Inadequate DNA repair is implicated in the pathogenesis of chronic obstructive pulmonary disease (25); however, the mechanisms that underlie inadequate DNA repair in COPD are poorly understood. In the present study, we investigated the expression levels of key enzymes of the base excision repair pathway that initiate the repair of oxidatively damaged bases in cohorts of stable COPD subjects and subjects suffering from AECOPD. The levels of NEIL2 mRNA were significantly decreased in stable COPD subjects and further reduced in the AECOPD group. Reduced levels of NEIL2 were also associated with significantly increased DNA damage in the transcriptionally active genome in both stable COPD and AECOPD subjects compared to that in controls, with significantly increased damage in the AECOPD cohort compared to that in stable COPD group. This finding is consistent with findings in NEIL2 knockout mice that demonstrated cumulative oxidative damage in the transcriptionally active genome over time (16).
Reduced NEIL2 levels in stable subjects without evidence of exacerbation are associated with persistent oxidative damage that could result in genome instability and dysfunctional transcription and contribute to the development of malignancy. A reduction in NEIL2 activity caused by dysfunctional polymorphisms has been associated with an increased risk of malignancy (17, 18). Acute exacerbations result in further reductions in Neil 2 levels, suggesting that this may be a mechanism that contributes to the increased risk of malignancy in COPD subjects with frequent exacerbations.
The most common cause of exacerbation is infection, which may explain the further suppression of Neil 2 levels and increased inflammation, as observed in RSV- and SARS-CoV-2-infected animal models (22, 23). Alternately, this group may have lower chronic depression of NEIL2 levels, which in turn increases their susceptibility to acute exacerbations and increased genomic damage. Although there was no significant correlation between the degree of airflow obstruction and NEIL2 mRNA, those subjects with AECOPD had a significantly lower FEV1 than did those in the stable group. Additionally, as reported previously, NEIL2 plays an essential role in maintaining immune response homeostasis, where NEIL2 blocks unwarranted NF-kB-mediated proinflammatory gene expression and protects hosts from an uncontrolled inflammatory response (19, 23). Dysregulation of inflammation due to the production of abnormal proteins could also impair the healing process and lead to more rapid disease progression. The cause of the reduction in NEIL2 in stable COPD subjects is unclear. There was no clinical evidence of ongoing infection or a history of lung malignancy in this group. However, the adverse effect on oxidative damage in the transcriptional genome was quite clear. As this was a cross-sectional study, we are unable to determine whether the levels might recover with time, but given that most were former smokers and had no recent exacerbations, this seems unlikely.
A recent meta-analysis using two large COPD cohorts did not demonstrate the predictive value of any cytokine for acute exacerbations (26). Similarly, we examined 7 cytokines, including IL6, CXCL 1, 8, 9, 10, CCL2, and CCL11, in our COPD population and found no significant differences between the COPD groups. The positive relationship between blood eosinophil levels and NEIL2 expression was unexpected. A recent analysis of blood eosinophil levels in the Spiromics COPD cohort revealed that GOLD D subjects with eosinophil counts less than 100/µl who did not receive steroids had more exacerbations and a more rapid decline in lung function (27). Therefore, blood eosinophils may be an indicator of low NEIL2 expression, which results in persistent inflammation and worsening genomic damage. The relationship between NEIL2 and eosinophil levels needs further investigation.
Our study is limited by its cross-sectional nature and use of circulating blood leukocytes. A follow-up study examining sampling of airway epithelium and leukocyte subgroups may clarify the role of the NEIL2-BER pathway. Given the low levels we detected, the use of recombinant NEIL2 as a therapeutic intervention is possible. We previously showed that recombinant NEIL2 modulates the inflammatory response by inhibiting NF-kB-mediated gene expression and decreasing viral replication (19, 22, 23). Thus, restoring NEIL2 levels could improve therapy for severe exacerbations, slow the progression of COPD in subjects at risk for a more rapid decline in lung function and/or frequent exacerbations and reduce the risk of malignancy in certain subgroups.