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Lambertus A. (Bart) Kiemeney, Professor of Cancer Epidemiology, Department of Epidemiology, Biostatistics & HTA (133 EBH), Department of Urology (659 URO), Radboud University, Nijmegen Medical Centre, PO Box 9101, 6500 HB Nijmegen, The Netherlands. e-mail: firstname.lastname@example.org
The incidence of bladder cancer varies considerably among countries; the highest incidence rates are in Western communities. The variation in occurrence can partly be explained by differences in registration and coding practices of pTa tumours. Factors that modify the occurrence of bladder cancer are smoking and exposure to many kinds of carcinogenic substances in the workplace. Evidence also exists for radiotherapy to the pelvis, infection with Schistosoma haematobium, and certain medications as risk factors for bladder cancer. Despite enormous efforts, other important environmental or lifestyle factors that clearly and consistently increase or decrease the risk of bladder cancer have not been identified. Bladder cancer in first-degree relatives doubles the risk of bladder cancer; this increased risk might be due to high-penetrance susceptibility genes in a small subset of families, but most of this risk is probably caused by common lower-penetrance DNA variants that influence risk through one or more different cancer pathways. In the next 2 years genome-wide association scans will probably yield important new information on such variants. This might also facilitate new studies on lifestyle factors restricted to groups of susceptible people. In the future it will also be necessary to pay more attention to potential risk factors for different types of bladder cancer, more specifically low- vs high-grade cancer. The ultimate goal is to build a risk-prediction model by integrating environmental and genetic factors that can project individualized probabilities of developing bladder cancer.
European Prospective Investigation on Nutrition and Cancer
NADPH quinine oxidoreductase-1
(non-synonymous) single-nucleotide polymorphism
classification and regression tree
benzo[a]pyrene diol epoxide
peripheral blood lymphocyte
area under the curve.
Worldwide, bladder cancer is the seventh most common malignancy in men and the 17th in women. In 2002, ≈357 000 new cases of bladder cancer were diagnosed, while 145 000 patients died from the disease . In Europe and the USA, bladder cancer accounts for 5–10% of all malignancies among males . The age-adjusted incidence rates of bladder cancer (Fig. 1)  are particularly high in Southern Europe, Northern Africa, North America and Western Europe; incidence rates are low in Japan, Korea and China. Part of the differences among countries is caused by differences in registration or reporting of (low-grade) pTa tumours. Unfortunately, this makes the comparison between countries very difficult.
Age-standardized (world) mortality rates are 2–10 per 100 000 males and 0.5–4 per 100 000 females. Bladder cancer is three to four times more common among males than among females . The excess of bladder cancer in males is largely but possibly not fully explained by gender differences in smoking habits and occupation (the two strongest risk factors for bladder cancer). African-Americans have a lower incidence of bladder cancer, but a higher mortality than Caucasians. African-Americans are being diagnosed with more aggressive and more advanced tumours  Non-urothelial bladder cancer is also found more commonly in African-Americans, and this might adversely affect survival. Not all survival differences between African-Americans and Caucasians can be explained by differences in stage at presentation. Also racial biological variations and within-race individual differences might modify various phases of carcinogenesis, such as the capacity to convert pro-carcinogens to carcinogens, to detoxify carcinogens, and to repair DNA.
The most well established risk factor for bladder cancer is cigarette smoking, although the association is not as strong as that for smoking and respiratory tract cancers. Compared to those who have never smoked cigarettes, people who have ever smoked cigarettes have two to four times the risk of bladder cancer [5–7]; 30–50% of all bladder cancer is caused by cigarette smoking [5,6]. Bladder cancer risk tends to increase with both increasing duration and increasing intensity of cigarette smoking [5,8]. Stopping smoking decreases the risk of bladder cancer immediately, and by 30% after 1–4 years, and 60% after 25 years . Smokers of unfiltered cigarettes have a 35–50% higher risk of bladder cancer than to smokers of unfiltered cigarettes . The risk associated with smoking ‘black tobacco’ cigarettes is higher than that of ‘white tobacco’ cigarettes, although studies disagree about the level of this difference in risk [9–11].
4-aminobiphenyl has been suggested to be the most important carcinogen in cigarette smoke, but other factors such as β-naphthylamine, benzene, cadmium, chromium, radon, vinyl chloride, nickel and >60 other carcinogenic substances are also important.
