Retrotransposition Of Long Interspersed Element-1 Research Articles

ZNF689 deficiency promotes intratumor heterogeneity and immunotherapy resistance in triple-negative breast cancer.

Triple-negative breast cancer (TNBC) is an aggressive disease characterized by remarkable intratumor heterogeneity (ITH), which poses therapeutic challenges. However, the clinical relevance and key determinant of ITH in TNBC are poorly understood. Here, we comprehensively characterized ITH levels using multi-omics data across our center's cohort (n = 260), The Cancer Genome Atlas cohort (n = 134), and four immunotherapy-treated cohorts (n = 109). Our results revealed that high ITH was associated with poor patient survival and immunotherapy resistance. Importantly, we identified zinc finger protein 689 (ZNF689) deficiency as a crucial determinant of ITH formation. Mechanistically, the ZNF689-TRIM28 complex was found to directly bind to the promoter of long interspersed element-1 (LINE-1), inducing H3K9me3-mediated transcriptional silencing. ZNF689 deficiency reactivated LINE-1 retrotransposition to exacerbate genomic instability, which fostered ITH. Single-cell RNA sequencing, spatially resolved transcriptomics and flow cytometry analysis confirmed that ZNF689 deficiency-induced ITH inhibited antigen presentation and T-cell activation, conferring immunotherapy resistance. Pharmacological inhibition of LINE-1 significantly reduced ITH, enhanced antitumor immunity, and eventually sensitized ZNF689-deficient tumors to immunotherapy in vivo. Consistently, ZNF689 expression positively correlated with favorable prognosis and immunotherapy response in clinical samples. Altogether, our study uncovers a previously unrecognized mechanism underlying ZNF689 deficiency-induced ITH and suggests LINE-1 inhibition combined with immunotherapy as a novel treatment strategy for TNBC.

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.

Schlafen-5 inhibits LINE-1 retrotransposition

Long interspersed element 1 (LINE-1) is the only currently known active autonomous transposon in humans, and its retrotransposition may cause deleterious effects on the structure and function of host cell genomes and result in sporadic genetic diseases. Host cells therefore developed defense strategies to restrict LINE-1 mobilization. In this study, we demonstrated that IFN-inducible Schlafen5 (SLFN5) inhibits LINE-1 retrotransposition. Mechanistic studies revealed that SLFN5 interrupts LINE-1 ribonucleoprotein particle (RNP) formation, thus diminishing nuclear entry of the LINE-1 RNA template and subsequent LINE-1 cDNA production. The ability of SLFN5 to bind to LINE-1 RNA and the involvement of the helicase domain of SLFN5 in its inhibitory activity suggest a mechanism that SLFN5 binds to LINE-1 RNA followed by dissociation of ORF1p through its helicase activity, resulting in impaired RNP formation. These data highlight a new mechanism of host cells to restrict LINE-1 mobilization.

MOV10 recruits DCP2 to decap human LINE-1 RNA by forming large cytoplasmic granules with phase separation properties.

Long interspersed element 1 (LINE-1) is the only active autonomous mobile element in the human genome. Its transposition can exert deleterious effects on the structure and function of the host genome and cause sporadic genetic diseases. Tight control of LINE-1 mobilization by the host is crucial for genetic stability. In this study, we report that MOV10 recruits the main decapping enzyme DCP2 to LINE-1 RNA and forms a complex of MOV10, DCP2, and LINE-1 RNP, exhibiting liquid-liquid phase separation (LLPS) properties. DCP2 cooperates with MOV10 to decap LINE-1 RNA, which causes degradation of LINE-1 RNA and thus reduces LINE-1 retrotransposition. We here identify DCP2 as one of the key effector proteins determining LINE-1 replication, and elucidate an LLPS mechanism that facilitates the anti-LINE-1 action of MOV10 and DCP2.

