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Biomedical subjects

H van Steeg

Publications and source records attributed to H van Steeg.

At least 19 recordsLinked to original sources
PubMed2006

Deoxyribonucleic acid damage and spontaneous mutagenesis in the thyroid gland of rats and mice.

Thyroid tumors are a frequent finding not only in iodine-deficient regions. They are predominantly characterized by somatic genetic changes (e.g. point mutations or rearrangements). Because slow thyroid proliferation is a apparent contradiction to a high frequency of tumor initiation, we characterized mutational events in thyroid. First we studied the frequency of certain base exchanges in somatic TSH receptor (TSHR) mutations and determined the spontaneous mutation rate in thyroid and liver. Then we applied different protocols of the comet assay to quantify genomic DNA damage and conducted immunohistochemistry for 8-oxoguanine as a molecular marker for oxidative stress. Among 184 somatic mutations of the human TSHR found in thyroid tumors, C-->T transitions had a unexpectedly high frequency (>32%). The mutation rate in thyroid is 8-10 times higher than in other organs. The comet assay detected increased levels of oxidized pyrimidine (2- to 3-fold) and purine (2- to 4-fold) in thyroid, compared with liver and lung, and a 1.6-fold increase of oxidized purine, compared with spleen. Immunohistochemistry revealed high levels of 8-oxoguanine in thyroid epithelial cells. We have shown a strikingly high mutation rate in the thyroid. Furthermore, results of the comet assay as well as immunohistochemistry suggest that oxidative DNA modifications are a likely cause of the higher mutation rate. It is possible that free radicals resulting from reactive oxygen species in the thyroid generate mutations more frequently. This is also supported by the spectrum of somatic mutations in the TSHR because more frequent base changes could stem from oxidized base adducts that we detected in the comet assay and with immunohistochemistry.

Animals↗
PubMed2003

Transgenic tumor models for carcinogen identification: the heterozygous Trp53-deficient and RasH2 mouse lines.

Genetically altered mouse models (GAMM) for human cancers have been critical to the investigation and characterization of oncogene and tumor suppressor gene expression and function and the associated cancer phenotype. Similarly, several of the mouse models with defined genetic alterations have shown promise for identification of potential human carcinogens and investigation of mechanisms of carcinogen-gene interactions and tumorigenesis. In particular, both the B6.129N5-Trp53 mouse, heterozygous for a p53 null allele, and the CB6F1-RasH2 mouse, hemizygous for the human H-ras transgene, have been extensively investigated. Using 26-week exposure protocols at or approaching the maximum tolerated dose, the summary results to date indicate the potential for GAMM to identify and, possibly, classify chemicals of potential risk to humans using short-term carcinogenicity experiments. This IWGT session focused on: (1) the development of recommendations for genetic/molecular characterization required in animals, tissues, and tumors before and after treatment for identification of presumptive human carcinogens based on the current state of knowledge, (2) identification of data gaps in our current state of knowledge, and (3) development of recommendations for research strategies for further development of our knowledge base of these particular models. By optimization of protocols and identification of significant outcomes and responses to chemical exposure in appropriate short-term mechanism-based genetically altered rodent models, strategies for prevention and intervention may be developed and employed to the benefit of public health.

Animals↗
PubMed2001

The role of nucleotide excision repair and loss of p53 in mutagenesis and carcinogenesis.

Xpa mice, which have a completely defective nucleotide excision repair (NER) pathway, have a cancer predisposition when exposed to several carcinogens. NER is one of the major DNA repair pathways in the mammalian cell, and is involved in the removal of a wide variety of DNA lesions, such as those induced by UV light, bulky adducts and DNA crosslinks. To study the role of NER in both mutagenesis and carcinogenesis, NER-defective Xpa mice were crossed with transgenic lacZ/pUR288 mutation-indicator mice. Furthermore, the relationship between the tumor suppressor gene p53, NER, induction of mutations and tumor development was studied in Xpa/p53+/-/lacZ triple transgenic mice. Using the genotoxic carcinogens benzo[a]pyrene (B[a]P) and 2-acetylaminofluorene (2-AAF), it is shown that mutations in the inactive (non-transcribed) lacZ reporter gene reliably predict cancer risk. In tissues at risk for the development of tumors, increased mutant frequencies could be found at much earlier stages. A heterozygous loss of p53 appears to act synergistically to a NER defect, both in mutation- as well as tumor-induction. Surprisingly, however, the effect of a heterozygous loss of p53 appeared to be tissue-restricted, being apparent in the bladder but absent in liver and spleen.

