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CACNA1C is one of the most universally reproduced risk genes for neuropsychiatric disorders including bipolar disorder, schizophrenia, and major depressive disorder. CACNA1C is also recently identified in the largest human genome-wide association study to date as one of only two genes presenting a common risk factor across five major forms of neuropsychiatric illness: major depression, bipolar disorder, autism, schizophrenia, and attention deficit hyperactivity disorder.
Figure 1. Calcium channel
Many neuropsychiatric disorders are highly heritable, indicating that genetics is certain to have a role in pathogenesis. These epidemiological data are supported by the more recent findings that specific genetic risk variants cross the diagnostic boundaries as well. While there is a credible prediction that identifying susceptibility genes will eventually lead to targeted ”cure therapeutics”, there first remains a critical demand to definitively identify such genes and to subsequently understand functional consequences of the associated genetic variations (Figure 2). However, identifying underlying susceptibility genes that are associated with psychiatric disorders in multiple populations has proven to be difficult due to a number of factors including less than adequate sample sizes, the polygenetic and multifactorial nature of the disorders, uncertainty or lack of precision in the diagnoses, and significant overlap both in terms of symptoms and likely pathophysiology.
Figure 2. Psychiatric disorders and possible novel treatments.
CACNA1C in mental disorders
CACNA1C codes for the pore-forming α1C subunit of L-type voltage-gated calcium channel (LTCC), referred to as Cav1.2. Cav1.2 couples transient activation of inward calcium current to transcriptional regulation and plays a significant role in dendritic development, neuronal survival, memory formation, synaptic plasticity, behavior and learning.
It is evident that functional mutations in the CACNA1C gene predispose to autism spectrum disorders. A rare human autosomal-dominant multisystem disorder, Timothy Syndrome, presenting with webbed digits, arrhythmias, congenital heart disease, and autism spectrum symptoms is associated with a mutation in the exon 8A splice variant of CACNA1C that comprises about 23% of Cav1.2 in heart and brain. This mutation greatly reduces voltage-dependent inactivation of the calcium channel with little effect on calcium-dependent inactivation, and thus may cause a pathogenic calcium overload in neuronal and cardiac cells. An analogous mutation in alternative exon 8, G402S and a corresponding reduction in voltage-dependent inactivation are observed in Cav1.2 of an individual with a severe Timothy Syndrome variant. Taken together, these results show a pathogenic effect of naturally occurring gain-of-function mutations probably affecting Cav1.2 molecular determinant of slow inactivation.
CACNA1C is a common risk factor for which genetic evidence indicates that the identified intronic SNPs have a vital correlation with pathological behaviors. The associated genetic changes in CACNA1C likely act through a common developmental and/or physiological process of central nervous system function in a manner which is common across current diagnostic categories and at a level upstream from symptoms. But the mechanisms underlying how genetic changes in CACNA1C modify risk for developing psychiatric disorders are still unclear. In fact, defining the implications of CACNA1C genetic change in a context relevant to psychiatric disorder pathophysiology is essential.
Effects of CACNA1C on human brain function
Supporting the rodent data implicating Cav1.2 in basic brain function, a number of recent reports have associated a primary disease-associated SNP in CACNA1C, rs1006737, with variation in human brain structure and function in subjects who have no diagnosable psychiatric illness. While polymorphisms in CACNA1C are significantly associated with bipolar disorder, schizophrenia and depression, such genetic changes only increase probability of disease, and are not deterministic. Thus, a large percentage of the population who do not have disease also carry disease associated SNPs in CACNA1C, which allows for studies to dissect the influences of SNPs on brain function and structure.
A large number of functional neuroimaging studies have revealed the relation between rs1006737 genotype and brain activity in healthy controls while undertaking specific tasks. To date these findings include changes in brain activity while performing an attention network task, amygdala activity during emotional processing, bilateral hippocampal activation during episodic memory recall and diminished functional coupling between left and right hippocampal regions, increased activation of the left inferior frontal gyrus as well as the left precuneus during a separate semantic verbal fluency task, mediotemporal emotional processing and prefrontocortical working memory processing, and reduced corticolimbic/frontotemporal functional connectivity during emotional face-processing. And a study has also found that this same SNP is associated with significant modulation of effective connectivity from medial frontal gyrus to left putamen significantly reduced during perception of fearful faces, but that the effect is primarily in BPD subjects.
L-type calcium channel antagonists in the treatment of psychiatric disease
LTCC antagonists are mainly used clinically for treating high blood pressure, angina, and abnormal heart rhythms. They are represented by three different structural classes in chemistry: dihydropyridines, phenylalkylamines and benzothiazepines, which all act by binding to different sites on Cav1.2 and blocking the calcium current. LTCC antagonists within the same structural class may vary in their affinity for the same calcium channel subtype, as well as for their penetration of the blood-brain barrier. LTCC antagonists have been assessed to treat bipolar disorder and unipolar depression. However, studies have yielded mixed results suggesting response in only a subset of patients or that these treatments have not yet been optimized. Most of the early studies are performed with the phenylalkamine verapamil, which has low blood-brain barrier permeability. More recent double-blind placebo studies with dihydropyridines including nimodipine and nicardipine that more efficiently cross blood-brain barrier have suggested that this class of calcium channel medication has efficacy as an adjunctive antidepressant treatment in combination with electroconvulsive therapy or selective serotonin reuptake inhibitors (SSRIs). However, LTCC antagonists have not been assessed for efficacy in the treatment of schizophrenia. More extensive investigation with LTCC antagonists that readily cross the blood brain barrier are clearly necessary to define specific profile of the class of medications in bipolar disorder, as well as depression and schizophrenia.
In the current study, researchers use induced human neurons to reveal a functional phenotype associated with this psychiatric risk variant. The studies demonstrate that the risk genotype at rs1006737 is associated with significant functional alterations in human iNs, and may direct future efforts at developing novel therapeutics for the treatment of psychiatric disease. The use of a renewable source of patient and unaffected human neurons to demonstrate a specific neuronal function is altered by a psychiatric risk haplotype is a milestone in the field of psychiatric biology. Several pharmacological studies have implicated the L-type VGCC in psychiatric disease. L-type calcium currents are downregulated in the mouse brain in response to antipsychotics, and calcium channel antagonists, such as verapamil and isradipine, have been used therapeutically in bipolar disease treatment. Moreover, recent studies have shown an impact of the rs1006737 CACNA1C haplotype in task-based human behaviors, such as spatial working memory and attention, as well as on morphological changes such as grey matter volume of specific regions of human brain. The current study suggests a potential strategy with which to study these psychiatric disorders at the cellular and molecular level, a strategy that may help prioritize experiments for the downstream identification and validation of psychiatric risk genes, and impact the development of novel therapeutic approaches for these common disorders.
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