Compensatory mutations have been suggested to promote multidrug-resistant tuberculosis (MDR-TB) transmission, but their role in facilitating the recent transmission of MDR-TB is unclear. To investigate the epidemiological signiﬁcance of compensatory mutations, we analyzed a four-year population-based collection of MDR-TB strains from Shanghai (the most populous city in China) and 1346 published global MDR-TB strains. We report that MDR-TB strains with compensatory mutations in the rpoA, rpoB,or rpoC genes were neither more frequently clustered nor found in larger clusters than those without compensatory mutations. Our results suggest that compensatory mutations are not a major contributor to the current epidemic of MDR-TB. Introduction outbreaks in HIV-negative patients were regarded as It had been thought that drug-resistance mutations successful examples of compensatory evolution . would introduce a ﬁtness cost into resistant Mycobacter- Recent studies have investigated the role of compensa- ium tuberculosis (M. tb) tory mutations in MDR-TB transmission by comparing , and the resulting reductions in virulence and transmissibility would prevent multidrug- the presence of compensatory mutations in clustered or 8,9 resistant tuberculosis (MDR-TB) strains from dis- nonclustered strains and found that putative compen- 2,3 seminating widely . Early mathematical models pre- satory mutations in rpoC and rpoA were more common in dicted that MDR-TB should remain a local problem , but VNTR-clustered strains. However, these studies did not the steady growth of the MDR-TB epidemic worldwide determine whether the putative compensatory mutations has contradicted these early expectations. A subsequent accumulated before or after the MDR-TB strains were study suggested that even if the average ﬁtness of MDR- transmitted. If these compensatory mutations accumu- TB strains is low, a small proportion of resistant strains lated after transmission, they should not be considered as that are relatively more ﬁt will outcompete the less ﬁt and factors that promote or facilitate transmission. Moreover, drug-susceptible strains . These more ﬁt, resistant strains the sampling methods used in these studies were not were thought to contain compensatory mutations that population based, and the IS6110/MIRU-VNTR deﬁned would restore ﬁtness and thus constitute an important clusters could have perhaps been further separated by 5,6 factor in the spread of MDR-TB strains , and MDR-TB whole-genome sequencing (WGS). To avoid these pitfalls, we examined the role of compensatory mutations in MDR-TB transmission by using WGS to determine Correspondence: Qian Gao (firstname.lastname@example.org) transmission clusters in the MDR-TB strains collected in Shenzhen Center for Chronic Disease Control, Shenzhen, China Key Laboratory of Characteristic Infectious Disease & Bio-safety Development Shanghai over a 4-year period (2009–2012). Our ﬁndings of Guizhou Province Education Department, Zunyi Medical University, Zunyi, contradict the previous inferences and do not support a Guizhou, China Full list of author information is available at the end of the article. These authors contributed equally: Qingyun Liu, Tianyu Zuo. © The Author(s) 2018 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to theCreativeCommons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/. 1234567890():,; 1234567890():,; Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 2 of 8 signiﬁcant role of compensatory mutations in promoting (compensated)-type clusters, in which all strains harbor the ongoing MDR-TB epidemic. the same compensatory mutation—indicating transmis- sion of a compensated MDR-TB strain (compensatory Results mutations occurring before transmission); (2) N (non- Collection of MDR-TB isolates compensated)-type clusters, containing only strains with From 2009 to 2012, a total of 324 MDR-TB isolates no putative compensatory mutation—indicating trans- mission of a noncompensated MDR-TB strain; (3) M were collected from 31 tuberculosis (TB) hospitals in Shanghai, China. All of these isolates were MIRU-VNTR (mixed)-type clusters, in which MDR-TB strains harbored genotyped, and 122 clustered MDR-TB strains had different compensatory mutations or only a proportion of undergone WGS previously . Here, to determine the the strains in the cluster contained compensatory muta- presence of compensatory mutations in VNTR non- tions, indicating that the compensatory mutations had clustered MDR-TB strains, we randomly selected 105 occurred after transmission (see more details about isolates from the remaining 202 VNTR nonclustered M-type clusters in the discussion). The groups so deﬁned MDR-TB isolates to sequence the full-length genes rpoA, are theoretically similar to the three prototypical types of rpoB, and rpoC. The combined results of the 227 clusters used to evaluate the role of drug resistance in Shanghai MDR-TB strains were termed the “Shanghai transmission . In the Shanghai dataset, 12 transmission dataset”. Meanwhile, we obtained WGS records of 8331 clusters were determined