Germ-line mutations of the BRCA1 gene predispose women to early-onset breast and ovarian cancer by compromising the gene's presumptive function as a tumor suppressor. Although the biochemical properties of BRCA1 polypeptides are not understood, their expression pattern and subcellular localization suggest a role in cell-cycle regulation. When resting cells are induced to proliferate, the steady-state levels of BRCA1 increase in late G 1 and reach a maximum during S phase. Moreover, in S phase cells, BRCA1 polypeptides are hyperphosphorylated and accumulate into discrete subnuclear foci termed ''BRCA1 nuclear dots.'' BRCA1 associates in vivo with a structurally related protein termed BARD1. Here we show that the steady-state levels of BARD1, unlike those of BRCA1, remain relatively constant during cell cycle progression. However, immunostaining revealed that BARD1 resides within BRCA1 nuclear dots during S phase of the cell cycle, but not during the G 1 phase. Nevertheless, BARD1 polypeptides are found exclusively in the nuclear fractions of both G 1 -and S-phase cells. Therefore, progression to S phase is accompanied by the aggregation of nuclear BARD1 polypeptides into BRCA1 nuclear dots. This cell cycle-dependent colocalization of BARD1 and BRCA1 indicates a role for BARD1 in BRCA1-mediated tumor suppression.The BRCA1 tumor suppressor has been implicated in familial cases of early-onset breast and ovarian cancer (1, 2). However, the biochemical functions of its protein product are not defined and the mechanism by which it counters tumor formation during normal development is not understood. The major isoform of BRCA1 is a polypeptide of Ϸ220 kDa that bears several recognizable amino acid motifs: these include a zinc-binding RING domain that lies near the amino terminus, two nuclear localization signals, and two tandem copies of the BRCT motif that reside at the carboxyl terminus (2-5). BRCA1 associates in vivo with BARD1, a protein that also contains an amino-terminal RING domain and two carboxylterminal BRCT motifs (6). The interaction between these proteins is abolished by tumorigenic missense mutations in the RING domain of BRCA1, raising the possibility that tumor suppression is mediated by a heteromeric complex of BRCA1 and BARD1.Products of the BRCA1 gene are found in a broad spectrum of cell and tissue types (2,7,8); however, the expression of this gene in most (9-12), but not all (13), cell types is tightly regulated during cell cycle progression. In resting cells, the levels of BRCA1 transcripts and polypeptides are either low or undetectable. However, after these cells receive a mitotic stimulus the steady-state levels of BRCA1 products rise in late G 1 , peak just prior to the onset of DNA synthesis, and persist for the duration of S phase and most of M phase. In addition, BRCA1 polypeptides become hyperphosphorylated as they begin to accumulate in late G 1 (9). While not conclusive, these findings suggest that BRCA1 may be involved in some aspect of cell cycle regulation (9-12).While there ...

Human O6-methylguanine-DNA methyltransferase (MGMT) repairs DNA by transferring alkyl (R-) adducts from O6-alkylguanine (6RG) in DNA to its own cysteine residue at codon 145 (formation of R-MGMT). We show here that R-MGMT in cell extracts, which is sensitive to protease V8 cleavage at the glutamic acid residues at codons 30 (E30) and 172 (E172), can be specifically immunoprecipitated with an MGMT monoclonal antibody, Mab.3C7. This Mab recognizes an epitope of human MGMT including the lysine 107 (K107) which is within the most basic region that is highly conserved among mammalian MGMTs. Surprisingly, the K107L mutant protein is repair-deficient and readily cleaved by protease V8 similar to R-MGMT. We propose that R-MGMT adopted an altered conformation which exposed the Mab.3C7 epitope and rendered that protein sensitive to protease V8 attack. This proposal could be explained by the disruption of a structural "salt-link" within the molecule based on the available structural and biochemical data. The specific binding of Mab.3C7 to R-MGMT has been compared with the protease V8 method in the detection of R-MGMT in extracts of cells treated with low dosages of methyliodide (SN2) and O6-benzylguanine. Their identical behaviors in producing protease V8 sensitive R-MGMT and Mab.3C7 immunoprecipitates suggest that probably methyl iodide (an ineffective agent in producing 6RG in DNA) can directly alkylate the active site of cellular MGMT similar to O6-benzylguanine. The effectiveness of MeI in producing R-MGMT, i.e., inactivation of cellular MGMT, indicates that this agent can increase the effectiveness of environmental and endogenously produced alkylating carcinogens in producing the mutagenic O6-alkylguanine residues in DNA in vivo.

