Project #3: Transposon-host interaction
TEs are best described as molecular parasites with the potential to give rise to a variety of genetic alterations. Such alterations include mutational damage to gene function, but can also provide useful genetic variability in host genomes. Thus, in contrast to most viruses, transposable elements and host have coevolved in a way that permits propagation of the transposon, but minimizes damage to the host. TEs make several interactions to host cellular machineries, piggyback and modify certain basic mechanisms of the host organism.
Transposon-host interaction as a model to investigate stress signalling and response in human cells
Dr. Suneel Narayanvari
TEs are often envisioned as stress-responsive “genomic noise-generators”. Our earlier studies clearly indicate that Sleeping Beauty transposition is sensitive to stress and stress signaling, and its response to these signals involves a complex, interactive regulatory platform involving evolutionary conserved cellular mechanisms. To model how a vertebrate-specific transposon senses and responds to stress signals in human cells, we study molecular interactions of a transposon with host cellular mechanisms to understand how stress-signaling and response triggers transposon activation. This experimental setup mimics a situation of a new TE colonizing a naïve genome.
Transposition and cell cycle
Dr. Suneel Narayanvari, Andrea Smith*
A temporary arrest at the G1/S transition phase of the cell-cycle was found to enhance transposition, suggesting that SB transposition is favored in the G1 phase of the cell-cycle, where the NHEJ pathway of DNA repair is preferentially active (Izsvak, 2004; Walisko, 2006). However, our recent data challenge this explanation. Our preliminary results indicate that, in addition to G1/S arrest, a temporal arrest in G2/M might also induce transposition. Notably, in contrast to certain viruses, severe DNA damage (e. g. DSBs) does not seem to trigger SB transposition, reflecting different strategies of various parasites that piggyback cellular processes.
Repairing transposition-inflicted DNA lesions
Yongming Wang*, Ilija Bilic, Helena Escobar
Cellular mechanisms that are directly involved in repairing transposition-inflicted DNA lesions or can attenuate the damage should have crucial role in establishing stable host-transposon co-existence. TEs can transpose by replicative or non-replicative processes. Non-replicative “cut & paste” type transposons, such as SB, excise from one genomic locus and insert into another. This model, however, does not provide explanations for the significant copy numbers these elements can reach in genomes (~3% in the human genome). Thus, how cut & paste transposons were amplified and propagated in vertebrate genomes is not yet clear. In principle, a transposon can take advantage of the cellular repair machinery to amplify its own genome. These studies will help us to understand how TEs similar to SB amplified in the human genome, and what factors influenced this process.
HMGXB4 targets transposition
We have shown earlier that HMGXB4/HMG2l1 (a component of the Wnt-signalling pathway) is able to physically associate with either the transposon DNA or the transposase protein, providing a negative feedback loop regulation to transposase expression (Walisko, 2008). New data suggest that HMXB4 has a high expression level in undifferentiated cells, but its expression level is dropping in differentiated cells in various models. We propose to validate the working hypothesis that HMGXB4 targets transposition to a well-defined developmental phase. The regulation of SB transposition in association with Wnt-signaling could be fundamentally different from LINE-1 (retrotransposon) regulation that is regulated primarily epigenetically in the human genome.
Heat shock response-associated mechanism of regulating transposition
Dawid Grzela*, Dr. Attila Szvetnik
Cellular mechanisms that are directly involved in development or stress-response have crucial role in establishing stable host-transposon co-existence. Our recent study sheds light on a heat shock (HS) response-associated mechanism of regulating transposition. Our data suggest that SB transposition is regulated by heat shock on different levels. The SB transposase protein is a relatively stable protein, but is sensitive to misfolding (Mates, 2009), arguing for a potential regulation via cellular trafficking and/or folding. The SB transposition might piggyback the HS response pathway that normally reactivates heat-aggregated, nuclear proteins. Thus, transposons might exist in a “latent” form in the genome and are able to sense developmental and environmental changes and manipulate stress signaling.