Abstract GS5-05: ZNF689 deficiency promotes intratumor heterogeneity and resistance to immune checkpoint blockade in triple-negative breast cancer

Abstract Background: Triple-negative breast cancer (TNBC) is an aggressive disease characterized by remarkable intratumor heterogeneity (ITH), which poses a significant therapeutic challenge. However, the key determinants and underlying mechanisms of ITH in TNBC remain to be fully elucidated. Methods: We used multi-omics data from our cohort (n = 260) and The Cancer Genome Atlas (n = 134) cohort to comprehensively characterize ITH at the genetic and histologic levels. Transcriptomic differences between high ITH and low ITH tumors were compared to identify the core genes contributing ITH in TNBC. Xenograft models were used to examine the role of key determinants in TNBC ITH. The molecular mechanism was investigated by mass spectrometry, coimmunoprecipitation, pull-down, RNA-seq, long interspersed element-1 (LINE-1) reporter, ATAC-seq, luciferase reporter assays, chromatin immunoprecipitation, flow cytometry and coculture assay. Results: We found that high ITH was associated with poor patient survival and immune checkpoint blockade (ICB) resistance, which were validated in four independent ICB-treated trials. Further analysis indicated zinc finger protein 689 (ZNF689) deficiency as an important determinant of TNBC ITH. Mechanistically, the ZNF689-TRIM28 complex directly bound to the promoter of LINE-1, inducing H3K9me3-mediated transcriptional silencing. ZNF689 deficiency reactivated LINE-1 retrotransposition to exacerbate genomic instability, which promoted ITH. ZNF689 deficiency-induced ITH inhibited antigen presentation and CD8+ T cell infiltration, leading to ICB resistance. Pharmacological inhibition of LINE-1 retrotransposition reduced ITH, augmented antitumor immunity, and eventually sensitized ZNF689-deficient tumors to ICB. Consistently, ZNF689 expression positively correlated with favorable prognosis and ICB responsiveness in clinical samples. Conclusions: Our study uncovers a new mechanism underlying ZNF689 deficiency-induced ITH and suggests LINE-1 inhibition combined with ICB as a novel treatment strategy in TNBC. Citation Format: Li-Ping Ge, Xi Jin, Gen-Hong Di, Yi-Zhou Jiang, Zhi-Ming Shao. ZNF689 deficiency promotes intratumor heterogeneity and resistance to immune checkpoint blockade in triple-negative breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr GS5-05.

The Zinc-Finger protein ZCCHC3 inhibits LINE-1 retrotransposition.

Long-interspersed element 1 (LINE-1) is an autonomous non-LTR retrotransposon. Its replication can cause mutation and rearrangement of host genomic DNA, which may result in serious genetic diseases. Host cells therefore developed defense strategies to restrict LINE-1 mobilization. In this study, we reported that CCHC-type zinc-finger protein ZCCHC3 can repress LINE-1 retrotransposition, and this activity is closely related to its zinc-finger domain. Further studies show that ZCCHC3 can post-transcriptionally diminish the LINE-1 RNA level. The association of ZCCHC3 with both LINE-1 RNA and ORF1 suggests that ZCCHC3 interacts with LINE-1 RNP and consequently causes its RNA degradation. These data demonstrate collectively that ZCCHC3 contributes to the cellular control of LINE-1 replication.

Research and application on LINE-1 in diagnosis and treatment of tumorigenesis.

Long interspersed element-1 (LINE-1), which is the only autonomous retrotransposon in human genome, makes up about 17% of the human genome. For LINE-1 retrotransposition may result in genome instability, it was strictly restricted by organisms, and its expression was therefore barely detected in normal somatic cells. However, the expression of LINE-1 is a common phenomenon in most tumor or cancer tissues, suggesting a close relationship between LINE-1 expression and cancer development. Differentially expressed LINE-1 in cancer tissues can be used as a biomarker for tumor diagnosis and an important indicator of prognosis after cancer therapeutics. Meanwhile, the feasibility of LINE-1 as a potential drug target for tumor treatment has also been evaluating and verifying in clinicals. In this review, we introduce the application of LINE-1 as a biomarker in tumor diagnosis and prognostic, as well as the research progress in LINE-1 as potential drug target for tumor treatment, in order to provide some references for clinical application in cancer treatment.

LINE-1 Retrotransposition Promotes the Development and Progression of Lung Squamous Cell Carcinoma by Disrupting the Tumor-Suppressor Gene FGGY.

