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Human genetic association studies in individuals with traumatic brain injury (TBI) have increased rapidly over the past few years. Recently, several review articles evaluated the association of genetics with outcomes after TBI. However, almost all of the articles discussed in these reviews focused on adult TBI. The primary objective of this review is to gain a better understanding of which genes and/or genetic polymorphisms have been evaluated in pediatric TBI. Our initial search identified 113 articles. After review of these articles only 5 genetic association studies specific to pediatric TBI were identified. All five of these studies evaluated the apolipoprotein (APOE) gene. The study design and methods of these identified papers will be discussed. An additional search was then performed to evaluate genes beyond APOE that have been evaluated in adult TBI; findings from these studies are highlighted. Larger genetic studies will need to be performed in the future to better elucidate the association of APOE and other genes with outcomes after TBI in children. There is great potential to utilized genetic information to inform prognosis and management after TBI in children; however, we have much work ahead of us to reach the goal of individualized management.
Traumatic brain injury (TBI) is a significant cause of morbidity and mortality. Approximately 1.7 million people in the US alone sustain a TBI annually . However, individuals with seemingly very similar injuries often have vastly different outcomes. Genetic factors may explain, in part, differential recovery trajectories. As our genetic and genomic technologies have improved, genetic analysis has become more accessible for use in the study of outcomes after TBI.
Genetic association studies typically fall under two types of approaches; candidate gene approaches or genome-wide association studies (GWAS) . In candidate gene studies, prior knowledge is required to form hypotheses related to specific functions of genes and the phenotype or outcome of interest . Single Nucleotide Polymorphisms (SNPs) are genetic variants that correspond to a difference in a single base pair in deoxyribonucleic acid (DNA). SNPs may be located in critical areas in genes and may ultimately lead to changes in a protein hypothesized to be important in a disease specific pathway. Alternatively, in GWAS, prior knowledge about a disease specific gene is not required . GWAS are more exploratory in nature and often consist of the screening of millions of SNPs for an association with an outcome of interest. GWAS are particularly useful in identifying groups of genetic variants or regions of interest within a specific gene that can be further analyzed to better characterize pathophysiologic pathways. However, a major limitation for GWAS is the large number (> 100,000 cases) of samples required to attain statistical significance.
To our knowledge, there have been no GWAS performed to evaluate outcomes after TBI likely in part due to the large sample size required. However, various review articles published recently have discussed specific genes potentially important to outcomes after TBI. Due to the complex nature of TBI, there are various pathophysiologic pathways that could be influenced by genetics and potentially contribute to varied outcomes after injury. A recent review by McAllister  postulated several domains that may be influenced by genetics and are important to the modulation of outcomes after neurotrauma, including pre-injury risk factors, response to neurotrauma, repair and plasticity, pre- and post-injury cognitive and neurobehavioral capacity/reserve, and epigenetic factors . In particular, there has been an extensive study of the role of the apolipoprotein (APOE) gene in outcomes and recovery after TBI . APOE is an attractive target because it is thought to play an important role in synaptic repair, remodeling, and neuron protection . Three commonly reported alleles of the APOE gene include APOE e2, e3, and e4. The presence of the APOE e4 allele has been associated with poorer global functional outcomes after adult TBI; however, the association is small to modest in magnitude  and likely only explains a portion of the variation in recovery after TBI. Since brain injury is a complex process, multiple other molecular pathways may be related to outcomes or recovery after injury. Due to the oxidative stress, inflammatory processes, and cell death that occur after TBI, genes associated with these pathways are also attractive to evaluate. Several studies have identified an association between genes specific to cell regulation and outcomes after TBI in adults [7–10]. Additionally, because cognitive and behavioral sequelae are common after brain injury, catecholamine-related genes important to regulation of behavior, attention, and executive function may also be associated with outcomes after TBI. Several studies in adult TBI demonstrated an association among dopamine-related genes and cognitive and behavioral outcomes after TBI in adults [11–15]. Overall, the study of the association of genetics with outcomes after TBI is in the early stages. Multiple genes are likely to influence recovery; therefore, it will be important to continue to evaluate the influence of other genes beyond APOE in recovery after TBI.
