Genes and Environment, Vol. 34, No. 1 pp. 1–8 (2012) Review Genotoxicity of Acrylamide and Glycidamide: A Review of the Studies by HPRT Gene and TK Gene Mutation Assays Lin Ao1,2 and Jia Cao1,2,3 1Institute of Toxicology, College of Preventive Medicine, Third Military Medical University, Chongqing, PR China 2Key Laboratory of Electromagnetic Radiation Damage and Medical Protection, Ministry of Education of China, Chongqing, PR China (Received September 19, 2011; Revised December 6, 2011; Accepted December 28, 2011) Acrylamide (AA), a proven rodent carcinogen, is found very high temperatures, as a result of Maillard reactions in a variety of commonly consumed human foods, which involving asparagine and reducing sugars (3). Total die- has raised public health concerns. AA is largely oxidized to tary daily AA intake has been estimated to correspond the chemically reactive epoxide, glycidamide (GA), by to about 0.5 mg AA per kg body weight on the basis of a cytochrome P450 2E1. The genotoxic eŠects of AA and normal western diet (4,5). In 2002, a consultation held GA have been extensively evaluated. However, the results by the Food and Agriculture Organization of the United in mammalian gene mutation tests were inconsistent, es- Nations(FAO)andtheWorldHealthOrganization pecially the genotoxic eŠects at the HPRT gene and TK gene. In this article, the relevant mutations induced by AA (WHO) concluded that AA represented a ``major prob- and GA on both gene loci in various test systems involving lem'', because of the relative high levels in these foods in vivo and in vitro tests are reviewed. It is conˆrmed that (6). The ˆnding has prompted renewed interest in its AA acts directly as a clastogen and produces weakly potential for toxicity in humans. mutagenic eŠects at the HPRT gene probably by metabolic AA exhibits carcinogenicity in experimental animals, conversion of AA to GA. On the other hand, GA is a strong causing tumors at multiple organ sites in mice and rats mutagen with high reactivity to DNA, inducing when given in drinking water or by other means (6). predominantly point mutations. The molecular mutation However, epidemiologic studies have failed to establish spectra of AA and GA at the HPRT and TK genes are also a relationship between AA exposure and an increased compared and summarized here, for better clarifying the risk for cancer, in either occupationally exposed wor- mechanisms of mutation induced by these two com- pounds. These data would help to understand the muta- kers or the general populations of several countries in genicity of AA and its contribution to human cancers. which AA is present in certain foods and beverages (7). In 1994, AA was classiˆed as a probable human carcino- Key words: acrylamide, glycidamide, genotoxicity, HPRT gen by the International Agency for Research on Cancer mutation, TK mutation (IARC, Group 2A), based on the signiˆcant evidence of carcinogenicity in experimental rodent models and DNA adduct formation in human bronchial cells (8,9). Because of the known human exposure and carcinogen- Introduction ic eŠects induced by AA in animals, it is critical to un- Acrylamide (AA), a water-soluble vinyl monomer, derstand its potential for inducing genetic damages in has been produced since the 1950s by hydration of animalsaswellasinhumans. acrylonitrile (1). It has multiple chemical and industrial In both humans and experimental animals, AA is lar- applications around the world, such as the use in water gely oxidized to the chemically reactive epoxide, treatment, manufacture of paper, plastics and textiles, glycidamide (GA), by cytochrome P450 2E1 (CYP2E1) and polyacrylamide gel preparation in research labora- (10). A broad body of evidence has demonstrated the tories (2). It is estimated human exposure from these ap- genotoxicity of AA and its reactive metabolite epoxide plications is very limited. More recently, the concerns GA (9,11,12). AA exhibits clastogenic eŠects by induc- about the health risks of AA were transferred to the general population from the occupational exposure in 3Correspondence to: Jia Cao, Institute of Toxicology, College of the past, since it is found to form naturally during the Preventive Medicine, Third Military Medical University, Chongqing, frying, roasting or baking of carbohydrate-rich foods at PR China. Fax: ＋86-23-68772533, E-mail: caojia1962＠126.com The Japanese Environmental Mutagen Society 1 Lin Ao and Jia Cao ing structural chromosomal aberrations, micronucleus as point mutations and small deletions (24), while the formations, sister chromatid exchanges and mitotic dis- latter is capable of detecting a wide range of mutational turbances in mammalian cells in vitro with or without events, including point mutations, large scale chro- metabolic activation. In the comet assay, AA could in- mosomal changes, recombination, aneuploidy and duce DNA damage in the FRTL5 and PC Cl3 rat others (25). Additionally, there is a striking feature ex- thyroid cell lines (13), as well as in human HepG2 cells