2-Acetylaminofluorene↗
PubMed2001

Early p53-positive foci as indicators of tumor risk in ultraviolet-exposed hairless mice: kinetics of induction, effects of DNA repair deficiency, and p53 heterozygosity.

p53 mutations appear to be early events in skin carcinogenesis induced by chronic UVB irradiation. Clusters of epidermal cells that express p53 in mutant conformation ("p53 positive foci") are easily detected by immunohistochemical staining long before the appearance of skin carcinomas or their precursor lesions. In a hairless mouse model, we determined the dose-time dependency of the induction of these p53+ foci and investigated the relationship with the induction of skin carcinomas. The density of p53+ foci may be a good direct indicator of tumor risk. Hairless SKH1 mice were exposed to either of two regimens of daily UVB (500 or 250 J/m2 broadband UV from Philips TL12 lamps; 54% UVB 280-315 nm). With the high-dose regimen, the average number of p53+ foci in a dorsal skin area (7.2 cm2) increased rapidly from 9.0 +/- 2.1 (SE) at 15 days to 470 +/- 80 (SE) at 40 days. At half that daily dose, the induction of p53+ foci was slower by a factor of 1.49 +/- 0.15, very similar to a previously observed slower induction of squamous cell carcinomas by a factor of 1.54 +/- 0.02. In a double-log plot of the average number of p53 + foci versus time, the curves for the two exposure regimens ran parallel (slope, 3.7 +/- 0.7), similar to the curves for the number of tumors versus time (slope, 6.9 +/- 0.8). The difference in slopes (3.7 versus 6.9) is in line with the contention that more rate-limiting steps are needed to develop a tumor than a p53+ focus. By the time the first tumors appear (around 7-8 weeks with the high daily dose), the dorsal skin contains >100 p53+ foci/cm2. To further validate the density of p53+ foci as a direct measure of tumor risk, we carried out experiments with transgenic mice with an enhanced susceptibility to UV carcinogenesis, homozygous Xpa knockout mice (deficient in nucleotide excision repair) and heterozygousp53 knockout mice (i.a. partially deficient in apoptosis). In both of these cancer-prone strains, the p53+ foci were induced at markedly increased rates, corresponding to increased rates of carcinoma formation. Therefore, the frequency of p53+ foci appears to correlate well with UVB-induced tumor risk.

Animals↗
PubMed2001

Differential ultraviolet-B-induced immunomodulation in XPA, XPC, and CSB DNA repair-deficient mice.

Ultraviolet B irradiation has serious consequences for cellular immunity and can suppress the rejection of skin tumors and the resistance to infectious diseases. DNA damage plays a crucial role in these immunomodulatory effects of ultraviolet B, as impaired repair of ultraviolet-B-induced DNA damage has been shown to cause suppression of cellular immunity. Ultraviolet-B-induced DNA damage is repaired by the nucleotide excision repair mechanism very efficiently. Nucleotide excision repair comprises two subpathways: transcription-coupled and global genome repair. In this study the immunologic consequences of specific nucleotide excision repair defects in three mouse models, XPA, XPC, and CSB mutant mice, were investigated. XPA mice carry a total nucleotide excision repair defect, whereas XPC and CSB mice only lack global genome and transcription-coupled nucleotide excision repair, respectively. Our data demonstrate that cellular immune parameters in XPA, XPC, and CSB mice are normal compared with their wild-type (control) littermates. This may indicate that the reported altered cellular responses in xeroderma pigmentosum patients are not constitutive but could be due to external factors, such as ultraviolet B. Upon exposure to ultraviolet B, only XPA mice are very sensitive to ultraviolet-B-induced inhibition of Th1-mediated contact hypersensitivity responses and interferon-gamma production in skin draining lymph nodes. Lipopolysaccharide-stimulated tumor necrosis factor alpha and interleukin-10 production are significantly augmented in both XPA and CSB mice after ultraviolet B exposure. Lymph node cell numbers were increased very significantly in XPA, mildly increased in CSB, and not in XPC mice. In general XPC mice do not exhibit any indication of enhanced ultraviolet B susceptibility with regard to the immune parameters analyzed. These data suggest that both global genome repair and transcription-coupled repair are needed to prevent immunomodulation by ultraviolet B, whereas transcription-coupled repair is the major DNA repair subpathway of nucleotide excision repair that prevents the acute ultraviolet-B-induced effects such as erythema.

