genes have been identified so far, the products of which function in the FA/BRCA pathway. A key event in the pathway is the monoubiquitination of the FANCD2 protein, which depends on a multiprotein FA core complex. In a number of patients, spontaneous genetic reversion can correct FA mutations, leading to somatic mosaicism. We analyzed the FA/BRCA pathway in 53 FA patients by FANCD2 immunoblots and chromosome breakage tests. Strikingly, FANCD2 monoubiquitination was detected in peripheral blood lymphocytes (PBLs) in 8 (15%) patients. FA reversion was further shown in these patients by comparison of primary fibro-blasts and PBLs. Reversion was associated with higher blood counts and clinical stability or improvement. Once constitutional FANCD2 patterns were determined, patients could be classified based on the level of FA/BRCA pathway disruption, as "FA core" (upstream inactivation; n = 47, 89%), FA-D2 (n = 4, 8%), and an unidentified downstream group (n = 2, 4%). FA-D2 and unidentified group patients were therefore relatively common, and they had more severe congenital phenotypes. These results show that specific analysis of the FA/BRCA pathway, combined with clinical and chromosome breakage data, allows a comprehensive characterization of FA patients.

A role for the Fanconi anemia C protein in maintaining the DNA damage-induced G2 checkpoint

Fanconi anemia (FA) is a complex, heterogeneous genetic disorder composed of at least 11 complementation groups. The FA proteins have recently been found to functionally interact with the cell cycle regulatory proteins ATM and BRCA1; however, the function of the FA proteins in cell cycle control remains incompletely understood. Here we show that the Fanconi anemia complementation group C protein (Fancc) is necessary for proper function of the DNA damage-induced G2/M checkpoint in vitro and in vivo. Despite apparently normal induction of the G2/M checkpoint after ionizing radiation, murine and human cells lacking functional FANCC did not maintain the G2 checkpoint as compared with wild-type cells. The increased rate of mitotic entry seen in Fancc-/-mouse embryo fibroblasts correlated with decreased inhibitory phosphorylation of cdc2 kinase on tyrosine 15. An increased inability to maintain the DNA damage-induced G2 checkpoint was observed in Fancc -/-; Trp53 -/-cells compared with Fancc -/-cells, indicating that Fancc and p53 cooperated to maintain the G2 checkpoint. In contrast, genetic disruption of both Fancc and Atm did not cooperate in the G2 checkpoint. These data indicate that Fancc and p53 in separate pathways converge to regulate the G2 checkpoint. Finally, fibroblasts lacking FANCD2 were found to have a G2 checkpoint phenotype similar to FANCC-deficient cells, indicating that FANCD2, which is activated by the FA complex, was also required to maintain the G2 checkpoint. Because a proper checkpoint function is critical for the maintenance of genomic stability and is intricately related to the function and integrity of the DNA repair process, these data have implications in understanding both the function of FA proteins and the mechanism of genomic instability in FA.

ATR couples FANCD2 monoubiquitination to the DNA-damage response

Fanconi anemia (FA) is a multigenic autosomal recessive cancer susceptibility syndrome. The FA pathway regulates the monoubiquitination of FANCD2 and the assembly of damage-associated FANCD2 nuclear foci. How FANCD2 monoubiquitination is coupled to the DNA-damage response has remained undetermined. Here, we demonstrate that the ATR checkpoint kinase and RPA1 are required for efficient FANCD2 monoubiquitination. Deficiency of ATR function, either in Seckel syndrome, which clinically resembles Fanconi anemia, or by siRNA silencing, results in the formation of radial chromosomes in response to the DNA cross-linker, mitomycin C (MMC), thus mimicking the chromosome instability of FA cells.

Telomere dysfunction in genome instability syndromes

Telomeres are nucleoprotein complexes located at the end of eukaryotic chromosomes. They have essential roles in preventing terminal fusions, protecting chromosome ends from degradation, and in chromosome positioning in the nucleus. These terminal structures consist of a tandemly repeated DNA sequence (TTAGGG in vertebrates) that varies in length from 5 to 15 kb in humans. Several proteins are attached to this telomeric DNA, some of which are also involved in different DNA damage response pathways, including Ku80, Mre11, NBS and BLM, among others. Mutations in the genes encoding these proteins cause a number of rare genetic syndromes characterized by chromosome and/or genetic instability and cancer predisposition. Deletions or mutations in any of these genes may also cause a telomere defect resulting in accelerated telomere shortening, lack of end-capping function, and/or end-to-end chromosome fusions. This telomere phenotype is also known to promote chromosomal instability and carcinogenesis. Therefore, it is essential to understand the interplay between telomere biology and genome stability. This review is focused in the dual role of chromosome fragility proteins in telomere maintenance.

First case of aplastic anemia in a Japanese child with a homozygous missense mutation in the NBS1 gene (I171V) associated with genomic instability

The NBS1 gene is strongly linked to several factors involved in genome integrity. Functional disruption of NBS1 could therefore induce genomic instability and carcinogenesis. Four children with acute lymphoblastic leukemia have been reported to be heterozygous for a germline and/or somatic missense mutation in NBS1, leading to the I171V substitution. We screened healthy controls and pediatric patients with hematological malignancies and aplastic anemia (AA) for the presence of I171V. Of the 62 patients, one individual with AA was confirmed to harbor a homozygous I171V mutation. Genetic analysis of NBS1 in this patient and her healthy parents indicated that she inherited the germline I171V mutation from her father and the wild-type allele from her mother, and that the second I171V hit occurred on the wild-type allele early in embryonic development. Furthermore, cytogenetic analysis of lymphoblastic cell lines from the patient indicated a remarkable increase in numerical and structural chromosomal aberrations in the absence of clastogens, suggesting that she potentially carried genomic instability. This is the first report of AA with a homozygous I171V mutation. We hypothesize that NBS1 may play an important role in the pathogenesis of AA.

