Abstract
Pharmacogenomics holds promise as a critical component of precision medicine. Yet, the use of pharmacogenomics in routine clinical care is minimal, partly due to the lack of efficient and effective use of existing evidence. This paper describes the design, development, implementation and evaluation of a knowledge-based system that fulfills three critical features: a) providing clinically relevant evidence, b) applying an evidence-based approach, and c) using semantically computable formalism, to facilitate efficient evidence assessment to support timely decisions on adoption of pharmacogenomics in clinical care. To illustrate functionality, the system was piloted in the context of clopidogrel and warfarin pharmacogenomics. In contrast to existing pharmacogenomics knowledge bases, the developed system is the first to exploit the expressivity and reasoning power of logic-based representation formalism to enable unambiguous expression and automatic retrieval of pharmacogenomics evidence to support systematic review with meta-analysis.
Introduction
Pharmacogenomics is the study of how genetic variants affect a person’s response to a drug. The rapid advances in pharmacogenomics research have made pharmacogenomics one of the genomics-based innovations that has great potential to contribute to improving people’s health and reducing health care costs by increasing drug efficacy and safety1. Yet, the adoption of pharmacogenomics in routine clinical care is relatively low2, partly due to the perception that there is insufficient evidence to determine the value of pharmacogenomics and the lack of effective and efficient use of already existing evidence3,4.
Systematic review with meta-analysis is a well-established methodology used in evidence-based medicine that assesses the findings of a collection of studies that address a similar research question of interest in order to provide a more precise estimate of the effect of interventions or risk factors on patients’ outcomes5. Generally, the review process is time-consuming and labor-intensive and involves the following generally manual steps6,7: a) conducting a comprehensive literature search, b) screening articles to identify relevant studies, c) extracting quantitative data and other essential elements from included studies, d) synthesizing the extracted data when they are acquired from sufficiently similar clinical context, e) rating the quality and strength of evidence, and f) interpreting the synthesized results. Informatics approaches such as natural language processing, machine learning and text mining have been applied to improve the efficiency of conducting a systematic review by reducing the burden of manual efforts in tasks of literature screening and data extraction8-10. However, there remains considerable room for further improvement, particularly in the area of representing the extracted primary evidence in a semantically computable formalism to enable intelligent support in initial and ongoing updating of evidence retrieval, synthesis and interpretation. In particular a system that leverages semantically computable formalisms would greatly facilitate the addition of new evidence and the reassessment of the conclusions factoring in the new evidence.
Knowledge representation and reasoning is a sub-domain of artificial intelligence that is concerned with encoding knowledge into semantically computable formalisms that can be efficiently manipulated by reasoning programs so that computers can demonstrate human-like abilities. During the past decade, Web Ontology Language (OWL) has been developed by combining the Semantic Web technologies and logic-based representation formalisms to advance computer interpretability of Web content11. OWL-encoded ontologies provide shared conceptualizations and controlled vocabularies of a domain of interest which allow for formal representation and automatic reasoning. Because of its expressivity and reasoning ability, research efforts are encouraged to exploit the advanced features of OWL in developing more intelligent systems that assist human decision making.
Considering the time-consuming and knowledge-intensive nature of pharmacogenomics evidence assessment, the idea of developing a knowledge-based system for intelligent initial and ongoing support in evidence assessment emerges intuitively from the perspective of biomedical informatics. We hypothesized that a knowledge-based system with the following three critical features can assist effective and efficient evidence assessment, and therefore facilitate timely decisions on adoption of pharmacogenomics in clinical practice. First, the information provided by the knowledge-based system should be clinically relevant evidence, which means that evidence related to clinical validity and clinical utility of pharmacogenomics should be accumulated in the system. Second, the information provided by the knowledge-based system should be acquired through an evidence-based approach, which means that primary evidence acquired from empirical research should be collected and synthesized through methodologies established in comprehensive systematic reviews. Third, the information provided by the knowledge-based system should be semantically computable, which means that a knowledge-based system should take full advantage of the expressivity and reasoning power of logic-based knowledge representation formalisms such as OWL 2 DL11 so that pharmacogenomics knowledge is unambiguously represented and accumulated in a knowledge base which allows for automatic reasoning.
