A single nucleotide polymorphism in the 5' untranslated region of RAD51 and risk of cancer among BRCA1/2 mutation carriers.

RAD51 colocalizes with both BRCA1 and BRCA2, and genetic variants in RAD51 would be candidate BRCA1/2 modifiers. We searched for RAD51 polymorphisms by sequencing 20 individuals. We compared the polymorphism allele frequencies between female BRCA1/2 mutation carriers with and without breast or ovarian cancer and between population-based ovarian cancer cases with BRCA1/2 mutations to cases and controls without mutations. We discovered two single nucleotide polymorphisms (SNPs) at positions 135 g-->c and 172 g-->t of the 5' untranslated region. In an initial group of BRCA1/2 mutation carriers, 14 (21%) of 67 breast cancer cases carried a "c" allele at RAD51:135 g-->c, whereas 8 (7%) of 119 women without breast cancer carried this allele. In a second set of 466 mutation carriers from three centers, the association of RAD51:135 g-->c with breast cancer risk was not confirmed. Analyses restricted to the 216 BRCA2 mutation carriers, however, showed a statistically significant association of the 135 "c" allele with the risk of breast cancer (adjusted odds ratio, 3.2; 95% confidence limit, 1.4-40). BRCA1/2 mutation carriers with ovarian cancer were only about one half as likely to carry the RAD51:135 g-->c SNP. Analysis of the RAD51:135 g-->c SNP in 738 subjects from an Israeli ovarian cancer case-control study was consistent with a lower risk of ovarian cancer among BRCA1/2 mutation carriers with the "c" allele. We have identified a RAD51 5' untranslated region SNP that may be associated with an increased risk of breast cancer and a lower risk of ovarian cancer among BRCA2 mutation carriers. The biochemical basis of this risk modifier is currently unknown.

Absence of post-transcriptional RNA modifications of BCL10 in human malignant mesothelioma and colorectal cancer.

The BCL10 gene, located at 1p22, has been implicated in a number of human malignancies, including malignant mesotheliomas (MMs) and colorectal carcinomas. Subsequent reports, however, have revealed an absence of BCL10 mutations in genomic DNA from such tumors. It has been proposed that some abnormalities of this gene may be found only in RNA and not in genomic DNA, suggesting that BCL10 may be mutated post-transcriptionally, rather than at the genomic level. To explore this possibility, we performed SSCP mutation analysis and direct sequencing of cDNA from 17 MM cell lines displaying LOH in 1p22, 12 MM tumor specimens, and 11 colon carcinoma cell lines. SSCP revealed several different band shifts in these samples. The nucleotide changes observed in the cDNA samples were also seen in matched genomic DNA and corresponded to known polymorphisms in the general population. Thus, we conclude the BCL10 mutations are absent at the cDNA level, and that this gene does not undergo "molecular misreading." Since BCL10 also does not possess mutations at the genomic DNA level, it can be ruled out as a gene involved in the pathogenesis of MM and colorectal cancer.

TbetaR-I(6A) is a candidate tumor susceptibility allele.

We have previously described a type I transforming growth factor (TGF)-beta receptor (TbetaR-I) polymorphic allele, TbetaR-I(6A), that has a deletion of three alanines from a nine-alanine stretch. We observed a higher than expected number of TbetaR-I(6A) homozygotes among tumor and nontumor DNA from patients with a diagnosis of cancer. To test the hypothesis that TbetaR-I(6A) homozygosity is associated with cancer, we performed a case-control study in patients with a diagnosis of cancer and matched healthy individuals with no history of cancer and who were identical in their gender and their geographical and ethnic background to determine the relative germ-line frequencies of this allele. We found nine TbetaR-I(6A) homozygotes among 851 patients with cancer. In comparison, there were no TbetaR-I(6A) homozygotes among 735 healthy volunteers (P < 0.01). We also observed an excess of TbetaR-I(6A) heterozygotes in cancer cases compared to controls (14.6% versus 10.6%; P = 0.02, Fisher's exact test). A subset analysis revealed that 4 of 112 patients with colorectal cancer were TbetaR-I(6A) homozygotes (P < 0.01). Using mink lung epithelial cell lines devoid of TbetaR-I, we established stably transfected TbetaR-I and TbetaR-I(6A) cell lines. We found that, compared to TbetaR-I, TbetaR-I(6A) was impaired as a mediator of TGF-beta antiproliferative signals. We conclude that TbetaR-I(6A) acts as a tumor susceptibility allele that may contribute to the development of cancer, especially colon cancer, by means of reduced TGF-beta-mediated growth inhibition.

