Introduction

Many publications in the current issue of Oncogene describe in detail the phenomenon of radiation-induced genomic instability in somatic cells. So far, the experimental evidence for elevations in mutation rate in the descendants of irradiated cells for many divisions after the initial exposure has been quite diverse, ranging from compelling evidence in the in vitro model (reviewed in Morgan et al., 1996; Morgan, 2003a), to a rather sketchy in the in vivo model (Ullrich and Davis, 1999; Morgan, 2003b) and highly controversial in humans (Nakanishi et al., 2001; Tawn et al., 2000). Apart from the studies on mutation rates in somatic cells, considerable progress has also been made in the analysis of radiation-induced instability in the mammalian germ line, where the effects of radiation exposure were investigated among the offspring of irradiated parents. These transgenerational studies were designed to test the hypothesis that radiation-induced instability in the germ line of irradiated parents could be transmitted to the offspring and may, in turn, affect their mutations rates. Here, I review a number of publications addressing transgenerational instability in mammals.

Transgenerational effects in somatic tissues

To date, a considerable number of publications have characterized the transgenerational changes in somatic tissues. In these studies, the offspring of irradiated parents were analysed using a spectacular variety of phenotypic traits, ranging from the incidence of cancer to behaviour patterns (Auroux et al., 1988) and fertilisation rate (Burruel et al., 1997). It should be noted that the genetic component of some phenotypic characteristics examined in these publications is either low or unknown, which substantially complicates their interpretation. Over the years, most progress has been made in the analysis of transgenerational changes in cancer predisposition and somatic mutation rate.

Cancer predisposition

The incidence of cancer in the offspring of irradiated male mice has been extensively analysed in a number of publications (early publications are reviewed in Napalkov et al., 1989). Some of them were clearly initiated by the findings showing clustering of childhood leukaemia in the vicinity of the Sellafield nuclear reprocessing plant (Gardner et al., 1990) and a substantial increase in the incidence of tumours in the first-generation offspring (F₁) of male mice exposed to X-rays or uretane (Nomura, 1982). As far as the study of Nomura is concerned, it should be stressed that further studies failed to confirm the results, and showed that this phenomenon could partially be attributed to the seasonal variation in tumour incidence in mouse colonies (Cattanach et al., 1995, 1998). Owing to the lack of measurable increases in tumour incidence in the offspring of exposed male mice, the later studies focused on the morphology of the tumours in the offspring of exposed males. Figure 1a presents the results of one such study assessing the effects of parental exposure to benz(a)pyrene (Turusov et al., 1990). From these data, it follows that the incidence of cancer among the offspring of exposed parents does not exceed those of the control, whereas the mean number of lung adenomas per tumour in the offspring of exposed parents remains persistently elevated over several generations.

Figure 1.

Transgenerational effects in somatic tissues. (a) Incidence of lung tumours in the nonexposed offspring of benz(a)pyrene-treated mice (data from Turusov et al., 1990). (b) Promotion of skin tumours by 12-O-tetradecanoylphorbol-13-acetate (TPA) in the offspring of male mice acutely exposed to 4.2 Gy of X-rays (data from Vorobtsova et al., 1993). (c) The frequency of somatic reversions of the pink-eyed unstable mutation in the offspring of male mice acutely exposed to X-rays. The data for the F₁ alleles derived from the irradiated and nonirradiated fathers are presented separately (data from Shiraishi et al., 2002). The 95% confidence intervals (CI) are shown

Full figure and legend (68K)

Other transgenerational studies have characterized the incidence of cancer in the offspring of irradiated male mice exposed to recognized carcinogens (Nomura, 1983; Vorobtsova et al., 1993; Lord et al., 1998; Hoyes et al., 2001). In contrast to the data obtained on the nontreated offspring, the results of these studies showed an elevated incidence of cancer among the carcinogen-challenged offspring of irradiated males (Figure 1b). The pattern of malignancy among the treated offspring of irradiated males was also modified (Lord et al., 1998). Thus, the treatment shortened latent period for the leukaemia and resulted in a switch from the predominant thymic lymphoma in the controls to a predominance of leukaemia in the offspring of irradiated males.

