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Journal of Clinical Microbiology, March 2003, p. 993-997, Vol. 41, No. 3
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.3.993-997.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Rapid Detection of rpoB Gene Mutations in Rifampin-Resistant Mycobacterium tuberculosis Isolates in Shanghai by Using the Amplification Refractory Mutation System
Xiao-Yong Fan,1* Zhong-Yi Hu,2 Fan-Hong Xu,1 Zhi-Qiang Yan,3 Sheng-Qi Guo,1 and Zhong-Ming Li4
Molecular Biology Laboratory, Shanghai Institute of Biological Products,1
Tuberculosis Laboratory, Shanghai Lung Hospital,2
Shanghai Research Center of Biotechnology, Chinese Academy of Sciences, Shanghai, China ,3
Synergy America, Inc., Rockville, Maryland4
Received 8 August 2002/
Returned for modification 10 October 2002/
Accepted 12 December 2002

ABSTRACT
Rapid detection of drug resistance in
Mycobacterium tuberculosis is essential for efficient treatment and control of this pathogen.
The amplification refractory mutation system (ARMS) was used
to detect mutations in the rifampin resistance-determining region
of the
rpoB gene. A total of 39 rifampin-resistant
M. tuberculosis isolates in Shanghai were analyzed by this assay, resulting
in 92.3% sensitivity (36 of 39) and 87.2% concordance (34 of
39) relative to DNA sequencing, by which 41 mutations of 11
different types, including 9 point mutations and 2 deletions,
were identified in the
rpoB gene. The most frequent mutations
were those associated with codon 531 (21 of 39 [53.8%]) and
codon 526 (9 of 39 [23.1%]). The results suggest that the ARMS
assay is rapid and simple to implement and could be performed
for detection of rifampin resistance in
M. tuberculosis to complement
conventional culture-based methods.

INTRODUCTION
Tuberculosis (TB), though curable, still remains a major public
health concern worldwide. According to the report of the World
Health Organization, about one-third of the world's population
(1.86 billion people) are infected with
Mycobacterium tuberculosis and are at risk of having the infection develop into clinical
TB. Approximately 8 million new cases occur each year, resulting
in 3 million deaths around the world (
15,
19). It has been estimated
that among 6 million active TB patients at present, the disease
causes 250,000 deaths every year in China (
28). Furthermore,
control of TB has been further complicated by the emergence
of multidrug-resistant (MDR)
M. tuberculosis strains and the
human immunodeficiency virus epidemic (
4,
9).
Rifampin, introduced in 1971, has proven to be one of the most potent antituberculosis agents (2). Rifampin is an effective bactericidal against M. tuberculosis, interacting with DNA-dependent RNA polymerase to inhibit transcription and elongation of RNA (11, 12), and the use of this drug has greatly shortened the duration of chemotherapy. The molecular mechanism of rifampin resistance in M. tuberculosis was first characterized in 1993 (21). Ninety-six percent of rifampin-resistant (Rifr) M. tuberculosis strains possess genetic alterations within an 81-bp rifampin resistance-determining region (RRDR) in the rpoB gene (16, 18), corresponding to codons 507 to533 (Escherichia coli numbering system). In addition, rifampin resistance can be assumed to be a surrogate marker for MDR TB, since more than 90% of Rifr isolates are also isoniazid resistant (6).
Early diagnosis of TB and rapid detection of rifampin resistance are essential for efficient treatment and control of M. tuberculosis. However, culture-based methods for detection of M. tuberculosis infection and testing of drug susceptibility usually take more than 1 month. Although several different genotypic methods, such as PCR single-strand conformational length polymorphism (21, 22), dideoxy fingerprinting (7), heteroduplex analysis (26), and DNA sequencing (13), have been used for analysis of rpoB gene muations associated with rifampin resistance, these procedures are labor-intensive and time-consuming. In the present study, a simple, rapid, and reliable method, the amplification refractory mutation system (ARMS), was developed to detect mutations in the rpoB genes of M. tuberculosis isolates. ARMS is a general technique for the analysis of any point mutation or small deletion (8, 17) and has already been used for the detection of several genetic polymorphisms including
1-antitrypsin deficiency (17), CFTR gene mutation (8), apolipoprotein E genotypes (5), and K-ras mutation (3).
The objectives of this study were to evaluate an assay for identification of rifampin resistance based on ARMS PCR amplification of the rpoB gene and to identify rpoB mutations associated with rifampin resistance in a panel of M. tuberculosis strains isolated in Shanghai.