Benzene increases aneuploidy in the lymphocytes of exposed workers: a comparison of data obtained by fluorescence in situ hybridization in interphase and metaphase cells.

Benzene is an established human leukemogen that increases the level of chromosome aberrations in lymphocytes of exposed workers. Numerical aberrations (aneusomy) can be observed by fluorescence in situ hybridization (FISH) in both interphase and metaphase cells. Whereas interphase FISH allows nondividing cells to be analyzed, one advantage of metaphase FISH is that it can also detect structural changes. The present study compares the abilities of metaphase and interphase FISH to detect aneusomy of chromosomes 7 and 8 in healthy benzene-exposed human subjects. Metaphase and interphase cells from the peripheral blood of 43 workers exposed to benzene (median = 31 ppm, 8-hr TWA) and 44 frequency-matched controls were analyzed by FISH. Normal diploid cells contained two hybridization signals, whereas those that were potentially monosomic contained one, trisomic 3 and tetrasomic 4. The frequency of cells with one hybridization signal for chromosome 7 in metaphase spreads rose from 2.72 +/- 0.19 (%, mean +/- SE) in controls to 3.79 +/- 0.63 in workers exposed to 31 or fewer ppm benzene and 5.9 +/- 0.85 in those exposed to more than 31 ppm (P(trend) < 0.0001). No similar dose-dependent increase in the frequency of cells with one hybridization signal was observed by interphase FISH, probably because of probe overlap artifact. Although significant dose-dependent increases in the frequency of cells with three hybridization signals for chromosome 7 were detected by both methods in the higher-exposed group, a larger, more significant difference was detected by metaphase FISH between controls and workers exposed to 31 or fewer ppm. Similar data were obtained for chromosome 8. Interphase and metaphase FISH were moderately correlated for three hybridization signals but not for one hybridization signal in chromosome 7 or 8. In general, metaphase FISH was more sensitive in detecting both monosomy and trisomy in the lymphocytes of exposed workers.

Increased aneusomy and long arm deletion of chromosomes 5 and 7 in the lymphocytes of Chinese workers exposed to benzene.

Two of the most common cytogenetic changes in therapy- and chemical-related leukemia are the loss and long (q) arm deletion of chromosomes 5 and 7. The detection of these aberrations in lymphocytes of individuals exposed to potential leukemogens may serve as useful biomarkers of increased leukemia risk. We have used a novel fluorescence in situ hybridization (FISH) procedure to determine if specific aberrations in chromosomes 1, 5 and 7 occur at an elevated rate in the blood cells of workers exposed to benzene. Forty-three healthy workers exposed to a wide range of benzene concentrations (median 31 p.p.m., 8 h time-weighted average) and 44 unexposed controls from Shanghai were studied. Whole blood was cultured and metaphase spreads were harvested at 72 h. Benzene exposure was associated with increases in the rates of monosomy 5 and 7 but not monosomy 1 (P < 0.001, P < 0.0001 and P = 0.94, respectively) and with increases in trisomy and tetrasomy frequencies of all three chromosomes. Long arm deletion of chromosomes 5 and 7 was increased in a dose-dependent fashion (P = 0.014 and P < 0.0001) up to 3.5-fold in the exposed workers. These results demonstrate that leukemia-specific changes in chromosomes 5 and 7 can be detected by FISH in the peripheral blood of otherwise healthy benzene-exposed workers. We suggest that aberrations in chromosomes 5 and 7 may be useful biomarkers of early biological effect for benzene exposure.

Increased translocations and aneusomy in chromosomes 8 and 21 among workers exposed to benzene.

