An active alternative splicing isoform of human mitochondrial 8-oxoguanine DNA glycosylase (OGG1)

Eight alternatively spliced isoforms of human 8-oxoguanine DNA glycosylase (OGG1) (OGG1-1a, −1b, −1c, −2a, −2b, −2c, −2d and −2e) are registered at the National Center for Biotechnology Information (NCBI). OGG1-1a is present in the nucleus, whereas the other seven isoforms are present in the mitochondria. Recombinant OGG1-1a has been purified and enzyme kinetics determined. OGG1(s) in mitochondria have not been fully characterized biochemically until recently. The major mitochondrial OGG1 isoform, OGG1-2a (also named β-OGG1), has also been expressed and purified; however, its activity is unresolved. Recently, we purified recombinant mitochondrial OGG1-1b and found that it was an active OGG1 enzyme. We reported its enzyme kinetics and compared the results with those of OGG1-1a. The reaction rate constant of OGG1-1b 8-oxoG glycosylase activity (kg) was 8-oxoG:C > > 8-oxoG:T > > 8-oxoG:G > 8-oxoG:A and was similar to that of OGG1-1a under single-turnover conditions ([E] > [S]). Both OGG1-1b and OGG1-1a showed high specificity towards 8-oxoG:C. The reaction rate constant of OGG1-1b N-glycosylase/DNA lyase activity (kgl) was 8-oxoG:C > 8-oxoG:T ≃ 8-oxoG:G > > 8-oxoG:A and that of OGG1-1a was 8-oxoG:C > 8-oxoG:T, 8-oxoG:G and 8-oxoG:A. The kgl of OGG1-1b and OGG1-1a is one order of magnitude lower than the corresponding kg value. OGG1-1b showed an especially low kgl towards 8-oxoG:A. Comparable expression of OGG1-1a and OGG1-1b was detected by RT-PCR in normal human lung tissue and lung cell lines. These results suggest that OGG1-1b is associated with 8-oxoG cleavage in human lung mitochondria and that the mechanism of this repair is similar to that of nuclear OGG1-1a. Currently, the other five mitochondrial OGG1 isoforms have not been isolated. I summarize information on OGG1 isoform mRNAs, coding DNA sequences and amino acid sequences that are archived by the National Center for Biotechnology Information.


