Prostate cancer risk regions at 8q24 and 17q24 are differentially associated with somatic TMPRSS2:ERG fusion status.

Molecular and epidemiological differences have been described between TMPRSS2:ERG fusion-positive and fusion-negative prostate cancer (PrCa). Assuming two molecularly distinct subtypes, we have examined 27 common PrCa risk variants, previously identified in genome-wide association studies, for subtype specific associations in a total of 1221 TMPRSS2:ERG phenotyped PrCa cases. In meta-analyses of a discovery set of 552 cases with TMPRSS2:ERG data and 7650 unaffected men from five centers we have found support for the hypothesis that several common risk variants are associated with one particular subtype rather than with PrCa in general. Risk variants were analyzed in case-case comparisons (296 TMPRSS2:ERG fusion-positive versus 256 fusion-negative cases) and an independent set of 669 cases with TMPRSS2:ERG data was established to replicate the top five candidates. Significant differences (P < 0.00185) between the two subtypes were observed for rs16901979 (8q24) and rs1859962 (17q24), which were enriched in TMPRSS2:ERG fusion-negative (OR = 0.53, P = 0.0007) and TMPRSS2:ERG fusion-positive PrCa (OR = 1.30, P = 0.0016), respectively. Expression quantitative trait locus analysis was performed to investigate mechanistic links between risk variants, fusion status and target gene mRNA levels. For rs1859962 at 17q24, genotype dependent expression was observed for the candidate target gene SOX9 in TMPRSS2:ERG fusion-positive PrCa, which was not evident in TMPRSS2:ERG negative tumors. The present study established evidence for the first two common PrCa risk variants differentially associated with TMPRSS2:ERG fusion status. TMPRSS2:ERG phenotyping of larger studies is required to determine comprehensive sets of variants with subtype-specific roles in PrCa.

Introduction

Prostate cancer (PrCa) is a complex disease with a considerable degree of heritability involved in its etiology (1). Although high-risk gene discovery has proven difficult against a background of disease and locus heterogeneity, genome-wide association studies (GWAS) and substantial validation efforts have identified more than 100 common variants with weak to moderate contributions to PrCa risk (2–11). These common risk variants are postulated to explain about 33% of the familial risk of PrCa (12).

Somatically, PrCa can be classified into two major molecular subtypes, where the presence or absence of oncogenic E-twenty-six (ETS) gene fusions is the crucial distinctive feature. ETS rearrangements are present in ∼50% of PrCa tissues (13) and their occurrence is considered an early event in PrCa tumorigenesis (14). In over 90% of ETS fusion-positive cases, the fusion partners are the androgen-regulated gene TMPRSS2 (transmembrane protease, serine 2), which is highly expressed in the prostate, and the oncogene ERG (v-ets avian erythroblastosis virus E26 oncogene homolog), both located on the long arm of chromosome 21 (13).

Since the discovery of ETS gene fusions in PrCa multiple studies have provided evidence for the molecular and epidemiological distinctness of TMPRSS2:ERG fusion-positive and negative tumors. Epigenetic profiling has revealed distinct DNA methylation patterns for TMPRSS2:ERG fusion-positive and negative PrCa tissues (15–17) and analyses of benign and tumor tissues suggest that hypermethylation is more pronounced in TMPRSS2:ERG fusion-negative PrCa compared with TMPRSS2:ERG fusion-positive tumors, which mostly show moderately elevated DNA methylation (16,17). During tumor evolution of fusion-positive PrCa interdependent complex rearrangements (chromoplexy) occur at transcriptionally active—predominantly androgen regulated—loci of multiple chromosomes, while fusion-negative tumors tend to undergo single fatal genetic restructuring events (chromothripsis) (18,19). In addition to tumor architecture, differences in clinical and epidemiological characteristics have also been investigated for TMPRSS2:ERG positive and negative PrCa. Although a correlation of more aggressive PrCa with fusion status has not been reported consistently (20), TMPRSS2:ERG fusions have been found more frequently in early onset PrCa (21,22). Interestingly, the frequency of TMPRSS2:ERG fusions varies among ethnicities with the highest prevalence in cases of European ancestry (23). Moreover, individual physiologic and metabolic factors appear to have different risk modifying effects for TMPRSS2:ERG positive and negative PrCa (24,25).

