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Advances in Brief |
Departments of Surgical Oncology [G. I. S., T. M., A. S., T. K. D. G.] and Pathology [M. F.], College of Medicine, and Department of Mathematics, Statistics, and Computer Sciences [D. M.], University of Illinois at Chicago, Chicago, Illinois 60612
| ABSTRACT |
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50% Mitf expression
(P = 0.04). Our data suggest that Mitf
may be a new molecular prognostic marker in patients with
intermediate-thickness melanoma. | Introduction |
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A number of genetic and cell biological models indicate that pigment cell development and function depend on both cell-extrinsic and -intrinsic factors. Although how these factors are functionally integrated is poorly understood, it is generally agreed that the basic helix-loop-helix-zipper Mitf3 relates to the downstream targets responsible for cell proliferation, fate determinism, and melanin biosynthesis (5 , 6) . Mutations in Mitf are found from fishes to man. In fact, Mitf is critical not only for neural crest-derived melanocytes of the integument and inner ear (7) but also for the cells of the retinal pigment layer, which develop from the neural epithelium of the optic cup.
Mouse mutations at the micropthalmia locus (mi) are
characterized by small nonpigmented eyes, lack of integumentary
melanocytes, and deficiencies in the inner ears. Also, these mutations
are associated with a deficiency in mast cells and osteoclasts,
resulting in a condition similar to osteopetrosis in humans. In humans,
Mitf mutation is characterized by hearing deficiencies, white forelock,
and ocular anomalies (Waardenburg syndrome IIA; Ref. 8
).
It is believed that Mitf can induce pigment cell-specific
transactivation of tyrosinase and tyrosinase-related protein 1
(9, 10, 11)
. In addition,
-MSH has been shown to
up-regulate the pigment gene expression via a cyclic AMP-mediated
signal transduction pathway that stimulates the expression of Mitf
(12)
. Moreover, it was reported that the c-Kit signaling
pathway in melanocytes targets transcription factor for micropthalmia
simultaneously for short-lived activation and net degradation
(13)
. Recently, King et al. (14)
reported that Mitf is a sensitive and specific melanocyte marker for
melanoma diagnosis.
Because Mitf appears to be involved in melanocyte development and differentiation, we hypothesized that its expression correlates with survival in patients with intermediate-thickness (14 mm) melanoma. Our findings suggest that Mitf expression in the primary tumor correlates with patient survival.
| Materials and Methods |
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Immunohistochemistry.
Four-µm-thick sections were used for immunocytochemical studies.
Standard avidin-biotin-peroxidase immunohistochemistry was performed
with the mouse monoclonal IgG1 micropthalmia antibody Ab-2
(Neomarkers, Union City, CA). Antigen retrieval was accomplished with 1
mM EDTA buffer. We used 1% dried skim milk to block
nonspecific binding before incubation with the primary antibody for
2 h (1:25 dilution). Aminoethylcarbazine was used as a chromagen.
Evaluation of Staining.
The stained slides were graded as follows according to the percentage
of cells whose nuclei stained positive for Mitf: (a) 0, 0%;
(b) +1, 125%; (c) +2, 2650%; (d)
+3, 5175%; and (e) +4, >75%. Cytoplasmic staining
was excluded from the analysis. Representative
immunohistochemical staining of two different primary melanomas is
shown in Fig. 1
.
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25%)
versus +2 to +4 (>25%); and (c) 0 to +2
(
50%) versus +3 to +4 (>50%). Pairwise association
between Mitf staining and known prognostic variables such as thickness,
histology, location of primary tumor, number of
mitoses/mm2, ulceration, and nodal status was
also evaluated. Differences in distribution of variables were assessed
by the Mann-Whitney and Kruskal-Wallis tests. Multivariate analysis was
performed by using the Cox proportional hazards model. P
<0.05 was considered statistically significant. Death from melanoma
was considered the only event for OS. DFS events included regional
recurrences and distant metastases. | Results |
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The median follow-up was 50 months. The mean DFS (Fig. 2A)
was 168.53 ± 13.96 months (5-year DFS,
74.1%). Mean OS (Fig. 2B)
was 171.9 ± 13.12
months (5-year OS, 74.5%).
|
In this small series of 63 patients, only thickness significantly influenced survival when evaluated by univariate analysis. The survival advantage was most marked when patients had primary melanoma thicker than 2.6 mm. In those instances, the DFS and OS were 66.48 months (5-year DFS, 60%) and 67.1 months (5-year OS, 58%), respectively, whereas the DFS and OS were 180.52 months (5-year DFS, 76.5%; P = 0.067) and 183.24 months (5-year OS, 78%; P = 0.039%), respectively, in patients with primary lesions <2.6 mm thick.
Curiously, when stratified according to the presence or absence of ulceration in the primary tumor, ulceration was significant only in patients whose melanomas were >2 mm thick. In patients with ulceration, the DFS and OS were 119.41 months (5-year DFS, 58.9%) and 123.20 months (5-year OS, 60%), respectively, compared with 160 months (5-year DFS, 80.7%; P = 0.086) and 165.14 months (5-year OS, 82.7%; P = 0.05) in patients with nonulcerated primaries.