To date, genetic association studies described in recent reviews have focused on the adult TBI population. TBIs in children occur when the brain is at a significantly different stage of development and maturation compared to TBI in adults; therefore, genes or polymorphisms important to outcomes or recovery after adult TBI may be significantly different for pediatric TBI. The aim of this article is to review human genetic association studies specific to TBI in children. This review will evaluate the methodology of these studies and discuss them in the context of genetic association studies performed in adult TBI. The potential future directions of genetic association studies after TBI in children will also be discussed.
An initial literature search was performed using Pub Med on February 21, 2012. The search terms included (gene or genotype or polymorphism) and traumatic brain injury (TBI) The search was limited to “Human” studies of children 0–18 years published in “English” using the Pub Med limits function. The overriding inclusion criterion for articles was that they had to evaluate the association of “genes”, “genotypes”, or “polymorphisms” with outcomes after pediatric TBI. Articles were excluded if the primary study population included non-traumatic brain injuries (e.g., perinatal brain injury, cerebral palsy, anoxic brain injury) or did not have a study population with an average age below 18 years.
A follow-up literature search was performed using The Human Genome Epidemiology Network (HuGE Net) (http://hugenavigator.net/HuGENavigator/startPagePhenoPedia.do) on March 12, 2012 [16,17]. HuGENet is a global collaborative effort coordinated by the US Centers for Disease Control and Prevention (CDC). The HuGE Navigator is a searchable online database of published data on human genetic associations and human genome epidemiology studies published since 2000. The database is updated weekly and is based on MeSH indexing of PubMed records. The HuGE Navigator uses a computerized literature search screening tool based on machine-learning techniques and has a sensitivity of 97.5% and specificity of 98.3% . A curator then manually reviews to verify that the articles meet criteria for indexing on HuGENet.
The primary search term used in the navigator was “Brain Injuries”. Articles specific to TBI were included, while articles that primarily evaluated other types of brain injuries were excluded (e.g., anoxic brain injury, stroke, etc). Additional MeSh search terms (“Brain Injury, Chronic”; “Cerebral Hemorrhage, Traumatic”; “Epilepsy, Post-Traumatic”; “Brain Concussion”; “Brain Hemorrhage, Traumatic”) were also used in the HuGE Navigator to ensure that all articles specific to TBI were identified. All of the studies identified in our initial search for pediatric specific articles were also identified using the HuGE navigator and no new studies were identified.
Our pediatric specific search initially identified 113 studies. Based on review of article titles, abstracts, and methods sections of the papers, 4 articles met the inclusion criteria. Articles were excluded for two primary reasons: (1) the mean age of the population was above 18 years of age and (2) TBI was not the focus of the study (e.g., perinatal or anoxic brain injury). All 4 articles evaluated the association of the APOE genotype with of outcomes after pediatric TBI. Additionally, one review article entitled “Apoliprotein and brain injury: implications for children” was identified. On review of the text, one unpublished study was described in the article that evaluated the association of Apoliprotein E and outcomes after pediatric TBI. After reviewing the references of the 5 articles identified, 1 additional paper was identified that included both adult and pediatric participants, but stratified analysis by age. A follow-up search with the HuGE Navigator did not reveal any additional articles. All of the studies included in this review evaluated the association of the APOE gene with outcomes after pediatric TBI. Given the limited number of studies examining genetic influences in pediatric TBI, we chose to include all of them despite considerable heterogeneity in study design and outcomes. Detailed descriptions of the studies’ methodologies and findings are below.
Quinn et al.  evaluated the association of APOE e4 allele and post-traumatic brain swelling in children who died following TBI . The study consisted of 165 cases from 1962–2000 of children between the ages of 2–19 years who survived 1 hour to 5 months (median 3 days) after injury. The median age was 13 years and 76% were male. Race or ethnicity of the study population was not described, but the study was conducted in Glasgow, Scotland and Southampton, UK. 78% of the injures were due to road traffic accident, 12% due to falls, 8% due to assault, and 2% were secondary to other causes. APOE genotyping was performed using postmortem tissue in 64% (106/165) of the cases Genotyping was performed “blind” to histological assessments. The primary outcome of presence of cerebral swelling (unilateral or bilateral) was determined by macroscopic and microscopic analysis of post-mortem brains. Twenty-five percent (27/106) possessed the APOE e4 allele. However, approximately equal proportions of individuals with (66%) and without the e4 allele (65%) had evidence of brain swelling on autopsy. Multiple other outcomes were measured in the study, including presence of skull fracture, raised intracranial pressure, ischemic damage, hematomas, contusion index, survival time, and microscopically graded diffuse axonal injury; however, the association of the e4 allele with these other outcomes was not reported. Overall, this study did not find an association of the APOE e4 allele with the presence or absence of cerebral swelling after pediatric TBI. One of the primary limitations of the study was that it only included post-mortem cases, thus biasing the study towards individuals that likely had more severe injuries and a greater likelihood of poor outcomes.
