DNA Repair: Part two

More about the article entitled 'Life without DNA Repair' by David M. Wilson III and Larry H. Thompson which can be accessed at the following address.

http://www.pnas.org/cgi/content/full/94/24/12754

Some snippets from the article and related comments follow. The article focuses on a deficiency in a base excision repair (BER) component, AAG, a DNA glycosylase that excises damaged DNA bases.

"DNA glycosylases can be separated into two groups: those that possess only an N-glycosidic cleaving activity, and those that possess both an activity to remove substrate bases and an activity to incise the phosphodiester backbone immediately 3 of the resulting AP site via a -lyase mechanism (reviewed in ref. 9). The biological significance of the AP lyase activity, which produces a normal 5-phosphate and an obstructive 3-end (i.e., a 3-deoxyribose moiety or a 3-phosphate), is currently unclear. Furthermore, how, if at all, the type of initiating DNA glycosylase dictates downstream events during BER is unknown. It seems likely, however, that any glycosylase-initiated repair event would proceed through the short-patch pathway in which APE would act as the 3-repair diesterase to remove the abnormal AP lyase-generated 3-terminus before gap filling and ligation."

[Bradford]: This reminds us how many parts there are to the base excision repair mechanism. Not only are there multiple proteins, but there can be multiple active sites too.

"Engelward, Weeda, and colleagues (8) have genetically engineered animals deficient in AAG, a DNA glycosylase that removes a broad spectrum of base damages, including, but likely not limited to, 3MeA, 3-methylguanine, 7-methylguanine, 1,N6-ethenoadenine, hypoxanthine, and 8-oxo-7,8-dihydroguanine; AAG does not possess an AP lyase activity. It is worth mentioning that the mouse and human AAG proteins are only moderately conserved (80% identity at the amino acid level) and display some differences in their substrate preferences (32). Given this fact and considering the notable disparities that have been observed between certain repair-deficient mice and their counterpart human subjects, we must proceed with caution when interpreting data gathered from animal models. However, this caveat does not diminish the incredible wealth of information that is being obtained from these models (1).

The First Glycosylase-Deficient Animal Model

Protein extracts from tissues of AAG (/) animals display essentially no detectable repair activity for 3MeA, 1,N6-ethenoadenine, and hypoxanthine base modifications, although a hint of a minor lung-specific glycosylase activity for 1,N6-ethenoadenine lesions was reported (8). Furthermore, the knockout embryonic stem cells show hypersensitivity to a variety of alkylating agents and, surprisingly, to mitomycin C (33). Thus, AAG likely represents the major repair glycosylase for alkylation base damages, whereas its role in protection against mitomycin C is unclear. The finding that AAG-deficient animals survive embryogenesis raises several issues, particularly in light of the embryonic lethality of the other BER knockouts (Table 1)."

[Bradford]: We see that AAG is likely the glycosylase repairing alkylation base damages but that AAG deficiency is not lethal during embryogenesis in contrast to other BER components. Next we find speculation as to the reason.

"The fifth, and perhaps most likely, explanation for the survival of these animals is that one or more of the other DNA repair systems substitutes for AAG in its absence. There may, in fact, be a minor DNA glycosylase activity that can cope with the normal level of alkylation base damage, but that goes undetected in the repair assays used. The ability to cross different genetically engineered repair-defective backgrounds may uncover any potential overlap of the various corrective systems. For instance, if two repair systems possess redundancy for a common cytotoxic lesion, then breeding the appropriate repair-deficient animals would lead to embryonic lethality of the double knockout. Measuring the distribution of the repair patch lengths in AAG (/) also may provide clues as to which pathway is adopted."

[Bradford]: The fifth possibility is thought to be perhaps the most likely. There may be functional redundancy that allows for other repair components to take up the AAG role. The DNA repair function is clearly a critical one and deserving of more attention from biology's natural historians.