A signature of ionizing radiation exposure is the induction of DNA clustered damaged sites, defined as two or more lesions within one to two helical turns of DNA by passage of a single radiation track. Clustered damage is made up of double strand breaks (DSB) with associated base lesions or abasic (AP) sites, and non-DSB clusters comprised of base lesions, AP sites and single strand breaks. This review will concentrate on the experimental findings of the processing of non-DSB clustered damaged sites. It has been shown that non-DSB clustered damaged sites compromise the base excision repair pathway leading to the lifetime extension of the lesions within the cluster, compared to isolated lesions, thus the likelihood that the lesions persist to replication and induce mutation is increased. In addition certain non-DSB clustered damaged sites are processed within the cell to form additional DSB. The use of E. coli to demonstrate that clustering of DNA lesions is the major cause of the detrimental consequences of ionizing radiation is also discussed. The delayed repair of non-DSB clustered damaged sites in humans can be seen as a “friend”, leading to cell killing in tumour cells or as a “foe”, resulting in the formation of mutations and genetic instability in normal tissue.

PURPOSE We present 5-year results from CheckMate 227 Part 1, in which nivolumab plus ipilimumab improved overall survival (OS) versus chemotherapy in patients with metastatic non-small cell lung cancer (mNSCLC), regardless of tumor programmed death ligand 1 (PD-L1) status. METHODS Adults with stage IV/recurrent NSCLC without EGFR mutations or ALK alterations and with tumor PD-L1 ≥1% or <1% (N=1739) were randomized. Patients with tumor PD-L1 ≥1% were randomized to first-line nivolumab plus ipilimumab, nivolumab alone, or chemotherapy. Patients with tumor PD-L1 <1% were randomized to nivolumab plus ipilimumab, nivolumab plus chemotherapy, or chemotherapy. Endpoints included exploratory 5-year results for efficacy, safety, and quality of life (QoL). RESULTS At 61.3 months’ minimum follow-up, 5-year OS rates were 24% versus 14% for nivolumab plus ipilimumab versus chemotherapy (PD-L1 ≥1%), and 19% versus 7% (PD-L1 <1%). Median duration of response was 24.5 versus 6.7 months (PD-L1 ≥1%) and 19.4 versus 4.8 months (PD-L1 <1%). Among patients surviving 5 years, 66% (PD-L1 ≥1%) and 64% (PD-L1 <1%) were off nivolumab plus ipilimumab without initiating subsequent systemic anticancer treatment by the 5-year time point. Survival benefit continued following nivolumab plus ipilimumab discontinuation due to treatment-related adverse events, with a 5-year OS rate of 39% (combined PD-L1 ≥1% and <1% populations). QoL in 5-year survivors treated with nivolumab plus ipilimumab was similar to the general US population through 5 years’ follow-up. No new safety signals were observed. CONCLUSION With all patients off immunotherapy treatment for ≥ 3 years, nivolumab plus ipilimumab increased 5-year survivorship versus chemotherapy, including long-term, durable clinical benefit regardless of tumor PD-L1 expression. These data support nivolumab plus ipilimumab as an effective first-line treatment for patients with mNSCLC.

Ionising radiation induces clustered DNA damage sites which pose a severe challenge to the cell’s repair machinery, particularly base excision repair. To date, most studies have focussed on two-lesion clusters. We have designed synthetic oligonucleotides to give a variety of three-lesion clusters containing abasic sites and 8-oxo-7, 8-dihydroguanine to investigate if the hierarchy of lesion processing dictates whether the cluster is cytotoxic or mutagenic. Clusters containing two tandem 8-oxoG lesions opposing an AP site showed retardation of repair of the AP site with nuclear extract and an elevated mutation frequency after transformation into wild-type or mutY Escherichia coli. Clusters containing bistranded AP sites with a vicinal 8-oxoG form DSBs with nuclear extract, as confirmed in vivo by transformation into wild-type E. coli. Using ung1 E. coli, we propose that DSBs arise via lesion processing rather than stalled replication in cycling cells. This study provides evidence that it is not only the prompt formation of DSBs that has implications on cell survival but also the conversion of non-DSB clusters into DSBs during processing and attempted repair. The inaccurate repair of such clusters has biological significance due to the ultimate risk of tumourigenesis or as potential cytotoxic lesions in tumour cells.

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