Worldwide, time trends in bladder cancer follow trends in smoking behaviour, comparable to time trends in lung cancer but with a longer delay. In most Western communities the bladder cancer incidence and mortality in men has decreased in the last decade. In Europe, bladder cancer age-standardized mortality rates declined by ≈16% in men and 12% in women in the last decade .
It has been estimated that occupational exposures could account for as much as 20% of all bladder cancer . Exposure to β-naphthylamine, 4-aminobiphenyl and benzidine, principally among workers in the textile dye and rubber tyre industry, are the only specific agents that have been unequivocally associated with bladder cancer, with extremely high relative risks (RRs) [5,14]. Currently these specific chemicals are banned from the workplace and contribute minimally to the current incidence of bladder cancer in Western countries. However, there remain many other strong candidates for bladder carcinogens, e.g. ortho-toluidine, which is still used in the manufacture of dyes, rubber, pharmaceuticals and pesticides [5,14,15].
Elevated risks are commonly reported among painters (exposure to possible carcinogenic constituents of paints, e.g. benzidine, polychlorinated biphenyls, formaldehyde, and asbestos, and solvents like benzene, dioxane and methylene chloride). A recent meta-analysis showed an increased risk (RR 1.17, 95% CI 1.11–1.23) of bladder cancer among painters . An increased risk was also found among leather workers and shoe makers, although the responsible agent is still unknown. A moderately increased risk of bladder cancer (odds ratio, OR, 1.11, 95% CI 1.04–1.18) was reported in aluminium-, iron- and steelworkers, which might be the result of exposure to aromatic amines and polycyclic aromatic hydrocarbons in coal-tar pitch volatiles .
Many studies have assessed the relation between bladder cancer and diesel exhaust exposure, and evidence is accumulating that diesel exhaust moderately increases the risk of bladder cancer. An increased bladder cancer risk was detected among drivers of trucks (OR 1.17, 95% CI 1.06–1.29) and buses (1.33, 1.22–1.45) . There seems to be a positive trend in risk with increasing duration of employment. Although an increased risk of bladder cancer has been reported for many other occupations, findings for most of these occupations are not consistent [19,20].
Drinking water quality and total fluid intake
Chlorinated drinking water: In the USA, Canada and many other countries, but not in Western Europe, drinking water is disinfected with chlorine. During the chlorination process, chlorine reacts with organic compounds in water, resulting in halogenated organic compounds (mainly trihalomethanes such as chloroform and bromoform). Bioassays and in vitro studies suggest that some of these halogenated compounds are mutagenic or carcinogenic. However, a study from Australia found that the trihalomethane concentration in chlorinated drinking water was not related to DNA damage in bladder cells .
Several studies [22–24] on chlorinated drinking water and bladder cancer all reported increased risks. The (smoking-adjusted) RRs varied from 1.4 to 2.2 for both sexes combined (exposure time 20–>60 years). In most studies the risks tend to increase with duration of exposure. Despite this, a report from the International Agency for Research on Cancer in 1999  concluded that there was inadequate evidence that the individual chlorination by-products such as chloroform and other trihalomethanes were carcinogenic. Although some studies had associated chlorinated drinking water intake with cancer, it was argued that single compounds could not be evaluated because these compounds occur in mixtures. A report of the WHO , published in 2000, concluded that the evidence was insufficient to determine whether observed associations were causal or to determine which specific by-product or contaminant plays a role.
Villanueva et al. reported two pooled analyses, in which primary data from six case-control studies with individual-based exposure assessments were pooled. These studies used trihalomethanes as a marker for the total mixture of chlorination by-products. The average trihalomethane levels in the studies was 10–30 µg/L. Exposure to trihalomethanes was associated with an excess risk among ever-exposed men (OR 1.32, 95% CI 1.10–1.59). The risk increased with increasing exposure. There was no detectable increased risk among women.
In conclusion, exposure to chlorinated drinking water might increase the risk of bladder cancer. The observed risks are relatively small, but if there is a greater risk then the attributable risk will be considerable, because of the size of the exposed population.