Based on their distinctness, we hypothesized that there may also be differences between TMPRSS2:ERG fusion-positive and negative PrCa at the underlying germline level. Within the framework of the PRACTICAL consortium, we have investigated the first confirmed 27 common risk variants, which were identified in PrCa GWAS studies (4), for fusion-specific associations. For this purpose, we have analyzed a set of 296 TMPRSS2:ERG positive and 256 negative cases for differences in variant allele frequencies between these subtypes, and additionally, both subgroups were compared with controls without PrCa (n = 7650). The five top-ranked candidate variants were then genotyped in an independent sample of 669 PrCa cases with known TMPRSS2:ERG status for replication purposes. For the highlighted risk regions, we considered mRNA expression analysis of candidate target genes in fusion-positive and negative tumor tissues, to investigate the mechanistic interplay between the somatic TMPRSS2:ERG phenotype and the germline genotype of associated risk variants.

Results

Quality control and eligibility of the hypothesis generating discovery dataset

The five participating studies (FHCRC, IPO-PORTO, TAMPERE, UKGPCS and ULM) consisted of a total number of 7650 controls and 8681 cases previously genotyped for the iCOGS study (7). From the available iCOGS array genotype data, we selected 27 variants, representing the initial set of confirmed common PrCa risk variants, for analyzing potential associations with TMPRSS2:ERG fusion status. None of these variants showed deviation from Hardy-Weinberg equilibrium (threshold P = 0.001) in any of the study populations.

A subgroup of 552 cases genotyped as part of the iCOGS dataset was somatically phenotyped for the TMPRSS2:ERG gene fusion with a mean TMPRSS2:ERG positive frequency of 54% (range 44–60%) across the study groups (Table 1). Since the patients with TMPRSS2:ERG data represented only a fraction of the total cases from each collaborating center, two validity issues were considered in supplemental analyses. First, we checked for potential bias that may have occurred in the course of subsampling tumor materials. For this question, risk allele frequencies for all 27 loci were compared between somatically phenotyped cases (n = 552) and the 8129 non-phenotyped cases from the same contributing sites by Mantel-Haenszel analysis (under a fixed-effects model). Using this approach, sampling bias was observed for one variant (rs7127900 at 11p15.5; P = 0.0056), which was consequently omitted from further analyses. For all other 26 variants, the phenotyped cases did not differ significantly from the untyped cases (P > 0.12; data not shown), and were therefore considered as representative of the entire case groups. Of note, no significant cancer-related sampling bias was indicated by clinical features, such as tumor stage (organ confined vs. advanced: P = 0.11) or tumor grade (Gleason Score ≤ 7 versus >7: P = 0.39).

Table 1.

 Distribution of PrCa cases based on study centers and TMPRSS2:ERG (T2E) fusion status

	Study	Total number of cases with T2E data	T2E positive cases	T2E negative cases	T2E positive frequency
Discovery sample	FHCRC I	174	91	83	0.52
	IPO-PORTO I	18	8	10	0.44
	TAMPERE	174	105	69	0.60
	UKGPCS	129	58	71	0.45
	ULM I	57	34	23	0.60
	Subtotal	552	296	256	0.54
Replication sample	FHCRC II	218	133	85	0.61
	IPO-PORTO II	146	79	67	0.54
	ULM II	107	65	42	0.61
	BERLIN	198	111	87	0.56
	Subtotal	669	388	281	0.58
	Total	1221	684	537	0.56

A second issue of validity was examined with respect to the relatively small effect sizes of common risk variants, questioning if subsampling may reduce our power for detecting any associations with overall risk of PrCa, or risk in the two PrCa subgroups stratified by fusion status. Using all 881 unselected cases in case-control comparisons, 20 out of the 26 ‘bona fide’ PrCa risk variants replicated at a threshold of P < 0.00185 (corresponding to Bonferroni correction for the 27 variants included in this study). However, after reduction to 552 TMPRSS2:ERG phenotyped cases, only six variants remained significantly associated with PrCa risk (Supplementary Material, Table S1), suggesting that larger sample sizes are likely required for the remaining variants to achieve adequate power for subset analyses.

Case-control comparisons according to TMPRSS2:ERG fusion status suggest common risk variants with subtype preference

Potential subtype preference for the 26 candidate variants were examined by comparing the groups of TMPRSS2:ERG fusion-positive (n = 296) and fusion-negative cases (n = 256) to the 7650 controls (Supplementary Material, Table S1). The six risk variants that were associated with PrCa by comparing all 552 TMPRSS2:ERG phenotyped cases to controls and two additional variants appeared to be associated with either TMPRSS2:ERG fusion-positive or fusion-negative PrCa. Four variants were associated with TMPRSS2:ERG positive PrCa and four with TMPRSS2:ERG negative PrCa at the study-wide significance threshold of P = 0.00185 (Supplementary Material, Table S1). The strongest associations were observed between TMPRSS2:ERG negative PrCa and two independent risk variants at 8q24 (rs16901979, region 2 (R2), P = 1.2 × 10−6; and rs1447295, region 1 (R1), P = 2.0 × 10−6). Figure 1 displays all variants with their significance in the total phenotyped sample (color codes), in fusion-positive cases (x-axis) and in fusion-negative cases (y-axis) as compared with controls, respectively. Variants with stronger effect sizes (as ranked in Supplementary Material, Table S1) tended towards having associations with one somatic subtype, but not with both. This view supports the hypothesis that subtype specific common germline variants most likely exist.

