Telomeres are nucleoprotein complexes located at chromosome ends. Although telomeres do not contain any genes, they play a pivotal role in protecting the natural chromosome ends from illegitimate DNA recombination, repair, and nucleolytic activities. Therefore, telomeres are essential for genome integrity and chromosome stability (which is important for preventing tumorigenesis in humans). Conventional DNA polymerases cannot fully replicate the ends of linear DNA molecules, leading to progressive telomere shortening in proliferating cells, causing the so-called "end replication problem" (which is an important cellular senescence mechanism in human somatic cells and has been implicated in organismal aging). Most eukaryotes use a specialized reverse transcriptase, telomerase, to synthesize the telomeric DNA de novo, which helps maintain a stable telomere length homeostasis. The core telomerase enzyme contains a catalytic protein subunit (TERT) and an RNA subunit (TR) that provides a short template for the telomeric DNA synthesis. Telomere maintenance mechanisms are essential for unlimited cell proliferation. In addition, telomeres often form a heterochromatic structure that suppresses the expression of genes located at subtelomeric regions, a phenomenon termed telomeric silencing. Furthermore, telomeric and subtelomeric regions are frequently fragile sites that experience relatively more frequent DNA recombination than most chromosome internal regions. Interestingly, in quite a few microbial pathogens that undergo antigenic variation, genes encoding the major surface antigens that are essential for the pathogen’s virulence are located at subtelomeric regions, presumably taking advantage of the plastic nature of the subtelomere.
My lab is interested in studying telomere functions in Trypanosoma brucei, a protozoan parasite that causes African trypanosomiasis in humans and livestock, which is frequently fatal without treatment. T. brucei regularly switches its major surface antigens (VSGs) to evade its mammalian host’s immune response. This antigenic variation is critical for T. brucei to establish a long-term infection. All VSG genes are located at subtelomeric loci, and VSGs are exclusively expressed from telomere-adjacent VSG expression sites in a strictly monoallelic manner. Our lab has demonstrated that T. brucei telomere proteins not only are essential for cell proliferation but also suppress VSG switching and are essential for VSG monoallelic expression. We have found that T. brucei RAP1, an essential telomere protein, is a key regulator of VSG monoallelic expression. We have also found that many telomere proteins help maintain the telomere integrity and suppress telomeric and subtelomeric DNA recombination. Interestingly, we have found that T. brucei TRF and RAP1 both bind the duplex telomeric DNA, although using very different binding mechanisms, and both help maintain the telomere integrity by suppressing the level of telomeric repeat containing RNA (TERRA) and the telomeric R-loop level. In contrast, PolIE, an essential telomere protein and an A family DNA polymerase, helps maintain the telomere integrity by coordinating the telomere G- and C-strand syntheses. We are currently investigating the underlying mechanisms of how telomere proteins maintain the telomere integrity and stability and how T. brucei RAP1 regulate VSG monoallelic expression.
We are also investigating the interaction between TERT and TR and its role in telomerase-mediated telomere maintenance in T. brucei.
Additionally, we are collaborating with Dr. Bin Su in the Chemistry Department at Cleveland State University to develope small compounds that specifically inhibits T. brucei growth without affecting its mammalian host. Our work has resulted in identification of several lead compounds that can inhibit T. brucei growth effectively both in vitro and in vivo.