Nonsynonymous somatic mitochondrial mutations occur in the majority of cutaneous melanomas

Targeting mitochondria in dermatological therapy: Beyond oxidative damage and skin aging

INTRODUCTION

The analysis of the role of the mitochondria in oxidative damage and skin aging is a significant aspect of dermatological research. Mitochondria generate most reactive oxygen species (ROS); however, excessive ROS are cytotoxic and DNA-damaging and promote (photo-)aging. ROS also possesses key physiological and regulatory functions and mitochondrial dysfunction is prominent in several skin diseases including skin cancers. Although many standard dermatotherapeutics modulate mitochondrial function, dermatological therapy rarely targets the mitochondria. Accordingly, there is a rationale for "mitochondrial dermatology"-based approaches to be applied to therapeutic research.

AREAS COVERED

This paper examines the functions of mitochondria in cutaneous physiology beyond energy (ATP) and ROS production. Keratinocyte differentiation and epidermal barrier maintenance, appendage morphogenesis and homeostasis, photoaging and skin cancer are considered. Based on related PubMed search results, the paper evaluates thyroid hormones, glucocorticoids, Vitamin D3 derivatives, retinoids, cannabinoid receptor agonists, PPARγ agonists, thyrotropin, and thyrotropin-releasing hormone as instructive lead compounds. Moreover, the mitochondrial protein MPZL3 as a promising new drug target for future "mitochondrial dermatology" is highlighted.

EXPERT OPINION

Future dermatological therapeutic research should have a mitochondrial medicine emphasis. Focusing on selected lead agents, protein targets, in silico drug design, and model diseases will fertilize a mito-centric approach.

Mitochondria in skin health, aging, and disease

The skin is a high turnover organ, and its constant renewal depends on the rapid proliferation of its progenitor cells. The energy requirement for these metabolically active cells is met by mitochondrial respiration, an ATP generating process driven by a series of protein complexes collectively known as the electron transport chain (ETC) that is located on the inner membrane of the mitochondria. However, reactive oxygen species (ROS) like superoxide, singlet oxygen, peroxides are inevitably produced during respiration and disrupt macromolecular and cellular structures if not quenched by the antioxidant system. The oxidative damage caused by mitochondrial ROS production has been established as the molecular basis of multiple pathophysiological conditions, including aging and cancer. Not surprisingly, the mitochondria are the primary organelle affected during chronological and UV-induced skin aging, the phenotypic manifestations of which are the direct consequence of mitochondrial dysfunction. Also, deletions and other aberrations in the mitochondrial DNA (mtDNA) are frequent in photo-aged skin and skin cancer lesions. Recent studies have revealed a more innate role of the mitochondria in maintaining skin homeostasis and pigmentation, which are affected when the essential mitochondrial functions are impaired. Some common and rare skin disorders have a mitochondrial involvement and include dermal manifestations of primary mitochondrial diseases as well as congenital skin diseases caused by damaged mitochondria. With studies increasingly supporting the close association between mitochondria and skin health, its therapeutic targeting in the skin-either via an ATP production boost or free radical scavenging-has gained attention from clinicians and aestheticians alike. Numerous bioactive compounds have been identified that improve mitochondrial functions and have proved effective against aged and diseased skin. In this review, we discuss the essential role of mitochondria in regulating normal and abnormal skin physiology and the possibility of targeting this organelle in various skin disorders.

Mitochondrial miR-762 regulates apoptosis and myocardial infarction by impairing ND2

Mitochondrial dysfunction plays a major role in the pathogenesis of cardiovascular diseases. MicroRNAs (miRNAs) are small RNAs that act as negative regulators of gene expression, but how miRNAs affect mitochondrial function in the heart is unclear. Using a miRNA microarray assay, we found that miR-762 predominantly translocated in the mitochondria and was significantly upregulated upon anoxia/reoxygenation (A/R) treatment. Knockdown of endogenous miR-762 significantly attenuated the decrease in intracellular ATP levels, the increase in ROS levels, the decrease in mitochondrial complex I enzyme activity and the increase in apoptotic cell death in cardiomyocytes, which was induced by A/R treatment. In addition, knockdown of miR-762 ameliorated myocardial ischemia/reperfusion (I/R) injury in mice. Mechanistically, we showed that enforced expression of miR-762 dramatically decreased the protein levels of endogenous NADH dehydrogenase subunit 2 (ND2) but had no effect on the transcript levels of ND2. The luciferase reporter assay showed that miR-762 bound to the coding sequence of ND2. In addition, knockdown of endogenous ND2 significantly decreased intracellular ATP levels, increased ROS levels, reduced mitochondrial complex I enzyme activity and increased apoptotic cell death in cardiomyocytes, which was induced by A/R treatment. Furthermore, we found that the inhibitory effect of miR-762 downregulation was attenuated by ND2 knockdown. Thus, our findings suggest that miR-762 participates in the regulation of mitochondrial function and cardiomyocyte apoptosis by ND2, a core assembly subunit of mitochondrial complex I. Our results revealed that mitochondrial miR-762, as a new player in mitochondrial dysfunction, may provide a new therapeutic target for myocardial infarction.

