Blockade of Base Excision Repair: Inhibition of Small Lesions Results in Big Consequences to Cancer Cells - DNA Repair in Cancer Therapy - page 34

TABLE 3.1 Mammalian DNA Glycosylases and Their Substrates

Glycosylase Enzyme Substrate Type Reference

UNGs UNG U Monofunctional 266

TDG U:G, T:G 267

SMUG1 U, OHmeU 268

HhH MBD4 U:G and T:G in CpG

sites

Monofunctional 269

OGG1 8-oxoG:C, faPyA,

faPyG

Bifunctional 270

MYH A:8oxoG Monofunctional 271

NTH1 TG, DHU, faPy Bifunctional 272

H2TH NEIL1 faPyA, faPyG, DHU,

TG, 8-oxoG

Bifunctional 273

NEIL2 5-OHU, DHU 55

NEIL3 Unknown Bifunctional? 57

AAG AAG 3-meA, 7-meG,

hypoxanthine

Monofunctional 59

emerging theme is a twist about the 3'-phosphate during
flipping of some bases, first noticed for the direct
damage repair protein AGT 37 but also present in some
other enzymes that flip the lesion site extrahelically.
Other protein:DNA complexes contain an extrahelical
nucleotide and rotated 3'-phosphate, including the
bacterial DNA glycosylases AlkA, 38 OGG1, 39 MUG, 40
UNG, 41 the endonucleases APE1, 42 and endonuclease
IV. 43 A third and as yet incompletely understood feature
of DNA glycosylases is their coordination with the
enzymes that follow them in the BER pathway. AP sites
or nicked DNA strands left unrepaired are more cyto-
toxic than base lesions, 13 therefore most glycosylases
remain bound to their product until the next enzyme
binds the substrate continuing the repair pathway.
Although the exact nature of this transfer is not yet clear,
it is likely that protein

(HhH) family, which contains such DNA glycosylases
as MBD4 (methyl-CpG binding domain 4), OGG1
(8-oxoguanine glycosylase), MYH (MutY glycosylase
homologue), and NTH1 (homologue of E. coli EndoIII);
the helix-2 turn-helix (H2TH) family, which is composed
by the three mammalian Nei-like proteins NEIL1, NEIL2,
NEIL3; and finally AAG (alkyladenine DNA glycosy-
lase), which at present

is unique in structure (see

Tabl e 3 .1 ). 7

Uracil-DNA N-Glycosylase
The UNGs subfamily combines a prominent and
highly important group of repair enzymes. Although
members of this subfamily share limited sequence simi-
larity, structural analysis highlighted the presence of
a common core fold. Moreover, all these enzymes vary
in substrate specificity and subcellular localization.
UNG, the first uracil DNA-glycosylase isolated from
E. coli 45 prevents mutagenesis by eliminating uracil
from DNA as a consequence of cytosine deamination
or misincorporation of dUTP residues during DNA
synthesis. TDG is responsible for the recognition of
G:U and G:T mismatches, and can also remove T when
present opposite to guanine. SMUG1 prefers single-
stranded DNA as substrate and has a broader specificity,
recognizing several oxidized pyrimidines 46 as well as
uracil. 47

DNA interaction surfaces play
a large role coupled to protein

e

protein interactions
and steric displacements. The basis for this handoff is
proposed to involve tight binding of DNA repair
enzymes to their products resulting in the formation of
stable enzyme:damaged DNA product complexes for
recognition by subsequent enzymes in the repair
pathway. 44
Although DNA glycosylases share very little
homology, they can be divided into four structural
subfamilies and within these subfamilies, into more
groups based on the substrate specificity and subcellular
localization: the UNG family, which includes UNG
(uracil DNA N-glycosylase), TDG (thymine-DNA glyco-
sylase) and SMUG1 (single-strand-selective monofunc-
tional uracil-DNA glycosylase); the helix

e

Helix e Hairpin e Helix
The helix helix (HhH) motif was first

discovered in NTH as a sequence-independent DNA
binding motif. The N-terminal domain typically has

hairpin

e e

e hairpin e helix