Our Science – Aladjem Website

Mirit Aladjem, Ph.D.

Laboratory of Molecular Pharmacology Head, DNA Replication Group Senior Investigator Building 37, Room 5068D National Cancer Institute 37 Convent Drive Bethesda, MD 20892 Phone:   301-435-2848 Fax:   301-402-0752 E-Mail:   aladjemm@mail.nih.gov Link: Other Homepage

Biography

Dr. Aladjem received her Ph.D. from Tel Aviv University. She was a research associate at the Weizmann Institute of Science and then a postdoctoral fellow at the Salk Institute, where her studies focused on the initiation of DNA replication and cell cycle control in mammalian cells. Dr. Aladjem joined the Laboratory of Molecular Pharmacology in October 1999.

Research

Initiation of DNA Replication in Mammalian Cells

The DNA replication group studies the regulation of DNA replication in mammalian cells. The initiation of DNA replication is a point of convergence for cellular signaling networks that insure proper cell cycle progression and preserve genomic stability. Disrupting the intricate balance between components of the cell cycle signaling network may lead to cancer, as manifested by the many cancer-predisposing mutations in genes involved in DNA replication. Conversely, anti-cancer drugs often target DNA replication and interfere with the cell cycle signaling network. Variations in cell cycle signaling pathways contribute to the wide range of sensitivity of various cancers to anti-cancer drugs.

In eukaryotes, different parts of a chromosome can replicate at different times during the S phase of the cell cycle, yet it is striking that each and every genomic locus replicates exactly once per cell cycle. To achieve the high precision of DNA replication, the cell cycle machinery must interact with chromatin to initiate replication at the correct location and the exact time. The cell cycle machinery must also receive signals from replicating chromatin to detect errors that might cause genotoxic lesions. Our goal is to identify and characterize interactions between mammalian cell cycle regulators and specific regions on chromatin that determine where and when replication initiates. Studying those interactions will reveal cell cycle signaling pathways that regulate DNA replication and coordinate replication with gene expression and chromatin condensation. Below is a short summary of our main findings.


Replication Sites: In previous studies we have identified the region between the two adult beta-like globin loci as a replicator, a DNA sequence that determines the location of replication initiation events. More recently, we have shown that two independent DNA sequences within the human beta globin locus can dictate the location of replication initiation events (hence, act as replicators). These studies imply that replicators tend to cluster in mammalian cells and that the degree of clustering differs in each locus and determines the initiation profile.
We have also dissected the requirements for initiation in each of the two replicators and identified point mutations that do not initiate replication. We have shown that essential modules within the two replicators can combine in several ways to form functional elements that dictate the location of initiation events. We have identified two classes of elements that need to interact to start replication and characterized the contribution of each element to initiation of DNA replication.

Recent Pertinent References:

Wang, L., Lin, C.M., Brooks, S., Cimbora, D., Groudine, M. and Aladjem, M.I. The human ?-globin replication initiation region consists of two modular independent replicators. Mol Cell Biol. 24:3373-86. 2004.

Wang, L, Lin, C.M., Lopreiato, J. O, Aladjem, M.I. Cooperative Sequence Modules Determine Replication Initiation Sites at the Human ?-Globin Locus. Human Molecular Genetics 15:2613-22. 2006.

Replication Timing: We have established a novel protocol that enabled the use of genetics to identify DNA sequences that affected replication timing in mammalian cells. We have found that an element within the human beta globin locus dictates replication timing and have characterized epigenetic factors that contributed to the establishment of replication timing. This work represents the first genetic analysis of replication timer sequences.
We have shown that inclusion of replicators, which were characterized based on their ability to dictate the location of initiation events, can also change the timing of replication and the condensation of chromatin in their vicinity. These studies suggested that replicators might stabilize gene expression in chromosomal regions prone to gene silencing. These studies also suggest that replicator sequences might be used to improve gene therapy vectors.

Recent Pertinent References:
Lin, C.M., Fu, H., Martinovsky, N., Bouhassira, E. and Aladjem, M.I. Dynamic Alterations of Replication Timing in Mammalian Cells. Current Biology 13, 1019-28. 2003

Fu, H., Wang, L., Lin, C.M., Singhania, S., Bouhassira, E.E, Aladjem, M.I. Preventing gene silencing with human replicators. Nature Biotechnology 24:572-6. 2006.

Response to Perturbed Replication: We have started to investigate how cells respond to perturbation of DNA replication. We have shown that exposure to mild drug-induced perturbation of DNA replication, which is below the threshold of the cell cycle checkpoint response, can rapidly induce DNA breaks. In cells that contain an intact nonhomologous end joining pathway, those DNA breaks are transient and cells rapidly resume replication in the presence of the inhibitor, albeit at a slow rate. However, DNA breaks persist in cells that are deficient in components of the pathway such as DNA-PK and XRCC4; such cells are unable to resume DNA replication and activate a cell cycle checkpoint response after a mild inhibition of DNA synthesis.

Recent Pertinent References:

Shimura, T., Martin, M.M, Torres, M. J, Gu, C. Pluth, J. M., DeBernardi, M., McDonald, J. S. Aladjem, M.I. DNA-PK is involved in repairing a transient surge of DNA breaks induced by deceleration of DNA replication. Journal of Molecular Biology 367:665-680. 2007

A systems approach to cell cycle and DNA replication studies. Understanding how cells control DNA replication is challenging, illustrating the growing gap between the rapidly accumulating data about signaling pathways and the ability of the scientific community to make sense of those data. To understand complex regulatory pathways involved in cell cycle signaling, we organized the molecular interactions involved in the early stages of DNA replication using the Molecular Interaction Map (MIM) notation first proposed by Dr. Kurt Kohn. We created MIMs of DNA replication pathways and developed electronic MIMs, which are available on the internet and allow easy access to annotations and databases. Those maps can operate as educational tools, but they can also serve as guides to simulation based studies aimed at understanding the control principles underlying bioregulatory networks (this project is a collaboration with Kurt Kohn, Yves Pommier and John Weinstein; for more information: http://discover.nci.nih.gov/mim/index.jsp).

Recent pertinent References:

Aladjem. M.I, Pasa S, Parodi S, Weinstein, J.N,, Pommier, Y and Kohn, KW. Molecular Interaction Maps - a diagrammatic language for bioregulatory and signal transduction networks. Science STKE 2004(222), pe8. 2004.

Kohn, K.W., Aladjem, M.I., Weinstein, J.N., and Pommier, Y. Molecular interaction maps of bioregulatory networks: a general rubric for systems biology. Molecular Biology of the Cell. 17:1-13. 2006.

This page was last updated on 6/11/2008.