# CRISPR-Cas Systems: Prospects for Use in Medicine
## 3 Genome Editing
### Application in Gene Therapy  
CRISPR-Cas systems are of considerable interest in the treatment of inherited disorders. Gene therapy implies making changes (correcting mutations, adding genes, or altering their expression) in the genome in order to cure diseases or eliminate a predisposition to them. It can be carried out both at the stage of embryo formation (fetal gene therapy) or in somatic cells of an adult patient. Fetal gene therapy has both pros (the introduction of corrective changes at an early stage leads to their transmission to almost all body cells) and cons--even a single mistake, caused by the off-target effect of the currently implied CRISPR-Cas systems, made at this stage can become fatal. Thus, the application of state-of-the-art genome editing techniques for fetal gene therapy raises multiple ethical questions. However, in 2018, fetal gene therapy was performed to confer HIV resistance by modifying the CCR5 receptor. The two children, born as a result of the experiment, were reported to be healthy and have an innate resistance to HIV, but these results have caused fierce polemics and both their ethical and scientific aspects are still debated [69]. In this respect, somatic cell therapy does not present such an ethical dilemma. As of today, the treatment of inherited disorders such as hemophilia, Duchenne muscular dystrophy, and rhinitis pigmentosa has been carried out on model animals [70; 71; 72; 73; 74]. Mutations in the \(\beta\)-globin (\(HBB\)) gene lead to thalassemia, a type of anemia. It can be cured either by editing \(HBB\) using CRISPR-Cas9 or by reactivating fetal hemoglobin (\(HbF\)) expression by knocking out the \(BCL11A\) gene, which suppresses \(HbF\) expression [75; 76]. The sickle-cell anemia, an inherited blood disease, upon which the normal hemoglobin A due to an SNP (L-glutamic acid in position 6 of the \(\beta\)-chain globin is substituted for L-valine) acquires a special shape, hemoglobin S, prone to crystallization instead of the formation of normal quaternary structure, can also be treated with CRISPR-Cas9 genome editing [75]. The use of CRISPR-Cas is also very promising in cancer therapy. Cancer is currently the second leading cause of death after cardiovascular diseases, and is quite often not responding well to chemotherapy [77]. On the one hand, CRISPR-Cas can be used to overcome acquired drug resistance in tumors; on the other hand, CRISPR-Cas can be used directly for targeted knock-out of oncogenes--genes whose expression can stimulate the formation of a malignant tumor. When proto-oncogene (a common gene capable of converting into an oncogene) becomes an oncogene as a result of mutations, CRISPR-Cas can precisely target malignant cells [78]. In addition, a number of cancers are associated with viruses, such as the human papillomavirus (HPV), which may cause cervical cancer. The HPV16 genes E6 and E7 are responsible for viral replication in epithelial cells and have oncogenic activity by activating the telomerase and suppressing the antitumor suppressor pRB [79]. Specific sgRNAs targeting the E6 and E7 genes blocks their expression and leads to the destruction of the virus [80]. However, genome-edited cells themselves were reported to be able to undergo malignant transformation. CRISPR-Cas-mediated DSBs were shown to activate the p53 signaling pathway in two independent studies [81; 82]. Although there is still no direct evidence to support the link between CRISPR-Cas-mediated genome editing and carcinogenesis, further research is required to eliminate even minimal risks before genomic editing can be applied to humans. Current successful applications of genome editing in humans include the use of TALEN in the treatment of CD19+ acute lymphoblastic leukemia in an 11-month-old infant, and ZFNs for the treatment of type II mucopolysaccharidescharidosis (Hunter's syndrome) [83; 84].