Upon reviewing existing pharmacogenomics knowledge bases including the Pharmacogenomics Knowledgebase (PharmGKB)12, the PharmacoGenomics Mutation Database (PGMD)13 and the DrugBank database14, we discerned that none of them fully meets the critical features of our envisioned pharmacogenomics knowledge-based system (Table 1). This gap motivated us to design and develop the knowledge-based system described in this paper de novo, aiming to provide intelligent assistance for pharmacogenomics evidence assessment.
Table 1:
Overview of identified gaps in current pharmacogenomics knowledge bases
| Features of the envisioned pharmacogenomics knowledge-based system | PharmGKB | PGMD | DrugBank | |
|---|---|---|---|---|
| Clinically relevant evidence | Clinical validity | Y | Y | Y |
| Clinical utility | Y | N | N | |
| Evidence-based approach | Primary evidence | Y | Y | Y |
| Sufficient information for meta-analysis | N | Y | N | |
| Risk of bias assessment | N | N | N | |
| Synthesized evidence | Y | N | N | |
| Explicit inclusion criteria | N | N | N | |
| Semantically computable formalism | Logic-based formalized ontology | N | N | N |
| Ontology-committed knowledge base | N | N | N | |
| Question answering by automatic reasoning | N | N | N | |
PharmGKB: the Pharmacogenomics Knowledgebase; PGMD: the PharmacoGenomics Mutation Database; DrugBank: the DrugBank database.
Y: abbreviation of “yes”, indicating that the knowledge base meets the specified features, N: abbreviation of “no”, indicating that the knowledge base does not meet the specified features.
Methods
Conceptual modeling of the domain of pharmacogenomics evidence assessment
To address the aforementioned features of clinically relevant evidence and an evidence-based approach, we proposed a basic information structure for developing the conceptual model of the domain of pharmacogenomics evidence assessment using a faceted analysis approach15. This information structure is composed of five building blocks, namely, information entities, information components, concepts, relations and terms. Figure 1 illustrates that in the domain of pharmacogenomics evidence assessment, an information entity is composed of information components, an information component is expressed by relation-concept pairs, and relations and concepts are substantiated by terms to express the intended meaning. Based on the information needs in conducting systematic reviews with meta-analyses6,7, the information entities in the conceptual model include publication, study, and evidence, and the minimal set of information components to describe these intended information entities include study population, drug therapy, comparison, outcome, genetic variation, study design, effect estimation, risk of bias assessment and bibliographical information of publication.
Figure 1:
Basic structure of the conceptual model and its building blocks for conceptualization of the domain of pharmacogenomics evidence assessment
We created operational definitions of evidence of clinical validity and utility and deployed a fine-grained characterization of these two types of pharmacogenomics evidence acquired from empirical pharmacogenomics studies in clopidogrel and warfarin therapies to identify concepts, relations and terms that are essential for modeling the domain of pharmacogenomics evidence assessment. References cited by the Clinical Pharmacogenetics Implementation Consortium (CPIC) guidelines for clopidogrel and warfarin therapies16,17 were used as the major sources from which we selected original research articles for manual extraction of concepts, relations and terms. Review articles cited in the two CPIC guidelines were used for backward citation tracking to identify relevant articles that did not directly cited in the guidelines’ reference list. Articles recently published after the release of the CPIC guidelines were also sought.
Implementation of a pharmacogenomics knowledge-based system
Our knowledge-based system for pharmacogenomics evidence assessment consists of three core components, i.e., an ontology, a knowledge base and a reasoner (Figure 2). The primary aim of the knowledge-based system is to enable formal representation and automatic retrieval of pharmacogenomics evidence to assist in meta-analysis, which lays the foundation for further applications in systematic review such as classification of homogeneous evidence and interpretation of clinical significance of evidence. We adopted OWL 2 DL11 as our formal representation language, used Prot¹g¹18 as an ontology editor, and leveraged HermiT19 as a reasoner to implement the knowledge-based system.
Figure 2:
Fundamental architecture and intended application scenarios of the developed pharmacogenomics knowledge-based system. The two applications highlighted by grey blocks are proposed for future research.
