In medicine, we have come to accept the hypothesis that genetic differences can greatly affect pain responses and specific responses to pain therapies.1 Over the past decade, a systematic effort has focused on understanding the role of the genetic and environmental factors among adult populations that affect an individual’s pain experience. Yet pediatric studies are scarce. Children are less likely to be enrolled as volunteers in experimental studies, and age-developmental differences in pharmacokinetics and pharmacodynamics add variability to the design of research studies in this population that is often unaccounted for. Nevertheless, there is increased interest in individualizing pain treatment for pediatric patients, and research efforts have expanded to understand the genetic and environmental factors that affect a child’s experience of pain.
Genetic studies indicate that racial and ethnic self-identification are linked to markers of ancestry and, therefore, have biological correlation.2,3 In addition, racial and ethnic identification are cultural constructs linked to important environmental factors that can affect an individual’s pain experience. Among geneticists worldwide, there is an increased awareness of the need to study more diverse racial and ethnic populations, rather than restricting studies to racially and ethnically homogeneous groups.4 Interpopulation and population-specific genetic studies allow researchers to learn about rare genetic variants, which tend to be population specific, and could be important predictors of disease. In addition, environmental factors that vary by race and ethnicity could influence gene expression, and might help explain variability in drug response between patients of different racial and ethnic backgrounds.
Simultaneously, research in pain has gone far beyond the idea that genetics are solely responsible for an individual’s pain responses. The evolution of fields such as epigenetics, proteomics, transcriptomics, metabolomics, and lipidomics has enhanced our appreciation of the complexity of the pain response and its relationship to individual genetics (Fig. 1).
We here discuss the use of interpopulation and population-specific pharmacogenetics; epigenetics, and –omics approaches to individualize pediatric pain treatment. Because of the limited number of genetic studies in pediatric pain and the absence of epigenetic and –omics studies in pediatrics, this article will focus on providing a review of the small but emerging body of literature regarding pharmacogenetic studies of pediatric pain, and epigenetic and –omics definitions.
INTERPOPULATION AND POPULATION-SPECIFIC PHARMACOGENETICS
Pharmacogenetics is the study of associations between an individual’s genotype and his/her response to a specific drug. Its primary goal is to individualize therapy to maximize drug response and minimize drug toxicity. Pharmacogenomics is another term used to describe the study of the human genome and the interaction between genes to predict drug response. Both terms are used interchangeably, but pharmacogenetics most often refers to the study of individual genes, while pharmacogenomics refers to the study of the whole genome and the interactions between genes. A comprehensive review of pharmacogenetics for pediatric anesthesiologists was previously published in this journal.5
This review aims to discuss the role of race and ethnicity in the context of pharmacogenetic studies in pediatric pain. Following the trend of adult pharmacogenetic studies, pediatric studies are including more racially and ethnically diverse populations to understand variability in drug response. Two terms frequently used within this context are “population-specific” and “interpopulation” pharmacogenetic studies. Population-specific studies are pharmacogenetic studies done in specific racial or ethnic population groups; interpopulation studies are those that include subjects of different racial or ethnic backgrounds. Both approaches acknowledge that humans are genetically similar, but support the study of diverse populations to better understand drug response, given that genetic variations occur in varying frequencies among different racial/ethnic groups.