Taken together, the results of these studies suggest that paternal exposure to ionizing radiation could somehow predispose the offspring of irradiated males to cancer.

Somatic mutation rates

The data showing a greater predisposition in the offspring of irradiated male mice to cancer raised the possibility of transgenerational increases in somatic mutation rates. Theoretically, genomic instability can greatly enhance tumour progression that is attributed to the accumulation of oncogenic mutations in somatic cells (Little, 2000). To date, somatic mutation rates in the offspring of irradiated male mice have been analysed in a number of publications. An elevated frequency of chromosome aberrations was found in the F₁ offspring of irradiated male mice and rats (Lord et al., 1998; Vorobtsova, 2000; Kropacova et al., 2002). Transgenerational changes in somatic mutation rates were also observed by studying the frequency of micronuclei and lacI mutations in the F₁ offspring of irradiated male mice (Luke et al., 1997; Fomenko et al., 2001). The most compelling data addressing somatic instability in the first-generation offspring of irradiated males were obtained from the analysis of somatic reversions of the pink-eyed unstable mutation (Carls and Schiestl, 1999; Shiraishi et al., 2002). The authors reported elevated frequencies of somatic reversions among the nonexposed F₁ offspring of irradiated male mice. Further analysis of the offspring revealed elevated frequencies of reversions in both alleles derived from the irradiated fathers and the unexposed mothers, implying a genome-wide destabilisation after fertilisation (Figure 1c).

Transgenerational effects in the germ line

The results of the above-mentioned studies suggest that a radiation-induced signal in the germ line of exposed males can be transmitted (via sperm) to the offspring that results in an elevated mutation rate detectable in somatic cells. These data also raise the possibility that the same effects may also persist in the germ line of the offspring of irradiated parents.

The first evidence for the transgenerational increases in germ line mutation rates was obtained in 1976 by Luning's group (Luning et al., 1976). The authors analysed the frequency dominant lethal mutations in the germ line of directly irradiated male mice and their first-generation offspring, and demonstrated that the F₁ germline mutation rates were highly elevated (Figure 2a). The results of this study also showed that the frequency of dominant mutations resulting in both early and late mortality is equally elevated in the offspring of irradiated males. Similar data were later obtained from the analysis of the F₁ offspring of male rats treated by cyclophosphamide (Hales et al., 1992). The results showing decreased proliferation of early embryonic cells and increased frequency of malformations in the F₂ offspring of irradiated parents are also consistent with these observations (Lyon and Renshaw, 1986; Wiley et al., 1997; Pils et al., 1999). It should be noted that the dominant-lethality test measures an excess embryonic mortality, which is potentially influenced by numerous environmental and genetic factors, and thus these results can only provide indirect evidence for an elevated mutation rate in the offspring of exposed parents. It is therefore clear that the analysis of transgenerational instability in the germ line requires a technique capable of directly measuring mutation rate in the mouse germ line. To date, the majority of the mouse data on spontaneous and induced germline mutation have been generated using the specific locus method (Russell 7-locus test, reviewed in Searle, 1974). However, because of the very low mutation rate at these loci, enormous samples are required to detect any changes in germline mutation, and therefore the sensitivity of the specific locus method is not high enough for detecting transgenerational increases in the mouse germ line.

Figure 2.

