Sadie Lou

Introduction:  The Complex Balancing Act

Often taken for granted and overlooked, the skin, and its associated structures, are one of the body’s most critical lines of primary defense. Much more than a mere container, skin conveys a variety of significant functions: resistance to trauma and infection, a barricade against water loss and excess absorption, vitamin D synthesis, and sensation.  It is the interface between us and the world at large.    However, like most other tissues within the body, it too can, via a series of genetic mutations, enter into a state of uncontrolled growth.  There are three forms of skin cancer:  squamous cell carcinoma (SCC), basal cell carcinoma (BCC), and melanoma, all identified and labeled according to the epidermal cell type in which it develops.  It is common knowledge that the rate of incidence of skin cancer has increased dramatically in the past five decades.  This increase has been generally attributed to the change of clothing styles, recreational activities, longevity, and other aspects of current lifestyles that are a result of an increased exposure to sunlight.  However, throughout human evolutionary history, an insight to the nature of how this increase has developed resides in the evolution of the variation of skin pigmentation expressed by humans. 

Contrary to the former theory that the variety of skin pigments in humans was a result of selective pressure of early hominids to prevent skin cancers, a stunning and elegant new theory of the evolution of human skin pigmentation is offered by anthropologist Nina Jablonski.  It instead maintains that as a result of human expansion and migration from Africa, humans developed lighter pigmented skin in order to maintain a complex balancing act between preserving the body’s essential B-vitamin folate stores while at the same time permitting vitamin D synthesis¹.  The crucial, key factor that facilitates this balancing act is the amount of ultraviolet radiation (UV-radiation) the skin receives.

Folate is a water-soluble B vitamin that is necessary for the synthesis of DNA; therefore, any process that involves rapid cell proliferation requires it.  In fact, studies utilizing erythroleukemic⁼ cells show that the ability of the cells to replicate in the absence of exogenous folate supply was directly proportional to the amount of intracellular folate:  When folate levels decrease, cell replication decreases². This explains why expectant mothers who are folate-deficient are at a higher risk for having babies with developmental neural tube defects, such as spina bifida, and why efforts to spread awareness about this vital compound and supplement foods with it have become prevalent in recent years¹.  It has been found that ultraviolet radiation and other high-energy radiation can destroy and deplete the natural folate stores within the body³.  According to Jablonski, if UV radiation can destroy folate, a substance so essential to all human life and reproduction, then it must be certain that some defense mechanism had to evolve via natural selection to help maintain the precious folate levels within the body:  This mechanism manifested itself in the form of “natural sunscreen”, or pigmentation.

Although there are manifold negative effects imposed on the skin via UV radiation (some of these will be discussed in detail later), it does provide one crucial positive function: the production of vitamin D.  The active form of vitamin D is utilized throughout the body for an array of reasons, but what mainly makes the compound so vital is its regulation of phosphorous and calcium metabolism, which is the basis of a strong skeleton.  Vitamin D is necessary for the growth of bones because it allows the body to absorb calcium from food³. 

Therefore, when early, darkly pigmented Homo sapiens began to branch out and migrate to latitudes away from the equator with a lower percentage of UV-radiation, an adaptive mechanism had to develop that would allow enough of the diminished UV-radiation to penetrate the epidermal layers and facilitate the synthesis of precious vitamin-D^{1, 3}.  This mechanism was the development of lighter-pigmented skin; the lighter pigment offers less protection from UV rays; however, this would not be a too much of a concern in a geographic region far from the equator (either north or south), as the low UV levels recived would not be high enough to threaten the depletion of the essential folate stores.  An extrapolation of this theory holds that in our modern and globally expanding world, as people move from an area with one pattern of UV radiation to another region, biological and cultural adaptations will not be able to keep pace.  This directly relates to part of the current skin cancer problem:  Light-skinned, sun-seeking individuals who vacation or move to tropical climates to bask in the sun are increasingly paying the price in the form of premature aging of the skin and skin cancers⁼.

Figure 1. The map on the left depicts the global UV-radiation pattern, with areas of darker shading corresponding to higher levels of UV-radiation.  Accordingly, the map on the right depicts the distribution of skin pigmentation of native populations around the world.  According to anthropologist Nina Jablonski’s theory, as early modern humans expanded and migrated to more temperate climes (higher latitudes), nearly 100,000 years ago, their skin adapted to the environmental conditions (namely the amount of ultraviolet radiation) that prevailed in different regions².  