Figure 1.

Mantel-Haenszel analysis showing associations between common PrCa risk variants and TMPRSS2:ERG (T2E) positive (x-axis) and T2E negative cases (y-axis) compared with controls. Analyses included n = 296 T2E fusion-positive and n = 256 T2E fusion-negative PrCa cases, which were separately compared with n = 7650 controls. Threshold lines correspond to nominal significance (P = 0.05, inner dashed square) and study significance adjusted for 27 variants (P = 0.00185, outer dashed square). Circles are colored based on separate analyses, where the variants were pre-checked for overall association with PrCa risk in all phenotyped cases (n = 552) versus all controls (Open circles: P > 0.05; gray: P < 0.05; black: P < 0.00185; Supplementary Material, Table S1). The majority of common risk variants was not associated with PrCa risk in the T2E phenotyped sample as compared with controls, and these remain unrelated to molecular subtype. Candidates significantly associated with PrCa risk showed stronger or unique associations for either T2E positive or negative PrCa. No variant was significantly associated with both subtypes. The highest ranked candidate variants, which were later genotyped in a replication dataset, are annotated with variant rs ID numbers.

TMPRSS2:ERG fusion-positive PrCa versus fusion-negative PrCa revealed differentially associated loci at 8q24 and 17q24

We then assessed differences in risk allele frequencies between the two somatic subtypes by case-case comparisons of the 296 TMPRSS2:ERG positive and the 256 TMPRSS2:ERG negative cases. Mantel-Haenszel results for all variants are presented in Supplementary Material, Table S2. No strong evidence for heterogeneity between study centers was observed. Nominally significant differences between TMPRSS2:ERG positive and negative cases were present for four variants. These include three variants with a higher risk allele frequency in TMPRSS2:ERG positive cases: rs10993994 at 10q11 (P = 0.015), rs2735839 at 19q13 (P = 0.0035) and rs1859962 at 17q24 (P = 0.038). One risk variant at 8q24 (rs16901979, R2) was more frequent in fusion-negative cases (P = 0.021). The second variant at 8q24 (rs1447295, R1), which was strongly associated with TMPRSS2:ERG negative PrCa when compared with controls, showed a similar tendency towards enrichment of the risk allele in TMPRSS2:ERG negative versus positive PrCa, although this result was not significant (P = 0.0891).

To substantiate findings of differential associations from the hypothesis generating dataset, an additional 669 independent cases with TMPRSS2:ERG phenotype data were used for case-case comparisons. The patients from four different study centers, FHCRC, IPO-PORTO, ULM and BERLIN, contained similar proportions of TMPRSS2:ERG positive (n = 388; 58%) and negative cases (n = 281; 42%) as the initial discovery set (Table 1). For genotyping, the top five candidate variants were selected based on results from the initial TMPRSS2:ERG subgroup case-control analyses (Supplementary Material, Table S1) and from case-case comparisons as ranked in Supplementary Material, Table S2. In this independent patient dataset, case-case comparisons found nominally significant associations between three variants and TMPRSS2:ERG subtype, each in the same direction as observed in the discovery sample (Table 2 and Supplementary Material, Fig. S1). The strongest associations were seen for rs1447295 (8q24, R1; P = 0.0085) and rs16901979 (8q24, R2; P = 0.012), where the risk alleles were enriched in TMPRSS2:ERG negative cases, and rs1859962 (17q24), where the risk allele was enriched in TMPRSS2:ERG positive cases (P = 0.018). The results for variants rs10993994 (10q11) and rs2735839 (19q13) were not confirmed in the independent dataset. In combined analyses of all 1221 phenotyped cases from the discovery and the replication sets, rs16901979 (8q24 R2; P = 0.0007) and rs1859962 (17q24; P = 0.0016) reached study significance (P < 0.00185), while rs1447295 (8q24 R1; P = 0.0025) was close to this threshold.

Table 2.

