DNA DamageH. Singh Ph.D.NTRS 467

DNA damageDNA damage is a major topic of research within cancer biology. Damage not only causes cancer, it is used as a means to cure certain cancers through radiotherapy or chemotherapy and is also responsible for the side effects of these treatments. http://people.bath.ac.uk/pr1cemb/DNAdamage.htm

DNA damageOver 74,000 damage incidences occur in DNA per cell per day, mostly by oxidation, hydrolysis, alkylation, radiation or toxic chemicals that can either directly damage one of the 3 billion bases contained in DNA or create breaks in the phosphodiester backbone that the bases sit on.

The result can be mutations in genes which are transferred the gene product (protein). If these mutations are in genes that normally control cell proliferation or suppress tumour growth, the cells may start to grow uncontrollably. Cells have therefore developed mechanisms to repair DNA damage but when they stop working efficiently, the number of mutations in our genome increases and cancer can develop.

Agents that Damage DNA

Certain wavelengths of radiation ionizing radiation such as gamma rays and x-rays ultraviolet rays, especially the UV-C rays (~260 nm) that are absorbed strongly by DNA but also the longer-wavelength UV-B that penetrates the ozone shield . Highly-reactive oxygen radicals produced during normal cellular respiration as well as by other biochemical pathways.

Who is responsible? Major causeReactive Oxygen Species (ROS)

Reactive Nitrogen Species (RNS)

Free radicals

O2-SuperoxideHydroxylHOHydroperoxylHO2Hydrogen peroxideH2O2Lipid peroxideLO2HReactive nitrogen speciesThiyl

http://www.benbest.com/lifeext/aging.html#radical

Types of DNA Damage

All four of the bases in DNA (A, T, C, G) can be covalently modified at various positions.

One of the most frequent is the loss of an amino group ("deamination") resulting, for example, in a C being converted to a U.

Mismatches of the normal bases because of a failure of proofreading during DNA replication.

Common example: incorporation of the pyrimidine U (normally found only in RNA) instead of T.

Other damagesBreaks in the backbone. Can be limited to one of the two strands (a single-stranded break, SSB) or on both strands (a double-stranded break (DSB). Ionizing radiation is a frequent cause, but some chemicals produce breaks as well. Crosslinks Covalent linkages can be formed between bases on the same DNA strand ("intrastrand") or on the opposite strand ("interstrand").Several chemotherapeutic drugs used against cancers crosslink DNA [Link].

WhereMitochondria more than nuclear DNAIntracellular source is Mitochondrial electron transport may generate radicals ROSPrevented by low calorie intake and free radical inhibitorsFe and Cu are associated with ROS

MembranesMitochondriaEnzymesChromosomesDNAScientific American, Dec. 1992

Repairing Damaged Bases

Damaged or inappropriate bases can be repaired by several mechanisms: Direct chemical reversal of the damage Excision Repair, in which the damaged base or bases are removed and then replaced with the correct ones in a localized burst of DNA synthesis. There are three modes of excision repair, each of which employs specialized sets of enzymes. Base Excision Repair (BER) Nucleotide Excision Repair (NER) Mismatch Repair (MMR)

Cancer: What can we do about it?Cancer Chemotherapy The hallmark of all cancers is continuous cell division. Each division requires both the replication of the cell's DNA (in S phase) and transcription and translation of many genes needed for continued growth.So, any chemical that damages DNA has the potential to inhibit the spread of a cancer. Many (but not all) drugs used for cancer therapy do their work by damaging DNA.

What is the problem?Sadly, the cancer patient has many other cell types that are also proliferating rapidly, e.g., cells of the intestinal endothelium bone marrow hair folliclesand anticancer drugs also damage these producing many of the unpleasant side effects of "chemo". Agents that damage DNA are themselves carcinogenic, and chemotherapy poses a significant risk of creating a new cancer, often a leukemia.

Cancer Preventionexample: Human papilloma viruses (HPV) common virus responsible for > 70% of cervical cancer effective vaccine developed (2005) politicized issue over its use

Early Diagnosis is Important in Cancer TreatmentGoal: Identify tumorbefore transitionto malignant stage

Traditional Treatments for TumorsLogic: Treatment must be targeted against difference between normal and tumor cells

A) Fast Growing

B) Loss of Cell Cycle Control

What is our nature doing?