It’s no mystery that the frequency of skin cancer incidence has increased dramatically; in fact, over the past two decades, a worldwide increase in the incidence of skin cancer to near epidemic proportions has led to increased morbidity and appreciating cost.  Squamous cell carcinoma and basal cell carcinoma occur much more frequently than does melanoma, and have much more defined link to ultraviolet radiation (UVR) as a causative agent.  However, melanoma, although not as frequently encountered as its counterparts is a more lethal and aggressive cancer, and its incidence is increasing. Melanoma is much more illusive and mysterious than it’s the other skin cancers:  The causative link between UVR and melanoma is less understood and highly debated.  Some sources claim that there is no link between UVR and melanoma while others in fact, downplay this association--- some even to claim that UV-radiation has no causative link to melanoma at all.  Before deeply dissecting and examining this disparity in melanoma etiology, it is necessary to review a brief overview of the anatomy and physiology of the cell type in which this cancer manifests itself---the melanocyte.

Nature’s Sunblock:  Melanocytes and Melanin Production

The skin is a highly stratified, stunningly dynamic organ:  It is comprised of layers with different physical and chemical characteristics.  The outer layer, or the epidermis, is a stratified mosaic of different cell types.  Keratinocytes, the prevailing cell type, confer strength, resistance, and are responsible for the stretchability of the skin’s surface⁴. They are comprised of the vital protein keratin, which confers strength and flexibility to the skin. The epidermis also contains a variety of immigrant cells, so called on account of their migration into the epidermis from other parts of the body during early development³.  A crucial component of this family is the melanocytes, derived from the neural crest, which flank the neural tube or early spine, in early embryonic development³.  Once they arrive in the epidermis, they situate themselves near the interface of the dermis (the second, internal layer of the skin) and epidermis and initiate their major physiological function:  the production of melanin and associated UV-radiation-protective qualities.

Melanocytes produce melanin pigment in small membrane-bound packets called melanosomes, which are then pushed out of melanocytes and into the keratinocytes of the epidermis by spindle-like extensions of the melanocytes known as dendrites.  The exquisite array of skin color pigmentation throughout the Homo sapiens species exists because people differ in the amount of melanin pigment their skin contains and the ways in which it is packaged³.  The size and shape of melanosomes influence their ability to protect the skin and underlying tissues from UV-radiation.  In darkly pigmented skin, the melanosomes are larger and dispersed evenly and frequently throughout the keratinocytes.  This arrangement allows them to absorb more energy than the smaller and less dense melanosomes of lightly pigmented skin. 

The production of melanin is governed by many factors, including pigmentation genes, hormones and the presence of UV-radiation.  The total count of melanocytes, in people with a normal range of pigmentation, is similar from one person’s body to another; however, not all melanocytes in all people are actively producing melanin.  Some light-skinned individuals produce very little melanin in melanocytes, while darker-skinned individuals produce more.  The production of melanin can be heightened in response to sun exposure⁴.  This is the basis of the process known as tanning; an important part of the body’s protective response to UV-radiation.

Mammalian melanin comes in two distinct varieties:  brownish-black eumelanin, and reddish-yellow phaeomelanin⁴. High concentrations of eumelanin are what make skin appear dark; it is also the type of melanin produced in the tanning response.  The concentration of phaeomelanin is much more variable, although it is more common and prevalent among red-haired northern Europeans, where it contributes to the total melanin composition of the skin⁴.  Despite the different and distinctive properties of each respective type of melanin, both eumelanin and pheomelanin both arise from the amino acid tyrosine and thus the same chemical pathway governs both of their production, where some of the intermediates differ⁴ (see fig.2). The preference of the melanocytes for producing eumelanin or pheomelanin is regulated by melanocortin 1 (MC1R) receptors present on the surface of melanocytes.  The binding of melanocyte-stimulating-hormone (MSH) to MC1R results in the formation of eumelanin.  The binding of another ligand, an agouti signaling protein, results in the production of pheomelanin.  Ultimately, genetic factors⁼ will play a role in determining which pigment is produced.

