NHEJ Inhibitor SCR7 and its Different Forms: Promising CRISPR Tools for Genome Engineering

Ujjayinee Ray, Supriya V. Vartak, Sathees C. Raghavan PII: S0378-1119(20)30666-1
Reference: GENE 144997

To appear in: Gene Gene
Received Date: 27 April 2020
Accepted Date: 21 July 2020

Please cite this article as: U. Ray, S.V. Vartak, S.C. Raghavan, NHEJ Inhibitor SCR7 and its Different Forms: Promising CRISPR Tools for Genome Engineering, Gene Gene (2020), doi: 2020.144997

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The CRISPR-Cas system currently stands as one of the best multifaceted tools for site-specific genome engineering in mammals. An important aspect of research in this field focusses on improving the specificity and efficacy of precise genome editing in multiple model systems. The cornerstone of this mini-review is one of the extensively investigated small molecule inhibitor, SCR7, which abrogates NHEJ, a Ligase IV-dependent DSB repair pathway, thus guiding integration of the foreign DNA fragment via the more precise SCR7, an inhibitor of nonhomologous DNA end joining.

In 2012, we reported a novel small molecule inhibitor, SCR7, which abrogated nonhomologous DNA end joining (NHEJ), the major DNA double-strand break repair pathway in mammals (Srivastava et al., 2012). SCR7 specifically bound to the DNA binding domain (DBD) of Ligase IV, thus preventing its binding to broken chromatin, and subsequently abolishing end-joining. NHEJ is often upregulated in many cancers which helps them evade several chemo and radiotherapy treatment modalities (Srivastava et al., 2012; Srivastava and Raghavan, 2015; Vartak and Raghavan, 2015). Interestingly, SCR7 caused tumor regression in three out of four mouse tumor models tested, without significant side-effects. One of the most important observations that came out from the study was the excellent combinatorial potential of SCR7 with radio and chemotherapeutic agents that introduce DNA strand-breaks as intermediates in their path of action (Srivastava et al., 2012). Further research on SCR7 resulted in water-soluble version of SCR7 prepared by encapsulating it using a pluronic copolymer nanomaterial (John et al., 2015a; John et al., 2015b). Encapsulated SCR7 showed ~5 times more efficacy than the parental compound, possibly owing to the increased bioavailability of the molecule.

SCR7 soon became the molecule of choice for studies involving biochemical inhibition of the NHEJ pathway as it inhibited DNA end joining in a Ligase IV dependent manner. In two independent studies, authors characterized the regulation and dynamics between broken chromatin ends, and Ligase IV-XRCC4-XLF interaction at these sites, using sophisticated single-molecule techniques (Reid et al., 2017; Reid et al., 2015). Both the studies employed SCR7 as the bona fide biochemical inhibitor of Ligase IV, to abrogate NHEJ. A recent study reported the role of BLM helicase in regulating homologous recombination (HR) and NHEJ pathways in a cell cycle phase-specific manner (Tripathi et al., 2018). Elegant experiments using specific inhibitors (SCR7 against NHEJ, ML216 and B02 against HR), revealed that BLM primarily mediated the recruitment of HR and NHEJ factors onto the broken chromatin, during the S and G1 phase, respectively. In an independent study, authors employed SCR7-mediated NHEJ inhibition as one of the strategies to validate that chromosome territory relocation during DSB repair inside the cell is a process mediated by early DDR sensing mechanisms such as -H2AX signalling, and does not depend on late events of the pathway (Kulashreshtha et al., 2016).

SCR7 in cancer therapy

The encouraging findings of SCR7 as a potential chemotherapeutic agent have given boost to further investigations in this area. Studies revealed that SCR7 can be used either as a single agent, or in combination with other routinely used therapeutic modalities. Our studies showed that SCR7 enhanced the action of radiation in cells, both in vitro and in vivo (VG/SCR, under consideration). This observation is clinically important as SCR7 contributed to bring down the dose of radiation employed during cancer therapy, thus alleviating IR- induced side effects, a principle concern in recent radiotherapy regimes. In a different study, we observed that SCR7 could improve the efficacy of temozolomide in glioblastoma primary patient cells (unpublished). Besides, Gkotzamanidou and colleagues reported that a combination of DSB repair inhibitors (NU7026, SCR7 for NHEJ, and RS-1 for HR) with melphalan worked remarkably well against multiple myeloma (MM) patient cells, SCR7 showing the best effect (Gkotzamanidou et al., 2016). This study also gives hope for melphalan resistant cancer patient treatment, wherein use of these inhibitors can revive sensitivity of cancer cells to chemotherapy. Further, SCR7 could inhibit proliferation of diffuse large B cell lymphoma (DLBCL) and potentiate the effect of ionizing radiation (Gopalakrishnan, 2018). In cervical cancer, enhanced cytotoxicity (15-50%) was observed, when SCR7 was used along with low dose of genotoxic drug doxorubicin (Kumar et al., 2017).