Moran et al.  evaluated whether APOE alleles were a predictor of outcomes in children after mild TBI . The study included children ages 8–15 years that presented to the emergency department and were diagnosed with concussion. Inclusion criteria included loss of consciousness (LOC) or a Glasgow Coma Scale (GCS) score of 13 or 14, or two or more acute signs or symptoms of concussion noted by emergency department personnel. Exclusion criteria included LOC more than 30 minutes, any GCS below 13, delayed neurological decline, Abbreviated Injury Severity (AIS) score above 3, any surgical intervention, previous head injury requiring medical treatment, history of severe psychiatric illness resulting in hospitalization, premorbid neurological disorders or mental retardation, hypoxia, hypertension, shock during or following injury, injury resulting from child abuse or assault, or injuries that would interfere with neuropsychological testing. Outcomes were assessed in the emergency department and at 2 weeks, 3 months, and 12 months post-injury. Outcome measures included assessments of memory (California Verbal Learning Test, child version), development (Developmental Test of Visual-Motor Integration), neuropsychological functioning (Cambridge Neuropsychological Test Automated Battery), general intelligence (Wechsler Abbreviated Scale of Intelligence), and academic achievement (Wide Range Achievement Test). Post-concussive symptoms were also assessed with the Post-Concussive Symptom (PCS) Interview and the Health and Behavior Inventory (HBI). 387 children were initially eligible for the study, 186 agreed to participate, and 99 were genotyped. Of the 99 participants 28 had a least one APOE e4 allele and 71 did not have any APOE e4 alleles. The average age of the APOE e4 allele group was 11.73 years and the non-e4 allele group was 12.06 years. 71% and 69% of the e4 allele and non-e4 allele groups were male, respectively. Both groups were 82% Caucasian. The e4 allele carrier group was more likely to have presented with a GCS score below 15. Additionally, the e4 group performed better on the Developmental Test of Visual-Motor Integration than the non e4 group (P < 0.05). The groups did not differ on any of the other measures evaluated. The overall conclusion of the study was that presence of at least one APOE e4 allele was not significantly associated with differential outcomes on neuropsychological testing or post concussive ratings after mild TBI in children.
Lo et al.  evaluated the association of the APOE alleles with cerebral perfusion pressure (CPP) and global outcome in 65 critically ill children admitted to the intensive care unit after TBI . Information specific to age and GCS scores was not reported for the participants . Additionally, participant ethnicity was not reported; however, the study was performed in the United Kingdom. Forty-five of 65 children had intracranial pressure monitoring performed. Outcome measures assessed included CPP, modified GCS, and modified Glasgow Outcome Scale (GOS). 21% were APOE e4 allele carriers and 78% were non-carriers. 27% of the APOE e4 carriers had a poor outcome versus 11% of the non-carriers, but this was not statistically significant (p = 0.35). However, increased CPP insult was associated with a poor outcome as measured by GOS (poor outcome = score of 1, 2, or 3). Poor outcomes 6 months after injury were more likely in carriers of the APOE 4 allele with less CPP insult compared to those without the e4 allele (p = 0.03). Overall, the Lo et al.  study suggests that individuals that are carriers of the APOE e4 allele may be less tolerant to increases in cerebral perfusion pressure compared to children that do not carry the e4 allele. Some limitations to the study include the lack of demographic information provided specific to age of injury, severity of injury, and ethnicity of the population, thus making it difficult to make inferences about the generalizability of the findings in the study. Additionally, the cohort in the study included a higher proportion of individuals with the e2 allele compared to the general population, which may have confounded the results.
