Mesothelioma and mineral fibers

Frequent homozygous deletion of Cdkn2a/2b in tremolite-induced malignant mesothelioma in rats

The onset of malignant mesothelioma (MM) is linked to exposure to asbestos fibers. Asbestos fibers are classified as serpentine (chrysotile) or amphibole, which includes the crocidolite, amosite, anthophyllite, tremolite, and actinolite types. Although few studies have been undertaken, anthophyllite has been shown to be associated with mesothelioma, and tremolite, a contaminant in talc and chrysotile, is a risk factor for carcinogenicity. Here, after characterizing the length and width of these fibers by scanning electron microscopy, we explored the cytotoxicity induced by tremolite and anthophyllite in cells from an immortalized human mesothelial cell line (MeT5A), murine macrophages (RAW264.7), and in a rat model. Tremolite and short anthophyllite fibers were phagocytosed and localized to vacuoles, whereas the long anthophyllite fibers were caught on the pseudopod of the MeT5A and Raw 264.7 cells, according to transmission electron microscopy. The results from a 2-day time-lapse study revealed that tremolite was engulfed and damaged the MeT5A and RAW264.7 cells, but anthophyllite was not cytotoxic to these cells. Intraperitoneal injection of tremolite in rats induced diffuse serosal thickening, whereas anthophyllite formed focal fibrosis and granulomas on peritoneal serosal surfaces. Furthermore, the loss of Cdkn2a/2b, which are the most frequently lost foci in human MM, were observed in 8 cases of rat MM (homozygous deletion [5/8] and loss of heterozygosity [3/8]) by array-based comparative genomic hybridization techniques. These results indicate that tremolite initiates mesothelial injury and persistently frustrates phagocytes, causing subsequent peritoneal fibrosis and MM. The possible mechanisms of carcinogenicity based on fiber diameter/length are discussed.

[Asbestos and disease: review of the scientific knowledge and a rationale for urgent change in the current Brazilian policy about this question]

This paper is a state-of-the-art review of scientific knowledge on both the health effects of asbestos fiber inhalation and possibilities for safe and sustainable prevention, from an ethical, political, and technological point of view. The author provides scientific background and arguments from the ongoing discussion in Brazil concerning the need to redefine current asbestos policy, in order to establish a more advanced and appropriate policy whose priority is the protection of life, human health, and the environment. The first part deals with several technological and economic aspects of asbestos-chrysotile. In the second part, the author presents and discusses a bibliographic review of the construction of scientific knowledge on the health effects of asbestos fibers, first within an international perspective, and then (in the third part), from a Brazilian view. The fourth part analyzes the current debate on the fibrogenicity and carcinogenicity of asbestos-chrysotile. Some current responses from the international community towards the asbestos-chrysotile ban are also discussed. Finally, the author discusses the historical inadequacy of Brazilian asbestos policy and the urgent need to revise it to include a ban on asbestos-chrysotile in this country.

Mesotheliomas induced in rats by the fibrous mineral erionite are independent from p53 alterations

The development of human malignant mesothelioma (MM) is strongly associated with occupational or environmental exposure to certain natural mineral fibers, although the genetic mechanisms underlying this malignancy remain unclear. Although the p53 gene is frequently mutated in various tumors, human asbestos-associated MMs appear to develop independently from p53 alterations. The high mesotheliomagenic potency of natural fibrous mineral erionite is well established in humans and rodents, but no data regarding genetic alterations in erionite-associated tumors are currently available. Previous speculations that the oncogenic mechanisms underlying asbestos and erionite carcinogenesis may differ led us to examine whether the p53 gene is targeted in erionite carcinogenesis. Fifteen erionite-induced rat MMs as well as six cell lines derived from asbestos-induced and spontaneous rat MM were analyzed for p53 mutations by direct DNA sequencing and immunohistochemical analysis. Both approaches did not reveal p53 alterations in rat MM samples used in the study indicating that, similar to asbestos carcinogenesis, erionite carcinogenesis does not target the p53 tumor suppressor gene.

