UV RADIATION, DNA DAMAGE, MUTATIONS AND SKIN CANCER - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 245

for the protein p14 ARF (p19 ARF in mice), which does not appear to inhibit any CDK but
binds to MDM2 and thus interferes with the degradation of p53 43 .
Partial or complete homozygous loss of INK4a is observed in a majority
(about 60%) of cell lines derived from sporadic CM, and most of the remaining cell
lines (e.g., 8 out of 11) bear point mutations that are typical of UV radiation, i.e., C to T
transitions at dipyrimidine sites 44 . Although 60-70% of sporadic melanomas (n=62)
show a lack of p16 INK4a expression, and all (n=5) of the metastases, the high number of
homozygous losses and mutations of INK4a found in cell lines is not reproduced in
primary CM. Homozygous deletions are found in approximately 10% and reported
mutation rates range from 0 to 25 % 42 . The reason for this discrepancy is not entirely
clear but it could be due to a high selection for a loss or mutation of INK4a in
generating the cell lines. It should, however, also be noted that LOH at the INK4a locus
in CM is quite common, and is even frequently observed in microdissected dysplastic
nevi (in 75%), potential precursors of CM, as is LOH at the locus of P53 (in 60%) 45 .
Signaling pathways related to p16 INK4a apparently play an important role in the
pathogenesis of CM. And, although the INK4a locus often shows LOH and less
frequently mutations in primary CM, it is not clear if and to what extent solar UV
radiation is responsible for these pertinent genetic changes.

RTK growth-stimulating pathway and INK4a in CM

Another family of genes that is implicated in CM are the RAS oncogenes, more
specifically N-RAS . 25-70% of CM from regularly sun-exposed sites have been reported
to carry activating point mutations in N-RAS , whereas none of the CM from irregularly
exposed sites carried such mutations 46-47 . In a comparative study the percentage of
N-RAS mutated CM from sun-exposed sites was higher in an Australian population
(24%) than in a European population (12%) 48 . These mutations occur at dipyrimidine
sites, the typical UV targets, but they are not dominated by C to T transitions.
As mentioned earlier, the RAS proteins function in mitogenic pathways which
start by activation of RTK at the cell membrane, e.g. the receptor for epidermal growth
factor, EGF-R. It is well known that oncogenic RAS will transform most immortal cell
lines and make them tumorigenic upon transplantation into nude mice. Surprisingly,
Serrano et al. 46 found that expression of oncogenic RAS (producing an activated H -
RAS G12V ) in primary human or rodent cells results in a state that is phenotypically
characterized as “senescence”: the cells are viable and metabolically active but remain
in the G1-phase of the cell cycle. This oncogenic RAS-induced arrest in G1 is
accompanied by an accumulation of both p16 and p53. The link between these
pathways is likely to be mediated by p14 ARF 49 . Inactivation of either p16 or p53
prevented this G1 arrest: the arrest did not occur in p53-/- cells, p16-/- cells, cells
transfected with a dominant negative p53 mutant ( p53 175H ) and cells with mutant
Cdk4 R24C insensitive to p16. Thus, cells immortalized by dysfunctional p16 or p53 will
not go into senescence upon RAS activation, but may progress to a tumorigenic state.
In a fish model (with hybrids of Xiphophorus maculatis and helleri ) an RTK
gene ( Xmrk ) of EGF-R family and an Ink4a homolog ( CdknX or DIFF ) appear to be
important for hereditary CM 50-51 . This provides experimental evidence for the
cooperation of an RTK mitogenic pathway and dysfunctional Ink4a

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