Brichtova and Kozak  evaluated the association of the APOE alleles with outcomes after TBI in 70 children . The cohort consisted of 48 boys (69%) and 22 girls (31%), ranging in age from 1 month to 17 years (mean age 9.47 years, SD: 4.87). Fifteen (21%) had mild TBI (GCS 13–15), 10 (14%) had moderate TBI (GCS 9–12), and 45 (64%) had severe TBI (GCS 3–8). Ethnicity was not reported in the study; however, the study was conducted in the Czech Republic. The primary outcome measure was GOS at 12 months post-injury. 27% of the APOE e4 carriers had a poor outcome based on GOS (scores of 1, 2, or 3) compared to 14% of the non-APOE e4 carriers. Specifically, the authors found that the APOE e4 genotype was associated with an unfavorable outcome when compared to e2/e3 and e3/e3 genotypes. Overall, this study found that the presence of the APOE e4 allele was associated with poor global outcomes as measured on the GOS.
Teasdale et al.  evaluated whether the APOE e4 allele was associated with poor global outcomes after TBI in individuals age 0–93 years (mean age 37 years) . 1094 individuals were initially included in the study with 984 participants undergoing genotyping. Of the 984 participants, 81% were male, 28% had a mild TBI (GCS 13–15), 19% had a moderate TBI (GCS 9–12), and 54% had a severe TBI (GCS 3–8). Race and ethnicity were not described, but the study was performed in Southampton, UK. There was no overall association found between the APOE genotype and outcome after injury. 36% of APOE e4 carriers had an unfavorable outcome compared with 33% of non-carriers. However, an interaction between age and outcome was observed, with younger age being associated with a greater likelihood of a poor outcome. In children less than 16 years, there was a 3.06 (95% confidence interval: 1.22–7.65) greater odds of an unfavorable outcome for carriers of the APOE e4 allele compared to non-carriers. Of the 212 children less than 16 years in the study, 94% non-carriers of the APOE e4 allele and 83% of the carriers of the APOE e4 allele had a favorable outcome. Demographics specific to the child participants were not reported. Overall, this study indicated that there may be an age effect related to the presence of the APOE e4 genotype, with the presence of the APOE e4 allele in younger individuals being associated with a greater risk for an unfavorable outcome.
The final study identified was an unpublished study that was described in the context of a larger review article by Blackman et al. . The study included 71 children with a TBI requiring inpatient rehabilitation admission. The mean age was 13 years and 2 months. Further demographic variables were not reported. The primary outcome was the Functional Improvement Measure for Children quotient (WeeFIM, version 5.01) on discharge from inpatient rehabilitation. The WeeFIM Quotient was better for APOE e4 carriers compared to non-carriers. However, the e4 frequency in the population studied was only 4%, which may have limited the ability of the authors to make definitive conclusions.
Because the Brichtova and Kozak , Lo et al. , and Teasdale et al.  articles all used GOS as an outcome measure, we were able to combine the data from these studies to assess to association of the APOE e4 allele on global outcomes 6–12 months post TBI in children. When combining the data from these papers, 77/95 carriers (81%) and 230/252 non-carriers (91%) of APOE e4 had a good outcome (GOS scores of 4 or 5) and 18/95 carriers (19%) and 22/252 (9%) non-carriers of APOE e4 had a poor outcome (GOS scores of 1, 2, or 3) 6 or 12 months after injury. Analysis of the combined data using SAS enterprise guide 4.3 revealed a 2.44 (95% confidence interval: 1.25–4.80) greater odds of having a poor outcome in children that are carriers of the APOE e4 allele compared to non-carriers.
Our review indicates that there is a paucity of genetic association studies specific to TBI in children. The only gene evaluated to date is APOE and the findings are mixed. However, the combined data from 3 studies that used GOS as a common outcome measure suggests that the APOE e4 allele is associated with poor global outcomes 6–12 months after TBI in children. This is in agreement with a meta-analysis performed in adult TBI that found the APOE e4 allele to be significantly associated with poor outcomes assessed by GOS 6 months after injury (relative risk = 1.36; 95% confidence interval 1.04–1.78) . Our combined analysis demonstrated a slightly larger association (odds ratio = 2.44); however, the individuals in our analysis likely consisted of children with more severe injuries, which may bias the results towards the more severe population. Future research is needed to determine if the association of the APOE e4 allele with poor outcomes holds true across all severities of injury. Additionally, future studies will need to evaluate if there are age effects within childhood TBI, especially because the brain is actively developing and changing throughout childhood and there may be differential genetic effects for younger children versus older children.