Environmental asbestos exposure and malignant pleural mesothelioma

Asbestos-related benign and malignant pleural diseases are endemic in some rural parts of central Turkey because of environmental exposure to asbestos fibres. We report here epidemiological data on 113 patients with diffuse malignant pleural mesothelioma (DMPM) diagnosed in our clinic in Eskişehir, located in central Turkey. Of the 113 patients, 59 were men and 54 women (male:female ratio = 1). Ninety-seven patients (86%) had non-occupational asbestos exposure; all were living in villages. Their mean age was 56 years. As the patients had been exposed to asbestos from birth, the latency period was equivalent to the age of the patients. Twenty-eight patients (29%) had lived in villages their entire lives. The other 69 (71%) had been born in a village but migrated to the city or had given up white-soil usage for various reasons. The mean exposure time was 55 years for those with a long exposure period and 25 years for those with a short exposure period, but there was no significant difference between the age of the disease appearance for both groups (55 and 56 years, respectively). Thus, the latency time of mesothelioma due to environmental exposure to asbestos was longer than that due to occupational exposure, but independent of the length of exposure. Soil samples from 67 villages were analysed, comprising a population of 10,120 villagers. Tremolite and some other types of asbestos were found. In conclusion, DMPM in our region is due to mainly to environmental exposure to asbestos. The risk is substantial as a large proportion of the villagers are exposed. After smoking, asbestos exposure is one of the most serious health hazards in our rural population.

Case-control study of mesothelioma in South Africa

BACKGROUND

South Africa has, uniquely, mined, transported, and used crocidolite, amosite, and chrysotile. A multicenter case-control study was done in South Africa to examine the details of asbestos exposure in cases and controls, and to calculate relative risks for level of certainty of asbestos exposure, nature of exposure (e.g., environmental, occupational) and fiber type.

METHODS

Cases and controls (one cancer and one medical per case) were collected by six study centers from referral hospitals, and exposure information was collected by interviewing cases and controls in life.

RESULTS

One hundred and twenty-three cases were accepted into the study. None had purely chrysotile exposure. Twenty-three cases had mined Cape crocidolite; three had mined amosite; and three Transvaal crocidolite plus amosite. A minimum of 22 of the cases had exclusively environmental exposure, 20 were from the NW Cape crocidolite mining area. The relative risks associated with environmental exposure in the NW Cape (crocidolite) were larger than for environmental exposure in the NE Transvaal (amosite and crocidolite): 21.9 vs. 7.1 and 50.9 vs. 12.0 for the cancer control and medical control datasets, respectively.

CONCLUSIONS

The results confirm the importance of environmental exposure in the Cape crocidolite mining area, the relative paucity of cases linked to amosite, the rarity of chrysotile cases and are consistent with a fiber gradient in mesotheliomagenic potential for South African asbestos with crocidolite > amosite > chrysotile.

Tyler asbestos workers: mortality experience in a cohort exposed to amosite

OBJECTIVES

To examine the causes of death among 1130 former workers of a plant in Tyler, Texas dedicated to the manufacture of asbestos pipe insulation materials. This cohort is important and unusual because it used amosite as the only asbestiform mineral in the production process. High level exposure of such a specific type was documented through industrial hygiene surveys in the plant.

METHODS

Deaths were ascertained through various sources including data tapes from the Texas Department of Health and the national death index files. As many death certificates as possible were secured (304/315) and cause of death assigned. After select exclusions, 222 death certificates were used in the analysis. Causes of death were compared with age, race, and sex specific mortalities for the United States population with a commercial software package (OCMAP Version 2.0).

RESULTS

There was an excess of deaths from respiratory cancer including the bronchus, trachea, and lung (standardised mortality ratio (SMR) 277 with 95% confidence interval (95% CI) 193 to 385). Four pleural mesotheliomas and two peritoneal mesotheliomas were identified. The analysis also showed an increasing risk of respiratory malignancy with increased duration of exposure including a significant excess of total deaths from respiratory cancer with less than six months of work at the plant (SMR 268 with 95% CI 172 to 399).

CONCLUSIONS

The importance of the cohort lies with the pure amosite exposure which took place in the plant and the extended period of latency which has followed. The death certificate analysis indicates the pathogenicity of amosite, the predominant commercial amphibole used in the United States. These data confirm a link between amosite asbestos and respiratory malignancy as well as mesothelioma.

Analysis of asbestos fiber burden in lung tissue from mesothelioma patients

Mesothelioma is a rare neoplasm that occurs most frequently in individuals with previous asbestos exposure. Differences for risk of development of asbestos-related mesothelioma and lung cancer have been attributed to the various types of asbestos, as well as to the dimension of the inhaled fibers. In the present study, 55 individuals with the pathological diagnosis of mesothelioma were evaluated as to ferruginous body and fiber content in lung tissue. The procedures used in the analysis included tissue digestion and analysis of the collected material for ferruginous bodies by light microscopy and for uncoated fibers by analytical transmission electron microscopy. Forty-six of the samples had ferruginous body concentrations of over 1000/per gram dry weight of lung tissue. The majority of the cores of these ferruginous bodies were amosite. Likewise, the most common uncoated asbestos fiber in the tissue was amosite. Only a small percentage of each type of asbestos would have been visible by light microscopy or even potentially by electron microscopy if the magnification was not sufficient to detect those with thin (< 0.2 micron) diameters. The consistent finding in most of the cases was a considerable presence of asbestos, often of mixed types.

