4. SUSCEPTIBILITY TO BRAIN TUMORS The most generally accepted current model of carcinogenesis holds that cancers develop through accumulation of genetical alterations which allow the cells to grow out of control of normal regulatory mechanisms and/or escape destruction by the immune system. Some inherited alterations in crucial cell-cycle control genes, such as p53, as well as chemical, physical, and biological agents that damage DNA, are therefore considered candidate carcinogens. Although rapid advances in molecular biology, genetics, and virology promise to help elucidate the molecular causes of brain tumors, continued epidemiological work will be necessary to clarify the relative roles of different mechanisms in the full scope of human brain tumors. Genetical and familial factors implicated in brain tumors have been the subject of many studies and were previously reviewed by us (82).
4.1. Familial Aggregation Because only a small proportion of brain tumors are due solely to heredity, most are probably due to gene-environment interactions. Although findings of familial cancer aggregation may suggest a genetical etiology, such aggregations can be the result of common familial exposure to environmental agents. Some epidemiological studies that compare family medical histories of brain tumor cases with those of controls find significantly increased family histories both of brain tumors and of other cancers. Other studies find no increase for any cancer, with a relative risk ranging from 1 to 1.8 and from 1 to 9 for brain tumors (82-85). These contradictions might be explained by differences in study methodologies, sample size, types of relatives included in the study, how cancers were ascertained and validated, and the country where the study was conducted.
Also supporting a genetical role in etiology are studies of cases reporting a high frequency of siblings with brain tumors, although twin studies have not. In a family study of 250 childhood brain tumor patients, we
(82) showed by segregation analysis that familial aggregation, although small, supported multifactorial inheritance, not chance alone. Segregation analyses of the families of more than 600 adult glioma patients revealed that a polygenic environment-interactive model best explained the pattern of occurrence of brain tumors (86). Segregation analyses of 2,141 first-degree relatives of 297 glioma families did not reject a multifactorial model, but an autosomal recessive model provided the best fit (87). The study estimated that 5% of all glioma cases were familial. Grossman et al. (88) showed that brain tumors can occur in families without a known predisposing hereditary disease and that the pattern of occurrence in many families suggests environmental causes. Given the previously described complexities of environmental impact and the multiplicity of possible heritable factors, more work will be required to delineate how genetical susceptibility affects brain cancer risk.
4.2. Hereditary Syndromes A few rare genes and chromosomal abnormalities can greatly increase the chances of developing brain tumors. Numerous case reports have associated central nervous system tumors with gross malformations, including medulloblastoma with gastrointestinal and genitourinary system abnormalities, ependymoma with multisystem abnormalities, astrocytoma with arteriove-nus malformation of the overlying meninges, and glioblastoma multiforme with adjacent arteriovenous angiomatous malformation and pulmonary arteriovenous fistula. Central nervous system tumors may also be associated with Down’s syndrome, a disorder involving chromosome 21. Three epidemiological studies have found that brain tumor cases are two to five times more likely than controls to have a mentally retarded relative although the result was statistically significant in only one study [reviewed in Ref. (82)]. The heritability of brain tumors is also suggested by many reports of these tumors in individuals with hereditary syndromes such as tuberous sclerosis, neurofibromatosis types land 2, nevoid basal cell carcinoma syndrome, and syndromes involving adenomatous polyps [reviewed in Ref. (82)].
Although there is convincing evidence that genetics plays a role in most cancers, including brain tumors, inherited predisposition through high penetrant genetical traits to brain tumors probably accounts for only a very small percentage (5-10%) of these tumors (89). In a review of 16,564 cases of childhood cancers diagnosed from 1971 to 1983, and reported to the National Registry of Childhood Tumors in Great Britain, Narod et al. (89) estimated that the heritable fraction of childhood brain tumors was about 2%. In a population-based study of nearly 500 adults with glioma, only four individuals (less than 1%), all of whom were diagnosed in their thirties, reported having a known heritable syndrome (three had neurofibromatosis and one had tuberous sclerosis) (83).
Another class of heritable conditions are the cancer family syndromes [such as the Li-Fraumeni syndrome (LFS)], so called because individuals in affected families have an increased risk of developing certain types of cancers. In LFS, the cancers include brain tumors, sarcomas, breast cancer, and cancer of the adrenal gland. Individuals with LFS have inherited at least one copy of a defective gene-which can be passed from parent to child.
In some families, LFS has been linked to a gene mutation in p53 on chromosome 17p (82). In addition, germline p53 mutations were found to be more frequent in patients with multifocal glioma, glioma and another primary malignancy, and a family history of cancer. In a population-based study of malignant glioma, Li et al. (90) reported that p53 mutation-positive patients were more likely to have a first-degree relative affected with cancer (58% vs. 42%) or a personal history of a previous cancer (17% vs. 8%). Further research needs to be done to determine the role of heredity, the frequency of p53 mutations, and whether specific p53 mutations correlate with specific exposures.
