Causation of nervous system tumors in children: insights from traditional and genetically engineered animal models

A meta-analysis of parental smoking and the risk of childhood brain tumors

OBJECTIVE

Previous studies regarding the association between parental smoking and the risk of childhood brain tumors (CBT) have reported inconsistent results. We performed a meta-analysis to summarize evidence on this association and to quantify the potential dose-response relationship.

METHODS

A systematic literature search was conducted in the Medline and Embase databases. The summary relative risks (RRs) with 95% confidence intervals (CIs) were calculated. Dose-response meta-analysis was also performed for studies that reported categorical risk estimates for a series of smoking exposure levels.

RESULTS

A total of 17 studies fulfilled the inclusion criteria. In the meta-analyses, the summary RRs (95% CIs) of CBT for maternal smoking during pregnancy, paternal smoking during pregnancy, maternal smoking before pregnancy, and paternal smoking before pregnancy were 0.96 (0.86-1.07), 1.09 (0.97-1.22), 0.93 (0.85-1.00), and 1.09 (1.00-1.20), respectively. Dose-response meta-analysis also showed no significant association between parental smoking and the risk of CBT.

CONCLUSIONS

Findings from our meta-analysis indicate that parental smoking may not be associated with a risk of CBT.

Space-time clustering in childhood nervous system tumors in the Region of Murcia, Spain, 1998-2009

AIMS

The aims of this study are the following: first, to analyze incidence, trends, and survival of nervous system tumors in children under the age of 15 in the Region of Murcia, Spain, during the years 1998-2009 and second, to evaluate if certain environmental exposures may be involved in the etiology of childhood nervous system tumors. The study was performed on the spatial and temporo-spatial distribution of the observed cases.

METHODS

The Environment and Pediatric Cancer in the Region of Murcia is an ongoing research project aimed at carefully collecting pediatric environmental history (PEH) and to use geographical information systems to map the incidence and to analyze the geographical distribution of pediatric cancer incidence in our region. Between 1998 and 2009, 125 patients were diagnosed with nervous system tumors. The spatial and temporal space clusters were evaluated using Kulldorff's statistics. Address at diagnosis was the main feature evaluated.

RESULTS

The incidence (cases/million children) for central nervous system (CNS) tumors was 34.2, that for sympathetic nervous system tumors was 10.9, and that for retinoblastoma was 1.9. There was evidence of space clustering for medulloblastoma and space-time clustering for all tumors, CNS tumors, astrocytoma, and neuroblastoma.

CONCLUSIONS

The incidence and survival for each type and subtype of nervous system tumors were within the reported values for the European region. There is evidence that spatial and spatial-temporal distribution in these cases is not random. The development of a careful PEH in these patients will help to reinforce geographical information system studies and to ascertain the importance of associated risk factors.

Parental smoking, maternal alcohol, coffee and tea consumption during pregnancy and childhood malignant central nervous system tumours: the ESCALE study (SFCE)

Parental smoking and maternal alcohol and caffeinated beverage consumption are prevalent exposures which may play a role, either directly or through their influence on metabolism, in the aetiology of childhood malignant central nervous system (CNS) tumours. The hypothesis was investigated in the Epidemiological Study on childhood Cancer and Leukemia ESCALE study, a national population-based case-control study carried out in France in 2003-2004. The study included 209 incident cases of CNS tumours and 1681 population-based controls, frequency matched with the cases by age and sex. The data were collected through a standardized telephone interview of the biological mothers. No association between maternal smoking during pregnancy and CNS tumours [odds ratio (OR): 1.1 (0.8-1.6)] was observed. Paternal smoking during the year before birth was associated with CNS tumours (P for trend=0.04), particularly astrocytomas [OR: 3.1 (1.3-7.6)]. Maternal alcohol consumption during pregnancy was not associated with CNS tumours. Associations between ependymomas and the highest consumption of coffee [OR: 2.7 (0.9-8.1)] and tea [OR: 2.5 (1.1-5.9)] were observed. A strong association between CNS tumours and the highest maternal consumption of both coffee and tea during pregnancy was observed [OR: 4.4 (1.5-13)]. The results constitute additional evidence for a role of paternal smoking and suggest that maternal coffee and tea consumption during pregnancy may also increase the risk of CNS tumours. The study does not suggest an increased risk of CNS tumours related to alcohol consumption during pregnancy.

