**3. Implications/conclusions**

Collectively, the studies summarized in this chapter indicate that it will likely take more than just targeting a particular dysregulated DNA repair pathway in the context of chemotherapy to cure many relapsed and aggressive sarcomas. As mentioned above, targeting the dysregulated process of replication stress and genomic instability which promotes tumorigenicity in many cancers such as sarcoma is an area of intense interest [132]. The use of small molecule inhibitors that block not only DNA repair mechanisms but other global networks that may be connected to or independent of DNA repair mechanisms may be key to improving clinical outcomes. As such, our group used a systems biology approach to discover risk signatures and potential biomarkers of therapeutic response in pediatric adolescent and young

**155**

other human ailments.

**Acknowledgements**

*DNA Repair Defects in Sarcomas*

*DOI: http://dx.doi.org/10.5772/intechopen.94881*

adults with aggressive osteosarcoma. We found that the MYC-RAD21 copy number gain correlated with poor overall survival and was a potential marker of replication stress. We demonstrated that an increase in replication stress via a combination therapy consisting of BET and CHK1 inhibitors in xenograft models of pediatric and AYA osteosarcomas that have copy number gains of MYC and RAD21, was efficacious and well tolerated [126]. Furthermore, to obtain insight into other potential treatments where DNA repair inhibitors can be combined, numerous efforts have focused on investigating and understanding of the cross-talk between the various DNA damage-repair pathways as well as with the tumor microenvironment so that novel therapeutic combinations can be identified [133]. For instance, it has reported that hypoxic conditions within the tumor microenvironment impairs the fidelity of DNA repair pathways [133]. Furthermore, increased immune response to tumor neoepitopes have been observed in cancer with impaired/dysregulated DNA repair pathways [133]. Therefore, preclinical and clinical validation of using DNA repair inhibitors in combination with anti-hypoxic or immunomodulatory therapies warrants additional investigation. Notably, DNA repair mechanisms clearly contribute to tumor resistance [134]. In fact, one mechanism, by which tumor resistance is regulated involves cancer stem cells (CSC) which have increased DNA repair capacity [134]. Additionally, it has been reported that chromatin structure (euchromatin vs. heterochromatin) impacts the efficacy of DNA repair [135]. Thus, combination therapy targeting DNA repair pathways with agents targeting CSC or epigenetic proteins that regulate chromatin also require further evaluation. Several studies have shown associations between DNA repair pathways. With advancements in next-generation sequencing and use of precision genomics one clinical implication is that it may be possible to identify germline and/or somatic mutations involved in DNA repair proteins that could help delineate subsets of sarcoma patient-population that are predisposed to factors such as likelihood of getting the disease, onset of relapse/metastasis/recurrence, or possibility of therapeutic resistance to certain treatments [136]. Furthermore, within the patient population genetic polymorphisms associated with efficacy for DNA repair also become evident [137].

With progress in scientific technology, characterizing and profiling key components of the repair pathways is now more feasible. This results in increased preclinical validation studies using DNA repair inhibitors to improve therapeutic outcomes for otherwise therapeutically plateaued cancers like sarcomas. Development and implementation of novel therapeutic interventions involving DNA-repair proteins in combination with other targeted therapies and/or standard-of-care agents may help improve clinical outcomes in the patients. Furthermore, the role of DNA repair proteins and damaged cellular DNA are not only relevant in sarcomas but are pertinent to other cancers as well as contributing to the pathogenesis of many other diseases [138]. Therefore, identification of novel therapeutic combination involving DNA repair proteins is of high clinical value as it may be applicable for treating

The Tyler Trent Research Endowment for the Riley Hospital for Children IU-Health was founded in memory of Mr. Tyler Trent. Tyler battled osteosarcoma courageously. He will always be remembered for his inquisitive nature and strong advocacy for pediatric cancer research. Grainne and Clay Owen founded Curing Kids Cancer, a 501(c) nonprofit devoted to funding cutting edge pediatric cancer therapies, after they lost their son, Killian, to leukemia in 2003. He was nine years old. Since it was founded in 2005, Curing Kids Cancer has raised more than \$17 million

#### *DNA Repair Defects in Sarcomas DOI: http://dx.doi.org/10.5772/intechopen.94881*

*DNA - Damages and Repair Mechanisms*

and induce apoptosis [118].

at a higher level of evidence.

**3. Implications/conclusions**

arrest [118]. Phosphorylation of the CDC25 dual specificity phosphatase family mediated by CHK1 causes phosphatase degradation, resulting in increased phosphorylation and inhibition of multiple cyclin dependent kinase (CDK) proteins, positive regulators of the cell cycle [119]. In addition to CDC25 phosphatases, WEE1 kinase is phosphorylated and activated by CHK1, subsequently leading to the inhibitory phosphorylation of CDK1 [120]. It is therefore logical that inhibitors of CHK1 in cancer treatment could facilitate cell cycle progression with damaged DNA