 Association results for the top five PrCa risk variants and TMPRSS2:ERG fusion status in the discovery sample, replication sample and both samples combined calculated by Mantel-Haenszel analyses

Variant	Discovery sample		Replication sample		Combined analysis
	ORb [95% CI]	P-value	ORb [95% CI]	P-value	ORb [95% CI]	P-value
rs16901979	0.53 [0.31–0.91]	0.0214	0.53 [0.33–0.87]	0.0121	0.53 [0.37–0.76]	0.0007
rs1447295	0.76 [0.56–1.04]	0.0891	0.63 [0.44–0.89]	0.0085	0.70 [0.55–0.88]	0.0025
rs10993994	1.35 [1.06–1.72]	0.0151	1.10 [0.89–1.37]	0.3789	1.21 [1.03–1.42]	0.0226
rs2735839	1.73 [1.20–2.51]	0.0035	1.03 [0.76–1.39]	0.8650	1.27 [1.00–1.59]	0.0455
rs1859962	1.29 [1.01–1.64]	0.0375	1.30 [1.05–1.62]	0.0178	1.30 [1.10–1.52]	0.0016

aSample numbers are given in Table 1, corresponding forest plots and study heterogeneity are shown in Supplementary Material, Figure S1.

bORs < 1 imply an overrepresentation of PrCa risk alleles in TMPRSS2:ERG fusion-negative cases, whereas ORs above 1 indicate an overrepresentation in TMPRSS2:ERG fusion-positive cases.

The main analysis addressed allelic association only, regardless of genetic models on genotypes. However, the crude TMPRSS2:ERG fusion frequencies in cases displayed by genotypes revealed additive effect tendencies (Supplementary Material, Fig. S2). This observation is particularly striking for the 8q24 variants associated with fusion-negative PrCa, where homozygous carriers showed a TMPRSS2:ERG frequency of only one-third, in contrast to the overall frequency of 56%.

Potential confounders

As previous studies have reported that TMPRSS2:ERG fusions have a higher prevalence in cases with early onset PrCa, we investigated whether age at diagnosis was potentially confounding the observed results of our confirmed variants, rs16901979 and rs1859962. In our dataset, age at diagnosis was significantly associated with TMPRSS2:ERG status (crude OR = 0.96 per year, P = 4.7 × 10−5; Supplementary Material, Table S3). Of note, the two variants at 8q24 and 17q24 were not associated with age at diagnosis (rs16901979: P = 0.38; rs1859962: P = 0.88). In multivariable logistic regression analyses, the association between age at diagnosis and TMPRSS2:ERG status did not change when adjusted for each variant (Supplementary Material, Table S3). Similarly, the associations between fusion status and the variants rs16901979 and rs1859962 were not modified when age at diagnosis was included in the model. In conclusion, age at diagnosis and the risk variants, 16901979 and rs1859962, are independent predictors of TMPRSS2:ERG status.

As a potential technical confounder, we considered differences in TMPRSS2:ERG detection methods among study samples. The inclusion of different TMPRSS2:ERG detection methods (i.e. fluorescence in situ hybridization (FISH) or RT-PCR) as a covariable, in addition to study center, revealed little differences in the associations between TMPRSS2:ERG subtype and the variants, rs16901979 and rs1859962 (Supplementary Material, Table S3).

EQTL analyses suggest subtype and genotype specific SOX9 mRNA expression at 17q24

The variants rs16901979 (8q24) and rs1859962 (17q24) are both located within gene deserts, where long-range interactions have been assumed between transcriptional regulatory elements and distant genes, such as MYC at 8q24 and SOX9 at 17q24. Expression levels of target genes could provide useful insights into how germline risk variants exert their effects, in particular in tumor subtypes according to TMPRSS2:ERG fusion status. From three cohorts, 262 fresh-frozen tumor samples were available for expression quantitative trait locus (eQTL) analysis and 70 matched sample pairs for comparing gene expression between tumor and adjacent benign tissue. With regards to the 8q24 variant, rs16901979, the rarity of the risk allele (frequency 0.04 in TMPRSS2:ERG fusion-positive and 0.07 in fusion-negative cases) resulted in insufficient genotype counts for generating adequate eQTL categories in the two subtypes, thus, this locus could not be investigated. For the 17q24 locus, we chose SOX9 as a candidate target gene based on previous studies (26) and assessed whether the observed differential association between rs1859962 and TMPRSS2:ERG status is reflected in subtype- and genotype-specific mRNA expression levels.