MtDNA As a Cancer Marker: A Finally Closed Chapter?

Sequence alterations of the mitochondrial DNA (mtDNA) have been identified in many tu-mor types. Their nature is not entirely clear. Somatic mutation or shifts of heteroplasmic mtDNA vari-ants may play a role. These sequence alterations exhibit a sufficient frequency in all tumor types investi-gated thus far to justify their use as a tumor marker. This statement is supported by the high copy num-ber of mtDNA, which facilitates the detection of aberrant tumor-derived DNA in bodily fluids. This will be of special interest in tumors, which release a relatively high number of cells into bodily fluids, which are easily accessible, most strikingly in urinary bladder carcinoma. Due to the wide distribution of the observed base substitutions, deletions or insertions within the mitochondrial genome, high efforts for whole mtDNA sequencing (16.5 kb) from bodily fluids would be required, if the method would be in-tended for initial tumor screening. However, the usage of mtDNA for sensitive surveillance of known tumor diseases is a meaningful option, which may allow an improved non-invasive follow-up for the urinary bladder carcinoma, as compared to the currently existing cytological or molecular methods. Fol-lowing a short general introduction into mtDNA, this review demonstrates that the scenario of a sensi-tive cancer follow-up by mtDNA-analysis deserves more attention. It would be most important to inves-tigate precisely in the most relevant tumor types, if sequencing approaches in combination with simple PCR-assays for deletions/insertions in homopolymeric tracts has sufficient sensitivity to find most tu-mor-derived mtDNAs in bodily fluids.

High-performance detection of somatic D-loop mutation in urothelial cell carcinoma patients by polymorphism ratio sequencing

Utilizing a polymorphism ratio sequencing platform, we performed a complete somatic mutation analysis of the mitochondrial D-loop region in 14 urothelial cell carcinomas. A total of 28 somatic mutations, all heteroplasmic, were detected in 8 of 14 individuals (57.1 %). Insertion/deletion changes in unstable mono- and dinucleotide repeat segments comprise the most pervasive class of mutations (9 of 28), while two recurring single-base substitution loci were identified. Seven variants, mostly insertion/deletions, represent population shifts from a heteroplasmic germline toward dominance in the tumor. In four cases, DNA from matched urine samples was similarly analyzed, with all somatic variants present in associated tumors readily detectable in the bodily fluid. Consistent with previous findings, mutant populations in urine were similar to those detected in tumor and in three of four cases were more prominent in urine.

KEY MESSAGES

PRS accurately detects high mtDNA mutations in UCCs and their body fluids. mtDNA mutations are universally heteroplasmic and often appear at low levels. The PRS technology could be a viable approach to develop mitochondrial biomarkers.

Mitochondrial dysfunction: a neglected component of skin diseases

Aberrant mitochondrial structure and function influence tissue homeostasis and thereby contribute to multiple human disorders and ageing. Ten per cent of patients with primary mitochondrial disorders present skin manifestations that can be categorized into hair abnormalities, rashes, pigmentation abnormalities and acrocyanosis. Less attention has been paid to the fact that several disorders of the skin are linked to alterations of mitochondrial energy metabolism. This review article summarizes the contribution of mitochondrial pathology to both common and rare skin diseases. We explore the intriguing observation that a wide array of skin disorders presents with primary or secondary mitochondrial pathology and that a variety of molecular defects can cause dysfunctional mitochondria. Among them are mutations in mitochondrial- and nuclear DNA-encoded subunits and assembly factors of oxidative phosphorylation (OXPHOS) complexes; mutations in intermediate filament proteins involved in linking, moving and shaping of mitochondria; and disorders of mitochondrial DNA metabolism, fatty acid metabolism and heme synthesis. Thus, we assume that mitochondrial involvement is the rule rather than the exception in skin diseases. We conclude the article by discussing how improving mitochondrial function can be beneficial for aged skin and can be used as an adjunct therapy for certain skin disorders. Consideration of mitochondrial energy metabolism in the skin creates a new perspective for both dermatologists and experts in metabolic disease.

