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.

Cancer predisposition in mutant mice defective in multiple genetic pathways: uncovering important genetic interactions.

Mouse models that mimic the human skin cancer-prone disease xeroderma pigmentosum (XP) provide an useful experimental system with which to study the relationship between the DNA repair process of nucleotide excision repair (NER) and ultraviolet- (UV) induced skin carcinogenesis. We have generated Xpc mutant mice and documented their deficiency in the process of NER of UV-induced DNA damage. Xpc mutant mice are highly predisposed to UV-B radiation-induced skin cancer, both in the homozygous and the heterozygous state. The combination of Xpc and Trp53 mutations enhances this predisposition and alters the tumor spectrum observed in single mutant mice. These results suggest a synergism between NER and the function of Trp53 in suppression of cancer. We have examined the mutational spectrum in the Trp53 gene from skin cancers in Trp53+/+ and Trp53+/- mice of all three Xpc genotypes and have found evidence for signature mutations associated with defective NER. In addition, we have demonstrated that Xpc mutant mice are highly predisposed to the induction of lung and liver cancers by treatment with 2-acetylaminofluorene (2-AAF) and N-OH-2-AAF. By combining the Xpc mutation with other mutations in genes involved in repair of DNA damage we have identified additional genetic interactions important in carcinogenesis. The mouse Apex gene is a critical component of the base excision repair (BER) pathway as well as the redox regulation of transcription factors important in growth control and the cellular response to DNA damage. By combining mutations in Xpc, Trp53 and Apex we have obtained genetic evidence for a functional interaction between Apex and Trp53 which probably involves the activation of the Trp53 protein by Apex. Mutations in the mismatch repair (MMR) gene Msh2 also influence the carcinogenesis observed in Xpc Trp53 mutant mice. Our results demonstrate that multiple repair pathways operate in prevention of tumor formation.

Genotype-specific Trp53 mutational analysis in ultraviolet B radiation-induced skin cancers in Xpc and Xpc Trp53 mutant mice.

We have examined the mutational spectrum in the Trp53 gene from UVB radiation-induced skin cancers in Trp53+/+ and Trp53+/- mutant mice of all three possible Xpc genotypes. Mutations were detected in exons 2-10 of the Trp53 coding region in approximately 90% of >80 different skin cancers examined. In contrast to Trp53+/+ mice in which most mutations in the Trp53 gene were located in exons 5-8, the majority of the mutations in Trp53+/- mice were at other exons. We observed a high predilection for C-->T transition mutations at a unique CpG site in codon 122 (exon 4) of the Trp53 gene in Xpc-/- Trp53+/- mice. This site is not part of a pyrimidine dinucleotide. Mutations at this codon, as well as in codons 124 and 210, were observed exclusively in Xpc-/- or Xpc+/- mice. Mutations at the corresponding codons (127 and 213) in the human p53 gene have been reported in skin tumors from human patients with xeroderma pigmentosum. Hence, mutations at codons 122 (125), 124 (127), and 210 (213) may constitute signatures for defective or deficient nucleotide excision repair in mice (humans). In Xpc-/- mice, the majority of mutations were located at C residues in CpG sites, in which the C is presumably methylated. A similar bias can be deduced from studies in human XP individuals.

Ultraviolet B radiation-induced skin cancer in mice defective in the Xpc, Trp53, and Apex (HAP1) genes: genotype-specific effects on cancer predisposition and pathology of tumors.

Mutations in nucleotide excision repair (NER) genes in humans result in the UV-induced skin cancer-prone disease xeroderma pigmentosum (XP). Mouse models that mimic XP have provided an informative experimental system with which to study DNA repair, as well as the molecular pathology of UV radiation-induced skin cancer. We reported previously that mice defective in the Xpc gene (Xpc-/-) are highly predisposed to UVB radiation-induced skin cancer and that the appearance of skin cancer is more rapid in Xpc Trp53 double mutants. Extended studies now demonstrate an increased predisposition to UVB radiation-induced skin cancers in Xpc heterozygous mice compared with normal mice. We also show that Xpc Trp53 double heterozygous mutants are more predisposed to skin cancer than Trp53 single heterozygous mice. No mutations were detected in the cDNA of the remaining Xpc allele, suggesting that haploinsufficiency of the Xpc gene may be operating and is a risk factor for UVB radiation-induced skin cancer in mice. Skin tumors from Xpc-/- mice were exclusively well or moderately well-differentiated squamous cell carcinomas. In Xpc+/+ and Xpc+/- mice, many of the squamous cell carcinomas were less well differentiated. We also documented previously increased predisposition to UV radiation-induced skin cancers in Xpc-/- Apex+/- mice. Here we show the absence of mutations in the cDNA of the remaining Apex allele, a further suggestive indication of haploinsufficiency and its resulting predisposition to skin cancer. The Trp53 and Apex heterozygous conditions altered the skin tumor spectrum to more poorly differentiated forms in all Xpc genotypes.

