Verena Jendrossek

Diffuse midline glioma H3K27M-altered (DMG) remains a fatal pediatric brain cancer driven by a global loss of histone H3K27 trimethylation. Radiotherapy comprises the most important treatment modality and significantly improves overall survival. Novel therapeutic strategies for DMG patients without or with radiotherapy are urgently needed. Here, we aimed to gain insights into potential radiation response modulators. To identify modulators of radiation response, we performed a high-throughput drug screening (HTS) in seven representative DMG cell lines using conventional chemotherapeutic drugs and phase I-IV drugs ( n = 687), followed by irradiation with 0 or 2 × 4 Gray (Gy). The ataxia–telangiectasia and Rad3-related (ATR) inhibitor berzosertib emerged as a potent radiosensitizer. Its effects were validated in three DMG cell lines using short-term proliferation assays, long-term limiting dilution assays (LDA), 3D spheroid cultures, and the chorioallantoic membrane (CAM) assay in ovo . Across all three tested DMG cell line models, berzosertib enhanced the antineoplastic effects of clinically relevant radiation doses, significantly reducing proliferation and clonogenic survival, delaying spheroid growth, and suppressing tumor formation in ovo . These findings provide strong preclinical evidence that ATR inhibition increases the sensitivity of DMG cells to radiotherapy. They highlight a novel therapeutic vulnerability and support further exploration of ATR inhibitors in rational combination strategies to improve radiotherapy efficacy for this deadly disease.

Background/Objectives: Analysis of the density and spatial distribution of pulmonary infiltrates of patients with high-grade (≥3) pneumonitis after radiochemotherapy and durvalumab consolidation (RT/CTx + IO) was performed in order to define dosimetric hallmarks of the development of infiltrates following this multimodality treatment. Methods: Consecutive patients treated with RT/CTx + IO for stage III NSCLC were retrospectively reviewed with respect to the occurrence of grade ≥ 3 pneumonitis. Lung infiltrates were contoured on follow-up CT scans acquired around the time of maximum pneumonitis expression. The applied dose distribution was overlaid with the follow-up CT using elastic deformation, and infiltrates were binned according to their density in density strata of 50 HU. The dose and density dependence of partial infiltrate volumes per unit lung volume was analyzed using a mixed fixed and random effect model adjusting for patient, density and dose-dependent random effects. Results: Six patients with grade ≥ 3 pneumonitis were identified from 132 patients treated with RT/CT + IO at a comprehensive cancer center. Partial volumes of lung infiltrates captured by follow-up CT with maximum pneumonitis expression ranged from 15.5 to 60.0% (median 39.8%). A significant, systematic dose–response relationship was found for partial lung infiltrate volumes per dose and density bin. A unimodal density distribution of partial lung infiltrate volumes was also found over the infiltrate density range of −1000 to 100 HU. This was determined using a mixed model that adjusted for random effects (p < 0.0001 for both effects, F-test). There was no interaction effect between systematic dose and infiltrate density dependence of the partial infiltrate volumes. The proportion of infiltrate volumes that are attributable to the systematic dose–response relation amounts to a mean of 16.6% of the total infiltrate volume per patient according to this model. Compared to patients with pneumonitis of grade ≤ 2, patients with high-risk pneumonitis had higher partial infiltrate volumes, particularly in the low-dose regions in five grade dose bins up to 20 Gy (AUC = 1.0, p < 0.0001, likelihood-ratio test). Conclusions: Dose-dependent and -independent partial lung infiltrate volumes were found in patients with high-grade pneumonitis after RT/CTx + IO. These results indicate that pneumonitis involves contributions from both radiochemotherapy-induced and immunotherapy-related mechanisms.

Protons are conventionally regarded as a low-linear energy transfer (low-LET) radiation modality with a relative biological effectiveness (RBE) of 1.1, suggesting direct mechanistic similarity to X-rays in the underpinning biological effects. However, exposure to spread-out Bragg peak (SOBP) protons reveals instructive deviations from this assumption. Indeed, proton beams have a maximum LET of ~5 keV/µm but display reduced reliance on classical non-homologous end joining (c-NHEJ) as well as an increased dependence on homologous recombination (HR) and alternative end joining (alt-EJ). These features are well described in cells exposed to high-LET radiation and typically manifest between 100 and 150 keV/µm. We hypothesized that this apparent discrepancy reflects biological consequences of proton-beam properties that remain uncharacterized. In the present study, we outline exploratory experiments aiming at uncovering such mechanisms. We begin by investigating for both entrance and SOBP protons the dose-dependent engagement of HR we recently showed for X-rays. Consistent with our previous findings with X-rays, HR engagement after exposure to both types of proton beams declined with dose, from ~80% at 0.2 Gy to less than 20% at higher doses. RAD51/γH2AX foci ratios, reflecting HR engagement, were modestly higher following proton irradiation, in line with increased HR utilization. G₂-checkpoint activation, previously linked to HR, was also stronger after exposure to protons, as was DNA end resection. Moreover, the formation of structural chromosomal abnormalities (SCAs) was higher for SOBP than entrance protons and X-rays. Collectively, our results suggest quasi-high-LET characteristics for proton beams and raise the question as to the physical proton properties that underpin them. We discuss that the commonly employed definition of LET may be insufficient for this purpose.

Summary of ATAC-seq sequence reads by sample.

HDAC3 functional loss radiosensitizes SCLC cell lines.

Cells with HDAC3 loss have increased generation and persistence of DNA DSBs.

Supplementary materials and methods.

Small cell lung cancer (SCLC) is an aggressive malignancy, with most patients presenting with prognostically poor extensive-stage disease. Limited progress in standard care stresses the urgent need for novel therapies. Radiotherapy offers some survival benefit for selected patients with SCLC but could be enhanced with radiosensitizers. In this study, we identify HDAC3 as a novel radiosensitizing target in SCLC using a CRISPR knockout screen and demonstrate its efficacy and mechanism. SBC5 cells were transduced with a custom EpiDrug single-guide RNA library and treated with ionizing radiation (IR) to identify radiosensitizing genes. HDAC3 emerged as a candidate and was validated through genetic knockdown and pharmacologic inhibition (RGFP966) in multiple SCLC cell lines. Both approaches enhanced radiosensitivity, as shown by cell viability (dose modification factor10 = 1.14–1.69) and clonogenic assays (dose modification factor10 = 1.16–1.41). We assessed changes in chromatin accessibility by assay for transposase-accessible chromatin using sequencing and IR-induced DNA damage and repair using γH2AX foci detection, double-strand break (DSB) repair assays, and immunoblotting of repair proteins. HDAC3-deficient cells exhibited increased chromatin accessibility, greater IR-induced DSBs, and impaired repair capacity, resulting in persistent DNA damage. This repair defect sensitized cells to PARP inhibitors, for which combining RGFP966 with olaparib or talazoparib produced additive to synergistic effects. In SCLC xenograft models, HDAC3 knockdown or RGFP966, combined with IR, achieved significant tumor growth inhibition. Collectively, we identified HDAC3 as a novel radiosensitizing target in SCLC. Its functional loss increased the generation and persistence of IR-induced DNA DSBs, effectively sensitizing SCLC cell lines and xenografts to IR, providing a potential radiosensitization strategy to treat SCLC.