Carcinogenic mechanisms related to immunosuppressive therapy

Sleeping Beauty Mouse Models of Cancer: Microenvironmental Influences on Cancer Genetics

The Sleeping Beauty (SB) transposon insertional mutagenesis system offers a streamlined approach to identify genetic drivers of cancer. With a relatively random insertion profile, SB is uniquely positioned for conducting unbiased forward genetic screens. Indeed, SB mouse models of cancer have revealed insights into the genetics of tumorigenesis. In this review, we highlight experiments that have exploited the SB system to interrogate the genetics of cancer in distinct biological contexts. We also propose experimental designs that could further our understanding of the relationship between tumor microenvironment and tumor progression.

Cancer Recurrence Following Immune-Suppressive Therapies in Patients With Immune-Mediated Diseases: A Systematic Review and Meta-analysis

BACKGROUND & AIMS

Physicians frequently encounter patients with immune-mediated diseases and a history of malignancy. There are limited data on the safety of immunosuppressive therapy for these patients. Published studies have been small with few events, precluding robust estimates of risk.

METHODS

We searched Medline, EMBASE, and conference proceedings for terms related to immune-mediated disease, immune-suppressive therapy, and cancer recurrence from inception to April 2015. We included 16 studies (9 of patients with rheumatoid arthritis, 8 of patients with inflammatory bowel disease, and 1 of patients with psoriasis) and stratified studies by type of immune-suppressive therapy (monoclonal antibodies to tumor necrosis factor [anti-TNF], conventional immune-modulatory agents, or no immune suppression). A random-effects meta-analysis was performed to calculate the pooled incidence rates as well as risk differences between the various treatments.

RESULTS

Our analysis included 11,702 persons contributing 31,258 person-years (p-y) of follow-up evaluation after a prior diagnosis of cancer. Rates of cancer recurrence were similar among individuals receiving anti-TNF therapy (33.8 per 1000 p-y), immune-modulator therapy (36.2 per 1000 p-y), or no immunosuppression (37.5 per 1000 p-y), but were numerically higher among patients receiving combination immune suppression (54.5 per 1000 p-y) (P > .1 for all). Subgroup analysis of new and recurrent cancers separately, type of immune-modulator therapy, or immune-mediated disease showed similar results, with no increase in risk. We found similar pooled incidence values for new or primary cancers when immunosuppression was initiated within 6 years (33.6 per 1000 p-y for immune-modulatory agents and 43.7 per 1000 p-y for anti-TNF agents) vs more than 6 years after the index cancer (32.9 per 1000 p-y for immune-modulatory agents, P = .86; and 21.0 per 1000 p-y for anti-TNF agents, P = .43).

CONCLUSIONS

In a meta-analysis of 16 studies, we observed similar rates of cancer recurrence among individuals with prior cancer who received no immunosuppression, anti-TNF therapy, immune-modulator therapy, or combination treatments. Prospective studies are needed to ascertain optimal intervals for re-initiation of immune-suppressive therapies for individuals with specific cancers.

Adaptive immunity does not strongly suppress spontaneous tumors in a Sleeping Beauty model of cancer

The tumor immunosurveillance hypothesis describes a process by which the immune system recognizes and suppresses the growth of transformed cancer cells. A variety of epidemiological and experimental evidence supports this hypothesis. Nevertheless, there are a number of conflicting reports regarding the degree of immune protection conferred, the immune cell types responsible for protection, and the potential contributions of immunosuppressive therapies to tumor induction. The purpose of this study was to determine whether the adaptive immune system actively suppresses tumorigenesis in a Sleeping Beauty (SB) mouse model of cancer. SB transposon mutagenesis was performed in either a wild-type or immunocompromised (Rag2-null) background. Tumor latency and multiplicity were remarkably similar in both immune cohorts, suggesting that the adaptive immune system is not efficiently suppressing tumor formation in our model. Exceptions included skin tumors, which displayed increased multiplicity in wild-type animals, and leukemias, which developed with shorter latency in immune-deficient mice. Overall tumor distribution was also altered such that tumors affecting the gastrointestinal tract were more frequent and hemangiosarcomas were less frequent in immune-deficient mice compared with wild-type mice. Finally, genetic profiling of transposon-induced mutations identified significant differences in mutation prevalence for a number of genes, including Uba1. Taken together, these results indicate that B and T cells function to shape the genetic profile of tumors in various tumor types, despite being ineffective at clearing SB-induced tumors. To our knowledge, this study represents the first forward genetic screen designed to examine tumor immunosurveillance mechanisms.

