Intact piRNA pathway prevents L1 mobilization in male meiosis.

The PIWI-interacting RNA (piRNA) pathway is essential for retrotransposon silencing. In piRNA-deficient mice, L1-overexpressing male germ cells exhibit excessive DNA damage and meiotic defects. It remains unknown whether L1 expression simply highlights piRNA deficiency or actually drives the germ-cell demise. Specifically, the sheer abundance of genomic L1 copies prevents reliable quantification of new insertions. Here, we developed a codon-optimized L1 transgene that is controlled by an endogenous mouse L1 promoter. Importantly, DNA methylation dynamics of a single-copy transgene were indistinguishable from those of endogenous L1s. Analysis of Mov10l1-/- testes established that de novo methylation of the L1 transgene required the intact piRNA pathway. Consistent with loss of DNA methylation and programmed reduction of H3K9me2 at meiotic onset, the transgene showed 1,400-fold increase in RNA expression and consequently 70-fold increase in retrotransposition in postnatal day 14 Mov10l1-/- germ cells compared with the wild-type. Analysis of adult Mov10l1-/- germ-cell fractions indicated a stage-specific increase of retrotransposition in the early meiotic prophase. However, extrapolation of the transgene data to endogenous L1s suggests that it is unlikely insertional mutagenesis alone accounts for the Mov10l1-/- phenotype. Indeed, pharmacological inhibition of reverse transcription did not rescue the meiotic defect. Cumulatively, these results establish the occurrence of productive L1 mobilization in the absence of an intact piRNA pathway but leave open the possibility of processes preceding L1 integration in triggering meiotic checkpoints and germ-cell death. Additionally, our data suggest that many heritable L1 insertions originate from individuals with partially compromised piRNA defense.

Retrotransposition creates sloping shores: a graded influence of hypomethylated CpG islands on flanking CpG sites.

Long interspersed elements (LINEs), through both self-mobilization and trans-mobilization of short interspersed elements and processed pseudogenes, have made an indelible impact on the structure and function of the human genome. One consequence is the creation of new CpG islands (CGIs). In fact, more than half of all CGIs in the genome are associated with repetitive DNA, three-quarters of which are derived from retrotransposons. However, little is known about the epigenetic impact of newly inserted CGIs. We utilized a transgenic LINE-1 mouse model and tracked DNA methylation dynamics of individual germline insertions during mouse development. The retrotransposed GFP marker sequence, a strong CGI, is hypomethylated in male germ cells but hypermethylated in somatic tissues, regardless of genomic location. The GFP marker is similarly methylated when delivered into the genome via the Sleeping Beauty DNA transposon, suggesting that the observed methylation pattern may be independent of the mode of insertion. Comparative analyses between insertion- and non-insertion-containing alleles further reveal a graded influence of the retrotransposed CGI on flanking CpG sites, a phenomenon that we described as "sloping shores." Computational analyses of human and mouse methylomic data at single-base resolution confirm that sloping shores are universal for hypomethylated CGIs in sperm and somatic tissues. Additionally, the slope of a hypomethylated CGI can be affected by closely positioned CGI neighbors. Finally, by tracing sloping shore dynamics through embryonic and germ cell reprogramming, we found evidence of bookmarking, a mechanism that likely determines which CGIs will be eventually hyper- or hypomethylated.

LINE-1-derived poly(A) microsatellites undergo rapid shortening and create somatic and germline mosaicism in mice.

Interspersed and tandem repeat sequences comprise the bulk of mammalian genomes. Interspersed repeats result from successive replication by transposable elements, such as Alu and long interspersed element type 1 (L1). Microsatellites are tandem repeats of 1-6 base pairs, among which poly(A) microsatellites are the most abundant in the human genome. The rise and fall of a microsatellite has been depicted as a life cycle. Previous studies have demonstrated that Alu and L1 insertions are a major source of A-rich microsatellites owing to the concurrent formation of a poly(A) DNA tract at the 3'-end of each insertion. The fate of such poly(A) tracts has been studied by surveying the length distribution of genomic resident Alu and L1 insertions. However, these cross-sectional studies provide no information about the tempo of mutation immediately after birth. In this study, de novo L1 insertions were created using a transgenic L1 mouse model and traced through generations to investigate the early life of poly(A) microsatellites. High frequencies of intra-individual and intergenerational shortening were observed for long poly(A) tracts, creating somatic and germline mosaicism at the insertion site, whereas little variation was observed for short poly(A) alleles. As poly(A) microsatellites are the major intrinsic signal for nucleosome positioning, their remarkable abundance and variability make them a significant source of epigenetic variation. Thus, the birth of poly(A) microsatellites from retrotransposons and the subsequent rapid and variable shortening represent a new way with which retrotransposons can modify the genetic and epigenetic architecture of our genome.

Characterization of L1 retrotransposition with high-throughput dual-luciferase assays.

