Nuclear expression of BAP-1 in transvitreal incisional biopsies and subsequent enucleation of eyes with posterior choroidal melanoma

Abstract

BackgroundAs a majority of patients with choroidal melanoma do not undergo enucleation, tumour tissue for prognostic testing has to be obtained with alternate methods. Transvitreal incisional biopsies enable histological examination as well as immunohistochemical staining of BRCA1-associated protein-1 (BAP-1).MethodsFifty-nine patients diagnosed with choroidal melanoma in transvitreal biopsies between years 2003 and 2019 were included. Twenty-one of these patients subsequently underwent enucleation. The level of nuclear expression of BAP-1 in transvitreal biopsies and enucleations was evaluated and the concordance calculated. Metastasis-free survival and HR for metastasis were analysed.ResultsThe mean tumour thickness and diameter at biopsy was 3.8 mm (SD 2.1) and 9.3 mm (SD 4.8), respectively. For biopsies, 37 of 59 tumours (63%) were classified as having high nuclear BAP-1 expression, and 22 (37%) as low. For enucleations, 13 of 21 tumours (62%) were classified as having high nuclear BAP-1 expression, and 8 (38%) as low. Eighty-six per cent of biopsies had an identical BAP-1 classification as the subsequent enucleation, yielding a Cohen’s kappa coefficient of 0.70. Patients with low nuclear BAP-1 expression in transvitreal biopsies had a significantly shorter metastasis-free survival (p=0.001), with a size-adjusted Cox regression HR for metastasis of 13.0 (95% CI 3.1 to 54.4, p=0.0004).ConclusionLoss of nuclear BAP-1 expression occurred in a large proportion of the small tumours included in this study. BAP-1 immunoreactivity in transvitreal incisional biopsies of choroidal melanoma is substantially concordant with immunoreactivity in enucleated specimens and identifies patients with poor metastasis-free survival.

Intratumor Heterogeneity in Uveal Melanoma BAP-1 Expression

“…Our results confirm the remarkable prognostic value in analysis of BAP-1 expression in uveal melanoma [ 6 , 7 , 8 , 10 , 11 , 12 , 26 ]. In fact, it added significant prognostic value to both tumor size category and gene expression class regardless of in which region it was scored.…”

“…Further, even if the eye has been enucleated and the full tumor cross-section is available for immunohistochemical analysis, a pathologist without the aid of digital image analysis cannot realistically evaluate every single cell. Instead, previously published methods for counting the number of BAP-1 positive cells have focused on a smaller fraction of the tumor, e.g., three high power fields or similar [ 7 , 10 , 11 , 12 ]. The selection of area for analysis could therefore impact the results of BAP-1 scoring significantly.…”

“…Regression analysis and curve fitting was used to test if the difference in BAP-1 expression between hot spots and cold spots could was a function of tumor volume. The volume of tumors was estimated assuming a semi ellipsoid shape, where t is the apical thickness and lbd the largest basal diameter of a tumor [ 12 , 16 ]: …”

“…Immunohistochemical staining of the BAP-1 protein in tumor tissue and assessment of its level of nuclear expression is a relatively quick and inexpensive alternative to genetic testing, with similar prognostic value [ 10 , 11 , 12 ]. If digital image analysis or deep learning algorithms are utilized for this assessment, reproducibility is increased [ 7 , 8 ].…”

“…Previous studies have shown that there is substantial intratumor heterogeneity in both uveal melanoma phenotype and genotype [ 13 , 14 , 15 , 16 ]. If only a smaller portion of a tumor is sampled for prognostic testing including assessment of level of BAP-1 expression, the presence of such heterogeneity may impact the results significantly [ 12 , 17 , 18 , 19 ]. In this paper, we therefore investigate how the proportion of BAP-1 positive cells varies across the cut surface of uveal melanomas, and the prognostic weight of different tumor regions.…”