[Immunosuppressive drugs and the development of skin cancer after organ transplantation]

BACKGROUND

Patients with organ transplants have a significantly increased risk of skin cancer, especially squamous cell carcinoma, as a result of long-term treatment with immunosuppressive drugs. This paper provides a brief overview of the assumed underlying mechanisms.

METHOD

The paper builds on relevant articles and studies identified in the course of many years of interest in immunopharmacology and skin cancer after organ transplantation.

RESULTS

Reduced immunological tumour surveillance as a result of chronic immunosuppression has long been assumed to underlie the increased risk of skin cancer after organ transplants. Recent studies indicate that immunosuppressive drugs may also have specific carcinogenic effects. Aziatropine, which inhibits proliferation of lymphocytes, increases oxidative DNA damage caused by UV radiation. Ciclosporin and tacrolimus, which have an immunosuppressive effect by inhibiting calcineurin, promote malignant phenotypes in cell culture and tumour growth in mouse models. Calcineurin has proved to be necessary in order for p53 protein to have a protective effect against skin cancer. A relatively new class of immunosuppressive drugs, mTOR inhibitors, have antineoplastic properties and are associated with less risk of skin cancer. A number of randomised studies are currently in progress to see whether mTOR inhibitors can reduce the risk of skin cancer after organ transplantation.

INTERPRETATION

Immunosuppressive drugs contribute to skin cancer after organ transplantation, either as a result of immunosuppression or through specific carcinogenic mechanisms. Immunosuppressive drugs with antineoplastic properties are now starting to be used.

Quantification of Thiopurine/UVA-Induced Singlet Oxygen Production

Thiopurines were examined for their ability to produce singlet oxygen ((1)O(2)) with UVA light. The target compounds were three thiopurine prodrugs, azathioprine (Aza), 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG), and their S-methylated derivatives of 6-methylmercaptopurine (me6-MP) and 6-methylthioguanine (me6-TG). Our results showed that these thiopurines were efficient (1)O(2) sensitizers under UVA irradiation but rapidly lost their photoactivities for (1)O(2) production over time by a self-sensitized photooxidation of sulfur atoms in the presence of oxygen and UVA light. The initial quantum yields of (1)O(2) production were determined to be in the range of 0.30-0.6 in aqueous solutions. Substitution of a hydrogen atom with a nitroimidazole or methyl group at S decreased the efficacy of photosensitized (1)O(2) production as found for Aza, me6-MP and me6-TG. (1)O(2)-induced formation of 8-oxo-7,8-dihydro-2'-dexyguanosine (8-oxodGuo) was assessed by incubation of 6-methylthiopurine/UVA-treated calf thymus DNA with human repair enzyme 8-oxodGuo DNA glycosylase (hOGG1), followed by apurinic (AP) site determination. Because more 8-oxodGuo was formed in Tris D(2)O than in Tris H(2)O, (1)O(2) is implicated as a key species in the reaction. These findings provided quantitative information on the photosensitization efficacy of thiopurines and to some extent revealed the correlations between photoactivity and phototoxicity.

Direct observation and quantitative characterization of singlet oxygen in aqueous solution upon UVA excitation of 6-thioguanines

The incorporation of 6-thioguanine (6-TG) into DNA increases the risk of (1)O(2)-initiated skin cancer. We herein provide the first report on quantitative characterization of the photoactivity of 6-thioguanines including 6-TG and 6-thioguanosine. Time-resolved singlet oxygen luminescence was observed directly for the first time after UVA irradiation of 6-thioguanines in both CHCN(3) and aqueous solutions. Their photosensitization was characterized by the quantum yield of singlet oxygen production, showing a dramatic decrease over time from the initial 0.49-0.58 to zero. Experiments performed on both 6-TG and 6-thioguanosine did not show any significant difference in the quantum yield of singlet oxygen production, indicating that there was no potential participation of 7H- and 9H-tautomers. Our findings provide a primary basis for a better understanding of molecular events of thiopurine drugs in biological systems.