Transgenerational effects in the germ line. (a) Dominant lethal mutations in the germ line of irradiated males and their offspring. The data for dominant mutations resulting in the early preimplantation and late postimplantation losses are shown separately (data from Luning et al., 1976). (b) ESTR mutation rate in the germ line of F₁ and F₂ offspring of irradiated male mice (data from Barber et al., 2002)

Full figure and legend (158K)

We have previously developed a new sensitive technique for monitoring mutation induction in the mouse germ line by ionizing radiation and chemical mutagens (Dubrova et al., 1993, 1998, 2000a; Barber et al., 2000; Dubrova and Plumb, 2002; Vilarino-Guell et al., 2003). This technique employs highly unstable expanded simple tandem repeat (ESTR) loci. These loci were originally termed minisatellites, but have recently been renamed to distinguish them from the much more stable true minisatellites in the mouse genome (Bois et al., 1998). Unstable ESTR loci consist of homogenous arrays of relatively short repeats (4–6 bp) and show a very high spontaneous mutation rates both in germline and somatic cells (Kelly et al., 1989; Gibbs et al., 1993; Bois et al., 1998; Yauk et al., 2002).

In two recent studies, we have used this technique to evaluate ESTR mutation rates in the germ line of F₁ and F₂ offspring of irradiated male mice (Dubrova et al., 2000b; Barber et al., 2002). The analysis of the F₁ offspring of a male mouse exposed to fission neutrons showed that their germline mutation rates did not return to the mutation rates seen in unexposed individuals, but remained similar to those observed in directly exposed males (Dubrova et al., 2000b). The increase was observed in most F₁ offspring and was in part attributable to increased mutational mosaicism in the germ line, therefore indicating that transgenerational destabilization should occur either immediately after fertilization or on the very early stages of the developing F₁ germ line.

To verify these results and to gain some insights on the mechanisms of transgenerational instability in the mouse germ line, we analysed ESTR mutation rates in the germ line of first- and second-generation offspring of inbred male CBA/H, C57BL/6 and BALB/c mice exposed to either high-LET fission neutrons or low-LET X-rays (Barber et al., 2002). Figure 2b presents the main result of this study, showing that paternal exposure to ionizing radiation results in increased mutation rates in the germ line of two subsequent generations of all inbred strains, demonstrating that transgenerational instability is not restricted to one particular inbred strain of mice. Our data revealed clear-cut inter-strain differences in the transgenerational effects, demonstrating that ESTR mutations rates in the F₁ and F₂ germ line of BALB/c and CBA/H mice were significantly higher than in those of C57BL/6 mice. These data are consistent with the results of previous studies showing that that BALB/c and CBA mice are significantly more radiosensitive, and display higher levels of radiation-induced genomic instability in somatic cells than C57BL/6 mice (Roderick, 1963; Ponnaiya et al., 1997; Watson et al., 1997). The high level of radiation-induced genomic instability observed in BALB/c mice could potentially be explained by the strain-specific amino-acid substitutions affecting the activity of the p16^ink4a cyclin-dependent kinase inhibitor and the catalytic subunit of the DNA-dependent protein kinase (Zhang et al., 1998; Yu et al., 2001). The elevated radiosensitivity of CBA/H mice still remains unknown.

In this study, we also compared the transgenerational effects of paternal exposure to high-LET fission neutrons and low-LET X-rays. It is well established that high-LET radiation produces highly complex and localized initial DNA damage, which is different to the sparse damage produced by low-LET radiation, resulting in the unique final biological effects of these different radiation sources (Goodhead, 1988). However, it appears that exposure to both types of radiation is capable of inducing genomic instability in somatic cells, although some studies have failed to detect the effects of low-LET exposure (reviewed in Limoli et al., 2000). Our data also demonstrated that paternal exposure to either high-LET fission neutrons or low-LET X-rays results in increased mutation rates in the F₁ and F₂ germ lines (Barber et al., 2002).

The mechanisms of transgenerational instability

The data described here show that the phenomenon of radiation-induced genomic instability is not restricted to somatic cells and is also manifested in the germ line, resulting in elevated somatic and germline mutation rates in the offspring of exposed parents. The results of these studies also provide important clues on the possible mechanisms of transgenerational instability.