Introduction
According to the National Center for Biotechnology Information (NCBI), the human 8-oxoguanine DNA glycosylase (OGG1) gene encodes the enzyme responsible for the excision of 8-oxoguanine (8-oxoG), a mutagenic base byproduct that occurs as a result of exposure to reactive oxygen (http://www.ncbi.nlm.nih.gov/gene/4968). 8-oxoG was first described in 1984 by Kasai et al. [1] and is an abundant DNA adduct caused by oxidative stress [2]. The action of OGG1 includes lyase activity for chain cleavage.
A review of all eight alternatively spliced isoforms has not been published; therefore, in this review I present the published data on mainly mitochondrial OGG1 isoforms and summarize the information on eight alternative splicing isoforms archived by the NCBI.
Recently, we purified recombinant OGG1-1b and OGG1-1a using commercial human lung total RNA as the starting material, and showed that OGG1-1b was an active OGG1 enzyme. We compared the enzyme kinetics of mitochondrial OGG1-1b with the nuclear OGG1-1a protein [11], as described in the next section.
Inconsistent findings regarding OGG1-2a protein have been published. Hashiguchi et al. [10] purified recombinant OGG1-2a (β-OGG1) and reported that OGG1-2a did not show any significant OGG1 activity in vitro. They examined OGG1 activity with 100 nM OGG1-2a and 10 nM oligonucleotide as the substrate, and found no activity. In the control experiment, they examined 1 nM OGG1-1a and 10 nM oligonucleotide substrate and found active OGG1 activity. Roldán-Arjona et al. [9] reported the purification of recombinant OGG1-2a and showed OGG1 activity against 8-oxoG:C oligonucleotide with 1 μM enzyme and 5 nM substrate. The OGG1 activity of OGG1-2a in this experiment was very low, because they used an unusually high enzyme concentration.
Recently, Su et al. suggested that OGG1-2a (written as β-OGG1) was an accessory factor in mitochondrial Complex I function and was related to mitochondrial base excision repair [17].
Other mitochondrial isoforms OGG1-1c was cloned as an alternative splicing isoform of OGG1 in 1997 by Abratani et al. [3]. Expression of OGG1-1c was demonstrated by RT-PCR in some human tissues including the colon [3]. Localization was demonstrated by expressing epitope-tagged OGG1-1c in COS-7 cells [5]. The expression of OGG1-2b, −2c, −2d, and −2e was demonstrated by RT-PCR in a small number of human tissues including the cerebrum and kidney, and in the Jurkat cell line by Nishioka et al. [4]. These proteins have not been purified.
Analysis of information on the eight alternatively spliced isoforms of OGG1 archived with the NCBI Table 1 summarizes the mRNA accession number, nucleotide (nt) length, position of the 5′-UTR, coding DNA sequence (CDS) and 3′-UTR, exons, position of exon 1, 2, 3, 4, 5, 6, 7, and 8 of the eight alternative splicing isoforms of OGG1, as derived from the gene and nucleotide database of the NCBI (http://www.ncbi.nlm.nih.gov/gene/ 4968). Table 2 summarizes alternative splicing isoforms of OGG1 CDS, nt length, identity to OGG1-1a CDS and identity to OGG1-2a CDS according to the NCBI and examined by BLAST (http://blast.ncbi.nlm.nih.gov/Blast.cgi). Table 3 summarizes the protein accession number, amino acid length, identity to the OGG1-1a protein, identity to the OGG1-2a protein (as determined by BLAST), position of mitochondrial and nuclear localization signals, and OGG1 activity.
The OGG1-1a mRNA has exons 1-7 and no exon 8. The CDS begins at nt 344 in exon 1, and the nt sequence of 1292-1381 (90 bp) in exon7 is the last part of the CDS. The OGG1-1a CDS is composed of part of exon 1, all of exons 2, 3, 4, 5, 6 and part of exon 7.
Exon 6 is the last exon of OGG1-1b mRNA. The nt sequence 1242-1318 (77 bp) in exon 6 is the last part of the CDS. Although the nt sequence of 1536-1882 (347 bp) in exon 6 of the OGG1-1b mRNA is the same as the entire exon 7 of the OGG1-1a mRNA (nt sequence 1292-1638, 347 bp), the former represents part of the 3′-UTR. As for the OGG1-1b CDS, the nt sequence of 1242-1291 from exon 6 of the OGG1-1b mRNA is identical to the whole exon 6 CDS of the OGG1-1a mRNA. The OGG1-1b mRNA nt sequence of 1292-1294 is identical to the first part of the exon 7 CDS of the OGG1-1a mRNA. The OGG1-1b mRNA 1295-1318 sequence (24 bp), which encodes seven amino acids and the stop codon, differs from the 1295-1318 sequence (CDS) from exon 7 of the OGG1-1a mRNA, resulting in a different amino acid sequence for the last seven amino acids of OGG1-1b when compared with the OGG1-1a sequence.
The OGG1-1c mRNA has exon 7, but the nt sequence of this exon is different from that of OGG1-1a mRNA. It also has no exon 8. The nt sequence 1292-1576 (285 bp) from exon 7 of the OGG1-1c mRNA is the last part of the CDS, but differs from the 1292-1381 CDS (90 bp) from exon 7 of the OGG1-1a mRNA. The nt sequence of 1309-1398 (90 bp) of the OGG1-1c mRNA, a part of the CDS from exon 7, is identical to 1292-1381 (90 bp) of the OGG1-1a mRNA, the entire exon 7 nt sequence.
Only type 2 OGG1 mRNAs have exon 8. All type 2 OGG1 mRNAs have the same exon 8 nt sequence (861 bp). The OGG1-2a mRNA has no exon 7. The nt sequence 1292-1618 (327 bp) in exon 8 of the OGG1-2a mRNA is the last part of the CDS and the sequence 1619-2158 (540 bp) is the 3′ UTR.
The OGG1-2b mRNA has no exons 5-7. The nt sequence 1091-1417 (327 bp) in exon 8 of the OGG1-2b mRNA is the last part of the CDS and is identical to the OGG1-2a CDS, resulting in an identical amino acid sequence for the last 108 amino acids of OGG1-2a and OGG1-2b.
The OGG1-2c mRNA has no exons 4-7. The nucleotide sequence 909-931 (23 bp: the first two nucleotides cross a   The OGG1-2d mRNA has exons 7-8. The whole nt sequence of exon 7, 1292-1391 (100 bp), in the OGG1-2d mRNA is a CDS. The nucleotide sequence of 1392-1414 (23 bp: the first two nucleotides cross a splice junction from exon 7, plus six amino acids and the stop codon) from exon 8 of the OGG1-2d mRNA is the last part of the CDS, and gives rise to the same six amino acid sequence as that of the OGG1-2c protein. The nt sequence of 1415-2258 (844 bp) is the 3′-UTR.
The OGG1-2e mRNA has exons 7 and 8. The nucleotide sequence 1292-1312 (21 bp for six amino acids and the stop codon) from the first part of exon 7 of the OGG1-2e mRNA is the last part of the OGG1-2e CDS, resulting in an amino acid sequence that differs from the exon 7 CDS of OGG1-1a, OGG1-1c, and OGG1-2d. The nt sequence 1313-1344 is a part of the 3′-UTR. Exon 8 (861 bp) of the OGG1-2e mRNA is a continuous 3′-UTR. The whole exon 7 nt sequence 1292-1344 (53 bp) of the OGG1-2e mRNA is identical to part of the nt sequence 1339-1391 (53 bp) in exon 7 of the OGG1-2d mRNA.

Conclusions
Eight alternatively spliced isoforms of human 8-oxoguanine DNA glycosylase (OGG1) are registered with the NCBI. OGG1-1a is present in the nucleus, whereas the other seven isoforms are present in mitochondria. Recombinant OGG1-1a has been purified and its enzyme kinetics studied. The mitochondrial major OGG1 isoform, OGG1-2a (also named β-OGG1), has been purified; however, the OGG1 activity of this enzyme was unusual and has not been determined. Recently, we purified recombinant mitochondrial OGG1-1b and showed that it is an active OGG1 enzyme. We reported its enzyme kinetics and compared the results with the corresponding kinetics of OGG1-1a. The OGG1 activity of OGG1-1b was similar to that of OGG1-1a, except for the k gl against 8-oxoG:A. The OGG1-1b mRNA was detected by RT-PCR in normal human lung tissue and lung cells lines. These results suggest that OGG1-1b is associated with 8-oxoG cleavage at least in human lung mitochondria, and the repair mechanism is similar to that of nuclear OGG1-1a. Currently, the other five mitochondrial OGG1 isoforms have not been purified.
Abbreviations CDS: coding DNA sequence; 5′-UTR: Five prime untranslated region; OGG1: Human 8-oxoguanine DNA glycosylase; nt: Nucleotide; NCBI: the National Center for Biotechnology Information; k g : the reaction rate constant of the 8-oxoG glycosylase activity; k gl : the reaction rate constant of N-glycosylase/DNA lyase activity; 3′-UTR: Three prime untranslated region.