Adjuvants, Immunologic↗
PubMed2001

The DNA damage signal for Mdm2 regulation, Trp53 induction, and sunburn cell formation in vivo originates from actively transcribed genes.

The stratum corneum and DNA repair do not completely protect keratinocytes from ultraviolet B. A third defense prevents cells with DNA photoproducts from becoming precancerous mutant cells: apoptosis of ultraviolet-damaged keratinocytes ("sunburn cells"). As signals for ultraviolet-induced apoptosis, some studies implicate DNA photoproducts in actively transcribed genes; other studies implicate non-nuclear signals. We traced and quantitated the in vivo DNA signal through several steps in the apoptosis-signaling pathway in haired mice. Homozygous inactivation of Xpa, Csb, or Xpc nucleotide excision repair genes directed the accumulation of DNA photoproducts to specific genome regions. Repair-defective Xpa-/- mice were 7-10-fold more sensitive to sunburn cell induction than wild-type mice, indicating that 86-90% of the ultraviolet B signal for keratinocyte apoptosis involved repairable photoproducts in DNA; the remainder involves unrepaired DNA lesions or nongenomic targets. Csb-/- mice, defective only in excising photoproducts from actively transcribed genes, were as sensitive as Xpa-/-, indicating that virtually all of the DNA signal originates from photoproducts in active genes. Conversely, Xpc-/- mice, defective in repairing the untranscribed majority of the genome, were as resistant to apoptosis as wild type. Sunburn cell formation requires the Trp53 tumor suppressor protein; 90-96% of the signal for its induction in vivo involved transcribed genes. Mdm2, which regulates the stability of Trp53 through degradation, was induced in vivo by low ultraviolet B doses but was suppressed at erythemal doses. DNA photoproducts in actively transcribed genes were involved in approximately 89% of the Mdm2 response.

Animals↗
PubMed2001

Xpa and Xpa/p53+/- knockout mice: overview of available data.

DNA repair deficient Xpa-/- and Xpa-/-/p53+/- knock-out mice in a C57BL/6 genetic background, referred to as respectively the XPA and XPA/p53 model, were investigated in the international collaborative research program coordinated by International Life Sciences Institute (ILSI)/Health and Environmental Science Institute. From the selected list of 21 ILSI compounds, 13 were tested in the XPA model, and 10 in the XPA/p53 model. With one exception, all studies had a duration of 9 months (39 weeks). The observed spontaneous tumor incidence for the XPA model after 9 months was comparable to that of wild-type mice (total 6%). For the XPA/p53 model, this was somewhat higher (9%/13% for males/females). The 3 positive control compounds used, B[a]P, p-cresidine, and 2-AAF, gave positive and consistent tumor responses in both the XPA and XPA/p53 model, but no or lower responses in wild-type mice. From the 13 ILSI compounds tested, the single genotoxic carcinogen (phenacetin) was negative in both the XPA and XPA/p53 model. Positive tumor responses were observed for 4 compounds, the immunosuppressant cyclosporin A, the hormone carcinogens DES and estradiol, and the peroxisome proliferator WY-14,643. Negative results were obtained with 5 other nongenotoxic rodent carcinogens, and 2 noncarcinogens tested. As expected, both DNA repair deficient models respond to genotoxic carcinogens. Combined with previous results, 6 out of 7 (86%) of the genotoxic human and/or rodent carcinogens tested are positive in the XPA model. The positive results obtained with the 4 mentioned nongenotoxic ILSI compounds may point to other carcinogenic mechanisms involved, or may raise some doubts about their true nongenotoxic nature. In general. the XPA/p53 model appears to be more sensitive to carcinogens than the XPA model.

