Expression of L1 retrotransposon open reading frame protein 1 in gynecologic cancers

Chronic Viral Mimicry Induction following p53 Loss Promotes Immune Evasion

SIGNIFICANCE

Our landmark discovery of viral mimicry characterized repetitive elements as immunogenic stimuli that cull cancer cells. If expressed repetitive elements cull cancer cells, why does every human cancer express repetitive elements? Our report offers an exciting advancement toward understanding this paradox and how to exploit this mechanism for cancer interception. See related commentary by Murayama and Cañadas, p. 670.

LINE-1 ORF1p expression occurs in clear cell ovarian carcinoma precursors and is a candidate blood biomarker

Long interspersed element 1 (LINE-1) retrotransposons are repetitive sequences that can move within the genome by an autonomous mechanism. To limit their mutagenic potential, benign cells restrict LINE-1 expression through molecular mechanisms such as DNA methylation and histone modification, but these mechanisms are usually impaired in cancer. Clear cell ovarian carcinoma (CCOC) represents 5-10% of ovarian cancers and is thought to arise from endometriosis. Women with advanced CCOC face poor prognoses, highlighting the importance of understanding early disease pathogenesis. In our study, 33 of 40 cases (over 82%) of CCOC tumors express ORF1p, a LINE-1-encoded protein. We found that LINE-1 de-repression is an early event in CCOC, as ORF1p is enhanced during the transition from typical to atypical endometriosis and persists in invasive cancer. Finally, using single-molecule array (Simoa) assays, we detected ORF1p in patient blood, suggesting it as a potential minimally invasive biomarker for this disease.

LINE-1 ORF1p is a Promising Biomarker in Cervical Intraepithelial Neoplasia Degree Assessment

Cervical intraepithelial neoplasia (CIN) represents a spectrum of preinvasive squamous lesions within the cervical epithelium, whose identification is a diagnostic challenge due to subtle histomorphological differences among its categories. This study explores ORF1p, a nucleic acid-binding protein derived from long interspersed nuclear element-1 (LINE-1), as a potential biomarker for enhancing CIN diagnosis. A comprehensive analysis of 143 cervical specimens, encompassing CIN I (n=20), CIN II (n=46), CIN III (n=14), invasive cancer (n=32), and nondysplastic cases (normal cervical epithelia (n=24) and atrophy (n=7) were conducted. ORF1p, Ki67, and p16 expressions were evaluated using immunohistochemistry. ORF1p immunopositivity was detected in the vast majority [110/112 (98.2%)] of dysplastic and neoplastic (CIN and invasive cancer) specimens, whereas 19/24 (79.2%) of normal cervical specimens lacked ORF1p expression. The observed pattern of ORF1p expression showed a progressively increasing extent and intensity with advancing CIN grades. CIN I exhibited mild ORF1p expression in the lower one or two-thirds of the cervical epithelium [14/16 (87.5%)], whereas CIN II demonstrated moderate to strong ORF1p expression spanning the lower two-thirds [29/46 (63.0%)]. Pronounced transepithelial ORF1p immunopositivity characterized CIN III cases [13/14 (92.8%)] and cervical cancer [30/32 (93.8%)]. These findings propose ORF1p as a valuable indicator even for detecting CIN I, effectively discerning them from normal cervical tissue (p < 0.0001). Our findings underscore the potential of ORF1p as an early diagnostic marker for cervical neoplasia.

