Peripheral biomarkers of attention-deficit hyperactivity disorder: Current status and future perspective

Association of impulsive behavior and cerebrospinal fluid/plasma oxidation and antioxidation ratio in Chinese men

OBJECTIVES

Impulsive behavior is the precursor of many psychiatric and neurological conditions. High levels of impulsive behavior will increase health risk behavior and related injuries. Impulsive behavior is produced and regulated by central and peripheral biological factors, and oxidative stress (OS) can aggravate it. However, previous studies only showed that impulsive behavior was related to the level of the peripheral OS. Therefore, this study aims to clarify the relationship between OS and impulsive behavior in the brain and peripheral blood.

METHODS

We recruited 64 Chinese men. We measured superoxide dismutase (SOD) (including copper, zinc and manganese) and nitric oxide synthase (NOS) (including total, inducible and constitutive) in cerebrospinal fluid (CSF) and plasma. The Barratt Impulsiveness Scale version 11 (BIS-11) was used to evaluate impulsive behavior. The relationship between OS and impulsive behavior was evaluated by partial correlation analysis and stepwise multiple regression analysis.

RESULTS

Partial correlation analysis showed that the ratio of total NOS-to-MnSOD and iNOS-to-MnSOD in CSF were negatively correlated with the BIS-11 motor scores (r = -0.431, p = -0.001; r = -0.434, p = -0.001). Stepwise multiple regression analysis showed that the ratio of CSF iNOS-to-MnSOD was the most influential variable on the BIS-11 motor scores(β = -0.434, t = -3.433, 95 %CI(-0.374, -0.098), p = 0.001).

CONCLUSIONS AND RELEVANCE

The imbalance of central oxidation and antioxidation is related to impulsive behavior, which broadens our understanding of the correlation between impulsive behavior and OS.

Human transcriptome array analysis and diffusion tensor imaging in attention-deficit/hyperactivity disorder

The mRNA markers identified using microarray assay and diffusion tensor magnetic resonance imaging (DTI) were applied to elucidate the pathophysiology of attention-deficit hyperactivity disorder (ADHD). First, we obtained total RNA from leukocytes from three children with ADHD and three healthy controls for analysis with microarray assays. Subsequently, we applied real-time quantitative polymerase chain reaction (qRT‒PCR) assays to validate the differential expression of 7 genes (COX7B, CYCS, TFAM, UTP14A, ZNF280C, IFT57 and NDUFB5) between 130 ADHD patients and 70 controls, and we built an ADHD prediction model based on the ΔCt values of aforementioned seven genes (AUROC = 0.98). Finally, in a validation group (28 patients with ADHD and 27 healthy controls), mRNA expression of the above seven genes also significantly differentiated ADHD patients from controls (AUROC value = 0.91). The DTI analysis showed increased fractional anisotropy (FA) of the forceps minor, superior corona radiata, posterior corona radiata and anterior corona radiata in ADHD patients. Moreover, the FA of the right superior corona radiata tract was positively correlated with ΔCt levels of the COX7B gene and the IFT57 gene. The results shed a new light on a genetic profile of ADHD that may help in deciphering the white matter microstructural features in disease pathogenesis.

MicroRNAs as potential biomarkers for diagnosis of attention deficit hyperactivity disorder