Somatic long interspersed element-1 (LINE-1) retrotransposition is a genomic process that relates to gene disruption and tumor occurrence. However, the expression and function of LINE-1 retrotransposition in lung squamous cell carcinoma (LUSC) remain unclear. We analyzed the transcriptomes of LUSC samples in The Cancer Genome Atlas and observed LINE-1 retrotransposition in 90% of tumor samples. Thirteen LINE-1 retrotranspositions of high occurrence were identified and further validated from an independent Chinese LUSC cohort. Among them, LINE-1-FGGY (L1-FGGY) was identified as the most frequent LINE-1 retrotransposition in the Chinese cohort and significantly correlated with poor clinical outcome. L1-FGGY occurred with smoke-induced hypomethylation of the LINE-1 promoter and contributed to the development of local immune evasion and dysfunctional metabolism. Overexpression of L1-FGGY or knockdown of FGGY promoted cell proliferation and invasion in vitro, facilitated tumorigenesis in vivo, and dysregulated cell energy metabolism and cytokine/chemotaxin transcription. Importantly, specific reverse transcription inhibitors, nevirapine and efavirenz, dramatically countered L1-FGGY abundance, inhibited tumor growth, recovered metabolism dysfunction, and improved the local immune evasion. In conclusion, hypomethylation-induced L1-FGGY expression is a frequent genomic event that promotes the development and progression of LUSC and represents a promising predictive biomarker and therapeutic target in LUSC. SIGNIFICANCE: LINE-1-FGGY is a prognosis predictive biomarker and potential therapeutic target to overcome local immune evasion in lung squamous cell carcinoma.

Transposable Elements and Their Epigenetic Regulation in Mental Disorders: Current Evidence in the Field.

Transposable elements (TEs) are highly repetitive DNA sequences in the human genome that are the relics of previous retrotransposition events. Although the majority of TEs are transcriptionally inactive due to acquired mutations or epigenetic processes, around 8% of TEs exert transcriptional activity. It has been found that TEs contribute to somatic mosaicism that accounts for functional specification of various brain cells. Indeed, autonomous retrotransposition of long interspersed element-1 (LINE-1) sequences has been reported in the neural rat progenitor cells from the hippocampus, the human fetal brain and the human embryonic stem cells. Moreover, expression of TEs has been found to regulate immune-inflammatory responses, conditioning immunity against exogenous infections. Therefore, aberrant epigenetic regulation and expression of TEs emerged as a potential mechanism underlying the development of various mental disorders, including autism spectrum disorders (ASD), schizophrenia, bipolar disorder, major depression, and Alzheimer’s disease (AD). Consequently, some studies revealed that expression of some sequences of human endogenous retroviruses (HERVs) appears only in a certain group of patients with mental disorders (especially those with schizophrenia, bipolar disorder, and ASD) but not in healthy controls. In addition, it has been found that expression of HERVs might be related to subclinical inflammation observed in mental disorders. In this article, we provide an overview of detrimental effects of transposition on the brain development and immune mechanisms with relevance to mental disorders. We show that transposition is not the only mechanism, explaining the way TEs might shape the phenotype of mental disorders. Other mechanisms include the regulation of gene expression and the impact on genomic stability. Next, we review current evidence from studies investigating expression and epigenetic regulation of specific TEs in various mental disorders. Most consistently, these studies indicate altered expression of HERVs and methylation of LINE-1 sequences in patients with ASD, schizophrenia, and mood disorders. However, the contribution of TEs to the etiology of AD is poorly documented. Future studies should further investigate the mechanisms linking epigenetic processes, specific TEs and the phenotype of mental disorders to disentangle causal associations.

Latent infection with Kaposi's sarcoma-associated herpesvirus enhances retrotransposition of long interspersed element-1.