The importance of surgical staging in the treatment of malignant pleural mesothelioma

OBJECTIVES

Progress in the therapy of malignant pleural mesothelioma is limited by the lack of an adequate staging system and controversy about prognostic factors. This surgical series was analyzed to determine whether a new TNM staging system proposed by the International Mesothelioma Interest Group and certain prognostic factors could stratify patients in future clinical trials.

METHODS

Thoracotomy was performed if computed tomographic scans showed resectable tumor confined to one hemithorax. Pleurectomy/decortication was done if visceral pleural tumor was minimal, and extrapleural pneumonectomy was done for more locally advanced disease. Complete resection was defined as no gross residual tumor. Adjuvant therapy was given as required by serial clinical trials. Patients had computed tomographic scans every 3 months until death. Prognostic factors were examined by log-rank and Cox regression analyses.

RESULTS

From October 1983 to July 1994, a total of 131 thoracotomies were performed, resulting in 101 resections, 72 of which were complete. Extrapleural pneumonectomy was done in 50 patients and pleurectomy/decortication in 51. The ratio of men to women was 108:23. Median age was 63 years (range 32 to 80 years). Operative mortality was five of 131 patients (3.8%), three of 50 in the group having extrapleural pneumonectomy (6%). Ninety-five of the 131 tumors were epithelial. Fifty-one of 89 patients (57%) having node dissections had diseased nodes, 45 (50%) N2. By univariate analysis, type of resection, T and N status, stage, histologic type, and adjuvant therapy, but no gender or age, significantly affected survival. Type of resection, stage, and histologic type were significant in a multivariate analysis. Local recurrence occurred mainly after pleurectomy/decortication, and distant metastases developed after extrapleural pneumonectomy.

CONCLUSIONS

(1) N2 nodal disease is more frequent than previously reported; (2) the prognostic importance of histologic type is confirmed; (3) both T and N status influence outcome, and the International Mesothelioma Interest Group staging system successfully identifies patients whose prognosis is poor; (4) despite more locally advanced disease in most patients with extrapleural pneumonectomy, that approach provided better local control than pleurectomy/decortication but failed to improve survival because of distant metastatic disease. Contrary to past practice, future clinical trials should stratify for histologic type, must control for TNM stage, and must consider the impact of type of surgical resection on the pattern of relapse.

Retention patterns of asbestos fibres in lung tissue among asbestos cement workers

Retention patterns in lung tissue (determined by transmission electron microscopy and energy dispersive spectrometry) of chrysotile, tremolite, and crocidolite fibres were analysed in 69 dead asbestos cement workers and 96 referents. There was an accumulation of tremolite with time of employment. Among workers who died within three years of the end of exposure, the 13 with high tremolite concentrations had a significantly longer duration of exposure than seven in a low to intermediate category (medians 32 v 20 years; p = 0.018, one sided). Crocidolite showed similar patterns of accumulation. In workers who died more than three years after the end of exposure, there were no correlations between concentrations of amphibole fibres and time between the end of exposure and death. Chrysotile concentrations among workers who died shortly after the end of exposure were higher than among the referents (median difference in concentrations 13 million fibres (f)/g dry weight; p = 0.033, one sided). No quantitative differences in exposure (duration or intensity) could be shown between workers with high and low to intermediate concentrations. Interestingly, all seven workers who had had a high intensity at the end of exposure (> 2.5 f/ml), had low to intermediate chrysotile concentrations at death, whereas those with low exposure were evenly distributed (31 subjects in both concentration categories); hence, there was a dependence between last intensity of exposure and chrysotile concentration (p = 0.014). Among 14 workers with a high average intensity of exposure, both those (n = 5) with high tissue concentrations of chrysotile and those (n = 10) with high tissue concentrations of tremolite fibres had more pronounced fibrosis than those with low to intermediate concentrations (median fibrosis grades for chrysotile: 2 v 1, p = 0.021; for tremolite: 2 v 0.5, p = 0.012). Additionally, workers who died shortly after the end of exposure with high concentrations of chrysotile and crocidolite had smoked more than those with low intermediate concentrations (medians for chrysotile 35 v 15 pack-years, p = 0.030; for crocidolite 37 v 15 pack-years, p = 0.012). The present data indicate that chrysotile has a relatively rapid turnover in human lungs, whereas the amphiboles, tremolite and crocidolite, have a slower turnover. Further, chrysotile retention may be dependent on dose rate. Chrysotile and crocidolite deposition and retention may be increased by tobacco smoking; chrysotile and tremolite by fibrosis.

