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Research Paper

Modulation of the activity of methyl binding domain protein 4 (MBD4/MED1) while processing iododeoxyuridine generated DNA mispairs

Mohammad Azhar Aziz, Jane E. Schupp and Timothy J. Kinsella

DNA glycosylases function to remove endogenous and exogenous base damage
and thus contribute to the maintenance of genomic integrity. This function gains clinical
relevance when base mispairs introduced by chemotherapy or radiosensitizing drugs
become their substrate. This report describes the action of DNA glycosylases on the
mispairs generated by iododeoxyuridine (IUdR) – a radiosensitizer. A non-radioactive
fluorescent dye-based in vitro glycosylase assay was employed to quantitatively measure
the enzymatic activities of functionally related DNA glycosylases on IUdR generated
mispairs including G:IU and A:IU. Thymine DNA glycosylase (TDG) and methyl
binding domain protein 4 (MBD4/MED1) are found to act on G:IU (but not A:IU)
mispairs and are functionally complementary to each other. However, uracil DNA
glycosylase (UDG) does not show any activity on these mispairs. The methyl binding
domain of MBD4/MED1 was found to specifically inhibit the activity of MBD4/MED1
as well as the glycosylase domain, when the G:IU mispairs were located in a methylated
CpG context. However, inhibition of TDG activity on methylated G:IU mispairs by the
methyl binding domain was not observed.

Authors

Mohammad Azhar Aziz

Department of Radiation Oncology, University Hospitals Case Medical Center and the Case Integrative Cancer Biology Program, Case Western Reserve University, Cleveland, OH 44106

Jane E. Schupp

Department of Radiation Oncology, University Hospitals Case Medical Center and the Case Integrative Cancer Biology Program, Case Western Reserve University, Cleveland, OH 44106

Timothy J. Kinsella Corresponding author: timothy.kinsella@stonybrook.edu

Vincent K. Smith Chair and Professor Department of Radiation Oncology Case Western Reserve University