(238d) Ultrasensitive Single-Molecule Detection of a Retrotransposon-Encoded Protein As a Multi-Cancer Blood Biomarker

The early detection of many cancers remains a critical challenge towards improving patient outcomes. Liquid biopsies are highly promising, as they provide abundant molecular information via circulating biomarkers and are minimally invasive, thus facilitating widespread screening and serial sampling. However, many current circulating cancer biomarkers have insufficient clinical sensitivity and specificity, and can exist at very low, often undetectable levels in the blood, particularly during early disease stages. Here, we demonstrate the detection of a long interspersed element-1 (LINE-1) retrotransposon-encoded protein, open reading frame 1 protein (ORF1p), as a blood-based biomarker for multiple cancers. While ORF1p expression has been shown to be elevated in tumor tissues in multiple cancers, circulating ORF1p has remained largely unexplored. An ultrasensitive digital enzyme-linked immunosorbent assay (ELISA) platform, Single Molecule Arrays (Simoa), was developed to detect ORF1p with femtomolar sensitivity. The exceptional sensitivity of the assay enabled the successful detection of ORF1p in the plasma of patients with various cancers, with notably elevated levels in ovarian and colorectal cancers. Importantly, we validate circulating ORF1p as a highly specific biomarker for cancer, with largely undetectable levels in healthy individuals. Our results show that the ORF1p Simoa assay can potentially serve as a single biomarker multi-cancer screening test that is amenable to widespread clinical implementation.