Kaposi's sarcoma (KS)-associated herpesvirus (KSHV), a gamma-2 herpesvirus, is the causative agent of KS, primary effusion lymphoma (PEL), and a plasma cell variant of multicentric Castleman's disease. Although KSHV latency is detected in KS-related tumors, oncogenic pathways activated by KSHV latent infection are not fully understood. Here, we found that retrotransposition of long interspersed element-1 (L1), a retrotransposon in the human genome, was enhanced in PEL cells. Among the KSHV latent genes, viral FLICE-inhibitory protein (vFLIP) enhanced L1 retrotransposition in an NF-κB-dependent manner. Intracellular cell adhesion molecule-1 (ICAM-1), an NF-κB target, regulated the vFLIP-mediated enhancement of L1 retrotransposition. Furthermore, ICAM-1 downregulated the expression of Moloney leukemia virus 10 (MOV10), an L1 restriction factor. Knockdown of ICAM-1 or overexpression of MOV10 relieved the vFLIP-mediated enhancement of L1 retrotransposition. Collectively, during KSHV latency, vFLIP upregulates ICAM-1 in an NF-κB-dependent manner, which, in turn, downregulates MOV10 expression and thereby enhances L1 retrotransposition. Because active L1 retrotransposition can lead to genomic instability, which is commonly found in KS and PEL, activation of L1 retrotransposition during KSHV latency may accelerate oncogenic processes through enhancing genomic instability. Our results suggest that L1 retrotransposition may be a novel target for impeding tumor development in KSHV-infected patients.

Abstract 3096: LINE-1 retrotransposition L1-FGGY promotes the occurrence and progression of lung squamous cell carcinoma by inhibiting the tumor-suppressor gene FGGY

Abstract Retrotransposition is a kind of chromatin rearrangement containing the non-coding region, which makes up about half of the human genome. Long interspersed element-1 (LINE-1) retrotransposition is the only currently known active autonomous transposon in humans and might cause genetic instability, which was reported to occur with high frequency in a variety of tumor tissues, including colon cancer, prostate cancer, ovarian cancer, hepatocellular carcinoma, etc. However, the relationship between LINE-1 and lung squamous cell carcinoma (LUSC) is unclear. In this study, we analyzed 504 cases of LUSC samples based on the RNA-seq database in The Cancer Genome Atlas (TCGA), and found that 1/3 of tumor samples possessed LINE-1 inserted retrotransposons. We screened out 14 LINE-1 retrotransposons with the highest occurrence frequency and detected the expression of LINE-1 retrotransposons in 110 clinical LUSC samples and 52 matched adjacent normal tissue samples. The results showed that among 14 screened retrotransposon genes, the expression of 13 retrotransposon genes in LUSC was significantly higher than that in the corresponding para-carcinoma tissues, indicating that the presence of LINE-1 retrotransposon was related to the occurrence of LUSC. Furthermore, by analyzing the correlationship between the expression levels of LINE-1 retrotransposons and overall survival time of patients, we also found that the survival time of patients with low retrotransposon gene expression was long, while the survival time of patients with high retrotransposon gene expression was short. Among them, L1-FGGY and L1-ATP8B1 showed significant differences. In this study, we focused on one LINE-1 retrotransposon, i.e. L1-FGGY. Then we further investigated the mechanism of the LINE-1 retrotransposition in regulating the occurrence and progression of LUSC and discovered that the expression of L1-FGGY and FGGY was negatively correlated in LUSC patients (coefficient of association=-0.232, p<0.05). In order to further explore the function of FGGY in cancer development, we constructed 3 different siRNAs which could specifically interfere with the expression of FGGY and discovered that knockdown of FGGY could promote cell proliferation, inhibit cell apoptosis, as well as facilitate cell migration and invasion in vitro. The further study showed that inhibition of FGGY could promote tumorigenesis and tumor metastasis in vivo, which collectively indicated that FGGY might function as a tumor-suppressor gene. In conclusion, we not only uncovered that the LINE-1 retrotransposition L1-FGGY promotes the development of LUSC by inhibiting the expression and function of the tumor-suppressor gene FGGY, but also revealed that LINE-1 retrotransposon might be a new biomarker for early diagnosis, prognosis evaluation, and targeted therapy in the future clinical translation. Citation Format: Rui Zhang, Jinpu Yu. LINE-1 retrotransposition L1-FGGY promotes the occurrence and progression of lung squamous cell carcinoma by inhibiting the tumor-suppressor gene FGGY [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3096.