MATERIALS AND METHODS
M. tuberculosis strains and drug susceptibility testing.
All clinical isolates originated from the Shanghai Lung Hospital
and were grown on Löwenstein-Jensen medium. Rifampin resistance
testing was performed by the absolute concentration method described
by Kim and Hong (
14).
Primer design and ARMS PCR protocol.
The rationale of ARMS PCR is that a single nucleotide mismatch at the 3'-OH extremity of the annealed forward primer renders Taq DNA polymerase unable to extend the primer in the PCR under appropriate conditions (8, 17). Thus, the absence of the specific PCR product, with a positive result for the internal control, reveals a deviation from the wild-type DNA sequence. An additional deliberate mismatch adjacent to the 3'-OH terminus of the ARMS primer was introduced in order to enhance discrimination between normal and mutant alleles. Sequences of the ARMS primers for mutation detection are shown in Fig. 1. Other primers used are as follows: control forward primer, 5'-CGAATATCTGGTCCGCTTGC-3' (positions 2090 to 2109; GenBank accession no. L27989); common reverse primer, 5'-GTCGACCACCTTGCGGTACG-3' (positions 2627 to 2608). In each PCR, one ARMS primer and the common reverse primer were used for mutation detection, which generated a short PCR product from the wild-type gene but failed to amplify from a mutant allele with a corresponding mutation at the location covered by the mismatch positions on the ARMS primer. A control forward primer that is expected to anneal efficiently to all alleles was used in conjunction with the common reverse primer to generate a longer PCR product as an internal control.
PCR amplification.
Bacterial suspensions containing approximately 10
5 bacteria
in 100 µl of distilled water were prepared from
M. tuberculosis isolates grown on Löwenstein-Jensen slants for 3 to 4 weeks
and then treated with 100 µl of a 10% suspension of Chelex
100 as described previously (
23). For each PCR, 5 µl of
a supernatant containing genomic DNA as a template was added
to a final volume of 50 µl containing 0.10 µM control
forward primer, 0.15 µM ARMS primer, 0.20 µM common
reverse primer, 50 mM KCl, 10 mM Tris-HCl (pH 8.3), 1.2 mM MgCl
2,
200 µM deoxynucleoside triphosphates, and 2.5 U of
Taq DNA polymerase (MBI Fermentas, Vilnius, Lithuania). The reaction
was carried out in a DNA Thermolyne (Gene-Cycler; Bio-Rad, Richmond,
Calif.) with the following program: initial denaturation at
94°C for 5 min, followed by 35 cycles of denaturation at
94°C for 35 s, annealing at 56°C for 35 s, and extension
at 72°C for 35 s, with an additional extension step at 72°C
for 10 min. PCR products were analyzed on a 1.5% agarose gel
in 1
x TAE buffer (40 mM Tris-acetate, 1 mM EDTA [pH 8.0]) stained
with ethidium bromide, and visualized under UV light. Each isolate
was employed in three single PCRs with three different primer
sets; then the PCR products were analyzed by agarose gel electrophoresis
at the same time. All amplifications were repeated at least
twice.
DNA sequencing of the rpoB gene.
The RRDR of the rpoB gene was sequenced after PCR amplification in order to analyze the mutations associated with rifampin resistance. PCR was performed by using another forward primer (5'-TGGTCGCCGCGATCAAG-3') and the common reverse primer (5'-GTCGACCACCTTGCGGTACG-3') to generate a 296-bp fragment from nucleotide 2332 to 2627 (GenBank accession no. L27989); then the amplification products were directly sequenced on an ABI Prism model 3700 DNA sequencer (Applied Biosystems, Foster City, Calif.) by Shanghai GeneCore Biotechnologies Co., Ltd.
Nucleotide sequence accession numbers.
The new alleles found in this study have been deposited in GenBank under accession no. AF532616 and AF532617.

RESULTS
Evaluation of the ARMS assay for analysis of rpoB gene mutations.
Each ARMS primer is complementary to the corresponding sequence
of the wild-type gene except for one additional deliberate mismatch
at the fourth nucleotide from the 3'-OH terminus of the primer.
However, there are two mismatched nucleotides at the 3' end
between the ARMS primer and the mutant allele. A single mismatch
at the fourth nucleotide from the 3' terminus of the ARMS primer
has little influence on the yield of PCR products, whereas the
mismatch at the 3'-OH extremity of the primer is refractory
to extension by the
Taq DNA polymerase so that the yield of
product from the mutant allele is not detectable (
8,
17). ARMS
PCR products of some strains with typical mutations in the RRDR
of the
rpoB gene are shown in Fig.
2.
A total of 45 clinical isolates were analyzed by the ARMS assay;
the results indicated that all 6 Rif
s strains including H
37Rv
showed no changes in the
rpoB gene. Among 39 Rif
r isolates,
36 were found to have mutations in the
rpoB gene: 21 had mutations
at codon 531, 9 had mutations at codon 526, and 6 had mutations
at codon 516. No mutation was detected in the other three Rif
r strains (Table
1).
View this table:
[in this window]
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TABLE 1. Rapid detection of rpoB gene mutations of M. tuberculosis isolates in Shanghai by ARMS compared with DNA sequencing resultsa
|
Mutations in the rpoB gene of Rifr M. tuberculosis isolates in Shanghai.
DNA sequencing analysis of the 39 Rif
r isolates showed that
33 strains have a single mutation, 3 strains have double mutations,
and 2 strains have gene deletions in the 81-bp RRDR of the
rpoB gene. One strain was identified as having no mutation, although
it tested repeatedly as a Rif
r isolate (Table
1). A total of
41 mutations of 11 different types, including 9 point mutations
and 2 deletions, were identified (Table
3).