Angiogenesis and cell proliferation in human craniopharyngioma xenografts in nude mice

OBJECT

Craniopharyngioma is one of the most common congenital tumors of the sellar and suprasellar regions and accounts for between 4 and 6% of all intracranial tumors. Its oncogenesis and biological behavior have not been well studied, and neither a cell line nor an animal model have been established. To better understand the tumor and improve its clinical management, the authors investigated the angiogenesis and cellular proliferation in subcutaneous craniopharyngioma xenografts obtained by implanting human tumor cells into athymic nude mice.

METHODS

Human craniopharyngioma cells obtained from surgical specimens were subcutaneously implanted into BALB/c-nu/nu nude mice to establish a preliminary animal model of a transplanted tumor. Immunohistochemical staining with streptavidin-peroxidase complex was used to identify the cell phenotype and to evaluate the angiogenesis and proliferation in the xenografts. Expression of cytokeratin, minichromosome maintenance deficient 6 (MCM6) protein, and endothelial cell marker CD34 on the xenograft sections were assayed quantitatively by computer-assisted microscopy. Twenty-seven surviving subcutaneous xenografts were obtained in 15 nude mice. The total implantation success rate was 28.12% (adamantine epithelioma [AE], 37.50%; squamous papillary tumor [SPT], 18.75%). Formation of capillaries and cell proliferation were observed in all of these xenografts. Microvessel density and degree of MCM6 immunostaining were positively correlated in the surviving grafts (r = 0.410, p < 0.05), but there was no significant difference in these variables between the AE and SPT groups (p > 0.05).

CONCLUSIONS

A preliminary animal model of human craniopharyngioma was established in the nude mouse by heterotopic implantation. Surviving xenografts maintained their vascularization and proliferation activities until harvesting at 12 weeks.

Environmental genotoxicants/carcinogens and childhood cancer: Bridgeable gaps in scientific knowledge

Cancer in children is a major concern in many countries. An important question is whether these childhood cancers are caused by something, or are just tragic random events. Causation of at least some children's cancers is suggested by direct and indirect evidence, including epidemiological data, and animal studies that predict early life sensitivity of humans to carcinogenic effects. Candidate risk factors include genotoxic agents (chemicals and radiation), but also diet/nutrition, and infectious agents/immune responses. With regard to likelihood of risks posed by genotoxicants, there are pros and cons. The biological properties of fetuses and infants are consistent with sensitivity to preneoplastic genotoxic damage. Recent studies of genetic polymorphisms in carcinogen-metabolizing enzymes confirm a role for chemicals. On the other hand, in numerous epidemiological studies, associations between childhood cancers and exposure to genotoxicants, including tobacco smoke, have been weak and hard to reproduce. Possibly, sensitive genetic or ontogenetic subpopulations, and/or co-exposure situations need to be discovered to allow identification of susceptible individuals and their risk factors. Among the critical knowledge gaps needing to be bridged to aid in this effort include detailed tissue and cellular ontogeny of carcinogen metabolism and DNA repair enzymes, and associations of polymorphisms in DNA repair enzymes with childhood cancers. Perinatal bioassays in animals of specific environmental candidates, for example, benzene, could help guide epidemiology. Genetically engineered animal models could be useful for identification of chemical effects on specific genes. Investigations of interactions between factors may be key to understanding risk. Finally, fathers and newborn infants should receive more attention as especially sensitive targets.

Monitoring the growth effect of xenotransplanted human medulloblastoma in an immunocompromised mouse model using in vitro and ex vivo green fluorescent protein imaging

INTRODUCTION

Medulloblastoma (MB) is one of the most common malignant brain tumors in children. It is a radiosensitive tumor. At 5 years after radical surgical excision and craniospinal axis irradiation, the tumor-free survival rate is from 50 to 70% [Halperin EC, Constine LS, Tarbell NJ, Kun LE. Pediatric radiation oncology (2005)].

CASE REPORT

In this study, we established xenotransplanted human MB (hMB) cells - isochromosome 17q - in a severe combined immunodeficiency (SCID) mouse model. We further transduced green fluorescent protein (GFP) into hMB cells to evaluate these hMB cells grafted in SCID mice.

RESULTS

The result of an ex vivo GFP imaging system showed that a small lesion of the third-week-hMB-transplanted graft presented "green" signals with a clear tumor margin before any tumor-related symptoms were noted. We also demonstrated that the tumor progression could be monitored by GFP imaging for up to 12 weeks post-transplantation.

CONCLUSIONS

This novel approach of GFP imaging assessment provides more accurate information of tumor status for experimental brain tumor studies. Because MB is sensitive to radiation and also response to chemotherapy, this SCID mouse model will be helpful for preclinical studies in the future.