Defective nucleotide excision repair in xpc mutant mice and its association with cancer predisposition.

Mice that are genetically engineered are becoming increasingly more powerful tools for understanding the molecular pathology of many human hereditary diseases, especially those that confer an increased predisposition to cancer. We have generated mouse strains defective in the Xpc gene, which is required for nucleotide excision repair (NER) of DNA. Homozygous mutant mice are highly prone to skin cancer following exposure to UVB radiation, and to liver and lung cancer following exposure to the chemical carcinogen acetylaminofluorene (AAF). Skin cancer predisposition is significantly augmented when mice are additionally defective in Trp53 (p53) gene function. We also present the results of studies with mice that are heterozygous mutant in the Apex (Hap1, Ref-1) gene required for base excision repair and with mice that are defective in the mismatch repair gene Msh2. Double and triple mutant mice mutated in multiple DNA repair genes have revealed several interesting overlapping roles of DNA repair pathways in the prevention of mutation and cancer.

Mutational inactivation of the xeroderma pigmentosum group C gene confers predisposition to 2-acetylaminofluorene-induced liver and lung cancer and to spontaneous testicular cancer in Trp53-/- mice.

Mice that are genetically engineered to mimic the human hereditary cancer-prone DNA repair-defective disease xeroderma pigmentosum (XP) are highly predisposed to UV radiation-induced skin cancer. It is not clear, however, whether XP mice or humans are predisposed to cancers in other tissues associated with exposure to environmental carcinogens. To test the importance of nucleotide excision repair in protection against chemical carcinogenesis in internal organs, we treated XPC mutant (XPC-/-) mice with 2-acetylaminofluorene and NOH-2-acetylaminofluorene. We observed a significantly higher incidence of chemically induced liver and lung tumors in XPC-/- mice compared with normal and heterozygous littermates In addition, the progression of liver tumors in XPC-/- Trp53+/- mice is accelerated compared with XPC-/- Trp53+/+ animals. Finally, we demonstrate a higher incidence of spontaneous testicular tumors in XPC-/- TrpS3-/- double mutant mice compared with XPC+/+ Trp53-/- mice.

Characterization of defective nucleotide excision repair in XPC mutant mice.

Nucleotide excision repair (NER) is a fundamental process required for maintaining the integrity of the genome in cells exposed to environmental DNA damage. Humans defective in NER suffer from the hereditary cancer-prone disease xeroderma pigmentosum. In order to model this disease in mice a mutation in the mouse XPC gene was generated and used to replace a wild-type XPC allele in mouse embryonic stem cells by homologous recombination. These cells were used to derive XPC mutant mice. Fibroblasts from mutant embryos were more sensitive to the cytotoxic effects of ultraviolet light than wild-type and heterozygous cells. Repair synthesis of DNA following irradiation with ultraviolet light was reduced in these cells, indicating a defect in NER. Additionally, XPC mutant embryo fibroblasts were specifically defective in the removal of pyrimidine (6-4) pyrimidone photoproducts from the non-transcribed strand of the transcriptionally active p53 gene. Mice defective in the XPC gene appear to be an excellent model for studying the role of NER and its interaction with other proteins in the molecular pathogenesis of cancer in mammals following exposure to environmental carcinogens.

Synergistic interactions between XPC and p53 mutations in double-mutant mice: neural tube abnormalities and accelerated UV radiation-induced skin cancer.