Arsenic in drinking water: Several large studies have evaluated the association between the ingestion of arsenic in drinking water and the risk of bladder cancer. Several studies were reported from the endemic area of Taiwan. Between 1930 and the mid-1960s, the population in this region was exposed to highly contaminated well-water (arsenic levels of 170–800 µg/L; the current regulation is a maximum of 10 µg/L in Taiwan). These studies showed a clear dose–response relationship with bladder cancer . Bangladesh and West Bengal have a chronic problem with very high concentrations of arsenic in drinking water, which in some sources exceed 2000–4000 µg/L . Arsenic levels in drinking water in the USA and Europe are much lower than those reported in regions of Asia. Overall, no clear association was found between low-intermediate exposure to arsenic in drinking water and the risk of bladder cancer.
Total fluid intake: High consumption of fluids might reduce the exposure to carcinogens by diluting the urine and reducing the contact time through increased frequency of urination. Results from fluid intake studies are inconsistent [30–33], but there is a tendency towards a reduced risk with high intake of fluids, regardless of type. Confounding might be caused by a possible harmful effect of chlorinated or arsenic-containing drinking water. A recent pooled analysis based on six case-control studies found an increased risk between tap-water and bladder cancer, but none for fluid intake other than tap- water . These findings suggest that contaminants in tap-water might be responsible for the excess risk. Confounding might also be caused by a possible harmful effect of coffee, as well as a possible beneficial effect of fruit and vegetable juices.
Chronic UTI is associated with the development of bladder cancer, especially invasive squamous cell carcinomas (SCC) [34–36]. This type of cancer can occur in patients with spinal-cord injury in whom chronic cystitis is inevitable. This might be the result of the formation of nitrites and nitrosamines by bacterial flora, and/or the inflammatory process, which leads to increased cell proliferation, providing more opportunities for spontaneous DNA replication errors.
Heavy consumption of phenacetin-containing analgesics increases the risk of renal pelvis and ureteric urothelial cell carcinoma, but has only a marginal effect on bladder cancer risk [37,38]. Because of its carcinogenic properties, the drug was initially banned from general use in the 1960s and 70s in most Western countries. The ban was later revoked in some countries, but its legal use (the drug is used as a cutting agent for cocaine) is highly restricted because of the dangers it poses.
Cyclophosphamide, an alkylating agent that is used in the treatment of malignant neoplasms, increases the risk of bladder cancer (mainly urothelial cell carcinoma) with a clear dose–response relationship . It is acutely toxic to the bladder mucosa and produces cellular abnormalities in the epithelium. Most cyclophosphamide-induced tumours present as muscle-infiltrating lesions at the time of diagnosis, with a relatively short latency period (6–13 years). Of four known metabolites of cyclophosphamide, acrolein and phosphamide mustard bind to DNA, and acrolein is known to be responsible for its bladder toxicity. Radiotherapy is also a known risk factor for bladder cancer. Kaldor et al. reported a case-control study of tumours of the bladder in women who had previously been treated for ovarian cancer. The risk of bladder tumours was greater for patients who had been treated with radiotherapy or chemotherapy (thiotepa and melphalan) than in those treated with surgery. Moreover, the risk seemed to be much higher in patients who received both. In another study, Neugut et al. also found an increased risk of bladder cancer (RR 1.5, 95% CI 1.1–2.0) several years after radiotherapy for prostate carcinoma.
SCC of the urinary bladder has been associated with Schistosoma haematobium infection for many years. The epidemiological association is based both on case-control studies [42,43] and on the close correlation of bladder cancer incidence with the prevalence of S. haematobium infection within different geographical areas. S. haematobium is found throughout much of Africa and the Middle East [43,44]. The life cycle of S. haematobium requires water-borne transmission of infection between man and snail. Individuals with chronic schistosomiasis might eventually develop SCC, probably as a result of a higher amount of carcinogenic nitroso compounds in the urine and/or a depressed immunocompetence of infected patients.
Fruit and vegetables
In observational studies a high consumption of vegetables and fruit has been associated with a decrease in risk of almost all cancers, including bladder cancer [45–47], although data are inconsistent. A role of diet in bladder carcinogenesis is plausible as most substances and metabolites, including (pre)-carcinogens, are excreted by the urinary tract. A possible biological mechanism is that several antioxidants (vitamin A, C, E, retinol, selenium and folate) detoxify free radicals and thereby decrease the cancer risk.
