ADHD is a common neurodevelopmental disorder, which interferes with an individual’s cognitive and language performance. Understanding the etiology and pathophysiology of this disorder, in addition to its possible influence on the aptitudes of children with ADHD, could offer attainable therapeutic targets for better intervention. It is necessary to explore patterns of expression of miRNAs in different populations, considering the complex nature of ADHD and the interaction between genes and environment regarding the development of its symptoms (Nuzziello et al. 2019).

In this study, memory abilities and knowledge were found to be the least developed among participants. These results indicated the disadvantages of this disorder for the cognitive abilities of children. These findings are in agreement with Kilany et al. (2022), who reported that working memory scores were the lowest among the IQ subitems. Nevertheless, it was the visuospatial ability, not the knowledge, which also revealed low scores in their sample. The memory problems could be related to the altered levels of BDNF, which was involved in memory consolidation. BDNF was reported to enhance neuronal growth and synaptic plasticity (Miranda et al. 2019). It has been involved in pre- and post-synaptic control of neurotransmitter release and regulation such as serotonin, glutamate, GABA, dopamine, and catecholamines (Colucci-D’Amato et al. 2020; Lima Giacobbo et al. 2019; Wang et al. 2022a). MiR-34 plays a critical role in memory formation, which is processed by the amygdala (Murphy and Singewald 2019). MiR-138 was found to control neuronal connections in the hippocampus in mice. The hippocampus has an essential role in memory consolidation (Daswani et al. 2022).

Receptive and expressive language skills in the current study were found to be delayed in children younger than 8 years old. The absence of discrepancies between receptive and expressive language ages in those with delayed language performance indicated that the language problems in these children were delays in development rather than a specific language impairment. The presence of language delays is in agreement with Hawkins et al. (2016), who reported that impairments in cognitive functions such as executive function interfere with language development in children with ADHD. On the other hand, basic language skills measured by the used language scale in children older than 8 years, including syntax and semantics, were not delayed. Goh et al. (2020) emphasized the relationship between early language problems and ADHD, which could be related to problems in some cognitive abilities in these children.

Considering that language and cognition are interrelated, alterations in essential neurobiological processes could impact both of them. The elevated levels of BDNF noticed in the ADHD participants of this study could have contributed to the deficits noticed in the abilities and behavior of participants with ADHD. High levels of BDNF could have led to deficits in the pruning of synapses responsible for the processing of memory and executive functions or to changes in the neurotransmitters’ levels, which were interrelated with BDNF and/or influenced by its action. Changes in synaptic quality influence connectivity between different brain areas involved in cognitive, language, and behavioral development (Liao et al. 2023). Increased interhemispheric connectivity was associated with reduced connectivity in the prefrontal cortex bilaterally and the right frontostriatal and frontoparietal neural networks (Chen et al. 2023). Furthermore, Yeom et al. (2016) reported that high BDNF levels had a negative influence on IQ and on indicators of behavioral problems in preschool children. The behavioral and cognitive problems in children with ADHD could also be related to changes in the levels of serotonin, glutamate, GABA, and dopamine, which were reported to be reduced in individuals with ADHD in previous reports, whereas levels of catecholamines were reported to be elevated. These neurotransmitters are essential for proper brain functioning (Banerjee and Nandagopal 2015; Dvořáková et al. 2007; Maltezos et al. 2014).

Heinrich et al. (2017) and Srivastav et al. (2018) depicted the regulatory role of miRNAs on BDNF gene expression. The downregulation of miR-138-1 could have led to overexpression of BDNF, increasing its level compared to controls. In the current study, we reported higher level of plasma BDNF in children with ADHD compared to their neurotypical peers with significant statistical difference. This finding aligned with previous studies, which targeted Egyptian children (El Ghamry et al. 2021) or other ethnic cohorts with ADHD (Gumus et al. 2022; Shim et al. 2008).

The reduced expression of miR-138-1 was in agreement with Wu et al. (2017), who reported downregulation of miR-138-1 in the prefrontal cortex of animal models with ADHD. They suggested that this downregulation was related to abnormalities in the miR-138-1 gene. This gene has been involved in visual perception processing. Interestingly, they discovered that the overexpression of the Nr3c1 gene was related to this miRNA downregulation. This gene has been linked to the hypothalamic-pituitary-adrenal axis and low cortisol levels, which were involved in ADHD development (Chen et al. 2019). The prefrontal cortex was among the brain areas that were reported to be altered in individuals with ADHD. This area was involved in attention and executive function control and processing (Wu et al. 2023). Martinez and Peplow (2024) reported that miR-138-1 expression levels were correlated with IQ scores, which could indicate its involvement in cognitive functioning. In this study, no correlations were detected between miR-138-1 and clinical measures. On the other hand, miR-138 was found by Zadehbagheri et al. (2019) to be upregulated in Iranian children with ADHD, which disagrees with the results of the present study. These differences in expression patterns highlight the importance of investigating miRNAs expression levels in different populations and in different gender distributions.

