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Antimicrobial Agents and Chemotherapy, January 2005, p. 144-147, Vol. 49, No. 1
0066-4804/05/$08.00+0 doi:10.1128/AAC.49.1.144-147.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Mycobacterial Reference Laboratory, Servicio de Bacteriología, Centro Nacional de Microbiología, Instituto de Salud Carlos III, Madrid, Spain
Received 4 February 2004/ Returned for modification 26 June 2004/ Accepted 11 September 2004
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ACC (serine to threonine) mutation in the katG gene and a 15 C-to-T substitution (inhAC15T) at the 5' end of a presumed ribosome binding site in the promoter of the mabA-inhA operon. These mutations have been reported in the majority of previous studies as the most frequent mutations involved in the resistance to isoniazid (INH) of Mycobacterium tuberculosis clinical strains with high levels of resistance. The method was optimized and validated after an analysis of 30 M. tuberculosis clinical isolates with known sequences of the relevant part of the katG gene and the regulatory region of the mabA-inhA operon. We analyzed 297 INH-resistant M. tuberculosis isolates collected in Spain from 1996 to 2003 by PCR-restriction fragment length polymorphism (using the katG gene), DNA sequencing, and the newly developed multiplex PCR. The results were concordant for all 297 isolates tested. The analysis revealed that 204 (68.7%) of the isolates carried one or both of the mutations. This finding suggests that with further development this multiplex PCR will be able to detect the majority of the INH-resistant M. tuberculosis clinical isolates from Spain and other countries where a high frequency of similar mutations occur. |
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Resistance to RIF has been shown to be caused by an alteration of the ß subunit of RNA polymerase, which is encoded by the rpoB gene. More than 95% of RIF-resistant strains are associated with mutations within an 81-bp region of the rpoB gene (encoding the RNA polymerase ß subunit). Specific mutations, insertions, and deletions have been described in several countries by several authors, and this 81-bp region has been termed the rifampin resistance determinant region (7, 14, 20, 26, 27, 30). Numerous methods exist to detect resistance to rifampin (10, 12, 18, 23).
In contrast, resistance to INH is more complicated, as mutations in several genes can lead to drug resistance. For most INH-resistant strains, mutations have been found in two genes, i.e., the katG gene, encoding catalase-peroxidase (31), and the mabA-inhA regulon (4), encoding a target of activated prodrug, enoyl-acyl carrier protein reductase (1-3, 11, 13, 15, 17, 21, 27). For some other INH-resistant strains, however, mutations in the ahpC promoter region (located in the 105-bp oxyR-ahpC intergenic region) or within the ß-ketocyl acyl carrier protein synthase gene kasA have also been reported (19, 25). Most studies have examined the mutations present in these genes by DNA sequencing or analyses of a portion of the katG gene after PCR amplification and digestion with the restriction enzyme MspI or SatI (2, 3, 17).
Molecular methods have been developed to detect resistance to INH and RIF as an alternative to conventional tests because of their ability to provide results rapidly. Upon the elucidation of the genes involved in resistance to RIF and INH, several studies describing various PCR-based molecular genetic techniques for the detection of resistance were published (12).
In the present study, we report a simple, rapid, and inexpensive assay based on allele-specific PCR methodology targeting an AGC
ACC mutation in the katG gene and an inhAC15T mutation in the regulatory region of the mabA-inhA operon to detect INH-resistant M. tuberculosis strains and to identify the M. tuberculosis complex in the same PCR tube for each sample.
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ACC mutation, 10 strains; inhAC15T mutation, 10 strains; and both the AGC
ACC and inhAC15T mutations, 4 strains. (ii) Set two. A total of 297 isolates collected from 1996 to 2003 in Spain and identified as INH-resistant M. tuberculosis strains by standard biochemical methods served to validate the developed multiplex PCR assay. Susceptibility testing with INH (0.2 and 1.0 µg/ml) was performed with Lowenstein-Jensen medium by the proportional method of Canetti et al. (6). A total of 50 M. tuberculosis strains that were susceptible to isoniazid were used as controls.
Extraction of DNAs for PCR. DNAs were extracted from cultures grown in Lowenstein-Jensen medium. A loop of culture was suspended in 500 µl of distilled water and boiled for 5 min, followed by centrifugation for 5 min at 12,000 x g, and 5 µl of the supernatant was used as a source of genomic DNA for amplification of the katG gene and the mabA-inhA operon.
PCR-RFLP. katG restriction fragment length polymorphism (RFLP) analysis to detect codon 315 mutations was performed by the use of previously described protocols and reaction components (23). A 620-bp region of the katG gene was amplified by use of the following primer set described by Uhl et al. (28): katG904, 5'-AGCTCGTATGGCACCGGAAC; and katG1523, 5'-TTGACCTCCCACCCGACTTG. After amplification, 10 µl of the product was digested with MspI (MBI Fermentas, Madrid, Spain) and analyzed by 2% MS8 agarose (8066; Pronadisa, Madrid, Spain) gel electrophoresis. Ten microliters of the same katG-specific PCR product was digested with SatI (MBI Fermentas) and analyzed in the same run with products generated by restriction with MspI (Fig. 1). SatI contains an overlapping GC sequence at a single recognition site of the triplet AGC at position 315, and any mutation at this position eliminates cleavage of the katG fragment.