In 1997, an international panel concluded that there was significant evidence that a high intake of vegetables decreased the risk of bladder cancer, although observational studies of fruit and vegetable consumption are inconclusive . In a meta-analysis by Steinmaus et al. increased risks of bladder cancer were associated with diets low in fruit intake (RR 1.4, 95% CI 1.08–1.83), and with diets of low vegetable intake (RR 1.16, 95% CI 1.01–1.34). These results suggest that a diet high in fruits and vegetables might help to prevent bladder cancer. In a more recent meta-analysis, there was an inverse association for fruit consumption but vegetable intake was not related to bladder cancer risk . In 2005, the Nurses’ Health Study evaluated the effect of fruits and vegetables, carotenoids, folate, and vitamin A, C, E on the risk of bladder cancer. In this prospective study there were no associations between any of the fruits, vegetables or vitamin intake and bladder cancer risk among women . The World Cancer research Fund/American Institute of Cancer Research report  on the relation between food, nutrition, physical activity and the prevention of cancer concluded that there is limited evidence of a protective effect of fruit and vegetables on bladder cancer risk.
Although fruits and vegetables are sources of many vitamins, minerals and other bioactive compounds which might prevent cancer, plant foods can also contain carcinogenic pesticides. When pesticides are used to increase crop yield, residues or traces sometimes remain. The inconsistent results of studies assessing the effect of fruits and vegetables on the risk of bladder cancer might be confounded by a harmful effect of pesticides in fruits and vegetables. However, the evidence of the carcinogenic effect of pesticides comes from animal studies only. A recent study  found no association between pesticide exposure and bladder cancer risk. There is no epidemiological evidence that these small amounts of pesticides in food can increase cancer risk .
To elucidate the role of diet in cancer, large-scale, structurally supported prospective cohort studies are required, preferably with multicentre and multinational participation to increase variation in exposure. The European Prospective Investigation on Nutrition and Cancer (EPIC) cohort data offers a unique opportunity to conduct large prospective cohort analyses on a wide variety of risk factors. Analyses in the EPIC cohort on serum biomarkers for fruit and vegetable consumption in relation to bladder cancer are ongoing.
Other dietary risk factors
Several other possible risk factors have been suggested, e.g. coffee and alcohol consumption. However, the evidence is very inconsistent [31,53,54]. The positive findings in some studies might be the result of residual confounding by smoking (several case-control studies did not adjust for smoking habits) or chance. Evidence for an increase in bladder cancer risk because of the use of artificial sweeteners [55,56] and a high total fat intake is insufficient . Drinking of black tea has been associated with a slightly decreased risk of bladder cancer, although the evidence is also inconsistent [33,57]. Physical activity was associated with an increase in bladder cancer in two large cohort studies [58,59].
The risk of bladder cancer through the use of hair-colouring products has been studied since the late 1970s but received much interest in the last few years. Some hair dyes contain aromatic amines such as 4-aminobiphenyl, of which small amounts might be absorbed percutaneously . Occupational exposure to hair dyes by hairdressers, barbers and beauticians has been suggested to moderately increase the risk of bladder cancer . Two cohort studies from the USA (the Nurses Health Study and the American Cancer Society CSP-II study) found no association between personal use of permanent dyes and bladder cancer. Also, several case-control studies found no evidence that the use of hair dyes was associated with bladder cancer risk [62–64]. However, in a study from California, Gago-Dominguez et al. reported that women who used permanent hair dyes at least once a month had twice the risk of bladder cancer relative to non-users. They further showed that genetic variations in arylamine activation and detoxification pathways, especially N-acetyltransferase (NAT)-2, -1, and cytochrome (CYP) 1A2, modify the relationship between permanent hair dyes and bladder cancer in women . These results were not confirmed in the large case-control study from Spain . It is therefore difficult to arrive at a definitive conclusion on the role of hair dyes in the causation of bladder cancer.
FAMILIAL BLADDER CANCER
Familial bladder cancer is a fairly rare phenomenon compared to the familial occurrence of many other tumour sites. Nevertheless, numerous striking case-reports described familial clustering of bladder cancer. Several of these show an extremely early age at onset, suggesting a genetic component . Epidemiological studies from the Netherlands and Spain showed that the risk of bladder cancer is about doubled in the case of a first-degree relative with bladder cancer [67,68]. The cause of this familial clustering is still speculative but several lines of evidence suggest a contributing genetic factor. There is probably a large heterogeneity in risk, with the cause in some high-risk families being a rare but highly penetrant gene , and in other families being the combined effects of several more common but less-penetrant genes. It is expected that genome-wide association studies will report their findings within the next 2 years. Such findings might also direct the search for genetic causes in families with bladder cancer.
