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Journal of Clinical Microbiology, October 2002, p. 3789-3792, Vol. 40, No. 10
0095-1137/02/$04.00+0 DOI: 10.1128/JCM.40.10.3789-3792.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Laboratory of Molecular Microbiology, Biomedical Research and Study Centre, University of Latvia,1 Latvian State Center for Tuberculosis and Lung Diseases, District of Riga,2 Latvian Academy of Medicine, Riga, Latvia3
Received 29 March 2002/ Returned for modification 21 June 2002/ Accepted 12 July 2002
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ACC) of the katG gene was found in 48 of 51 multidrug-resistant samples by sequencing. Furthermore, katG gene restriction fragment length polymorphism analysis with endonuclease AciI confirmed the nucleotide change in codon 315. |
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Thr) (7). In this study, we characterized the rpoB and katG gene mutations that are dominant in the Latvian isolates of multidrug-resistant (MDR) M. tuberculosis (16) by commercial Line Probe assay (LiPA), direct sequencing of PCR products, and single-strand conformation polymorphism (SSCP) and PCR-restriction fragment length polymorphism analyses. We suggest that this approach using molecular markers could be useful for predicting multidrug resistance in M. tuberculosis. Seventy patients from different regions of Latvia admitted to the State Centre for Tuberculosis and Lung Diseases between 1999 and 2000 with clinical symptoms of lung or renal tuberculosis or tuberculous meningitis were included in this study. Diagnosis was confirmed by microscopy and by culturing (18). Patients with a primary infection were preferably selected. They were typical of TB patients in Latvia in that they ranged in age from 25 to 60 years and in that two-thirds of them were male. Cultures of the M. tuberculosis complex were grown on Löwenstein-Jensen medium for 4 to 6 weeks. Drug susceptibility was determined by using the absolute concentration method on slants with the H37Rv strain of M. tuberculosis as the positive control and by using the BACTEC system (6, 13). With the absolute concentration method, resistance was defined as growth on solid media containing graded concentrations of drugs with more than 20 CFU at a specific drug concentration. The breakpoints for INH were 0.2 and 2.0 µg/ml on Löwenstein-Jensen medium and 0.1 µg/ml on the BACTEC system; for RIF, they were 40.0 µg/ml on Löwenstein-Jensen medium and 2.0 µg/ml on the BACTEC system. Of the 70 strains examined, 19 were drug susceptible and 51 were MDR, i.e., resistant to RIF and INH at least. Due to the small amount of DNA from some of the isolates, it was not possible to perform all of the analyses on all 70 isolates. Therefore, only 19 drug-susceptible and 34 MDR M. tuberculosis isolates were analyzed by LiPA. All 51 MDR isolates, however, were analyzed for the katG gene mutations by nucleotide sequencing. High-molecular-weight genomic DNA was isolated from the 70 mycobacterial cultures by the lysozyme/proteinase K cetyltrimethylammonium bromide procedure and precipitated with isopropanol (20). Purified DNA was dissolved in 20 to 50 µl of TE buffer (10 mM Tris HCl-1 mM EDTA [pH 8]). DNA isolated from M. tuberculosis MT14323 and Mycobacterium bovis BCG strains was used for controls.
A commercial INNO-LiPA Rif kit (Innogenetics NV, Ghent, Belgium) was used for the detection of rpoB gene mutations. First, a 256-bp fragment of the gene was amplified with biotinylated primers 71.B (5'-GGTCGGCATGTCGCGGATGG-3') and 72.B (5'-GCACGTCGCGGACCTCCAGC-3') flanking the 81-bp region of the rpoB gene (14). Ten to twenty nanograms of DNA diluted in TE buffer was added to the PCR mixture to a final volume of 50 µl [containing PCR buffer with (NH4)2SO4, 2.5 mM MgCl2, 200 µM concentrations of each dNTP, and 1 U of Taq DNA polymerase (Fermentas, Vilnius, Lithuania)]. Ten picomoles of each primer was used for one reaction mixture. The optimized cycling protocol was used in a Progene thermal cycler (Techne, Cambridge, England). PCR products were analyzed in 1.2% agarose gel. In each set of reactions, one negative control and one positive control (from a sensitive strain of M. bovis BCG) were included. Ten microliters of each PCR product was used for the LiPA hybridization, which was performed according to the manufacturer's instructions. Each of the LiPA strips used contained one probe specific for the M. tuberculosis complex, five partially overlapping wild-type probes, and four probes specific for the most common rpoB mutations within the 509- to 534-amino-acid region (D516V, H526Y, H526D, and S531L) (Innogenetics NV). Manual sequencing of the rpoB gene was performed by using a Cycle Reader DNA sequencing kit (Fermentas). Radioactively labeled primers used for manual sequencing were identical to those used for the PCR amplification.