The observed transgenerational increases in cancer predisposition and mutation rates are attributed to some yet unknown signal transmitted via a single sperm from irradiated males to their offspring. It has previously been suggested that the persistently elevated concentrations of free radicals found in the progeny of irradiated somatic cells may modulate the molecular events leading to genomic instability (Clutton et al., 1996; Limoli et al., 2001). However, given that the negligible cytoplasmic component of the mature sperm is unable to carry substantial amounts of such long-lived free radicals or other radiation-induced species to zygote, it therefore appears that the observed transgenerational instability must be caused by a DNA-dependent signal transmitted from the irradiated father.

The results of recent publications suggest that an increased mutation rate in the offspring of irradiated males may be attributed to a very early onset of instability during the F₁ development. For example, similarly elevated mutation rates in the F₁ germ line and somatic tissues were found at the alleles derived from both the irradiated male and the nonirradiated female F₀ parents (Dubrova et al., 2000b; Niwa and Kominami, 2001; Barber et al., 2002; Shiraishi et al., 2002; see Figure 1c). Moreover, an unusually high level of mutational mosaicism was found in the germ line and somatic tissues of F₁ mice (Dubrova et al., 2000b; Niwa and Kominami, 2001). Taken together, these data clearly imply a genome-wide elevation of mutation rate, acting at the very early stages of development.

Our data do not support the hypothesis of direct targeting of any specific set of genes in the F₀ male, causing for example mutational inactivation of a DNA repair gene and consequent destabilization of the F₁ germ line. As similarly elevated mutation rates were found in most of the F₁ and F₂ offspring, it is highly unlikely that the same genes in these offspring might be affected by parental irradiation (Dubrova et al., 2000b; Barber et al., 2002). Moreover, the persistence of elevated germline mutation rates in the F₁ and F₂ offspring of irradiated males rules out the possibility that transgenerational effects are due to radiation-induced mutations at any specific set of genes in the exposed F₀ males (Barber et al., 2002). It therefore appears that the radiation exposure signal could be inherited through sperm in an epigenetic fashion.

It is currently assumed that DNA methylation and chromatin condensation may represent the main mechanisms by which DNA is epigenetically marked (Rakyan et al., 2001; Reik and Walter, 2001). Methylation is known to survive the reprogramming of DNA methylation during spermatogenesis and early development (Roemer et al., 1997; Constancia et al., 1998) and can be transmissible through many cell divisions (Holliday, 1987). It can also influence DNA repair functions, as in the case of microsatellite instability in colorectal carcinomas resulting from loss of hMLH1 mismatch repair activity due to promoter hypermethylation (Herman et al., 1998; Wheeler et al., 1999). We have therefore suggested that DNA methylation is a strong candidate for the epigenetic signal leading to transgenerational mutagenesis (Dubrova et al., 2000b). Altering of DNA methylation pattern in the directly exposed cells could be attributed to a cellular response to ionizing radiation and might affect methylation status of genes responsible for maintaining genomic integrity. If this epigenetic signal can be transmitted to the fertilized egg, it could then affect the stability of the early embryo and result in transgenerational instability. According to this model, the transgenerational increases should be attributed to the change in the expression patterns of genes involved in DNA repair or cell cycle checkpoints in the offspring of irradiated males. Indeed, recent data showed persistently altered pattern of expression of some genes in the offspring of irradiated male mice (Daher et al., 1998; Baulch et al., 2001; Vance et al., 2002).

