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Research

Nuclear Architecture

Nuclear Architecture

Alterations of nuclear architecture affect nuclear function. In particular, mutations in the LMNA gene encoding the nuclear proteins A-type lamins lead to a variety of changes of nuclear structure and chromatin modifications. Over 300 mutations in the LMNA gene have been associated with degenerative disorders, broadly termed laminopathies. These include muscular dystrophies, neuropathies, lypodystrophies, and premature aging syndromes. Hutchinson Gilford Progeria Syndrome is the most devastating lamin-related disease, with kids exhibiting phenotypes of premature aging and dying at adolescence from severe cardiovascular complications. Despite recent advances in the identification of pathways altered in HGPS, a clear cause or efficient therapy has not been found yet. One or our projects aims to determine whether genomic instability contributes to the pathophysiology of premature aging laminopathies, and characterize the molecular mechanisms involved. Our goal is to find therapeutic strategies that could ameliorate the disease phenotype.

In addition to mutations, changes in the expression of A-type lamins are emerging as a factor contributing to tumorigenesis. In particular, silencing of A-type lamins has been linked to poor prognosis in different types of cancer. We are using cells from the LMNA knock-out mouse model to determine the molecular mechanisms by which loss of A-type lamins contribute to cancer progression. Our studies indicate that A-type lamins deficiency impacts on the maintenance of telomere structure, length and function as well as on the DNA damage response pathway. We are now placing special emphasis on understanding how A-type lamins impact on these pathways.


Genomic instabilility: defects in telomere maintenance and DNA repair

Genomic instabilility: defects in telomere maintenance and DNA repair

Alterations in the DNA damage response pathway and in mechanisms of DNA repair as well as defects in telomere maintenance are among the leading causes of genomic instability, and clear contributors to aging and cancer. Defective DNA repair during early stages of tumorigenesis facilitates the acquisition of additional mutations over time, but can also be exploited for cancer treatment. Radiotherapy and chemotherapy preferentially kill tumor cells by generating extensive amounts of DNA damage that promotes cell death in repair-compromised tumors, with less effect in the surrounding non-neoplastic tissue where the repair pathways are intact. Thus, these therapies are important components in the management of breast cancer. However, radiation therapy causes inflammation and chronic oxidative stress, factors that contribute to tissue dysfunction and vascular injury. This toxicity limits the dose of radiation that can be delivered to cancer patients. As part of the Radiation Oncology Department, one of our goals is to find therapeutic strategies that enhance the response of the tumors to radiation therapy while ameliorating the toxic effects associated with radiation. To achieve this goal, we need to identify novel molecular pathways contributing to genomic instability in cancer that provide new targets for the development of radiosensitizers.


Chromatin structure: alterations of epigenetic mechanisms in cancer and aging

Chromation Structure

Cancer and aging are multifaceted processes characterized by genetic and epigenetic changes in the genome. The genetic component of aging received initially all the attention. However, epigenetic mechanisms have now emerged as key contributors to the alterations of genome structure and function that accompany aging and cancer. The three pillars of epigenetic regulation are DNA methylation, histone modifications and non-coding RNA species. Alterations of these epigenetic mechanisms affect the vast majority of nuclear processes including gene transcription and silencing, DNA replication and repair, cell cycle progression, and telomere and centromere structure and function. One of our research goals is to understand how epigenetic defects contribute to the pathophysiology of aging and aging-related diseases, especially cancer. Epigenetic changes may initiate aging and cancer phenotypes, or prime cells in such a way as to make them more susceptible to subsequent genetic or epigenetic alterations. The accumulation of further genetic or epigenetic changes over time would promote the progression of aging and cancer phenotypes.

ChromationStructure2

The basic unit of chromatin, the nucleosome, consists of DNA that wraps around an octamer of histones. The histone tails protrude from the nucleosome core and are subjected to different post-translational modifications. Nucleosomal DNA can also be methylated by DNA methyltransferase activities. The types of post-translational modifications of histones and the degree of DNA methylation determine a specific chromatin structure. Two morphologically distinct types of chromatin can be distinguished, namely euchromatin and heterochromatin. Euchromatin is associated with gene-rich and transcriptionally active domains of dispersed appearance characterized biochemically by hyperacetylation of histones and hypomethylation of both histones and DNA. In contrast, highly condensed heterochromatin is linked to gene-poor and transcriptionally inactive domains such as centromeres and telomeres. Biochemically, heterochromatin is characterized by hypoacetylation of histones, and hypermethylation of histones and DNA. In addition, non-coding RNAs have been recently recognized as players in chromatin remodeling, primarily involved in heterochromatin formation and transcriptional or post-transcriptional gene silencing.

Chromation Structure3

Epigenetic alterations, mainly in DNA methylation, modification of histones, and expression of non-coding RNAs are recognized as mechanisms contributing to malignancy. One of the aims of our research is the characterization of epigenetic mechanisms that regulate chromatin structure and the impact that epigenetic alterations have on chromosome stability, tumorigenesis and radiosensitivity. Our specific goals are:

(1) Characterize novel mechanisms regulating telomere chromatin structure and function in mammalian cells. A number of chromatin-modifying activities have been identified that participate in the acquisition of a heterochromatic structure at mouse telomeres. Abrogation of the different activities leads to telomere length deregulation. Given that telomere homeostasis is essential for genomic stability and regulation of cellular proliferative potential, it is of crucial importance to identify mechanisms regulating telomeric function as well as their contribution to the development and progression of tumoral processes.

(2) Evaluate the impact of chromatin alterations on the DNA damage response pathway. Recent evidence supports a role for chromatin-modifying activities in the DNA damage response. In addition, chromatin alterations that disrupt telomere function might trigger telomere-damage induced activation of the DNA damage response pathway. By comparing ionizing radiation sensitivity and activation of a DNA damage response between epigenetically normal and altered cells, we are investigating the implication of different chromatin modifications on DNA repair.
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Copyright 2009 Washington University School of Medicine Department Radiation Oncology.