SSCP analysis is based on the ability of single-stranded DNA to undergo a conformational change when a single nucleotide is altered. Such alterations or mutations can then be detected because of the change in DNA mobility in polyacrylamide gel. Here, the 256-bp rpoB fragment was used for SSCP analysis. For better strand separation, we used one biotinylated (reverse) primer, 72.B (17). The biotinylated DNA strand was separated from the unbiotinylated strand by Dynal streptavidin magnetic beads by using the single-stranded DNA purification kit (Labsystems, Helsinki, Finland). In brief, 80 µl of the beads, suspended in 40 µl of binding buffer, was mixed with an equal volume of the PCR product at room temperature for 15 min. After washing the beads with 80 µl of washing buffer, the bound DNA was denatured and incubated for 10 min at room temperature in 80 µl of 0.1 M NaOH. The alkaline solution containing the unbiotinylated strand was aspirated and used for SSCP analysis. The beads with the bound biotinylated strand were washed once with washing buffer and then suspended in 20 µl of concentrating solution. After being heated for 5 min at 96°C with an equal volume of loading buffer (0.05% bromophenol blue, 0.05% xylene cyanol, and 95% formamide), the biotinylated and unbiotinylated DNA samples were snap-cooled and immediately loaded onto the gel. The SSCP analysis of DNA fragments was performed in 6% polyacrylamide gel containing 5% glycerin at 35 mA and 500 V on a 17- by 28-cm gel plate cooled to 5°C (Bio-Rad, Hercules, Calif.). After 8 h of electrophoresis, the separated DNA bands were visualized by silver staining.
A 704-bp fragment of the katG gene (GenBank accession no. X68081) (23) was amplified by PCR using direct primer 74(5'-CGGGATCCGCTGGAGCAGATGGGC-3'), targetingthe 52 terminal nucleotides before codon 315, and reverse primer 75 (5'-CGGAATTCCAGGGTGCGAATGACCT-3'), positioned 158 nucleotides downstream from codon 463 (4). The 50 µl of the PCR mixture that was used contained PCR buffer with (NH4)2SO4, 2.5 mM MgCl2, 200 µM concentrations of each dNTP, and 1 U of Taq DNA polymerase (Fermentas). A 12.5-pmol portion of each primer was used for one reaction mixture. The standard cycling protocol was applied in a Progene thermal cycler (Techne). Amplification products were visualized in 1.5% agarose gel, purified, and used for sequencing and for digestion reactions.
Sequencing of the 704-bp katG fragment was performed with fluorescence-labeled dideoxynucleotide terminators by using an ABI PRISM Big Dye terminator cycle sequencing ready reaction kit (Applied Biosystems, Foster City, Calif.) and the same primers as those used for the PCR amplification. Nucleotide sequences were analyzed by an ABI PRISM 310 genetic analyzer (Applied Biosystems). PCR was performed on the relevant 704-bp katG fragment, followed by restriction with endonuclease AciI (New England BioLabs, Beverly, Mass.). One INH-sensitive M. tuberculosis sample, BCG, and the MT14323 strain were used as controls. AciI is a rare endonuclease that can recognize 5'-C'CGC-3' and 5'-GCG'G-3' sequences and can therefore be used for determining changes in codon 315. Four microliters of PCR product was added to the restriction mixture containing 1 µl of 10x restriction buffer 3 (New England BioLabs), 1 µl of AciI endonuclease (5 U/µl), and 4 µl of H2O. After 2 h of incubation at 37°C, the AciI digestion products were separated in 6% polyacrylamide gel and stained with ethidium bromide.
Nineteen drug-susceptible and 34 MDR DNA isolates were analyzed by the reverse hybridization-based LiPA to determine RIF susceptibility. The remaining 17 RIF-resistant samples from a total of 51 MDR patients were not analyzed by LiPA because of insufficient amounts of DNA.
The LiPA confirmed that the 19 drug-susceptible isolates were RIF susceptible and that the 34 MDR isolates were RIF resistant. The presence of M. tuberculosis complex was confirmed by identification of the oligonucleotide zone specific for this complex. The results obtained with LiPA correlated completely with the in vitro susceptibility results obtained from the culturing. One of the analyzed isolates, however, was a mixture of wild-type and mutant (H526Y) strains. Nine different LiPA patterns were found for the 34 RIF-resistant samples (Table 1). The most frequent rpoB gene mutations were S531L (found in 14 of the 34 isolates), D516V (7 of 34), and H526D (4 of 34). One isolate showed the H526Y mutation in rpoB. Mutations were clearly demonstrated in 27 of the 34 RIF-resistant samples by LiPA. The remaining seven RIF-resistant samples with unclear LiPA patterns were examined further by manual nucleotide sequencing (Table 1). The sequencing results showed the D516V plus P535S double mutation in four of these isolates and the Q510H plus H526Y double mutation in another isolate; these double mutations have not been described previously (11). Altogether, five double mutations of the rpoB gene were found in the 34 MDR isolates examined. Sequencing results for the remaining two (of the 34) samples with unclear but similar LiPA patterns (
S5, R4a, and R4b; faint zones) showed the L533P mutation.
