Human DNA glycosylase NEILl exhibits a superior ability to remove oxidized guanine lesions guanidinohydantoin (Gh) and spiroiminodihydantoin (Sp) from duplex DNA in comparison to other substrates. In this work, Gh and Sp lesions in bubble, bulge, and single-stranded DNA were found to be good substrates for NEIL1 but were typically excised at much slower rates than from canonical duplex substrates. A notable exception was the activity of NEIL1 on removal of Gh in bubble structures which approaches that of the normal duplex substrate. The cleavage of Gh in the template strand of a replication or transcription bubble may prevent mutations associated with Gh during replication or transcription. However, removal of hydantoin lesions in the absence of an opposite base may also result in strand breaks and potentially deletion and frameshift mutations. Consistent with this as a potential mechanism leading to an N-1 frameshift mutation, the nick left after the removal of the Gh lesion in a DNA bulge by NEIL1 was efficiently religated in the presence of polynucleotide kinase (PNK) and human DNA ligase III (Lig III). These results indicate that NEIL1 does not require a base opposite to identify and remove hydantoin lesions. Depending on the context, the glycosylase activity of NEIL1 may stall replication and prevent mutations or lead to inappropriate removal that may contribute to the mutational spectrum of these unusual lesions.
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