Schizophrenia (SZ), which is characterized by positive symptoms, negative symptoms, and cognitive deficits, is a devastating psychiatric illness (Tandon et al., 2013). The estimated heritability for SZ is 65–80% (Hilker et al., 2018). Although the impact of genetic factors on brain function in SZ has repeatedly been reported (Romme et al., 2017), the neurobiological mechanisms of dopamine risk genotypes remain unclear.
Resting-state brain activity evaluated by the blood oxygen level-dependent (BOLD) signal could reflect the intrinsic characteristics of brain fluctuations. Intrinsic brain activity is crucial for understanding the neuropathology and neurophysiology of mental disease; for example, abnormal energy consumption in a region may suggest decreased or excessive resting-state metabolic rates or concentrations of neuromodulators (Fox and Raichle, 2007). The amplitude of low frequency fluctuation (ALFF) is an established method for characterizing spontaneous neural activity and effectively exploring the potential pathophysiological mechanisms of psychiatric disorders (Yang et al., 2007). Recently, ALFF has been conducted to investigate abnormal neural activity in first-episode drug-naïve schizophrenia (FES), reporting increased ALFF in the middle temporal gyrus and precuneus regions, as well as decreased ALFF in the precentral gyrus and cerebellar regions (Guo et al., 2018; Wu et al., 2018). However, previous studies rely on the implicit assumption that brain activity remains stationary during typical resting functional magnetic resonance imaging (fMRI) scanning. This assumption may neglect the dynamic and time-varying changes in brain activity. An accumulating number of studies have reported that the human brain inherently rapidly changes neural interactions over time with nonstationary brain activity (Buzsáki and Freeman, 2015; Hutchison et al., 2013). In fact, human neural activity is highly dynamic over time (Calhoun et al., 2014; Li et al., 2019). Therefore, static or time-averaged ALFF may provide limited information on the functional activity of the brain in association with the pathophysiology of psychiatric disease (Calhoun et al., 2014). Combining the ALFF with ‘sliding-window’ methods, the dynamic ALFF (dALFF) provides a new avenue to measure the temporal variability of intrinsic brain activity. Several studies have reported that dALFF is a more stable and sensitive indicator than static ALFF (Ma et al., 2020; Tang et al., 2018). Cui et al. discovered that compared with static ALFF abnormalities, dALFF contributes more than static ALFF in differentiating between generalized anxiety disorder and healthy controls (HCs) (Cui et al., 2020). The conduct of dALFF will probably provide a more accurate assessment of brain activity, enabling us to more cleanly capture information related to the disease.
Considering the high heritability of SZ, a considerable effort has been made to discover the causative genetic factors, candidate gene studies have been a major approach in this area. Selecting candidate genes based on prevailing theories of the etiology in SZ, such as the dopamine hypothesis or/and antipsychotic pharmacology, is an important strategy in identifying genetic variation. The dopamine hypothesis has been one of the most enduring ideas in SZ. This hypothesis has one major implication for clinical treatment methods (Howes and Kapur, 2009). Existing first and second generation antipsychotic drugs for SZ act via the dopamine system (Horacek et al., 2006). Therefore, dopamine-related genes have traditionally been prime candidates for genetic studies of SZ. Because the list of genes relating to dopaminergic function is potentially long, we involved five frequently studied genes, including catechol-O-methyltransferase (COMT), dopamine receptor D1 (DRD1), dopamine receptor D2 (DRD2), dopamine receptor D3 (DRD3), and ankyrin repeat and kinase domain containing 1 (ANKK1) genes. COMT is involved in the catabolic clearance of dopamine. Two meta-analyses report an association between rs4680 polymorphism and SZ, with the G allele may be a reliable risk factor for SZ (Glatt et al., 2003; Lohmueller et al., 2003). Recent association studies have investigated several other single nucleotide polymorphism (SNPs) of the COMT gene. A large sample study revealed a highly significant association between rs165599 and rs737865 and SZ, with SZ displaying an excess of G/G genotype in these two SNPs (Shifman et al., 2002). Wang et.al reported the rs4633 T allele was associated with susceptibility to SZ (Wang et al., 2009). Zhu et.al found the T allele of rs686 of DRD1 gene was associated with a higher risk of SZ (Zhu et al., 2011). The DRD2 is a logical target for association studies because of the effect of therapeutic agents. One large case-control study detected a significant association between the G allele of rs6277 and SZ (Betcheva et al., 2009), a result that has been confirmed by a meta-analysis including 12 articles involving 3079 SZ and 3851 HCs (Liu et al., 2014). The association between the A allele of rs6275(Lawford et al., 2016) and the A allele of rs1076560 (Cohen et al., 2016) with SZ was also reported. Numerous studies have sought association at DRD3, and most have focused on rs6280. Lochman et.al reported the T allele of rs6280 is likely to be a risk factor for SZ (Lochman et al., 2013). A case-control study, followed by a 108 trios family-based association analysis for replication, reported an association between the ANKK1 rs1800497 G allele and SZ (Dubertret et al., 2010).
Dopamine is an important neuromodulator that influences oscillatory information processing (Zaldivar et al., 2018). Dysregulated dopaminergic modulation of brain activity is fundamental to many studies that attempt to explain the mechanisms underlying the clinical symptoms of SZ (Howes and Kapur, 2009; Kraguljac et al., 2021). Multiple imaging genetic studies have observed an association between single-nucleotide polymorphisms (SNPs) of dopamine candidate genes and functionally or structurally derived brain imaging phenotypes. Vink et al. found that compared with noncarriers of DRD2 rs2514218 in SZ, risk genotype carriers exhibit a diminished striatal response to increasing proactive inhibitory control demands (Vink et al., 2016). Vercammen et al. found that an increased number of risk alleles of COMT rs4680 and DRD2 rs2283265 predict decreased prefrontal activation during response inhibition in healthy adults (Vercammen et al., 2014). This study suggested a significant genotype effect of dopamine risk alleles on prefrontal activation in healthy subjects. SZ has a polygenic architecture in which hundreds or even thousands of risk genotypes collectively contribute to risk (Foley et al., 2017). The collective effects of the dopamine SNP should result in greater statistical sensitivity than that of studies using a single locus (Arslan, 2018). Thus, there remains a dearth of knowledge about whether and how dopamine-related risk genetic variations combine to affect dALFF in FES.
In the present study, we aimed to determine the extent to which ten dopamine-related SNPs from the COMT, DRD1, DRD2, DRD3 and ANKK1 genes combine to influence dALFF in FES patients and HCs. New evidence regarding the action of the dopamine gene on brain activity will help us elucidate the pathological mechanisms underlying SZ to facilitate more effective therapeutic development.