The BRCA1 gene encodes a tumor suppressor that has been implicated in hereditary forms of breast and ovarian cancer. During S phase of the cell cycle, BRCA1 polypeptides are found in discrete nuclear bodies (`BRCA1 nuclear dots') together with HsRad51, a human homolog of the E. coli recA protein, and BARD1, a protein that interacts with BRCA1 to form a stable heterodimer. BARD1 is structurally similar to BRCA1 in that both molecules harbor an amino-terminal RING domain and two carboxy-terminal BRCT domains. Here we describe the amino acid sequence and expression pattern of murine Bard1. A comparison of the mouse and human sequences reveals that the recognizable protein motifs of BARD1 are well conserved, including the RING domain, the three tandem ankyrin repeats, and, to a lesser extent, the two BRCT domains. However, the remaining sequences of BARD1 display a markedly lower degree of phylogenetic conservation, comparable to those reported for BRCA1 and BRCA2. Moreover, murine Bard1 retains the ability to associate in vivo with BRCA1, and its expression pattern in adult mice mirrors that of Brca1, with elevated levels of RNA transcripts found in the testes and spleen. These data suggest that BRCA1 and BARD1 have co-evolved to participate in a common pathway of tumor suppression.

DNA lesions that halt RNA polymerase during transcription are preferentially repaired by the nucleotide excision repair pathway. This transcription-coupled repair is initiated by the arrested RNA polymerase at the DNA lesion. However, the mutagenic O 6 -methylguanine (6MG) lesion which is bypassed by RNA polymerase is also preferentially repaired at the transcriptionally active DNA. We report here a plausible explanation for this observation: the human 6MG repair enzyme O 6 -methylguanine-DNA methyltransferase (MGMT) is present as speckles concentrated at active transcription sites (as revealed by polyclonal antibodies specific for its N and C termini). Upon treatment of cells with low dosages of N-methylnitrosourea, which produces 6MG lesions in the DNA, these speckles rapidly disappear, accompanied by the formation of active-site methylated MGMT (the repair product of 6MG by MGMT). The ability of MGMT to target itself to active transcription sites, thus providing an effective means of repairing 6MG lesions, possibly at transcriptionally active DNA, indicates its crucial role in human cancer and chemotherapy by alkylating agents.DNA in unwound (active) chromatin at sites of transcription or replication is vulnerable to damage induced by chemicals and irradiation (3,7,32,34). Left unrepaired, these DNA lesions affect cell survival. First, they either inhibit DNA polymerase (11) or are miscoded by the polymerase during DNA replication (1,30,31). Second, they can halt RNA polymerase during transcription of active genes (44). For example, to overcome the possible lethal blockage of transcription due to the arrested RNA polymerase at the thymine-thymine (T-T) photodimer (or bulky DNA lesions) formed in the transcribing DNA strand by irradiation, bacteria use the MFD protein (a transcription repair coupling [TRC] factor), which interacts with the arrested RNA polymerase at the lesion and recruits the uvrABC repair proteins (bacterial nucleotide excision repair [NER] proteins) for its repair (41,42). In eukaryotes, similar preferential repair of bulky DNA lesions in the transcribing DNA strand by the NER pathway, i.e., TRC, has been reported. However, the details of the mechanism appear to be much more complicated than the prokaryotic counterpart since coupling of eukaryotic DNA repair to transcription should involve several stages, such as nucleosome remodelling (e.g., the yeast RAD26 protein as a Swi2/Snf2-like ATPase [50]), assembling of the multicomponent preinitiation complex (e.g., the RAD25 helicase as a subunit of TFIIH [50]), and possibly others (e.g., the unestablished role of the human ERCC6 protein as a DNA-dependent ATPase [43]). Furthermore, TRC may be interrelated between different DNA repair pathways as mismatch repair-defective human cells may lack TRC of the T-T photodimer by NER (33).The N-nitroso compounds are carcinogens to which we are all exposed because they are synthesized naturally in our gastrointestinal tract. They are also cytotoxic, and some of them, notably bis-chloroethylnitrosourea, ar...

No abstract