as C-type, 18 as N-type, and six M. tb isolates of global origin from the European as M-type (Fig. 2a); in the Global dataset, 26 transmission Nucleotide Archive (ENA) (Supplementary Table 1), and clusters were determined as C-type, 84 as N-type, and 1346 of these strains were identiﬁed as MDR-TB (Sup- seven as M-type. plementary Table 3). Of these MDR-TB strains, 602 were collected through retrospective cohorts or a population- Compensated MDR-TB strains did not cause larger based approach (Supplementary Table 1) and thus were transmission clusters appropriate for the subsequent analysis. The data from If the putative compensatory mutations stimulated the the global MDR-TB isolates were termed the “Global transmission of MDR-TB, then C-type clusters would be dataset”. expected to be larger than N-type clusters, but in the Shanghai strains, the C-type clusters were not larger than Identiﬁcation of putative compensatory mutations and either the N-type or the M-type clusters (Wilcoxon rank- sum test, P = 0.4053 and 0.9587, respectively) (Fig. 2b). transmission clusters Through phylogenetic reconstruction of the MDR-TB An analysis of the size of the clusters in the global MDR- strains obtained above, we found 60 nonsynonymous TB strains yielded similar results (Wilcoxon rank-sum mutations in the rpoA, rpoB,or rpoC genes that had arisen test, P = 0.3816 and 0.2277, respectively) (Fig. 2c). Nota- at least twice in parallel and were identiﬁed as putative bly, the largest clusters observed in both the Shanghai and compensatory mutations (Fig. 1, Supplementary Table 4). Global datasets (with seven and 23 linked cases, respec- Of these mutations, six were in rpoA,16in rpoB, and 38 in tively) were caused by MDR-TB strains without com- rpoC. We set “mutation parallelism” as a criterion to pensatory mutations (Fig. 2b, c). Moreover, we did not exclude lineage mutations or neutral polymorphisms that ﬁnd any particular compensatory mutation that was were ﬁxed due to genetic drift. Although this ﬁlter might associated with larger transmission clusters in either also exclude some uncommon compensatory mutations, dataset. These results suggest that compensated MDR-TB it increased the level of conﬁdence that the mutations strains are not prone to generate more secondary cases identiﬁed were truly compensatory mutations. We inclu- than noncompensated MDR-TB strains do. ded rpoB in the search for putative compensatory muta- tions because additional mutations in rpoB have been Compensated MDR-TB strains were not more likely to be reported to both restore ﬁtness and increase the level of clustered rifampicin resistance . MDR-TB strains with less than 12 Previous studies suggested that MDR-TB strains with Single-nucleotide polymorphisms (SNPs) difference were compensatory mutations were more frequently clus- 8,9 considered to constitute a transmission cluster. A total of tered . However, these studies treated “compensatory 36 such clusters were found in the Shanghai dataset, and mutations” as a static feature and ignored the possibility 117 clusters were identiﬁed in the Global dataset. that compensatory mutations could occur in secondary cases that were initially infected with noncompensated Three types of transmission clusters MDR-TB strains. This is probably what has occurred in To distinguish whether the compensatory mutations the had occurred before or after transmission, we divided the M-type clusters (Fig. 2a), and therefore, it should MDR-TB clusters into three types (Fig. 2a): (1) C not be considered as representing the transmission of Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 3 of 8 Fig. 1 Putative compensatory mutations in the rpoA, rpoB, and rpoC genes identiﬁed in this study. Each putative compensatory mutation was supported by at least two independent evolution events Fig. 2 Compensated MDR-TB strains were not associated with larger transmission clusters. a A maximum likelihood phylogenetic tree showing genomic clusters in the Shanghai dataset. The strain identiﬁers were “year, strain number, and compensatory mutation type”. The three genomic cluster types are illustrated with different colors, as indicated. The isolate names in gray represent the VNTR-clustered strains that were separated by WGS. Comparison of cluster sizes in C-type, M-type, and N-type clusters in the Shanghai MDR-TB dataset (b) and the Global MDR-TB dataset (c); each dot represents a genomic cluster identiﬁed by WGS Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 4 of 8 Table 1 Ratios of compensated strains in clustered and nonclustered MDR-TB groups a a 2 Groups Total With CMs Without CMs χ P value Clustered MDR-TB (%) 78 32 (41.0%) 46 (59.0%) 2.260 0.133 Nonclustered MDR-TB (%) 133 41 (30.8%) 92 (69.2%) CMs compensatory mutations Excluded M-type clusters Fig. 3 Recently formed MDR-TB strains would decrease the ratio of compensatory mutations in the nonclustered MDR-TB group. a A theoretical scheme shows that recently formed MDR-TB strains would be included, while the secondary clustered strains resulting from transmission would occur