In another interesting study, authors proposed early embryonic Zebra fish cells as a model system to screen potent NHEJ inhibitors, by taking SCR7 as one of the examples (Yang et al., 2018). By introducing a SV40 transcriptional terminator in embryonic cells, authors observed that embryonic lethality in these cells was alleviated upon SCR7 treatment, hence serving as a screening methodology for other NHEJ inhibitors as potent anti-cancer agents. More recently, SCR7 was reported to evade HSP-110 induced resistance to oxaliplatin in vitro as well as in patient biopsies (Causse et al., 2019). Abolishment of NHEJ in oxaliplatin and 5-fluorouracil resistant colorectal cancer with SCR7 is a promising avenue for further exploration in clinical cancer therapy.

One of the road blocks for SCR7 to reach the clinics is the high IC50 (in micromolar range) observed in cancer cell lines. However, its property to cause tumor regression in mouse models at a dose of just 10 mg/kg body weight (6 doses), without signs of relapse, is remarkable (Srivastava et al., 2012), and warrants further investigation to take this molecule forward.

SCR7 exists in different forms

Careful examination of the structure of SCR7 revealed that the two imine rings, in the structure make the molecule less stable, and it can get autocyclized to a more stable cyclized form of SCR7 (Figure 1). Chemical characterization showed that parental SCR7 and the cyclized version are one and the same molecule, having the same molecular weight, molecular formula, melting point and number of protons (Vartak et al., 2018). Upon oxidation, the cyclized form gets converted to SCR7-pyrazine, which deviates with respect to molecular weight, molecular formula, melting point and number of protons (Vartak et al., 2018). (Molecular weight difference: 332 for SCR7-pyrazine, and 334 for SCR7) (Figure 1). Biological characterization revealed that both SCR7 and SCR7-pyrazine inhibit NHEJ in a Ligase IV dependent manner (Vartak et al., 2018), unlike reported by another study (Greco et al., 2016). Both the molecules were potent inhibitors of residual DSB repair in cells, thereby causing accumulation of unrepaired DSBs, ultimately leading to cell death. However, unlike SCR7, SCR7-pyrazine is less specific inside the cell as it showed sensitivity in Ligase IV null cells (Vartak et al., 2018). These observations are important and warrant special attention because of the increasing number of companies selling SCR7 and its forms, which reflects its increasing usage in CRISPR-Cas mediated precise genome editing by several labs.

More recently, a water-soluble version of SCR7 (sodium salt of SCR7) was also identified, which can block NHEJ and induce cell death in a Ligase IV dependent manner (UR/SCR, under consideration). Water-soluble form of SCR7-pyrazine viz. sodium salt of SCR7 pyrazine (Na-SCR7-P) was also recently characterized (Pandey, 2019). Although it effectively prevented tumor progression in mice, it inhibited end joining catalysed by all three DNA ligases. Keeping these observations in mind, the different forms should ideally work in enhancing the efficiency of CRISPR-Cas mediated precise gene editing, however SCR7 would be more specific.

Further, a novel SCR7-based small molecule inhibitor of NHEJ, SCR130 was identified recently, which showed 20 fold better efficacy compared to SCR7 in inducing cytotoxicity in a Ligase IV-dependent manner within cells (Ray et al., 2020). SCR130 inhibited NHEJ with higher efficacy, and potentiated effect of radiation.

