MSH6, a key component of the MSH2–MSH6 complex, plays a fundamental role in the repair of mismatched DNA bases. Herein, we report that MSH6 is a novel Ku70-interacting protein identified by yeast two-hybrid screening. Ku70 and Ku86 are two key regulatory subunits of the DNA-dependent protein kinase, which plays an essential role in repair of DNA double-strand breaks (DSBs) through the non-homologous end-joining (NEHJ) pathway. We found that association of Ku70 with MSH6 is enhanced in response to treatment with the radiomimetic drug neocarzinostatin (NCS) or ionizing radiation (IR), a potent inducer of DSBs. Furthermore, MSH6 exhibited diffuse nuclear staining in the majority of untreated cells and forms discrete nuclear foci after NCS or IR treatment. MSH6 colocalizes with γ-H2AX at sites of DNA damage after NCS or IR treatment. Cells depleted of MSH6 accumulate high levels of persistent DSBs, as detected by formation of γ-H2AX foci and by the comet assay. Moreover, MSH6-deficient cells were also shown to exhibit impaired NHEJ, which could be rescued by MSH6 overexpression. MSH6-deficient cells were hypersensitive to NCS- or IR-induced cell death, as revealed by a clonogenic cell-survival assay. These results suggest a potential role for MSH6 in DSB repair through upregulation of NHEJ by association with Ku70.

External-beam radiotherapy plays a critical role in the treatment of most pediatric solid tumors. Particularly in children, achieving an optimal therapeutic index to avoid damage to normal tissue is extremely important. Consequently, in metastatic disease, the utility of external-beam radiotherapy is limited. Molecular radiotherapy with tumor-targeted radionuclides may overcome some of these challenges, but to date there exists no single cancer-selective agent capable of treating various pediatric malignancies independently of their histopathologic origin. We tested the therapeutic potential of the clinical-grade alkyl-phospholipid ether analog CLR1404, 18-(-iodophenyl)octadecyl phosphocholine, as a scaffold for tumor-targeted radiotherapy of pediatric malignancies. Uptake of CLR1404 by pediatric solid tumor cells was tested in vitro by flow cytometry and in vivo by PET/CT imaging and dosimetry. The therapeutic potential ofI-CLR1404 was evaluated in xenograft models. In vitro, fluorescent CLR1404-BODIPY showed significant selective uptake in a variety of pediatric cancer lines compared with normal controls. In vivo tumor-targeted uptake in mouse xenograft models usingI-CLR1404 was confirmed by imaging. Single-dose intravenous injection of I-CLR1404 significantly delayed tumor growth in all rodent pediatric xenograft models and extended animal survival while demonstrating a favorable side effect profile.I-CLR1404 has the potential to become a tumor-targeted radiotherapeutic drug with broad applicability in pediatric oncology. Because I-CLR1404 has entered clinical trials in adults, our data warrant the development of pediatric clinical trials for this particularly vulnerable patient population.

Antitumor alkyl phospholipid (APL) analogs comprise a group of structurally related molecules with remarkable tumor selectivity. Some of these compounds have shown radiosensitizing capabilities. CLR127 is a novel, clinical-grade antitumor APL ether analog, a subtype of synthetic APL broadly targeting cancer cells with limited uptake in normal tissues. The purpose of this study was to investigate the effect of CLR127 to modulate radiation response across several adult and pediatric cancer types as well as in murine xenograft models of human prostate adenocarcinoma, neuroblastoma, Ewing sarcoma, and rhabdomyosarcoma., CLR127 demonstrated selective uptake in cancer cells compared to normal cells. In cancer cells, CLR127 treatment prior to radiation significantly decreased clonogenic survival , and led to increased radiation-induced double-stranded DNA (dsDNA) breakage compared with radiation alone, which was not observed in normal controls. In animal models, CLR127 effectively increased the antitumor response to fractionated radiotherapy and led to delayed tumor regrowth at potentially clinically achievable doses. In conclusion, our study highlights the ability of CLR127 to increase radiation response in several cancer types. Given almost universal uptake of CLR127 in malignant cells, future research should test whether the observed effects can be extended to other tumor types. Our data provide a strong rationale for clinical testing of CLR127 as a tumor-targeted radiosensitizing agent..

No abstract