The Health Effects Institute
http://www.healtheffects.org/Pubs/st103.htm
STATEMENT Synopsis of Research Report 103
The Nature of Chromosomal Alterations and How They Are Induced by
Benzene in Mice and Humans
This Statement, prepared by the Health Effects Institute, summarizes a
research project funded by HEI and conducted by Dr David Eastmond of
the University of California, Riverside. The following Research Report
contains both the detailed Investigators’ Report and a Commentary on
the study prepared by the Institute’s Health Review Committee.
EASTMOND 103
INTRODUCTION
Exposure to high levels of benzene is associated with the development
of leukemia and other blood disorders, but the effects of exposure to
low levels of benzene are not well understood. In the 1990s, the
Health Effects Institute initiated its Air Toxics Re-search Program to
address uncertainties about the health effects that may result from
exposure to ambient levels of benzene and other air toxics derived
from mobile sources. One of the goals of this program was to develop
and validate biomarkers of benzene exposure.
Benzene can induce changes in the structure and function of
chromosomes, although the relevance of these findings to the
development of clinical conditions has not been fully established. HEI
funded Dr David Eastmond to investigate two related approaches to
determining whether such chromosomal changes could be used as
biomarkers of benzene exposure in mice and humans. HEI also thought
that Eastmond’s study would provide useful data to compare benzene’s
effects in two species.
APPROACH
The first part of the study involved detecting chromosomal alterations
in cells using a modification of a molecular cytogenetic technique
known as fluorescence in situ hybridization (FISH). Eastmond used two
different fluorescently labeled DNA sequences ("tandem labeled
probes") that would bind to unique regions of particular chromosomes.
This approach, if successful, may be better than other cytogenetic
methods for estimating benzene’s effects because it is potentially
highly sensitive and may be useful in large population studies. It
could also provide information about how different chromosomal
alterations arise. Eastmond and colleagues evaluated the frequency of
such chromosomal aberrations in the erythrocytes (red blood cells)
from the bone marrow of mice exposed to various doses of benzene (50
to 450 mg/kg of body weight per day) and for different exposure
durations (2, 6, or 12 weeks). The investigators also tested
aberrations in chromosomes 1 and 9 of peripheral blood cells from two
groups of humans occupationally exposed to benzene who were matched
with control subjects. One exposed population comprised 44 Chinese
workers who were either currently being exposed to median levels of 31
parts per million (ppm) benzene, or had formerly been exposed to such
high levels that they had become "benzene poisoned." The other exposed
population was made up of 17 Estonian workers; 12 subjects were in
benzene production (exposed to about 1.3 ppm) and 5 were operating a
coke oven (exposed to about 0.3 ppm benzene).
The second part of Eastmond’s proposal was to determine whether
benzene or its metabolites affect DNA indirectly, acting through the
nuclear enzyme topoisomerase II. This enzyme plays a key role in
maintaining the chromosomal structure, so inhibiting topoisomerase II
function might lead to chromosomal damage or to the development of
aberrations. The investigators tested a number of benzene metabolites
in vitro to assess their inhibitory effects on the purified human
enzyme and on the enzyme’s activity in a human cell line. They also
tested whether administering benzene orally to mice would inhibit the
enzyme’s activity in vivo. This part of the study was expected to
provide novel information about what mechanisms may be relevant to the
carcinogenic effects of benzene, which are not well understood.
RESULTS AND INTERPRETATION
Eastmond and colleagues addressed several important goals in their
study. Using tandem labeled fluorescent probes they demonstrated that
they could detect some types of benzene-induced chromosomal
alterations in mice and humans. Controlled exposure studies in mice
suggested that benzene-induced increases in chromosomal alterations in
bone marrow erythrocytes are dependent on both dose and duration of
exposure. By contrast, the results of human biomonitoring were not as
clearcut: Chromosomal alterations in the highly exposed population of
Chinese workers did not differ from control levels, but the smaller
group of Estonian workers who were exposed to lower levels did show
chromosomal changes related to benzene exposure. A number of reasons
may explain why the investigators found higher numbers of aberrations
in the chromosomes of Estonian workers than in Chinese workers. For
example, an agent or agents distinct from benzene in the Estonian work
environment (such as polycyclic aromatic hydrocarbons) may be a
factor; differences in lifestyle (such as diet or medications) may be
influential; or an unusual dose-response curve for benzene, in which
lower doses would induce higher numbers of aberrations, is an option.
An additional explanation is that these two groups of workers could
express different types of enzymes that may metabolize benzene along
distinct pathways to harmful or less harmful metabolites. The binding
of the fluorescent DNA probes to cells is also likely to be critically
influenced by the way in which the slides of cell samples are
pre-pared. Because slides for the two studies were pre-pared in
different countries, it is quite probable that differences in
preparation conditions might also have affected the results. Thus,
although the results obtained by Eastmond and his colleagues indicate
the feasibility of the approach tested, they also underline important
limitations in the use of the tandem labeled FISH assay in large human
studies.
These investigators were the first to show that benzene administration
to mice in vivo, and some benzene metabolites or potential metabolites
in vitro, can inhibit the nuclear enzyme topoisomerase II. These
findings suggest a potential mechanism by which benzene may induce
genotoxic and carcinogenic effects. Because the results of the in
vitro assay of topoisomerase II activity were not linear in the
dilution range tested, however, the assay cannot be used at present as
an indicator of early benzene effects.
The investigators also were able to conduct initial tests of new
biomarkers of benzene exposure and effects in humans. Additional
studies will help to determine whether using FISH with tandem probes
or measuring topoisomerase II activity will be useful biomarkers for
assessing ambient or occupational exposures to benzene.
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What is the Health Effects Institute?
The Health Effects Institute (HEI) is an independent, nonprofit
corporation chartered in 1980 to provide high-quality, impartial, and
relevant science on the health effects of pollutants from motor
vehicles and from other sources in the environment. Supported jointly
by the U.S. Environmental Protection Agency (EPA) and industry, HEI
has funded over 170 studies and published over 100 Research Reports,
and several Special Reports, producing important research findings on
the health effects of a variety of pollutants, including carbon
monoxide, methanol and aldehydes, nitrogen oxides, diesel exhaust,
ozone, and most recently, particulate air pollution.
--
Gary N. Greenberg, MD MPH Sysop / Moderator Occ-Env-Med-L MailList
gary.greenberg at duke.edu Duke Occupat, Environ, Int & Fam Medicine
OEM-L Maillist Website: http://occhealthnews.net
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