Chromosomal rearrangements and instability caused by the LINE-1 retrotransposon

LINE-1 (L1) retrotransposition is widespread in many cancers, especially those with a high burden of chromosomal rearrangements. However, whether and to what degree L1 activity directly impacts genome integrity is unclear. Here, we apply whole-genome sequencing to experimental models of L1 expression to comprehensively define the spectrum of genomic changes caused by L1. We provide definitive evidence that L1 expression frequently and directly causes both local and long-range chromosomal rearrangements, small and large segmental copy-number alterations, and subclonal copy-number heterogeneity due to ongoing chromosomal instability. Mechanistically, all these alterations arise from DNA double-strand breaks (DSBs) generated by L1-encoded ORF2p. The processing of ORF2p-generated DSB ends prior to their ligation can produce diverse rearrangements of the target sequences. Ligation between DSB ends generated at distal loci can generate either stable chromosomes or unstable dicentric, acentric, or ring chromosomes that undergo subsequent evolution through breakage-fusion bridge cycles or DNA fragmentation. Together, these findings suggest L1 is a potent mutagenic force capable of driving genome evolution beyond simple insertions.

LINE-1 retrotransposition and its deregulation in cancers: implications for therapeutic opportunities

Long interspersed element 1 (LINE-1) is the only protein-coding transposon that is active in humans. LINE-1 propagates in the genome using RNA intermediates via retrotransposition. This activity has resulted in LINE-1 sequences occupying approximately one-fifth of our genome. Although most copies of LINE-1 are immobile, ∼100 copies are retrotransposition-competent. Retrotransposition is normally limited via epigenetic silencing, DNA repair, and other host defense mechanisms. In contrast, LINE-1 overexpression and retrotransposition are hallmarks of cancers. Here, we review mechanisms of LINE-1 regulation and how LINE-1 may promote genetic heterogeneity in tumors. Finally, we discuss therapeutic strategies to exploit LINE-1 biology in cancers.

Ultrasensitive Detection of Circulating LINE-1 ORF1p as a Specific Multicancer Biomarker

Improved biomarkers are needed for early cancer detection, risk stratification, treatment selection, and monitoring treatment response. Although proteins can be useful blood-based biomarkers, many have limited sensitivity or specificity for these applications. Long INterspersed Element-1 (LINE-1) open reading frame 1 protein (ORF1p) is a transposable element protein overexpressed in carcinomas and high-risk precursors during carcinogenesis with negligible expression in normal tissues, suggesting ORF1p could be a highly specific cancer biomarker. To explore ORF1p as a blood-based biomarker, we engineered ultrasensitive digital immunoassays that detect mid-attomolar (10-17 mol/L) ORF1p concentrations in plasma across multiple cancers with high specificity. Plasma ORF1p shows promise for early detection of ovarian cancer, improves diagnostic performance in a multianalyte panel, provides early therapeutic response monitoring in gastroesophageal cancers, and is prognostic for overall survival in gastroesophageal and colorectal cancers. Together, these observations nominate ORF1p as a multicancer biomarker with potential utility for disease detection and monitoring.

SIGNIFICANCE

The LINE-1 ORF1p transposon protein is pervasively expressed in many cancers and is a highly specific biomarker of multiple common, lethal carcinomas and their high-risk precursors in tissue and blood. Ultrasensitive ORF1p assays from as little as 25 μL plasma are novel, rapid, cost-effective tools in cancer detection and monitoring. See related commentary by Doucet and Cristofari, p. 2502. This article is featured in Selected Articles from This Issue, p. 2489.

Proceedings of the inaugural Dark Genome Symposium: November 2022

In November 2022 the first Dark Genome Symposium was held in Boston, USA. The meeting was hosted by Rome Therapeutics and Enara Bio, two biotechnology companies working on translating our growing understanding of this vast genetic landscape into therapies for human disease. The spirit and ambition of the meeting was one of shared knowledge, looking to strengthen the network of researchers engaged in the field. The meeting opened with a welcome from Rosana Kapeller and Kevin Pojasek followed by a first session of field defining talks from key academics in the space. A series of panels, bringing together academia and industry views, were then convened covering a wide range of pertinent topics. Finally, Richard Young and David Ting gave their views on the future direction and promise for patient impact inherent in the growing understanding of the Dark Genome.

