Detection of phosphatidylserine-positive exosomes for the diagnosis of early-stage malignancies

Background: There has been increasing interest in the detection of tumour exosomes in blood for cancer diagnostics. Most studies have focussed on miRNA and protein signatures that are surrogate markers for specific tumour types. Because tumour cells and tumour-derived exosomes display phosphatidylserine (PS) in their outer membrane leaflet, we developed a highly sensitive ELISA-based system that detects picogram amounts of exosomal phospholipid in plasma as a cancer biomarker. Methods: This report describes the development of a highly specific and sensitive ELISA for the capture of PS-expressing tumour exosomes in the blood of tumour-bearing mice. To monitor the relationship between tumour burden and tumour exosome plasma concentrations, plasma from one transplantable breast cancer model (MDA-MB-231) and three genetic mouse models (MMTV-PyMT; breast and KIC and KPC; pancreatic) were screened for captured exosomal phospholipid. Results: We show that quantitative assessment of PS-expressing tumour exosomes detected very early-stage malignancies before clinical evidence of disease in all four model systems. Tumour exosome levels showed significant increases by day 7 after tumour implantation in the MDA-MB-231 model while palpable tumours appeared only after day 27. For the MMTV-PyMT and KIC models, tumour exosome levels increased significantly by day 49 (P <= 0.0002) and day 21 (P <= 0.001) while tumours developed only after days 60 and 40, respectively. For the KPC model, a significant increase in blood exosome levels was detected by day 70 (P = 0.023) when only preinvasive lesions are microscopically detectable. Conclusions: These data indicate that blood PS exosome levels is a specific indicator of cancer and suggest that blood PS is a biomarker for early-stage malignancies.