the converge consortium has obtained a large number of samples ( including 5,303 cases of mdd and 5,337 controls , all chinese subjects ) for inclusion in genome - wide association studies ( gwas ) .
snp rs35936514 in the lhpp gene has been reported to be significantly associated with mdd at a genome - wide level .
lhpp encodes an enzyme known as phospholysine phosphohistidine inorganic pyrophosphate phosphatase ( lhpp ) , which was originally purified from swine brain tissue   and shows high levels of expression in the brain [ 3 , 4 ] .
this region of the genome has been implicated in the etiology of mdd using a combination of linkage and association analysis .
lhpp or a product of a collinear brain - specific transcript , therefore , may interact with htr1a in the pathogenesis of major depression .
a recent study has extended the findings of previous literature by supporting the role of lhpp in risk for mdd .
a single study has shown the expression of lhppase to be associated with thyroid function , which could be interesting given that thyroid disorder is thought to mediate the functional regulation of mdd patients . a link between thyroid function and depression
any impairment of thyroid function supply to the developing cns causes severe changes to the overall architecture and function of human brain , leading to various neurological dysfunctions [ 912 ] .
recently , genetic imaging has been used to investigate the association of genetic variation with neuroimaging endophenotypes .
genetic imaging is an emerging field that is rapidly identifying genes that influence the brain , cognition , or risk for diseases . along with clinical diagnosis ,
it has fostered new enthusiasm in depression research , because this approach allows for assessing the neural impact of candidate genes in vivo and thus provides for a new level of evidence [ 17 , 18 ] .
resting - state functional magnetic resonance imaging ( rs - fmri ) is often used to examine mental disorders .
the amplitude of low - frequency fluctuations ( alff ) is thought to detect the intensity of spontaneous neural activity at rest .
it has been considered a reliable and sensitive measure to study both healthy and clinical populations [ 2022 ] .
the effects of the neural system and their association with lhpp variation in the context of mdd are not yet understood .
identifying previously unknown physiological pathways can open new avenues for the development of novel depression drugs . in the present study
, the association of lhpp rs35936514 with the resting - state brain activity was measured using alff .
it is here hypothesized that variations in rs35936514 may influence spontaneous brain activity at rest in mdd patients .
45 mdd patients ( mean age 27.13  sd  12.65 years , range 1359 years , 71% female ) were recruited from the outpatients at the department of psychiatry , the first hospital of china medical university , and the mental health center of shenyang .
the diagnosis of mdd was confirmed by 2 trained psychiatrists using the structured clinical interview for dsm - iv disorders ( scid ) .
scores on the 17-item hamilton rating scale for depression ( hamd ) were obtained from each participant except for one , who did not complete these evaluations . here , 115 healthy control ( hc ) subjects ( mean age 33.96  sd  14.25 years , range 758 years , 57% female ) without any personal history of any axis i disorder or first - degree family member with a major mood or psychotic disorders were recruited through surrounding communities .
exclusion criteria for both groups included history of neurological disease , loss of consciousness 5  min , any medical condition that might affect neurovascular function including hypertension , current substance abuse , or dependence , and having contraindications for mri .
all participants provided written informed consent after receiving a complete description of the study as approved by the institutional review board of the china medical university .
subjects were further divided into two groups : a cc group homozygous for the c allele ( 23 mdd , 57 hc ; mean age = 34.06  14.45 years , 60% female ) and risk t - carrier group ( ct / tt genotypes ; = 22 mdd , 58 hc ; mean age = 30.02  13.57 years , 61% female ) .
genotype frequencies were consistent with hardy - weinberg equilibrium expectation ( mdd :  = 2.894 , p = 0.08 ; hc :  = 2.52 , p = 0.11 ) .
the mri data were acquired using a ge signa hdx 3.0 t mri scanner ( general electric , us ) in the department of radiology of the first hospital of china medical university .
soft pads and earplugs were used when scanning to restrict head motion and reduce scanner noise .
participants were asked to relax with their eyes closed but remain awake throughout the resting - state scan .