Radiation-induced epigenetic changes may occur not only in the germ line of an irradiated parent, but could also take place after fertilization. Several recent publications report transgenerational changes in the offspring of male mice irradiated during the late postmeiotic stages of spermatogenesis, where most of pathways maintaining genomic integrity are shut down (Vorobtsova et al., 1993; Vorobtsova, 2000; Niwa and Kominami, 2001; Shiraishi et al., 2002). Theoretically, the targeting of these metabolically inactive stages of mouse spermatogenesis should not result in epigenetic alterations. However, it has been established that premutational lesions in sperm DNA are effectively repaired within a few hours of fertilization (Generoso et al., 1979; Brandriff and Pedersen, 1981). The recognition and repair of premutational damage in the fertilized egg is also accompanied by a suppression of DNA synthesis in both the irradiated male and nonirradiated female pronuclei (Shimura et al., 2002) and alters the expression of DNA repair genes in the preimplantation embryo (Harrouk et al., 2000). It would therefore appear that radiation-induced damage to sperm DNA could later trigger a cascade of epigenetic events in the fertilized egg, similar to that in the repair-proficient diploid spermatogonia and finally result in epigenetic modifications. The results of our study showing similarly elevated mutation rates in the germ line of F₁ and F₂ offspring of exposed males conceived from either postmeiotic (3 weeks) or premeiotic (6 weeks) stages of spermatogenesis (Barber et al., 2002) are consistent with this hypothesis.

Conclusions

The data reviewed here raise the important issue of the delayed effects of ionizing radiation and suggest that persistent transgenerational instability could lead to a significant increase in the mutation load in exposed populations. Using a model of a diploid population (Crow and Kimura, 1970), it is possible to estimate the impact of transgenerational instability on the frequency of mutant alleles in the exposed population. According to this model, the frequency of mutant allele in generation i+1 is q(i+1)=(m p(i) + (1–s₁) p(i) q(i) + (1–s₂) q(i)²) w^-1, where p(i) and q(i) are the frequencies of nonmutant and mutant alleles in the previous generation, s₁ and s₂ are the coefficients of selection against heterozygotes and homozygotes, m is mutation rate, and w=1–2s₁ p(i) q(i)–s₂ q(i)² (Crow and Kimura, 1970). Here, the effects of transgenerational instability are described for recessive (s₁=0) and dominant (s₁=s₂) mutations.

Figure 3 presents the frequency of mutant allele in populations. Model A in this graph describes the impact of mutation induction alone, where mutation rate is only elevated in the germ line of the directly exposed generation and then returns to the initial spontaneous level. The two other models describe transgenerational instability, assuming that mutation rate is equally elevated in the exposed generation and over one (Model B) or two generations (Model C) following exposure. Finally, Model D addresses the possibility that transgenerational instability can persist over multiple generations. In Model D, the initially elevated mutation rate is steadily declining in the following generations, finally returning to the initial spontaneous level, m₀, (m(i)=m₀ (1+be^-ai). The results of modelling show that neither radiation-induced increase in mutation rate nor the following transgenerational instability can noticeable affect the high equilibrium frequency of recessive mutations in a population. In contrast, the frequency of dominant mutations is dramatically affected by transgenerational instability. Given that the elevated rate of dominant lethals is elevated in the offspring of irradiated males (Luning et al., 1976; see Figure 2a), it is therefore conceivable that such effects may persist in population over some generations after the initial exposure to ionizing radiation.

Figure 3.

The impact of transgenerational instability on the frequency of mutant allele in the exposed diploid population. For recessive mutations s₁=0, s₂=0.5, for dominant mutations s₁=s₂=0.5 (see text for details)

Full figure and legend (216K)

It should be stressed, however, that the lack of reliable experimental evidence for transgenerational effects in humans currently complicates the assessment of delayed risk factors. However, the results of some epidemiological studies may be explained by radiation-induced instability manifested in the offspring of exposed parents. For example, the data collected on the families of male radiation workers from the Sellafiled nuclear reprocessing plant showed an elevated incidence of leukaemia and stillbirths in their offspring (Gardner et al., 1990; Dickinson and Parker, 2002; Pearce et al., 2002). It appears rather tempting to speculate that the delayed transgenerational effects of ionizing radiation could provide a plausible explanation for these data.

Finally, the results of publications discussed here raise a number of important questions concerning the transgenerational effects of ionizing radiation. Firstly, it remains to be seen whether human exposure to ionizing radiation can also result in elevated mutation rates in the offspring of directly exposed parents. Secondly, given the widespread use of anticancer drugs, some of which possess clear mutagenic potential, additional studies are clearly needed to evaluate the transgenerational effects of parental exposure to chemical mutagens. Thirdly, the still unknown mechanisms of transgenerational instability have to be elucidated. Future work should address these important issues.