Recent studies employing genome-wide approaches have provided an unprecedented view of the scope of L1 activities on structural variations in the human genome, and further reinforced the role of L1s as one of the major driving forces behind human genome evolution. The rapid identification of novel L1 elements by these high-throughput approaches demands improved L1 functional assays. However, the existing assays use antibiotic selection markers or fluorescent proteins as reporters; neither is amenable to miniaturization. To increase assay sensitivity and throughput, we have developed a third generation assay by using dual-luciferase reporters, in which firefly luciferase is used as the retrotransposition indicator and Renilla luciferase is encoded on the same or separate plasmid for normalization. This novel assay is highly sensitive and has a broad dynamic range. Quantitative data with high signal-to-noise ratios can be obtained from 24- up to 96-well plates in 2-4 days after transfection. Using the dual-luciferase assays, we have characterized profiles of retrotransposition by various human and mouse L1 elements, and detailed the kinetics of L1 retrotransposition in cultured cells. Its high-throughput and short assay timeframe make it well suited for routine tests as well as large-scale screening efforts.

Rationally Designed Transgene-Encoded Cell-Surface Polypeptide Tag for Multiplexed Programming of CAR T-cell Synthetic Outputs.

Synthetic immunology, as exemplified by chimeric antigen receptor (CAR) T-cell immunotherapy, has transformed the treatment of relapsed/refractory B cell-lineage malignancies. However, there are substantial barriers-including limited tumor homing, lack of retention of function within a suppressive tumor microenvironment, and antigen heterogeneity/escape-to using this technology to effectively treat solid tumors. A multiplexed engineering approach is needed to equip effector T cells with synthetic countermeasures to overcome these barriers. This, in turn, necessitates combinatorial use of lentiviruses because of the limited payload size of current lentiviral vectors. Accordingly, there is a need for cell-surface human molecular constructs that mark multi-vector cotransduced T cells, to enable their purification ex vivo and their tracking in vivo. To this end, we engineered a cell surface-localizing polypeptide tag based on human HER2, designated HER2t, that was truncated in its extracellular and intracellular domains to eliminate ligand binding and signaling, respectively, and retained the membrane-proximal binding epitope of the HER2-specific mAb trastuzumab. We linked HER2t to CAR coexpression in lentivirally transduced T cells and showed that co-transduction with a second lentivirus expressing our previously described EGFRt tag linked to a second CAR efficiently generated bispecific dual-CAR T cells. Using the same approach, we generated T cells expressing a CAR and a second module, a chimeric cytokine receptor. The HER2txEGFRt multiplexing strategy is now being deployed for the manufacture of CD19xCD22 bispecific CAR T-cell products for the treatment of acute lymphoblastic leukemia (NCT03330691).

L1 expression and regulation in humans and rodents.

Long interspersed elements type 1 (LINE-1s, or L1s) have impacted mammalian genomes at multiple levels. L1 transcription is mainly controlled by its 5' untranslated region (5'UTR), which differs significantly among active human and rodent L1 families. In this review, L1 expression and its regulation are examined in the context of human and rodent development. First, endogenous L1 expression patterns in three different species-human, rat, and mouse-are compared and contrasted. A detailed account of relevant experimental evidence is presented according to the source material, such as cell lines, tumors, and normal somatic and germline tissues from different developmental stages. Second, factors involved in the regulation of L1 expression at both transcriptional and posttranscriptional levels are discussed. These include transcription factors, DNA methylation, PIWI-interacting RNAs (piRNAs), RNA interference (RNAi), and posttranscriptional host factors. Similarities and differences between human and rodent L1s are highlighted. Third, recent findings from transgenic mouse models of L1 are summarized and contrasted with those from endogenous L1 studies. Finally, the challenges and opportunities for L1 mouse models are discussed.

Repeat-induced gene silencing of L1 transgenes is correlated with differential promoter methylation.

Recent transgenic studies on L1 retrotransposons have afforded exciting insights into L1 biology, and a unique opportunity to model their function and regulation in vivo. Thus far, the majority of the transgenic L1 mouse lines are constructed via pronuclear microinjection, a procedure that typically results in the integration of tandem arrayed transgenes. Transgene arrays are susceptible to repeat-induced gene silencing (RIGS) in both plants and animals. In order to examine the potential impact of RIGS on L1 retrotransposition, we derived a cohort of animals carrying reduced copies of ORFeus transgene at the same genomic locus by Cre-mediated recombination. The copy number reduction of ORFeus transgenes did not decrease the overall retrotransposition activity. Using a sensitive and reproducible quantitative PCR assay, an average frequency of 0.45 insertions per cell was observed for animals carrying the donor transgene at a single copy, representing a 9-fold increase of retrotransposition frequency on a per-copy basis. DNA methylation analyses revealed that the observed retrotransposition activity was correlated with differential CpG methylation at the heterologous promoter: the promoter region was largely methylated in animals with the high-copy array but significantly hypomethylated in animals with the single-copy counterpart. In contrast, the ORF2 region, which represents the body of the ORFeus transgene, and the 3' end of the transgene showed high level of methylation in both high-copy and single-copy samples. The observed methylation patterns were metastable across generations. In summary, our data suggest that tandem arrayed L1 transgenes are subject to RIGS, and transgenes present at a single copy in the genome are thus recommended for modeling L1 in animals.