Figure 2- Illustrated here is the metabolic melanin pathway, which occurs in the melanocytes.  This process initially enlists help from the tyrosinase enzyme to convert the amino acid tyrosine into intermediary dopa and then to dopaquinone.  Depending on what ligand is bound to the melanocortin receptor present on the surface of the melanocyte , either melanocyte-stimulating-hormone or agouti signaling protein respectively, will determine if either eumelanin or pheomelanin is ultimately produced.  (Image courtesy of http://albinism.med.umn.edu/factpath.gif) 

Melanoma is a malignancy of melanocytes that exhibit a series of molecular events that result in a stepwise progression from dysplasia to invasion to metastasis in these cells⁵.  Histologically, melanomas are usually large lesions that display an irregular silhouette and a lack of symmetry.  At presentation, afflicted patients usually report a new skin growth or change in the size, shape, or color of an existing mole, all contained within the American Skin Cancer’s diagnostic mnemonic device “ABCD”:  asymmetry, border irregularity, color change, and a diameter of greater than 6mm ⁵.  Pigment may be present within the tumor cells, although pigmentless, or amelanotic tumors are not uncommon⁹.  Approximately 62,000 new cases of melanoma are reported annually in the United States, and these are responsible for about 7,900 deaths⁵.  It is far rarer than the non-melanoma-skin-cancers (NMSCs), accounting for only 5% of all dermatological cancers diagnosed; however, it poses a substantially greater threat as it is responsible for 80% of deaths from skin cancer⁴. 

Figure 3-   Histology of Melanoma.  Left:  Melanoma in situ is defined in part by single atypical melanocytes, nests, and migration of melanocytes above the basal zone; it is when the basement membrane of the epidermis is not penetrated (high chance of survival if caught in this stage).  Right: A feature of melanoma is extension of the atypical cells down adnexal structures.⁵   

Melanoma:  Sun-Induced After-all?

Generally lumped together with the other types of skin cancers, melanoma incidence is often considered to be a by-product of excessive sunlight exposure.  There are many factors that have lead researchers to this assumption; many of them entail the results of epidemiological studies (to be discussed thoroughly later).  After all, melanoma, like the NMSCs, does occur more frequently in individuals with lighter-pigmented skin.    However, this assumption, that melanoma is caused by sunlight, has spawned an epistemological minefield that has come under siege and inspired a debate on the etiology of melanoma.  Although at the broad, population-level pattern of melanoma induction it appears as if sunlight bears a direct causative link to melanoma, it is deeper, at the cellular and genetic level where this view falls short. The first question to be asked is if sunlight and UV-radiation cause melanoma, how and by what mechanisms do they induce it? 

Approximately 5% of the solar radiation on Earth is composed of ultraviolet radiation, which is defined as wavelengths between 100 and 400 nm⁶.  This range is subdivided into UVA (315-400nm), UVB (280-315nm), and UVC (100-280nm) ⁶.  Both UVA and UVB have wavelengths that induce direct DNA damage.  While UVB has been long known to cause DNA damage in the skin, UVA’s role has only recently been illuminated. The characteristic UV damage inflicted upon DNA is the generation of dimeric photoproducts between neighboring pyrimidine bases.  UV-induced DNA linkage between two adjacent pyrimidines (e.g. CC or TT, where C= cytosine and T= thymine) on the same DNA strand is usually repaired by nucleotide excision repair enzymes before replication⁶.  However, if the repair is incomplete or delayed, the pyrimidine dimers form a bulge in the DNA molecule, rendering it a poorly functioning molecule.  Also, DNA polymerase will insert an adenine dinucleotide (AA) opposite of the unrepaired pyrimidine dimers.  The pairing of AA with TT is normal, and therefore no mutations occur, but if AA erroneously pairs with CC or CT linked photoproducts, mutations are observed---- “lesions” form in the normally linear, anti-parallel strands.  These mutations are referred to as “UV signature mutations” since they are only produced by UV and not by any other mutagen.

In response to the disastrous UV-induced DNA lesions, the cell upregulates a 53-kDa protein called p53.  This protein, in a normally functioning cell, usually interacts with a protein called MDM2, which causes p53 to dissociate⁸.  However in a cell with UV-induced stress and damage, a protein kinase called ATM phosphorylates and activates p53, where it then enters the nucleus and binds to DNA.  The p53 protein is a transcription factor that activates the transcription of gene p21.  The subsequent protein, p21, will bind to the cyclinD-CDK4 complex and inhibit it, causing an arrest of the cell cycle at G₁.  This allows the repair of DNA damage before its replication in the S phase.  Therefore, activated p53 facilitates DNA damage repair by regulation of the cell cycle and by directly inducing pathways of excision repair⁶.  If the damage is beyond repair, p53 also directly upregulates expression of proapoptotic genes, promoting apoptosis, or controlled cell suicide, of UV-radiation-damaged cells¹¹. 