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TABLE 1. Mutations in the rpoB gene of 34 RIF-resistant isolates
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TTG), D516V (GAC
GTC), and H526D (CAC
GAC), were analyzed. Double mutations were not included in this modeling. Computer modeling altered the folding of single-stranded DNA in the fragments with the D516V and H526D mutations but not in the fragment with the S531L mutation, where the TCG
TTG change hypothetically should not influence loop interaction (data not shown). The SSCP analysis showed a strand mobility difference between the RIF-susceptible and RIF-resistant samples, except for the five samples with the S531L mutation (Fig. 1). Since S531L is the most common rpoB mutation in our isolates, the employment of the SSCP method for rapid rpoB screening for RIF susceptibility cannot be recommended, at least in Latvia.
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FIG. 1. SSCP analysis of the 256-nucleotide rpoB gene fragments from RIF-resistant M. tuberculosis isolates. Lanes: 1, 3, 4, 5, and 7, rpoB gene fragments with the D516Y plus P535S (lane 1), S531L (lanes 3 and 7), D516V (lane 4), and H526D (lane 5) mutations; 2 and 6, fragments from the RIF-sensitive H37Rv strain of M. tuberculosis.
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View this table: [in a new window] |
TABLE 2. Mutations in codon 315 of the katG gene detected by nucleotide sequencing of 51 INH-resistant samples
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The high percentage of mutations at position 315 of the katG gene (48 of 51 isolates, or 91%) demonstrates the importance of this codon for the development of INH resistance in M. tuberculosis in Latvia. A high percentage of mutations at codon 315 has also been observed in Russia, where 22 of 24 INH-resistant isolates carried a mutation at codon 315 (Ser
Thr), 1 had a mutation at codon 88 (Gly
Arg), and 1 had a mutation at codon 155 (Tyr
Ser) (10). According to another study, the Arg
Leu mutation at position 463 in the katG gene was found in 30 to 40% of the INH-resistant isolates (15). Further studies of the mutations at codon 315 seem to hold more promise for understanding INH resistance, since the mutation at codon 463 was also found in M. bovis BCG, a strain that is inherently INH sensitive. One MDR isolate showed a lack of mutations in the katG gene fragment analyzed here. According to previous studies, there are more genes responsible for INH resistance, such as the inhA, ahpC, and kasA genes (8, 22) as well as the recently described ndh gene (9). Therefore, mutations responsible for INH resistance in this isolate may be present in any of these genes. The restriction enzyme AciI was used on the 704-kb PCR product and confirmed the high frequency of nucleotide change at codon 315 in the INH-resistant isolates. The AGC
ACC (Ser
Thr) nucleotide change is the mutation most often observed, but other substitutions at codon 315 [AGC
AAC (Ser
Asn), AGC
ATC (Ser
Ile), and AGC
CGC (Ser
Arg)] have been found in some studies (5). The AGC
CGC (Ser
Arg) substitution cannot be distinguished by AciI restriction analysis. Therefore, nucleotide sequencing of the 704-kb katG gene fragment seems to be a more reliable method for detecting the gene responsible for INH resistance. Taking into account the high prevalence of substitutions at codon 315, we propose the use of a shorter katG fragment for the detection of resistance to INH. The high prevalence of the S315 mutation in the katG gene in INH-resistant isolates is clearly associated with multidrug resistance, since all of the isolates with this mutation were also found to be MDR in in vitro susceptibility tests. These data are consistent with the results of a study from The Netherlands where the S315 mutation was found in more than 50% of the INH-resistant isolates (21).
We speculate that one of the reasons for the high level of drug resistance in M. tuberculosis isolates in Latvia may be the selection of strains that develop drug resistance more rapidly. This may explain the high percentage of certain katG gene mutations usually associated with a high level of multidrug resistance in our samples. More-detailed molecular studies with strictly defined patient groups may further elucidate this hypothesis. Molecular markers are a valuable tool for predicting drug resistance in epidemiological studies. At present, unfortunately, most molecular typing and mutation detection methods can only be applied to cultivated bacteria [about 105 copies of the gene(s) are needed to obtain good results in these assays]. Development of methods for uncultivated clinical material will accelerate their introduction in clinical practice throughout the world.
We thank Ilva Pole for the DNA isolation and Liana Pliss for technical assistance. We are also grateful to Petras Stakenas and Daiva Bakonyte (Institute of Biotechnology, Vilnius, Lithuania) for advice and to Eskild Peterson (Statens Serum Institut, Copenhagen, Denmark) and Vitauts Kalnins (University of Toronto, Toronto, Canada) for their help in preparing the manuscript.
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