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FIG. 1. Schematic presentation of multiplex allele-specific PCR to detect the AGC ACC mutation in codon 315 of the katG gene (A) and the 15 C-to-T substitution (inhAC15T) at the 5' end of a presumed ribosome binding site in the promoter of the mabA-inhA operon (B). Short arrows indicate the primers, and long double-headed arrows depict the resulting PCR fragments. The targeted sequences are shown in shaded boxes. The mutated bases are underlined and in bold, and the mutated bases are underlined.
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PCRs were performed with a pureTaq Ready-To-Go PCR bead kit (Amersham Biosciences, Piscataway, N.J.). PCRs were performed in a 9600 thermocycler (Perkin-Elmer) under the same PCR conditions for both katG and mabA-inhA (an initial denaturation step of 5 min at 94°C, followed by 30 cycles of 1 min at 94°C, 1 min at 65°C, and 90 s at 72°C and ending with a final elongation step of 10 min at 72°C). Efficient PCR amplifications were confirmed by gel electrophoresis in 0.7% MS8 agarose gels (8066; Pronadisa). PCR products were purified by use of a GFX PCR DNA and gel band purification kit (Amersham Biosciences) and were sequenced by use of a Big Dye Terminator cycle sequencing ready reaction kit (Applied Biosystems Inc., Foster City, Calif.) in an ABI Prism 377 automated DNA sequencer (Applied Biosystems). The following thermocycler parameters were used: denaturation at 94°C for 3 min, followed by 25 cycles of denaturation at 96°C for 10 s and primer annealing at 60°C for 4 min. The primer sequences are shown in Table 1.
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TABLE 1. Primer sequences
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Multiplex PCR.
Specially designed primers were used to detect the AGC
ACC and inhAC15T mutations. The reverse primer R315mut was positioned so that its 3'-OH end paired with two changed bases, i.e., a GC-to-CT substitution (Fig. 1A). Consequently, in the presence of the AGC
ACC mutation at this position in katG, a 296-bp fragment was amplified by the forward primer katg0F (described previously by Mokrousov et al. [22]). On the other hand, the reverse primer inhARmut was positioned so that its 3'-OH end paired with two changed bases, a 15 C-to-T substitution (inhAC15T) and a 14 G-to-A substitution at the 5' end of a presumed ribosome binding site in the promoter of the mabA-inhA operon (Fig. 1B). If the inhAC15T mutation occurred, a 146-bp fragment was amplified by the forward primer mabAF. In the absence of a mutation, the result would be two mismatches at the 3' end of the reverse primers (R315mut and inhARmut). Thus, under appropriate PCR conditions, resistant strains with these two mutations resulted in 296-bp and/or 146-bp PCR products. The MTUBf and MTUBr primers (previously reported by Kasai et al. [16]) produced a 1,020-bp fragment by amplification of a partial sequence of the gyrB gene. This fragment was the individual positive control and needed to be present in every sample. The absence of this amplification product would show that the PCR was inhibited and that the result should be ignored.
The PCR mix consisted of 10 mM Tris-HCl (pH 9), 50 mM KCl, 1.5 mM MgCl2, a 200 µM concentration of each deoxynucleoside triphosphate, 2.5 U of pureTaq DNA polymerase and reaction buffer, which were included in the pureTaq Ready-To-Go PCR bead kit (Amersham Biosciences), and PCR primers to a final volume of 25 µl. The concentrations of the primers were 200 mM for katG0F and 400 mM for R315mut (to detect the AGC
ACC mutation), 400 mM for both mabAF and inhARmut (to detect the inhAC15T mutation), and 400 mM for both MTUBf and MTUBr. All primer sequences are shown in Table 1.
For PCRs, a GenAmp PCR system 9600 thermocycler (Applied Biosystems) was used, beginning with a 5-min denaturation at 95°C and followed by 30 cycles of 1 min at 95°C, 1 min at 68°C, and 45 s at 72°C. The final cycle was followed by an extension time of 10 min at 72°C. Efficient PCR amplifications and the sizes of the products were estimated after electrophoresis in 2% agarose gels by staining with ethidium bromide.
