When Autophagy Fails: ATG7 Variants and Their Impact on Radiation Lung Injury

Radiation therapy is a common treatment strategy for cancer that uses high-energy X-rays to target and destroy cancer cells or impair their ability to proliferate (1). However, radiation pneumonitis (RP) is an adverse outcome in approximately 10–30% of patients who undergo radiation therapy for the treatment of lung, breast, and esophageal cancers (2). Patients may experience symptoms such as dry cough, fever, respiratory distress, lung fibrosis, and, in severe cases, death (3). Despite the current use of radioprotectors and mitigators, it is critical to understand the effect of genetic susceptibility on the individual adverse response to radiation to accurately predict the development of RP and to maximize the therapeutic benefits of radiation while minimizing its potential adverse effects.

In this issue of the Journal, Liu and colleagues (pp. 221–231) identified a genetic variance of the ATG7 gene that is associated with the risk of developing RP following standard cancer radiation treatment (4). In these studies, 301 patients with non–small-cell lung cancer or small-cell lung cancer were included in genotyping analysis. Eligible patients received a radiation dose of ≥45 Gy. Thirteen SNPs from five genes in the autophagy pathway—ATG5, ATG7, ATG10, ATG12, and BECN1—were evaluated for their relationship with RP. Using Kaplan-Meier plots and/or multiple Cox proportional hazard analyses, the investigators conclude that patients who carried the AA genotype of ATG7:rs2122031 are significantly more likely to develop grade ⩾2 or grade ⩾3 RP. Further analyses indicate that ATG7:rs2122031 is a predictor of RP independent of the mean lung dose of radiation or the volume of normal lung receiving ≥20 Gy radiation. Mechanistically, in silico analysis and luciferase reporter assays demonstrated that the rs2122031-A allele can downregulate the expression of ATG7 through preventing the binding of a transcription factor, EGR1 (Figure 1). Overall, these clinical studies are the first to document the genotype of an autophagy gene, ATG7, in relation to its expression and the risk of developing RP.

Figure 1.

ATG7 rs2122031 variants are associated with the risk of radiation pneumonitis.

In animal model studies, mice received a single fraction of 16 Gy X-ray to the thorax. The investigators identified a localized increase in ATG7 expression in PDGFR-β⁺ myofibroblasts after radiation. They used a Pdgfrβ-Cre mouse model to conditionally knock out ATG7 in the myofibroblast compartment and showed that these conditional-knockout mice are more susceptible to RP and subsequent lung fibrosis. Single-cell RNA sequencing also revealed the enrichment of autophagy and collagen-containing extracellular matrix genes after irradiation stress in ATG7 conditional-knockout mice. This suggests that physiological (myo)fibroblast autophagy may play a protective role in mitigating RP and the development of lung fibrosis following radiation.

Autophagy is a cellular process in which a cell engulfs and degrades its own components, such as damaged proteins, organelles, and intracellular pathogens, through a lysosomal degradation system that involves the reorganization of subcellular membranes to form autophagosomes (5). In the context of fibrotic lung diseases, previous studies indicate that lung tissues from patients with idiopathic pulmonary fibrosis have decreased autophagic activity (6, 7). Moreover, experiments using animal models of lung fibrosis demonstrate that epithelial cell senescence and myofibroblast differentiation are consequences of insufficient autophagy (8), highlighting a crucial role of autophagy and potential therapeutic strategies targeting autophagy in fibrotic lung diseases (9). The study by Liu and colleagues contributes to a growing body of literature indicating that ATG7 could be a targetable autophagy protein for the treatment of lung fibrosis resulting from radiation therapy.

RP is characterized by early-stage epithelial cell injury, inflammation, and late-phase lung fibrosis. Fibroblasts are critical in late-stage lung fibrosis development. However, increasing evidence highlights their role in contributing to lung inflammation by acting as inflammatory cells themselves or recruiting leukocytes to promote chronic inflammation (10). Recent findings reveal alveolar fibroblasts as the primary source of emerging fibroblast subsets following lung injury. These fibroblasts differentiate into inflammatory fibroblasts in response to inflammatory cytokines during the initial phase of injury and later into profibrotic fibroblasts under the influence of profibrotic cytokines (11). In the current studies, although ATG7 knockout in the myofibroblast compartment is expected to lead to increased fibrosis at later time points, modulating autophagy in fibroblasts during earlier stages of inflammation could also significantly affect lung inflammation and therefore deserves equal attention. Single-cell sequencing data in the study of Liu and colleagues shows that fibroblasts from ATG7 conditional-knockout mice exhibit increased expression of TNF-α, IL-1β, and IL-6. This suggests that fibroblasts are a major source of circulating proinflammatory cytokines. It would be interesting to investigate the role of autophagy in specific fibroblast subsets, particularly SCUBE2⁺ alveolar fibroblasts and CTHRC1⁺ fibrotic fibroblasts in the pathogenesis of RP, and if autophagy changes the balance between emergent inflammatory and fibrotic fibroblast subsets.

There are a few limitations of the study. First, immunostaining of ATG7 using human lung tissues does not have the resolution to demonstrate the cell types in which ATG7 is decreased in association with the ATG7:rs2122031 AA genotype. In fact, it appears that ATG7 is expressed in most lung cells, as it should be, including lung epithelial cells. It is possible the ATG7:rs2122031 variant will affect its expression in a variety of lung epithelial populations, and the role of these epithelial cell populations, particularly alveolar type 2 cells and associated transitional cell types (12, 13), may contribute to the development of RP. In addition, it would be helpful to compare the levels of ATG7 in patients with the same ATG7:rs2122031 variants who did or did not experience RP. With these results, it would be possible to understand whether ATG7 expression can be modulated by radiation therapy itself and, if so, in which cellular compartments. Finally, in animal studies, although ATG7 upregulation in fibroblasts or other cell compartments of the lungs following radiation may serve as a feedback-protective mechanism, more clarity could emerge from the use of various epithelial cell–specific Cre drivers to dissect the role of epithelial cell autophagy in the development of RP.

Overall, the investigators convincingly demonstrate that patients with unfavorable ATG7 genotypes exhibit reduced ATG7 expression, which could impair autophagic flux following irradiation stress, potentially increasing the risk of RP (Figure 1). The ATG7:rs2122031 AA genotype may serve as a predictive marker for RP, given its significant association with an increased incidence of grade ⩾2 or grade ⩾3 RP. In mice, the loss of autophagy through ATG7 deletion in fibroblasts further supports that autophagy plays a protective role in RP pathogenesis, suggesting that ATG7 could be a potential therapeutic target for RP.