Academies and Institutes↗
PubMed2001

DNA repair-deficient Xpa and Xpa/p53+/- knock-out mice: nature of the models.

Xeroderma pigmentosum (XP) is a rare autosomal recessive disease in which repair of ultraviolet (UV)-induced DNA damage is impaired or is totally absent due to mutations in genes controlling the DNA repair pathway known as nucleotide excision repair (NER). XP is characterized, in part, by extreme sensitivity of the skin to sunlight, and XP patients have a more than 1000-fold increased risk of developing cancer at sun-exposed areas of the skin. To study the role of NER in chemical-induced tumorigenesis in more detail, the authors developed Xpa-/- homozygous knockout mice with a complete defect in NER (designated as Xpa mice or XPA model). Xpa mice develop skin tumors at high frequency when exposed to UV light, and as such, they mimic the phenotype of human XP. Moreover, the Xpa mice also appear to be susceptible to genotoxic carcinogens given orally. Based on these phenotypic characteristics, the Xpa mice were considered to be an attractive candidate mouse model for use in identifying human carcinogens. In an attempt to further increase both the sensitivity and specificity of the XPA model in carcinogenicity testing, the authors crossed Xpa mice with mice having a heterozygous defect in the tumor suppressor gene p53. Xpa/p53+/- double knockout mice develop tumors earlier and with higher incidences upon exposure to carcinogens as compared to their single knockout counterparts. Here the authors describe the development and features of the Xpa mouse and present some examples of the Xpa and Xpa/p53+/- mouse models' sensitivity towards genotoxic carcinogens. It appeared that the Xpa/p53+/- double knockout mouse model is favorable over both the Xpa and p53+/- single knockout models in short-term carcinogenicity testing. In addition to the fact that the double knockout mice respond more robustly to carcinogens, they also appear to respond in a very discriminative way. All compounds identified thus far are true (human) carcinogens, and, therefore, the authors believe that the Xpa/p53+/- mouse model is an excellent candidate for a future replacement of the chronic mouse bioassay, at least for certain classes of chemicals.

Animal Testing Alternatives↗
PubMed2001

Intestinal toxicity and carcinogenic potential of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) in DNA repair deficient XPA-/- mice.

The effects of the food mutagen 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were studied in DNA repair deficient XPA(-/-) mice. The nullizygous XPA-knockout mice, which lack a functional nucleotide excision repair (NER) pathway, were exposed to dietary concentrations ranging from 10 to 200 p.p.m. The results show that PhIP is extremely toxic to XPA(-/-) mice, even at doses 10-fold lower than tolerated by wild-type C57BL/6 mice. XPA(-/-) mice rapidly lost weight and died within 2 and 6 weeks upon administration of 200 and 100 p.p.m., respectively. Intestinal abnormalities like distended and overfilled ileum and caecum, together with clear signs of starvation, suggests that the small intestines were the primary target tissue for the severe toxic effects. Mutation analysis in XPA(-/-) mice carrying a lacZ reporter gene, indicated that the observed toxicity of PhIP might be caused by genotoxic effects in the small intestine. LacZ mutant frequencies appeared to be selectively and dose-dependently increased in the intestinal DNA of treated XPA(-/-) mice. Furthermore, DNA repair deficient XPC(-/-) mice, which are still able to repair DNA damage in actively transcribed genes, did not display any toxicity upon treatment with PhIP (100 p.p.m.). This suggests that transcription coupled repair of DNA damage (PhIP adducts) in active genes plays a crucial role in preventing the intestinal toxicity of PhIP. Finally, PhIP appeared to be carcinogenic for XPA(-/-) mice at subtoxic doses. Upon treatment of the mice for 6 months with 10 or 25 p.p.m. PhIP, significantly increased tumour incidences were observed after a total observation period of one year. At 10 p.p.m. only lymphomas were found, whereas at 25 p.p.m. some intestinal tumours (adenomas and adenocarcinomas) were also observed.

