Re: Ming-Jun Shi, Xiang-Yu Meng, Philippe Lamy, et al. APOBEC-mediated Mutagenesis as a Likely Cause of FGFR3 S249C Mutation Over-representation in Bladder Cancer. Eur Urol 2019;76:9-13

FGFR3 Mutational Activation Can Induce Luminal-like Papillary Bladder Tumor Formation and Favors a Male Sex Bias

BACKGROUND

Bladder cancer (BCa) is more common in men and presents differences in molecular subtypes based on sex. Fibroblast growth factor receptor 3 (FGFR3) mutations are enriched in the luminal papillary muscle-invasive BCa (MIBC) and non-MIBC subtypes.

OBJECTIVE

To determine whether FGFR3 mutations initiate BCa and impact BCa male sex bias.

DESIGN, SETTING, AND PARTICIPANTS

We developed a transgenic mouse model expressing the most frequent FGFR3 mutation, FGFR3-S249C, in urothelial cells. Bladder tumorigenesis was monitored in transgenic mice, with and without carcinogen exposure. Mouse and human BCa transcriptomic data were compared.

INTERVENTION

Mutant FGFR3 overexpression in mouse urothelium and siRNA knockdown in cell lines, and N-butyl-N(4-hydroxybutyl)-nitrosamine (BBN) exposure.

OUTCOME MEASUREMENTS AND STATISTICAL ANALYSIS

Impact of transgene dosage on tumor frequency, synergy with BBN treatment, and FGFR3 pathway activation were analyzed. The sex-specific incidence of FGFR3-mutated tumors was evaluated in mice and humans. FGFR3 expression in FGFR3-S249C mouse urothelium and in various human epithelia was measured. Mutant FGFR3 regulation of androgen (AR) and estrogen (ESR1) receptor activity was evaluated, through target gene expression (regulon) and reporter assays.

RESULTS AND LIMITATIONS

FGFR3-S249C expression in mice induced low-grade papillary BCa resembling human luminal counterpart at histological, genomic, and transcriptomic levels, and promoted BBN-induced basal BCa formation. Mutant FGFR3 expression levels impacted tumor incidence in mice, and mutant FGFR3-driven human tumors were restricted to epithelia presenting high normal FGFR3 expression levels. BCa male sex bias, also found in our model, was even higher in human FGFR3-mutated tumors compared with wild-type tumors and was associated with higher AR and lower ESR1 regulon activity. Mutant FGFR3 expression inhibited both ESR1 and AR activity in mouse tumors and human cell lines, demonstrating causation only between FGFR3 activation and low ESR1 activity in tumors.

CONCLUSIONS

Mutant FGFR3 initiates luminal papillary BCa formation and favors BCa male sex bias, potentially through FGFR3-dependent ESR1 downregulation. Patients with premalignant lesions or early-stage BCa could thus potentially benefit from FGFR3 targeting. FGFR3 expression level in epithelia could account for FGFR3-driven carcinoma tissue specificity.

PATIENT SUMMARY

By developing a transgenic mouse model, we showed that gain-of-function mutations of FGFR3 receptor, among the most frequent genetic alterations in bladder cancer (BCa), initiate BCa formation. Our results could support noninvasive detection of FGFR3 mutations and FGFR3 targeting in early-stage bladder lesions.

Identification of new driver and passenger mutations within APOBEC-induced hotspot mutations in bladder cancer

BACKGROUND

APOBEC-driven mutagenesis and functional positive selection of mutated genes may synergistically drive the higher frequency of some hotspot driver mutations compared to other mutations within the same gene, as we reported for FGFR3 S249C. Only a few APOBEC-associated driver hotspot mutations have been identified in bladder cancer (BCa). Here, we systematically looked for and characterised APOBEC-associated hotspots in BCa.

METHODS

We analysed 602 published exome-sequenced BCas, for part of which gene expression data were also available. APOBEC-associated hotspots were identified by motif-mapping, mutation signature fitting and APOBEC-mediated mutagenesis comparison. Joint analysis of DNA hairpin stability and gene expression was performed to predict driver or passenger hotspots. Aryl hydrocarbon receptor (AhR) activity was calculated based on its target genes expression. Effects of AhR knockout/inhibition on BCa cell viability were analysed.

RESULTS

We established a panel of 44 APOBEC-associated hotspot mutations in BCa, which accounted for about half of the hotspot mutations. Fourteen of them overlapped with the hotspots found in other cancer types with high APOBEC activity. They mostly occurred in the DNA lagging-strand templates and the loop of DNA hairpins. APOBEC-associated hotspots presented systematically a higher prevalence than the other mutations within each APOBEC-target gene, independently of their functional impact. A combined analysis of DNA loop stability and gene expression allowed to distinguish known passenger from known driver hotspot mutations in BCa, including loss-of-function mutations affecting tumour suppressor genes, and to predict new candidate drivers, such as AHR Q383H. We further characterised AHR Q383H as an activating driver mutation associated with high AhR activity in luminal tumours. High AhR activity was also found in tumours presenting amplifications of AHR and its co-receptor ARNT. We finally showed that BCa cells presenting those different genetic alterations were sensitive to AhR inhibition.

CONCLUSIONS

Our study identified novel potential drivers within APOBEC-associated hotspot mutations in BCa reinforcing the importance of APOBEC mutagenesis in BCa. It could allow a better understanding of BCa biology and aetiology and have clinical implications such as AhR as a potential therapeutic target. Our results also challenge the dogma that all hotspot mutations are drivers and mostly gain-of-function mutations affecting oncogenes.