three - dimensional t1-weighted images were obtained using a fast spoiled gradient - echo ( fspgr ) sequence : repetition time ( tr ) = 7.1  ms , echo time ( te ) = 3.2  ms , field of view ( fov ) = 24  cm  24  cm , flip angle = 15 , matrix = 240  240 , slice thickness = 1  mm , and no gap .
functional images were acquired using a gradient echo planar imaging ( epi ) : tr = 2000  ms , te = 30  ms , fov = 24  cm  24  cm , flip angle = 90 , matrix = 64  64 , slice thickness = 3  mm , no gap , and slices = 35 .
the first 10 volumes of each subject were discarded to allow participants to adapt to the scanning environment .
the remaining data were preprocessed including slice timing , head motion correction , and spatial normalization to the standard montreal neurological institute ( mni ) space ( resampling to 3  3  3  mm ) .
subsequently , the images were spatially smoothed with a 6  mm full - width at half - maximum ( fwhm ) gaussian kernel .
participants with head motion less than 3.0  mm in any dimension or 3 rotation at any point during the course of the scan were included for further analysis .
subsequent data preprocessing included removal of linear trends and temporal filtering ( band pass , 0.010.08  hz ) to reduce the effects of low - frequency drift and high - frequency noise .
the square root was calculated at each frequency of the power spectrum and averaged square root was between 0.01 and 0.08  hz .
the alff of each voxel was divided by the global mean alff value within a brain mask .
resting - state fmri data analysis toolkit ( rest ) ( http://www.restfmri.net/ ) was performed for further data processing and alff analysis .
demographic data ( sex , age , education , and hamd ) were analyzed using chi - square tests and two - way analysis of variance ( anova ) with diagnostic group ( hc , mdd ) and genotype group ( cc , ct / tt ) as between subject factors . a two - sample t - test was used to compare the durations of illness across genotypes within the mdd group .
a voxelwise anova ( 2  2 anova : diagnosis  genotypes ) was used to determine the effects of diagnosis and genotype on alff values , and age and sex were considered as covariates .
post hoc t - test was used to explore the details of the main effects and interactions .
correction for multiple comparisons was based on monte carlo simulation [ alphasim , analysis of functional neuroimages ( afni ) , cluster size > 3321  mm ] .
pearson correlation analyses and spearman correlation analyses were performed to determine the correlation of alff values between regions showing significant differences with the hamd scores , education , and illness duration in the two diagnostic groups .
there was no significant effect of diagnosis , genotype , or interaction between diagnosis and genotype for age , sex , or education .
the effect of diagnosis in hamd was significant , with significantly higher hamd scores in the mdd group than in the hc group .
two - sample t - tests showed no difference in the duration of illness between the mdd subgroups ( table 1 ) .
the influence of diagnosis and genotypes on alff in the cc and ct / tt genetic subgroups in the mdd and control subjects is listed in table 2 . after performing a two - way anova on the alff maps ,
a significant main diagnosis group effect was observed ( f = 56.39 , p < 0.001 , corrected ) .
mdd patients showed significantly increased alff in the left middle temporal gyrus and left occipital cortex compared with individuals in hc group ( figure 1 ) .
there was also a significant main effect of genotype ( f = 31.80 , p < 0.001 , corrected ) .
the t - carrier group showed increased alff in the left superior temporal gyrus ( figure 2 ) .
significant diagnosis by genotype interaction was noted ( f = 22.24 , p < 0.001 , corrected ) . among patients with mdd ,
the t - carrier group had significantly lower alff values in the bilateral lingual gyri than the cc group ( f = 2.92 , p = 0.006 ) .
within - genotype comparisons showed that t - carriers with mdd had significantly lower alff in the bilateral lingual gyri than healthy controls who were t - carriers ( f = 4.82 , p = 0.02 ) .
in addition , alff were highest in the brain regions of the bilateral dorsal lateral frontal cortex ( dlpfc ) and left medial prefrontal gyrus ( mpfc ) in mdd rs35936514 t - carriers compared to mdd rs35936514 cc homozygotes and both control genotype groups ( p < 0.001 ) ( figure 3 ) .