Inducible and transmissible genetic events and pediatric tumors of the nervous system

Tumors of the nervous system most often occur in both children and adults as sporadic events with no family history of the disease, but they are also among the clinical manifestations of a significant number of familial cancer syndromes, including familial retinoblastoma, neurofibromatosis 1 and 2, tuberous sclerosis, and Cowden, Turcot, Li-Fraumeni and nevoid basal cell carcinoma (Gorlin) syndromes. All of these syndromes involve transmissible genetic risk resulting from loss of a functional allele, or inheritance of a structurally defective allele, of a specific gene. These genes include RB1, NF1, NF2, TSC1, TSC2, TP53, PTEN, APC, hMLH1, hPSM2, and PTCH, most of which function as tumor suppressor genes. The same genes are also observed in mutated and inactive forms, or are deleted, in tumor cells in sporadic cases of the same tumors. The nature of the mutational events that give rise to these inactivated alleles suggests a possible role of environmental mutagens in their causation. However, only external ionizing radiation at high doses is clearly established as an environmental cause of brain, nerve and meningeal tumors in humans. Transplacental carcinogenesis studies in rodents and other species emphasize the extraordinary susceptibility of the developing mammalian nervous system to carcinogenesis, but the inverse relationship of latency to dose suggests that low transplacental exposures to genotoxicants are more likely to result in brain tumors late in life, rather than in childhood. While not all neurogenic tumor-related genes in humans have similar effects in experimental rodents, genetically engineered mice (GEM) increasingly provide useful insights into the combined effects of multiple tumor suppressor genes and of gene-environment interactions in the genesis of brain tumors, especially pediatric brain tumors such as medulloblastoma.

Predictive values of traditional animal bioassay studies for human perinatal carcinogenesis risk determination

The many physiological, biochemical, and structure differences between rodents and humans, especially with regard to gestation and fetal development, invite questions as to the utility of rodent models for the prediction of risk of perinatal carcinogenesis in humans and for extrapolation of mechanistic studies. Here, the relevance of basic generalities, derived from rodent perinatal studies, to human contexts is considered. The cross-species usefulness of these generalities was upheld by the example of carcinogen activation and detoxification as determining factors. These have been established in rodent studies and recently indicted in humans by investigations of genetic polymorphisms in cytochromes P450, N-acetyltransferase, myeloperoxidase, quinone reductase, and glutathione S-transferase. Also, published data have been analyzed comparatively for diethylstilbestrol and irradiation, the two known human transplacental carcinogenic agents. At similar doses to those experienced by humans, both diethylstilbestrol and X- and gamma-irradiation in rodents and dogs yielded increased tumors at rates similar to those for humans. In rodents, there was a clearly negative relationship between total diethylstilbestrol dose and tumors per dose unit, and a similar pattern was suggested for radiation. Diethylstilbestrol had transgenerational effects that did not diminish over three generations. Overall, this analysis of the published literature indicates that there are basic qualitative and quantitative similarities in the responsiveness of human and rodent fetuses to carcinogens, and that dose effects may be complex and in need of further investigation.

Introduction and overview. Perinatal carcinogenesis: growing a node for epidemiology, risk management, and animal studies

Perinatal carcinogenesis as a cross-disciplinary concern is the subject of this special issue of Toxicology and Applied Pharmacology, which consists of a total of eight reviews or commentaries in the areas of epidemiology, risk assessment, and animal models. Some of the conclusions from these articles, and the Questions and Answers section that follows most of them, are summarized here. There is adequate reason to suspect that perinatal exposures contribute to human cancer risk, both childhood cancers, and those appearing later in life. The latter type of risk may actually be quantitatively the more important, and involve a wide range of types of effects, but has received only limited attention. With regard to childhood cancers, fetal irradiation and diethylstilbestrol exposure are known etiological agents, and it is likely, but not yet certain, there are additional external causes of a portion of these. Some current focal points of interest here include nitroso compounds, DNA topoisomerase inhibitors, viruses, anti-AIDS drugs, and endocrine disruptors. Regulatory agencies must rely heavily on animal data for estimation of human risk due to perinatal exposures to chemicals, and the quantity and quality of these data presently available for this purpose are greatly limiting. Correctly designed conventional animal studies with suspect chemicals are still needed. Furthermore, genetically engineered mouse models for childhood cancers, especially medulloblastoma, have become available, and could be used for screening of candidate causative agents for this cancer type, and for better understanding of gene-environment interactions.