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ACC mutation (S315T) at codon 315 of katG or the less frequent mutations in that codon. The AGC
ACC missense mutation was observed in 130 (43.8%) INH-resistant strains by PCR-RFLP with an MspI endonuclease treatment (Fig. 2, lanes 2 and 3), and 17 INH-resistant strains showed a different mutation in codon 315 by PCR-RFLP with a SatI endonuclease treatment (Fig. 2, lanes 10 and 11). The use of the MspI endonuclease only detected the AGC
ACC mutation, whereas SatI was able to detect any mutation in codon 315 that affected the GC nucleotides at that position. Of the 17 strains with different mutations, 12 (4.0%) showed an AGC
AAC mutation at codon 315 of katG and 5 (1.7%) showed an AGC
ACA mutation by automated DNA sequencing. The results are summarized in Table 2.
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FIG. 2. Profiles generated by PCR-restriction digestion with MspI and SatI endonucleases. Lanes 1 to 6, strains digested with MspI; lane 1, wild-type strain; lane 2, AGC ACC mutation (S315T); lane 3, S315T mutation and R463L polymorphism; lane 4, AGC AAC mutation at katG codon 315; lane 5, AGC ACA mutation at katG codon 315; lane 6, wild type; lanes 7 to 12, the same strains digested with the SatI endonuclease; M, 50-bp DNA ladder (Amersham Biosciences).
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TABLE 2. Comparison of INH susceptibility test results obtained by the different methods used in this study
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ACC mutation as detected by PCR-RFLP, and in none of the 50 INH-susceptible strains (Table 2).
The multiplex PCR that we developed was evaluated for its detection of the AGC
ACC mutation in codon 315 of the katG gene and the inhAC15T mutation in the promoter of the mabA-inhA operon in the same PCR tube for each sample. The 130 INH-resistant strains with the AGC
ACC mutation produced a 296-bp band (Fig. 3, lanes 1 to 4), while the 70 strains with the inhAC15T mutation produced a 146-bp band (Fig. 3, lanes 5 to 12). The four strains with both mutations produced two bands, one at 296 bp and the other at 146 bp (Fig. 3, lanes 13 and 14). All of the strains, including the susceptible strains (Fig. 2, lanes 16 to 18), produced a 1,020-bp band by amplification of a partial sequence of the gyrB gene. The multiplex PCR results were concordant with those generated by PCR-RFLP analysis and DNA sequencing for all of the strains tested (Table 2). The specificity of the multiplex PCR for the detection of INH resistance in resistant strains carrying the AGC
ACC mutation at codon 315 of katG and/or the inhAC15T mutation in our setting was 100%. The sensitivity of the multiplex PCR for the detection of INH resistance depended on the prevalence of these two mutations among the INH-resistant strains in each geographic area. In our setting, the sensitivity was 68.7%. The negative predictive value was 35%, and the positive predictive value was 100%. In Spain, the prevalence rates of the AGC
ACC allele at codon 315 of katG and of the inhAC15T mutation are 47.2 and 25%, respectively (unpublished data), whilst other mutations in codon 315 are found in 6.6% of INH-resistant strains. Consequently, 72.2% of the INH-resistant strains in our country could be detected with the multiplex PCR.
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FIG. 3. Profiles generated by multiplex PCR assay. Lanes 1 to 4, strains with the AGC ACC mutation at katG codon 315; lanes 5 to 13, strains with the 15C T substitution in the promoter of the mabA-inhA operon; lanes 14 to 15, strains with both mutations; lanes 16 to 18, INH-susceptible strains; M, 100-bp DNA ladder (Bio Tools).
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ACC and inhAC15T, respectively, may be responsible for 70% of INH-resistant M. tuberculosis cases. The prevalence of mutations in codon 315 of katG varies depending on the geographical region studied, with percentages ranging from 35% in Beirut (2) to 91% in Latvia and Russia (21, 27), while the prevalence of the inhAC15T mutation varied from 32.7% in a study by Bakonyte et al. (3) to 3.3% in a study by Van Rie et al. (29), with several studied strains being similar to those in Bakonyte et al.'s study. Torres et al. (26) reported that this mutation was found with a frequency of 4.3% in strains isolated from Seville, Spain, but the present study shows a much higher frequency of 23.6%. The frequency of other mutations in codon 315 (5.7%) was found to be lower than those in other studies (1, 11).
The high prevalence of the AGC
ACC mutation in the katG gene and of the inhAC15T mutation in the promoter of the mabA-inhA operon in the strains isolated from Spain suggests that the multiplex PCR technique can be used as a single, rapid tool for detecting INH resistance in M. tuberculosis clinical strains with a high probability. The assay is easy to perform and interpret and could be implemented as part of the routine practices of clinical laboratories in areas with a high prevalence of multidrug-resistant TB strains. For easier and unambiguous interpretations of multiplex PCR profiles, each run should include a wild-type strain (as a positive control for no amplification due to mutation) and one strain with a known mutation (as a positive control of amplification due to mutation). Furthermore, the procedure is inexpensive and requires only standard PCR and electrophoresis equipment. However, all negative results should be confirmed by conventional methods based on cell culture.
This work was supported by Instituto de Salud Carlos III (0017/99).
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