This new test is a low-cost, multi-cancer detector that can pick up the presence of the telltale protein, known as LINE-1 ORF1p, in a tiny amount of blood in less than two hours.
Cancer biomarker detection is a young and growing field. Recently, an important new biomarker for earlier detection may have emerged: the LINE-1 ORF1 protein (ORF1p). LINE-1 is a retrotransposon, a virus-like element, present in every human cell, that replicates through a copy-and-paste mechanism, resulting in a new copy in a new position in the genome. ORF1p is a protein it produces at high levels in cancer but that is not present at readily detectable levels in healthy cells.
“Transposons are normally expressed in sperm and egg and during embryogenesis, so there are some circumstances where you have nonpathobiological expression of transposons,” says LaCava, a co-author on the paper, who specializes in LINE-1 research. “But otherwise, these ‘jumping genes’ are silenced within the genome, because their activity creates stress and insults in the cell.”
Most of the time, the body keeps LINE-1 in check. “There are layers of mechanisms that prevent LINE–1 from being expressed and producing ORF1p, so we can use the presence of the protein as a proxy for an unhealthy cell that no longer has control over its transcriptome,” LaCava notes. “You shouldn’t find ORF1p in the bloodstream of a healthy person.”
Over the past five years, he adds, “it’s become abundantly clear that these proteins are highly elevated in most cancers,” including many of the most common and lethal cancers of the esophagus, colon, lung, breast, prostate, ovaries, uterus, pancreas, and head and neck.
Because carcinoma cells make ORF1p from the onset of disease, researchers have long sought a sensitive, accurate test to detect ORF1p as early as possible. The ability to spot it in patients before a cancer has a chance to spread could potentially save lives.
The researchers developed a fast, low-cost assay able to detect ORF1p in plasma, which accounts for more than half of the content of human blood. The new study uses a single-molecule-based detection technology developed by the team of senior author Prof. David Walt, at Harvard University and Brigham and Women’s Hospital in Boston, MA. It costs less than $3 to produce and returns fast results.
The single-molecule-based detection technology is very sensitive but is based on antibodies' ability to accurately bind with their intended target. "This accuracy is not to be taken for granted,” says LaCava, who helped develop the antibodies used in the study. Because it is possible that antibodies may sometimes recognize the wrong target, LaCava and Wolters worked together with other members of the international team to corroborate the results of the single-molecule method, using an alternative method called targeted mass spectrometry, in which Wolters is an expert. Targeted mass spectrometry was able to confirm that the identity of the antibody-bound target was ORF1p, demonstrating that the single-molecule assay was itself highly accurate at detecting ORF1p in the blood samples of patients with a variety of cancers.
The ultrasensitive assessment tool successfully quantified ORF1p in cancer patient blood samples, but only in very rare ‘healthy’ patients, supporting the specificity for cancer detection. Studies in tissue samples from ~200 colon cancer and 75 esophageal biopsies, obtained from patients at different disease stages, showed that ORF1p tissue expression was prevalent in carcinomas and high-risk precursor lesions.
“We were shocked by how well this test worked across cancer types,” says lead author Martin Taylor, of the Department of Pathology at Massachusetts General Hospital.
The researchers also analyzed the plasma of 400 healthy people aged 20–90 who’d donated blood to the Mass General Brigham Biobank; ORF1p was undetectable in 97–99% of them. Of the five people who did have detectable ORF1p, the person with the highest level was found six months later to have advanced prostate cancer. In one part of the study, the researchers studied 19 patients being treated for gastroesophageal cancer; in the 13 people who responded to the treatment, levels of ORF1p fell below the detection limit of the assay.
“We’ve known since the 1980s that transposable elements were active in some cancers, and nearly 10 years ago we reported that ORF1p was a pervasive cancer biomarker, but, until now, we haven’t had the ability to detect it in blood tests,” said senior author Prof. Kathleen Burns, Chair of the Department of Pathology at Dana-Farber Cancer Institute in Boston. “Having a technology capable of detecting ORF1p in blood opens so many possibilities for clinical applications. We were fortunate to assemble this tremendous team to push the limits of these assays and obtain and test these precious samples. There’s a lot of excitement as our work continues.”
