High MCM3 expression is an independent biomarker of poor prognosis and correlates with reduced RBM3 expression in a prospective cohort of malignant melanoma

Abstract

Background: Malignant melanoma is the most lethal form of skin cancer with a variable clinical course even in patients with thin melanomas and localized disease. Despite increasing insights into melanoma biology, no prognostic biomarkers have yet been incorporated into clinical protocols. Reduced expression of the RNA binding motif protein 3 (RBM3) has been shown to correlate with tumour progression and poor prognosis in melanoma and several other cancer forms. In ovarian cancer, an inverse association was found between expression of RBM3 and the minichromosome maintenance 3 (MCM3) gene and protein. In melanoma, gene expression analysis and immunohistochemical validation has uncovered MCM3 as a putative prognostic biomarker. The aim of the present study was to examine the associations of MCM3 expression with clinical outcome and RBM3 expression in a prospective, population-based cohort of melanoma.

Methods: Immunohistochemical MCM3 expression was examined in 224 incident cases of primary melanoma from the Malmö Diet and Cancer Study, previously analysed for RBM3 expression. Spearman's Rho and Chi-Square tests were used to explore correlations between MCM3 expression, clinicopathological factors, and expression of RBM3 and Ki67. Kaplan Meier analysis, the log rank test, and univariable and multivariable Cox proportional hazards modelling were used to assess the impact of MCM3 expression on disease-free survival (DFS) and melanoma-specific survival (MSS).

Results: High MCM3 expression was significantly associated with unfavourable clinicopathological features and high Ki67 expression. A significant inverse correlation was seen between expression of MCM3 and RBM3 (p = 0.025). High MCM3 expression was associated with a reduced DFS (HR = 5.62) and MSS (HR = 6.03), and these associations remained significant in multivariable analysis, adjusted for all other factors (HR = 5.01 for DFS and HR = 4.96 for MSS). RBM3 expression remained an independent prognostic factor for MSS but not DFS in the multivariable model.

Conclusions: These findings provide validation of the utility of MCM3 expression as an independent biomarker for prognostication of patients with primary melanoma. Moreover, the inverse association and prognostic impact of MCM3 and RBM3 expression indicate a possible interaction of these proteins in melanoma progression, the functional basis for which merits further study.

Virtual slides: The virtual slide(s) for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1814908129755401.

Figure 1

Distribution and sample immunohistochemical images of MCM3 expression in primary and metastatic melanoma. Distribution of immunohistochemical MCM3 expression denoted as the multiplier of fraction and intensity of staining (nuclear score = NS) in a) primary melanoma and b) metastases. Sample images of MCM3 expression in primary tumour and paired metachronous metastasis: c) nodular melanoma on left ear (NS = 2) and d) lymph node metastasis in left groin (NS = 0); e) superficial spreading melanoma on lower left leg (NS = 6) and g) cutaneous metastasis on back of neck (NS = 4); nodular melanoma on lower right leg (NS = 6) and cutaneous metastasis on lower right leg (NS = 9).

Figure 2

Kaplan-Meier estimates of the impact of MCM3 expression on disease free and melanoma-specific survival. Disease survival in strata according to (a) the fraction of staining, logrank p <0.001, (b) intensity of staining, logrank p =0.001, (c) nuclear score, logrank p < 0.001, and (d) low vs high staining, logrank p < 0.001. Melanoma-specific survival in strata according to (e) the fraction of staining, logrank p <0.001, (f) intensity of staining, logrank p = 0.053, (g) nuclear score, logrank p < 0.003, and (h) high vs low staining, logrank p < 0.001. The nuclear score refers to a multiplier of fraction and intensity. Low MCM3 expression refers to tumours with a nuclear score ≤3 and high expression to tumours with a nuclear score >3.

Figure 3

Kaplan-Meier estimates of the impact of MCM3 expression on disease free and melanoma-specific survival in thin melanomas. The impact of low and high MCM3 expression on (a) disease free survival, logrank p = 0.007, and (b) melanoma-specific survival, logrank p = 0.031, in thin melanoma (<= 1 mm).

Figure 4

Kaplan-Meier estimates of disease free and melanoma-specific survival in combined strata of high and low MCM3 and RBM3 expression. The impact of combined categories low and high expression of MCM3 and RBM3 expression, respectively, on (a) disease free survival and (b) melanoma-specific survival.