Comparison of adjacent benign and tumor tissue revealed a significant increase in SOX9 mRNA expression in TMPRSS2:ERG fusion-positive tumors (P = 0.0012), while the expression of SOX9 in fusion-negative tumors resembled that of benign tissue (P = 0.60, Fig. 2A). Regarding the hypothesized eQTL manifestation of rs1859962 (Fig. 2B), linear regression analysis showed a significant association between SOX9 mRNA levels and the presence of the risk allele G (effect per G allele = 0.21, P = 0.0019). When split by fusion status, the genotype dependency was evident in the TMPRSS2:ERG positive subset (effect per G allele = 0.23, P = 0.014). No significant association was observed in TMPRSS2:ERG negative tumors (effect per G allele = 0.09, P = 0.39). To further investigate, whether the correlation structure between rs1859962 and SOX9 mRNA levels statistically differ between TMPRSS2:ERG fusion-positive and negative tumors, we added an interaction term for genotype and TMPRSS2:ERG fusion status to the linear regression model with these two main factors. Though underpowered, this extended model demonstrated a significant impact of rs1859962 genotype (P = 0.021) and fusion status (P = 0.036) on SOX9 mRNA levels, but could not formally prove their interrelationship (P = 0.31).

Figure 2.

Expression levels of SOX9 according to TMPRSS2:ERG fusion status in adjacent benign and matched tumor tissues (A) and in eQTL analyses of rs1859962 (B). Mean values of log2 expression levels are presented with corresponding 95% CI. Significant P-values are in bold-type. (A) SOX9 expression levels in pairs of 70 tumor and adjacent benign tissues for TMPRSS2:ERG fusion-negative (open circles) and fusion-positive cases (black circles). P-values are derived from paired t-tests. (B) SOX9 expression levels according to rs1859962 genotype for all tumor samples (n = 262; gray circles), TMPRSS2:ERG fusion-negative samples (n = 22; open circles) and fusion-positive samples (n = 140; black circles). P-values correspond to the association between risk alleles and expression levels in a linear regression model.

Discussion

Since the discovery of ETS gene rearrangements in PrCa, numerous efforts have sought to determine whether fusion-positive and fusion-negative tumors differ with respect to clinical significance, pathology and tumorigenesis itself. While comprehensive analyses of genomic and epigenomic alterations provide supportive evidence for distinct molecular mechanisms in the pathogenesis of fusion-positive and negative tumors (15,16,18), little is known to what extent these molecular subtypes are linked to the apparent heritable background of PrCa. Nevertheless, several previous reports have supported the hypothesis of genetically distinct tumor entities. In familial PrCa pedigrees, we have observed that relatives are more likely to share the same TMPRSS2:ERG subtype (27), and have found rare variants in DNA repair genes to be associated with fusion status (28). Intriguingly, after the recent identification of the hereditary PrCa gene HOXB13 (29), in-depth pathology examination subsequently revealed subtype specific predisposition, as 83% of HOXB13 germline mutation carriers had TMPRSS2:ERG negative tumors (30). With respect to common risk-modifying variants, the Physicians’ Health Study (PHS) and Health Professionals Follow-up Study (HPFS) recently examined 39 variants for subtype preference in a cohort of 227 fusion-positive and 260 negative cases (31). The authors found nominally significant associations between TMPRSS2:ERG fusion status and PrCa risk variants at 4q24, 5p15, 8p21, 17q24, 19q13 and 22q13. Although not withstanding correction for multiple testing, these six variants exceeded the number of associations expected by chance. In this study, consisting of a large sample of cases with TMPRSS2:ERG fusion data, we have substantiated the hypothesis that common risk variants are involved in particular molecular subtypes of PrCa, rather than in PrCa risk in general, and have found significant evidence that variants at 8q24 and 17q24 are differentially associated with TMPRSS2:ERG fusion status.