Perhaps one of the most clinically significant demonstrations of pharmacogenetic interpopulation variability in pediatrics is the variability in analgesic response to codeine. As a prodrug, codeine’s analgesic properties depend entirely on its conversion to the active metabolites morphine and morphine-6-glucuronide. Most codeine administered is converted to inactive metabolites by glucuronidation and N-demethylation. For most patients only 10%–20% of codeine is converted to the active metabolite morphine by O-demethylation by the CYP2D6 (Cytochrome P450 2D6) enzyme. The CYP2D6 gene is highly polymorphic with more than 100 CYP2D6 different alleles consisting of single nucleotide polymorphisms, insertions, and deletions. The activity level of the CYP2D6 enzyme is determined by the combination of alleles (diplotype) that each patient has. Alleles are characterized as normal function (wild type), reduced function, and nonfunctional; each with an activity score ranging from 1.0, normal function, to 0, nonfunctional. The total activity score for the CYP2D6 enzyme is the sum of the activity scores of all alleles. Patients with activity scores lower than 0.5 are considered poor metabolizers, those between 0.5 and 1 are intermediate metabolizers, those between 1.0 and 2.0 are extensive metabolizers, and those with a score greater than 2.0 are ultrarapid metabolizers (in patients with more than two alleles). While poor metabolizers do not have analgesia after codeine administration, ultrarapid metabolizers experience a high risk of serious adverse effects, such as respiratory depression due to high levels of the morphine metabolite. Allele frequencies vary significantly between racial and ethnic groups. Normal function alleles (wild type) are more frequent among European Caucasians (71%), compared with Asians and African-Americans (50%).6 Genetic studies among subjects from East Africa (Ethiopians) show that up to 29% of studied subjects exhibit duplications of the CYP2D6 gene, which is indicative of ultrarapid metabolism, making them prone to serious adverse reactions.7,8
The clinical implications of CYP2D6 polymorphisms are clear. Clinical studies among pediatric patients with sickle cell disease show that patients with reduced functioning alleles are 4 times more likely to experience codeine analgesic failure when compared with those with normal functioning alleles.9 Guidelines for the use of the CYP2D6 genotype to direct the clinical administration of codeine and other opioids with CYP2D6 metabolism, such as tramadol, hydrocodone, and oxycodone, have already been developed.10 Of importance, these guidelines specifically address the racial and ethnic variation of CYP2D6 polymorphisms, raising awareness among clinicians of a higher risk of adverse events among specific population groups.
While there are several polymorphisms in genes coding for enzymes involved in other opioid metabolic pathways, the clinical significance of these findings is less apparent. Nonetheless, some studies among pediatric patients have yielded interesting results. Most studies have focused on investigating genetic polymorphisms in genes related to morphine metabolism, transport across membranes, and effector site. Morphine’s main metabolic pathway is glucuronidation. The uridine glucuronyl transferase (UGT) enzymes are subdivided into 4 families, and each of these into subfamilies. The UGT2B7 isoform of the UGT2 family is the principal isoform responsible for morphine metabolism and, in adult studies, polymorphisms of the UGT2B7 gene have been associated with variations in clearance of morphine.11 Other genes studied include the OPRM1 gene (the gene coding for mu opioid receptor type 1) and genes coding for transport proteins such as ATP binding cassette, subfamily B, member 1 (ABCB1, which functions as a transporter in the blood-brain barrier) and organic cationic transporter 1 (OCT1, which functions as a hepatic uptake transporter).
Pharmacokinetic studies among pediatric patients of diverse racial and ethnic backgrounds have shown differences in side effects after morphine administration for postoperative pain management. Among this group of studies, a study on 194 children (160 Caucasian and 34 African American) receiving morphine for pain treatment after tonsillectomy and adenoidectomy procedures has shown that Caucasian children, compared with African American children, experience more side effects and lower morphine clearance after morphine administration.12 A subsequent study within this same cohort of patients did not find an association between morphine clearance and UGT2B7 polymorphisms, which could explain the differences in morphine clearance between the 2 racial groups.13 However, an association between polymorphisms in the OCT1 gene and the presence of side effects was found among this cohort of children; this association is supported by a 3% reduction in morphine clearance for the African American group, when the OCT1 genotype was included in the statistical analyses.14
Another study on a cohort of 70 children of Latino (n = 35) and non-Latino Caucasian (n = 35) ethnicity receiving morphine for postoperative pain after tonsillectomy and adenoidectomy found significant differences in side effects between these 2 ethnic groups.15 In this study, Latino children experienced a 3-fold greater incidence and increased severity of vomiting, pruritus, and respiratory depression. This study is in agreement with the results of an earlier experimental study among adult volunteers, which showed a greater depression of ventilatory response to CO2 in South American Native Indians, compared with Latinos, who had more ventilatory depression than Caucasians after administration of IV morphine.16
The study among Latino and Caucasian children did not find differences in plasma levels of morphine or its glucuronides that could help explain the differences in side effects between the 2 groups. Further analysis, adjusting for the effect of polymorphisms in 8 candidate genes including UGT2B7 and ABCB1, did not show attenuation of the difference in side effects between the 2 ethnic groups, which suggests that the studied polymorphisms do not explain the differences in the occurrence of side effects between groups.15
There are 2 additional pediatric studies that are not strictly pharmacogenetic, but are pertinent to the discussion of individualizing pain treatment in children. The first is an experimental study that evaluated differences in pain sensitivity between children of diverse racial and ethnic backgrounds.17 This study, which included 214 volunteer children 8–18 years of age (98 Caucasian, 58 Latino, 34 African American and 24 Asian), found that pressure and heat pain sensitivity was highest among Asians, followed by Caucasians, followed by African Americans and Latinos. These differences persisted even after adjusting for age, sex, socioeconomic status, and the investigator’s ethnicity. The results from this study are not consistent with those reported by adult studies, which show higher pain sensitivity among all minority groups when compared with Caucasians.18–24 However, it is worth noting that the adult studies used different experimental models that preclude direct comparisons with the pediatric study.