Ultrastructure of mesothelial regeneration after intraperitoneal injection of asbestos fibres on rat omentum

In order to describe the ultrastructural features of the early phases of regenerating mesothelium in rat peritoneum, 69 cases were examined after intraperitoneal injection of 0.05-15 mg crocidolite, chrysotile B and other mineral and synthetic fibers. The findings show the presence of intermediate or transition cells between proliferating submesothelial connective tissue cells bearing the ultrastructural phenotype of myofibroblasts and mature fully regenerated mesothelium. Our results and data accumulated in the literature provide strong support for the hypothesis of submesothelial cells origin for regenerating mesothelium.

The incidence of malignant mesothelioma in Australia 1982-1988

From 1980 to 1985, the Australian Mesothelioma Surveillance Program, and since 1986, the Australian Mesothelioma Register, have been collecting data on all cases of malignant mesothelioma that could be ascertained in Australia. Incidence rates were calculated on 854 Program and 696 Register cases (total 1271) diagnosed in Australia between January 1, 1982 and December 31, 1988. Australia has one of the highest national rates of mesothelioma in the world (15.8 cases per million of population aged 20 years and older), and the rate is rising. The rate is far higher in males (28.3) than females (3.3). The Western Australian rate (28.9) is the highest among the states, as may be expected because of the crocidolite mine at Wittenoom; however, the largest numbers of cases occur in the more populous and industrial New South Wales. The high incidence rate, its expected continuing increase, and absence of a history of exposure to asbestos in approximately 28% of cases, demand consideration of potential environmental factors other than asbestos in the causation of this tumor, as well as continued surveillance.

The incidence of malignant mesothelioma in Australia, 1947-1980

Details of patients with malignant mesothelioma that was diagnosed in Australia before 1981 were obtained by searching all possible sources throughout Australia as far into the past as possible and up to and including 1980. The earliest patient with mesothelioma who was identified was diagnosed in Victoria in 1947. By 1980, 535 (81%) men and 123 (19%) women had been diagnosed with the disease; only 14 persons were aged less than 35 years at the time of diagnosis (the youngest person was 15 years of age). The incidence rate in subjects who were 35 years or older at diagnosis was less than 1.0 cases per million person-years until 1964-1968, and then it rose progressively to 15.5 cases per million person-years in 1979-1980. The highest rate (69.7 cases per million person-years) was observed in 65- to 74-year-old men in 1979-1980. The incidence rate in Western Australia was greater than were the rates in other states of Australia after the mid 1960s. Pleural mesotheliomas accounted for 88% of cases in which the site of the tumour was known; peritoneal mesotheliomas accounted for 10% of such cases and "other" sites for 2% of such cases. In 6% of cases the site was not specified. The exposure to asbestos was stated as "definite" in 59% of the cases with a recorded history of exposure: 8% of all the cases in the study had been exposed to crocidolite (blue asbestos) from Wittenoom Gorge in Western Australia. The age at diagnosis of patients with known exposure to asbestos was similar to that in those without known exposure. The increases in the incidence of malignant mesothelioma in Australia follow the published trends in the production and use of the amphibole varieties of asbestos in this country after a lag period of between 20 and 30 years.

Relative risk of mesothelioma among railroad machinists exposed to chrysotile

This study challenges the assertion of low relative risk of chrysotile in the causation of mesothelioma. Data are provided on the time period use of various types of asbestos in the United States and in insulation materials. The focus of the study is on mesothelioma among railroad machinists employed in the steam locomotive era who were exposed to chrysotile. Within a cohort of machinists alive January 1, 1945, a sub-cohort method was applied to all successive machinists hired in each of the respective years (1920-1929) followed through 1986. The total cohort was 181 and the number of deaths 156, with 14 mesotheliomas identified among 41 cancer deaths. The findings demonstrated an extremely high relative risk for machinists exposed to chrysotile for the induction of mesothelioma in the individual year of hire cohorts. A similar observation was noted for other crafts hired in the same time period. One mesothelioma occurred for every 13 machinists hired in the succeeding years (1920-1929) and constituted 34% of all cancer deaths. It is concluded that chrysotile is far more hazardous in the induction of mesotheliomas and that the asbestos cancer risk in the steam locomotive eras was much higher than had been previously estimated.