GPRC5A基因杂合性缺失在肺癌发生中的作用机理和意义

Lung cancer is the leading cause of cancer deaths. Lung carcinogenesis is associated with oncogenic driver mutantions, such as those on EGFR and K-Ras. Identification of oncogenic driver mutations is critically important for cancer therapy. There are still about half of lung cancer without identified driver mutations. GPRC5A is a newly identified lung tumor suppressor gene. GPRC5A is frequently repressed in human lung cancer. Gprc5a-knockout (ko) mice develop spontaneous lung tumor, which resembles the pathological features of human lung cancer. We hypothesize that loss of heterozygosity (LOH) of GPRC5A gene is frequent occurred in human lung cancer; and LOH of GPRC5A would confer a selection pressure for certain types of genetic mutations or pathways during lung carcinogenesis. In this research proposal, we are to study three specific aims and answer related questions: First, whether LOH of GPRC5A is occurred in human lung cancer? What is the frequency? Secondly, whether is there a common genetic or epigenetic pathway shared by Gprc5a-ko mouse lung cancer cells and human lung cancer (GPRC5A deficient) cells? Finally, by reconstitution and in vitro characterization of the newly identified oncogenic mutants, whether we could find the novel biochemical activities and biological pathways related to carcinogenesis in these lung cancer cells. Taken together, the proposed study will not only reveal new genetic mutations and reveal a novel mechanism of lung cancer, but also provide a novel strategy for targeted therapy of lung cancer.

肺癌是致死人数最多的癌症。肺癌的发生与癌基因驱动突变紧密相关。癌基因（如EGFR）驱动突变的鉴定对肺癌的靶向治疗至关重要。GPRC5A是新发现的肺肿瘤抑制基因，其在人肺癌组织中常被抑制。Gprc5a敲除小鼠会产生自发性肺肿瘤，许多病理变化与人肺癌的十分相似。我们假设，人肺癌中会发生GPRC5A基因杂合性缺失；该基因的缺失会予肺癌细胞以某种选择压力，使得其选择某类遗传突变或癌信号通路。本研究方案集中三个方面：首先，鉴定人肺癌组织中是否有GPRC5A基因杂合性缺失？发生频率是多少？其次，利用Gprc5a-ko小鼠肺癌动物模型，筛选Gprc5a基因缺失背景下肺癌细胞中的驱动突变癌基因，并检验人肺癌中是否有类似的遗传突变。最后，在体外重建这些突变体，分析其生化活性及生物学功能的改变。总之，我们欲利用Gprc5a敲除小鼠肺癌模型，揭示人肺癌中尚未探明的新机理，为肺癌的靶向治疗提供新靶标。

癌基因驱动突变在肺癌发生过程中起到关键性作用。靶向驱动癌基因（如EGFR）对肺癌的靶向治疗至关重要。然而，仍有一半肺癌因无已知的驱动癌基因，而缺乏靶向治疗的方案。GPRC5A是新发现的肺肿瘤抑制基因，在人肺癌组织中普遍被抑制，然而机制不清。我们认为，找到抑癌基因的驱动突变导致的癌信号通路依旧可成为治疗肺癌的靶标。对523例NSCLC组织样品进行了研究，我们发现有283例有杂合性缺失(LOH)，占54.11%。其中，282例肺腺癌组织中有162例（57.44%）发生GPRC5A-LOH，和241例肺鳞癌组织中有121例（50.21%）发生GPRC5A-LOH。我们还发现，GPRC5A-LOH与12p-LOH的发生频率高度重叠，包括肺腺癌 (P=0.0007) ，肺鳞癌 (P=0.0001)。这说明GPRC5A是染色体12p上的一个肿瘤抑制基因a haplo-insufficient tumor suppressor。肺腺癌中GPRC5A-LOH的发生与EGFR突变呈现显著正相关(P=0.0119)，这提示，二者在肺癌发生发展过程中可能存在协同作用。但GPRC5A-LOH与KRAS突变没有相关性(P=0.96699)，以及在肺鳞癌中与驱动癌基因没有相关性。GPRC5A-LOH的发生与肺癌的肿瘤分期没有关系，包括肺腺癌(P=0.0532)，肺鳞癌(P=0.5822)。这提示，GPRC5A-LOH发生在肺癌发生的早期，或启动阶段。最后，根据523例肺癌患者中417例的随访资料，我们发现，GPRC5A-LOH突变患者的总生存期(overall survival, OS)预后更短（P=0.037）；在GPRC5A-LOH兼有EGFR驱动突变的情况下，总生存期(overall survival, OS)预后更差（P=0.005）。因此，GPRC5A-LOH是一个独立的总生存率的危险指标。