Several preclinical studies and a few clinical studies demonstrate that CHK1 could be a therapeutic target in sarcoma treatment [115, 116, 121–124]. Larocheclary et al., conducted a study to investigate the role of CHK1 protein kinase in p53 mutant and wild-type STS [122]. They performed a systematic screening of a panel of 10 STS cell lines after combination treatment of CHK1 inhibitor (GDC-0575) with gemcitabine [122]. They showed that GDC-0575 induced apoptosis by abrogating DNA damage-induced S and G2–M checkpoints [122]. Moreover, they observed a synergistic or additive effect of GDC-0575 in combination with gemcitabine *in vitro* and *in vivo* in TP53-proficient but not in TP53-deficient sarcoma models [122]. Before conducting the mentioned study, they had analyzed the expression profile of a series of 339 complex genomics sarcomas and 108 translocation-related sarcomas, they showed that CHK1 expression is significantly associated with poor prognosis in sarcoma patients [125]. Moreover, they evaluated the efficacy of CHK1 inhibition in STS patients in a phase 1 clinical study with 3 STS patients (two with p53 mutation and one without p53 mutation) [122]. Two STS patients who had p53 mutation demonstrated promising response to the combination of gemcitabine and GDC-0575, while the other patient displayed no clinical benefit [122]. In conclusion, they provided pre-clinical and clinical evidence of the significance of CHK1 activity in STS and revealed that combination of CHK1 inhibitors with chemotherapy could be a promising treatment strategy for p53-mutant STS patients [122, 125]. There are also numbers of studies which have highlighted the important role of CHK1 activity in osteosarcoma progression and drug resistance and showed that CHK1 inhibitors either as a single agent or in combination with other drugs could be considered as a promising therapeutic target for treatment of osteosarcoma patients [126–129]. Regarding the role of CHK1 in Ewing sarcoma progression, some studies demonstrated that CHK1 protein is over-activated in Ewing sarcoma and showed that Ewing sarcoma cells are sensitive to CHK1 inhibitors either as a single agent or in combination with other drugs *in vivo* and *in vitro* [116, 121, 130, 131]. Further clinical investigations are needed to confirm whether treatment of sarcoma with CHK1 inhibition is efficacious therapeutic approach to improve sarcoma patient outcomes

Collectively, the studies summarized in this chapter indicate that it will likely take more than just targeting a particular dysregulated DNA repair pathway in the context of chemotherapy to cure many relapsed and aggressive sarcomas. As mentioned above, targeting the dysregulated process of replication stress and genomic instability which promotes tumorigenicity in many cancers such as sarcoma is an area of intense interest [132]. The use of small molecule inhibitors that block not only DNA repair mechanisms but other global networks that may be connected to or independent of DNA repair mechanisms may be key to improving clinical outcomes. As such, our group used a systems biology approach to discover risk signatures and potential biomarkers of therapeutic response in pediatric adolescent and young

**154**

adults with aggressive osteosarcoma. We found that the MYC-RAD21 copy number gain correlated with poor overall survival and was a potential marker of replication stress. We demonstrated that an increase in replication stress via a combination therapy consisting of BET and CHK1 inhibitors in xenograft models of pediatric and AYA osteosarcomas that have copy number gains of MYC and RAD21, was efficacious and well tolerated [126]. Furthermore, to obtain insight into other potential treatments where DNA repair inhibitors can be combined, numerous efforts have focused on investigating and understanding of the cross-talk between the various DNA damage-repair pathways as well as with the tumor microenvironment so that novel therapeutic combinations can be identified [133]. For instance, it has reported that hypoxic conditions within the tumor microenvironment impairs the fidelity of DNA repair pathways [133]. Furthermore, increased immune response to tumor neoepitopes have been observed in cancer with impaired/dysregulated DNA repair pathways [133]. Therefore, preclinical and clinical validation of using DNA repair inhibitors in combination with anti-hypoxic or immunomodulatory therapies warrants additional investigation. Notably, DNA repair mechanisms clearly contribute to tumor resistance [134]. In fact, one mechanism, by which tumor resistance is regulated involves cancer stem cells (CSC) which have increased DNA repair capacity [134]. Additionally, it has been reported that chromatin structure (euchromatin vs. heterochromatin) impacts the efficacy of DNA repair [135]. Thus, combination therapy targeting DNA repair pathways with agents targeting CSC or epigenetic proteins that regulate chromatin also require further evaluation. Several studies have shown associations between DNA repair pathways. With advancements in next-generation sequencing and use of precision genomics one clinical implication is that it may be possible to identify germline and/or somatic mutations involved in DNA repair proteins that could help delineate subsets of sarcoma patient-population that are predisposed to factors such as likelihood of getting the disease, onset of relapse/metastasis/recurrence, or possibility of therapeutic resistance to certain treatments [136]. Furthermore, within the patient population genetic polymorphisms associated with efficacy for DNA repair also become evident [137].

With progress in scientific technology, characterizing and profiling key components of the repair pathways is now more feasible. This results in increased preclinical validation studies using DNA repair inhibitors to improve therapeutic outcomes for otherwise therapeutically plateaued cancers like sarcomas. Development and implementation of novel therapeutic interventions involving DNA-repair proteins in combination with other targeted therapies and/or standard-of-care agents may help improve clinical outcomes in the patients. Furthermore, the role of DNA repair proteins and damaged cellular DNA are not only relevant in sarcomas but are pertinent to other cancers as well as contributing to the pathogenesis of many other diseases [138]. Therefore, identification of novel therapeutic combination involving DNA repair proteins is of high clinical value as it may be applicable for treating other human ailments.
