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Molecular Oncology, Markers, Clinical Correlates |
Departments of Dermatology [P. E. H., J. E. O.] and Plastic Surgery [P. N. M.], Leicester Royal Infirmary, Leicester LE1 5WW; Department of Dermatology [J. T. L., A. G. S.], North Staffordshire Hospital, Stoke-on-Trent, Staffordshire ST4 7PA; Department of Dermatology, Royal Cornwall Hospitals, Truro, Cornwall TR1 3LJ [P. W. B.]; and Department of Mathematics [P. W. J.] and Centre for Pathology and Molecular Medicine, School of Postgraduate Medicine [R. C. S., A. A. F], University of Keele, Staffordshire ST5 5BG, United Kingdom
| ABSTRACT |
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3.5 mm; P = 0.001; odds
ratio = 31.5). Thus, polymorphisms of the VDR gene,
which would be expected to result in impaired function, are associated
with susceptibility and prognosis in MM. These data suggest that
1,25(OH)2D3, the ligand of the VDR, may have a
protective influence in MM, as has been proposed for other
malignancies. | INTRODUCTION |
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Breslow thickness at presentation remains the most important single
prognostic factor for patients with cutaneous MM (3)
. In
general, patients with thin tumors have a much longer survival than
those with thick lesions; the 5-year survival rate for lesions <1.5-mm
thick is 93%, compared with 67% for 1.5 mm-3.49 mm and 37% for
3.5
mm (4)
. Risk factors for thicker tumors, and hence poorer
prognosis, include age at initial presentation and tumor site.
Relatively little is known of the genetic factors that mediate susceptibility to, and outcome of, sporadic MM. Several putatively important genes, including the susceptibility genes melanocyte stimulating hormone receptor (5 , 6) , glutathione S-transferase GSTM1 (7) , and cytochrome P450 CYP2D6 (8 , 9) as well as the cancer candidate genes, p16INK4a and p15INK4b (10) , have been studied, although thus far only the CYP2D6 PM genotype has been associated with increased risk in independent studies.
We propose that the VDR gene may influence susceptibility and outcome in MM. This view is supported by data showing that 1,25(OH)2D3 (the hormonal derivative of vitamin D3 and the ligand of the VDR) has antiproliferative and prodifferentiation effects in VDR-expressing cell types (11, 12, 13, 14) . Furthermore, associations have been identified between 1,25(OH)2D3 and susceptibility to, and outcome of, systemic malignancies such as breast, prostate, and colon. These include association with both serum vitamin D/1,25(OH)2D3 levels as well as with polymorphisms in the VDR gene (15, 16, 17, 18, 19) .
Similar supportive data exist for MM. Thus, melanocytes and MM cells express the VDR, and 1,25(OH)2D3 has an antiproliferative effect in vitro (20 , 21) . For example, stimulation of tyrosinase activity, a specific prodifferentiation stimulus, has been reported in melanocytes exposed to 1,25(OH)2D3 (21) . In vivo, there is currently little evidence of involvement of vitamin D3, although low serum levels of 1,25(OH)2D3 have been reported in patients with MM (22) . The role of sun exposure in MM is unclear. Current literature remains controversial, with most clinicians advocating a causative association between UV exposure and risk, whereas other studies support the view of a possible protective effect of vitamin D (generated at least in part by UV). For example, use of sunscreens is associated with increased MM risk, an all-year tan appears protective, and outdoor occupation appears to demonstrate no association with susceptibility to MM.
Five polymorphic sites have been identified in the VDR. These comprise RFLP in exon 2 (FokI restriction site), the last intron (BsmI and ApaI restriction sites), and an adjacent area of exon 9 (TaqI restriction site) as well as a poly(A) microsatellite length polymorphism in the 3' untranslated region. The FokI polymorphism results in an altered translation start site and has been shown to be functionally relevant (23) . The other four sites demonstrate linkage disequilibrium, and there is evidence to suggest functional consequences of these polymorphisms (24) .
Because these data support the view that polymorphism in the VDR gene may be an important determinant of susceptibility and outcome in patients with MM, the aim of the present study was to investigate the relationship between the VDR polymorphisms and susceptibility to and prognosis (as estimated by Breslow thickness) of MM. Because there is no evidence of linkage disequilibrium between the FokI RFLP and the cluster of polymorphisms at the 3' end of the gene and there is evidence to suggest functional consequences of each of these polymorphic regions, we have concentrated on the FokI polymorphism and a representative example of the 3' cluster (TaqI RFLP).
| PATIENTS AND METHODS |
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80% of all
eligible patients and represents a typical sample of MM patients
presenting to dermatologists in the participating centers. The controls
(n = 108) comprised randomly recruited, hospital-based
Northern European Caucasians attending these Dermatology departments
with basal cell papillomas and without clinical or histological
evidence of malignancy. Subjects with a history of inflammatory
pathology were also excluded. The study was performed with local
Ethical Committee approval, and informed consent was obtained from all
of the individuals recruited.