Adenocarcinoma↗
PubMed2000

The relationship between benzo[a]pyrene-induced mutagenesis and carcinogenesis in repair-deficient Cockayne syndrome group B mice.

Cockayne syndrome (CS) patients are deficient in the transcription coupled repair (TCR) subpathway of nucleotide excision repair (NER) but in contrast to xeroderma pigmentosum patients, who have a defect in the global genome repair subpathway of NER, CS patients do not have an elevated cancer incidence. To determine to what extent a TCR deficiency affects carcinogen-induced mutagenesis and carcinogenesis, CS group B correcting gene (CSB)-deficient mice were treated with the genotoxic carcinogen benzo(a)pyrene (B[a]P) at an oral dose of 13 mg/kg body weight, three times a week. At different time points, mutant frequencies at the inactive lacZ gene (in spleen, liver, and lung) as well as at the active hypoxanthine phosphoribosyltransferase (Hprt) gene (in spleen) were determined to compare mutagenesis at inactive versus active genes. B[a]P treatment gave rise to increased mutant frequencies at lacZ in all of the organs tested without a significant difference between CSB-/- and wild-type mice, whereas B[a]P-induced Hprt mutant frequencies in splenic T-lymphocytes were significantly more enhanced in CSB-/- mice than in control mice. The sequence data obtained from Hprt mutants indicate that B[a]P adducts at guanine residues were preferentially removed from the transcribed strand of the Hprt gene in control mice but not in CSB-/- mice. On oral treatment with B[a]P, the tumor incidence increased in both wild-type and CSB-deficient animals. However, no differences in tumor rate were observed between TCR-deficient CSB-/- mice and wild-type mice, which is in line with the normal cancer susceptibility of CS patients. The mutagenic response at lacZ, in contrast to Hprt, correlated well with the cancer incidence in CSB-/- mice after B[a]P treatment, which suggests that mutations in the bulk of the DNA (inactive genes) are a better predictive marker for carcinogen-induced tumorigenesis than mutations in genes that are actively transcribed. Thus, the global genome repair pathway of NER appears to play an important role in the prevention of cancer.

Animals↗
PubMed2000

Age-dependent spontaneous mutagenesis in Xpc mice defective in nucleotide excision repair.

DNA damages caused by cellular metabolites and environmental agents induce mutations, that may predispose to cancer. Nucleotide excision repair (NER) is a major cellular defence mechanism acting on a variety of DNA lesions. Here, we show that spontaneous mutant frequencies at the Hprt gene increased 30-fold in T-lymphocytes of 1 year old Xpc-/- mice, possessing only functional transcription-coupled repair (TCR). Hprt mutant frequencies in Xpa-/- and Csb-/- mice that both have a defect in this NER subpathway, remained low during ageing. In contrast to current models, the elevated mutation rate in Xpc-/- mice does not lead to an increased tumour incidence or premature ageing. Oncogene (2000) 19, 5034 - 5037

Aging↗
PubMed2000

Differential role of transcription-coupled repair in UVB-induced G2 arrest and apoptosis in mouse epidermis.

