s (yellow), along with
cell nuclei (blue), actin (red), and an actin regulator (green) The earliest stage of melanoma starts when melanocytes begin out-of-control growth. Melanocytes are found between the outer layer of the skin (the
epidermis) and the next layer (the
dermis). This early stage of the disease is called the radial growth phase, when the tumor is less than 1 mm thick, and spreads at the level of the basal epidermis. Because the cancer cells have not yet reached the blood vessels deeper in the skin, it is very unlikely that this early-stage melanoma will spread to other parts of the body. If the melanoma is detected at this stage, then it can usually be completely removed with surgery. When the tumor cells start to move in a different direction – vertically up into the epidermis and into the
papillary dermis – cell behaviour changes dramatically. which is judged by the presence and activity of the
tumor infiltrating lymphocytes (TILs). These cells sometimes completely destroy the primary tumor; this is called regression, which is the latest stage of development. In certain cases, the primary tumor is completely destroyed and only the metastatic tumor is discovered. About 40% of human melanomas contain activating mutations affecting the structure of the B-Raf
protein, resulting in
constitutive signaling through the Raf to
MAP kinase pathway. A cause common to most cancers is damage to DNA. UVA light mainly causes
thymine dimers. UVA also produces
reactive oxygen species and these inflict other DNA damage, primarily single-strand breaks, oxidized
pyrimidines and the oxidized
purine 8-oxoguanine (a mutagenic DNA change) at 1/10, 1/10, and 1/3rd the frequencies of UVA-induced thymine dimers, respectively. If unrepaired, cyclobutane pyrimidine dimer (CPD) photoproducts can lead to mutations by inaccurate
translesion synthesis during DNA replication or repair. The most frequent mutations due to inaccurate synthesis past CPDs are cytosine to thymine (C>T) or CC>TT
transition mutations. These are commonly referred to as UV fingerprint mutations, as they are the most specific mutation caused by UV, being frequently found in sun-exposed skin, but rarely found in internal organs. Errors in DNA repair of UV photoproducts, or inaccurate synthesis past these photoproducts, can also lead to deletions, insertions, and
chromosomal translocations. The entire genomes of 25 melanomas were sequenced. On average, about 80,000 mutated bases (mostly C>T transitions) and about 100 structural rearrangements were found per melanoma genome. This is much higher than the roughly 70 mutations across generations (parent to child). Among the 25 melanomas, about 6,000 protein-coding genes had
missense,
nonsense, or
splice site mutations. The transcriptomes of over 100 melanomas has also been sequenced and analyzed. Almost 70% of all human protein-coding genes are expressed in melanoma. Most of these genes are also expressed in other normal and cancer tissues, with some 200 genes showing a more specific expression pattern in melanoma compared to other forms of cancer. Examples of melanoma specific genes are
tyrosinase,
MLANA, and
PMEL. UV radiation causes
damage to the DNA of cells, typically thymine dimerization, which, when unrepaired, can create mutations in the cell's genes. This strong mutagenic factor makes cutaneous melanoma the tumor type with the highest number of mutations. When the cell
divides, these mutations are propagated to new generations of cells. If the mutations occur in
protooncogenes or
tumor suppressor genes, the rate of
mitosis in the mutation-bearing cells can become uncontrolled, leading to the formation of a
tumor. Data from patients suggest that aberrant levels of activating transcription factor in the nucleus of melanoma cells are associated with increased metastatic activity of melanoma cells; studies from mice on skin cancer tend to confirm a role for activating transcription factor-2 in cancer progression.
Cancer stem cells may also be involved.
Gene mutations Large-scale studies, such as
The Cancer Genome Atlas, have characterized recurrent
somatic alterations likely driving initiation and development of cutaneous melanoma. The Cancer Genome Atlas study has established four subtypes:
BRAF mutant,
RAS mutant,
NF1 mutant, and triple wild-type. The most frequent mutation occurs in the 600th codon of
BRAF (50% of cases).
BRAF is normally involved in cell growth, and this specific mutation renders the protein constitutively active and independent of normal physiological regulation, thus fostering tumor growth. RAS genes (
NRAS,
HRAS and
KRAS) are also recurrently mutated (30% of TCGA cases) and mutations in the 61st or 12th codons trigger oncogenic activity. Loss-of-function mutations often affect
tumor suppressor genes such as
NF1,
TP53 and
CDKN2A. Other oncogenic alterations include fusions involving various kinases such as BRAF, RAF1, ALK, RET, ROS1, NTRK1., NTRK3 and MET
BRAF, RAS, and
NF1 mutations and kinase fusions are remarkably mutually exclusive, as they occur in different subsets of patients. Assessment of mutation status can, therefore, improve patient stratification and inform targeted therapy with specific inhibitors. In some cases (3–7%), mutated versions of
BRAF and
NRAS undergo
copy-number amplification. In another experiment they found that
elasticity of melanoma cells is important for its metastasis and growth: non-pigmented tumors were bigger than pigmented and it was much easier for them to spread. They showed that there are both pigmented and non-pigmented cells in melanoma
tumors, so that they can both be
drug-resistant and metastatic. ==Diagnosis==