References

1. Auroux MR, Dulioust EJ, Nawar NN, Yacoub SG, Mayaux MJ, Schwartz D and David G. (1988). J. Androl., 9, 153–159. | PubMed |
2. Barber R, Plumb MA, Boulton E, Roux I and Dubrova YE. (2002). Proc. Natl. Acad. Sci. USA, 99, 6877–6882. | Article | PubMed | ChemPort |
3. Barber R, Plumb MA, Smith AG, Cesar CE, Boulton E, Jeffreys AJ and Dubrova YE. (2000). Mutat. Res., 457, 79–91. | PubMed |
4. Baulch JE, Raabe OG and Wiley LM. (2001). Mutagenesis, 16, 17–23. | PubMed |
5. Bois P, Williamson J, Brown J, Dubrova YE and Jeffreys AJ. (1998). Genomics, 49, 122–128. | PubMed |
6. Brandriff B and Pedersen RA. (1981). Science, 211, 1431–1433. | PubMed |
7. Burruel VR, Raabe OG and Wiley LM. (1997). Mutat. Res., 381, 59–66. | Article | PubMed | ISI | ChemPort |
8. Carls N and Schiestl RH. (1999). Carcinogenesis, 20, 2351–2354. | PubMed |
9. Cattanach BM, Papworth D, Patrick G, Goodhead DT, Hacker T, Cobb L and Whitehill E. (1998). Mutat. Res., 403, 1–12. | PubMed |
10. Cattanach BM, Patrick G, Papworth D, Goodhead DT, Hacker T, Cobb L and Whitehill E. (1995). Int. J. Radiat. Biol., 67, 607–615. | PubMed |
11. Clutton SM, Townsend KM, Walker C, Ansell JD and Wright EG. (1996). Carcinogenesis, 17, 1633–1639. | Article | PubMed | ISI | ChemPort |
12. Constancia M, Pickard B, Kelsey G and Reik W. (1998). Genome Res., 8, 881–900. | PubMed | ISI | ChemPort |
13. Crow JF and Kimura M. (1970). An Introduction to Population Genetics Theory. Harper: New York.
14. Daher A, Varin M, Lamontagne Y and Oth D. (1998). Carcinogenesis, 19, 1553–1558. | PubMed |
15. Dickinson HO and Parker L. (2002). Int. J. Cancer, 99, 437–444. | Article | PubMed | ISI | ChemPort |
16. Dubrova YE, Jeffreys AJ and Malashenko AM. (1993). Nat. Genet., 5, 92–94. | Article | PubMed | ChemPort |
17. Dubrova YE and Plumb M. (2002). Mutat. Res., 499, 143–150. | PubMed |
18. Dubrova YE, Plumb M, Brown J, Boulton E, Goodhead D and Jeffreys AJ. (2000a). Mutat. Res., 453, 17–24. | PubMed |
19. Dubrova YE, Plumb M, Brown J, Fennelly J, Bois P, Goodhead D and Jeffreys AJ. (1998). Proc. Natl. Acad. Sci. USA, 95, 6251–6255. | Article | PubMed | ChemPort |
20. Dubrova YE, Plumb M, Gutierrez B, Boulton E and Jeffreys AJ. (2000b). Nature, 405, 37. | Article | PubMed | ChemPort |
21. Fomenko LA, Vasiljeva GV and Bezlepkin VG. (2001). Biol. Bull., 28, 419–423.
22. Gardner MJ, Snee MP, Hall AJ, Powell CA, Downes S and Terrell JD. (1990). Br. Med. J., 300, 423–429. | ISI | ChemPort |
23. Generoso WM, Cain KT, Krishna M and Huff SW. (1979). Proc. Natl. Acad. Sci. USA, 76, 435–437. | Article | PubMed | ChemPort |
24. Gibbs M, Collick A, Kelly RG and Jeffreys AJ. (1993). Genomics, 17, 121–128. | Article | PubMed | ISI | ChemPort |