Chromosome aberrations in peripheral blood lymphocytes have been used for many years to monitor human populations exposed to potential carcinogens. Recent reports have confirmed the validity of this approach by demonstrating that elevated levels of chromosome aberrations in lymphocytes are associated with subsequent increased cancer risk, especially for increased mortality from hematological malignancies including acute myeloid leukemia (AML). We postulated that this approach could be improved in two ways: (a) by detecting oncogenic disease-specific aberrations; and (b) by using chromosome painting so that many more metaphases could be analyzed. Numerical and structural aberrations in chromosomes 8 and 21 are commonly observed in AML. In the present study, we painted chromosomes 8 and 21 in lymphocyte metaphases from 43 healthy workers exposed to benzene, an established cause of AML, and from 44 matched controls. To examine dose-response relationships the workers were divided into two groups at the median exposure level, a lower-exposed group (< or = 31 ppm; n = 21), and a higher-exposed group (> 31 ppm; n = 22). Benzene exposure was associated with significant increases in hyperdiploidy of chromosomes 8 (1.2, 1.5, and 2.4 per 100 metaphases; P < 0.0001) and 21 (0.9, 1.1, and 1.9 per 100 metaphases; P < 0.0001). Translocations between chromosomes 8 and 21 were increased up to 15-fold in highly exposed workers (0.01, 0.04, and 0.16 per 100 metaphases; P < 0.0001). In one highly exposed individual, these translocations were reciprocal and were detectable by reverse transcriptase-PCR. These data indicate a potential role for t(8;21) in benzene-induced leukemogenesis and are consistent with the hypothesis that detection of specific chromosome aberrations may be a powerful approach to identify populations at increased risk of leukemia.

Genotoxicity of malathion in human lymphocytes assessed using the micronucleus assay in vitro and in vivo: a study of malathion-exposed workers.

The aerial application of malathion, a widely used organophosphate insecticide, has raised public concerns about potential adverse health effects. We therefore studied micronucleus formation in human lymphocytes as a biomarker of genotoxicity both in vitro and in vivo. Lymphocytes were cultured either as whole blood or after Ficoll isolation and treated with malathion in doses from 5 to 100 micrograms/ml for 48 h. A significant increase in micronucleated cells (47.5/1000 versus 16.0/1000 in DMSO control, p < 0.001) was found in isolated lymphocytes at high dose levels (75-100 micrograms/ml), concurrent with cytotoxicity and a strong inhibition of proliferation (p < 0.001). Many of the treated cells also possessed multiple micronuclei. Antikinetochore-antibody staining revealed that the majority of malathion-induced micronuclei were kinetochore-negative. A significant dose-response was also observed in whole blood cultures, although the increase in micronucleated cells was lower than in isolated lymphocyte cultures (p = 0.03). When the same technique was applied to lymphocytes of 38 intermittently malathion-exposed workers involved in the Mediterranean Fruit Fly Eradication Program in California, no change in either proliferation or micronucleus level was observed compared with an unexposed control group. We conclude that malathion has a relatively low potential to cause chromosome damage in vitro, and corresponding doses are much higher than ones that even professional applicators are likely to be exposed to in vivo. The potential risk of chromosome damage for malathion exposure in vivo is therefore relatively low. More studies are needed to assess the possibility of interaction of malathion with other pesticides through combined exposure.

An epidemiologic study of early biologic effects of benzene in Chinese workers.

Benzene is a recognized hematotoxin and leukemogen, but its mechanisms of action in humans are still uncertain. To provide insight into these processes, we carried out a cross-sectional study of 44 healthy workers currently exposed to benzene (median 8-hr time-weighted average; 31 ppm), and unexposed controls in Shanghai, China. Here we provide an overview of the study results on peripheral blood cells levels and somatic cell mutation frequency measured by the glycophorin A (GPA) gene loss assay and report on peripheral cytokine levels. All peripheral blood cells levels (i.e., total white blood cells, absolute lymphocyte count, platelets, red blood cells, and hemoglobin) were decreased among exposed workers compared to controls, with the exception of the red blood cell mean corpuscular volume, which was higher among exposed subjects. In contrast, peripheral cytokine levels (interleukin-3, interleukin-6, erythropoietin, granulocyte colony-stimulating factor, tissue necrosis factor-alpha) in a subset of the most highly exposed workers (n = 11) were similar to values in controls (n = 11), suggesting that benzene does not affect these growth factor levels in peripheral blood. The GPA assay measures stem cell or precursor erythroid cell mutations expressed in peripheral red blood cells of MN heterozygous subjects, identifying NN variants, which result from loss of the GPA M allele and duplication of the N allele, and N phi variants, which arise from gene inactivation. The NN (but not N phi) GPA variant cell frequency was elevated in the exposed workers compared with controls (mean +/- SD, 13.9 +/- 8.4 mutants per million cells versus 7.4 +/- 5.2 per million cells, (respectively; p = 0.0002), suggesting that benzene produces gene-duplicating but not gene-inactivating mutations at the GPA locus in bone marrow cells of exposed humans. These findings, combined with ongoing analyses of benzene macromolecular adducts and chromosomal aberrations, will provide an opportunity to comprehensively evaluate a wide range of early biologic effects associated with benzene exposure in humans.