Research progress of LINE-1 in the diagnosis, prognosis, and treatment of gynecologic tumors

The retrotransposon known as long interspersed nuclear element-1 (LINE-1), which is currently the sole autonomously mobile transposon in the human genome, can result in insertional mutations, chromosomal rearrangements, and genomic instability. In recent years, numerous studies have shown that LINE-1 is involved in the development of various diseases and also plays an important role in the immune regulation of the organism. The expression of LINE-1 in gynecologic tumors suggests that it is expected to be an independent indicator for early diagnosis and prognosis, and also, as a therapeutic target, LINE-1 is closely associated with gynecologic tumor prognosis. This article discusses the function of LINE-1 in the diagnosis, treatment, and prognosis of ovarian, cervical, and endometrial malignancies, as well as other gynecologic malignancies. It offers fresh perspectives on the early detection of tumors and the creation of novel anti-tumor medications.

LINE-1 ORF1p as a candidate biomarker in high grade serous ovarian carcinoma

Long interspersed element 1 (LINE-1) open reading frame 1 protein (ORF1p) expression is a common feature of many cancer types, including high-grade serous ovarian carcinoma (HGSOC). Here, we report that ORF1p is not only expressed but also released by ovarian cancer and primary tumor cells. Immuno-multiple reaction monitoring-mass spectrometry assays showed that released ORF1p is confidently detectable in conditioned media, ascites, and patients' plasma, implicating ORF1p as a potential biomarker. Interestingly, ORF1p expression is detectable in fallopian tube (FT) epithelial precursors of HGSOC but not in benign FT, suggesting that ORF1p expression in an early event in HGSOC development. Finally, treatment of FT cells with DNA methyltransferase inhibitors led to robust expression and release of ORF1p, validating the regulatory role of DNA methylation in LINE-1 repression in non-tumorigenic tissue.

The proteome of clear cell ovarian carcinoma

Clear cell ovarian carcinoma (CCOC) is the second most common subtype of epithelial ovarian carcinoma. Late-stage CCOC is not responsive to gold-standard chemotherapy and results in suboptimal outcomes for patients. In-depth molecular insight is urgently needed to stratify the disease and drive therapeutic development. We conducted global proteomics for 192 cases of CCOC and compared these with other epithelial ovarian carcinoma subtypes. Our results showed distinct proteomic differences in CCOC compared with other epithelial ovarian cancer subtypes including alterations in lipid and purine metabolism pathways. Furthermore, we report potential clinically significant proteomic subgroups within CCOC, suggesting the biologic plausibility of stratified treatment for this cancer. Taken together, our results provide a comprehensive understanding of the CCOC proteomic landscape to facilitate future understanding and research of this disease. © 2022 The Pathological Society of Great Britain and Ireland.

Transposable element regulation and expression in cancer

Approximately 45% of the human genome is composed of transposable elements (TEs). Expression of these elements is tightly regulated during normal development. TEs may be expressed at high levels in embryonic stem cells but are epigenetically silenced in terminally differentiated cells. As part of the global 'epigenetic dysregulation' that cells undergo during transformation from normal to cancer, TEs can lose epigenetic silencing and become transcribed, and, in some cases, active. Here, we summarize recent advances detailing the consequences of TE activation in cancer and describe how these understudied residents of our genome can both aid tumorigenesis and potentially be harnessed for anticancer therapies.

Cell fitness screens reveal a conflict between LINE-1 retrotransposition and DNA replication

LINE-1 retrotransposon overexpression is a hallmark of human cancers. We identified a colorectal cancer wherein a fast-growing tumor subclone downregulated LINE-1, prompting us to examine how LINE-1 expression affects cell growth. We find that nontransformed cells undergo a TP53-dependent growth arrest and activate interferon signaling in response to LINE-1. TP53 inhibition allows LINE-1+ cells to grow, and genome-wide-knockout screens show that these cells require replication-coupled DNA-repair pathways, replication-stress signaling and replication-fork restart factors. Our findings demonstrate that LINE-1 expression creates specific molecular vulnerabilities and reveal a retrotransposition-replication conflict that may be an important determinant of cancer growth.