The second study looked at associations between polymorphisms in 6 candidate genes and postoperative pain intensity among a prospective cohort of 168 children 6–18 years of age of diverse European ancestry.25 This study found a significant association between polymorphisms in the ABCB1 gene, the OPRM1 gene and intensity of postoperative pain measured by self-reported pain scores, and morphine PCA consumption.25 These results are in partial agreement with a prior study in adult volunteers in which polymorphisms in the OPRM1 gene were associated with pain sensitivity.26 Although both studies report an association between the A118G single nucleotide polymorphism in the OPRM1 gene and pain sensitivity, the adult study found that Caucasian subjects who carry the variant G-allele were less sensitive to different experimental pain modalities, whereas the pediatric study found that children who carry the G-allele had higher postoperative pain scores.
As a whole, this small body of literature constitutes a further step in understanding the causes of racial and ethnic differences in children’s pain experience, and reinforces the idea of considering race and ethnicity when evaluating and treating pain in children.
BEYOND GENOMICS: OTHER –OMIC TECHNOLOGIES THAT HAVE THE POTENTIAL TO OPEN THE DOOR TO A BETTER UNDERSTANDING OF INDIVIDUAL RESPONSE
Epigenetics, proteomics, transcriptomics, metabolomics, and lipidomics are techniques that use complex assays and computational models for sorting through huge data sets to explore pain at a mechanistic level.
Epigenetics is the study of noninherited influences on gene expression and/or cellular phenotype. These changes do not affect or alter DNA sequence. The influences are often environmental factors and can lead to broad changes at a very macrolevel. A common environmentally influenced change is on DNA methylation at genomic areas that contain a high frequency CpG sites (CpG is the phosphodiester bond between cytosine and guanine in DNA). Changes in methylation can be due to factors such as age, smoking, and exposure to drugs. An example of how changes in methylation may affect clinical pain can be seen in a recent study in which DNA methylation in the OPRM1 gene coding for mu-opioid receptors and a more global methylation site, LINE-1, were compared and correlated to pain sensitivity in chronic pain patients, both naïve to and chronically exposed to opioids.27 This study found significantly increased methylation in both the OPRM1 gene and the LINE-1 site in patients exposed to chronic opioids. The more global LINE-1 site, but not the OPRM1 gene, increased methylation also correlated with increased chronic pain scores and further suggests that opioids have a notable effect on DNA methylation. This effect suggests that opioids may have an inhibitory effect on the transcription of still unspecified nocifensive gene products. It also implies that opioids may be causally associated with increased genome-wide DNA methylation. This research demonstrates that opioids may enhance pain through a direct effect on DNA transcription.
Transcriptomics is the study of the transcriptome. Transcription products, all RNA molecules, including mRNA, rRNA, tRNA, and other noncoding RNA produced in one or a population of cells, allow the production of proteins. Thus, minor changes in the transcriptome, or the coding for the transcriptome, can have pronounced effects on function. The effect of chronic pain on the transcriptome at a specific area of injury in an animal model28 is addressed by a study in which rats with a spinal nerve ligation at L5 had their mRNA quantified at the L4 dorsal nerve root ganglion. This study found that “12.4% of known genes were induced and 7% suppressed in the dysfunctional (but anatomically intact) L4 dorsal root ganglion 2 weeks after spinal nerve ligation. These alterations persisted chronically (2 months).” Further, 10,464 novel exons were found, of which almost 22% were dysregulated. Although this approach cannot be duplicated in a human model, it is easy to appreciate the potential for techniques examining the transcriptome after specific injuries to find new targets for novel therapeutics.