Cases and controls were interviewed by a dermatologist (J. E. O.,
J. T. L., A. G. S., or P. W. B.). The following demographic and
clinical data were recorded: patient age at presentation, gender, skin
type in terms of propensity to sun burning and tanning using the
Fitzpatrick classification (25)
, eye and hair color at age
21 years, tumor site, and Breslow thickness. Breslow thickness (defined
as the vertical thickness of the tumor from the granular layer of the
epidermis to the deepest part of the melanoma) was determined by
specialist pathologists. On the basis of Breslow thickness, patients
were divided into five categories: in situ, <0.75 mm,
0.751.49 mm, 1.53.49 mm, and
3.5 mm. Table 1
shows the distribution of these
clinical parameters in the total case group. As also indicated in Table 1
, complete clinical data could not be obtained from all patients
because of insufficient time in busy clinics (7495% for
TaqI and 7292% for FokI genotyped cases).
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Statistical Analysis.
Statistical analysis was undertaken using the Stata software
package (version 5.0; Stata Corp., College Station, TX).
2 tests were used to test for homogeneity
between and within cases and controls (28)
. Because some
frequencies were small, the StatXact-Turbo statistical package (version
3; Cytel Software Corp., Cambridge, MA) was used to obtain exact
significance levels (Ps). Logistic regression analysis was
used to examine differences between cases and controls while
simultaneously correcting for imbalances in age and gender. Logistic
regression was also used to examine differences in genotype frequencies
between cases stratified by Breslow thickness, while correcting for age
at presentation and gender. Significant associations of combined
genotypes (e.g., ttff) were only accepted if they
remained significant in the presence of the main effects
(i.e., a model including ttff, tt, and
ff). If the significance of the combined genotype
disappeared, this would suggest that the factors were acting
independently, and the significance of the combined effect was driven
by the strength of either (or both) of the component factors.
Associations of Breslow thickness were confirmed using linear
regression after transformation of thickness values to normality and
correction for age at presentation and gender. Because some tumors were
in situ (0 mm thick), transformation was performed using the
formula: ln (Breslow thickness + 1).
| RESULTS |
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Case-Control Analysis.
Table 2A
shows the allele frequencies of TaqI and
FokI alleles in controls and MM cases. All allele
frequencies conformed to Hardy-Weinberg equilibrium. The F
allele was significantly less common in MM cases than controls
(P = 0.029; OR, 0.69; 95% CI, 0.500.96). Table 2B
shows the relationship between TaqI and FokI
genotypes in cases and controls. No significant correlations between
genotypes at the two sites were identified in either controls
(P = 0.365) or cases (P = 0.847),
suggesting that the two polymorphisms did not demonstrate linkage
disequilibrium. Table 1
shows frequencies
of FokI and TaqI genotypes in controls compared
with MM cases. There was a decreased proportion of individuals with the
FokI FF genotype in cases versus
controls. Thus, for FF versus other
FokI genotypes, the uncorrected OR for MM was 0.60
(P = 0.026). The findings remained significant after
correction for age and gender using multivariate logistic regression
(OR, 0.59; P = 0.029). The estimated risk reduction
attributable to the FF genotype was 23.7% (95% CI,
1.251.3%).
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21 = 5.31 and
P = 0.040,
21
= 4.21, respectively).
Association for VDR Genotypes with Breslow Thickness.
Patients were categorized by Breslow thickness (Table 1)
. In tumors
1.5-mm thick, for TaqI, there was an increased proportion
of the tt genotype (P = 0.047), but there
was no obvious effect for FokI (P = 0.701).
Homozygosity for variant alleles at either FokI and
TaqI loci (tt or ff genotypes) was
associated with an increased proportion of tumors
3.5 mm thick,
although this did not achieve statistical significance (tt:
P = 0.105; OR, 2.84; and ff:
P = 0.266; OR, 1.99, uncorrected).