isting in the TK mutant colonies, which is the induction (14). In the micronucleus (MN) test, positive results of large colonies growing at normal growth rates and were observed in rodents (15–17) Meanwhile, several small colonies growing at slower rates. The relative fre- studies reported the induction of chromosomal aberra- quency of the two colony classes is mutagen-dependent, tions and mitotic disruptions following AA exposure of with the chemicals that mainly induce gross chromo- cultured mammalian cells, mostly Chinese hamster cell some aberrations primarily producing small colony lines (11). mutants and chemicals that mainly induce point muta- While the direct-acting clastogenic eŠects of AA have tions primarily forming large colonies (26,27). been clearly elucidated, the ˆndings in point mutation A variety of cell types are available for use in HPRT assays are inconsistent. As reported by IARC and and/or TK gene mutation assays. In the assessment of FAO/WHO, AA did not induce gene mutations in mutagenic eŠects induced by AA and its metabolite GA, Ames test with or without metabolic activation (18), but the cell subclones employed in TK and HPRT gene mu- GA was always mutagenic in the same assay even in the tation assays include L5178Y TK＋/－ mouse lymphoma absence of an exogenous activating system (19). On the cell (18,28–30), human lymphoblastoid cell TK6 (31), other hand, AA has exhibited equivocal, negative, Chinese hamster ovary (CHO) cell (32), Chinese ham- weakly or mildly positive response in mammalian cell ster V79 cell (33–35) and human promyelocytic leukae- gene mutations assays (20). Most of the results above miacellsHL–60andNB4(36).Morerecently,thede- mentioned come from the studies measuring mutations velopment of the mammalian cell gene assays employing at the speciˆc loci which code thymidine kinase (TK) rodent lymphocytes has also contributed signiˆcantly to and hypoxanthine-guanine phosphoribosyl transferase the understanding of in vivo somatic cell mutagenesis (HPRT), respectively. In this article, the relevant muta- induced by AA, including the assays performed in Big tions induced by AA and GA on both gene loci in vari- Blue mice, B6C3F1/TK mice and Big Blue rats (37–39). ous test systems are reviewed; these data would help to understand the mutagenicity of AA and its contribution Mutagenicity of AA and GA at the HPRT Locus to human cancers. in DiŠerent Test Systems GA is consistently mutagenic at HPRT mutation as- Mammalian Gene Mutation Assays at the HPRT says in various test systems involving mammalian cell and TK Loci lines (32,34,35) and rodents (37–39). On the contrary, A wide range of assays are now available which ena- some studies suggested that AA may not induce gene ble the eŠective detection of the mutagenic and more mutation in HPRT test. Tsuda et al.(33)andBaumet generally genotoxic activity of individual chemicals and al. (34) reported the negative responses of AA in mixtures. Among these assays, the mammalian cell gene Chinese hamster V79 cells, independently. In the ˆrst mutation assays on the HPRT gene and the TK gene experiment, AA did not induce any gene mutation in have been widely used to determine the genotoxic poten- HprtlocusevenwhenV79cellsweretreatedwith7mM tial of various agents. The HPRT mutation assay uses AA (equal to 500 mg/l) for 24 h. In the second experi- the HPRT gene as a reporter of mutation. This is a sin- ment, AA was inactive up to a concentration of 10 mM gle copy gene at position Xq26–27 in humans, codes for (equal to 710 mg/L) for 24 h. These results may support HPRT, an enzyme in the nucleotide salvage pathway the concept of AA by itself not acting as a mutagenic a- (21). Mutations at this locus are recognized phenotypi- gent at exposure levels to be expected from food inges- cally by resistance to 8-azaguanine (8-AZA) or 6-thio- tion. The concept has also been conˆrmed by the evi- guanine (6-TG), purine nucleotide analogues that selec- dence that AA was non-mutagenic in bacterial gene mu- tively kill wild-type cells or cells with normal HPRT ac- tation assays (11,18), in which AA consistently exhibit- tivity (22). Similarly, in the TK mutation test, cells ed negative results using diŠerent strains of Salmonella deˆcient in thymidine kinase due to the mutation of typhimurium, including TA1535, TA1537, TA1538, TK＋/－ to TK－/－ are resistant to the cytotoxic eŠects of TA97, TA98, TA100 and TA102 in the presence or the the pyrimidine analogue tri‰uorothymidine (TFT). absence of an exogenous activating system. Thus mutant cells are selected in the presence of TFT However, studies at the HPRT gene have exhibited (23). However, the HPRT gene mutation assay and TK inconsistent mutagenic eŠects induced by AA in diŠer- gene mutation assay detect diŠerent spectra of genetic e- ent models. Results obtained in some test models indi- vents: the former detects only intragenic mutations such cate the weakly or moderately positive responses of AA 2 Genotoxicity of Acrylamide and Glycidamide at Hprt locus.