**4. Targeting mitochondria for cancer therapy**

Numerous studies suggested that mtDNA alterations may contribute to chemotherapy resistance and affect radiotherapy outcome. For instance, Guerra et al. [88] showed that mutations in the NADH dehydrogenase subunit 4 (MT-ND4) lead to acquired chemoresistance during treatment with paclitaxel carboplatin.

In the last few years, spindle transfer, a promising emerging strategy aimed at generating clinical germline gene therapy against inherited mitochondrial disorders, has supported the idea of a possible gene therapy approach for the editing of somatic mtDNA alterations [89]. Ideally, repairing the mutated mtDNA sequence would also restore the normal mitochondrial function and likely induce tumor regression. Taylor et al. [90] proposed a strategy that aimed to specifically block the replication of the mutant mtDNA by peptide nucleic acid (PNA), thereby allowing the selective propagation of the wild-type DNA. Moreover, mitochondrial dysfunction might also be restored by stimulating the mitophagy process in order to eliminate the deleterious mtDNA variants [91]. Targeting DNA repair enzymes to mitochondria may be a suitable strategy to correct mtDNA mutations. For instance, cell transfection with an expression vector containing the gene coding the DNA repair enzyme human 8-oxoguanine DNA glycosylase/apurinic lyase (hOGG1) has been used to reduce free fatty acids (FFAs)-induced mtDNA damage [92]. Furthermore, overexpression of hOGG1 in mitochondria has been shown to attenuate breast cancer progression and metastasis in transgenic mice [93]. Although hOGG1 has been the most frequently employed enzyme to enhance mtDNA repair, alternative strategies targeting other proteins transferred to mitochondria, such as endonuclease III (EndoIII) and endonuclease VIII (EndoVIII), have been proposed in the last years [94–96]. Other therapeutic approaches for patients carrying mtDNA mutations are based on allotopic gene expression, as preliminary demonstrated in different mitochondrial disorders [97], and targeted restriction endonucleases. In this regard, SmaI and PstI have been used as a powerful tool for treatment of mitochondrial dysfunction, resulting in the elimination of the mutant mtDNA and restoration of normal mitochondrial functionality [98]. In the last decade, many other approaches and compounds targeting dysfunctional mitochondria have been experienced, such as signal peptides. Lipophilic cations, cell-penetrating peptides and nanoparticles. A promising approach is based on the reprogramming of energy metabolism in colorectal cancer cells, through specific mitochondria-targeting agents, such as the second-generation rosamine analogs that target complex II and ATP synthase activities of the mitochondrial oxidative phosphorylation pathway [99]. More recently, it has been argued that mitochondria of tumor-initiating cells (TICs), which play a prominent role in cancer initiation, metastasis and resistance to therapy, may be targeted by mitocan vitamin E succinate in a complex II-dependent manner [100]. Another original approach has been developed to trigger cell death signaling pathways in colorectal cancer cells [101], such as ROS-dependent apoptosis and autophagy [102]. The recent improvement of high-throughput drug-screening platforms allowed the identification of novel non-toxic mitochondrial inhibitors, as in the case of diphenyleneiodonium chloride (DPI), a strong inhibitor of mitochondrial complex I and II flavin-containing enzymes, which effectively depletes cancer stem-like cells (CSCs), one of the main drivers of poor clinical outcome in a wide variety of tumor types and especially in advanced disease states [103]. Interestingly, mitochondrial inhibition with VLX600 has also been proposed in combination with imatinib in the treatment of drug-resistant gastrointestinal stromal tumors (GISTs) [104].

Moreover, oxidative stress may also affect the expression of nuclear genes involved in tumorigenic and invasive phenotypes [87]. Altogether these findings suggest that targeting the retro-

Numerous studies suggested that mtDNA alterations may contribute to chemotherapy resistance and affect radiotherapy outcome. For instance, Guerra et al. [88] showed that mutations in the NADH dehydrogenase subunit 4 (MT-ND4) lead to acquired chemoresistance during

In the last few years, spindle transfer, a promising emerging strategy aimed at generating clinical germline gene therapy against inherited mitochondrial disorders, has supported the idea of a possible gene therapy approach for the editing of somatic mtDNA alterations [89]. Ideally, repairing the mutated mtDNA sequence would also restore the normal mitochondrial function and likely induce tumor regression. Taylor et al. [90] proposed a strategy that aimed to specifically block the replication of the mutant mtDNA by peptide nucleic acid (PNA), thereby allowing the selective propagation of the wild-type DNA. Moreover, mitochondrial dysfunction might also be restored by stimulating the mitophagy process in order to eliminate the deleterious mtDNA variants [91]. Targeting DNA repair enzymes to mitochondria may be a suitable strategy to correct mtDNA mutations. For instance, cell transfection with an expression vector containing the gene coding the DNA repair enzyme human 8-oxoguanine DNA glycosylase/apurinic lyase (hOGG1) has been used to reduce free fatty acids (FFAs)-induced mtDNA damage [92]. Furthermore, overexpression of hOGG1 in mitochondria has been shown to attenuate breast cancer progression and metastasis in transgenic mice [93]. Although hOGG1 has been the most frequently employed enzyme to enhance mtDNA repair, alternative strategies targeting other proteins transferred to mitochondria, such as endonuclease III (EndoIII) and endonuclease VIII (EndoVIII), have been proposed in the last years [94–96]. Other therapeutic approaches for patients carrying mtDNA mutations are based on allotopic gene expression, as preliminary demonstrated in different mitochondrial disorders [97], and targeted restriction endonucleases. In this regard, SmaI and PstI have been used as a powerful tool for treatment of mitochondrial dysfunction, resulting in the elimination of the mutant mtDNA and restoration of normal mitochondrial functionality [98]. In the last decade, many other approaches and compounds targeting dysfunctional mitochondria have been experienced, such as signal peptides. Lipophilic cations, cell-penetrating peptides and nanoparticles. A promising approach is based on the reprogramming of energy metabolism in colorectal cancer cells, through specific mitochondria-targeting agents, such as the second-generation rosamine analogs that target complex II and ATP synthase activities of the mitochondrial oxidative phosphorylation pathway [99]. More recently, it has been argued that mitochondria of tumor-initiating cells (TICs), which play a prominent role in cancer initiation, metastasis and resistance to therapy, may be targeted by mitocan vitamin E succinate in a complex II-dependent manner [100]. Another original approach has been developed to trigger cell death signaling pathways in colorectal cancer cells [101], such as ROS-dependent apoptosis and autophagy [102]. The recent improvement of high-throughput drug-screening platforms allowed the identification of novel non-toxic mitochondrial inhibitors, as in the

grade signaling could be a successful therapeutic strategy for cancer.

**4. Targeting mitochondria for cancer therapy**

treatment with paclitaxel carboplatin.

202 Mitochondrial DNA - New Insights

Recently, morphological and ultrastructural changes in the mitochondrial cristae structure (cristae remodeling), for example, through the optic atrophy 1 (OPA1) pathway, represent an important step in apoptosis and autophagy, and a potential target for future pharmacological modulation in cancer [105].

Chromosomal translocations generating in-frame oncogenic gene fusions also represent successful examples of targeted cancer therapies, and recently it has been shown that the FGFR3- TACC3 (F3–T3) gene fusion—initially discovered in human glioblastoma and then reported in many other cancers—promotes oxidative phosphorylation, mitochondrial biogenesis and tumor growth [106–108].
