Viral oncoprotein antibodies as a marker for recurrence of Merkel cell carcinoma: A prospective validation study

Abstract

Background: Merkel cell carcinoma (MCC) is an aggressive skin cancer with a recurrence rate of >40%. Of the 2000 MCC cases per year in the United States, most are caused by the Merkel cell polyomavirus (MCPyV). Antibodies to MCPyV oncoprotein (T-antigens) have been correlated with MCC tumor burden. The present study assesses the clinical utility of MCPyV-oncoprotein antibody titers for MCC prognostication and surveillance.

Methods: MCPyV-oncoprotein antibody detection was optimized in a clinical laboratory. A cohort of 219 patients with newly diagnosed MCC were followed prospectively (median follow-up, 1.9 years). Among the seropositive patients, antibody titer and disease status were serially tracked.

Results: Antibodies to MCPyV oncoproteins were rare among healthy individuals (1%) but were present in most patients with MCC (114 of 219 patients [52%]; P < .01). Seropositivity at diagnosis independently predicted decreased recurrence risk (hazard ratio, 0.58; P = .04) in multivariate analyses adjusted for age, sex, stage, and immunosuppression. After initial treatment, seropositive patients whose disease did not recur had rapidly falling titers that became negative by a median of 8.4 months. Among seropositive patients who underwent serial evaluation (71 patients; 282 time points), an increasing oncoprotein titer had a positive predictive value of 66% for clinically evident recurrence, whereas a decreasing titer had a negative predictive value of 97%.

Conclusions: Determination of oncoprotein antibody titer assists in the clinical management of patients with newly diagnosed MCC by stratifying them into a higher risk seronegative cohort, in which radiologic imaging may play a more prominent role, and into a lower risk seropositive cohort, in which disease status can be tracked in part by oncoprotein antibody titer. Cancer 2017;123:1464-1474. © 2016 American Cancer Society.

Keywords: Merkel cell carcinoma (MCC); Merkel cell polyomavirus (MCPyV); T antigen; oncoprotein; serology; skin cancer.

Figure 1. Rationale for a viral serologic assay for Merkel cell carcinoma (MCC) recurrence

Panel A: Clinical and microscopic characteristics of a Merkel cell polyomavirus (MCPyV) positive MCC arising on sun-exposed skin. Tumor sections containing stroma (pink on H/E staining) demonstrate MCC-specific expression of cytokeratin-20 (CK20) in a perinuclear dot-like pattern and express viral large T-Ag oncoprotein (CM2B4 antibody) (scale bar = 50 μm). Panel B: Schematic of the Merkel cell polyomavirus (MCPyV) genome and oncoproteins that are persistently expressed in human MCCs. The small and large T-Ag oncoproteins share an amino-terminal domain (common T-Ag) that is recognized by antibodies produced by the majority of patients with MCPyV-positive tumors. The ‘X’ symbols indicate the region in which truncating mutations clonally occur in individual tumors. Panel C: Schematic of MCC development and relative MCPyV-oncoprotein antibody titers. Panel D: Distribution of antibody titers among control subjects and MCC patients. 1% of healthy blood donors (n=100) are seropositive as compared with 52% of MCC patients (n=219) at the time of diagnosis.

Figure 2. Patient inclusion schematic for demographic and outcomes analyses

219 patients were evaluable for demographic and survival analyses and 71 patients were both seropositive and had serial draws and thus could be included in recurrence analyses. An additional 43 patients were seropositive at diagnosis but excluded from survival analyses due to lack of a follow-up draw (38 patients), inability to receive definitive therapy (4 patients), or recurrence prior to first draw (1 patient).

Figure 3. Prospective validation of serial MCPyV antibody levels as a marker for recurrence among patients who are antibody positive at diagnosis

A total of 71 patients were antibody positive at diagnosis, definitively treated, and had serial follow-up draws (n=282 draws: 71 initial and 211 follow-up) during the study period. These patients were followed prospectively to determine whether MCPyV antibody trend between draws could be used clinically as a tumor marker/biomarker for recurrence. A) Association of titer trend with recurrence. A total of 176 draws were falling by >20% as compared to previous draw. In 98% of cases, patients were without evidence of disease by clinical assessment (exam and/or scans). *After statistically accounting for multiple draws in an individual subject, the negative predictive value (likelihood that a falling titer represented no progression) was 97%. Conversely, there were 17 draws from 13 patients associated with a rising titer. In 59% of cases, recurrence could be detected at the time of the positive/rising titer while an additional 29% developed overt metastasis after the study period. The positive predictive value for detectable recurrence at the time of rising titer was 66%. **p<0.05 for proportion of patients in recurrence, comparing patients with rising titer(59%) with patients with falling/negative titer (2%). The number of patients in the three groups (stable, failing, or rising titers) is greater than 71 because some patients were evaluable in more than one category (such as a patient who initially had a falling titer and then later had a rising titer). B) Examples of individual patients who did not recur. 4 of the 54 patients who did not recur during the study period are shown. Approximately half of patients who did not recur became seronegative during the study period, at a median of 9 months. Note the logarithmic Y-axis (titer) and the X (time) axis in years. C) Examples of individual patients who recurred. 3 of the 17 patients who developed recurrence during the study period are shown. Note that the X (time) axis is now depicted in months.