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Figure 5. Repair of 1-methyladenine and 3-methylcyotsine. The reaction converts α-ketoglutarate to succinate with CO 2 release and
requires oxygen and Fe(II). There is also a release of formaldehyde to regenerate the normal base. 1-meA: 1-methyladenine; 3-meC:
3-methylcytosine.

[25]
is another major DNA repair pathway that is often defective in tumors . Hereditary nonpolyposis colon
cancer has defective MMR genes that result in genomic instability [26-28] , but other tumor types can also
evolve MMR deficiencies by mutations, although also by epigenetic silencing, particularly of MLH1 [29-34] .
The sensitivity of MMR-proficient cells in the absence of MGMT is due to the futile MMR cycles [35,36] that
6
are induced by O meG:T mispairing [Figures 2 and 3]. The role of MMR was demonstrated using Msh2-
[37]
deficient mouse embryonic stem cells and O6-benzylguanine that inactivates MGMT . Further mouse
model work demonstrated the resistance to alkylating agents that arises with the simultaneous inactivation
of both the Mgmt and Mlh1 and the increase in mutations upon treatment as compared to either of the
genes inactivated independently [38,39] .

ALKB HOMOLOG PATHWAY
The AlkB homolog (ALKBH) pathway in this review refers to both ALKBH2 and ALKBH3 (ALKBH),
which reversibly remove numerous modified bases but principally 1-methyladenine (1-meA) and
3-methylcytosine (3-meC) [40-42] . There are 9 proteins among the AlkB homologs, but the functions for most
are diverse and include removal of RNA modifications . Numerous protein groups use similar reaction
[43]
mechanisms, which include proline hydroxylation and ten-eleven translocation [44-47] . The ALKBH reaction
mechanism uses an oxidative demethylation with a Fe(II) that is coordinated to amino acids in the proteins,
along with α-ketoglutarate. The removal of the methyl group involves the conversion of α-ketoglutarate to
succinate with the release of carbon dioxide and formaldehyde [Figure 5].

Persistence of 1-meA and 3-meC can lead to mutations that are reported to be A→T transversions or
[48]
C→T transitions , which involve 1-meA pairing with T or possibly bypass of the 3-meC, because only
a single base pair can form with the 1-meC modification [Figure 6]. The bypass of C by translesion DNA
polymerases inserting A opposite to produce a C→T transition mutation is suggested by an in vitro steady
[49]
state kinetics study that showed a substrate preference for insertion of dATP, based on the k /K ratio .
cat
M
Persistent 1-methyladenine and 3-methylcytosine lesions and cell death
Currently, the consequences of the completion of replication with the base pair structures in Figure 6 is
unknown. On the basis of the link between MGMT and MMR, it is possible that mispairing occurs that
leads to cell death [Figure 2], but that is not reported. Moreover, understanding the links between the
MGMT and ALKBH pathways is important to the understanding of how tumor cells protect themselves
against alkylating agent chemotherapy.