Analysis of the Two Isoforms of the Human Alkyl Adenine DNA Glycosylase (HAAG) Gene: A Comparative Study of its Isoforms, its Protein and its Resistance to DNA Damage Agents

Bonanno, Kenneth C

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Analysis of the Two Isoforms of the Human Alkyl Adenine DNA Glycosylase (HAAG) Gene: A Comparative Study of its Isoforms, its Protein and its Resistance to DNA Damage Agents

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This study was conducted at the University of Massachusetts Medical Center in the Volkert laboratory. Human alkyl adenine DNA glycosylase (hAAG) is a DNA repair enzyme that repairs alkylated DNA bases. hAAG was cloned in 1991 and a second isoform was classified in 1994. The difference between the two isoforms of hAAG is an alternate spliced first exon. Both isoforms of the hAAG gene were present in the Volkert laboratory collection, however the second isoform (hAAG-2) was phenotypically different than the first and became the first focus of this study. Using the improperly functioning isoform as a template, and constructing a 5â€™ primer with the identical upstream sequence as the functioning isoform (hAAG-1), a phenotypically similar gene was constructed by PCR. The new isoform (hAAG-2) was cloned into an expression vector and its activity as a DNA repair agent was studied. A second version of hAAG-2 was also constructed, incorporating a histidine tag for protein purification and identification purposes. Efforts included using the ability of hAAG to complement glycosylase deficient alkA tagA E. coli double mutant strains to assess and to compare the ability of the two isoforms of hAAG and to determine if the histidine tag affected function. The ability of hAAG to rescue cells from exposure to a variety of DNA damaging agents was studied by inducing each isoform and analyzing the sensitivity of the cells to increased doses of DNA damaging agents. Both hAAG-1 and hAAG-2 were able to restore the wild type resistance of the alkA and tag genes when exposed to the alkylating agents MNNG and MMS. In order to study the ability of hAAG to repair alkyl lesions larger than methyl groups, it was necessary to inactivate the uvrA dependent nucleotide excision repair gene. In E. coli, methyl lesions are repaired primarily by glycosylases, while nucleotide excision repairs bulky lesions. Thus, in order to detect hAAG activity on these types of damage, it was necessary to inactivate the bacterial uvrA gene. Each isoform of hAAG was transformed into a triple mutant strain deficient in alkA tagA and uvrA, then exposed to CNU, BCNU, and Mitomycin C. Each of these DNA damaging agent caused increased toxicity in the presence of hAAG. hAAG-1 expressed in the alkA tag double mutant strain was exposed to Mitomycin C and showed greater resistance than hAAG-1 expressed in the alkA tag uvrA triple mutant. In fact, in the nucleotide excision proficient strain, expression increased Mitomycin C resistance above that seen in the control, suggesting that glycosylase activity may function in a partnership with nucleotide excision repair and that the two isoforms of hAAG have subtle differences. An ompT protease knockout host strain was constructed using P1-transduction and used to examine protein products. hAAG-2 was inserted into the pBlueScript plasmid so that the gene could be regulated by the T7 promoter for use beyond the scope of this thesis. A protein synthesis time course assay was conducted to determine the expression levels of hAAG-1 and hAAG-2 when induced by IPTG. Immunoblot detection of the histidine tag was used to measure expression levels of each isoform.

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