BLM and the FANC proteins collaborate in a common pathway in response to stalled replication forks

Fanconi anaemia (FA) and Bloom syndrome (BS) are autosomal recessive diseases characterised by chromosome fragility and cancer proneness. Here, we report that BLM and the FA pathway are activated in response to both crosslinked DNA and replication fork stall. We provide evidence that BLM and FANCD2 colocalise and co-immunoprecipitate following treatment with either DNA crosslinkers or agents inducing replication arrest. We also find that the FA core complex is necessary for BLM phosphorylation and assembly in nuclear foci in response to crosslinked DNA. Moreover, we show that knock-down of the MRE11 complex, whose function is also under the control of the FA core complex, enhances cellular and chromosomal sensitivity to DNA interstrand crosslinks in BS cells. These findings suggest the existence of a functional link between BLM and the FA pathway and that BLM and the MRE11 complex are in two separated branches of a pathway resulting in S-phase checkpoint activation, chromosome integrity and cell survival in response to crosslinked DNA.

Human disorders of ubiquitination and proteasomal degradation

PURPOSE OF REVIEW

The goal of this review is to provide an overview of rapidly evolving information on a new group of genetic inborn errors affecting ubiquitination and proteasomal degradation of proteins and to suggest a classification scheme for these disorders. The relevant genes encode ubiquitin, ubiquitin enzymes (E1 and many E2s and E3s), deubiquitinating enzymes, proteasomal subunits, and substrates undergoing ubiquitination.

RECENT FINDINGS

Since the initial recognition that Angelman syndrome is caused by maternal deficiency of the E6-AP ubiquitin E3 ligase (gene symbol UBE3A), several. other disorders of E3 ligases have been identified, including autosomal recessive juvenile Parkinson disease, the APECED form of autoimmune polyendocrinopathy syndrome, von Hippel-Lindau syndrome, and congenital polycythemia. Disorders that disturb ubiquitin regulatory signaling include at least two subtypes of Fanconi anemia, the BRCA1 and BRCA2 forms of breast and ovarian cancer susceptibility, incontinentia pigmenti, and cylindromatosis. Many disorders affect ubiquitin pathways secondarily.

SUMMARY

The authors propose both a genetic and a functional classification for disorders of ubiquitination and proteasomal degradation, as follows. Genetic classes include mutations in (1) the UBB ubiquitin gene; (2) enzymes of ubiquitination including E1, E2, E3, and related proteins; (3) deubiquitinases; (4) proteasomal subunits; and (5) substrates of ubiquitination. Functional classes include defects in (1) proteolytic degradation, (2) ubiquitin signaling, and (3) subcellular localization of substrates. Additional functional classes are likely to be defined, and individual disorders may involve multiple functional defects.

The Fanconi anemia core complex associates with chromatin during S phase

Fanconi anemia (FA) is an autosomal recessive disease marked by bone marrow failure, birth defects, and cancer. The FA proteins FANCA, FANCC, FANCE, FANCF, FANCG, and FANCL participate in a core complex. We previously have shown that several members of this complex bind to chromatin until mitosis and that this binding increases after DNA damage. The purpose of the present study was to determine the dynamics of complex movement between cytoplasm and nuclear compartments. Fluorescent-tagged versions of FANCA, FANCC, and FANCG colocalize in cytoplasm and nucleus, chiefly in chromatin. At the G1-S border, the FA core complex exists as foci on chromatin, progressively diffusing and migrating to the nuclear periphery and becoming completely excluded from condensed chromosomes by mitosis. Chromatin fiber analysis shows FA proteins diffusely staining along chromatin fibers during G1-S and S phase. Treatment with the DNA cross-linker mitomycin C results in a diffusion of foci and increased binding of complex proteins to chromatin, as well as diffuse and increased complex binding to chromatin fibers. These data are consistent with the idea that the FA proteins function at the level of chromatin during S phase to regulate and maintain genomic stability.

Involvement of G1/S checkpoint regulators during photodynamic-therapy-mediated cell cycle arrest in human colon carcinoma SW480 cells

AIM: To investigate the involvement of G₁/S regulators during ALA-PDT-mediated cell cycle arrest.

METHODS: Colon carcinoma SW480 cells were treated with 1 mmol/L ALA and 6 h later irradiated with 9 J/cm² 631 nm light from a laser. The analysis of cell cycle arrest by ALA-PDT was performed by flow cytometry. The effect of ALA-PDT on G₁/S cell cycle regulators Chk2, CyclinD1 and P21WAF1/Cip1/Sdi1 was examined by using immunocytochemical technique.

RESULTS: ALA-PDT resulted in G₀-G₁ phase arrest of the tumor cell cycle in post-PDT in a time-dependent manner. Immunocytochemical analysis revealed that PDT resulted in an induction of G₁/S checkpoint regulation factors such as Chk2 and P21WAF1/Cip1/Sdi1, and a down-regulation of CyclinD1 in a time-dependent manner.

CONCLUSION: ALA-PDT causes G₀-G₁ phase arrest of SW480 cell cycle in post-PDT earlier period; PDT-mediated induction of G₁/S checkpoint regulators Chk2 and P21WAF1/Cip1/Sdi1 results in the imposition of artificial checkpoint at G₁-S checkpoint thereby leading to an arrest of cells in G₀-G₁ phase of the cell cycle through inhibition in cyclinD1 in a time-dependent manner.