Cells Use Checkpoints to Fix DNA Damage Before Cell DivisionKey Point:Cell cycle arrestprovides time to fix damage before proceeding(Checkpoint protein & Tumor suppresor)Stopscell cycleDamaged DNAp53=

Normal Cells with Extensive Damage Commit Suicide (Apoptosis)Damaged DNAp53Too much damageTrigger cell death

Antioxidant EnzymesThe antioxidant enzymes are proteins with antioxidant properties. There are three known classes of antioxidant enzymes:Superoxide dismutases (SOD)Catalases (CAT)Peroxidases (Glutathione Peroxidase, GP)There are many forms of each class of protein. In general, cancer cells have low levels of these enzymes, when compared to an appropriate normal cell control.

Primary Antioxidant Enzyme System

http://lpi.oregonstate.edu/f-w97/reactive.html

Another important concept in cancer is Angiogenesis

What is Angiogenesis? Angiogenesis is the formation of new capillary blood vessels from quiescent vascular endothelium.

Formation of new blood vesselsWhen EC and smoothmuscle cells grow in response to specific signals

However, blood vessels donot grow in healthy adult

The process of Angiogenesis Local tissues require additional O2 and nutrients to sustain their growth.

Hypoxia results in the upregulation of genes for VEGF and enzymes that trigger the process.

The process of Angiogenesis For new blood vessels to sprout require that the surrounding area be broken down by metalloproteinase enzymes that carve out the pathway for vessel formation.

The process of Angiogenesis This creates local inflammation resulting in migration of macrophages and inflammatory cells to the site. Macrophages and monocytes release growth factors such as FGF that synergize with the VEGF activated by hypoxia.

The process of Angiogenesis The growth factors act upon receptors located on the surface of endothelial cells

The process of Angiogenesis The growth factors message is in turn carried into the cell by tyrosine kinases which turn on the machinery to initiate ANGIOGENESIS

Types of Angiogenesis Physiological Pathological

Physiological Angiogenesis A developing child in a mothers womb must create the vast network of arteries, veins, and capillaries that are found in the human body. A process called vasculogenesis creates the primary network of vascular endothelial cells that will become major blood vessels. Later on, angiogenesis remodels this network into the small new blood vessels or capillaries that complete the childs circulatory system.

Physiological Angiogenesis

The Bodys Control of AngiogenesisThe healthy body controls angiogenesis through a series of "on" and "off" switches: The main "on" switches are known as angiogenesis-stimulating growth factors The main "off switches" are known as angiogenesis inhibitors

Pathological Angiogenesis

How can food help?

Omega-3 fatty acids and cancer therapyW. Elaine Hardman, Ph.D.Department of Biochemistry and MicrobiologyMarshall University School of MedicineHuntington, West Virginia

Outline

What are omega 3 fatty acids?

Pre-clinical evidence for benefit of n-3 fatty acids during cancer therapy

Potential mechanisms for therapeutic benefit of n-3 fatty acids

Clinical evidence for benefit of n-3 fatty acids during cancer therapy

Major fat typesStearic18:0537911131517Saturated fatMonounsaturated fatOHC1291816OOleic (OA) 18:1n-9

Linoleic (LA) 18:2n-6O18C13 129OHOHC129O1815Linolenic(LNA) 18:3n-3Polyunsaturated fats

Pre-clinical evidence for benefit of omega-3 fatty acids during cancer therapySupplementing the diet with omega-3 fatty acids may suppress the growth of existing cancers and may prevent or slow metastasis

Omega 3 fatty acids may increase the efficacy of chemo- or radiation therapy

Hormone responsive tumors such as: breast, prostate and colon cancers seem especially sensitive to omega 3 fatty acids.

However, in animal models, lung cancer growth has been also slowed by omega 3 fatty acids.

Corn oil dietOmega 3 diet

***********Proliferation of new blood vessels also takes place in adults, although it is a relatively infrequent event. Physiological angiogenesis is necessary for the repair or regeneration of tissue during wound healing.**