Figure 4- This figure shows the known effects of direct UV-radiation DNA damage in skin cells and how they can ultimately lead to tumorigenesis.  The fact that these mutations are commonly found in SCC and BCC, but not melanoma, holds drastic implications in the debate concerning the etiology of melanoma. 

However, when the UV-induced mutations occur in the p53 gene itself, an early event in the development of a tumor begins (see figure 4).  Without the functional p53 protein, the cell will be unable to initiate the proper DNA repair pathways.  This results in thousands of p53 mutant cell clones that are found in normal-appearing sun-exposed skin.  UV-radiation also affects the expression of the Fas receptor and its ligand in the epidermis.  The Fas-Fas ligand interaction is a complex signaling cascade that activates many caspases⁼ within the cell, and is one pathway responsible for apoptosis.  In mice, it was found that transcriptional inhibition of Fas ligand expression occurs after 1 week of continuous UV-radiation exposure.  This results in a dramatic decrease in apoptotic cells.  The resistance to UVR-induced apoptosis and a proliferative advantage of p53 mutant keratinocytes contributes to clonal expansion of the p53 mutant cells under repeated UVR exposure.  Mutations and amplifications in Ras genes (for thorough explanation of Ras function, refer to fig. 5) have also been found in UVR-induced skin tumors in mice and in humans⁶. 

All of the previously mentioned pathways and mechanisms are the only known direct consequences of UV-radiation that implement cutaneous carcinogenesis and tumorigenesis.  These mutations are commonly found and expressed in the tumors of both squamous cell carcinoma and basal cell carcinoma; however, are NOT characteristic of melanoma.   This has drastic implications in the etiological debate concerning melanoma:  It can be definitively said that UV-radiation is not responsible for causing the direct DNA damage that is found in the NMSCs.   However, it is possible that UV-radiation could be implicated in a process that is not currently well defined.  For instance, in addition to the classic pyrimidine photoproduct mutations, DNA damage via UV-radiation can also be the direct result of reactive oxygen species.  UVA has a greater impact than UVB on oxidative stress in skin^{7, 11}.  UVA is also commonly associated as the ‘etiological wavelength’ of melanoma due to the fact that common SPF sunblocks do not protect against them.  UVA, in addition to forming pyrimidine dimers, induces oxygen and nitrogen species, which can damage DNA, proteins and lipids, and lead to energy loss from cells.  This damage may eventuate in tumor initiation, promotion, and progression; however, the molecular mechanisms that would be responsible for this are currently unknown¹¹.

The p53 mutation is not a trademark of melanoma; however, another prominent mutation occurs in 30-70% of all melanoma tumors (depending on what stage they are in).  This is a mutation in the BRAF oncogene. BRAF encodes a serine/threonine kinase involved in the transduction of mitogenic signals from the cell membrane to the nucleus (see figure 5 for further information). The RAF-MEK-ERK signaling pathway (activated by RAS binding to RAF-see figure 5.) is critical for cell survival, growth, and proliferation, but also can influence tumorigenesis. The mutations in the BRAF gene are usually clustered within the kinase domain and the most prominent mutation (85% of all reported) is a single base substitution T----> A¹⁵.  This substitution results in a change of the valine residue in the activation loop, ultimately leading to a constitutively active kinase. The causal relationship between UV-induced-DNA damage and B-RAF mutations has not been well defined; however, it is unlikely that a relationship exists because the substitution that brings about the mutation in B-Raf is not classically associated with the UV-induced pyrimidine dimer.  The identification of mutationally activated and transforming BRAF alleles in melanoma suggests that certain pharmacological inhibitors may be effective for the treatment of B-RAF mutation-positive melanoma.

