Biology Reference
In-Depth Information
shut off. In most tumors there is heterogeneity within
individual tumors in MGMT expression. In studies
evaluating MGMT expression in cohorts of human
tumors, high levels of expression have been noted in
colon cancer,
14,15
melanoma,
16,17
pancreatic carcinoma,
7
lung cancer,
18
and gliomas.
8,19,20
In many instances, the
MGMT activity in these tumors is higher than in their
normal tissue counterparts. For instance, in immunohis-
tochemical analysis of colon tumors and adjacent
mucosa, statistically higher levels of MGMTwere found
in tumors than in mucosa.
14
In this study, tumor grade
was not correlated with MGMT activity. In gliomas,
a wide range of MGMT activity was noted, with statisti-
cally higher activity in higher-grade tumors than in adja-
cent brain tissue collected at surgery.
8
In a number of
instances, the normal adjacent brain tissue did not
express MGMT, particularly in pediatric glioma
patients.
9
In our own studies of myeloma, CD38
þ
plasma cells had higher MGMT than normal CD34
þ
hematopoietic progenitors.
21
Numerous studies have
found a strong relationship between MGMT activity
and drug resistance in primary tumors and established
human tumor cell lines.
7,8,14
e
20
ME
CHANISMS OF MGMT REP
AIR
FIGURE 2.1
Sites of attack on guanine and the transition of mis-
matched base pair. (A) Letters in bold type mark the positions on
guanine that are frequently modified with adducts. (B) The mismatch
transition from G:C to O
6
-methylG:T.
O
6
-alkylguanine alkylation occurs when a reactive
alkylating agent transfers an alkyl group (typically
a small hydrocarbon side chain such as a methyl or ethyl
group, denoted as -CH
3
and -C
2
H
5
, respectively) to
a DNA base. The nitrogen atoms of the purine bases
(N
3
of adenine and N
7
of guanine) and the oxygen
atom of guanine (O
6
) are particularly susceptible to
alkylation in the form of methylation (
Figure 2.1
A).
Although alkylation of DNA can occur at various sites,
predominantly at N
7
G, the O
6
-alkylguanines, which
account for
The O
6
-methylguanine and O
6
-chloroethylguanine
lesions are induced by the methylating (temozolomide,
dacarbazine, and procarbazine) and chloroethylating
(BCNU and 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea)
agents.
28
e
31
The removal of O
6
-alkyl group from the
guanine results in the decrease in pharmacological effect
induced by these alkylating therapeutic agents. Thus,
MGMT is of particular interest in cancer research for its
role in cancer etiology and chemotherapy.
Although its physiological function in cells other than
removal of aberrant O
6
meG events remains unclear,
MGMT is well known as a DNA repair protein and its
mechanism of repair has been extensively studied.
32
The essential, conserved function of MGMT is the trans-
fer of alkyl groups from the O
6
-position of guanine to an
internal cysteine residue located at position 145 within
a highly conserved sequence of PCHR (Pro-Cys-His-
Arg). The crystal structure of MGMT shows that the
cysteine receptor site is buried in the protein and, in
order to react with a DNA substrate, a change in confor-
mation of either the substrate or the protein must
occur.
33,34
The most probable reaction involves the flip-
ping out of the substrate O
6
-alkylguanine out of the
10% of total alkylations bear strong
mutagenic and cytotoxic potential.
22
This is because
O
6
-alkylguanines exhibit distorted base pairing charac-
teristics in pairing with thymine, thereby, resulting in
G:C to A:T transitions upon DNA replication
22
(
Figure 2.1
B).
S
-adenosylmethionine (SAM), which is
required for normal cellular metabolism and is an
endogenous methyl donor, has been estimated to
generate 10
8
w
e
30 O
6
-methylguanine residues per day in
a mammalian cell.
23
SAM can methylate inappropriate
targets, such as adenine and guanine. Such endogenous
DNA-alkylation damage must be continually repaired
by MGMT; otherwise, mutation fixation can occur.
O
6
-alkylguanines caused mutation-related cancer
diseases have been well documented, including colon,
lung, breast, neck, and head.
24
e
27
O
6
-alkylguanines as critical lesions are also produced
by many clinically used anticancer alkylating agents.
e