Inappropriate levels of hyperactivity, impulsivity, and inattention characterize attention deficit hyperactivity disorder, a common childhood-onset neuropsychiatric disorder. The cognitive function and learning ability of children with attention deficit hyperactivity disorder are affected, and these symptoms may persist to adulthood if they are not treated. The diagnosis of attention deficit hyperactivity disorder is only based on symptoms and objective tests for attention deficit hyperactivity disorder are missing. Treatments for attention deficit hyperactivity disorder in children include medications, behavior therapy, counseling, and education services which can relieve many of the symptoms of attention deficit hyperactivity disorder but cannot cure it. There is a need for a molecular biomarker to distinguish attention deficit hyperactivity disorder from healthy subjects and other neurological conditions, which would allow for an earlier and more accurate diagnosis and appropriate treatment to be initiated. Abnormal expression of microRNAs is connected to brain development and disease and could provide novel biomarkers for the diagnosis and prognosis of attention deficit hyperactivity disorder. The recent studies reviewed had performed microRNA profiling in whole blood, white blood cells, blood plasma, and blood serum of children with attention deficit hyperactivity disorder. A large number of microRNAs were dysregulated when compared to healthy controls and with some overlap between individual studies. From the studies that had included a validation set of patients and controls, potential candidate biomarkers for attention deficit hyperactivity disorder in children could be miR-140-3p, let-7g-5p, -30e-5p, -223-3p, -142-5p, -486-5p, -151a-3p, -151a-5p, and -126-5p in total white blood cells, and miR-4516, -6090, -4763-3p, -4281, -4466, -101-3p, -130a-3p, -138-5p, -195-5p, and -106b-5p in blood serum. Further studies are warranted with children and adults with attention deficit hyperactivity disorder, and consideration should be given to utilizing rat models of attention deficit hyperactivity disorder. Animal studies could be used to confirm microRNA findings in human patients and to test the effects of targeting specific microRNAs on disease progression and behavior.

The Kynurenine Pathway in Attention-Deficit/Hyperactivity Disorder: A Systematic Review and Meta-Analysis of Blood Concentrations of Tryptophan and Its Catabolites

Preliminary evidence shows that the kynurenine pathway (KP) may be altered in attention-deficit/hyperactivity disorder (ADHD). We thus conducted a systematic review and meta-analysis exploring the peripheral blood concentrations of tryptophan catabolites (TRYCATs) in people with ADHD. We searched the main electronic databases up to 7th December 2023. Standardised mean differences (SMDs) with 95% confidence intervals (95%CIs) were used to compare TRYCAT concentrations between participants with ADHD and healthy controls (HCs). We included eight studies. Random-effects meta-analyses found higher kynurenine (SMD = 0.56; 95%CI: 0.04 to 1.08; p = 0.033; I2 = 90.3%) and lower kynurenic acid (SMD = -0.33; 95%CI: -0.49 to -0.17; p < 0.001; I2 = 0%) concentrations in people with ADHD compared to HCs. Additional analyses on drug-free children with ADHD showed higher tryptophan (SMD = 0.31; 95%CI: 0.11 to 0.50; p = 0.002; I2 = 0%) and kynurenine (SMD = 0.74; 95%CI: 0.30 to 1.17; p < 0.001; I2 = 76.5%), as well as lower kynurenic acid (SMD = -0.37; 95%CI: -0.59 to -0.15; p < 0.001; I2 = 0%) blood levels, as compared to HCs. Despite some limitations, our work provides preliminary evidence on KP alterations in ADHD that may suggest decreased neuroprotection. Further research is needed to clarify the role of the KP in ADHD.

Positron emission tomography studies in adult patients with attention-deficit/hyperactivity disorder

Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder characterized by inattention, motor hyperactivity, impulsivity, and psychosocial as well as cognitive dysfunction. Although characteristic clinical manifestations have been described, no definitive biomarkers to diagnose ADHD have been established. In this review article, we summarize positron emission tomography (PET) studies conducted in adult patients with ADHD. We found that, although, disturbances of dopamine, serotonin, and norepinephrine functions have been implicated in ADHD, no characteristic findings have been identified from PET studies in patients with ADHD. Several previous PET studies on the central dopaminergic transmission-related ligands in patients with ADHD have shown altered binding of dopamine markers in the basal ganglia. However, no consistent results were observed in the binding characteristics for dopamine transporters and receptors. Findings from PET studies with ligands related to serotonin and norepinephrine pathways showed either unclear clinical significance or low replicability. Therefore, whether alterations of monoamine function may be involved in the pathophysiological mechanism remains to be clarified. The limitations of previous PET studies include their small sample sizes, focus on several kinds of existing ligands, and a questionable validity of the diagnosis (lack of biological diagnostic criteria). To determine the characteristic findings for diagnosing ADHD, further research is needed, and particularly, studies that evaluate new active ligands with specific binding to monoamine pathways should be undertaken.

Is ADHD a way of conceptualizing long-term emotional stress and social disadvantage?

