Stable inheritance of epigenetic states is essential for the maintenance of tissue and cell type specific functions. Aberrations of epigenetic regulation often lead to cancer and other human diseases. Our laboratory is interested in epigenetic regulation by histone demethylases (figure 1) in cancer and stem cells. In particular, we focus on the roles and regulatory mechanisms of histone demethylases from the JARID1/KDM5 protein family (figure 2). These enzymes can remove tri- and di- methyl mark from lysine 4 in histone H3 (H3K4me3 and H3K4me2) (figure 2), the epigenetic marks for transcriptionally active chromatin.
Figure 1. Histone demethylases are enzymes that remove methyl groups from histone tails.
Nucleosomes are shown in yellow, DNA is shown in red, histone tails are shown in blue, and methyl groups are shown as “Me”.
Figure 2. The JARID1/KDM5 proteins are histone demethylases for H3K4me3 and H3K4me2.
All these proteins contain JmjN, ARID, PHD, JmjC and C5HC2 domains. JmjC domain is the catalytic domain for histone demethylases.
We have previously identified RBP2 (Retinoblastoma Binding Protein 2) as one of the first known histone demethylases for H3K4me3. To understand the in vivo function of this enzyme, we generated an RBP2-/- mouse model (figure 3), which is the first knockout mouse model for lysine demethylases. We are currently studying how this enzyme contributes to oncogenesis using highly integrated mouse genetic (figure 3), molecular and cellular biological (figure 4), and biochemical (figure 5) approaches. Its functions will be better understood if we combine our findings at the organismal, cellular and molecular levels.
The other research directions of our laboratory focus on another H3K4me3 histone demethylase PLU1. PLU1 is highly expressed in breast and prostate tumor samples and is a marker of melanoma "stem" cells. Similar approaches are undertaken to investigate its roles in cancer. Our current data suggest that these enzymes are potential drug targets for cancer therapy and we have identified specific inhibitors of these enzymes.
Figure 3. Generation of an rbp2-/- mouse model.
The elimination of exons 5 and 6 of the mouse rbp2 gene leads to the loss of a portion of the ARID domain and all the C-terminal domains.
Figure 4. Both H3K4me3 and H3K27me3 marks are critical for the maintenance and differentiation of stem cells.
H3K4me3 is the epigenetic mark for transcriptionally active chromatin, and H3K27me3 is the epigenetic mark for transcriptionally inactive chromatin. In stem cells and progenitor cells, many genes important for developmental control are marked by both H3K4me3 and H3K27me3 marks and therefore are poised for transcription activation or repression. Thus, the enzymes that regulate the levels of H3K4me3 and H3K27me3 are important for the homeostasis of the stem cells.
Figure 5. Isolation of the histone demethylase complexes using FLAG and biotin tags.
The red flag denotes the FLAG tag, and the yellow circle with B denotes the biotin tag.