Modulation of LINE-1 retrotransposition by a human SAMHD1 polymorphism

The HIV-1 restriction factor SAMHD1 has the ability to negatively modulate retrotransposition of the long interspersed element 1(LINE-1). By exploring the ability of human SAMHD1 polymorphisms to inhibit LINE-1, we found that the single nucleotide polymorphism S33A present in the Korean population lose the ability to inhibit LINE-1 retrotransposition. Because SAMHD1 residue S33 is phosphorylated in human cycling and non-cycling cells, we demonstrated that SAMHD1 requires to be either phosphorylated on position 33 or to contain a bulky residue in order to inhibit LINE-1 retrotransposition. Therefore this unique mutation uncouples functions in this important restriction factor.

Upregulated LINE-1 Activity in the Fanconi Anemia Cancer Susceptibility Syndrome Leads to Spontaneous Pro-inflammatory Cytokine Production

Fanconi Anemia (FA) is a genetic disorder characterized by elevated cancer susceptibility and pro-inflammatory cytokine production. Using SLX4FANCP deficiency as a working model, we questioned the trigger for chronic inflammation in FA. We found that absence of SLX4 caused cytoplasmic DNA accumulation, including sequences deriving from active Long INterspersed Element-1 (LINE-1), triggering the cGAS-STING pathway to elicit interferon (IFN) expression. In agreement, absence of SLX4 leads to upregulated LINE-1 retrotransposition. Importantly, similar results were obtained with the FANCD2 upstream activator of SLX4. Furthermore, treatment of FA cells with the Tenofovir reverse transcriptase inhibitor (RTi), that prevents endogenous retrotransposition, decreased both accumulation of cytoplasmic DNA and pro-inflammatory signaling. Collectively, our data suggest a contribution of endogenous RT activities to the generation of immunogenic cytoplasmic nucleic acids responsible for inflammation in FA. The additional observation that RTi decreased pro-inflammatory cytokine production induced by DNA replication stress-inducing drugs further demonstrates the contribution of endogenous RTs to sustaining chronic inflammation. Altogether, our data open perspectives in the prevention of adverse effects of chronic inflammation in tumorigenesis.

Conservation and Innovation of APOBEC3A Restriction Functions during Primate Evolution.

LINE-1 (long interspersed element-1) retroelements are the only active autonomous endogenous retroelements in human genomes. Their retrotransposition activity has created close to 50% of the current human genome. Due to the apparent costs of this proliferation, host genomes have evolved multiple mechanisms to curb LINE-1 retrotransposition. Here, we investigate the evolution and function of the LINE-1 restriction factor APOBEC3A, a member of the APOBEC3 cytidine deaminase gene family. We find that APOBEC3A genes have evolved rapidly under diversifying selection in primates, suggesting changes in APOBEC3A have been recurrently selected in a host-pathogen "arms race." Nonetheless, in contrast to previous reports, we find that the LINE-1 restriction activity of APOBEC3A proteins has been strictly conserved throughout simian primate evolution in spite of its pervasive diversifying selection. Based on these results, we conclude that LINE-1s have not driven the rapid evolution of APOBEC3A in primates. In contrast to this conserved LINE-1 restriction, we find that a subset of primate APOBEC3A genes have enhanced antiviral restriction. We trace this gain of antiviral restriction in APOBEC3A to the common ancestor of a subset of Old World monkeys. Thus, APOBEC3A has not only maintained its LINE-1 restriction ability, but also evolved a gain of antiviral specificity against other pathogens. Our findings suggest that while APOBEC3A has evolved to restrict additional pathogens, only those adaptive amino acid changes that leave LINE-1 restriction unperturbed have been tolerated.

The devil is in the details: Transposable element analysis of the Tasmanian devil genome.

The third marsupial genome was sequenced from the Tasmanian devil (Sarcophilus harrisii), a species that currently is driven to extinction by a rare transmissible cancer. The transposable element (TE) landscape of the Tasmanian devil genome revealed that the main driver of retrotransposition the Long INterspersed Element 1 (LINE1) seem to have become inactivated during the past 12 million years. Strangely, the Short INterspersed Elements (SINE), that normally hijacks the LINE1 retrotransposition system, became inactive prior to LINE1 at around 30 million years ago. The SINE inactivation was in vitro verified in several species. Here I discuss that the apparent LINE1 inactivation might be caused by a genome assembly artifact. The repetitive fraction of any genome is highly complex to assemble and the observed problems are not unique to the Tasmanian devil genome.

SAMHD1 Inhibits LINE-1 Retrotransposition by Promoting Stress Granule Formation.

