The eukaryotic cells undergoes a series of coordinated events to ensure faithful replication of their genome. This chain of events, called the ‘cell cycle’, ensures the correct segregation of genetic material in the two daughter cells. A disruption of these events may lead to cell death or oncogenic transformation. Hence, the processes of cell cycle are carefully regulated.
Elucidating the role of effector proteins in cell cycle progression
A key step in the eukaryotic cell cycle is the G1 to S phase transition and this step is tightly coupled to the transcriptional control of genes involved in growth and DNA replication.
Figure 1. Proposed model for activation and repression of S-phase promoters. . During G1/S phase transition, HCF-1 and its associated H3K4 HMTs are recruited to E2F-responsive promoters, replacing the pRb repressor complex. The E2F-HCF-H3K4HMT complex then deposits the H3K4 tri-methylation mark and activates transcription of S-phase genes. In early G1 phase, E2F4 and p130 recruit demethylase RBP2 to erase the H3K4 tri-methylation mark and repress these promoters. Hence H3K4 tri-methylation mark is dynamically regulated during the cell cycle.
In mammalian cells, the E2F family of transcription factors primarily controls this temporal gene expression. We had previously shown that HCF-1 is an important regulator of G1 to S-phase transition and plays a direct role in the activation of E2F-responsive promoters through the cell-cycle-specific recruitment of the Mixed Lineage leukemia (MLL)-family of Histone H3 lysine 4 (H3K4) histone methyltransferases. However, how and when the H3K4 tri-methylations marks are reset during the cell cycle was not known. Our recent work shows that E2F4 and pocket protein p130 recruit the demethylase KDM5A/RBP2 to these promoters in early G1 to erase the H3K4 tri-methylations marks and repress the E2F-responsive promoters (Figure 1). While this work has added new effectors, how E2F-responsive promoters are regulated is still poorly understood. In our laboratory, we aim to discover new effector proteins involved in regulation of E2F-responsive promoters and better understand how these effectors influence the chromatin modulation during cell cycle progression.
Cell cycle and cancer:
Key regulators of cell cycle play a central role in tumor development as well. For example, deregulation of the Rb-E2F pathway is one of the hallmarks of human cancers. Many genes involved in basic cell cycle processes are differentially expressed in more proliferative tumors. Characterization of the function of such genes vis-a-vis cell cycle is a critical step in understanding the basic cell cycle processes, and their role in cancer.
Delineating the role of chromatin modifying proteins in cell cycle regulation
MLL, the trithorax ortholog, is a well-characterized H3K4 methyltransferase that was first identified for its involvement in chromosomal translocations associated with acute leukemia in infants and adults. Later studies revealed its crucial role in proper regulation of the Hox genes, hematopoietic stem cell self-renewal and progenitor cell expansion during embryonic development.
MLL’s role as a histone methyltransferase in gene activation and in leukemic cells is extensively studied. However, the functions of MLL in cellular processes like cell-cycle regulation are not well allocated. Understanding how MLL regulates essential processes such as cell proliferation and genomic stability may provide a better understanding of its pathological condition.
We have previously reported a transcription- and methylation-independent role of MLL complex during mitosis. Our recent work reveals that MLL complex localizes to spindle microtubules, and RNAi of the MLL complex severely compromises chromosome alignment during metaphase. Our studies uncover interactions of the MLL complex with kinesin and dynein motor proteins. We characterized the role of the MLL complex in regulating spindle localization of kinesin-13 motor protein, Kif2A. Collectively, our results show how the MLL complex functions in pathways, other than transcription, to ensure proper division of genetic material, and even loss of function of MLL in this process may be conducive to tumorigenesis. Now we are in the process of understanding other functions of MLL in regulation of mitosis and how they can provide insights into MLL-rearranged leukemia.
Figure 2. Loss of MLL results in chromosome misalignment. MLL localizes to the spindle microtubules in mitosis. In control siRNA treated cells, MLL is present on the spindle, and chromosomes are aligned properly (metaphase cells are shown). Upon loss of MLL, the spindle is distorted and as a result chromosomes are unable to align, resulting in errors in segregation.