Cancers comprise a heterogeneous group of human diseases. Unifying characteristics include unchecked abilities of tumor cells to proliferate and spread anatomically, and the presence of clonal advantageous genetic changes. However, universal and highly specific tumor markers are unknown. Herein, we report widespread long interspersed element-1 (LINE-1) repeat expression in human cancers. We show that nearly half of all human cancers are immunoreactive for a LINE-1-encoded protein. LINE-1 protein expression is a common feature of many types of high-grade malignant cancers, is rarely detected in early stages of tumorigenesis, and is absent from normal somatic tissues. Studies have shown that LINE-1 contributes to genetic changes in cancers, with somatic LINE-1 insertions seen in selected types of human cancers, particularly colon cancer. We sought to correlate this observation with expression of the LINE-1-encoded protein, open reading frame 1 protein, and found that LINE-1 open reading frame 1 protein is a surprisingly broad, yet highly tumor-specific, antigen.

Barrett's esophagus (BE) is a common disease in which the lining of the esophagus transitions from stratified squamous epithelium to metaplastic columnar epithelium that predisposes individuals to developing esophageal adenocarcinoma (EAC). We hypothesized that BE provides a unique environment for increased long-interspersed element 1 (LINE-1 or L1) retrotransposition. To this end, we evaluated 5 patients with benign BE, 5 patients with BE and concomitant EAC, and 10 additional patients with EAC to determine L1 activity in this progressive disease. After L1-seq, we confirmed 118 somatic insertions by PCR in 10 of 20 individuals. We observed clonal amplification of several insertions which appeared to originate in normal esophagus (NE) or BE and were later clonally expanded in BE or in EAC. Additionally, we observed evidence of clonality within the EAC cases; specifically, 22 of 25 EAC-only insertions were present identically in distinct regions available from the same tumor, suggesting that these insertions occurred in the founding tumor cell of these lesions. L1 proteins must be expressed for retrotransposition to occur; therefore, we evaluated the expression of open reading frame 1 protein (ORF1p), a protein encoded by L1, in eight of the EAC cases for which formalin-fixed paraffin embedded tissue was available. With immunohistochemistry, we detected ORF1p in all tumors evaluated. Interestingly, we also observed dim ORF1p immunoreactivity in histologically NE of all patients. In summary, our data show that somatic retrotransposition occurs early in many patients with BE and EAC and indicate that early events occurring even in histologically NE cells may be clonally expanded in esophageal adenocarcinogenesis.retrotransposon | LINE-1 | Barrett's esophagus | esophageal adenocarcinoma | retrotransposition

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