Establishing the role of Raf in melanoma growth is important for the validation of the usage of  these pharmacologic inhibitors of the Raf-MEK-ERK signaling cascade for the treatment of these cancers.  Cancer development is a multistep process involving genetic alteration of six or more genes.  Therefore, a key concern in the preliminary use of anti-Raf strategies is whether this mutant gene serves a “hit-and-run-role”, or alternatively, is still continually required for supporting the aberrant growth of the BRAF mutation-positive tumor cells¹⁵.  Studies utilizing RNAi to suppress the expression of the mutated B-Raf molecule suggest that B-Raf expression is critical for the continued aberrant growth of B-Raf mutation-positive tumor cells.  Additionally, pharmacologic inhibitors of the Raf-MEK-ERK cascade (specifically a highly specific MEK1 and MEK2 inhibitor were used as well as two Raf inhibitors) were found to effectively impair the growth of melanoma cells in vitro¹⁵.  This loss over the proliferative capacity may be attributed to both the inhibition of cell cycle progression and/or the increased rate of apoptosis, and may vary in different tumor cell lines.  Ultimately, taken together, these results, that B-Raf activity is necessary for continued tumor growth and that its inhibition halted tumor growth, support the feasibility of pharmacologic inhibitors of these signaling targets for the treatment of B-RAF mutation-positive melanomas.  Of course, many more experiments and testing will have to be undergone before the release of any official clinical drugs, but the initial prospects are optimistic. 

Figure 5- Depicted here is the complete Map-Kinase phosphorylation pathway, from extracellular (in this case a growth factor), to gene expression.  First, the growth factor initially binds to one dimer of the receptor tyrosine kinase (RTK).  The binding event causes a conformational change in the protein, which causes it to dimerize with a nearby homodimer.  After this dimerization occurs, the dimers cross phosphorylate each other on tyrosine residues (phosphorylation events are signified by the yellow circles containing “P”).  A nearby tyrosine kinase, SHC (SRC-homology-complex) recognizes and interacts with the phosphorylated tyrosine residue of one of the RTK’s dimer, causing phosphorylation and activation of SHC itself.  SHC then phosphorylates another tyrosine kinase, GRB-2.  GRB-2 phosphorylates and activates the guanine exchange factor (GEF) SOS. SOS’s role is to cleave the nearby monomeric GTPase, Ras’ associated GDP, where it is exchanged for an intracellular GTP molecule.  Once GTP has bound, Ras undergoes a conformational change and is activated.  Active Ras interacts with nearby B-Raf serine/threonine kinase, and shuttles it to the cytoplasmic side of the plasma membrane, where it is then phosphorylated and activated.  B-Raf then phosphorylates the MEK serine/threonine kinase, which activates it.  Activated MEK phosphorylates ERK; activated ERK then enters the nucleus and phosphorylates ELK a transcription factor.  ELK promotes transcription of genes whose associated protein products induce cell proliferation.  In 30- 70% of melanoma tumors, a mutation in the B-Raf protein, which renders it constitutively active, is present¹³. 

A hint about UV-radiation’s possible role in melanoma induction can be gleaned by assessing melanoma incidence and tumor patterns in populations of people with different pigmentation.  As mentioned previously, individuals with lightly pigmented skin are more likely to be afflicted with melanoma than those with dark skin.  The general trend in incidence patterns for Caucasian populations is the closer to the equator; the higher the incidence rate climbs.  Countries such as Australia, New Zealand, and the United States follow this trend¹¹.  These data have often been interpreted as evidence that sunlight is in fact the cause of melanoma¹⁴.  An anomaly to this trend; yet, is evident within Europe, as the incidence rate is higher to the north of the equator, in Scandinavia and Scotland, as opposed to in countries closer to the equator such as France or Spain¹⁴. Various studies have tried to link this anomaly to causes from global warming to ethnic differences in sun sensitivity, all of which fall under the assumption that sunlight causes melanoma.  However, researchers may be missing the big picture (or small picture) as to what influences this trend.