Chemotherapy-induced changes in uterine volume: CT study.

OBJECTIVE: The goal of this study was to quantify changes in uterine volume during and after chemotherapy for breast cancer. MATERIALS AND METHODS: Fifty-eight patients (mean age 42 years) with advanced breast cancer undergoing treatment with FLAC (5-fluorouracil, leucovorin, Adriamycin, and cyclophosphamide) were studied with serial pelvic CT. The transverse, anteroposterior, and sagittal measurements of the uterus were taken in each of the CT scans. The uterine volumes were calculated, normalized to baseline volumes, and graphically displayed for each patient. The temporal changes in uterine volume were correlated to the dates of chemotherapy administration and menstrual status. RESULTS: There was a striking and consistent loss of uterine volume with chemotherapy. This phenomenon was observed in 55 (95%) of the 58 patients. The mean minimum uterine volume was 58.3 +/- 20.2% compared with the baseline. This loss of uterine volume began after the administration of the first cycle of chemotherapy and progressed during subsequent cycles. It coincided with loss of normal menses in all patients. After completion of chemotherapy, there was a recovery of the uterine volume in 12 of the 16 patients who had follow-up CT. The mean recovery uterine volume was 108.4 +/- 49.8% of baseline. Menses recovered in four of these. CONCLUSION: Chemotherapy causes loss of uterine volume, which usually recovers after the withdrawal of chemotherapy in premenopausal women. This change should not be mistaken for a therapeutic response of primary or secondary malignancy within the uterus or be confused with subsequent uterine changes due to tamoxifen.

Induction of kinetochore-positive micronuclei in human lymphocytes by the anti-fungal drug griseofulvin.

Griseofulvin (GF) is a widely used antifungal drug for the treatment of superficial dermatomycoses. However, because GF is carcinogenic and teratogenic in animal models there is considerable concern regarding its clinical application. Further, it produces numerical chromosome aberrations in human lymphocytes and cell lines. There are conflicting reports on the ability of GF to induce structural chromosomal aberrations. Here, we show GF induces micronucleus formation both in isolated peripheral lymphocytes and lymphocytes from whole blood cultures. An antikinetochore antibody was used to distinguish micronuclei with acentric chromosome fragments (kinetochore-negative) and from those containing whole chromosomes (kinetochore-positive). The micronuclei formed were 99% kinetochore-positive in isolated lymphocytes. In addition, GF was able to alter the cell cycle kinetics of lymphocytes, thereby increasing the percentage of triploid cells. We conclude that GF is a strong aneuploidy-inducing agent in peripheral human lymphocytes and produces effects at concentrations which should be detectable in the blood of persons undergoing therapy.

Benzene and its phenolic metabolites produce oxidative DNA damage in HL60 cells in vitro and in the bone marrow in vivo.

Benzene, an important industrial chemical, is myelotoxic and leukemogenic in humans. It is metabolized by cytochrome P450 2E1 to various phenolic metabolites which accumulate in the bone marrow. Bone marrow contains high levels of myeloperoxidase which can catalyze the further metabolism of the phenolic metabolites to reactive free radical species. Redox cycling of these free radical species produces active oxygen. This active oxygen may damage cellular DNA (known as oxidative DNA damage) and induce genotoxic effects. Here we report the induction of oxidative DNA damage by benzene and its phenolic metabolites in HL60 cells in vitro and in the bone marrow of C57BL/6 x C3H F1 mice in vivo utilizing 8-hydroxy-2'-deoxyguanosine as a marker. HL60 cells (a human leukemia cell line) contain high levels of myeloperoxidase and were used as an in vitro model system. Exposure of these cells to phenol, hydroquinone, and 1,2,4-benzenetriol resulted in an increased level of oxidative DNA damage. An increase in oxidative DNA damage was also observed in the mouse bone marrow in vivo 1 h after benzene administration. A dose of 200 mg/kg benzene produced a 5-fold increase in the 8-hydroxydeoxyguanosine level. Combinations of phenol, catechol, and hydroquinone also resulted in significant increases in steady state levels of oxidative DNA damage in the mouse bone marrow but were not effective when administered individually. Administration of 1,2,4-benzenetriol alone did, however, result in a significant increase in oxidative DNA damage. This represents the first direct demonstration of active oxygen production by benzene and its phenolic metabolites in vivo. The conversion of benzene to phenolic metabolites and the subsequent production of oxidative DNA damage may therefore play a role in the benzene-induced genotoxicity, myelotoxicity, and leukemia.

