Postdoctoral Fellow: Dr. Sabine Jung-Klawitter
Graduate Student: Anja Bock
Bachelor Student: Kerstin Dziewior
Technical Assistance: Ulrike Held
L1 elements are a major force shaping mammalian genomes. Research in our laboratory originates from an interest in interactons between human LINE-1 (Long INterspersed Element-1, L1) retrotransposons, which represent the currently active group of autonomous non-LTR retrotransposons in the human genome, and their host. We are investigating the consequences of L1-mediated retrotransposition for genomic destabilization, expression of host genes and diseases associated with L1 activity.
Although the human genome harbours ~106 L1 copies, only a subset of 80-100 L1 elements is functional and retrotransposition-competent. L1 elements replicate via an RNA intermediate using a ‘copy-and-paste’ mechanism. The L1-encoded protein machinery is responsible for the mobilization of L1 elements in cis, but also for the trans-mobilization of non-autonomous human non-LTR retrotransposon groups like SINEs (e.g. Alu) and SVA elements. Random cellular mRNAs can also be mobilized by L1-encoded proteins resulting in processed pseudogene formation. Taken together, the generation of approximately 34% of the current human genome is a consequence of the activity of L1 elements.
Human L1 was found to retrotranspose in numerous transformed somatic cells in culture, primary human cells, germ cells, embryonic stem cells, neural progenitor cells, and during early embryogenesis. L1 elements are mobilized early in development during the formation of the central nervous system and later during adult neurogenesis.
L1 activity induces local genomic instability and genomic rearrangements
Beyond their influence on genome evolution on the species level, L1-mediated retrotransposition reshuffles individual genomes in many different ways, a process which is often associated with the induction of local genomic instabilities. L1 insertions into genic functional sequences may be full length or 5’-truncated or contain inversions and can alter gene expression (e.g. by insertional mutagenesis). To date, ≥65 cases of different genetic disorders (e.g. haemophilia, cystic fibrosis, neurofibromatosis etc.) and tumorigenic diseases (e.g. breast cancer, colon cancer, etc.) were shown to be the consequence of such local genomic instabilities caused by L1, Alu and SVA de novo insertions. Roughly 20% of all L1 insertions are associated with structural rearrangements including deletions or insertions ranging from 1 bp to more than 130 kb.
So far, L1- and Alu-mediated non-allelic homologous recombination ( NAHR ) events were shown to be the cause for at least 73 reported cases of genetic and tumorigenic diseases . Additionally, the endonuclease activity of ORF2p has been shown to be associated with the induction of double-strand breaks in mammalian cell lines inducing various cellular responses including apoptosis, cellular senescence, cell-cycle checkpoints, and DNA repair responses. L1 expression was also suggested to affect cell proliferation, differentiation and tumor progression.
Impact of L1, Alu and SVA activity on gene expression
L1-mediated retrotransposition of L1, Alu and SVA can affect expression of nearby genes through several mechanisms: Intronic L1 sequences interfere with transcription elongation of the host gene due to the reduced ability of RNA polymerase II to read through L1 sequences. The retrotransposon can provide new splice sites that can promote exonization or alternative splicing, and they often carry accessory polyadenylation signals that induce transcriptional termination. Sense and antisense promoter of L1 elements are often used to initiate sense or antisense transcription through surrounding host genes.
Projects / Research Objectives
- LINE1-mediated retrotransposition in human pluripotent stem cells: Consequences for genomic stability and gene expression of human embryonic stem (hES) cells and human induced pluripotent stem (hIPS) cells and its derivatives
- Host-encoded mechanisms of intracellular defence against L1-mediated retrotransposition
- Connection between the tumorigenic process and the mobilization of endogenous retrotransposons
- Structure and composition of L1 ribonucleoprotein complexes
- Activity of endogenous L1 retrotransposons during embryonal and fetal developmental stages
- L1 – Host interactions involved in the L1 replication cycle
- SVA elements - Structural and functional characterization of a composite primate-specific retrotransposon
- Applying retrotransposable elements as tools for in vivo insertional mutagenesis and for targeted integration in gene therapy
Figure: L1 retrotransposition cycle
L1 mRNA (red) is exported into the cytoplasm, translated, and L1-encoded proteins (L1 ORF1p, L1 ORF2p) bind to their own mRNA (cis preference) and form ribonucleoprotein (RNP) complexes which are reimported into the nucleus. Subsequently, L1 RNA is reverse transcribed and the cDNA is inserted into the genome by a mechanism termed target-primed reverse transcription (TPRT). Frequently, reverse transcription fails to proceed to the 5’ end, resulting in truncated non-functional L1 de novo insertions.