As summarized above, bladder cancer is primarily a disease induced by environmental carcinogens. However, there is also a clear genetic component to the aetiology of bladder cancer, as evidenced by the familial association of bladder cancer. Identification of low-penetrance genetic predisposition loci provides additional strong evidence for a genetic component of bladder cancer. Although the relative risks are modest, genetic polymorphisms might account for a large fraction of all bladder cancers because of their high prevalence in the population. Because there is a good understanding of the pathways and genes involved in carcinogen metabolism and host defence mechanisms, numerous studies evaluated associations between genetic polymorphisms in these pathways and bladder cancer risk. There are a few consistent associations, but most of the reported genetic associations have not been supported by replication in independent populations. We will not enumerate all the genetic variants that have been evaluated, but only summarize the most consistent associations and data from larger studies.
Carcinogen metabolism genes
In general, the metabolism of carcinogens consists of two phases, phase I and II. Phase I enzymes, mainly CYP450, typically activate carcinogens, whereas phase II enzymes generally detoxify carcinogens. The balance between phase I and II enzymes often determines the accumulation of toxic reactive intermediates.
There are about 60 human CYP genes arranged in 18 families and 42 subfamilies . Human CYP genes are highly polymorphic. For example, CYP2D6 has >100 variant alleles and CYP2C9 has 37 different variant alleles. Numerous studies have investigated various CYP gene polymorphisms and bladder cancer risk, including CYP1A1, CYP1A2, CYP1B1, CYP2C19, CYP2D6 and CYP2E1[71,72]; however, none of the genotypes showed consistent associations with bladder cancer risk. There are many reasons for this, including small sample sizes, weak correlations between genotypes and phenotypes, redundancy in enzyme functions and overlap in enzyme substrates, and variable frequencies of the polymorphic alleles in different ethnic populations.
NADPH quinine oxidoreductase-1 (NQO1) is a cytosolic enzyme that catalyses two- or four-electron reductions of quinoid compounds into less toxic hydroquinones. A non-synonymous single-nucleotide polymorphism (nsSNP), Pro187Ser, has been widely studied in many cancer types. Several case-control studies of bladder cancer with small sample sizes (<300 cases) have produced contradictory results [72,73], but a meta-analysis of six studies among Caucasians suggested a modestly increased risk for the variant genotypes (OR 1.20, 95% CI, 1.00–1.43) .
Glutathione S-transferases (GSTs) are a group of Phase II detoxification enzymes that catalyse the conjugation of glutathione to a wide variety of xenobiotics and carcinogenic compounds. At least seven mammalian GST families (α, µ, ο, π, σ, τ and ζ) have been identified and several functional polymorphisms in GST family genes have been closely assessed in cancer-association studies . The largest case-control study to date (1150 cases and 1149 controls) reported that the null genotype of GSTM1 was associated with a significantly increased bladder cancer risk . A meta-analysis of 28 studies (5072 cases and 6466 controls) produced a summary odds ratio of 1.5 (95% CI 1.3–1.6) . The Ile105Val variant of GSTP1 might have a modest effect on bladder cancer risk, as suggested in a recent meta-analysis . Other common polymorphisms, such as the GSTT1 null genotype and the Val224Ile variant of GSTM3, appear to have a minimal effect on bladder cancer risk .
NATs catalyse the metabolic activation of aromatic and heterocyclic amine carcinogens by acetylation. NAT1 and NAT2 are two distinct NAT isozymes that exist in humans. The NAT1 and NAT2 genes have extensive polymorphisms that stratify the population into rapid-, intermediate- and slow-acetylator phenotypes . The association between NAT2 slow-acetylator genotype and increased bladder cancer risk is among the most consistent and robust, and has become a classical example of gene–environment interaction in the aetiology of sporadic cancer. The two largest case-control studies and a meta-analysis of 31 studies (5091 cases and 6501 controls) showed that, compared with the NAT2 rapid- or intermediate-acetylators, NAT2 slow-acetylators had a 40% increased risk of bladder cancer [75,78]. In addition, there was an interaction between NAT2 genotype and smoking. The association of NAT1 slow-acetylator genotypes with bladder cancer is controversial and warrants further clarification [72,75,78]. Similarly, polymorphisms in many other phase II enzyme genes, including sulfotransferases, UDP-glucuronosyltransferases, myeloperoxidase, catechol-o-methyltransferase, manganese superoxide dismutase and glutathione peroxidase 1, are either contradictory or evaluated in only a few small studies . Well-designed large studies are needed to clarify their roles in bladder cancer susceptibility.