outside of the study’s observation period. Thus, newly formed MDR-TB strains would be assigned to the nonclustered group. This schematic diagram also shows that transmission that occurred within the ﬁrst 2 years will be captured in our study, while the secondary cases resulting from transmission that occurred during the last 2 years could be beyond the study’s observation period. b Comparison of the number of drugs to which strains are resistant in clustered and nonclustered MDR-TB strains. c Comparison of the number of drug-resistance mutations in clustered and nonclustered MDR-TB strains. d Comparison of collection time distribution between clustered and nonclustered MDR-TB groups; the collection time of each isolate was counted as “days to the end of the study duration (31 December 2012)” compensated MDR-TB strains. Accordingly, we excluded mutations that occurred after transmission result in a M-type clusters from an analysis of the ratios of com- higher ratio of compensatory mutations in clustered than pensatory mutations in the clustered and nonclustered in nonclustered MDR-TB strains. groups, and compared only C-type and N-type clusters. Surprisingly, no signiﬁcant difference was observed in the The dilution effect of recently formed MDR-TB strains ratios of compensatory mutations in clustered compared Even when the M-type clusters were excluded, the to nonclustered groups, indicating that compensated clustered group had a nonsigniﬁcantly higher ratio of MDR-TB strains were not more frequently clustered compensatory mutations than that of the nonclustered (Table 1). group (41.0% versus 30.8%). One explanation might be To allow a comparison with previous studies, we repe- that new MDR-TB strains are continuously emerging ated the analysis without excluding the M-type clusters. during antibiotic treatment, and these new MDR-TB When the M-type clusters were included, the ratio of strains might not have had enough time to accumulate compensatory mutations in the clustered group was sig- compensatory mutations. For the same reason, within the niﬁcantly higher than that in the nonclustered group 4-year duration of the study, they might not have had (Supplementary Table 5), similar to the results of previous enough time to cause secondary TB cases (Fig. 3a). Thus, studies. In our M clusters, 62.5% (10/16) of MDR-TB these recently formed “young” MDR-TB strains would be strains harbored compensatory mutations, which was assigned to the nonclustered group and thereby decrease much higher than that of either clustered (41.0%) or the ratio of compensatory mutations in this group. In nonclustered (30.8%) groups. M-type clusters are those in contrast, all strains in MDR-TB clusters are relatively which compensatory mutations are present in only some “older” (had become MDR-TB prior to their transmis- of the strains in a cluster, presumably because the sion), and thus would have had a longer period to accu- mutations have occurred during the transmission of the mulate additional drug resistance and compensatory clustered strain. Therefore, when the M-type clusters are mutations than strains in the nonclustered group. If this is included with the clustered MDR-TB strains, the true, then clustered MDR-TB strains should have wider Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 5 of 8 drug-resistance spectra (number of drugs to which they We believe in the accuracy of the results of our study are resistant) than those of nonclustered MDR-TB strains. because it incorporated several methodological improve- To test this inference, we compared the drug-resistance ments. First, we used WGS to identify transmission spectra of the clustered versus nonclustered MDR-TB clusters, which is more precise than IS6110 or MIRU- strains in the Global dataset. This analysis demonstrated VNTR . In our data, a total of 28 VNTR-clustered strains that the median number of antibiotics to which (23% of the VNTR-clustered strains) were further sepa- nonclustered MDR-TB strains were resistant was 4.22 rated by WGS. Second, we set strict criteria to identify (4.10 ~ 4.34), while for clustered MDR-TB strains it was putative compensatory mutations and excluded phyloge- 5.13 (5.05 ~ 5.21) (Student’s t-test, P < 0.0001, Fig. 3b). In netic/lineage mutations or neutral polymorphisms. Third, addition, as a single M. tb strain can accumulate our analysis discriminated between the accumulation of multiple mutations that confer resistance to a single compensatory mutations before or after the transmission 12–14 antibiotic (evolution of high-level resistance) ,we of MDR-TB strains (i.e., C- versus M-type clusters) in the further compared the numbers of drug-resistance- analysis of cluster size and the ratio of compensatory conferring mutations in these two groups. Consistent mutations. with our prediction, the clustered MDR-TB strains had, The effect of drug resistance on transmission has been 4,17 on average, more drug-resistance mutations than the long debated , and different studies have reported nonclustered MDR-TB strains did (Student’s t-test, heterogeneous