When compared to incidence pattern of other populations are when these results seem the most puzzling.  For instance, within populations of darkly pigmented individuals in areas of high UV-radiation exposure (e.g. Africa), although melanoma is less likely to occur to these individuals than their lighter-skinned counterparts, the majority of melanoma cases (80%) occur on the sole of the foot.  As it is a site that is shielded from the sun, sunlight clearly plays no causative role in cases of melanoma on the sole of the foot⁹.  Also populations of intermediate pigmentation, such as China, Saudi Arabia, and India have a preponderance of melanoma on the sole of the foot as well⁹.   Studies of African albinos offer even more stunning results.   There are two main types of albinos:  tyrosinase-positive albinos, and tyrosinase-negative albinos.  The former are able to synthesize pheomelanin, while the latter produce no melanin whatsoever (refer to fig. 2 for melanin chemical pathway and the role of tyrosinase).  African albinos fall into the first category, and are nature’s perfect epidemiological study.  One would expect that, if sunlight were a true cause of melanoma, that these African albinos, who live in areas of intense UV-radiation with little to no protection offered by their skin, would be highly prone to developing melanomas.   However, one study of 111 individuals showed that although nearly one-quarter expressed NMSCs, there was not one single case of melanoma⁹.  In a similar study, it was found that among a total of 164 Tanzanian albinos there were seven cases of SCC, three cases of BCC, and no cases of melanoma¹⁰.  It seems the actual opposite of the predicted scenario:  Melanoma is rare in albinos.  These results offer a powerful paradox in the debate on the etiology of melanoma:  In other population groups, sun exposure and light color constitute a major risk for melanoma, whereas in these albinos sun exposure and light color conversely constitute a low risk for melanoma.  There must be an inherent susceptibility factor that accounts for the ability of the former population to be more likely to develop melanoma than the latter population.  Also from the results and incidence patterns throughout these three populations (fair-skinned, dark-skinned, African albino) there seems to be a subtype of melanoma tumors that involve chronic sun-induced damage and a subtype that involve low to no sun-induced damage whatsoever.  The plot thickens…  

The Missing Link:  BRAF & MC1R

Both epidemiological¹⁶ and molecular^{17, 18} studies suggest that different types of melanoma tumors can be distinguished on sun-exposed skin within Caucasian populations.   Tumors on skin with few or no histopathological signs of chronic sun-induced-damage (CSD) occur in younger individuals, and are found to exhibit frequent BRAF oncogene mutations (see figure 5. and related material).  Conversely, melanomas on skin that display signs of CSD are found to affect older individuals, have different patterns of chromosomal aberrations, and have a lower frequency of BRAF mutations¹¹.  The fact that melanomas that affect anatomic sites exposed to UV-radiation predominantly affect Caucasians, and that the non-CSD melanomas must occur at a relatively low UV doses, implies that the high frequency of BRAF mutations in this type of melanoma must be due to an inherent factor that makes Caucasian individuals more predisposed to be afflicted with.

Several of the well-known risk factors for melanoma have a genetic basis.  Fair skin, red hair, and freckling are genetically determined as complex traits with the input of many gene products in determining the final phenotype⁹.  The gene that has been studied most in this context is none other than the melanocortin 1 receptor gene.  As proposed by Landi, Bauer, et.al, [11] the MC1R turns out to be a promising candidate for this melanoma “susceptibility factor” within Caucasian populations.  MC1R is a G-protein coupled receptor on melanocytes that responds to alpha-melanocyte stimulating hormone (α-MSH) secreted in response to UVR (see figure 6).  This gene happens to be highly polymorphic within Caucasians, with sequence variants that can result in partial or complete loss of the receptor’s signaling ability^†. 

In order to verify whether there is an association between MC1R variants and the BRAF mutant melanoma, both the entire coding region for MC1R and the mutational hotspot for BRAF were sequenced in two samples:  the first, from 85 patients from a case-control study conducted in Italy from 1994 to 1999; the next from an independent set of 112 invasive primary cutaneous melanomas examined at the Department of Dermatology at the University of California, San Francisco, in 2004 and 2005.  The results showed that BRAF mutations were more frequent in the non-CSD melanoma cases with germline MC1R variants than in those with the wild-type alleles (with P-values of .001 and .02 for trend in Italians and Americans, respectively). In an analysis stratified by median age, the association between MC1R and melanoma risk by BRAF mutation s was stronger in the younger subjects of each respective sample. However, formal tests for interaction between age and MC1R were not significant (P = 0.22 and P = 0.13 in the Italian and U.S. populations, respectively). MC1R variation had no effect on the frequency of BRAF mutations in melanomas with CSD¹¹.  These results were dependent on the careful and diligent classification of melanomas into the CSD and non-CSD subtypes¹².

