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As previously mentioned, the genotoxic properties of nanomaterials (NMs) are often closely linked to oxidative damage to DNA and proteins caused by oxidative stress resulting from hyper production of reactive oxygen species (ROS) and reactive nitrogen species (RNS). NM-induced oxidative stress is perhaps the most broadly developed and accepted mechanism for the potential toxic activity of NPs (nanoparticles). NP-mediated ROS and RNS production mechanisms can be classified into three groups: intrinsic production, production by interaction with cell targets, and production mediated by the inflammatory reaction. The three groups share responsibility for most of the primary (direct or indirect) or secondary genotoxic effects so far observed with NMs [1]. A critical assessment of the current knowledge about NM genotoxicity revealed that some standard models of mutagenesis and genotoxicity are either not well adapted or even not suitable for the investigation of NMs, for instance, the bacterial cell wall possibly being a barrier for many NMs; accordingly, the Ames test [2] is not relevant for assessing the mutagenicity of NMs due to a high risk of false-negative results [3–5]. The bacterial tests are probably insufficient to ensure that bacterial DNA is exposed and in consequence cannot be considered as robust. In contrast, regulatory gene mutation tests in mammalian cells [6], using rodent cells (L5178Y, CHO, V79) presenting deficiencies (weak amount of detoxification enzymes, expression of p53, etc.), may overestimate observed effects and lead to an incorrect assessment. Furthermore, their nonhuman origin could also impede their relevance as rodent cell lines are considered more susceptible to give misleading positive results [7]. Therefore, the use of such gene mutation tests is highly debatable. For in vivo genotoxicity, the standard tests recommended by regulations in a first intention target hematopoietic cells [8, 9]. Nevertheless, the bone marrow does not represent the most exposed tissue to NMs; thus, the relevance of these tests is highly questionable. In return the in vivo comet assay and the micronucleus test appear more relevant when carried out on target organs either primary exposed, for instance, the intestine or lung [10], or exposed after translocation as the liver. Consequently, in vitro genotoxicity tests, which so far have most frequently and successfully been used, for genotoxicity testing of NMs were the comet assay and the micronucleus test [11, 12]. As a matter of fact, the in vitro micronucleus assay was seen to be advantageous as it is capable of detecting chromosomal damage in the form of clastogenicity (e.g., triggered by ROS) and aneugenic effects such as physical disturbance of spindles/mitotic apparatus [13]. The in vitro comet test appears to be well suited to point out primary DNA lesions. |