The aforementioned conceptual model of pharmacogenomics evidence assessment served as the blueprint for constructing the ontology. Based on a commonly cited guide for constructing an OWL 2 ontology11, we derived mapping principles to convert the varieties of building blocks of the conceptual model into the constructs of an OWL ontology, i.e., classes, properties and individuals. The individual information entities extracted for deriving the conceptual model served as the test materials to construct the knowledge base. Using the constructs encoded in the OWL ontology, and constructors (i.e., restrictions and operators) supported by OWL 2 DL, we derived representation patterns for asserting individual information entities with heterogeneous information content. Our major considerations while deriving the representation patterns were to avoid computational inefficiency caused by over-representation and irrelevant retrieval of individuals caused by under-representation.
Evaluation of the implemented knowledge-based system on ontology-driven pharmacogenomics evidence retrieval
In order to provide a proof-of-concept that the developed knowledge-based system is capable of providing intelligent support in retrieving relevant pieces of pharmacogenomics evidence for systematic review with meta-analysis, a convenience sample of 9 systematic reviews20-28 that investigated the association between genetic variations and responses to clopidogrel was obtained from the reference list of the CPIC clopidogrel guideline16. A collection of 33 meta-analyses were selected from these reviews and used as test cases to evaluate the precision and efficiency of the ontology-driven approach to evidence retrieval.
The reported criteria for including individual pieces of evidence into each test case of meta-analyses were extracted from the respective review articles. Ontology-driven evidence retrieval was implemented first by formally representing these inclusion criteria as the necessary and sufficient conditions of defined classes using the constructed OWL ontology. Then the HermiT reasoner embedded in Prot¹g¹ was manually triggered to perform instance checking over the implemented knowledge base to retrieve those individual pieces of evidence that match the definition of each defined class. The results of ontology-driven evidence retrieval were evaluated in terms of precision and efficiency. Precision was calculated as the percentage of retrieved individual pieces of evidence that are relevant to the inclusion criteria specified for each respective meta-analysis. The relevance was judged by BD, one of the authors of this paper. Efficiency was measured by the computing time taken by HermiT reasoner to perform the reasoning tasks, which was captured from Prot¹g¹ Command Prompt.
Results
Table 2 provides an overview of basic statistics on evidence source, conceptual model, ontology metrics and asserted individual information entities of the developed knowledge-based system. A total of 73 empirical research articles were selected as evidence source, from which three types of intended information entities were identified, including 73 pieces of publications, 82 pieces of studies and 445 pieces of evidence.
Table 2:
Overview of statistics of data source, conceptual model, ontology and asserted individual information entities of the developed knowledge-based system
| Evidence Source | Building Blocks of Conceptual Model | Metrics of Ontology and Asserted Individual IE | Ontology | Asserted Individual IEs | ||
|---|---|---|---|---|---|---|
| Publication | 73 | Information entity (IE) | 3 | DL expressivity | ALCRF(D) | ALCRQ(D) |
| -clopidogrel | 51 | Information component | 9 | Class | 306 | - |
| -warfarin | 22 | Concept | 30 | Object property | 69 | - |
| Study | 82 | Relation | 49 | Datatype property | 12 | - |
| - clopidogrel | 57 | Term | 282 | Individual | 9 | 667 |
| - warfarin | 25 | SubClassOf axioms | 289 | - | ||
| Evidence | 445 | EquivalentClasses axioms | 9 | - | ||
| - clopidogrel | 285 | SubObjectPropertyOf axioms | 27 | - | ||
| - warfarin | 160 | SubPropertyChainOf axioms | 11 | - | ||
| SubDatatypePropertyOf axioms | 5 | - | ||||
| FunctionalDatatypeProperty axioms | 7 | - | ||||
| DatatypePropertyRange axioms | 7 | - | ||||
| ClassAssertion axioms | 9 | 2670 | ||||
| ObjectPropertyAssertion axioms | - | 1187 | ||||
| DatatypePropertyAssertion axioms | - | 1522 | ||||
Conceptual model
Fine-grained characterization of this collection of individual information entities yielded 30 concepts, 49 relations, and 282 terms to describe the 9 intended information components. By organizing these extracted building blocks, we derived a conceptual model for representing the domain of pharmacogenomics evidence assessment (Figure 3). Three types of information entities are independent yet inter-related. Specifically, Evidence is related to Study via the relation of “is acquired from”, and Study is in turn related to Publication via “is reported in”. Each type of information entity is described by specific information component modules, with Publication described by publication module, Study described by modules of study population, study design, drug therapy, and risk of bias assessment, and Evidence described by modules of comparison, genetic variation, outcome, and effect estimation. Each information component module is expressed in a layered structure that is composed of multiple relation-concept pairs. When the conceptual model is used to express concrete information entities, concepts and relations are directly substantiated by terms commonly used in a variety of clinical, pharmacological or genomic domains. Thus the meaning of each individual real-world information entity could be explicitly and precisely expressed. For example, to describe a study population of “patients who were treated with clopidogrel for acute coronary syndrome”, the concept of Person is substantiated by the term of Patient, the concept of Drug by the term of Clopidogrel, and the concept of Disease by the term of Acute Coronary Syndrome. The developed conceptual model was validated by fitting two original articles29,30 and two systematic reviews20,31. The preliminary verification results showed that our model is adequate for annotating primary pharmacogenomics evidence and inclusion criteria for meta-analysis.