Benzene metabolite, 1,2,4-benzenetriol, induces micronuclei and oxidative DNA damage in human lymphocytes and HL60 cells.

The triphenolic metabolite of benzene, 1,2,4-benzenetriol (BT), is readily oxidized to its corresponding quinone via a semiquinone radical. During this process, active oxygen species are formed that may damage DNA and other cellular macromolecules. The ability of BT to induce micronuclei (MN) and oxidative DNA damage has been investigated in both human lymphocytes and HL60 cells. An antikinetochore antibody based micronucleus assay was used to distinguish MN containing kinetochores and potentially entire chromosomes (kinetochore-positive, K+) from those containing acentric chromosome fragments (kinetochore-negative, K-). BT increased the frequency of MN formation twofold in lymphocytes and eightfold in HL60 cells with the MN being 62% and 82% K+, respectively. A linear dose-related increase in total MN, mainly in K(+)-MN, was observed in both HL60 cells and lymphocytes. Addition of copper ions (Cu2+) potentiated the effect of BT on MN induction threefold in HL60 cells and altered the pattern of MN formation from predominantly K+ to K-. BT also increased the level of 8-hydroxy-2'-deoxyguanosine (8-OH-dG), a marker of active oxygen-induced DNA damage. Cu2+ again enhanced this effect. Thus, BT has the potential to cause both numerical and structural chromosomal changes in human cells. Further, it may cause point mutations indirectly by generating oxygen radicals. BT may therefore play an important role in benzene-induced leukemia.

Assay of excised oxidative DNA lesions: isolation of 8-oxoguanine and its nucleoside derivatives from biological fluids with a monoclonal antibody column.

An immunoaffinity column is described that facilitates the analysis of oxidative damage products of DNA and RNA in urine, blood plasma, and medium isolated from cultures of Escherichia coli. In intact animals, lesions (adducts) excised from DNA are transported from the cell through the circulation and excreted in urine. In bacteria, DNA adducts are excreted directly into the medium. In either case, the adducts can be assayed as a measure of oxidative damage to DNA. A monoclonal antibody that recognizes 8-oxo-7,8-dihydro-2'-deoxyguanosine (oxo8dG;8-hydroxy-2'-deoxyguanosine), a bio-marker of oxidative damage to DNA, has been isolated, and its substrate binding properties have been characterized. The relative binding affinities of this monoclonal antibody for oxo8dG, unmodified nucleosides, or derivatives of Gua made it suitable for the preparation of immunoaffinity columns that greatly facilitate the isolation of oxo8dG, 8-oxo-7,8-dihydroguanine, and 8-oxo-7,8-dihydroguanosine from various biological fluids. Quantitative analysis of these adducts in urine of rats fed a nucleic acid-free diet and in the medium from cultures of E. coli suggests that oxo8-7,8-dihydroguanine is the principal repair product from oxo8-dG in DNA of both eukaryotes and prokaryotes. The results support our previous estimate of about 10(5) oxidative lesions to DNA being formed and excised in an average rat cell per day.

Metabolism of hydroquinone by human myeloperoxidase: mechanisms of stimulation by other phenolic compounds.