To date, associations between common risk variants and TMPRSS2:ERG subtypes have been investigated by the PHS/HPFS study (31) and our present work, resulting in more than 1700 PrCa cases with somatic fusion status. As these two studies used different sets of candidate SNPs, several interesting loci cannot be checked for independent confirmation between the studies, such as 5p15 (rs12653946), 19q13 (rs11672691) and 22q13 (rs11704416), which were associated with nominal significance in the PHS/HPFS dataset, but were not genotyped directly or by a proxy SNP in our study. Two further findings in the PHS/HPFS cohorts, 4q24 (rs7679673) and 8p21 (rs1512268), were genotyped in the discovery dataset of the present work, but no significant associations were observed (P = 0.86 and 0.45, respectively). Notably, rs1859962 at 17q24 was included in both studies, and was identified in the PHS/HPFS dataset to be nominally associated with TMPRSS2:ERG fusion-positive PrCa (OR = 1.32; P = 0.04). We observed a similar association in both of our independent datasets (discovery: OR = 1.29; P = 0.04 and replication OR = 1.30; P = 0.02) with a study-wide significance in our combined analysis (OR = 1.30; P = 0.0016), providing strong evidence that the 17q24 variant is preferentially associated with TMPRSS2:ERG fusion-positive PrCa risk. Variant rs16901979 at 8q24 was found to be associated with TMPRSS2:ERG negative PrCa, in both the discovery and replication datasets in our study (OR = 0.53; P = 0.02 and OR = 0.53, P = 0.01, respectively; P = 0.0007 combined); however this was not the case in the PHS/HPFS cohorts (OR = 0.78; P = 0.48). Variant rs16901979 maps to the known 8q24 PrCa risk region 2, where a variant, rs1016343, with a more frequent risk allele was genotyped in the PHS/HPFS cohort. This variant shows linkage disequilibrium to rs16901979 (r2 = 0.11; D’ = 1) and was over-represented in the PHS/HPFS TMPRSS2:ERG negative PrCa cases (OR = 0.75) with borderline significance (P = 0.06). Also of interest was the fact that the risk alleles in the independent 8q24 risk regions 3 (rs6983267, OR = 0.85, P = 0.19; PHS/HPFS study) and 1 (rs1447295, OR = 0.70, P = 0.0025; present study) were also over-represented in TMPRSS2:ERG negative PrCa, although with different levels of significance. In summary, the consistent tendency of multiple 8q24 risk loci to be over-represented in TMPRSS2:ERG fusion-negative PrCa is intriguing, and requires the study of larger cohorts to confirm or disprove the involvement of 8q24 in the fusion-negative subtype.

The association found between TMPRSS2:ERG positive PrCa and rs1859962 at 17q24 suggests a molecular mechanism linking the risk region to the ERG pathway. For eQTL analysis, we considered SOX9 (SRY (sex determining region Y)-box 9), which is located in relatively close proximity (1 Mb) to the rs1859962 risk variant. SOX9 acts as a transcription factor in the development of prostate epithelia and its over-expression evidently plays a role in PrCa tumorigenesis (32,33). Long-range interactions between SOX9 and variants in LD with rs1859962 have been proposed previously in (26). SOX9 has also been identified as a downstream target of ERG (34) and a recent large histopathological study found a strong correlation between positive ERG status and moderate and high levels of SOX9 in PrCa tumor tissues (35). In line with SOX9 being a downstream target of ERG, we observed SOX9 over-expression only in fusion-positive tumors, while fusion-negative tumors have transcript levels similar to adjacent benign tissue. Remarkably, eQTL analysis stratified by fusion type demonstrated a positive correlation between SOX9 gene expression and the rs1859962 risk allele in TMPRSS2:ERG positive tumor tissue. In contrast, this correlation was not evident in the TMPRSS2:ERG negative subset. Of note, for normal prostate tissue, where ERG should not be overexpressed, no eQTL evidence between rs1859962 and SOX9 (P = 0.51) was retrieved from the GTex portal (www.http://www.gtexportal.org) (36). Taken together, these results suggest that germline risk alleles at 17q24 promote ERG-mediated changes in SOX9 expression only in TMPRSS2:ERG fusion-positive tumors, and the synergistic effect of these factors—risk variants and TMPRSS2:ERG fusion—render advantages to precursor cells in tumor formation.

Recent independent studies have found that TMPRSS2:ERG positive tumors are more frequent in patients with an earlier age at diagnosis of PrCa (21,22). The association with age at diagnosis was also present in our study population. Several explanations for the higher incidence of TMPRSS2:ERG fusions in early onset patients have been proposed, including a crucial role of higher androgen levels at younger ages (21), as well as the notion that TMPRSS2:ERG positive tumors may develop faster leading to earlier clinical manifestation, as compared with fusion-negative PrCa (22,37). The hypothesis that specific germline variants may predispose the development of early onset TMPRSS2:ERG-dependent PrCa is intriguing. Of note, the risk variant rs1859962 at 17q24 has been implicated in early onset PrCa previously (38). However, regression based analyses of the present study population revealed that age at diagnosis and rs1859962 were both associated with TMPRSS2:ERG fusion status, but were independent of each other.

