We investigated evidence of heterogeneity of genetic susceptibility to three well-established UCCB subphenotypes with different biologic behaviours and prognoses, leading to the identification of novel tumour subgroup-specific associations. These findings require replication in future studies. Our aim was to identify gene variants that differentially predispose bladder tumours to papillary low-grade, genomically stable (α), high-grade non–muscle-invasive (β
1), or muscle-invasive (β
2) subphenotypes (). We previously found that an inherited
FGFR3 variant was differentially associated with low-grade versus high-grade non–muscle-invasive tumours [
16].
Risk estimates for the two best established UCCB susceptibility genes,
GSTM1 and
NAT2, were similar across subphenotypes. Similarly,
VEGF-rs25648 and other polymorphisms previously identified to be associated with UCCB risk in our study [
12] conferred similar risks among subphenotypes.
Among the patients with non–muscle-invasive tumours, risk estimates were similar for the SNPs analysed, except for
MBL2-rs5030737 and
MMP1-rs10488, which conferred significantly decreased risk for heterozygotes in high-grade tumours. Mannose-binding lectin 2 (
MBL2) participates in innate immunity and rs5030737 (
R52C) is associated with impaired function [
20]. Other
MBL2 variants have been associated with risk of autoimmune and infectious diseases [
20] and cancer [
21,
22]. Recently, Pine et al reported the prognostic value of
MBL2 polymorphisms, including rs5030737, for lung cancer survival [
23].
MMP1 is involved in tissue remodelling, tumour invasion, and metastasis. Although associations found only for the heterozygous genotype are often disregarded because they are believed to be biologically implausible, there is precedent that some of them are detected consistently across studies [
24,
25]. Also, it is likely that these SNPs follow other modes of inheritance (eg, dominant or recessive) but, due to the exploratory nature of these analyses, only the codominant and additive modes of inheritance were considered. Another caveat is that, when stratified by subphenotypes, the genotype frequencies can be considerably low and thus add noise to the OR estimates (
Supplementary Table 2c).
The most notable differences in risk estimates occurred when comparing muscle-invasive and non–muscle-invasive tumours. Irrespective of the mode of inheritance, heterogeneity was observed for
DNAJC18-rs4315920, coding for a heat shock protein with methyl-transferase activity and SNPs in DNA repair genes,
PMS2-rs6463524,
BRCA2-rs1801406, and
XRCC5-rs828702. There is some evidence on the role of genetic variation in double-strand break DNA repair mechanisms in UCCB susceptibility [
13].
CD4-rs3213427 and
PMS2-rs6463524 showed neutral risk estimates (OR: ≈1) for the non–muscle-invasive groups and decreased risk for the muscle-invasive group ( and ). Other SNPs conferred risk estimates among subphenotypes in opposite directions. Although we cannot discard the possibility that this may be due to random variation or chance, it is possible that variants confer contrasting risks to different subgroups due to the complex mechanisms involved in the development/progression of tumours. Because the effect of each genetic variant is probably modulated by other genetic factors, both inherited and somatic, as well as by micro- and macroenvironmental exposures, such contrasting risks remain biologically plausible. Specific somatic alterations are involved in the different subphenotypes, possibly leading to distinct molecular profiles that may explain inherited factors in opposite directions. In fact, subgroup analyses in other studies have observed such crossover interactions [
26,
27], with some underlying debate about their biologic plausibility [
28].
An additional aspect regarding the interpretation of the results relates to the functionality of the variants differently associated with bladder cancer subphenotypes. Some of them are placed in coding regions leading to a change of amino acid and for whom an altered protein function has been proved (23). Other significant SNPs, although placed in coding regions, do not lead to an amino acid change. This fact does not rule out a functional effect because it has been reported that SNPs can be an important mechanism affecting splicing regulation (29,30). Furthermore, it is noteworthy that most of the very reproducible “hits” in genome-wide association studies identified over the past 2–3 yr indeed are placed in gene deserts or in areas of unknown function; the findings remain highly relevant (A Catalog of Published Genome-Wide Association Studies; available at:
http://www.genome.gov/26525384). Among them, that reported by Kiemeney et al was highly associated with bladder cancer risk [
31].
The study’s strengths include a large sample size, high participation rates, high quality of information on exposures and genotyping, and the uniform pathologic assessment of tumours. Furthermore, this study was prospectively designed to assess the role of genetic, environmental, and clinical factors in UCCB risk. Nonetheless, the results reported here should be interpreted with caution in light of several limitations. The genes/SNPs analysed were not selected to assess subphenotype heterogeneity. Some subjectivity in the pathologic evaluation of tumours cannot be ruled out, although it was reduced by ensuring a uniform tumour classification by trained pathologists. By considering molecular alterations occurring early in urothelial carcinogenesis, a more accurate subphenotyping classification may be applied in future studies. Furthermore, the study was based on a tumour development/progression model that is likely to be an oversimplification. Although rare, a few patients with low-grade tumours eventually invade the muscle; furthermore, confluence of tumours can also occur in the context of a multicentric neoplasm such as UCCB. Although we observed some evidence of heterogeneity in genetic susceptibility, the results were not robust to multiple testing due to limited statistical power. Although this is one of the largest UCCB studies carried out so far, it was not designed to have sufficient power to investigate heterogeneity by subphenotypes. However, the study provides valuable novel information on the heterogeneity in genetic susceptibility of UCCB that, jointly with the molecular and pathologic heterogeneity, will help to disentangle the complexity of this disease. This work should stimulate replication of our findings in independent UCCB case series.