The effects of combinations of the TaqI and
FokI polymorphisms are shown in Table 3
. There was an association of
ttff combined genotype with thicker tumors, using either
1.5 mm (P = 0.065) or
3.5 mm (P <
0.001) as the cutoff. These results retained similar significance,
particularly for tumors
3.5-mm thick, after correction for potential
confounding factors (age, gender, and tumor site). Thus, the mean
Breslow thickness in patients with the ttff genotype
combination was 2.9 mm compared with 1.1 mm in patients with other
genotype combinations. This association was further confirmed using
linear regression analysis, which showed that the ttff
genotype was correlated with Breslow thickness (P =
0.002, transformed to normality and corrected for age and gender).
Significant associations were also identified between Breslow thickness
and combinations of genotypes including the genotypes Ttff
and ttFf, although these were less effective at predicting
Breslow thickness.
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| DISCUSSION |
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Data for MM are similar, although more limited. Normal (38) and malignant melanocytes (20) express the VDR, and 1,25(OH)2D3 has been shown to inhibit normal (21) and malignant melanocyte (20) growth in vitro. In a study of 1,25(OH)2D3 serum levels in MM patients, lower levels were found compared with controls, although this did not achieve statistical significance (22) .
In our study, homozygosity for the wild-type (F) allele at the FokI restriction sites was associated with a reduced risk of MM, with a risk reduction attributable to the FF genotype estimated at 23.7%. Furthermore, the proportion of F alleles was significantly lower in the case group compared with controls. The number of controls, however, was relatively small, and larger cohorts would be required to reduce the risk of both type 1 and type 2 errors. In this initial study, we have used hospital-based controls. Selection of control subjects is always difficult, and although the use of "normal" volunteers or blood donors would reduce the risk of potential bias because of occult associations with other disease processes, since they are generally not examined by a clinician, the possibility of undetected malignant or inflammatory pathologies cannot be excluded. By use of hospital-based controls, it was possible to focus only on controls who were clinically free of other malignant or inflammatory pathologies. Furthermore, the control genotype frequencies were similar to those described in other studies (18 , 26 , 27 , 39) , supporting the view that our control group is representative of the normal population.
The FokI RFLP has been reported previously to be associated
with breast cancer (16)
, where the FF genotype
was associated with a decreased risk of
50% in certain racial
groups. The poly(A) polymorphism (classified into long, L,
or short, S) has been associated with altered risk of breast
(16)
and prostate (18)
cancer. In breast
cancer, LL and LS alleles were also associated
with a
50% reduction in risk (16)
. However, in
prostate cancer, the presence of L, whether in the
heterozygous (LL) or homozygous (LS) state, was
associated with a 45-fold increased risk of prostate cancer
(18
, 37)
. Because the TaqI restriction site is
in strong linkage disequilibrium with the poly(A) polymorphism
(T demonstrates linkage disequilibrium with L;
Ref. 39
), the findings in breast cancer are comparable
with our findings in MM, although our data on the TaqI RFLP
did not achieve statistical significance.
Our data also identified an association between VDR genotypes and red hair in patients with MM. There was insufficient hair color data on our control subjects to examine whether this was a general phenomenon. Although the mechanism for this association is not known, other studies have identified links between polymorphism at other loci and hair color in MM (6 , 8) . These data suggest that the molecular route by which patients with red hair develop MM may differ from patients with other hair colors, supporting the view that these patients represent a high risk subgroup. However, these data require confirmation in independent studies, including in control individuals.
More significantly, we have identified significant associations
between VDR genotypes and outcome in patients with MM. Thus, our data
suggest that VDR polymorphism is a better determinant of outcome in MM
than of its initiation. Melanoma depth is well recognized as an
important prognostic indicator with respect to risk of metastatic
disease and survival (40)
. In general, for both
restriction sites, the proportion of thick MMs (with either
1.5 or
3.5 mm cutoff) increased with increasing number of variant alleles.
The effect of VDR genotypes on Breslow thickness was markedly increased
when the two polymorphic sites were considered together. Thus, the
combined ttff genotype was associated with tumors
1.5-mm
thick but particularly those
3.5-mm thick (P <
0.001; Table 3
). We also corrected the data for the
potential confounding effects of gender, tumor site, and age at
presentation because thicker tumors are associated with male gender,
head/neck tumor site, and older age. The association of the
ttff genotype remained significant, suggesting that the
effect on Breslow thickness is independent of these factors. Similar
results were obtained with other genotype combinations, although the
magnitudes of the effects were smaller, suggesting that the
heterozygote genotypes were of intermediate importance in determining
Breslow thickness. Similarly, in carcinoma of the prostate, poly(A)
microsatellite variants are reported to be associated with more
advanced disease (37)
. In addition, low serum levels of
1,25(OH)2D3 have been
implicated in metastatic rather than in situ disease in
prostatic cancer, suggesting an impact on tumor progression rather than
development (17)
.