4.3. Metabolic Susceptibility Genetic traits involved in susceptibility refer to more common genetic alterations that influence oxidative metabolism, carcinogen detoxification, and DNA stability and repair. The role of genetic polymorphisms (alternative states of genes established in the population) in modulating susceptibility to carcinogenic exposures has been explored in some detail for tobacco-related neoplasms but much less so for other neoplasms including gliomas. Due to rapid developments in genetic technology, an increasing number of potentially relevant polymorphisms are available for epidemiological evaluation, including genes involved in carcinogen detoxification, oxidative metabolism, and DNA repair. The first study to report the role of metabolic polymorphisms in brain tumor risk found that the variants of cytochrome P450 2D6 (CYP2D6) and glutathione transferase (GSTT1) were significantly associated with increased risk of brain tumors (91). Kelsey et al. (92) were unable to find an association of adult onset glioma with either the GSTT1 null genotype or homozygosity for the CYP2D6 variant poor-metabolizer genotype. However, when they stratified the data by histological subtype, there was a significant threefold increased risk for oligodendroglioma associated with the GSTT1 null genotype. Trizna et al. (93) found no statistically significant associations between the null genotypes of glutathione transferase m, GSTT1, and CYP1A1 and the risk of adult gliomas. However, they observed an intriguing pattern with N-acetyltransferase acetylation status, with a nearly twofold increased risk for rapid acetylation and a 30% increased risk for intermediate acetylation.
It is unlikely that any single polymorphism will be sufficiently predictive of brain tumor risk. Therefore, a panel of relevant markers integrated with epidemiological data should be assessed in a large number of study participants to clarify the role of genetic polymorphisms and brain tumor risk.
4.4. Mutagen Sensitivity Cytogenetical assays of peripheral blood lymphocytes have been extensively used to determine response to genotoxic agents. The basis for these cytogenetical assays is that genetical damage reflects critical events in carcinogenesis in the affected tissue. To test this hypothesis, Hsu et al. (94) developed a mutagen sensitivity assay in which the frequency of in vitro bleomycin-induced breaks in short-term lymphocyte cultures is used to measure genetical susceptibility. We (95) have modified the assay by using gamma radiation to induce chromosome breaks because radiation is a risk factor for brain tumors and can produce double-stranded DNA breaks and mutations. It is believed that mutagen sensitivity indirectly assesses the effectiveness of one or more DNA repair mechanisms. The following observations support this hypothesis. First, the relationship between chromosome instability syndromes and cancer susceptibility is well established (96). Patients with these syndromes also have defective DNA repair systems (97). Furthermore, patients with ataxia telangiectasia, who are extremely sensitive to the clastogenic effects of x-irradiation and bleomycin, differ from normal people in the speed with which aberrations induced by these agents are repaired but not in the number of aberrations produced (98).
Gamma-radiation-induced mutagen sensitivity is one of the few significant independent risk factors for brain tumors (95). DNA repair capability and predisposition to cancer are hallmarks of rare chromosome instability syndromes, and are related to differences in radiosensitivity. An in vitro study showed that individuals vary in lymphocyte radiosensitivity, which correlates with DNA repair capacity (95). Therefore, it is biologically plausible that increased sensitivity to gamma radiation results in increased risk of developing brain tumors because of individuals’ inability to repair radiation damage. However, this finding needs to be tested in a larger study to determine the roles of mutagen sensitivity and radiation exposure in the risk of developing gliomas. The mutagen sensitivity assay has been shown to be an independent risk factor for other cancers including head and neck and lung, suggesting that the phenotype is constitutional (99). The breaks are not affected by smoking status or dietary factors (micronutrients) (100).
4.5. Chromosome Instability A number of chromosomal loci have been reported to play a role in brain tumorigenesis because of the numerous gains and losses in those loci. For example, Bigner et al., (101) reported gain of chromosome 7 and loss of chromosome 10 in malignant gliomas and structural abnormalities involving chromosomes 1, 6p, 9p, and 19q; Bello et al. (102) reported involvement of chromosome 1 in oligodendrogliomas and meningiomas; and Magnani et al. (103) demonstrated involvement of chromosomes 1, 7, 10, and 19 in anaplastic gliomas and glioblastomas. Loss of heterozygosity for loci on chromosome 17p (104) and 11p15 (105) has also been reported.
There are few data on chromosomal alterations in the peripheral blood lymphocytes of brain tumor patients. Information on such changes might shed light on premalignant changes that lead to tumor development. We (95) demonstrated that compared with controls, glioma cases have less efficient DNA repair, measured by increased chromosome sensitivity to gamma radiation in stimulated peripheral blood lymphocytes. This inefficiency was shown to be an independent risk factor for glioma (95). Recently, we investigated whether glioma patients have increased chromosomal instability that could account for their increased susceptibility to cancer (106). Using fluorescent in situ hybridization methods, background instability in these patients was measured at hyper-breakable regions in the genome. Reports indicate that the human heterochromatin regions are frequently involved in stable chromosome rearrangements (107,108). Smith and Grosovsky
(109) and Grosovsky et al. (110) reported that breakage affecting the centromeric and pericentromeric heterochromatin regions of human chromosomes can lead to mutations and chromosomal rearrangements and increase genomic instability. Our (106) study demonstrated that individuals with a significantly higher level of background chromosomal instability have a 15-fold increased risk of development of gliomas. A significantly higher level of hyperdiploidy was also detected. Chromosome instability leading to aneu-ploidy has been observed in many cancer types (111). Although previous studies have demonstrated the presence of chromosomal instability in brain tumor tissues (112-115), our (105) study was the first study to investigate the role of background chromosomal instability in the peripheral blood lymphocytes of patients with gliomas. This suggests that accumulated chromosomal damage in peripheral blood lymphocytes may be an important biomarker for identifying individuals at risk of developing gliomas.