With TMPRSS2:ERG status as the main study focus, concerns arose as to whether different detection methods used by study groups could have biased results. Each method, i.e. FISH for formalin fixed paraffin embedded (FFPE) tissue or quantitative real-time PCR of RNA from fresh-frozen tissue, has its own spectrum of false-positive and false-negative outcomes. In particular, while the FISH break apart assay manages to detect almost every rearrangement involving TMPRSS2 and ERG, including those which do not lead to a functional TMPRSS2:ERG isoform (over-estimation of relevant TMPRSS2:ERG), qPCR detection of the most prevalent TMPRSS2:ERG transcript may misclassify tumors harboring only rare TMPRSS2:ERG isoforms (underestimation of relevant TMPRSS2:ERG). In addition, FISH on tissue micro arrays may miss TMPRSS2:ERG positive tumor foci, due to the limited area of analyzed tumor tissue, while qPCR on macro-dissected fresh-frozen tumor tissue could enable a more comprehensive evaluation. We believe, however, that the different detection methods have had little effect on the results of our study. First, the TMPRSS2:ERG fusion frequencies among individual studies were similar to each other and the meta-analyses of the present samples revealed little evidence for heterogeneity. Second, adjustment for the detection method in multivariable regression analyses had almost no impact on the observed associations between TMPRSS2:ERG status and common risk variants. As reported from detailed studies of the technical issues (39–41), TMPRSS2:ERG assessment methods yield very similar results, and we are therefore confident that our results are robust to misclassification. Apart from the detection method, cohort selection is also known to influence the detection rate of TMPRSS2:ERG fusions. Of note, the observed TMPRSS2:ERG frequency of 56% in the present work is above the consensus of 45–50% reported in literature (reviewed in (42,43)). This might be in part explained by the remarkably different prevalence of TMPRSS2:ERG fusions among ethnicities. Studies, which explicitly addressed the population issue, reported 50% or higher TMPRSS2:ERG frequencies in subjects of European descent, while significantly less fusions (13%) were observed in non-Europeans (44). Lower TMPRSS2:ERG fusion prevalence applied for African Americans (31%) as well as for Asians (16%) (23). The present association study was restricted to European ethnicity, in order to avoid population stratification within the genotype data sets. Therefore, our study only included individuals who have the highest prevalence of TMPRSS2:ERG by ethnic origin and in consequence we would expect our TMPRSS2:ERG frequencies to reach higher levels as compared with studies with mixed populations.

One important study limitation is the restricted number of cases that had tumor tissue available for somatic typing. Even when phenotyped case groups were compared with a considerable number of controls (n = 7650), power was limited for assessing PrCa risk variants and, thus, some true associations may have been missed. Conversely, the possibility of false-positive results should be considered.

Our finding that known PrCa risk variants at 8q24 and 17q24 are differentially associated with TMPRSS2:ERG fusion status further strengthens support for the existence of distinct molecular subtypes in PrCa development. Importantly, this finding should encourage researchers conducting large genetic association studies to ascertain fusion status in order to identify comprehensive sets of subtype-specific risk variants. Recently, genetic epidemiologists have been considering a multifactorial model of PrCa risk, where genotypes of known common variants are converged into polygenic risk scores. Although this approach has promise, the predictive utility of these models is still limited. The knowledge that some risk variants are associated with a particular molecular subtype of PrCa could be incorporated into multifactorial models, thereby refining and improving their ability to identify specific PrCa risk groups.

Materials and methods

Study sample

The study samples for each collaborating center are described in detail in the Supplementary Materials. The hypothesis generating discovery sample consisted of PrCa cases and controls genotyped in 2011 using the ‘iCOGS’ array (7), including 27 variants previously shown to influence PrCa risk. Individuals were pre-selected for European ancestry, which was confirmed by principal component analyses of genotyping data. For analyzing the phenotype of interest, the TMPRSS2:ERG fusion status, selection criteria for cases were: 1) the availability of primary tumor tissue for TMPRSS2:ERG assessment or 2) existing information on fusion status. From five eligible study centers in Finland (TAMPERE), Germany (ULM), the UK (UKGPCS), USA (FHCRC) and Portugal (IPO-PORTO), a total of 552 cases with genotypes (n = 27 variants) and somatic phenotype data were included. An independent sample of cases with available tumor tissue or known TMPRSS2:ERG status was used to replicate the results for the five highest ranked candidate variants. The sample comprised 669 cases from Germany (BERLIN and ULM), the UK (UKGPCS), USA (FHCRC) and Portugal (IPO-PORTO). Gene expression analysis of tumor materials was performed using fresh-frozen tissue collections from ULM (35 matched tumor and adjacent benign) and BERLIN (194 specimens, tumor only), and one additional center, ERLANGEN (35 tissue pairs), to increase simple size.

Genotyping

Genotyping was performed on DNA from peripheral blood lymphocytes. Initially, 27 PrCa risk-associated variants were genotyped by means of the custom Illumina iSelect genotyping array (the iCOGS chip), previously generated by the Collaborative Oncological Gene-Environment Study (COGS). A detailed procedure including genotype calling and quality control has been described earlier in (7).

Genotyping of the replication samples (BERLIN, IPO-PORTO, FHCRC and ULM) was performed using predesigned TaqMan Genotyping Assays for rs1447295, rs16901979, rs10993994, rs1859962 and rs2735839 (Life Technologies, Carlsbad, USA).

