The Relationships Between Cancer, Chemotherapy, and DNA Repair Shelly Beard, Fred Caton, Sarah Ivan, Edra London, and Ka

1. CONCLUSIONS: INTRODUCTION: Figure 4. MDC1 and BRCA1 deficient cells show similar reductions in homologous recombination. (Zhang et al., 2005) Figure 2. AGT repair process: a. covalent transfer of alkyl group; b. repair, G→A transition mutation, or stand break can result. (Gerson et al., 2004). DNA repair mechanisms are necessary to prevent DNA damage from progressing to a mutation that could lead to cancer, but these same repair mechanisms are used by cancer cells to repair DNA damaged caused by anti-cancer drugs rendering the drugs less effective. Left un-repaired, methylation of guanine at the O6 position would lead to a G-T base pair. MGMT can remove this methylation allowing for correct base pairing. New cancer therapies are looking at ways to inactivate MGMT, which would allow the DNA damaging chemotherapy agents to work better. MDC1 is a potential target for anti-cancer drugs to inhibit the HR pathway. Inhibition of this protein leads to a decrease in the signal for repair so the repair machinery to slow to respond to the damage. Inhibitors of these enzymes along with traditional therapies would increase the efficacy of the therapies, but the key is to inhibit the DNA repair mechanisms of the cancer cells without further damaging the rest of the cells in the body. Figure 1. Figure showing mismatch repair mechanism involving key enzymes. (Iyer et al., 2006) Figure 3. How different methylation backgrounds of MGMT affect the response to alkylating drugs. (Esteller et al., 2004) The Relationships Between Cancer, Chemotherapy, and DNA Repair Shelly Beard, Fred Caton, Sarah Ivan, Edra London, and Katie Nelson Biochemistry 465, Univ. of Maryland, College Park, MD 20742 According to the American Cancer Society, 45.67% of males and 38.09% of females developed invasive cancers from 2000-2002. In addition, it is estimated that there will be over 550,000 deaths attributed to cancer in 2006 in the United States. Prevention and treatment of these cancers is a hot topic. Manipulation of DNA damage and repair mechanisms is the basis for several treatment options. DNA damage can occur naturally and there are countless enzymes and pathways that serve to repair this damage in order to prevent any lasting effects. While some base insertions, deletions, or point mutations may be harmless due to redundancy and conservation of the genetic code, other mutations can cause cells to become cancerous. However, DNA damage can also be used to treat cancer by causing enough DNA damage to lead to the death of the tumor cell. DNA repair pathways include: nucleotide excision repair, base excision repair, non-homologous end-joining, mismatch repair, homologous recombination among others. This poster concentrates on the last two, providing specific examples of how mismatch repair and homologous recombination are related to cancer treatment. Cancer treatment involving radiation and/or chemotherapy is an interesting dichotomy because the ability of these agents to reduce cancerous cells is related to their ability to cause DNA damage, while these cells also have the ability to invoke pathways to repair this damage (Madhusudan and Hickson, 2005). If one can inhibit DNA repair in specific tumor cells, increased efficacy of the treatment can be achieved. O6-Methylguanine DNA Methyltransferase (MGMT) Role of MDC1 in Homologous Recombination Homologous recombination (HR) plays an integral role in the survival of tumor cells after radio- and chemotherapies. MDC1 is a protein that senses DNA damage and aids in the transport of other repair proteins to the site of damage. As shown in figure 4, deficiency in MDC1 is similar to BRCA1, which is a known tumor suppressing gene that aids in repairing DNA damage. Programmed DNA methylation plays an important role in the maintenance of normal gene expression and genome stability. O6-methylguanine DNA methyltransferase (MGMT) is the gene that encodes O6-alkylguanine DNA alkyl-transferase (AGT), a DNA repair protein that removes abnormal alkyl adducts from the O6 position of guanine and less often, thymine (Fig. 2a). MGMT protects DNA from mutagenic legions by transferring the alkyl adduct from the guanine to the active site of MGMT (Cys145), thereby inactivating the enzyme so that one MGMT molecule protects against one DNA lesion prior to its degradation. Downstream Effects of MDC1 Disruption • Homology-mediated repair is impaired • Increased sensitivity to cross-linking agents and ionizing radiation • Decreased stability and mobilization of key repair protein Rad51 Therapeutic Strategies Involving MDC1 Knockdown • siRNA is the treatment of choice in silencing the gene that encodes MDC1. • Ionizing Radiation can be coupled with a pharmacological treat-ment to finish off cells that have compromised repair mechanisms. • Chemotherapeutic agents such as mitomycin C become especially effective against hypersensitive cells. Mismatch Repair • MGMT’s Role in the Etiology of Cancer • AGT protects healthy cells against exogenous carcinogenic damage. • MGMT-promoter methylation shuts off MGMT expression which causes DNA hypermethylation, and in turn, can silence other genes including protective tumor suppressor genes such as p53. • MGMT and It’s Potential Applications in the Treatment of Cancer • MGMT overexpression has been shown to prevent specific thymus, lung, liver, skin, and intestine cancers induced by methylating agents in a mouse model. (Gerson, 2004) • Cancer Relation To Mismatch Repair • Inactivation of mismatch repair causes hereditary nonpolyposis colon cancer (HNPCC). • Mismatch repair deficiency also linked to development of certain sporadic tumours in various tissues. • Mismatch repair may function as a general sensor of DNA damage. • 6-TG, FdU and cisplatin are used as antitumor drugs. • MutSa or MutLa (deficiency of the mismatch repair enzymes) increases the rate of gene duplication 50-100 fold, which can lead to cancer predisposition. • MGMT hypermethlyation has a potential role in pharmaco-epigenomics as methylated tumors are more sensitive to the destructive effects of alkylating drug therapies. • AGT inhibitors can increase the sensitivity of tumors to alkylating agents, thereby increasing the effectiveness of such chemotherapeutic treatment. • MGMT is a target gene for the protection of hematopoietic stem cells during chemotherapy for cancer. Stem cells are protected from the toxic affects of methylating agents by MGMT expression. REFERENCES: Esteller M, Herman JG. Generating mutations but providing chemosensitivity: the role of O6-methylquanine DANA methyltransferase in human cancer. Oncogene 2004; 23:1-8. Gerson SL. MGMT: Its role in cancer aetiology and cancer therapeutics. Nature Rev. 2004; 4:296-306. Iyer, Ravi R., et al. DNA Mismatch Repair: Functions and Mechanisms. Chem. Rev. 2006; 106: 302-323. Madhusudan, S., and I.D. Hickson. 2005. DNA repair inhibition: a selective tumour targeting strategy. Trends in Molecular Medicine 2005; 11: 503-511. Zhang, Junran., et al. MDC1 interacts with Rad51 and facilitates homologous recombination. Nature: Structural & Molecular Biology 2005; 12: 902-909 • All are cytotoxic with intrinsic mutagenic activity. • Defects in mismatch repair lead to a resistance to these drugs. • Patients with mismatch repair deficient cancers do not respond well to these drugs.