The SAM domain and HD domain containing protein 1 (SAMHD1) inhibits retroviruses, DNA viruses and long interspersed element 1 (LINE-1). Given that in dividing cells, SAMHD1 loses its antiviral function yet still potently restricts LINE-1, we propose that, instead of blocking viral DNA synthesis by virtue of its dNTP triphosphohydrolase activity, SAMHD1 may exploit a different mechanism to control LINE-1. Here, we report a new activity of SAMHD1 in promoting cellular stress granule assembly, which correlates with increased phosphorylation of eIF2α and diminished eIF4A/eIF4G interaction. This function of SAMHD1 enhances sequestration of LINE-1 RNP in stress granules and consequent blockade to LINE-1 retrotransposition. In support of this new mechanism of action, depletion of stress granule marker proteins G3BP1 or TIA1 abrogates stress granule formation and overcomes SAMHD1 inhibition of LINE-1. Together, these data reveal a new mechanism for SAMHD1 to control LINE-1 by activating cellular stress granule pathway.

Viral protein R of HIV type-1 induces retrotransposition and upregulates glutamate synthesis by the signal transducer and activator of transcription 1 signaling pathway.

Viral protein R (Vpr) of HIV-1 plays an important role in viral replication in macrophages. Various lines of evidence suggest that expression of Vpr in macrophages causes immunopathogenesis; however, the underlying mechanism is not yet fully understood. In this study, it was shown that recombinant Vpr (rVpr) induces retrotransposition of long interspersed element-1 in RAW264.7, a macrophage-like cell line, and activates reverse transcriptase-dependent immunotoxic cascades including production of IFN-β and phosphorylation of signal transducer and activator of transcription 1 (STAT1). Knockout experiments based on the CRISPR/Cas9 nickase system further demonstrated that cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) and stimulator of interferon gene (STING) are responsible for IFN-β production and STAT1 phosphorylation, respectively. Moreover, rVpr was found to increase production of glutaminase C, a regulator of glutamate synthesis, which is also dependent on the cGAS-STING pathway. Taken together with reports that glutaminase C is involved in the pathogenesis of HIV-associated neurocognitive disorder (HAND) and that Vpr is detectable in the cerebrospinal fluid of HIV-1-positive patients, a possible role of Vpr-induced L1-RTP and immunotoxic cascades in the development of HAND is discussed.

Under-Expression of LINE-1 Retrotransposons in Chronic Myeloid Leukemia Patients: Effect of Tyrosine Kinase Inhibitors