Reviewing the somatic genetics of melanoma: from current to future analytical approaches

Metastatic melanoma has traditionally been difficult to treat, and although molecularly based targeted therapies have shown promising results, they have yet to show consistent improvements in overall survival rates. Thus, identifying the key mutation events underlying the etiology of metastatic melanoma will no doubt lead to the improvement of existing therapeutic approaches and the development of new treatment strategies. Significant advances toward understanding the complexity of the melanoma genome have recently been achieved using next-generation sequencing (NGS) technologies. However, identifying those mutations driving tumorigenesis will continue to be a challenge for researchers, in part because of the high rates of mutation compared to other cancers. This article will review the catalog of mutations identified in melanoma through a variety of approaches, including the use of unbiased exome and whole-genome NGS platforms, as well discuss complementary strategies for identifying driver mutations. The promise of personalized medicine afforded by better understanding these mutation events should provide impetus for increased activity and rapid advances in this field.

Investigating the role of melanin in UVA/UVB- and hydrogen peroxide-induced cellular and mitochondrial ROS production and mitochondrial DNA damage in human melanoma cells

Skin cancer incidence is dramatically increasing worldwide, with exposure to ultraviolet radiation (UVR) a predominant factor. The UVA component initiates oxidative stress in human skin, although its exact role in the initiation of skin cancer, particularly malignant melanoma, remains unclear and is controversial because there is evidence for a melanin-dependent mechanism in UVA-linked melanoma studies. Nonpigmented (CHL-1, A375), moderately pigmented (FM55, SKmel23), and highly pigmented (FM94, hyperpigmented FM55) human melanoma cell lines have been used to investigate UVA-induced production of reactive oxygen species using FACS analysis, at both the cellular (dihydrorhodamine-123) and the mitochondrial (MitoSOX) level, where most cellular stress is generated. For the first time, downstream mtDNA damage (utilizing a quantitative long-PCR assay) has been investigated. Using UVA, UVB, and H(2)O(2) as cellular stressors, we have explored the dual roles of melanin as a photoprotector and photosensitizer. The presence of melanin has no influence over cellular oxidative stress generation, whereas, in contrast, melanin protects against mitochondrial superoxide generation and mtDNA damage (one-way ANOVA with post hoc Tukey's analysis, P<0.001). We show that if melanin binds directly to DNA, it acts as a direct photosensitizer of mtDNA damage during UVA irradiation (P<0.001), providing evidence for the dual roles of melanin.

Mitochondrial haplogroups, control region polymorphisms and malignant melanoma: a study in middle European Caucasians

Background

Because mitochondria play an essential role in energy metabolism, generation of reactive oxygen species (ROS), and apoptosis, sequence variation in the mitochondrial genome has been postulated to be a contributing factor to the etiology of multifactorial age-related diseases, including cancer. The aim of the present study was to compare the frequencies of mitochondrial DNA (mtDNA) haplogroups as well as control region (CR) polymorphisms of patients with malignant melanoma (n = 351) versus those of healthy controls (n = 1598) in Middle Europe.

Methodology and Principal Findings

Using primer extension analysis and DNA sequencing, we identified all nine major European mitochondrial haplogroups and known CR polymorphisms. The frequencies of the major mitochondrial haplogroups did not differ significantly between patients and control subjects, whereas the frequencies of the one another linked CR polymorphisms A16183C, T16189C, C16192T, C16270T and T195C were significantly higher in patients with melanoma compared to the controls. Regarding clinical characteristics of the patient cohort, none of the nine major European haplogroups was associated with either Breslow thickness or distant metastasis. The CR polymorphisms A302CC-insertion and T310C-insertion were significantly associated with mean Breslow thickness, whereas the CR polymorphism T16519C was associated with metastasis.

Conclusions and Significance

Our results suggest that mtDNA variations could be involved in melanoma etiology and pathogenesis, although the functional consequence of CR polymorphisms remains to be elucidated.

ReseqChip: automated integration of multiple local context probe data from the MitoChip array in mitochondrial DNA sequence assembly

Background

The Affymetrix MitoChip v2.0 is an oligonucleotide tiling array for the resequencing of the human mitochondrial (mt) genome. For each of 16,569 nucleotide positions of the mt genome it holds two sets of four 25-mer probes each that match the heavy and the light strand of a reference mt genome and vary only at their central position to interrogate all four possible alleles. In addition, the MitoChip v2.0 carries alternative local context probes to account for known mtDNA variants. These probes have been neglected in most studies due to the lack of software for their automated analysis.

Results

We provide ReseqChip, a free software that automates the process of resequencing mtDNA using multiple local context probes on the MitoChip v2.0. ReseqChip significantly improves base call rate and sequence accuracy. ReseqChip is available at http://code.open-bio.org/svnweb/index.cgi/bioperl/browse/bioperl-live/trunk/Bio/Microarray/Tools/.

Conclusions

ReseqChip allows for the automated consolidation of base calls from alternative local mt genome context probes. It thereby improves the accuracy of resequencing, while reducing the number of non-called bases.