Landscape of L1 Retrotransposon Expression in Human Carcinomas
Nemanja Rodic, Rajni Sharma, Kathleen H Burns. Johns Hopkins Hospital, Baltimore, MD
Background: Over a third of our genome by weight is comprised of repeated sequences known as long interspersed elements (L1s). L1s are retrotransposons that comprise of two proteins: open reading frame 1 (ORF1p), which coats nascent messanger L1 RNA; and ORF2p, which reverse transcribes and integrates L1 sequence. Because L1 ORF2p encoded endonuclease can introduce double stranded breaks into DNA, L1s may be considered as endogenous mutagens that may contribute to tumorigenesis in human cancer tissue.
Design: Using tissue microarray technology, we examined L1 ORF1p expression in a broad range of normal and malignant tissues. We devised a standard categorical scoring system for assaying L1 ORF1p levels of antibody staining to include: 0=Negative (<1% tumor cells are immunoreactive); 1+=Low level (<24% tumor cells are immunoreactive); 2+=Medium level (25%-74% tumor cells are immunoreactive); and 3+=High level (>74% tumor cells are immunoreactive). We finally evaluated L1 ORF1p expression using a binomial scoring system, where 1+,2+, and 3+ antibody staining were considered positive and no antibody staining was considered negative.
Results: Unlike unaffected human somatic tissue, which we found to be devoid of any detectable L1 ORF1p expression, we found that a large fraction of human malignant epithelial neoplasms are immunoreactive for L1 ORF1p antigen. L1 ORF1p was detected in all carcinomas examined including: lung and bronchus (41 out of 81 were immunoreactive); breast (9 out of 47); colorectal (19 out of 39); ovarian (23 out of 25); cervical (3 out of 22); pancreatic (9 out of 13); bladder (24 out of 44); endometrial (23 out of 30); salivary (19 out of 35); and esophageal carcinoma (9 out of 14). Because L1 expression has a potential to induce double stranded breaks, we reasoned that concomitant loss of p53 tumor suppressor gene, may contribute to clonal selection of L1 ORF1p expressing cancer cells. In fact, we find a strong tendency for the loss of p53 tumor suppressor gene in L1 ORF1p positive neoplasms in lung and bronchus carcinomas (n = 78, p-value = 0.0051); pancreatic carcinomas (n = 39, p-value = 0.00000047); and ovarian carcinomas (n = 25, p-value = 0.0000089).
Conclusions: We find that L1 ORF1p immunoreactivity is common to many carcinomas including pulmonary, pancreatic, and gynecologic (especially ovarian) malignancies. We further note that in many of these malignancies there is a tendency for the concomitant loss of p53 tumor suppressor gene in L1 ORF1p expressing tumors. Our study lays a foundation in understanding how L1 biology may contribute to human tumorigenesis.
Monday, March 4, 2013 11:45 AM
Proffered Papers: Section G2, Monday Morning