Determination of the TMPRSS2:ERG fusion status

The study groups used FISH or RT-PCR for the assessment of the TMPRSS2:ERG fusion status. FISH was applied to FFPE tumor material. Detailed methods of the FISH based TMPRSS2:ERG assessment by break apart assays have been described previously for the samples of ULM (14,27), UKGPCS (45), FHCRC (24) and TAMPERE (46). Fresh-frozen material, collected by the BERLIN, ERLANGEN, IPO-PORTO and ULM study groups, was subjected to TMPRSS2:ERG detection via RT-PCR using TaqMan primers and probes specific for the most prevalent fusion transcript variant (T1G4, TMPRSS2:ERGa), which is found in ∼90% of TMPRSS2:ERG fusion-positive tumors (41,47). The IPO-PORTO samples were phenotyped as described by Paulo et al. (39). Tissues from BERLIN, ERLANGEN and ULM were macro-dissected, followed by RNA isolation using the RNeasy Mini Kit (QIAGEN, Hilden, Germany). The detection of the TMPRSS2:ERG fusion transcript was performed using QuantiFast Multiplex RT-PCR +R Kit (QIAGEN, Hilden, Germany) on a VIIA7 Fast Real-Time PCR System (Life Technologies, Carlsbad, USA). Reactions were set up in duplicate in a final volume of 20 µl. Cycling conditions were as follows: 50 °C for 20 min and 95 °C for 5 min for initial reverse transcription and hot start polymerase activation respectively, and subsequently 45 cycles of 94 °C for 15 s and 60 °C for 60 s. Primer and probe sequences are provided in Supplementary Material, Table S4.

Determination of SOX9 expression

The expression levels of ALAS1 (reference gene) and SOX9 were quantified with the QuantiFast Multiplex RT-PCR +R Kit (QIAGEN, Hilden, Germany) on a VIIA7 Fast Real-Time PCR System (Life Technologies, Carlsbad, USA). Reaction and cycling set up is described earlier. The primer and probe sequences are provided in Supplementary Material, Table S4.

Statistical analysis

Statistical analyses were performed with the Review Manager version 5.1.7 (Copenhagen: The Nordic Cochrane Centre, The Cochrane Collaboration, 2012) and SAS version 9.3.

As heterogeneity between study centers was of interest, we used a meta-analytic approach to assess associations in case-case and case-control comparisons. In detail, for each variant and study center, table-based per-allele odds ratios (ORs) were calculated and Mantel-Haenszel analyses were used to pool the ORs across centers. Fixed effects meta-analyses were preferred over random effect models because the inconsistency of association results across populations (as measured by I2) was mostly limited.

Discovery and replication analyses were based on comparisons between TMPRSS2:ERG fusion-positive and negative cases (case-case comparisons), where nominal thresholds were applied (P = 0.05) based on the limited number of cases available for the analyses. The candidate variant selection for the replication round was also guided by supplementary analysis of the more powerful comparison of case subtypes versus unaffected controls (threshold P = 0.00185, according to Bonferroni adjustment for 27 variants). Four variants fulfilled both criteria in the discovery sample (rs2735839, rs10993994, rs16901979 and rs1859962). The candidate variant list was expanded by one further variant (rs1447295) based on the case-case ranking of variants and rankings derived from cancer subtypes vs. controls. Formally, these five variants form the smallest subset of variants ranked ≤ n in case-case comparisons that have also rank ≤ n in cancer subtypes vs. controls. For the combined Mantel-Haenszel analyses of the discovery and replication stages the study wide significance level of P = 0.00185 was applied.

The relationship between TMPRSS2:ERG fusion status, risk alleles, age at diagnosis and gene fusion detection methods was assessed using multivariable logistic regression, adjusting for study center effects. For this purpose, TMPRSS2:ERG status was modeled as the dependent variable, whereas, in addition to center, either age at diagnosis and genotype, or detection method and genotype were included as covariables.

SOX9 expression levels were calculated by the ΔCt method using ALAS1 as reference gene, with subsequent log2 transformation to achieve normal distribution of the data for downstream analyses. The comparisons of gene expression between tumor and adjacent benign tissue were performed using the paired t-test. Genotype specific effects on SOX9 expression levels were tested using a regression model with genotype as an independent variable, adjusted for study center effects. The model was extended for the TMPRSS2:ERG status and an interaction term to test for differences between TMPRSS2:ERG fusion-positive and negative subsets with regard to the correlation structure of SOX9 mRNA expression levels and rs1859962 genotypes.

Supplementary Material

Supplementary Material is available at HMG online.

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