results suggesting that MDR-TB strains P < 0.0001, Fig. 3c). can be ten times more or ten times less transmissible than 11,18 Finally, to verify whether nonclustered MDR-TB strains drug-susceptible strains . However, the reasons tended to occur toward the end of the study period, i.e., underlying these variations could simply reﬂect the dif- were indeed younger, we compared the distributions of ferences in study settings and methodologies 4,18 the collection times of the initial cultured clinical speci- employed; the drug-resistant strains reported to have mens (counted as days to the end of the study duration) reduced transmissibility were found in settings with for the isolates in the clustered versus nonclustered effective TB control , while strains with increased groups. We found that the isolates in the nonclustered transmissibility were reported from regions with high TB 4,18 group had a shorter average time since collection than the burdens . Thus, it seems more likely that the hetero- isolates in the clustered group did (mean values: 762.08 geneous and discordant published results serve to versus 903.14, P < 0.0001, Wilcoxon rank-sum test, demonstrate that TB transmission is a process that is Fig. 3d), which suggests that the strains in the non- primarily inﬂuenced by environmental factors such as TB clustered group tended to be isolated toward the end of control policy, time with the illness before diagnosis, the study period. These analyses support our inference treatment efﬁcacy, and general quality of the health-care that clustered MDR-TB strains differ from nonclustered system . MDR-TB strains, suggesting a dilution effect from We consider that there are at least four factors that can recently formed MDR-TB strains that could in turn pro- obscure the function of compensatory mutations in MDR- vide an explanation for the relatively lower ratio of TB transmission. First, the efﬁcacy of the TB control compensatory mutations in the nonclustered MDR-TB program. In countries or regions with high MDR-TB group. prevalence and poor management of MDR-TB patients, the transmission of MDR-TB strains is highly likely, even Discussion in the absence of compensatory mutations. In such cir- Our ﬁndings suggest that, in contrast to published cumstances, outbreaks and epidemics of MDR-TB are 5,15 20,21 reports , MDR-TB strains with compensatory muta- mainly driven by environmental factors . Second, tions are not more frequently found in clusters, nor are MDR-TB patients are, on average, infectious for longer they more likely to belong to larger transmission clusters periods due to the long course of treatment and high 22,23 than the MDR-TB strains without these mutations do. treatment failure rates . Thus, there might be more Previous studies have made direct comparisons of the chances for MDR-TB patients to generate secondary number of strains with putative compensatory mutations cases, which could conceivably compensate for a modest in clustered versus nonclustered MDR-TB strains and decrease in ﬁtness. Third, the most common drug- reported an enrichment of compensatory mutations in the resistance mutations carried by clinical isolates are asso- 8,9 clustered group . However, we observed that M-type ciated with the lowest ﬁtness cost in vitro, and such slight clusters had an impact on this comparison because decreases in ﬁtness may not be sufﬁcient to affect trans- 1,6 inclusion or exclusion of these clusters would dramati- mission . Moreover, positive epistatic effects between cally alter the results. Hence, we infer that the previous different drug-resistance mutations could further ame- observations would probably change if they excluded liorate the ﬁtness cost imposed by individual drug- 17,24 M-type clusters. resistance mutations . Fourth, it is still possible that Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 6 of 8 the ﬁtness cost measured in vitro might not reduce the Analysis of WGS data ability of M. tb to transmit and establish in a new host We used a validated pipeline for the mapping of 25,26 10,29 in vivo . This idea was supported by a previous study sequencing reads to the reference genome . In brief, using tuberculin skin testing to trace infection that found the Sickle tool was used for trimming WGS data. an equal prevalence of infection among contacts exposed Sequencing reads with Phred base quality above 20 and to patients harboring both drug-resistant and drug- reads length longer than 30 were retained for analysis. susceptible strains . The whole-genome sequence of the M. tb H37Rv strain The objective of this study was to investigate the (NC_000962.2) was used as the reference template for effects of compensatory mutations on MDR-TB trans- mapping reads. Sequencing reads were mapped to the missions that occurred within three or fewer years (recent reference genome using Bowtie2 (v2.2.9) . SAMtools 28 32 transmission) , and the 4-year observation window (v1.3.1) was used for SNP calling with mapping quality should have captured most of the transmissions that greater than 30. Fixed