The significance of DNA repair to human health has been well documented by studies on xeroderma pigmentosum (XP) patients, who suffer a dramatically increased risk of cancer in sun-exposed areas of their skin [1,2]. This autosomal recessive disorder has been directly associated with a defect in nucleotide excision-repair (NER) [1,2]. Like human XP individuals, mice carrying homozygous mutations in XP genes manifest a predisposition to skin carcinogenesis following exposure to ultraviolet (UV) radiation [3-5]. Recent studies have suggested that, in addition to roles in apoptosis [6] and cell-cycle checkpoint control [7] in response to DNA damage, p53 protein may modulate NER [8]. Mutations in the p53 gene have been observed in 50% of all human tumors [9] and have been implicated in both the early [10] and late [11] stages of skin cancer. To examine the consequences of a combined deficiency of the XPC and the p53 proteins in mice, we generated double-mutant animals. We document a spectrum of neural tube defects in XPC p53 mutant embryos. Additionally, we show that, following exposure to UV-B radiation, XPC p53 mutant mice have more severe solar keratosis and suffer accelerated skin cancer compared with XPC mutant mice that are wild-type with respect to p53.

Changes in red blood cell methotrexate pharmacology and their impact on outcome when cytarabine is infused with methotrexate in the treatment of acute lymphocytic leukemia in children: a pediatric oncology group study.

Since it is unclear whether methotrexate and cytarabine are synergistic or antagonistic in the treatment of acute lymphoblastic leukemia, the Pediatric Oncology Group studied the prognostic significance of a potential interaction between these agents. RBC methotrexate concentrations were compared from 140 patients at lower risk of relapse randomized to two treatment groups: one receiving six methotrexate infusions with overlapping cytarabine; the other, six methotrexate infusions alone. Samples from 248 patients from all risk groups were studied to determine whether patients with extremely low RBC methotrexate concentrations had inferior outcomes. Among low-risk patients studied 3 weeks after the sixth infusion, median RBC methotrexate concentrations were 0.13 nmol/ml RBCs (n = 71) for the methotrexate-only group and 0.02 nmol/ml RBCs (n = 69) for the methotrexate/cytarabine-treated low-risk patients, P < 0.001 by the two-sided Wilcoxon test. For low- and high-risk patients receiving methotrexate/cytarabine infusions, event-free survival at 1 and 3 years after RBC sampling was 97 +/- 2% and 90 +/- 3% for patients with concentrations greater than the median, and 88 +/- 3% and 78 +/- 4% for those with concentrations at or below the median. Log rank comparisons of event-free survival in the first year and overall yielded P = 0.005 and P = 0.04, respectively. Cytarabine altered methotrexate pharmacology when the drugs were infused together. Patients whose levels were extremely low had an adverse prognosis. Although this study could not assess efficacy of the methotrexate/cytarabine combination, it appears that concurrent administration is not optimal.

Red blood cell methotrexate and folate levels in children with acute lymphoblastic leukemia undergoing therapy: a Pediatric Oncology Group pilot study.

We enrolled children with acute lymphoblastic leukemia (ALL) in a Pediatric Oncology Group (POG) pilot study to monitor erythrocyte (RBC) methotrexate (MTX) and folate (F) levels before and during treatment. The mean value for RBCF at diagnosis was 0.86 +/- 0.46 nmol/ml RBC in the 214 patients who achieved remission and 1.21 +/- 0.74 nmol/ml RBC in the 10 patients who did not (P = 0.020). Folate levels tended to increase during remission induction, but they dropped following an intensive consolidation with methotrexate to levels that were sustained throughout chemotherapy treatment. Methotrexate levels reached mean values of approximately 0.15 nmol/ml RBC at the end of an intensive methotrexate consolidation, then fell to levels that were sustained throughout maintenance therapy. There was a weak correlation between improved event-free survival and higher RBCMTX levels after consolidation, but no correlation was found between improved survival and the level of RBCMTX or RBCF during maintenance therapy. A larger study with more complete data is needed to determine whether RBCMTX or RBCF might be useful in predicting event-free survival in patients with ALL.

Intraocular penetration of cytosine arabinoside following intravenous administration in primates.

Cytosine arabinoside was administered intravenously to three monkeys. Blood, aqueous, and vitreous specimens were collected after 30 minutes, 1 hour, 2 hours, and 3 hours. Intraocular and intravenous drug concentrations were comparable at all time points, at levels likely to be effective against human tumors. This study suggests that therapeutic intraocular concentrations of cytosine arabinoside are attained following intravenous administration in primates.