Background

The prevalent, neuropsychiatric, deficit perspective on children and youth diagnosed with ADHD prohibits a multidimensional approach where socio-economic status, family stress and relationships within the families are relevant factors to examine. Assessments of ADHD through the use of rating scales and short-term interventions may lead not only to overdiagnosis but also to a reductionistic approach in the psychiatric field. This literature review aims to address research outside the prevailing discourse on ADHD as an organic brain dysfunction and broaden the perspectives on children's behavioral difficulties.

Methods

The articles included in this applied, mixed-method, systematic review includes 26 peer-reviewed articles, both English and French, with a search focus on ADHD in children and youth related to Attachment styles and relationships.

Results

In the studies reported, researchers approached correlations between ADHD and attachment in different ways, and in most cases, there was a caution to address causality. The role of parents was found to be both buffering and aggravating for the appearance of ADHD. In the French case studies, the diagnosis was conceptualized as a relational phenomenon where the child's behavior was inseparable from family member's suffering.

Discussion

This review article illustrates how children's difficulties in terms of ADHD symptoms can be addressed through a paradigm where emotional and cognitive dysregulation is understood through psychosocial factors rather than as a neurological condition. In our view, to avoid an overly reductionistic and medicalized approach to children's behavioral difficulties, it is time to reiterate the value of the biopsychosocial perspective.

Conclusion

Professionals and researchers need to acknowledge that becoming diagnosed with ADHD has a strong connection to economic disadvantage, social status, and familial care. The academic discourse of addressing brain dysfunctions might serve the unintended purpose of masking emotional stress and social disadvantage that manifests across generations. A biopsychosocial approach to ADHD including family, emotional history, and socio-economic issues could imply a lesser focus on medical treatment as a first choice.

Potential Inflammatory Biomarker in Patients with Attention Deficit Hyperactivity Disorder

Attention deficit hyperactivity disorder (ADHD) is a prevalent neurodevelopmental disorder that can diminish the quality of life of both children and adults in academic, occupational, and social contexts. The kynurenine pathway (KP) contains a set of enzymatic reactions involved in tryptophan (TRP) degradation. It is known to be associated with the risk of developing ADHD. This review will address the KP and underlying mechanism of inflammation in ADHD. Potential inflammatory biomarkers reported in the most recent studies are summarized. Although a strong neuroimmunological basis has been established due to the advances of recent neurobiological research, the pathophysiology of ADHD remains unclear.

Peripheral blood inflammatory markers in patients with attention deficit/hyperactivity disorder (ADHD): A systematic review and meta-analysis

It has been observed that subclinical inflammation might be involved in the pathophysiology of attention deficit/hyperactivity disorder (ADHD). However, studies investigating peripheral blood levels of immune-inflammatory markers have provided mixed findings. We performed a systematic review and meta-analysis of studies comparing unstimulated serum or plasma levels of C-reactive protein (CRP) and cytokines in subjects with ADHD and healthy controls (the PROSPERO registration number: CRD 42021276869). Online searches covered the publication period until 30th Sep 2021 and random-effects meta-analyses were carried out. Out of 1844 publication records identified, 10 studies were included. The levels of interleukin (IL)-6 were significantly higher in studies of participants up to the age of 18 years (k = 10, g = 0.70, 95%CI: 0.10-1.30, p = 0.023) and after including those above the age of 18 years (k = 10, g = 0.71, 95%CI: 0.12-1.31, p = 0.019). In turn, the levels of tumor necrosis factor-α (TNF-α) were significantly lower in subjects with ADHD compared to healthy controls (k = 7, g = -0.16, 95%CI: -0.30 - -0.03, p = 0.020). Individual studies had a high contribution to the overall effect, since the overall effect was no longer significant after removing single studies. No significant differences were found with respect to the levels of CRP, IL-1β, IL-10 and interferon-γ. The present findings indicate that individuals with ADHD tend to show elevated levels of IL-6 and reduced levels of TNF-α. Larger and longitudinal studies recording potential confounding factors and comorbid psychopathology are needed to confirm our findings.

