Relationship Between Suicidal Behavior And Genes Related With GABA-Glutamate System And Glucocorticoid Receptor

Suicide is a major public health concern worldwide.Each year,about 30,000 individuals die by suicide in the US and about one million die by suicide in the world.Suicide is a global public health problem.Suicide is an autonomous,complex behavioral disorders for the purpose of death,and its prediction,assessment,treatment and prevention are problems of the whole society.The main risk factor for suicide is the presence of mental illness,including substance abuse,as it is estimated that 90%of suicides are associated with some form of psychiatric disorder.About 60%of all suicides associated with mental illness occur in patients with mood disorders,and the lifetime risk for suicide in major depressive disorders(MDD)and bipolar illness may be similar.Although the presence of mental disorders is a major risk factor,suicidal behavior is multifactorial,and many other risk factors,such as psychosocial factors,are involved in suicide.Although a large proportion of suicides occur in those subjects who suffer from mood disorders,only a small number of these subjects attempt suicide and an even smaller number commit suicide.This suggests that,while the presence of mental illness is a major risk factor for suicide,other factors also play a role in suicide.Psychiatrists believe that suicide is not a symptom of mental illness,but an independent trait,when people with this trait suffer from certain psychiatric diseases or a major stimulus,then suicidal behavior will appear.One cannot always ask individuals if they are suicidal,as desire not to be stopped or future impulsive changes of mind may make their selfreport of feelinGLUL,thoughts and plans to be unreliable.Therefore,we should combine intrinsic,subjective assessment scales and external,objective biological testing together to get an accurate assessment of suicidal behavior.Currently,many studies verified that suicidal behavior was related with neurobiology changes of many system in the peripheral blood,saliva and brain tissue to a plurality of and the system,which included all aspects of genetic polymorphisms,gene expression and epigenetic mechanisms.The most studied is the serotonin system,serotonin and its signaling level changed in suicide population,and it has a unique genotype and expression pattern.Overall,although a lot of research has been made,given that the target organ biopsy can not be obtained,lack of a useful animal model and constraints of ethical and other aspects,the molecular mechanisms of suicidal behavior is still unclear.Recent years,the role of γ-aminobutyric acid(γ-aminobutyric acid,GABA)and glutamate system in the molecular mechanisms of suicidal behavior was gradually taken seriously,as the glutamate pathway targeting druGLUL(eg ketamine)is expected to treat suicidal ideation.Gamma-amino butyric acid(GABA)and glutamate are the major inhibitory and excitatory neurotransmitters in the mammalian central nervous system,respectively.In biochemistry,the glutamate-glutamine cycle is a sequence of events by which an adequate supply of the neurotransmitter glutamate is maintained in the central nervous system.Neurons are not able to perform new synthesis of the neurotransmitter glutamate andγ-aminobutyric acid(GABA)from glucose.The glutamate/GABA-glutamine cycle is a metabolic pathway that describes the release of glutamate or GABA from neurons which are then taken up into astrocytes(star shaped glial cells).In return,astrocytes release glutamine to be taken up into neurons for use as a precursor to the synthesis of glutamate or GABA.In neurons,glutamine is not only used for energy production and protein synthesis,as in other cells,but is also an essential precursor for biosynthesis of amino acid neurotransmitters.Many previous studies have suggested,GABA-glutamate system and suicidal behavior are closely linked.GABA neurons density decreased in auditory cortex in Schizophrenia patients with suicidal behavior.The majority of the GABA-glutamate system is associated with suicidal behavior in some form.In addition,almost all candidate endophenotypes of suicidal behavior(impulsive-agression trait,neurocognitive impairment,alteration in cortisol under stress,etc.)are closely related with the GABA-glutamate system.Many studies have shown that impulsivity and aggression traits are related with GABA levels in specific areas of the brain;and many studies have found that,GABA system dysfunction can lead to neurocognitive impairment including working memory,decision-making ability and response suppression;A series of studies have shown that,GABA can regulate HPA axis system and change individual’s response under stress.Hence,GABA-glutamate system related genes may play important roles in the pathological mechanisms of suicidal behavior.Stress response systems and suicidal behavior are closely linked,and researchers found that HPA axis related genes were related with suicidal behavior.Stress leads to the secretion of catecholamines and glucocorticoids,and the latter activate glucocorticoid or mineralocorticoid receptors(GR or MR).GR,also named NR3C1(nuclear receptor subfamily 3 group C member 1),typically found in the cytoplasm,but upon ligand binding,is transported into the nucleus.It is involved in inflammatory responses,cellular proliferation,and differentiation in target tissues.GR activation plays a variety of roles,including rapid transcriptional regulation,the molecule activator of hypothalamus-pituitary-adrenal(HPA)axis,and mediating negative feedback regulation of the HPA axis when cortisol levels are high as in response to stress.GR is regulated by many regulatory factors,wherein the single nucleotide polymorphism and molecular chaperones are two important factors.Researches show that themechanisms how the single nucleotide polymorphism of glucocorticoid receptor gene NR3C1 and molecular chaperones FKBP5 influence GR’s function include altering the expression of GR,the binding force between GR and glucocorticoids,GR’s activity to turn to nuclear,the binding force between GR and the DNA of target gene,the ratio of glucocorticoid receptor subtype