Inhaled crocidolite mutagenicity in lung DNA

Our plain-English summary of Inhaled crocidolite mutagenicity in lung DNA, by B. Rihn, C. Coulais, E. Kauffer, et al (Environmental Health Perspectives, 2000).

The study

• Authors: B. Rihn, C. Coulais, E. Kauffer, M.C. Bottin, P. Martin, F. Yvon, J.C. Vigneron, S. Binet, N. Monhoven, G. Steiblen, G. Keith
• Journal: Environmental Health Perspectives
• Published: 2000;108(4):341-6
• Cited by: 26 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

We used transgenic mice carrying the lacI reporter gene to study the mutagenesis potential of asbestos crocidolite. The animals were exposed by nose-only inhalation to an aerosol containing 5.75 mg/m(3) crocidolite dust for 6 hr/day and 5 consecutive days. After 1, 4, and 12 weeks, we examined four end points: the cytology of bronchoalveolar lavage, the lung load of crocidolite, the hydrophobic DNA adducts, and the mutations in the lacI reporter gene. Twelve weeks after exposure, nearly 10% of the inhaled fibers remained in the lung (227 +/- 103 ng/mg lung). There was evidence of a typical inflammatory response consisting of multinucleate macrophages at weeks 4 and 12, whereas immediately after the exposure, we observed numerous polymorphonuclear neutrophils. The mutant frequency significatively increased during the fourth week after the exposure: 13.5 [time] 10(-5) in the exposed group versus 6. 9 10(-5) in the control group. The induction factor, defined by the ratio of checked mutants of exposed mice to checked mutants of control mice, was 1.96. The mutation spectrum of control lung DNA and exposed lung DNA was similar, suggesting the possible involvement of a DNA repair decrease in crocidolite-treated animals. We used the (32)P-postlabeling method and did not detect any increase of either 5 mC or bulky adduct in treated mice. This is the first study that demonstrates asbestos mutagenicity in vivo after a nose-only inhalation.

In plain English

This animal experiment tested whether inhaled crocidolite asbestos causes gene mutations in lung DNA. Two-month-old male transgenic mice carrying a 'reporter' gene (lacI) breathed crocidolite dust through the nose only, averaging 5.75 mg/m3, for 6 hours a day over 5 days. Control mice were held in the same tubes without dust. The aerosol was made up mainly of short fibers. Four weeks after exposure, the mutant frequency in lung DNA was 13.5 x 10^-5 in exposed mice versus 6.9 x 10^-5 in controls, a ratio of 1.96 (p less than 0.05). At 1 and 12 weeks the two groups did not differ significantly. The types of mutations were similar in both groups. Exposed mice showed lung inflammation but no fibrosis. Twelve weeks after exposure their lungs still held 227 plus or minus 103 ng of fibers per mg of lung. No increase in bulky DNA adducts was found. The authors concluded that crocidolite roughly doubled the mutant frequency. They noted that their test cannot detect large DNA insertions or deletions, so the effect is probably underestimated.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Animal experiment: short-term inhalation study in transgenic mice carrying the lacI reporter gene, with unexposed controls, examined 1, 4 and 12 weeks after exposure
• Who was studied: Two-month-old transgenic male LacI (Big Blue, C57/B16) mice, studied at the Institut National de Recherche et de Sécurité in France. Lung fiber content was measured in four control and four exposed mice at each of 1 and 12 weeks.
• Exposure: Nose-only inhalation of crocidolite dust at an average of 5.75 plus or minus 0.87 mg/m3, 6 hours a day for 5 days. The average fiber density was 1,875 plus or minus 1,216 fibers/cm3, counting only fibers longer than 5 micrometers and thinner than 3 micrometers. Fibers meeting Stanton's criteria stayed below 3% of all fibers, so the aerosol was mainly short fibers. The authors estimated the total inhaled dose at 0.26 mg per mouse.
• Main results:
  • Four weeks after exposure, the lung DNA mutant frequency was 13.5 x 10^-5 (plus or minus 1.7 x 10^-5) in exposed mice versus 6.9 x 10^-5 (plus or minus 1.1 x 10^-5) in control mice. The induction factor was 1.96 (p less than 0.05).
  • At 1 week and at 12 weeks after exposure, mutant frequencies in exposed and control mice were not significantly different.
  • Among 60 sequenced mutants per group at 4 weeks, the types and positions of mutations did not differ significantly between control and exposed mice.
  • No fibers were detected in control lungs (less than 1 ng/mg lung). Exposed mice averaged 693 plus or minus 496 ng fibers/mg lung at 1 week and 227 plus or minus 103 ng/mg at 12 weeks. This corresponded to an estimated 17.1% and 6.6% of the total inhaled fibers.
  • At 1 week, exposed mice showed a large rise in neutrophils (polymorphonucleated cells) and lymphocytes in lung washings. Macrophages with two or more nuclei were increased at 4 and 12 weeks. Average Wagner scores were 1.0 in controls and 1.7 in exposed mice, which the authors read as inflammation without fibrosis.
  • No excess of bulky DNA adducts was found in exposed mice. Naturally occurring adducts (I-compounds) were slightly lower in exposed mice: 160 versus 60 per 10^9 nucleotides for control and treated mice, respectively. The DNA methylation ratio (m5dC/dC) was 3.0 plus or minus 0.7 in controls versus 3.5 plus or minus 0.4 in exposed mice, with no statistical difference.
• The authors' conclusion: The authors concluded that crocidolite exposure significantly increased the mutant frequency of lung DNA, to approximately twice that of controls. They saw mutagenesis as an early event and found that a low fiber dose was enough to observe it. They suggested the mutations could be due to oxidative stress acting randomly on DNA rather than a specific direct effect of the fibers.
• Limitations the authors state:
  • The Big Blue test cannot detect large DNA insertions and deletions, so the authors considered the true mutation frequency due to asbestos probably underestimated.
  • The DNA repair capacity of the cells, one of the proposed mechanisms, was not tested.
  • The adduct method used detects only bulky adducts.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Crocidolite.

Medical terms in this study, explained in our Lexicon: Crocidolite, Cytology, Bronchoalveolar Lavage (BAL), DNA adducts, Macrophage.

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