Figure 3:
Conceptual Model of Pharmacogenomics Evidence Assessment. Double-lined squares: information entities, single-lined squares: concepts, arrows: relations. Dotted lines divide the entire model into 9 modules, each corresponding to one information component.
Ontology
The constructed ontology contains 396 constructs, including 306 classes, 69 object properties, 12 data properties, and 9 individuals (Table 2). By following the information structure illustrated in Figure 3, these constructs could be used to formally represent publications, studies and evidence that were involved in assessing the evidence of clinical validity and utility in the domain of clopidogrel and warfarin pharmacogenomics.
As shown in Table 2, the ontology features the use of several axioms, including SubClassOf, EquivalentClass, SubObjectPropertyOf, and SubPropertyChainOf axioms, to facilitate reasoning for evidence retrieval. For example, we used SubClassOf axioms to construct the class of Disease with 6-level depth of class hierarchy, where the bottom-level classes are more specific than the top-level classes. Thus a piece of evidence annotated with specialized disease terms could be retrieved by inclusion criteria defined with broad disease terms. We used EquivalentClasses axioms to define acute coronary syndrome (ACS) as equivalent to the union of ST-segment elevation myocardial infarction (STE_MI), non-ST-segment elevation myocardial infarction (NSTE_MI) or unstable angina (UA). Thus inclusion criteria that are specified with ACS as disease characteristics of patients will retrieve not only evidence that is exactly annotated with ACS but also those annotated with STE_MI, NSTE_MI or UA. We used SubObjectPropertyOf axioms to represent more specific relations. For example, the object property hasDrugTherapy represents a general relation between a study and a drug therapy under investigation. Subproperties such as hasDrugTherapyObserved, hasDrugTherapyOI and hasDrugTherapyRef were created to specify a drug therapy that was investigated under an observational study, or given to the experimental arm, or given to the control arm respectively. Thus inclusion criteria that are specified with hasDrugTherapyObserved will retrieve exactly those evidence acquired from observational studies. We used SubPropertyChainOf axioms to connect individuals by a chain of properties. For example, an individual of evidence Ie is linked to an individual of study Is via object property isAcquiredFrom, and Is is linked to a risk-of-bias-assessment (ROBA) value low on random sequence generation via object property hasROBA_Cochrane_RandomSequenceGeneration, by linking these two properties to form a property chain, Ie will be automatically inferred the ROBA value of low.
Knowledge base
The constructed knowledge base contains 73, 82 and 445 individual pieces of asserted publications, studies and evidence respectively. These information entities were formally represented via class assertion axioms, object property assertion axioms, and datatype property assertion axioms (Table 2). Figures 4, 5 and 6 illustrate respectively the formal representation of individual pieces of publication, study and evidence that were extracted from the article [Kimmel et al., 2013]30.
Figure 4:
Example of assertion of an individual piece of publication. Screenshot extracted from Prot¹g¹.
Figure 5:
Example of assertion of an individual piece of study. Screenshot extracted from Prot¹g¹.