mutations (frequency ≥75%) were occurred within the ﬁrst 2 years of the study. For identiﬁed using VarScan (v2.3.9) with at least ten reads transmissions that occurred during the last 2 years supporting and strand bias ﬁlter option on. We excluded of the study, the secondary cases might have developed all SNPs located in noise or repetitive regions of the TB disease after our observation window. (Fig. 3a). genome (e.g., PPE/PE-PGRS family genes, phage sequen- However, we believe that the transmission pattern ces, insertions or mobile genetic elements). in the last 2 years should not substantially differ from that in the ﬁrst 2 years, and therefore, further extending the Identiﬁcation of putative compensatory mutations time frame of our study would not change its major The maximum likelihood method was used to recon- conclusion. struct the phylogenetic trees with 500 bootstrap repeats In conclusion, we did not observe a promoting inﬂuence using MEGA (v6.06) , and the phylogenetic tree was of compensatory mutations on the transmission of MDR- further visualized by FigTree (http://tree.bio.ed.ac.uk/ TB, and we suggest that the putative contribution of software/ﬁgtree/). Putative compensatory mutations in compensatory mutations may be overwhelmed by the the rpoA, rpoB,or rpoC genes were identiﬁed through the complex and more powerful effects of environmental following criteria: (1) nonsynonymous mutations in the factors. rpoA, rpoB or rpoC genes that were present in only rifampicin-resistant strains; (2) each putative compensa- tory mutation must have arisen at least twice indepen- Methods MDR-TB datasets dently (parallel selection). We wrote a Python script to A total of 324 MDR-TB isolates were collected from 1 implement these criteria with the phylogenetic tree and a January 2009 to 31 December 2012, in Shanghai (the most mutation matrix, and the Python script was uploaded to populous city in China) during a population-based GitHub. observational study . MIRU-VNTR genotyping (12 loci) was performed for all MDR-TB strains, and the clustered Genotypic drug resistance detection and identiﬁcation of strains were further subjected to WGS . In the present transmission clusters study, we further sequenced the entire gene sequences of Drug-resistance-associated mutations in M. tb were the rpoA, rpoB, and rpoC genes in 105 nonclustered obtained from the database TBDReaMDB and articles MDR-TB strains to detect putative compensatory muta- reporting rifampicin-resistance mutations (Supplemen- tions. The PCR amplicons were sent for Sanger sequen- tary Table 2). M. tb isolates carrying any characterized cing, and the results were analyzed to detect putative rifampicin-resistance mutation were identiﬁed as compensatory mutations using Geneious (http://www. rifampicin-resistant strains, whereas those isolates with geneious.com/). additional isoniazid-resistance mutations were deter- A total of 8331 published whole-genome sequences mined as MDR-TB strains. To determine the drug- of M. tb strains were downloaded from the ENA resistance spectra of the MDR-TB isolates in the Global (http://www.ebi.ac.uk/ena). The quality criteria for dataset, we additionally identiﬁed drug-resistance- data inclusion were set as follows: (1) the average associated mutations for nine drugs (amikacin, capreo- sequencing depth should be above 10-fold; (2) the mycin, ethambutol, ethionamide, ﬂuoroquinolones, genome coverage rate should be above 95%. To avoid isoniazid, kanamycin, para-aminosalicylic acid, pyr- false transmission clustering, we included only the last azinamide, streptomycin) by inspection of their whole- isolate of longitudinal isolates collected from the same genome sequences. Any isolates with genetic distances of patient. Information about the geographic isolation of less than 12 SNPs were classiﬁed into a cluster .To these isolates was obtained from the articles or the compare the collection time distributions between the authors. isolates in the clustered and nonclustered groups, we Liu et al. Emerging Microbes & Infections (2018) 7:98 Page 7 of 8 Publisher’s note determined the collection time of each isolate as “days to Springer Nature remains neutral with regard to jurisdictional claims in the end of the study duration (31 December 2012)”. For published maps and institutional afﬁliations. example, if an isolate was collected on 1 January 2011, Supplementary Information accompanies this paper at (https://doi.org/ then its collection would be counted as “365 (days in 10.1038/s41426-018-0101-6). 2010) + 366 (days in 2012) = 731 days”. A Python script was written for screening transmission Received: 24 February 2018 Revised: 26 April 2018 Accepted: 29 April 2018 clusters in both the Shanghai and Global datasets, and this Python script was uploaded to GitHub. Statistical analysis References The Wilcoxon nonparametric rank-sum test and Stu- 1. Gagneux, S. et al. 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Emerging Microbes & Infections – Springer Journals
Published: Jun 6, 2018
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