SCR7 in CRISPR-Cas mediated precise genome editing

CRISPR, abbreviated from Clustered Regularly Interspaced Short Palindromic Repeats, is an efficient and robust genome editing tool to render targeted modification of a gene (Jinek et al., 2012; Mali et al., 2013). The success of CRISPR-Cas9 mediated precise genome editing profoundly depends on how the DNA is cut and pasted. To insert a foreign DNA fragment into the genome, Cas9 should first make a target specific scission in the genome, followed by joining of the fragment, designed such that it is homologous to the flanking sequences of the joining junction (Barrangou and Doudna, 2016; Hsu et al., 2014; Jiang and Doudna, 2017). Repair can be mediated by one of the two double-strand break repair pathways; HR, which functions during the S and G2 phase of the cell cycle, and NHEJ which is active throughout the cell cycle (Deriano and Roth, 2013; Lieber, 2010). Although, the frequency of joining by NHEJ is 3 times higher than that by HR (Mao et al., 2008), this would not be the pathway of choice for precise genome editing, owing to the error-prone nature of NHEJ. Hence, decreasing NHEJ frequency/efficiency would push cells to take up the precise joining through HR, thereby ensuring maximum precision (Figure 2). Although KU inhibition or uses of DNA-Pkcs inhibitors have been reported to increase homology directed repair (HDR), these have not been used in genome editing (Fattah et al., 2008; Riesenberg and Maricic, 2018; Yeh et al., 2019). In 2015, an independent study demonstrated that SCR7 (5-100 μM) treatment tipped the balance of DSB repair in favour of HR in single-celled mouse embryos, in which the HR frequency was increased up to 10-fold in resulting pups without any adverse effect on blastocyst (Singh et al., 2015). The idea originated from genome editing in Drosophila embryos where Zinc finger nuclease and donor DNA increased HR by 70% in Ligase IV mutant cases (Beumer et al., 2008). This interesting strategy was further supported by two independent reports (Chu et al., 2015; Maruyama et al., 2015). Using three distinct approaches (use of small molecule inhibitor SCR7, Ligase IV gene silencing and expression of adenovirus proteins) to turn off/reduce NHEJ efficiency, authors demonstrated as a proof of principle that the efficiency could be improved by 4-19 fold, in mammalian cell lines and mice (Table 1). While Adenoviral 4 (Ad4) proteins E1B55k and E4orf6 affected NHEJ 93% by ubiquitinylating and degrading Ligase IV, SCR7 mediated inhibition was 70% (Chu et al., 2015). Cell synchronisation, together with regulated delivery of Cas9 and SCR7 have also demonstrated an increase in HR (Ma et al., 2016). In a contrasting report, in G2/M arrested hPSCs, the efficiency did not increase further upon addition of SCR7 (Yang et al., 2016).

In porcine fetal fibroblasts, SCR7 could boost knock-in efficiency by increasing HDR 2-fold (Li et al., 2017). The frequency of knock-in also increased twice that of control conditions in cell lines post treatment. Furthermore, a coalition of Rad52 expression and SCR7 treatment significantly improved CRISPR-mediated HR by about 40% in Saccharomyces cerevisiae (Shao et al., 2017). Counteracting NHEJ with SCR7, thereby increasing HDR was also investigated in genome editing of Herpes Simplex Virus Type-I (HSV-1) in human cells. This provided the first report of simultaneous replacement of two copies of a gene (ICP10) located at different positions of viral genome (double replacement) using CRISPR and SCR7, thus proving useful for the development of oncolytic virus (Lin et al., 2016). Another study elucidated increase in targeted efficiency approximately by 3-fold in cancer cell lines in presence of SCR7, single-stranded oligonucleotides and CRISPR/Cas9/eGFP coexpression vector (Hu et al., 2018). Directed mutation was corrected in GFP-silent MCF7/GFP-Mut-2 cell line and β-catenin mutated (∆Ser45) colon cancer cell line (HCT-116) in presence of SCR7, at concentrations as low as 5 and 10 μM. Since 10- 50% colorectal patients are reported to bear such β-catenin mutation, combinatorial effect of SCR7 and CRISPR will be a potential therapeutic improvement. 1.5-2 fold decrease in NHEJ by SCR7 was also reported to stimulate Cas9 mediated genome editing (Robert et al., 2015) (Table 1). Recently, in both iPSC and ESC, a ‘CRISPY’ mix containing SCR7 along with other small molecules (NU7026, Trichostatin A, AICAR, RS1, etc.) have been reported to successfully increase genome editing 2.8-6.7 fold alongside Cas9n and 2.9-4 fold alongside Cpf1 (Riesenberg and Maricic, 2018) (Table 1).

