ARID1A-deficiency in urothelial bladder cancer: No predictive biomarker for EZH2-inhibitor treatment response?

Bladder cancer therapy relies on aggressive treatments highlighting the need for new, targeted therapies with reduced side effects. SWI/SNF complexes are mutated in ~20% across human cancers and dependency of SWI/SNF-deficient tumors on EZH2 has been uncovered recently. To systematically dissect the frequency of genetic alterations in SWI/SNF complexes potentially contributing to their inactivation, mutations and copy number variations in 25 SWI/SNF subunit genes were analyzed making use of publicly available sequencing data for 408 muscle-invasive bladder carcinoma samples. ARID1A truncating mutations were identified as the by far most common alterations of SWI/SNF complexes in urothelial bladder cancer. As current ARID1A protein expression data in bladder cancer are inconsistent and incomplete we examined if the frequency of truncating ARID1A mutations translates into a similar frequency of cases showing ARID1A protein loss. We applied a validated ARID1A antibody conducting a comprehensive immunohistochemistry-based expression analysis in urothelial bladder cancer (n = 362) including carcinoma in situ (CIS) cases. While observing increased median ARID1A protein levels in all carcinoma subgroups compared to normal urothelial controls (n = 21), the percentage of cases showing ARID1A protein loss was positively correlated with increasing stage and grade culminating in a rate of 14.1% in muscle-invasive disease. ARID1A-depletion did neither increase EZH2 protein or trimethylated H3K27 levels in vitro nor did ARID1A expression correlate with EZH2 or H3K27me3 amounts in human bladder carcinomas. Importantly, ARID1A-deficiency was neither associated with enhanced sensitivity towards inhibition of EZH2 enzymatic activity nor depletion of EZH2 protein. In summary, ARID1A truncating mutations, potentially translating into ARID1A protein loss in a subset of high-grade bladder cancers, are the most common SWI/SNF genetic alterations in bladder cancer. Our data do not support ARID1A-deficiency as predictive biomarker for EZH2-inhibitor treatment response in bladder cancer underlining the need for future bladder cancer-specific, drug screens for successfull discovery of ARID1A-deficiency-based targeted drugs.

膀胱癌治疗目前依赖侵袭性疗法，这凸显了开发副作用更低的新型靶向治疗手段的迫切需求。SWI/SNF复合物（SWI/SNF complexes）在约20%的人类癌症中发生突变，且近期研究揭示了SWI/SNF缺陷型肿瘤对EZH2的依赖性。为系统剖析可能导致SWI/SNF复合物失活的遗传变异发生频率，本研究借助408份肌肉浸润性膀胱癌样本的公开测序数据，对25个SWI/SNF亚基基因的突变与拷贝数变异展开分析。ARID1A（ARID1A）截短突变被证实是尿路上皮膀胱癌中SWI/SNF复合物最常见的遗传变异。鉴于当前膀胱癌中ARID1A蛋白表达数据存在不一致且不完整的问题，本研究探究了ARID1A截短突变的发生频率是否对应着相似比例的ARID1A蛋白缺失病例。本研究使用经过验证的ARID1A抗体，对362例尿路上皮膀胱癌样本（包含原位癌（carcinoma in situ, CIS）病例）开展基于免疫组化的全面表达分析。相较于21例正常尿路上皮对照样本，所有癌亚组的ARID1A蛋白中位表达水平均有所升高；而ARID1A蛋白缺失病例的占比则随肿瘤分期与分级升高呈正相关，在肌肉浸润性膀胱癌中这一比例高达14.1%。在体外实验中，ARID1A敲减既不会升高EZH2蛋白或三甲基化组蛋白H3K27的水平，在人类膀胱癌组织中ARID1A的表达也与EZH2或H3K27me3的含量无相关性。重要的是，ARID1A缺陷既未表现出对EZH2酶活性抑制剂的敏感性增强，也未与EZH2蛋白敲减存在关联。综上，ARID1A截短突变是膀胱癌中最常见的SWI/SNF复合物遗传变异，该突变可能在部分高级别膀胱癌中导致ARID1A蛋白缺失。本研究数据不支持将ARID1A缺陷作为膀胱癌患者对EZH2抑制剂治疗响应的预测性生物标志物，这凸显了未来需开展膀胱癌特异性药物筛选，以成功开发基于ARID1A缺陷的靶向治疗药物的必要性。