a and glucocorticoid receptor subtype β,etc.,thus affecting the HPA axis function,neural plasticity and depressive behavior.FKBP5(FK506 binding protein 51),a gene encoding the 51kDa protein FKBP5,located on human chromosome 6(6p21.31),is an important negative regulator of GR activity.It is a co-chaperone of hsp90,which regulates GR sensitivity.Over-expression of FKBP5 can reduce the affinity of cortisol for GR,and impair the nuclear translocation of GR.Activation of GR increases FKBP5 expression and protein as part of a feedback loop.Furthermore,FKBP5 can further reduce GR signal by promoting the nuclear translocation of inactive β-isomer of GR.Overall,FKBP5 is an important gene regulator of stress response and can inhibit GR activity.SKA2(spindle and KT associated 2),also known as FAM33A(family with sequence similarity 33,member A),located on 17q22,is a recently described member of the SKA complex,which is essential for proper chromosomal segregation.Overexpression of SKA2 resulted in modest enhancement of GR transactivation,while knockdown of SKA2 markedly inhibits GR transactivation.This supports a functional interaction between the two proteins.There is a close relationship between suicidal behavior and stress,and FKBP5,SKA2 and NR3C1 are important regulators in stress response system,therefore they may also play important roles in the pathological mechanisms of suicidal behavior.In the present study,we made a preliminary analysis to explore the relationship between the genes in GABA-glutamate systems and glucocorticoid receptor system and suicidal behavior,screened out the genes with most parallel evidences related to suicidal behavior,and intends to do in-depth analysis of these biomarkers in further studies.Objectives:To examine the relationship between genotype,brain transcriptome and MDD/suicide for 24 genes involved in GABAergic and glutamatergic signaling and three genes that related with glucocorticoid receptor:FK506 binding protein 5(FKBP5),Spindle and kinetochore-associated protein 2(SKA2)and Glucocorticoid Receptor(NR3C1)in live subjects and postmortem samples,to find the most independent evidence gene,and to screen biomarkers of suicidal behavior.Methods:In part 1 of the study,119 candidate SNPs in 24 GABA-glutamate system related genes(four transporters,four enzymes and sixteen receptors)and 3 glucocorticoid receptor related genes(FKBP5,SKA2 and NR3C1)were tested for associations with MDD and suicidal behavior in 276 live participants(86 nonfatal suicide attempters with MDD and 190 non-attempters of whom 70%had MDD)and 209 postmortem cases(121 suicide deaths of whom 62%had MDD and 88 sudden death from other causes of whom 11%had MDD)using logistic regression adjusting for sex and age.In part 2,RNA-seq was used to assay isoform-level expression in dorsolateral prefrontal cortex of 59 postmortem samples(21 with MDD and suicide,9 MDD without suicide,and 29 sudden death non-suicides and noiv psychiatric illness)using robust regression adjusting for sex,age and RIN score.In part 3,SNPs with subthreshold(uncorrected)significance levels below 0.05 for an association with suicidal behavior and/or MDD in part 1 were tested for eQTL effects in prefrontal cortex using the Brain eQTL Almanac(www.braineac.org).Results:Among GABA-glutamated related genes,No SNPs or transcripts were significant after adjustment for multiple comparisons.However,And for glucocorticoid receptor related genes,Six SNPs on this gene,three SNPs on SKA2 and one near NR3C1 showed before-adjustment association with attempted suicide,and two SNPs of SKA2 with suicide death,but none stayed significant after adjustment for multiple testing.Only the SKA2 SNPs were related to expression in the prefrontal cortex.1.In the GABA-glutamate system and glucocorticoid receptor system,8 polymorphisms of 7 genes were associated with suicide death:rs6447520(GABRA4),rs2808536(GABBR2),rs424740(GABRG2),rs6849345(GRIA2),rs4780886(GRIN2A),rs1062246(SLC6A1),rs8082544(SKA2)and rs7502947(SKA2).2.In the GABA-glutamate system and glucocorticoid receptor system,15 polymorphisms of 8 genes were associated with suicide attempts:rs6447520(GABRA4),rs10836356(SLC1A2),rs1529461(SLC1A3),rs13035504(GLS),rs2236418(GAD2),rs9296158(FKBP5),rs3777747(FKBP5),rs4713902(FKBP5),rs7757037(FKBP5),rs737054(FKBP5),rs12945875(SKA2),rs8067682(SKA2),rs9380529(FKBP5),rs9911583(SKA2)and rs9324924(NR3C1).3.Because of the limited sample size,this study failed to use mediation analysis to prove the existence of eQTL.However,the expression of GABRG2 gene was decreased(P = 0.01)and the genotype AA of its SNP rs424740was associated with suicide death(P = 0.03),while the gene expression of the prefrontal cortex of AA genotype carriers was decreased(p = 0.016,0.04).This conclusion strongly suggests that rs424740 is a potential eQTL and also reflects the relationship between GABRG2 gene and suicidal behavior,and it is necessary to study further in larger samples.4.The expression of the RNA transcript of GABAA receptor subtypes GABRB1-001,GABRB3-006,GABRG2-003 in the suicide death group was lower than that in the control group.5.FKBP5 haplotype(structure:rs3800373,rs7757037;frequency = 47%)was correlated with increased risk of suicide attempts(OR = 1.58,T = 6.03,p value = 0.014).Individual who carries haplotype AA has higher suicide attempt risk.6.One mRNA transcription ENST00000394466(NR3C1-006)of NR3C1 had lower expression in suicide death(B =-0.48,SE = 0.12,T =-4.02,P value = 0.004after adjustment for multiple testing).Conclusion:The present study found a number of independent evidence in the association between GABA-glutamate and glucocorticoid receptor related genes and suicidal behavior,confirmed the important roles of GABA-glutamate systems and glucocorticoid receptor system in suicidal behavior.These preliminary results implicated GABRG2 in suicide,and identified an association of FKBP5 haplotype with risk of suicide attempt and found an association between suicide and altered NR3C1 gene expression in the prefrontal cortex.Our finding further implicated hypothalamic pituitary axis dysfunction in suicidal behavior and warrant further investigation and replication in larger samples.