INERVACION DE LAS ARTICULACIONES
1. Músculo estriado. Las fibras musculares presentan un citoplasma estriado transversalmente cuando es observado al microscopio de luz, en orientación longitudinal
Somatic mutations, like in other cancer types, are a main driving force in melanoma initiation and progression. UV-induced DNA damage can cause the formation of somatic mutations and, when these mutations are induced in key regulatory genes or oncogenes, the result is carcinogenesis. Mutations which upregulate murine sarcoma viral oncogene homolog B
(BRAF) are most common mutations in melanoma, occurring in nearly 65 % of all melanoma cases. The most common somatic mutation in the BRAF gene is a missense mutation from valine to glutamic acid at amino acid 600 (BRAFV600E) in the ATP-binding region of the protein. This specific mutation comprises of nearly 90 % of all BRAF-mutated melanomas, while other variations, including BRAFV600K, are rare [138]. These mutations occur somatically, since wild-type forms of both genes are also found in normal tissue of melanoma patients [139]. In fact, there is a very low incidence of BRAF mutations found in melanomas arising from non-sun exposed skin, suggesting that UV exposure plays a crucial role in inducing BRAF mutations in cutaneous melanoma [135].
Functionally, the BRAFV600E mutation causes a 10-12-fold increase in its activity, triggering the hyperactivation of the mitogen-activated protein kinase (MAPK) signaling cascade leading to cellular survival and proliferation [135]. Studies in immortalized mouse melanocytes showed this connection by expressing mutant BRAF and observing the induction of extracellular signal-regulated kinase (ERK) and the initiation of malignant transformation [140]. Moreover, upon treating human melanoma cell lines with BRAF mutations with mitogen-activated protein kinase kinase (MEK)-1/2 inhibitor U0126 the blockade of MAPK signaling and cell cycle progression was observed, but had no effect on melanoma cells harboring oncogenic neuroblastoma viral oncogene homolog (NRAS). Therefore, mutated BRAF can act as a potent oncogene in early stages of melanoma progression through activated MAPK signaling. However, mutated BRAF is not required for RAS-transformed melanocytes due to the innate redundancy within the pathway [140].
The BRAFV600E mutation is found in approximately 80 % of benign nevi. In zebrafish, expression of melanocyte-specific BRAF proteins induces ectopic proliferation of melanocytes, analogous to human nevi [141]. However, it is unknown why mutated BRAF leads to benign nevi formation and malignant formation. Multiple requirements are needed for transformation. A BRAF mutation alone is not sufficient to progress towards malignancy. In the zebrafish study described above, a combination of a BRAF mutation and inactivation of the tumor-suppressor gene p53 led to the malignant transformation of melanocytes [141]. Additionally, studies identified the association between mutant BRAFV600E and p16INK4a/p19ARF loss or mutations in p53 and PTEN leading to malignant transformation [142–144]. These studies provide an explanation for how BRAF mutations exist in nevi without directly inducing malignant transformation.
The RAS family is also affected by somatic mutations in melanoma and functions as a transducer of extracellular growth factor signals in the cell. Among the RAS gene family, NRAS is the most commonly mutated and represents the second most common mutation
found in melanoma, comprising of nearly 20 % of mutations in the disease. This mutation is a point mutation of amino acid 61 from a glutamine to a lysine (NRASQ61K) or arginine (NRASQ61R), but there are many variations. Mutations also occur in other RAS family members but mutations in NRAS seem to be the most detrimental and oncogenic. Studies in mice investigated the different consequences of HRAS and NRAS in melanocytes. The hyperactivation of HRAS in combination with loss-of-function mutations in CDKN2A and/or p53 led to non-metastatic melanomas in mice. Activation of NRAS together with CDKN2A loss led to the production of melanomas with severe metastatic spread to both lymph nodes and distant organs in mice [135, 145, 146].
As described above, MITF plays a crucial role in melanocyte development and differentiation. The function of this gene in malignant transformation and melanoma progression has become important and remains not well understood. The role of MITF in differentiation and cell cycle arrest in normal melanocytes is known, but conversely MITF in melanoma cells does not possess the same function. A large study investigated genomic changes in melanocytes using analysis of high-density single-nucleotide polymorphisms (SNPs) and discovered an increased copy number of a region on chromosome 3 that included the MITF locus [147]. Moreover, this increase was accompanied by increased expression of the MITF protein. Upon the overexpression of both MITF and BRAF, primary human melanocyte cultures were malignantly transformed. The amplification of MITF correlates to poor prognosis and is associated with melanoma therapy resistance [147]. Taken together, these results suggest that MITF is an oncogene and a lineage-survival gene in melanoma [147– 149].
There are many other somatic mutations found in melanoma [136, 137, 150–153]. For example, another gene susceptible to somatic mutations is the KIT oncogene. Approximately 1-2 % of all melanomas contain point mutations in the KIT receptor tyrosine kinase gene. Most of these mutations are found in mucosal and acral melanomas and also in constantly sun-damaged melanomas [138, 154]. An influential study observed that the MAPK signaling pathway was activated upon stimulating melanoma cells with the KIT ligand, SCF. This resulted in the phosphorylation of MITF and in the transactivation of the promoter of the pigmentation-related gene TYR [95]. This link between external signals transduced by KIT into gene regulation became an early indication for the importance of KIT in melanomagenesis.
Another gene susceptible to somatic mutations in melanoma is the phosphatase and tensin homolog (PTEN) gene. Somatic mutations in PTEN are found in 40-60 % of melanoma cell lines and in approximately 10-20 % of primary melanomas, where the majority of the
mutations occur in the phosphatase domain (reviewed in [132], [155]). However, the contrast between low mutation frequency detection and a high level of gene silencing can be explained by other mechanisms important for the inactivation of PTEN, including epigenetic silencing and altered subcellular location of this protein [135]. Several methylation sites have been found within the PTEN promoter. Hypermethylation of these sites reduce PTEN expression in melanoma [132]. Moreover, genetic studies have found that deletions in chromosome 10q, including the PTEN locus, occur at high frequencies in BRAF-mutated melanoma, while deletions in this chromosome were less common in NRAS-mutated melanomas [156, 157]. This work suggests that BRAF and PTEN may cooperate in melanoma progression. PTEN is commonly regarded as a tumor suppressor because it acts as an antagonist of the phosphatidylionositol-3 kinase (PI3K) pathway and MAPK signaling, which will be discussed further in the following sections.