25. Goodhead DT. (1988). Health Phys., 55, 231–240. | PubMed |
26. Hales BF, Crosman K and Robaire B. (1992). Teratology, 45, 671–678. | PubMed |
27. Harrouk W, Codrington A, Vinson R, Robaire B and Hales BF. (2000). Mutat. Res., 461, 229–241. | PubMed | ChemPort |
28. Herman JG, Umar A, Polyak K, Graff JR, Ahuja N, Issa J-PJ, Markowitz S, Willson JKV, Hamilton SR, Kinzler KW, Kane MF, Kolodner RD, Vogelstein B, Kunkel TA and Baylin SB. (1998). Proc. Natl. Acad. Sci. USA, 95, 6870–6875. | Article | PubMed | ChemPort |
29. Holliday R. (1987). Science, 238, 163–170. | Article | PubMed | ISI | ChemPort |
30. Hoyes KP, Lord BI, McCann C, Hendry JH and Morris ID. (2001). Radiat. Res., 156, 488–494. | PubMed |
31. Kelly R, Bulfield G, Collick A, Gibbs M and Jeffreys AJ. (1989). Genomics, 5, 844–856. | PubMed | ISI | ChemPort |
32. Kropacova K, Slovinska L and Misurova E. (2002). J. Radiat. Res., 43, 125–133. | PubMed |
33. Limoli CL, Kaplan MI, Giedzinski E and Morgan WF. (2001). Free Radic. Biol. Med., 31, 10–19. | PubMed |
34. Limoli CL, Ponnaiya B, Corcoran JJ, Giedzinski E, Kaplan MI, Hartmann A and Morgan WF. (2000). Adv. Space Res., 25, 2107–2117. | Article | PubMed | ISI | ChemPort |
35. Little JB. (2000). Carcinogenesis, 21, 397–404. | Article | PubMed | ISI | ChemPort |
36. Lord BI, Woolford LB, Wang L, Stones VA, McDonald D, Lorimore SA, Papworth D, Wright EG and Scott D. (1998). Br. J. Cancer, 78, 301–311. | PubMed | ISI | ChemPort |
37. Luke GA, Riches AC and Bryant PE. (1997). Mutagenesis, 12, 147–152. | PubMed | ISI | ChemPort |
38. Luning KG, Frolen H and Nilsson A. (1976). Mutat. Res., 34, 539–542. | PubMed |
39. Lyon MF and Renshaw R. (1986). In: Ramel C, Lambert B, Magnusson J (eds). Genetic Toxicology of Environmental Chemicals. Part B: Genetic Effects and Applied Mutagenesis. Alan R Liss: New York, pp. 449–458.
40. Morgan WF. (2003a). Radiat. Res., 159, 567–580. | Article | PubMed | ISI | ChemPort |
41. Morgan WF. (2003b). Radiat. Res., 159, 581–596. | Article | PubMed | ISI | ChemPort |
42. Morgan WF, Day JP, Kaplan MI, McGhee EM and Limoli CL. (1996). Radiat. Res., 146, 247–258. | PubMed | ISI | ChemPort |
43. Napalkov NP, Rice JM, Tomatis L and Yamasaki H (eds). (1989). Perinatal and Multigenerational Carcinogenesis. IARC: Lyon.
44. Nakanishi M, Tanaka K, Takahashi T, Kyo T, Dohy H, Fujiwara M and Kamada N. (2001). Int. J. Radiat. Biol., 77, 687–694. | Article | PubMed | ChemPort |
45. Niwa O and Kominami R. (2001). Proc. Natl. Acad. Sci. USA, 98, 1705–1710. | Article | PubMed | ChemPort |
46. Nomura T. (1982). Nature, 296, 575–577. | Article | PubMed | ISI | ChemPort |
47. Nomura T. (1983). Mutat. Res., 121, 59–65. | Article | PubMed | ISI | ChemPort |