Alterations in the expression patterns of miR-34-c, either over or under the normal expression, would lead to perturbations in the processes that it is involved in. Abnormalities in this type of miRNA have been linked to polymorphisms in genes that were previously implicated in ADHD, such as BCL2, MET, and CREB1 (Garcia-Martínez et al. 2016). These genes were related to neuron development, neurotransmission, axonal growth, and cellular projection, as well as to lipid biosynthesis and metabolism. These genes were involved in central nervous system derangements and psychiatric disorders, including ADHD. Furthermore, miR-34-c is an essential component of the calcium-triggering mechanism, which aids neurotransmitter release in ADHD (Kim et al. 2014). The results of the downregulation of miR-34-c disagree with Garcia-Martínez et al. (2016), who investigated the level of miRNAs in a Spanish population and reported upregulation of this type of miRNA. Their study was conducted on adults with ADHD, and the males’ distribution was less than this study, which could also contribute to disagreement besides the population heterogeneity.

Correlation analysis in this study revealed that miR-34-c levels were negatively correlated with miR-138-1 levels. The detected correlation could stem from the opposing actions of these two miRNAs. Neuronal development is induced by miR-34, whereas apoptosis is induced by miR-138, which also inhibits proliferation. Despite having opposing actions, these two miRNAs were reported to be involved in memory processing. The importance of the proliferation of neurons and dendrites is as essential as their apoptosis. A proper balance between long-term potentiation and long-term depression is required for the optimal functioning of neurons and other brain cells. This balance contributes to memory processing and the proper development of other cognitive functions, including language integration, in developing brains (Stacho and Manahan-Vaughan 2022). The correlation results detected in the ADHD group were not identified in the control group.

The absence of significant differences in expression levels of miR-155 and miR-296-5p disagrees with Kandemir et al. (2014) and Wu et al. (2017), who reported significant differences between individuals with ADHD and neurotypicals in the form of upregulation of miR-155 and downregulation of miR-296-5p. This disagreement could be related to differences in the populations included, who were Chinese and Turkish. Nonetheless, a significant negative correlation was detected in the present study between miR-296-5p and the total IQ. MiR-296-5p was reported to regulate BDNF activity via Bhlhb2 gene suppression. This gene was reported to be involved in ADHD pathogenesis (Wu et al. 2017). It is worth noting that the expression of miRNAs is regulated by various factors, such as miRNA biogenesis, transcription factors, DNA copy number, and DNA methylation (Misiewicz-Krzeminska et al. 2019). This could explain the absence of correlations between the targeted miRNAs and other clinical measures in this study.

Some differences between males and females regarding the features of ADHD were reported. Females with ADHD exhibited more inattention problems compared to males (Graetz et al. 2005). However, few studies investigated the biochemical and miRNA levels using gender-based analysis. Szakats et al. (2023) explored the miRNA expression differences in males and females in animals. They detected gender differences in the patterns of expression in seven out of ten types of miRNAs they investigated, such as miR-206-3p, miR-200c-3p, and miR-205-5p. Although these types were not investigated in this study, the detected gender differences indicated the influence of biological sex on the expression of some types of miRNAs. Interestingly, the same pattern was noticed in the expression levels of the miRNAs and BDNF levels. Notwithstanding, the expression of miR-155 was found to be upregulated in females only with significant statistical differences. This is in agreement with Kandemir et al. (2014), who reported upregulation of this miRNA. However, they did not examine gender-based differences in their study. Wang et al. (2019) investigated the BDNF levels in children with ADHD, followed by a stratified gender analysis of these levels. The overall levels of BDNF did not differ from the control group. Nevertheless, the males with ADHD showed higher BDNF levels compared to controls, whereas the females showed lower BDNF levels. This is partially in agreement with this study considering the higher levels of BDNF detected in males and females with ADHD compared to controls, but they showed a significant statistical difference only in males. The differences between males and females could be attributed to hormonal differences, differences in polymorphisms of the targeted genes, or differences in the mRNA levels that miRNAs are controlling (Szakats et al. 2023; Wang et al. 2019). These findings highlight the importance of investigating gender differences in neurodevelopmental disorders.

The small sample size in this study could be considered a limitation. The stringent inclusion and exclusion criteria could have contributed to reducing the number of participants. Furthermore, the possible environmental factors, such as prenatal and perinatal histories, have not been compared between the groups. The participants were recruited from one clinic. However, it is a referral center for children with special needs, and it is frequently visited by participants from all Egyptian governorates. Despite these limitations, the inclusion and exclusion criteria were determined to eliminate factors that could influence the miRNA expression patterns. Moreover, this study is the first to investigate the miRNA expression patterns in Egyptian children with ADHD. The gender difference influenced the expression patterns in children with ADHD despite the low number of female participants in the ADHD group, which necessitates investigating this issue with a larger sample size in future studies. The expression levels of miR-34c-3p and miR-138-1, in addition to plasma BDNF concentrations, showed good discrimination accuracy between subjects with and without ADHD. Therefore, miR-34c-3p, miR-138-1, and BDNF can be candidate diagnostic biomarkers for ADHD.