Andrew J. Modzelewski, BSc, PhD, or "Dr. Modz" as he is affectionately known, is an assistant professor in the Department of Biomedical Sciences who was recruited to Penn Vet last year. Dr. Modz received his BSc from Penn State University with a major in Biochemistry and Molecular Biology. During this time, under the mentorship of Dr. John Golbeck, he developed an interest in biophysics, technology development, and understanding basic mechanisms. Dr. Modz then went to Cornell University for his PhD in Genetics, Genomics and Development. Under the mentorship of Dr. Paula Cohen, Dr. Modz uncovered an unexpected nuclear role for RNAi in sex chromosome inactivation mouse oogenesis and spermatogenesis. It was during his time at Cornell that he developed an interest in reproduction and early embryonic development. Dr. Modz then moved to the University of California at Berkeley for his postdoctoral research in the lab of Dr. Lin He, an expert on miRNAs and cancer.
As a postdoc, Dr. Modz pivoted from his mentor’s major research to investigate a different kind of non-coding RNA in the context of early development. Dr. Modz modified and developed various tools to study the phenomenon of retrotransposon reactivation that occurs in all mammalian preimplantation embryos. One of these tools is an electroporation-based CRISPR/Cas9 delivery system called “CRISPR RNP Electroporation of Zygotes” (CRISPR-EZ). This tool is quick, easy, and importantly very affordable and outperforms the gold standard technique of microinjection in most zygote related manipulation techniques. In collaboration, Dr. Modz helped develop a single cell/embryo Western Blot-based technique called Tri-Blot that can measure DNA, RNA, and Protein for the same single cell. Dr. Modz used these tools to study one of the many potential mechanisms of retrotransposons. Despite being called “Junk DNA” or “Selfish DNA," Dr. Modz challenged the long standing “Us vs. Them” model that describes retrotransposons as parasites and mammalian genomes as hosts, and published evidence of the first essential retrotransposon in mammalian preimplantation development, suggesting instead a “symbiotic” relationship. After completing his postdoctoral training, Dr. Modz combined his background and training to further study the role of retrotransposon reactivation in development and extend this to instances of epigenetic breakdown that occur in aging, disease, and cancer, where retrotransposons frequently re-emerge and potentially contribute to the success of the malignancy.
This First Essential Retrotransposon
Approximately 50% of mammalian genomes originate from retrotransposons while only ~2% is associated with protein coding genes. Retrotransposons are effective at colonizing genomes, through hijacking cellular machineries to spread their own genomes using a ‘copy and paste’ mechanism for expansion. Over millions of years of co-evolution, most retrotransposons have been inactivated through mutation. Still, many retain regulatory and structural features that can influence nearby genes in cancer, immunity, and aging cells. While silenced in healthy adult tissues, retrotransposon reactivation is essential in preimplantation embryos, as disruption of their expression results in embryonic lethality, but the reason for this is unknown. The repetitive nature of retrotransposons makes studying individual functions difficult, however comparative analysis of eight publicly available mammalian preimplantation RNA-SEQ datasets revealed strikingly similar levels of dynamic retrotransposon family expression that are active during defined windows of time, sometimes spanning a single cell division. A subset of these splice with nearby protein coding genes, forming hundreds of novel embryo and species-specific promoters, exons, and polyA sites, called “Chimeric Transcripts.” Using CRISPR-EZ, Dr. Modz generated five retrotransposon deletions mouse lines, essentially restoring the genomes to a “pre-integration” ancestral state.
One retrotransposon deletion mouse line was fully characterized and revealed at least one essential function for retrotransposons. A mouse-specific retrotransposon called MT2B2 acts as a promoter to transiently drive a truncated Cdk2ap1 isoform (Cdk2ap1ΔN) that promotes proliferation just prior to implantation. In contrast, the canonical Cdk2ap1 represses cell proliferation but expresses after implanting. The MT2B2 promoter is essential, where deletion induces maternal and pup lethality in part by reduction of cell proliferation, impaired implantation, uterine crowding, and implantation into unsuitable uterine sites, reminiscent of the human pregnancy complication placenta previa. Surprisingly, Cdk2ap1ΔN is evolutionarily conserved in sequence and function, yet is driven by different retrotransposons promoters across mammals in an unusual case of convergent evolution. The distinct Cdk2ap1ΔN expression strongly correlates with the duration of preimplantation in each species. Hence, retrotransposon reactivation is an aspect of normal biology, where species-specific transposon promoters can yield evolutionarily conserved protein isoforms, bestowing novel functions and species-specific expression to govern essential biological processes.
Current efforts in the lab are based on characterizing the remaining retrotransposon deletion mouse lines that display developmental defects spanning fertility, global translation, and regulation of totipotency and pluripotency. The lab is also engaged on technology development and expanding the utility of CRISPR-EZ for more sophisticated genome editing strategies. Collectively, these efforts are aimed at developing a comprehensive atlas of retrotransposon functions and mechanisms to better understand and appreciate the complex nature of developmental biology.
Looking Forward
As the Modz Lab grows, Andrew aims to use the power of comparative biology to expand his studies to other species including humans using embryonic stem cells and novel “synthetic embryo” technologies that are rapidly emerging. Ultimately, the Modz Lab is interested in understanding the various retrotransposon-based mechanisms that exist in the developing embryo (where reactivation is intentional and essential) and apply this knowledge to instances where dysregulation of retrotransposons occur, where reactivation is instead spontaneous and contributes to the survival and success of the disease at the expense of the individual.
References:
Modzelewski, A. J., Chen, S., Willis, B. J., Lloyd, K. C. K., Wood, J. A., & He, L. (2018). Efficient mouse genome engineering by CRISPR-EZ technology. Nature Protocols, 13(6).
Diallo, C. K., Modzelewski, A. J. Efficient Genome Editing of Mice by CRISPR Electroporation of Zygotes. J. Vis. Exp. (190), e64302, doi:10.3791/64302 (2022).
Modzelewski, A. J., Gan Chong, J., Wang, T., & He, L. (2022). Mammalian genome innovation through transposon domestication. Nature Cell Biology 2022, 1–9.
Modzelewski, A. J., Shao, W., Chen, J., Lee, A., Qi, X., Noon, M., Tjokro, K., Sales, G., Biton, A., Anand, A., Speed, T. P., Xuan, Z., Wang, T., Risso, D., & He, L. (2021). A mouse-specific retrotransposon drives a conserved Cdk2ap1 isoform essential for development. Cell, 184(22), 5541-5558.e22.
Rosàs-Canyelles, E., Modzelewski, A. J., Geldert, A., He, L., & Herr, A. E. (2021). Multimodal detection of protein isoforms and nucleic acids from mouse pre-implantation embryos. Nature Protocols, 16(2), 1062–1088.