DISCUSSION
It has been known that ARMS is a simple and rapid method used
to detect gene mutations in many research fields. The mechanism
of this system is that oligonucleotides which are complementary
to a given DNA sequence except for a mismatch at their 3'-OH
residue will not function as primers in PCR under appropriate
conditions because of the absence of 3'-exonucleolytic proofreading
activity associated with
Taq DNA polymerase (
8,
17).
In this study, an ARMS assay for detection of rifampin resistance in M. tuberculosis was successfully developed to analyze 39 Rifr isolates in Shanghai. Thirty-six out of 39 (92.3%) Rifr strains were detected as having genetic alternations in one of the three most common codons: codon 531, 526, or 516. Furthermore, the ARMS protocol in this study differs from those in other reports (5, 8, 17). The ARMS assays reported previously were designed to amplify the mutated gene regions and to detect specific nucleotide changes at the corresponding particular positions, but the ARMS assay as described here was directed against the wild-type gene rather than a mutant allele. Targeting the wild-type sequence is a more comprehensive tactic than targeting a particular mutation. For example, three different kinds of mutations at codon 526 were detected in this study. In contrast, an ARMS assay with a primer targeting one certain codon 526 mutation could analyze only the corresponding allele (data not shown). A second, 537-bp internal control fragment was generated in all reactions in order to avoid false-negative results. A further deliberate mismatch close to the 3'-OH end of the ARMS primer was employed to enhance specificity (5, 8, 17).
The RRDR of the rpoB gene was sequenced after PCR amplification to analyze the mutations associated with rifampin resistance and to verify the detection results of the ARMS assay. The most frequent mutations were found at codons 531, 526, and 516, with frequencies of 53.8, 23.1, and 10.3%, respectively. Similar results have been reported in other papers (10, 20, 24, 27). Among all mutations, S531L (TCG
TTG) occurred at the highest frequency, 51.2%. Three different types of mutations (H526Y, H526D, and H526N) were seen at codon 526 (Table 3). Though the D516V mutation (GAC
GTC) occurred at frequencies of 37.9% in Hungarian isolates (1) and 13.3% in Asian isolates (10), a very low frequency of this mutation, only 2.4%, was found in this investigation. However, D516G (GAC
GGC) combined with the L511P change (CTG
CCG) occurred at a relatively high frequency (7.3%). It is noteworthy that all the mutations mentioned above were identified by the ARMS assay, resulting in an 87.2% concordance between the ARMS results and the results of phenotypic rifampin susceptibility testing and genetic DNA sequencing (Table 2). Two isolates with deletions at codons 514 and 515 or codons 516 to 520, and three isolates with double mutations (D516V and L511P), were also detected by the ARMS-516 primer, resulting in a 92.3% sensitivity. Three Rifr isolates which were demonstrated repeatedly to be rifampin resistant by conventional susceptibility testing gave false wild-type results by the ARMS assay due to limitations of the methodology: either their mutations are outside the detection range (Q513L and L533P) or no mutation exists in the RRDR of the rpoB gene even though the isolate (strain FK007) is resistant to rifampin. Similar observations have also been reported by others (13, 20-22, 24, 27), which suggests that mutations beyond the 81-bp region of the rpoB gene or the existence of at least one additional molecular mechanism may be involved in the rifampin resistance of M. tuberculosis.
The ARMS assay described here may be used for rapid detection
of the mutations in the
rpoB gene associated with the rifampin
resistance of
M. tuberculosis, although the method has limitations
as well. For example, (i) this assay can detect only the existence
of mutations, not their nature, so it cannot be a substitute
for DNA sequencing analysis, and (ii) it is impossible to detect
100% of
rpoB gene mutations, since more than 40 types of mutations
covering about 20 codons are involved in the RRDR of
M. tuberculosis (
16,
18). However, the sensitivity of this assay is 92.3%, which
is relatively high and roughly equivalent to those of the line
probe assay (89.7%) (
1) and the mismatch RNA/RNA assay (93.8%)
(
25). Furthermore, compared with those methods, the ARMS assay
is more convenient, less expensive, and easier to perform, since
it utilizes only commonly available reagents and equipment.
The entire procedure, including genome DNA extraction, ARMS
PCR amplification, and agarose gel electrophoresis, can be finished
within 1 day. In addition, if a biotin-labeled common probe
and enzyme immunosorbent assay are introduced to analyze the
ARMS PCR products, a large number of clinical samples will be
detected at the same time. Therefore, the application of such
a rapid and simple method for detection of the rifampin resistance
of
M. tuberculosis would be potentially valuable for efficient
treatment and control of
M. tuberculosis.

FOOTNOTES
* Corresponding author. Mailing address: Molecular Biology Laboratory, Shanghai Institute of Biological Products, 1262, Yan'an Rd. (W), Shanghai, 200052, China. Phone: 86-21-62803189-62. Fax: 86-21-54191054. E-mail:
xyfan008{at}hotmail.com.


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Journal of Clinical Microbiology, March 2003, p. 993-997, Vol. 41, No. 3
0095-1137/03/$08.00+0 DOI: 10.1128/JCM.41.3.993-997.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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