Hydroquinone, a metabolite of benzene, is converted by human myeloperoxidase to 1,4-benzoquinone, a highly toxic species. This conversion is stimulated by phenol, another metabolite of benzene. Here we report that peroxidase-dependent hydroquinone metabolism is also stimulated by catechol, resorcinol, o-cresol, m-cresol, p-cresol, guaiacol, histidine, and imidazole. In order to gain insights into the mechanisms of this stimulation, we have compared the kinetics of human myeloperoxidase-dependent phenol, hydroquinone, and catechol metabolism. The specificity (Vmax/Km) of hydroquinone for myeloperoxidase was found to be 5-fold greater than that of catechol and 16-fold greater than that of phenol. These specificities for myeloperoxidase-dependent metabolism inversely correlated with the respective one-electron oxidation potentials of hydroquinone, catechol, and phenol and suggested that phenol- and catechol-induced stimulation of myeloperoxidase-dependent hydroquinone metabolism cannot simply be explained by interaction of hydroquinone with stimulant-derived radicals. Phenol (100 microM), catechol (20 microM), and imidazole (50 mM) did, however, all increase the specificity (Vmax/Km) of hydroquinone for myeloperoxidase, indicating that these three compounds may be stimulating hydroquinone metabolism by a common mechanism. Interestingly, the stimulation of peroxidase-dependent hydroquinone metabolism by other phenolic compounds was pH-dependent, with the stimulating effect being higher under alkaline conditions. These results therefore suggest that the interaction of phenolic compounds, presumably by hydrogen-bonding, with the activity limiting distal amino acid residue(s) or with the ferryl oxygen of peroxidase may be an important contributing factor in the enhanced myeloperoxidase-dependent metabolism of hydroquinone in the presence of other phenolic compounds.

Metabolism of phenylhydroquinone by prostaglandin (H) synthase: possible implications in o-phenylphenol carcinogenesis.

o-Phenylphenol (OPP) and its sodium salt sodium ortho-phenylphenate (NaOPP) are broad spectrum fungicides and antibacterial agents. Both are urinary bladder and renal carcinogens in the Fischer 344 rat. OPP is converted by mixed-function oxidases in the liver to phenylhydroquinone (PHQ). Since appreciable amounts of prostaglandin (H) synthase (PGS) are found in rat bladder and kidney-medullary papilla, the target sites of OPP- and NaOPP-induced tumors, we hypothesized that a secondary PGS-mediated activation of PHQ to phenylbenzoquinone (PBQ) may occur in the bladder and kidney. We have studied the metabolism of PHQ by PGS in the presence of arachidonic acid and hydrogen peroxide as co-factors. These studies showed that PHQ is indeed metabolized to a product having identical spectral and electrochemical properties to PBQ. The disappearance of PHQ with time was stoichiometric to the formation of PBQ. Less than 10% of PHQ was converted to PBQ in the absence of enzyme, indicating that auto-oxidation may play only a minor role in the conversion of PHQ to PBQ. Similar results were obtained when PGS was replaced with either myeloperoxidase or horseradish peroxidase and hydrogen peroxide as co-factor. These studies suggest that the peroxidative metabolism of PHQ by PGS to the reactive PBQ could play an important role in OPP-induced urinary bladder and kidney carcinogenesis in rats.

Hydroxylation of phenol to hydroquinone catalyzed by a human myeloperoxidase-superoxide complex: possible implications in benzene-induced myelotoxicity.

Benzene, a known human myelotoxin and leukemogen is metabolized by liver cytochrome P-450 monooxygenase to phenol. Further hydroxylation of phenol by cytochrome P-450 monooxygenase results in the formation of mainly hydroquinone, which accumulates in the bone marrow. Bone marrow contains high levels of myeloperoxidase. Here we report that phenol hydroxylation to hydroquinone is also catalyzed by human myeloperoxidase in the presence of a superoxide anion radical generating system, hypoxanthine and xanthine oxidase. No hydroquinone formation was detected in the absence of myeloperoxidase. At low concentrations superoxide dismutase stimulated, but at high concentrations inhibited, the conversion of phenol to hydroquinone. The inhibitory effect at high superoxide dismutase concentrations indicates that the active hydroxylating species of myeloperoxidase is not derived from its interaction with hydrogen peroxide. Furthermore, catalase a hydrogen peroxide scavenger, was found to have no significant effect on hydroxylation of phenol to hydroquinone, supporting the lack of hydrogen peroxide involvement. Mannitol (a hydroxyl radical scavenger) was found to have no inhibitory effect, but histidine (a singlet oxygen scavenger) inhibited hydroquinone formation. Based on these results we postulate that a myeloperoxidase-superoxide complex spontaneously rearranges to generate singlet oxygen and that this singlet oxygen is responsible for phenol hydroxylation to hydroquinone. These results also suggest that myeloperoxidase dependent hydroquinone formation could play a role in the production and accumulation of hydroquinone in bone marrow, the target organ of benzene-induced myelotoxicity.