Association between gene expression profile, proliferation and metastasis in uveal melanoma

Abstract

Purpose: Uveal melanomas cluster into two molecular groups based on their gene expression profile. Tumors with the class 1 signature rarely metastasize, whereas those with the class 2 signature have a very high rate of metastasis. However, the biological basis for this metastatic propensity of class 2 tumors remains unclear. Towards such an explanation, this study was conducted to determine whether class 2 tumors have a higher proliferative rate than class 1 tumors.

Materials and methods: The study included 28 primary uveal melanomas with extensive clinical, pathologic, and genetic annotation, including age, gender, ciliary body involvement, tumor basal diameter, thickness, cell type, gene expression profile, status of chromosomes 3 and 8p, aneuploidy, and clinical outcome. Immunopositivity for Ki-67 was determined by counting all positive nuclei in representative whole tumor sections.

Results: Ki-67 positivity was significantly associated with class 2 gene expression profile, loss of chromosome 3 and increased aneuploidy (P = 0.04, P = 0.004, and P = 0.03, respectively). Ki-67 positivity showed a borderline significant association with epithelioid cell type (P = 0.07). Receiver operating characteristic (ROC) analysis of Ki-67 positivity, using the class 2 signature as an endpoint, identified a Ki-67 score of approximately 20 cells per high power field as the optimal cut-off point between low and high risk for metastasis (log rank test, P = 0.01).

Conclusions: On average, class 2 uveal melanomas have a higher proliferative rate than class 1 tumors. Further work is needed to determine whether loss of chromosome 3, increased aneuploidy, or other factors may be responsible for the increased proliferation.

Figure 1

Representative photomicrographs of tumors with low (A), medium (B) and high(C) Ki-67 positivity. Positive cells are indicated by dark blue nuclei. Sections are counterstained with nuclear fast red. Brown pigment is melanin.

Figure 2

Kaplan-Meier survival plot showing relationship between Ki-67 positivity and uveal melanoma metastasis. High Ki-67 was defined as ≥ 1.5 positive cells per unit area (approximately 20 positive cells per high power field).

Figure 3

Graphs showing relationship between Ki-67 positivity and gene expression class (A), loss of chromosome 3 (B) and aneuploidy (C). Significance levels calculated by Mann-Whitney test are indicated for A and B. Linear regression of data in C (line) indicates a positive relationship between tumor mitotic activity and aneuploidy.