One study identified in our review failed to find an association between APOE and neuropsychological recovery and post-concussion symptoms after mild TBI in children. Several studies evaluating the association of APOE genotype with outcomes after mild to moderate brain injury in adults have been performed, and the results have been mixed. Several studies did not identify a significant association between APOE genotypes and neuropsychological outcomes after mild to moderate TBI in adults [24–26]. However, other studies have demonstrated an increased risk of problems after mild to moderate TBI in adults who are APOE e4 carriers. One study found an association between APOE e4 carriers and poorer neuropsychological outcomes at 3 weeks, but not at 6 weeks post-injury ; another study demonstrated that there was an increased risk of fatigue in APOE e4 carriers after mild TBI in adults ; a third study found head injury in combination with the APOE e4 genotype increases the risk of dementia . Future studies will need to be performed to better elucidate the influence of APOE genotype on differing outcomes after varying severities of TBI.
Several studies have also evaluated the association of APOE with the risk of sustaining a concussion. Kristman et al.  performed a prospective evaluation in search for an association between the APOE e4 allele and concussion injuries in college athletes. They did not identify an association with a risk of sustaining a concussion between APOE e4 carriers and non-carriers . Similarly, Terrel et al.  did not find an association between the APOE e4 allele and self-reported history of multiple concussions in college athletes . However, a polymorphism in another area of the APOE gene, the promoter region, was associated with a graded increase in self-reported history of concussion . Most recently, Tierney et al.  demonstrated an association between the presence of both the APOE e4 allele and APOE promoter polymorphism with a history of at least one concussion injury . The findings from this study indicate that multiple polymorphisms in the APOE gene may interact with each other to determine overall risk of concussion. In the future, studies evaluating the association of the APOE gene with outcomes after mild TBI in children should consider evaluating the influence of the polymorphism in the APOE promoter region used in the evaluation of concussion injuries in college athletes. Additionally, the Tierney et al.  study highlights the potential importance of evaluating the influence of a combination of genetic polymorphisms within or across genes on outcomes in future genetic association studies.
The association of APOE with outcomes after TBI in adults has been well studied; however, the pathophysiology of TBI is complex and multiple other genes likely influence outcomes. Genes that influence a range of pathways including cell-cycle regulation and cognitive processing may also be important to recovery after TBI. Because the genetic association studies in pediatric TBI have only evaluated APOE, we also used the HuGE navigator to identify genes beyond APOE that have been associated with outcomes after TBI in adults. Since the primary goal of this review articles was to focus on TBI in children, findings from the adult search are briefly summarized below, but detailed in a table in Appendix A.
Using the HuGE navigator, 74 publications were identified and included 29 genes. Eight publications were excluded as they did not evaluate TBI specifically as a diagnosis [33–39] or only included penetrating brain injuries in the population evaluated . Forty-three of the publications were specific to the association of the APOE gene with outcomes after TBI [6,19–22, 24,25,28–30,32,41–72]; one was a meta-analysis . Twenty-three of the studies evaluated the association of genes besides APOE alleles with outcomes after TBI in adults [7–12,15,31,73–83] (Appendix A). A wide variety of genes beyond APOE potentially associated with outcomes after TBI in adults have been evaluated. Outcomes measured in these studies varied widely and included survival, global disability outcomes, higher cognitive skills, and medication response. The studies are fairly recent as they have all been published in 2005 or more recently. These studies highlight the potential for different genes to be important at different stages after TBI. For example, the interleukin genes, which are involved in the cellular proliferation, seem to be important in survival and global outcomes after injury [7,9,76,81,84] while dopamine related genes may be more important for longer-term recovery of higher level cognitive and behavioral functioning [11,12,15]. Future studies should evaluate the association of genes beyond APOE with outcomes after TBI in children.
