Date of Graduation
Doctor of Philosophy (PhD)
Gain-of-function Mouse Models to Investigate Biological Roles of PRMT6
Alessandra Di Lorenzo, Ph.D. Candidate
Mentor: Dr. Mark T. Bedford
Protein Arginine Methyltransferase 6 (PRMT6) is the histone tail writer that methylates the H3R2 (arginine 2 of histone H3) residue, which counteracts the activating H3K4me3 mark. PRMT6 has been shown to behave both as transcriptional co-repressor (i.e. trhrombospondin-1, p21, p53), and co-activator (nuclear receptors). The co-repressor function of PRMT6 is likely the result of H3K4me3 antagonism, while the mechanism by which PRMT6 exerts its co-activator function has yet to be elucidated. PRMT6 is over-expressed in several types of tumors including small cell lung cancer, lymphoma, cervical cancer, bladder cancer, oligodendroglioma, breast cancer, and osteosarcoma, suggesting an oncogenic role for this enzyme. Importantly, we have found a positive correlation between PRMT6 over-expression and H3R2me2a elevation in small cell-lung cancer tissue arrays.
The aim of this project was to study the consequence of PRMT6 over-expression and the effects of elevated histone arginine methylation at the organismal level in inducible PRMT6 gain-of-function mouse models. To this purpose, I generated two different systems. One mouse model is based on an ER*-Flag-PRMT6 fusion (under a beta-actin promoter), while the other model is based on a Cre-inducible system, generated by cloning a floxed transcriptional Stop cassette upstream a Flag-PRMT6 (under a beta-actin promoter). Upon PRMT6 induction, both mouse models show elevated asymmetric dimethylation of H3R2 compared to wild type mice, demonstrating for the first time that PRMT6 methylates this residue in vivo. The ER*-PRMT6 mice die upon stabilization of the chimera and serum analyses have revealed elevation of Interleukin 6 (IL-6) levels, a key interleukin regulated by the transcription factor NF-kB. This finding has led us to the discovery that PRMT6 functions as a co-activator for NF-kB. I have found that PRMT6 is in the NF-kB complex, and that its overexpression enhances NF-kB transcriptional activity in luciferase assays and quantitative Real-Time PCR experiments. The activity of PRMT6 is clearly necessary for the coactivator function. ChIP analysis demonstrated that PRMT6 was recruited to the IL-6 promoter upon TNF-alpha stimulation. Moreover overexpression of PRMT6 caused RelA shuttling into the nucleus in our system, which could justify, at least in part, the mechanism underlying the coactivator function. I have also found that PRMT6 can deposit the H4R3me2a activating mark in vivo, which could explain the co-activator role of this arginine methyltransferase.
The Cre-inducible PRMT6 transgenic mice have been crossed with a mouse line expressing a Cre recombinase under a Keratin 5 (K5) promoter, as well as with a MMTV-Cre mouse line. I have conducted carcinogenesis studies with these mice. The K5-Cre crossed mice have been subjected to skin carcinogenesis study using the DMBA/TPA system and to a p53+/- genetic cross. The MMTV-cre crossed mice have been subjected to ageing studies. I have also performed RNA-seq experiments on primary mammary epithelial cells from the MMTV-Cre crossed mice.
In conclusion, this work identifies PRMT6 as a novel regulator of the inflammatory process, indeed we found that it cooperates with the transcription factor NF-kB in activating early response inflammatory genes such as IL-6 and TNF-alpha.
PRMT6, Arginine methylation, Transgenic mice, NF-kB, cancer