48. Pearce MS, Dickinson HO, Aitkin M and Parker L. (2002). J. R. Statist. Soc. A, 165, 523–548.
49. Pils S, Muller W-U and Streffer C. (1999). Mutat. Res., 429, 85–92. | PubMed |
50. Ponnaiya B, Cornforth MN and Ullrich RL. (1997). Radiat. Res., 147, 121–125. | PubMed |
51. Rakyan VK, Preis J, Morgan HD and Whitelaw E. (2001). Biochem. J., 356, 1–10. | Article | PubMed | ISI | ChemPort |
52. Reik W and Walter J. (2001). Nat. Rev. Genet., 2, 21–32. | Article | PubMed | ISI | ChemPort |
53. Roderick TH. (1963). Radiat. Res., 20, 631–639. | PubMed |
54. Roemer I, Reik W, Dean W and Klose J. (1997). Curr. Biol., 7, 277–280. | Article | PubMed | ISI | ChemPort |
55. Searle AG. (1974). Adv. Radiat. Biol., 4, 131–207.
56. Shimura T, Inoue M, Taga M, Shiraishi K, Uematsu N, Takei N, Yuan Z-M, Shinohara T and Niwa O. (2002). Mol. Cell. Biol., 22, 2220–2228. | Article | PubMed | ISI | ChemPort |
57. Shiraishi K, Shimura T, Taga M, Uematsu N, Gondo Y, Ohtaki M, Kominami R and Niwa O. (2002). Radiat. Res., 157, 661–667. | Article | PubMed | ChemPort |
58. Tawn EJ, Whitehouse CA and Martin FA. (2000). Mutat. Res., 465, 45–51. | PubMed |
59. Turusov VS, Nikonova TV and Parfenov YD. (1990). Cancer Lett., 55, 227–231. | Article | PubMed | ChemPort |
60. Ullrich RL and Davis CM. (1999). Radiat. Res., 152, 170–173. | PubMed |
61. Vance MM, Baulch JE, Raabe OG, Wiley LM and Overstreet JM. (2002). Int. J. Radiat. Biol., 78, 513–526. | PubMed |
62. Vilarino-Guell C, Smith AG and Dubrova YE. (2003). Mutat. Res., 526, 63–73. | PubMed |
63. Vorobtsova IE. (2000). Mutagenesis, 15, 33–38. | PubMed |
64. Vorobtsova IE, Aliyakparova LM and Anisimov VN. (1993). Mutat. Res., 287, 207–216. | Article | PubMed | ISI | ChemPort |
65. Watson GE, Lorimore SA, Clutton SM, Kadhim MA and Wright EG. (1997). Int. J. Radiat. Biol., 71, 497–503. | Article | PubMed | ISI | ChemPort |
66. Wheeler JMD, Beck NE, Kim HC, Tomlinson IPM, Mortensen NJMcC and Bodmer WF. (1999). Proc. Natl. Acad. Sci. USA, 96, 10296–10301. | Article | PubMed | ChemPort |
67. Wiley LM, Baulch JE, Raabe OG and Straume T. (1997). Radiat. Res., 148, 145–151. | PubMed |
68. Yauk CL, Dubrova YE, Grant GR and Jeffreys AJ. (2002). Mutat. Res., 500, 147–156. | PubMed | ChemPort |
69. Yu Y, Okayasu R, Weil MM, Silver A, McCarthy M, Zabriskie R, Long S, Cox R and Ullrich RL. (2001). Cancer Res., 61, 1820–1824. | PubMed | ChemPort |
70. Zhang S, Ramsay ES and Mock BA. (1998). Proc. Natl. Acad. Sci. USA, 95, 2429–2434. | Article | PubMed | ChemPort |

Acknowledgements

I thank the many colleagues and collaborators for their important contribution to this work. This work was supported by grants from the Wellcome Trust.