**4. Closing remarks**

524 Selected Topics in DNA Repair

A large body has yielded data suggesting that oncogene activation creates a tumor state that increases the accumulation of DNA damage 66-69. This damage, if unrepaired, can be converted into DSBs that eventually lead to cell death. To compensate for this increased DNA damage, the tumor cells require increased utilization of DNA repair processes 69. In this context, the administration of radiation introduces additional DNA damage that further taxes the already over-utilized repair process. This situation, in turn, increases the likelihood of an unrepaired DSB causing cell death. The same effect can be achieved by inhibition of DSB repair. The following sections will review FDA-approved agents with such properties. It is important to

Conventional chemotherapy involves DNA damaging agents that are often used in conjunction with radiation. In this context, these FDA-approved agents often sensitize the tumoricidal effects of radiation. The mechanism of this sensitization is thought to be related to the generation of DNA damages that sequester critical DNA repair proteins. For instance, lesions generated by cisplatin bind to and sequester the Ku70/80 heterodimer and thereby compromise the efficiency of NHEJ 70. Further, most DNA damages induced by conventional chemotherapy are ultimately converted to DSBs 71. These DSBs will titrate away the repair proteins available to repair the DSBs induced by subsequent radiation. These types of mechanisms likely account for the increased glioblastoma patient survival observed in the context of concurrent radiation/ temozolomide treatment relative to

As a result of extreme aneuploidy, copy-number variation, and transcriptional alteration that are present in many cancer cells, there is increased stress on the chaperone pathways (such as heat shock proteins) to maintain folding of over-expressed proteins. When the capacity of these chaperone proteins becomes saturated, the unfolded proteins require degradation by the proteasome complex 72. Thus, tumor cells exhibit increased dependency on proteasome function. Indeed, proteasome inhibition has been demonstrated to selectively ablate cancer cells both *in vitro* and *in vivo* 73. The proteasome inhibitor bortezomib has attained FDA-approval as a treatment for multiple myeloma and mantle cell lymphoma. Recent studies implicate proteasome function in DSB repair. The yeast Sem1 protein is a subunit of the 19S proteasome that is required for efficient HR 74. The human Sem1 homologue, DSS1, physically interacts with the HR protein, BRCA2, and is required for its stability and function 75-77. Using the DR-GFP assay to directly assess HR efficiency, Murakawa et al. demonstrated that HR efficiency is significantly reduced by proteasome inhibition 78. As a whole, these studies suggest proteasome inhibition as a means to target

The mechanism by which proteasome inhibition modulates HR remains an area of investigation. One hypothesis frequently put forth is the following. The proteins destined for proteasome degradation are typically modified by attachment of multiple ubiquitin moieties 74. Processing of the tagged protein releases the tagged ubiquitin to replete the intracellular pool. Proteasome inhibition, thus, leads to accumulation of ubiquitinated proteins. This accumulation, in turn, depletes the intracellular ubiquitin pool. Since free ubiquitins are

required to activate HR, the repair process is compromised by proteasome inhibition.

note that these agents induce pleiotropic effects beyond DSB repair inhibition.

**3.2 DNA damaging agents** 

radiation treatment alone 3, 4.

**3.3 Proteasome inhibitors** 

HR in cancer therapy.

Radiotherapy is the most effective post-surgical treatment modality in the management of glioblastoma. Adjuvant radiotherapy alone provides a more than doubling of median survival. Incremental gains with additional medical therapy have proven elusive, with most agents showing moderate activity *in vitro* or with encouraging early clinical experience only to demonstrate a lack of benefit in larger trials. Attempts at treatment intensification with radiotherapy have been similarly disappointing. Molecular understanding of DNA damage response and repair, on the other hand, has now afforded novel therapeutic targets. These targets are particularly attractive in the context that oncogenes induce increased DNA damage accumulation and cause tumors to become hyper-dependent on DNA damage response pathways. Encouragingly, several FDA-approved agents modulate critical proteins in DNA damage response/repair, including conventional DNA damaging agents, proteasome inhibitors, and EGFR inhibitors. Clinical trials involving these and other agents modulating DNA damage response should be designed with this consideration.

DNA Damage Response and Repair: Insights into Strategies for Radiation Sensitization 527

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**24** 

*USA* 

**The Botanical Extract** 

**Feverfew PFE Reduces DNA** 

*The Johnson & Johnson Skin Research Center, CPPW,* 

**Damage and Induces DNA Repair Processes** 

*a division of Johnson & Johnson Consumer Companies, Inc. Skillman, New Jersey,* 

Our skin is equipped with specialized cells and mechanisms that defend our bodies against pathogens, heat, and water loss. Today, our skin is exposed to increased environmental stress including solar ultraviolet radiation (which results in direct and indirect DNA damage) and atmospheric pollutants. Ozone depletion from the earth's atmosphere as well as expanding industrial processes has led to increased exposure to pollutants including pesticides and cigarette smoke. While UV radiation, and in particular its UV-B component (280-315 nm), has several health benefits (including production of vitamin D3) (Reichrath,

The sun produces UV radiation classified into three broad bands. The highest energy UV-C (100-280 nm) radiation is largely absorbed by the earth's atmosphere and thus does not affect humans. Meanwhile, the UV-B component is partially absorbed by the atmosphere and UV-A (315-400 nm) is primarily unabsorbed. While lower energy UV-A radiation penetrates beyond the epidermis, higher energy UV-B radiation primarily affects the

Harm to the body's barrier can lead to DNA mutation or DNA replication inhibition in the skin and eyes (cataracts) and may lead to broader immunosuppression (Britt, 1995). The most serious skin cancer (malignant melanoma) occurs when excitation of a chromophore leads to either direct reaction of the excited molecule with DNA or in the production of a free radical which may also react with DNA. Since the body produces oxygen free radicals (ROS) as part of normal metabolism (during ATP production), it is able to combat oxidative

The body's protective system, however, may become overwhelmed and compromised by environmental factors, age, or disease. Aging leads not only to increased total exposure but also to a decrease in production of endogenous antioxidants (enzymes and vitamins) and an increased risk of DNA damage. Oxidative stress can also lead to damage to other cellular components including lipids and proteins. A naturally-derived method to enhance protection against environmental factors that eventually overwhelm the body's defense mechanisms is discussed. Another major risk of UV radiation is oxidative damage to lipids (peroxidation) and proteins. Cell membranes, which are composed of lipids, are especially

2008) continuous exposure is the primary source of UV-induced DNA damage.

**1. Introduction** 

outermost epidermal layer of skin.

stress through endogenous antioxidants.

Michael D. Southall, Simarna Kaur and Khalid Mahmood