Proteomics is the study of the end product of DNA and RNA production and coding. Proteins are large molecules that effect action and response in the living system. Proteomics involves looking at multiprotein systems at both micro- and macrolevels. Again, the study of proteins involves highly complex and expensive analytic techniques and large data reduction strategies. In a recent meta-analysis, gene expression was examined as a surrogate for protein expression in multiple studies of pain in rats.29 In this combined analysis, it was found that the 2 genes, CCL2 and Reg 3b, may be suitable drug targets, based on increased expression. These genes correlated to the proteins monocyte chemoattractant protein 1 and pancreatitis-associated protein, suggesting that these 2 proteins have potential as biomarkers of pain response in rats. Unfortunately, it is often difficult to translate these findings to human studies since the controlled nature of animal studies and animal pain models is impossible to replicate in humans. However, as our ability to assay multiple proteins at a time grows, so will our ability to examine proteomics in humans. Currently, this exploration is limited by the high cost and the evolution of our analytics.
Metabolomics is essentially the study of all small molecules (metabolites) that can be found in a living system. Metabolite profiles are often the result of protein breakdown. In examining the profiles, researchers often look at individual classes of molecules such as sugars, nucleotides, amino acids, and lipids. The study of lipids (lipidomics) has become especially appealing to pain researchers since profiling the lipidome is more feasible than examining the proteome or the transcriptome, as many of these lipid mediators are circulating modulators in blood, cerebrospinal fluid, and plasma. Lipid mediators in the dorsal horn of the spinal cord, dorsal root ganglia, tibial nerve, and plasma have been examined after tibial nerve transection in rats.30 Tibial nerve transection is a well-established model of chronic pain and allodynia. In 1 study, the ipsilateral dorsal horns of rats were examined after tibial nerve transection and characterization of the dysregulated lipid metabolites was performed, finding multiple alterations of the sphingomyelin-ceramide pathway.30 Among the metabolites examined, it was found that endogenous metabolite N, N-dimethylsphingosine (DMS) production (a catabolite of ceramide) was significantly altered. This was novel since DMS itself had not been identified or studied in a neuropathic pain model. In this same study, DMS was intrathecally injected into healthy rats and the development of mechanical allodynia in the hind paw was measured. Within 24 hours, rats developed persistent allodynia. These results demonstrate that DMS inhibition could be a novel target for blocking neuropathic pain and shows the potential for metabolomics to develop candidate targets after wide screening for new drugs and therapeutics.
In aggregate, these –omics technologies have the potential to change the way we research pain. As analytic and data-reduction strategies improve, examining large volumes of data to seek new pain therapeutics will become more of a reality. Examining the external effectors and end products of gene expression will further help explain very complicated systems, such as neuropathic pain. It will not be until all of these –omics strategies can be combined that a more global understanding of the pain response will be fully understood.
Unfortunately, we are far from understanding the implications of –omics, racial, and ethnic differences in children’s pain experience. Lack of research funding, lack of interest by industry, and a lack of focus on the topic of pediatric response variability by regulating agencies have been large barriers in studying these issues. Further, culture, socioeconomic status, access to health care, language barriers, and environmental exposures further muddy a difficult study field. The real challenge is in educating the public, general practitioners, and legislators that children may have very different responses than adults. Only through this education can we get to the point where we demand appropriate attention and funding to study these important issues.
Name: Nathalia Jimenez, MD, MPH.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Nathalia Jimenez approved the final manuscript.
Name: Jeffrey L. Galinkin, MD, FAAP.
Contribution: This author helped design the study, conduct the study, and write the manuscript.
Attestation: Jeffrey L. Galinkin approved the final manuscript.
This manuscript was handled by: James A. DiNardo, MD.
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