DNA repair genes
There are four major DNA repair systems in mammalian cells: nucleotide-excision, base-excision, double-strand break and mismatch repair. Tobacco and environmental carcinogens might cause varied DNA damage that requires distinct repair pathways. Polymorphisms in DNA repair pathways, particularly in the first three systems, might be important in the aetiology of bladder cancer. There are many small case-control studies suggesting such roles , but none of the commonly studied SNPs in DNA repair genes showed a consistent significant association with bladder cancer across large case-control studies published more recently [79–87]. These latter studies had large samples and took a pathway-based approach to genotype a panel of SNPs with potential functional impact (e.g. nsSNPs and SNPs in promoter and untranslated regions). Meta-analyses indicate that two XPD nsSNPs, Asp312Asn and Lys751Gln, might have modest effects on bladder cancer (García-Closas, personal communication). The same is true for the Thr214Met nsSNP in the XRCC3 gene (homozygous variant vs homozygous common genotype: OR 1.17, 95% CI, 1.00–1.36, from seven studies with 3086 cases and 3150 controls) .
Other pathways and candidate genes
In addition to carcinogen metabolism and DNA repair, there are numerous individual reports on candidate SNPs in cell cycle control , apoptosis , inflammatory response , cell-adhesion molecules [89,90], tumour microenvironment , folate metabolism [92,93], G proteins , angiogenesis , and other pathways in relation to bladder cancer risk , but the individual results need replication in independent populations. In addition, the rapid advance of high-throughput genotyping technology has allowed large scale genotyping. García-Closas et al. genotyped 1433 SNPs in 386 genes in a large case-control study, and found that a SNP in VEGF was the most significant one. They further analysed 29 additional SNPs in VEGF and found a few additional SNPs and two haplotype blocks encompassing the promoter and 5′ UTR associated with bladder cancer risk . We (X.W.) recently used the iSelect platform (Illumina, San Diego, CA, USA) and genotyped ≈10 000 SNPs in ≈1000 cancer-related genes in the Texas bladder cancer case-control study (data unpublished), and we will validate the top 10 promising loci in an independent population.
Gene-gene and gene-environment interaction
The candidate-gene approach has given a few examples of true associations (e.g. GSTM1 and NAT2), but also produces numerous inconsistent results. The trend in cancer-association studies is to move beyond a limited candidate approach, and apply a pathway-based genotyping and analytical approach, and ultimately genome-wide association (GWA) studies. Several recent bladder cancer case-control studies examined the combined effect of multiple genes, as well as gene-gene and gene-environment interactions, using nonparametric data-mining tools, e.g. classification and regression tree (CART) and multifactor dimensionality reduction [79–83]. Evaluating 44 SNPs in DNA repair and cell-cycle genes, Wu et al. found a significant gene-dosage effect for increasingly elevated risks of bladder cancer with increasing numbers of high-risk alleles. CART analysis revealed potential higher-order gene-gene and gene-smoking interactions, and categorized a few higher-risk subgroups for bladder cancer based on distinct genotype and smoking combinations. Moreover, subgroups with a higher cancer risk also had higher levels of induced genetic damage. It is apparent that gene-environment interaction is important in bladder cancer aetiology, particularly NER genes and smoking interactions. However, from current reports, the interactions identified from various posthoc data-mining tools in different populations do not overlap [79–83]. Unfortunately, validation of interactions is more demanding than that of individual associations and requires large sample sizes.
GWA study of bladder cancer
Some cancer susceptibility loci cannot be identified by a candidate-gene approach, as recent GWA studies of other cancers suggest. High-resolution GWA studies, with extensive replications of positive findings in other case and controls series, can map such susceptibility loci. For bladder cancer, there are at least three independent GWA studies ongoing, all using Illumina’s high-density SNP chips. We expect that there will be exciting results in the coming year or two.
Phenotypic markers and bladder cancer risk
Although the vast majority of modern reports focus on genotypes, because of clear advantages in terms of easy sample collection, invariable genotype data and robust technology, phenotypic assays remain important. For most known genotypes, the functional impact is either not clear or not strong enough to be biologically relevant, contributing to the inconsistent results of genotypic data. Phenotypic assays measure the combined effects of multiple genes as well as environmental factors. Phenotypic markers are generally more consistent if the technical reproducibility is established, and require many fewer samples.