Background: Transposable elements (TEs) are mobile pieces of DNA that can move ("jump") in our genome, thereby influencing a myriad of cellular processes. Long interspersed element-1 (LINE-1) retrotransposons comprise approximately 20 percent of the mammalian genome, and LINE-1 retrotransposition events can create genomic instability by unknown mechanism. In chronic myeloid leukemia (CML), BCR-ABL1 expression is associated with a high degree of chromosomal genetic instability. The aim of this study was to investigate LINE-1 retrotransposon ORF1 and ORF2 expression in CML patients and cell lines.Materials and methods: LINE-1 quantitative RT-PCR was performed on 48 CML patients and 35 control subjects before therapy and during imatinib therapy. In addition LINE-1 expression was carried out on BCR-ABL1 positive and negative cell lines treated by kinase inhibitors, depleted in BCR-ABL1 by RNA interference. The effect of gamma irradiation (50 Gy) on CML and non CML cell lines was investigated.Results: For CML patients at diagnosis, LINE-1 ORF1/ß-actin expression (median expression value 2.8 x 102) was significantly lower than for healthy donors (3.9 x 103) (P<0.0001). The same level of significance (P<0.0001) was observed in the comparison of LINE-1 ORF2 expression (median values of 1.7 x 102 for CML patients and 1.4 x 103for healthy donors). The median expression of LINE-1 ORF1 and ORF2 in CML patients were lower than in control subjects by 14-fold and 8-fold, respectively. To exclude that the difference observed was not due to the different blood cell composition, we first analyzed LINE-1 ORF1 and ORF2 expression in patients with other diseases such as BCR-ABL1 positive Acute Lymphoblastic Leukemia (n=3), BCR-ABL1 negative Acute Lymphoblastic Leukemia (n=2), Acute Myeloblastic Leukemia (n=5), Essential Thrombocythemia (n=2), and other diseases as mastocytosis, myelofibrosis, lymphocytosis and myeloma, and second we analyze LINE-1 expression in the different cells from peripheral blood. Collectively, these data indicate that LINE-1 expression is significantly reduced in CML patients compared to patients with other diseases and cannot be due to the cellular composition.Prospective analysis of LINE-1 expression in CML patients (22 responder, 5 non responder and 8 secondary resistant patients) during imatinib therapy showed a negative correlation between BCR-ABL1 and LINE-1 expression during the kinetics of response or relapse.LINE-1 expression was investigated on BCR-ABL1 cell lines (K562 and LAMA-84) treated or not by imatinib, depleted in BCR-ABL1 or not by RNA interference. No modification of LINE-1 expression was observed in the different cell lines.To try to modulate LINE-1 expression, BCR-ABL1 positive cell lines (K562 and LAMA-84) and BCR-ABL1 negative cell lines (HL60 and U937) were irradiated with gamma radiation (50Gy). After irradiation we notice a decrease of LINE-1 expression in the four cell lines. BCR-ABL1 expression was not modified in K562 and LAMA-84 cell lines after irradiation. In addition, we observed the appearance of a BCR-ABL1 transcript by quantitative PCR in HL60 and U937 cell lines as already reported by Deininger et al (Cancer Res. 1998;58:421).Conclusions: Herein, we show that the levels of LINE-1 in CML patients at diagnosis were significantly lower than in control subjects. In spite of LINE-1 expression was inversely correlated with BCR-ABL1 expression in CML patients, no modification of LINE-1 mRNA expression was observed in CML cell lines treated by kinase inhibitors or depleted in BCR-ABL1 by RNA interference. Gamma irradiation induces the appearance of BCR-ABL1 transcripts and a decrease of LINE-1 expression. These two events could be concomitant or one could precede the other. However, we cannot exclude a link between LINE-1 activity and the BCR-ABL1 expression.Further studies will be necessary to determine if LINE-1 under-expression may precede the active disease or occurs as a result of the disease. DisclosuresJosselin:Qiagen: Employment. Etienne:Novartis, BMS, Pfizer, Ariad: Honoraria. Hermitte:QIAGEN: Employment. Mahon:NOVARTIS PHARMA: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; BRISTOL MYERS SQUIBB: Honoraria, Membership on an entity’s Board of Directors or advisory committees, Research Funding; ARIAD: Honoraria; PFIZER: Honoraria.

Tumor suppressor proteins restricts LINE-1 retrotransposition

Background Long interspersed element 1 (LINE-1) is a retroelement comprising about 17% of the human genome, encoding ORF1 with an RNA interaction domain and ORF2 with endonuclease and reverse transcriptase activities. Similar to retroviruses, LINE-1 replicates in the host via an RNA intermediate that is reverse transcribed and integrated into the host genome. LINE-1 retrotransposition has resulted in genetic diseases. In fact, LINE-1 insertion was recently found in several human tumors. However, the life cycle of LINE-1 is not fully understood. Therefore, we examined identification and characterization of host factors affected the retrotransposition of LINE-1.

The Microprocessor controls the activity of mammalian retrotransposons

More than half of the human genome is made of Transposable Elements. Their ongoing mobilization is a driving force in genetic diversity; however, little is known about how the host regulates their activity. Here, we show that the Microprocessor (Drosha-DGCR8), which is required for microRNA biogenesis, also recognizes and binds RNAs derived from human LINE-1 (Long INterspersed Element 1), Alu and SVA retrotransposons. Expression analyses demonstrate that cells lacking a functional Microprocessor accumulate LINE-1 mRNA and encoded proteins. Furthermore, we show that structured regions of the LINE-1 mRNA can be cleaved in vitro by Drosha. Additionally, we used a cell culture-based assay to show that the Microprocessor negatively regulates LINE-1 and Alu retrotransposition in vivo. Altogether, these data reveal a new role for the Microprocessor as a post-transcriptional repressor of mammalian retrotransposons acting as a defender of human genome integrity.