The Role of Epigenetics in the Pathogenesis and Potential Treatment of Attention Deficit Hyperactivity Disorder

There is increasing evidence that dysregulated epigenetic mechanisms of gene expression are involved in the pathogenesis of attention deficit hyperactivity disorder (ADHD). This review presents a comprehensive summary of the current state of research on the role of epigenetics in the pathogenesis of ADHD. The potential role of epigenetic drugs in the treatment of ADHD is also reviewed. Several studies suggest that there are epigenetic abnormalities in preclinical models of ADHD and in ADHD patients. Regarding DNA methylation, many studies have reported DNA hypermethylation. There is evidence that there is increased histone deacetylation in ADHD patients. Abnormalities in the expression of microRNAs (miRNAs) in ADHD patients have also been found. Some currently used drugs for treating ADHD, in addition to their more well-established mechanisms of action, have been shown to alter epigenetic mechanisms of gene expression. Clinical trials of epigenetic drugs in patients with ADHD report favorable results. These data suggest that abnormal epigenetic mechanisms of gene expression may be involved in the pathogenesis of ADHD. Drugs acting on epigenetic mechanisms may be a potential new class of drugs for treating ADHD.

Training a machine learning classifier to identify ADHD based on real-world clinical data from medical records

The diagnostic process of attention deficit hyperactivity disorder (ADHD) is complex and relies on criteria sensitive to subjective biases. This may cause significant delays in appropriate treatment initiation. An automated analysis relying on subjective and objective measures might not only simplify the diagnostic process and reduce the time to diagnosis, but also improve reproducibility. While recent machine learning studies have succeeded at distinguishing ADHD from healthy controls, the clinical process requires differentiating among other or multiple psychiatric conditions. We trained a linear support vector machine (SVM) classifier to detect participants with ADHD in a population showing a broad spectrum of psychiatric conditions using anonymized data from clinical records (N = 299 participants). We differentiated children and adolescents with ADHD from those not having the condition with an accuracy of 66.1%. SVM using single features showed slight differences between features and overlapping standard deviations of the achieved accuracies. An automated feature selection achieved the best performance using a combination 19 features. Real-world clinical data from medical records can be used to automatically identify individuals with ADHD among help-seeking individuals using machine learning. The relevant diagnostic information can be reduced using an automated feature selection without loss of performance. A broad combination of symptoms across different domains, rather than specific domains, seems to indicate an ADHD diagnosis.

MicroRNAs serve as prediction and treatment-response biomarkers of attention-deficit/hyperactivity disorder and promote the differentiation of neuronal cells by repressing the apoptosis pathway

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable neurodevelopmental disorder. This study aimed to examine whether miRNA expression abundance in total white blood cells (WBCs) facilitated the identification of ADHD and reflected its response to treatment. Furthermore, whether miRNA markers facilitated the growth of the human cortical neuronal (HCN-2) cells was also investigated. Total WBC samples were collected from 145 patients and 83 controls, followed by RNA extraction and qPCR assays. Subsequently, WBC samples were also collected at the endpoint from ADHD patients who had undergone 12 months of methylphenidate treatment. The determined ΔCt values of 12 miRNAs were applied to develop an ADHD prediction model and to estimate the correlation with treatment response. The prediction model applying the ΔCt values of 12 examined miRNAs (using machine learning algorithm) demonstrated good validity in discriminating ADHD patients from controls (sensitivity: 96%; specificity: 94.2%). Among the 92 ADHD patients completing the 12-month follow-up, miR-140-3p, miR-27a-3p, miR-486-5p, and miR-151-5p showed differential trends of ΔCt values between treatment responders and non-responders. In addition, the in vitro cell model revealed that miR-140-3p and miR-126-5p promoted the differentiation of HCN-2 cells by enhancing the length of neurons and the number of junctions. Microarray and flow cytometry assays confirmed that this promotion was achieved by repressing apoptosis and/or necrosis. The findings of this study suggest that the expression levels of miRNAs have the potential to serve as both diagnostic and therapeutic biomarkers for ADHD. The possible biological mechanisms of these biomarker miRNAs in ADHD pathophysiology were also clarified.