Preliminary data analysis from our group indicates that catecholamine-related genes may be associated with executive functioning 12 months after TBI in early childhood . The studied consisted of 52 Caucasian children (16 with TBI and 36 with orthopedic injury) with a mean age of 5.44 years at the time injury. We found that polymorphisms in the catechol-O-methyl transferase gene and the dopamine receptor gene were associated with executive function outcomes 12 months after injury. Larger studies will need to be performed in the future to validate these findings; however, the results of this analysis indicate that catecholamine related genes may influence higher level cognitive and behavioral functioning after TBI in children.
Our review also highlights the importance of using common data elements with genetic studies in TBI to enable combined analysis and direct comparison among studies [86–91]. In this review, we were able to perform a combined analysis to evaluate the overall association of the APOE e4 allele with global outcomes after pediatric TBI measured with the GOS. However, the generalizability of the results is limited as demographic variables (e.g., age, race, and gender) and injury severity variables (e.g., GCS) were not consistently reported across studies. The combined analysis used one study that only included “critically ill” patients (GCS not reported) , another that included approximately 65% severe injuries , and the severity in the third was not reported for the child sub-analysis . Common data elements included in future studies will need to span across multiple domains, including global outcomes [91,92], neuropsychological [91,92], psychological and psychosocial [90–92], biospecimens and biomarkers , radiological imaging [86,87], and demographics and clinical assessment . Having comparable demographic, phenotypic, and environmental measures across studies will potentially improve cross-study comparison in human genetic association studies of TBI , thus improving our understanding of the association of genetics with outcomes after TBI and the generalizability of the findings. Additionally, tailoring outcome measure to the specific function or biological process associated with the genes being studied may help to streamline the design of human genetic association studies to better elucidate the relationship of genetics with recovery and outcomes after TBI.
The primary limitations of this review is that, to date, there have been very few studies that have addressed the association of genetics with outcomes after TBI in children; thus it is difficult to make definitive conclusions regarding the overall association of genetics with outcomes after pediatric TBI. However, this limitation is important to note because it highlights the need for more studies in this area in the future. Additionally, although the combined analysis reported in our review indicates that APOE may be associated with outcomes after pediatric TBI, the findings are limited because there was a lack of consistently reported demographic and injury-related information in the studies evaluated. Therefore, we were unable to account for potential confounders (e.g., age, race, gender, time since injury) and heterogeneity of the population in the analysis. Future studies are needed to confirm our findings, and should also attempt to clarify the role of potential confounding factors.
The evaluation of the association of genetics with outcomes after TBI in children is in the early stages. The literature related to genetic studies in children is currently very limited and has only assessed the influence of the APOE gene on outcomes after injury. APOE appears to be associated with global outcomes after TBI in children; however, larger studies should be performed to validate these findings.
Future studies should also evaluate the association of genes beyond APOE with outcomes after TBI in children. Studies that consider common functional pathways or gene-gene interactions should be performed because it is unlikely that one gene or polymorphism will explain the majority of recovery, but rather quite likely that combinations of genes or polymorphisms are more influential. Gene-environment interactions also need to be considered since environment is known to be related to recovery after TBI in children. Additionally, longitudinal pediatric specific studies should be developed in order to improve our understanding of the combined effect of TBI and genetics on brain development.
Methodological challenges related to TBI and genetic research will need to be addressed in the future as well. The genetic effects of polymorphisms are often small-to-modest in magnitude , thus future studies may need to be multi-center in nature to allow for recruitment of large sample sizes. Additionally, TBI is a very heterogeneous phenotype and detailed data need to be collected that characterize injury severity and outcomes precisely. Collection of common data elements would potentially improve the ability to combine data across multiple studies. Widespread biobanking and electronic medical records also provide promise as tools for developing large clinical genetic studies  to elucidate the role of genetics in recovery after TBI in children. As our genetic technologies continue to improve, there is the great potential to utilize genetic information to individualize prognosis and management after TBI in children; however, we have much work ahead of us to reach the goal of using genetic information to individualize management.
Support from this work was provided through the Rehabilitation Medicine Scientist Training Program (RMSTP) 7K12 HD001097-14, the National Institute of Mental Health (NIMH) 5R01MH073764-03, and H133B090010 from the National Institute on Disability and Rehabilitation Research, Department of Education.
Conflict of interest
The authors report no conflicts of interest.