Nucleotide excision repair (NER), apoptosis, and cell-cycle regulation are major defense mechanisms against the carcinogenic effects of UVB light. NER eliminates UVB-induced DNA photolesions via two subpathways: global genome repair (GGR) and transcription-coupled repair (TCR). Defects in NER result in the human disorders xeroderma pigmentosum (XP) and Cockayne syndrome (CS), displaying severe UV sensitivity and in the case of XP, cancer proneness. We investigated the impact of deficiencies in NER subpathways on apoptosis, hyperplasia, and cell cycle progression in the epidermis of UVB-exposed CS group B (Csb(-/-)) mice (no TCR), XP group C (Xpc(-/-)) mice (no GGR), and XP group A (Xpa(-/-)) mice (no TCR and no GGR). On UVB treatment (250 J/m(2)), Xpa(-/-) and Csb(-/-) mice revealed an extensive apoptotic response in the skin, a blockage of cell cycle progression of epidermal cells, and strong hyperplasia. Interestingly, the absence of this apoptotic response in the skin of wild-type and Xpc(-/-) mice coincided with the ability of epidermal cells to enter the S phase. However, only epidermal cells of Xpc(-/-) mice subsequently became arrested in the G(2) phase. Our data demonstrate that TCR (and/or restoration of UVB-inhibited transcription) enables damaged cells to progress through S phase and prevents the induction of apoptosis and hyperplasia. G(2) arrest is manifest only under conditions of proficient TCR in combination with deficient GGR, indicating that epidermal cells become arrested in the G(2) phase as a result of persisting damage in their genome.

Animals↗
PubMed2000

Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression.

Exposure to UV-B radiation impairs immune responses in mammals by inhibiting especially Th1-mediated contact hypersensitivity and delayed-type hypersensitivity. Immunomodulation is not restricted to the exposed skin, but is also observed at distant sites, indicating the existence of mediating factors such as products from exposed skin cells or photoactivated factors present in the superficial layers. DNA damage appears to play a key role, because enhanced nucleotide excision repair (NER) strongly counteracts immunosuppression. To determine the effects of the type and genomic location of UV-induced DNA damage on immunosuppression and acute skin reactions (edema and erythema) four congenic mouse strains carrying different defects in NER were compared: CSB and XPC mice lacking transcription-coupled or global genome NER, respectively, as well as XPA and TTD/XPD mice carrying complete or partial defects in both NER subpathways, respectively. The major conclusions are that 1) transcription-coupled DNA repair is the dominant determinant in protection against acute skin effects; 2) systemic immunomodulation is only affected when both NER subpathways are compromised; and 3) sunburn is not related to UV-B-induced immunosuppression.

Animals↗
PubMed2000

Mutagenesis and carcinogenesis in nucleotide excision repair-deficient XPA knock out mice.

Mice with a defect in the xeroderma pigmentosum group A (XPA) gene have a complete deficiency in nucleotide excision repair (NER). As such, these mice mimic the human XP phenotype in that they have a >1000-fold higher risk of developing UV-induced skin cancer. Besides being UV-sensitive, XPA(-/-) mice also develop internal tumors when they are exposed to chemical carcinogens. To investigate the effect of a total NER deficiency on the induction of gene mutations and tumor development, we crossed XPA(-/-) mice with transgenic lacZ/pUR288 mutation-indicator mice. The mice were treated with various agents and chemicals like UV-B, benzo[a]pyrene and 2-aceto-amino-fluorene. Gene mutation induction in several tumor target- and non-target tissues was determined in both the bacterial lacZ reporter gene and in the endogenous Hprt gene. Furthermore, alterations in the p53- and ras genes were determined in UV-induced skin tumors of XPA(-/-) mice. In this work, we review these results and discuss the applicability and reliability of enhanced gene mutant frequencies as early indicators of tumorigenesis.

2-Acetylaminofluorene↗
PubMed2000

Homologous and non-homologous recombination differentially affect DNA damage repair in mice.