Figure 6-  A basic outline of the MC1R G-protein signaling cascade.  The binding of alpha-MSH  changes (which is produced due to UV-radiation) the conformation of the receptor, which in turn alters the conformation of the G-protein that is bound to the receptor (not pictured here).  The α subunit of the G-protein allows it to exchange its GDP for GTP, which causes the subunits to disassemble into the α subunit and βγ subunit.  The active, GTP-bound α subunit then activates adenylyl cyclase (AC). Activated AC then coverts ATP to cyclic AMP (cAMP), and therefore an intercellular rise in cAMP is brought on.  This rise in cAMP levels activates PKA in the cytosol, and the subsequent releasing of catalytic subunits.  These subunits move into the nucleus, where they phosphorylate  the CREB gene regulatory protein.  Once phosphorylated, CREB recruits the coactivator, CBP, which stimulates gene transcription.  This pathway regulates the production of skin pigmentation in melanocytes.  Germline polymorphisms of the MC1R gene, however, can lead to an increased susceptibility of melanoma^{13, 12}. 

These results demonstrate that the variant MC1R alleles are at least a component of the hypothesized susceptibility.  Also, looking back, epidemiological studies often identify associations between melanoma cancer risk and environmental exposure based on the assumption that UV-radiation holds a certain role in melanoma induction [see 14 for an example].  However, this is an assumption that is largely unfounded!  Alternatively, shown by these results, this difference may be due to specific inheirited genetic polymorphisms.  Therefore, in the epidemiology studies of Caucasian populations, the European anomaly, instead of being explained in reasons that denote an association to UV-radiation can now be explained in a much more comprehensible way.  More so than environmentally stipulating explanations, the anomaly exists simply because the populations of Scandinavia and Scotland, at the genetic level, contain more MC1R variants than do the populations of countries closest to the equator, such as Spain and France.

Other population studies assess the incidence of melanoma in the context of other forms of irradiation. One of these is a compelling case study focused on Icelandic air crews [19]. The aim of the study was , by using a questionnaire, to evaluate whether a difference in the prevalence of risk factors for malignant melanoma in a random sample of the population and among pilots and cabin attendants could explain the increased incidence of melanoma which had been previously been found in studies of aircrews¹⁹.  The questionnaire contained questions to collect information about hair color, eye color, freckles, number of nevi, family history, skin type, history of sunburn, sunbed use, all sunscreen use, and the number (or frequency) of sunny vacations.  All persons in the studies were residents of Iceland, and were compared with the national population register⁼ and vital status for every member.  The difference in constitutional and behavioral risk factors for malignant melanoma between the aircrews and the population sample was not substantial, and therefore, it is unlikely that the increased incidence of malignant melanoma observed in the airline crew can be explained solely by excessive sun exposure of the aircrews compared to the general population.  In other words, the increased frequency of melanoma in the airline crews could not only be account of excessive sun exposure compared with the controls of the study, because there was not a substantial difference in the risk factors between the control group and the airline staff.   An explanation for this could be due to the presence of cosmic radiation, a mixture of gamma rays and neutrons, in which spending large quantities of time at a cruising altitude of 30000 feet of higher can increase the exposure of this radiation greatly.  Neutrons constitute 30-60% of the mixture and there is sufficient evidence that neutrons are carcinogenic to humans. Traditionally, pilots receive a dose of of ionizing radiation that is within regulated range. However, the estimates of the healthy dose are based on the traditional weighing factors of relative biological effectiveness of neutrons, which are actually unknown for humans.

Other studies focus on the incidence levels of atomic bomb survivors in Japan [cross-reference from 19].  One found 11 cases of malignant melanoma in a series of 140 skin cancer cases.  The other study found 10 cases of malignant melanoma in the combined populations of Hiroshima and Nagasaki.  At first,  seemingly unsignificant, these indications of an association are important in regard of the low background incidence of malignant melanoma in Japan.  

Conclusion:  Dilemma within Dilemma

In conclusion, “the skin cancer dilemma”, that is of increasing rates of incidence in today’s world, does not have one easy simplified answer, but instead has a complex, multifactorial explanation.  At least in the case with the non-melanoma skin cancers, a direct  causative link has been established between ultraviolet-radiation and induction.  This is far from the scenario when regarding malignant melanoma.  The etiology of melanoma has turned into a debate pitting those for UV radiation in one camp and those against it in another.  Holding assumptions, which are largely unfounded, will not pave the way for breakthroughs.  Instead, it is an issue which must be examined at many functional levels:  from those small in magnitude, such as genetic mechanisms and molecular/cellular mechanisms, but also from those large in magnitude, such as population and case-control studies.  The answer is not complete and is not an easy one, but new information is being learned everyday.  Time will only tell what the true answer is.                         

*Works Cited*

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