Expression of Mitf as determined immunohistochemically in the
primary lesion appears to significantly influence both DFS and OS
(Table 1
; Fig. 3
). Moreover, by applying semiquantitative methods, it appears that the
amount of Mitf expression could be directly correlated to both DFS and
OS. Mean DFS and OS in the 11 patients whose melanomas did not stain
positive for Mitf were 71.36 months (median, 40 months; 5-year
DFS, 37.6%) and 80.89 months (median, 51 months; 5-year OS, 42.1%),
respectively. These figures are significantly lower than those for
patients who showed evidence of Mitf expression, for whom the mean DFS
and OS were 186.78 months (5-year DFS, 83.1%; P = 0.0086) and 187.9 months (5-year OS, 84%;
P = 0.0054), respectively. Mean DFS and OS
with >25% of the melanoma cells staining positive for Mitf expression
were 137.84 months (5-year DFS, 92.2%; P = 0.023) and 147.32 months (5-year OS, 92%; P = 0.039) compared with cells in which <25% of the cells stained
positive for Mitf expression, in which the DFS and OS were 127.14
months (5-year DFS, 55.8%) and 136.68 months (5-year OS, 58%),
respectively. This observation was most marked in DFS where >50% of
the cells expressed Mitf. DFS was 212.57 months (5-year DFS, 92.3%) in
these patients as compared with patients in whom <50% of the cells
expressed Mitf, where the mean DFS was 127.27 months (5-year DFS,
64.8%). The P between these two survival figures was
0.035.
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When multivariate analysis was performed on known primary
prognostic variables, patients whose melanomas were >2.6 mm thick had
worse DFS (P = 0.005) and OS
(P = 0.032). When the nodal status was
included as a covariate, both thickness and nodal status significantly
influenced survival: (a) DFS, P = 0.078 and P = 0.05, respectively; and
(b) OS, P = 0.005 and
P = 0.048, respectively. When Mitf expression
with a cutoff value of 25% was included with the other primary tumoral
prognostic variables, it significantly affected DFS
(P = 0.014) and OS (P = 0.018). When all covariates were analyzed together (Table 2)
, Mitf expression emerged as the most significant variable
influencing DFS (P = 0.008). By stepwise Cox
regression, the final model consisted of Mitf expression
(P = 0.015) and thickness
(P = 0.035) influencing DFS, and regional
nodal status (P = 0.048) influencing OS.
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50% of melanoma cells expressed Mitf, the incidence of
nodal metastases was significantly lower. Of the 26 patients with nodal
metastases, 6 (23%) had primary lesions with >50% Mitf expression,
whereas 20 (77%) of the primary melanomas had <50% of the cells
expressing Mitf (P = 0.04). | Discussion |
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Clinically, nodal status is probably the most important prognostic
marker in cutaneous melanoma, and in the primary lesion, thickness and
ulceration are considered the most significant markers for survival. In
this study, we confirmed that all of these three aspects are clinically
reliable markers. However, in this small series, thickness and
ulceration became statistically significant when the lesions were
2
mm. Mitf expression, in contrast, remains significant in all instances
when compared by univariate analysis. In multivariate analysis, using
the stepwise Cox regression model, Mitf expression and nodal status
emerged as the most important variables influencing DFS and OS.
This prognostic correlation between nodal status and Mitf
expression suggests that Mitf expression or lack thereof in primary
lesions is an indicator of regional node metastases. This certainly was
the case in our study. Patients with Mitf expression of >50% had a
significantly lower incidence of nodal disease than those with a Mitf
expression of
50%. The biological role of Mitf in melanocyte
differentiation continues to show new facets. Our findings suggest that
increased Mitf expression in patients with intermediate-thickness
melanomas may lead to more differentiated tumors. This, in turn, leads
to decreased relapse and nodal metastases and improved survival.
In the study reported by King et al. (14) , all melanomas evaluated stained positive for Mitf, but not for other melanocytic markers evaluated. In our study, 82.5% of melanomas stained positive for Mitf. It should be noted that in addition to tissue blocks, we examined archival specimens only for which slides from outside institutions were sent to us for evaluation. The difference in tissue handling may explain, in part, the difference in staining. In addition, we have used a commercially available antibody for immunostaining. However, it is germane to emphasize that Mitf expression is a sign of differentiation; thus, dedifferentiated tumors will either not express or will barely express Mitf, and therefore there will be some lesions in which no Mitf expression will be detected. Thus, lack of expression should not be attributed merely to random occurrence. Our data also suggest that Mitf expression may not be as highly conserved in melanomas as suggested previously.
In summary, the data presented above suggest for the first time that Mitf expression, as determined by immunocytochemistry, can be used as a new prognostic molecular marker in melanoma. Additional prospective studies are warranted to determine whether or not these findings will be validated.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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1 Supported by NIH Training Grant T32-CA09432 and
a grant from the Cancer Federation, Inc. ![]()
2 To whom requests for reprints should be
addressed, at Department of Surgical Oncology (M/C 820), College of
Medicine, University of Illinois at Chicago, 840 South Wood Street,
Chicago, IL 60612. Phone: (312) 996-3288; Fax: (312) 996-9365;
E-mail: geosalti{at}uic.edu ![]()
3 The abbreviations used are: Mitf, micropthalmia
transcription factor; OS, overall survival; DFS, disease-free
survival. ![]()
Received 6/ 1/00. Accepted 8/ 3/00.
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