Figure 6:
Example of assertion of an individual piece of evidence. Screenshot extracted from Prot¹g¹.
Figure 4 illustrates the formal representation of an individual publication labeled as pub_24251361, expressing that it is a full-text refereed journal article that was published in 2013 and its PubMed identifier is 24251361.
Figure 5 illustrates the formal representation of an individual study labeled as stu_1_pub_24251361, which was reported in the publication pub_24251361 that has been asserted in Figure 4. This study is expressed as a randomized and paralleled controlled clinical trial that aimed to investigate a genotype-guided warfarin therapy considering three genetic variants (CYP2C19*2 and CYP2C19*3 and VKORC1-1639G/A) versus clinically guided warfarin dosing in patients with atrial fibrillation or deep vein thrombosis or pulmonary embolism or deep vein thrombosis & pulmonary embolism. In addition, the risk of bias in this particular study was assessed using Cochrane assessment tool, with low risk of bias in each of the six criteria.
Figure 6 illustrates the formal representation of an individual piece of evidence labeled as evi_01_ pub_24251361_stu_1, which was acquired from the study stu_1_pub_24251361 that has been asserted in Figure 5. This evidence is expressed as comparison between two drug therapies (which could be known by its link to stu_1_pub_24251361). The outcome measure was the percentage of time of international normalized ratio in the therapeutic range up to the follow-up of 28 days. The effect was measured as absolute difference between two group means and was estimated as -0.2% with 95% confidence interval -3.4% to 3.1% and P-value of 0.91. In addition, some information was inferred for this evidence (as shown in the highlighted blocks) through its linkage with stu_1_pub_24251361, e.g., the publication pub_24251361 from which it was extracted, and the risk of bias assessment values of the study stu_1_pub_24251361 from which the evidence was acquired.
The construction of our knowledge base features the design of representation patterns to enable representation of heterogeneous information content of complicated information components. For example, the representation patterns for describing 6 types of information content in the drug therapy module are summarized in Table 3. Each type of information content is represented by an anonymous class expression, which is described by object properties, property restrictions, classes used as property values, and operators that link multiple property values, as appropriate. The exemplary drug therapies illustrated in Figure 5 were asserted based on these representation patterns. It is worth mentioning that these representation patterns are capable of describing the highly heterogeneous drug therapies investigated in clinical pharmacogenomics studies. In the pilot implementation of 82 individual studies, the representation patterns were successfully used to represent a total of 35 different types of drug therapies.
Table 3:
Representation patterns for describing information content of drug therapy
| Information content | Object property | Property restriction | Class used as property value (possible number of values) | Operator used to link multiple values |
| Drug therapy | hasDrugTherapy with subproperties | Existential restriction | DrugTherapy (single or multiple) | or |
| Drug therapy strategy | hasDrugTherapyStrategy | Existential restriction | DrugTherapyStrategy (single) | Not applicable |
| Genetic variant considered in genotype-guided strategy | considersGeneticVariant | Qualified cardinality restriction | GeneticVariant (single or multiple) | and |
| Alternative drug therapy in genotype-guided drug selection | hasAlternativeDrugTherapy | Existential restriction | DrugTherapy (single or multiple) | or |
| Pharmacodynamic parameter monitored | monitorsPharmacodynamics Parameter | Existential restriction | PharmacodynamicsParameter (single) | Not applicable |
| Drug regimen | hasDrugRegimen | Existential restriction | DrugRegimen (single or multiple) | and/or |
Performance of ontology-driven evidence retrieval
Table 4 illustrates the implementation and result of ontology-driven evidence retrieval, using one test case meta-analysis selected from the review article of [Singh et al., 2012]27 as an example. The inclusion criteria extracted from the review article are summarized in the upper left part. Ontology-based formal representation of the extracted inclusion criteria is presented in the lower left part. Implementation of 9 defined classes to represent the inclusion criteria of 9 meta-analyses is presented in the middle part (marked by brackets). After triggering the HermiT reasoner, the retrieved relevant evidence could be viewed by clicking each specific defined class. As shown in the right part, 22 pieces of relevant evidence were retrieved for the defined class named as MACE_CYP2C19star2_CADandPCI_Singh.