NHEJ is an evolutionarily conserved pathway for DSB repair. Small molecule NHEJ inhibitors originally designed against human proteins can thus be used in other organisms owing to the conserved nature of DSB repair proteins (Aravind et al., 1999; Cole et al., 2010). Recent studies have harnessed SCR7 for CRISPR-Cas genome editing in various model organisms such as Zebra fish and Xenopus, apart from rodents and mammalian cells (Table 1). In an interesting study, authors reported CRISPR-mediated point mutations in Zebra fish, and developed efficient ways to screen mutant individuals from those harbouring whole gene knockouts with increased efficiency (Zhang et al., 2018). While individual treatments of SCR7 and RS-1 (RAD51 stimulator) showed enhanced HR efficiencies of ~55 and 24% respectively, a combination of both exhibited HR efficiency of ~74%. Studies have also reported RS-1 being more potent than SCR7 (Pinder et al., 2015; Song et al., 2016), stressing the system-to-system variation in this technology. Besides SCR7, SCR7-pyrazine has also been employed in CRISPR-Cas mediated gene editing techniques in recent studies. In 2017, Killian and group devised a simple and effective colony count based technique for characterization of gene editing events. A combination of Diphtheria toxin and Puromycin selection enabled quantification comparison of the specificity and efficacy of gene inactivation and cassette insertion events (Killian et al., 2017). SCR7-pyrazine added 4 hours prior to transfection significantly increased the number of integrations as compared to vehicle control samples, thus highlighting the efficacy of the pyrazine form in abrogating NHEJ, and redirecting the DNA end-joining process toward the more precise HR pathway. In another elegant study on Xenopus oocytes, authors showed that HR-mediated knock-in is feasible in this model system, and that use of SCR7-pyrazine to inhibit NHEJ can increase the efficiency of HR (Aslan et al., 2017). Treatment of SCR7-pyrazine at the oocyte stage increased the rate of successful knock in for two independent genes tested by an efficiency of ~7.4 to 22% respectively. However, embryo stage samples treated with SCR7 did not respond in a similar manner owing to the toxicity, which warrants further investigation. Deeper insights into this area of research in the future will help to bring about a revolution in disease modelling in Xenopus, by creating patient-specific mutations for several human diseases.


The CRISPR-Cas technology has revolutionized genome engineering in laboratories as well as clinics. One of the main advantages of using small molecule inhibitors in CRISPR-Cas technology is their chemical amenability that can subsequently influence their biological functions inside a cell. In conclusion, SCR7 and its several forms apart from being potent biochemical inhibitors and cancer therapeutic agents, can efficiently tackle the on-target non- specificity of CRISPR by tipping the balance of DSB repair toward the more precise HR pathway.


We thank Bibha Choudhary, Mridula Nambiar, Mrinal Srivastava and other members of SCR laboratory for critical reading and comments on the manuscript. This work was supported by grants from CEFIPRA (IFC/5203-4/2015/131), DAE (21/01/2016- BRNS/35074), DBT Glue-Grant (BT/PR23078/MED/29/1253/2017), IISc-DBT partnership programme (BT/PR27952-INF/22/212/2018) to SCR. UR is supported by Senior Research Fellowship (SRF) from CSIR, India.

Conflict of interest

Authors disclose that there is no conflict of interest.


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Figure Legends

Table 1. Summary of studies employing SCR7 and SCR7-pyrazine for enhancing the efficiency of CRISPR-Cas mediated precise genome editing in various model systems.Figure 1. Different forms of the Ligase IV inhibitor, SCR7. Less stable parental SCR7 can get autocyclized into a more stable form (SCR7-cyclized) having the same molecular weight, molecular mass, number of protons and melting point. Upon dehydrogenation, SCR7-cyclized gets converted into SCR7-pyrazine, possessing a different molecular weight, molecular mass, number of protons and melting point.

Figure 2. Impact of DSB repair inhibitors on genome editing. During the process of CRISPR-Cas9 mediated genome editing, the donor DNA fragment is joined with the parental DNA strands via two major DNA double-strand break (DSB) repair pathways, viz. NHEJ and HR. Majority (70%) of the breaks are repaired via the less precise NHEJ pathway (dark arrow), whereas a minor portion (30%) through the more precise HR pathway (light arrow). Small molecule inhibitors of NHEJ can abrogate binding of proteins to DNA break site. Treatment of SCR7 or SCR7-pyrazine abrogates Ligase IV function, thereby inhibiting NHEJ. Repair balance is now tipped towards HR, leading to increased precision. L755507 and RS-1 may also be used to increase HDR frequency.