Lung Basal Stem Cells Rapidly Repair DNA Damage Using the Error-Prone Nonhomologous End-Joining Pathway

Lung squamous cell carcinoma (SqCC), the second most common subtype of lung cancer, is strongly associated with tobacco smoking and exhibits genomic instability. The cellular origins and molecular processes that contribute to SqCC formation are largely unexplored. Here we show that human basal stem cells (BSCs) isolated from heavy smokers proliferate extensively, whereas their alveolar progenitor cell counterparts have limited colony-forming capacity. We demonstrate that this difference arises in part because of the ability of BSCs to repair their DNA more efficiently than alveolar cells following ionizing radiation or chemical-induced DNA damage. Analysis of mice harbouring a mutation in the DNA-dependent protein kinase catalytic subunit (DNA-PKcs), a key enzyme in DNA damage repair by nonhomologous end joining (NHEJ), indicated that BSCs preferentially repair their DNA by this error-prone process. Interestingly, polyploidy, a phenomenon associated with genetically unstable cells, was only observed in the human BSC subset. Expression signature analysis indicated that BSCs are the likely cells of origin of human SqCC and that high levels of NHEJ genes in SqCC are correlated with increasing genomic instability. Hence, our results favour a model in which heavy smoking promotes proliferation of BSCs, and their predilection for error-prone NHEJ could lead to the high mutagenic burden that culminates in SqCC. Targeting DNA repair processes may therefore have a role in the prevention and therapy of SqCC.

肺鳞状细胞癌（Lung squamous cell carcinoma, SqCC）是肺癌第二常见的亚型，与吸烟密切相关且具有基因组不稳定性特征。其发生的细胞起源与分子机制在很大程度上仍未被阐明。本研究发现，从重度吸烟者体内分离的人类基底干细胞（human basal stem cells, BSCs）可大量增殖，而其对应的肺泡祖细胞则仅具备有限的集落形成能力。我们证实，这种差异部分源于基底干细胞在电离辐射或化学诱导DNA损伤后，相比肺泡细胞拥有更高效的DNA修复能力。对携带DNA依赖蛋白激酶催化亚基（DNA-dependent protein kinase catalytic subunit, DNA-PKcs）——非同源末端连接（nonhomologous end joining, NHEJ）这一DNA损伤修复关键酶——突变的小鼠进行分析后发现，基底干细胞优先通过这一易出错的途径修复DNA。值得注意的是，多倍体——一种与基因组不稳定细胞相关的现象——仅在人类基底干细胞亚群中被观测到。表达特征分析显示，基底干细胞可能是人类肺鳞状细胞癌的起源细胞，且肺鳞状细胞癌中高水平的NHEJ基因表达与基因组不稳定性升高呈正相关。综上，本研究结果支持如下模型：重度吸烟可促进基底干细胞增殖，而它们倾向于使用易出错的非同源末端连接途径修复DNA，这可能导致高突变负荷，最终引发肺鳞状细胞癌。因此，靶向DNA修复过程或可用于肺鳞状细胞癌的预防与治疗。