Current Research

Role of chromatin structure in DNA repair

Chromatin structure plays a critical role in modulation of cellular response to DNA damage. We are examining how it influences the ability of proteins involved in the repair process to interact with damaged DNA. The cancer prone, DNA repair deficient genetic disorder, xeroderma pigmentosum (XP), is being used as a model for these studies. Cells from XP complementation group A (XPA) patients, unlike normal human cells are defective in ability to repair damaged DNA when it is in the form of nucleosomes. This is may be related to the mechanism by which DNA endonucleases involved in the repair process locate sites of damage. These studies indicate that the XPA protein may be involved in determining the mechanism of action utilized by these endonucleases. By investigating the role that the XPA protein plays in location of damage sites on DNA and the importance of this protein for location of target sites on nucleosomal DNA, we expect to obtain a clearer understanding of the mechanism of interaction of repair proteins with damaged nucleosomal DNA and the severe consequences, such as the production of numerous cancers, that occur in individuals when this mechanism is defective.

Figure 1. Localization of a SpII S * and FANCA in the nucleus of normal and FA-A cells after treatment with a DNA interstrand cross-linking agent, 8-MOP plus UVA light. (A) a SpII S * and FANCA co-localize in damaged normal cell nuclei, (B) decreased formation of a SpII S * foci and no FANCA foci in damaged FA-A cells, (C) a SpII S * and FANCA again co-localize in nuclear foci in 8-MOP plus UVA treated corrected FA-A cells.

Figure 2. Model for the role of a SpII S * in repair of DNA interstrand cross-links in (A) normal and (B) Fanconi anemia cells. In normal cells, FANC gene products regulate the stability of a SpII S *. a SpII S * binds to cross-linked DNA, acts as a scaffold and aids in the recruitment and alignment of repair proteins and FANC proteins to the site of damage thus enhancing the efficiency of the repair process. In FA cells, defects in the FANC genes or gene products lead to decreased levels of a SpII S * due to decreased stability of a SpII S *. Decreased levels of a SpII S * lead to decreased binding of this protein to damaged DNA and decreased recruitment and alignment of repair proteins at sites of damage, thus reducing the efficiency of DNA repair in FA cells.

Pre-Cancerous Genodermatoses

Damage to cellular DNA is among the most important causes of disease in man, with over 10,000 adducts in DNA believed to occur per cell per day due to normal metabolic events as well as a great many more due to environmental stress, exposure to mutagens, etc. Human cells respond to these events in at least three ways: 1) DNA repair, via a number of separate enzyme pathways, 2) stopping or slowing of replicative DNA synthesis, putatively to allow repair to occur, and 3) in cells damaged beyond certain thresholds, apoptosis. We have developed an extremely advanced computerized image analysis system which allows all of these events, and others, to be examined in cultured human cells simultaneously. This has been combined with very sophisticated cell culture techniques, a number of which are also unique to our laboratory, which allows us, for example to examine the function of certain DNA repair genes/proteins in repair deficient cell lines (e.g.,from patients with Xeroderma Pigmentosum or Fanconi Anemia) in culture following their introduction into these cells by electroporation. Using these methods we have also identified specific DNA repair defects in patients with other cellular degenerative disorders, in particular Amyotrophic Lateral Sclerosis and Alzheimer = s Disease. Recently the imaging probes have been further enhanced by the use of mathematical transforms, particularly Fourier transforms and fractal analysis, which have been applied to cultured cells as well as to biopsy specimens from humans and experimental animals. We have also developed methods to quantitate special configurations of DNA, such as triplex DNA, quadroplex DNA and Z-DNA, in cells undergoing these processes. This combination of special methods is allowing us to address fundamental questions, particularly related to DNA repair, at the cellular, molecular, and clinical level.  Using this technology, we have recently shown that the cell cycle defect characteristic of the important inherited blood disease, Fanconi anemia, is not in the G 2 phase, as has been believed and studied for over two decades, but is actually in the S phase. We have also shown that this defect is at least as important as, and may be more important than, the DNA repair defect in this disease. Studies are in progress to further characterize this newly discovered defect as well as to search for similar deficiencies in other diseases.