Table 4:
Example of ontology-driven evidence retrieval
 |
* Screenshots extracted from Prot¹g¹. The 9 defined classes marked by the brackets represent the inclusion criteria of 9 meta-analyses.
The evaluation of ontology-driven evidence retrieval in terms of precision and efficiency is encouraging. Overall, 33 test cases of ontology-based evidence retrieval achieved a precision rate of 100%. The computing time taken to retrieve relevant evidence for each systematic review approximately ranged from 9 to 23 seconds (Table 5).
Table 5:
Evaluation of efficiency of ontology-driven evidence retrieval
 |
Note: The retrievals were tested on a personal laptop (Intel Corei7-4700MQ 2.4GHz Processor, 16 GB DDR3 Ram and a 64-bit version of Windows 8.1).
Discussion
This paper presents a knowledge-based system that adopts OWL 2 DL as the representation language to enable ontology-based representation of primary evidence and ontology-driven retrieval of relevant pieces of evidence for conducting systematic review with meta-analysis. The unique features of this system are elaborated as follows.
First, the system was developed based on a conceptual model of pharmacogenomics evidence assessment that considers different dimensions of information needs and thus accommodates different types of information in a unified model. Considering that both clinical validity and clinical utility evidence are essential to integrate pharmacogenomics into clinical practice, the conceptual model was designed to enable annotation of evidence related to association between genetic variant and drug response as well as evidence related to effectiveness of genotype-guided drug therapies. Moreover, to address the information needs for conducting systematic reviews with meta-analyses, the conceptual model was designed to enable annotation of primary evidence along with its study context and provenance as well as to allow annotation of inclusion criteria for retrieving relevant evidence. To our best knowledge, none of the existing pharmacogenomics knowledge bases is capable of annotating all of these types of information in a single information model.
Second, the system exploited the expressivity and reasoning ability of OWL 2 DL to deliver an ontology and a number of representation patterns, which collectively allow complex and heterogeneous pharmacogenomics evidence to be unambiguously represented. Thereby, the formally represented primary evidence could be classified at different levels of specificity as defined by different research questions. Nevertheless, it was challenging to derive representation patterns that avoid computational inefficiency caused by over-representation and irrelevant retrieval caused by under-representation. We also identified some cases in that the meaning of our intended retrieval criteria were not expressible in the representation patterns we designed.
Third, the system could represent inclusion criteria as defined classes and embed them into the ontology, which allows the users of the system to acquire the most updated profile of evidence of their interests each time newly extracted pieces of evidence are added in the knowledge base. This is achieved via the OWL 2 DL reasoner’s capability of automatic reasoning. This feature is most beneficial in view of the evolving nature of the development of pharmacogenomics and the recurrent needs to assess the change of evidence over time.
Our preliminary work has several limitations. The scope of knowledge base was limited to clinical validity and utility of clopidogrel and warfarin pharmacogenomics. Some useful information was missing in the conceptual model, such as age and ethnicity of study population. The evidence asserted in the knowledge base is not exhaustive, but to serve as representative examples to provide a proof-of-concept of the design, development, implementation, and evaluation of the envisioned knowledge-based system. No informatics tool has been developed to automatically export retrieval results from Prot¹g¹ to existing statistical software that supports meta-analysis.
Through evaluation of its performance using real-world test cases, the preliminary pharmacogenomics knowledge-based system has proven to be an effective and efficient approach to retrieve relevant primary evidence for conducting systematic review with meta-analysis. Future research to enhance its applicability is proposed as follows. The scope that limited to clopidogrel and warfarin pharmacogenomics should be expanded to include other domains, particularly cancer pharmacogenomics. The information component modules should be refined to express more useful information. Moreover, the application scenarios should be extended to address the subsequent steps in the process of a comprehensive evidence assessment, such as formal representation of synthesized evidence to enable semantic computation of the clinical significance of genetic variants in predicting drug response and improving patient outcome. With the enhanced applicability, the knowledge-based system might greatly improve the efficiency of pharmacogenomics evidence assessment, and ultimately increase the adoption of pharmacogenomics in routine clinical care.
Acknowledgements
The authors are thankful to Dr. James F. Brinkley for helpful comments on the paper. This work was supported in part by U01 HG006507 (Tarczy-Hornoch).
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