Most of the phenotypic assays measure DNA damage level and DNA repair capacity. Schabath et al. applied the Comet assay to measure baseline, benzo[a]pyrene diol epoxide (BPDE)- and γ-radiation-induced DNA damage in peripheral blood lymphocytes (PBLs) of bladder cancer cases and controls. They found that higher levels of DNA damage at baseline (OR 1.84, 95% CI 1.07–3.15), after γ-radiation (1.81, 1.04–3.14) and after BPDE treatment (1.69, 0.98–2.93) were all associated with increased risks of bladder cancer. Mutagen sensitivity is an established genetic predisposing factor for cancer . A recent large case-control study showed that high BPDE and γ-radiation sensitivity was associated with significantly increased bladder cancer risks (OR 1.48, 1.33–1.64 and 1.92, 1.71–2.15, respectively) . Lin et al. developed a modified host-cell reactivation assay to measure DNA repair capacity for DNA damage induced by 4-aminobiphenyl, an aromatic amine and known bladder carcinogen, and found that poor DNA repair capacity was associated with a 3.42-fold increased bladder cancer risk.
Another consistent phenotypic marker is telomere length in PBLs. Three independent studies showed that constitutive telomere shortening is a cancer-susceptibility factor for bladder cancer. Wu et al. provided the first epidemiological evidence that telomeres in PBLs of cases were significantly shorter than those of controls for four different types of cancer, including lung, head and neck, bladder, and kidney cancer. Dichotomized at the 75th percentile value of telomere length in controls, individuals with shorter telomeres were associated with a significantly increased risk of these cancers (OR 4.51, 2.31–8.81). Broberg et al. showed that the risk of bladder cancer was significantly increased with decreasing telomere length, with an odds ratio of 4.5 for the shortest quartile compared to the longest quartile of telomere length. In a recent prospective case-control study nested within the Health Professionals Follow-up Study and the Nurses’ Health Study, McGrath et al. showed that the OR for bladder cancer was 1.88 (1.05–3.36) for the shortest quartile compared to the longest quartile of telomere length.
RISK ASSESSMENT MODEL FOR BLADDER CANCER
The ultimate goal is to build a risk-prediction model by integrating environmental and genetic factors that can project individualized probabilities of developing bladder cancer. As the first attempt, Wu et al. used epidemiological and genetic data from a large case-control study to build a prediction model for bladder cancer. The area under the curve (AUC) was used to evaluate the discriminatory ability of the models. Significant risk factors in the epidemiological model included pack-years smoked and exposures to diesel, aromatic amines, dry-cleaning fluids, radioactive materials, and arsenic. This model yielded good discriminatory ability (AUC 0.70, 95% CI 0.67–0.73). When mutagen sensitivity (BPDE and γ-radiation sensitivity) data were incorporated, the AUC increased to 0.80 (0.72–0.82). The enhanced epidemiological-genetic model with the addition of a single genetic component has excellent discriminatory ability, showing the promising future of cancer risk prediction by incorporating multiple genetic factors. Validation of this model in an external population is an essential next step towards its practical use in the clinical setting.
Smoking and exposure to carcinogenic substances at the workplace are the main causes of bladder cancer. In theory, at least half of all the cases of bladder cancer can be prevented with smoking-cessation programmes and protection from occupational exposures. Pelvic radiation, S. haematobium infection, and specific medications are confirmed risk factors, but their contribution to bladder cancer occurrence is much smaller. Other than that, important environmental or lifestyle factors have not been clearly and consistently associated with bladder cancer risk. A Mendelian hereditary form of bladder cancer is unknown, although RB1 carriers seem to be at greater risk. However, susceptibility to bladder cancer is modified by common lower-penetrance DNA variants in several cancer pathways. Only variants in NAT2 and deletions of GSTM1 have unequivocally been confirmed to be bladder cancer risk factors, but technical advances in this area of research will lead to the identification of dozens of other low-penetrance bladder cancer genes in the next few years. Future epidemiological research needs to pay more attention to different forms of bladder cancer, more specifically low- and high-grade cancer. Large international collaborations might be needed for sufficient power in such studies. The ultimate goal is to build a risk-prediction model by integrating environmental and genetic factors and that can project individualized probabilities of developing bladder cancer.