Ionizing radiation and interstrand DNA crosslinking compounds provide important treatments against cancer due to their extreme genotoxicity for proliferating cells. Both the efficacies of such treatments and the mutagenic potential of these agents are modulated by the ability of cells to repair the inflicted DNA damage. Here we demonstrate that homologous recombination-deficient mRAD54(-/-) mice are hypersensitive to ionizing radiation at the embryonic but, unexpectedly, not at the adult stage. However, at the adult stage mRAD54 deficiency dramatically aggravates the ionizing radiation sensitivity of severe combined immune deficiency (scid) mice that are impaired in DNA double-strand break repair through DNA end-joining. In contrast, regardless of developmental stage, mRAD54(-/-) mice are hypersensitive to the interstrand DNA crosslinking compound mitomycin C. These results demonstrate that the two major DNA double-strand break repair pathways in mammals have overlapping as well as specialized roles, and that the relative contribution of these pathways towards repair of ionizing radiation-induced DNA damage changes during development of the animal.

Animals↗
PubMed2000

Disruption of mouse SNM1 causes increased sensitivity to the DNA interstrand cross-linking agent mitomycin C.

DNA interstrand cross-links (ICLs) represent lethal DNA damage, because they block transcription, replication, and segregation of DNA. Because of their genotoxicity, agents inducing ICLs are often used in antitumor therapy. The repair of ICLs is complex and involves proteins belonging to nucleotide excision, recombination, and translesion DNA repair pathways in Escherichia coli, Saccharomyces cerevisiae, and mammals. We cloned and analyzed mammalian homologs of the S. cerevisiae gene SNM1 (PSO2), which is specifically involved in ICL repair. Human Snm1, a nuclear protein, was ubiquitously expressed at a very low level. We generated mouse SNM1(-/-) embryonic stem cells and showed that these cells were sensitive to mitomycin C. In contrast to S. cerevisiae snm1 mutants, they were not significantly sensitive to other ICL agents, probably due to redundancy in mammalian ICL repair and the existence of other SNM1 homologs. The sensitivity to mitomycin C was complemented by transfection of the human SNM1 cDNA and by targeting of a genomic cDNA-murine SNM1 fusion construct to the disrupted locus. We also generated mice deficient for murine SNM1. They were viable and fertile and showed no major abnormalities. However, they were sensitive to mitomycin C. The ICL sensitivity of the mammalian SNM1 mutant suggests that SNM1 function and, by implication, ICL repair are at least partially conserved between S. cerevisiae and mammals.

Amino Acid Sequence↗
PubMed1999

Mouse model for the DNA repair/basal transcription disorder trichothiodystrophy reveals cancer predisposition.

Patients with the nucleotide excision repair (NER) disorder xeroderma pigmentosum (XP) are highly predisposed to develop sunlight-induced skin cancer, in remarkable contrast to photosensitive NER-deficient trichothiodystrophy (TTD) patients carrying mutations in the same XPD gene. XPD encodes a helicase subunit of the dually functional DNA repair/basal transcription complex TFIIH. The pleiotropic disease phenotype is hypothesized to be, in part, derived from a repair defect causing UV sensitivity and, in part, from a subtle, viable basal transcription deficiency accounting for the cutaneous, developmental, and the typical brittle hair features of TTD. To understand the relationship between deficient NER and tumor susceptibility, we used a mouse model for TTD that mimics an XPD point mutation of a TTD patient in the mouse germline. Like the fibroblasts from the patient, mouse cells exhibit a partial NER defect, evident from the reduced UV-induced DNA repair synthesis (residual repair capacity approximately 25%), limited recovery of RNA synthesis after UV exposure, and a relatively mild hypersensitivity to cell killing by UV or 7,12-dimethylbenz[a]anthracene. In accordance with the cellular studies, TTD mice exhibit a modestly increased sensitivity to UV-induced inflammation and hyperplasia of the skin. In striking contrast to the human syndrome, TTD mice manifest a dear susceptibility to UV- and 7,12-dimethylbenz[a]anthracene-induced skin carcinogenesis, albeit not as pronounced as the totally NER-deficient XPA mice. These findings open up the possibility that TTD is associated with a so far unnoticed cancer predisposition and support the notion that a NER deficiency enhances cancer susceptibility. These findings have important implications for the etiology of the human disorder and for the impact of NER on carcinogenesis.

9,10-Dimethyl-1,2-benzanthracene↗