The polymorphism at the FokI restriction site (T-C transition) produces an ATG start codon resulting in translation initiation 10 bp upstream and therefore the production of a lengthened protein of 427 amino acids (26 , 41) . The F allele (restriction site absent, ACG), which results in a shorter protein, has been shown to be more effective at activating the transcription of a VDR reporter construct (23) , thereby indicating that the polymorphism is functionally significant. The cluster of polymorphisms at the 3' end of VDR, which includes TaqI, are in mutual tight linkage disequilibrium and a representative, Bsm, is known to be in linkage disequilibrium with the poly(A) microsatellite (39) . It has been suggested that the length of the poly(A) repeat affects mRNA stability or is tightly linked to a further functionally significant site (24) . The net effect of the ff and the tt polymorphisms can be envisaged as a reduction in the cellular effect of 1,25(OH)2D3 and therefore a growth advantage of the melanocytes. This conclusion is supported by the increased effect of combined homozygosity, which would be expected to have a more profound effect on the VDR protein.
These data support the hypothesis that the VDR genotype has a significant role in determining tumor occurrence and behavior in MM and indicate a role for vitamin D in melanoma cell cycle control and differentiation in vivo. There is evidence of a blocking effect of 1,25(OH)2D3 at the transition from G1 to S phase of the cell cycle via several mechanisms, such as stimulation of the CDK inhibitory proteins, P21 (42) , which contains a VDR response element (43) , and P27 (44) , and inhibition of cyclin D1 (45) . It is of interest that the other reported genetic changes associated with melanoma also have an impact at the G1 to S-phase check point. These include mutations of the CDK inhibitor genes, p16INK4a (46, 47, 48) and p15INK4b (10) , and mutations in the CDK4 gene (49 , 50) . This point in the cell cycle may therefore be of pivotal importance in the development and/or progression of MM.
It has been argued above that the effect of the VDR polymorphisms, reported here, is a functional, cellular deficiency of 1,25(OH)2D3. Decreased serum levels of 1,25(OH)2D3 have been reported in certain cancers (15 , 17 , 19) , including MM (22) . Furthermore, some studies suggest that vitamin D deficiency, resulting from decreased cutaneous production from solar irradiation, may be contributory to the development of carcinoma of the breast (51) , prostate (52) , and colon (53) . The role of sun exposure in MM is, however, more complex. On the one hand, it is firmly held by the majority of clinicians that solar radiation is causative in MM, and on the other, there is the potential deleterious effect of vitamin D deficiency, a cause of which is lack of sun exposure. Inhibition of MM cell growth in vitro by 1,25(OH)2D3 (20) , the effect of VDR polymorphisms described in this study, and low serum levels of vitamin D (22) implicate a possible role of vitamin D deficiency in MM pathogenesis. In addition, a protective effect of vitamin D (produced at least in part by sun exposure) might explain previous ambiguous results, such as the increased incidence of melanoma associated with sunscreen use (54) , the protective effect of an all-year tan (55) , and the lack of an association of MM with outdoor occupation (56) . The effect on vitamin D status in these circumstances warrants further investigation, particularly in the climates where these associations have been reported.
| FOOTNOTES |
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1 Supported by the Cancer Research Campaign
(Project Grants SP2207/0201 and SP2402/0101). ![]()
2 To whom requests for reprints should be
addressed, at Department of Dermatology, Leicester Royal Infirmary,
Leicester LE1 5WW, United Kingdom. Phone: 44-0116-258-5762; Fax:
44-0116-258-6792. ![]()
3 Present address: Department of Dermatology,
Bristol Royal Infirmary, Bristol, BS2 8HW, UK. ![]()
4 The abbreviations used are: MM, malignant
melanoma; VDR, vitamin D receptor; OR, odds ratio; 95% CI, 95%
confidence interval; calcitriol, 1,25(OH)2D3;
CDK, cyclin-dependent kinase. ![]()
Received 8/ 4/99; revised 10/20/99; accepted 10/25/99.
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,25-dihydroxyvitamin D3,
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,25-dihydroxyvitamin D3 on the growth of the renal carcinoma cell line. Kidney Int., 29: 834-840, 1986.[Medline]
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