correlation analyses did not show any significant associations between alff and hamd scores , illness duration , or education in mdd or hc groups .
based on the large number of samples studied on lhpp rs35936514 associated with mdd in chinese subjects , it was here noted that lhpp was associated with thyroid disorders , which may affect depression .
multiple studies have reported that both hypo- and hyperthyroidism may potentially increase the risk of cognitive impairment and neurodegeneration .
the association between hypothyroidism and depression might be explained by higher rates of hypercortisolism in depression , which might lead to changes in the hypothalamic - pituitary - thyroid / adrenal ( hpt / hpa ) axes .
dysfunctional hpt / hpa axes regulation might be a trait in mdd patients , producing changes in the monoaminergic pathways that modulate hormonal responses .
autoimmune thyroiditis which is marked by the presence of thyroid antibodies with normal or abnormal thyroid hormone level is also a major cause for mdd .
a bidirectional association between depression and the immune system has been also reported [ 33 , 34 ] .
depression has been linked with increased inflammatory markers and depression risk alleles have been found to be associated with regulating genes of the immune response .
however , the current work did not show apparent relationship between lhpp , thyroid disorders , and mdd .
our present study may provide a new imaging genetics approach to explore their relationships . in order to assess the effects on the neural system associated with lhpp variation and mdd
, the current work demonstrated that lhpp rs35936514 snp polymorphism may influence the spontaneous brain activity in individual subjects with mdd and also healthy controls .
the spontaneous low - frequency fluctuations ( lff ) of blood oxygenation level - dependent ( bold ) signals at rest have been identified as a biological measure of baseline spontaneous activity in the brain [ 21 , 22 , 36 ] .
abnormal alff can reflect pathophysiological states in the regional brain area and may help to locate specific impaired brain regions during the resting state . as an additional advantage
, alff can be used to assess neuronal activity within the entire brain . in particular , by rs - fmri , the cortico - limbic - striatal circuits ( including the pfc , hippocampus , amygdala , and striatum ) have been implicated in the dysfunctional regulation in mdd .
the current study showed significant interactions between diagnosis and genotype in the bilateral lingual gyri , bilateral dlpfc , and left mpfc .
the alterations of alff in the mdd t - carrier group suggest that the risk t allele is associated with abnormal spontaneous activity in mdd .
more research is needed to identify the mechanisms by which these regional activities in individuals with mdd may be more vulnerable to the effects of variations in rs35936514 , which render them more vulnerable to genetic liability .
however , it remains unknown when and how this interaction could change alff , although many factors could influence the status of mdd [ 3840 ] , which are well established as a risk for mdd .
furthermore , expression of multiple genes involved in mdd , and their interaction with the lhpp genotype could make a change in the neural circuits of mdd patients .
the parts of the brain in the mpfc are believed to be the neural site of self - referential processing , like the anterior node of the default mode network ( dmn ) .
alterations to the mpfc functional networks are involved in the development of mdd [ 43 , 44 ] .
altered resting state in the dlpfc of mdd could adversely affect activity during emotional or cognitive regulation .
the lingual gyrus has been reported to participate in the visual recognition network and to play a role in the perception of emotions [ 45 , 46 ] .
many studies have reported decreased activity in the lingual gyrus [ 45 , 47 , 48 ] in patients with mdd .
this may be attributable to different roles and levels of expression of the lhpp gene in different regions of the brain , different structural or functional bases ( activities ) of different brain regions , the combined effects of the t - carrier genotype , and other unknown factors . in this study ,
independent of genotype , increased alff was observed in the left middle temporal gyrus and left occipital cortex among mdd participants compared to that among hcs .
the increased spontaneous activity in the temporal and occipital lobe within mdd patients was consistent with the findings of rs - fmri studies examining depression [ 4951 ] .
genotype was found to have an effect on alff in the left superior temporal gyrus .
it was higher among t - carriers of the lhpp rs35936514 polymorphism in both the hc and mdd groups than in cc individuals within the respective diagnostic groups .
it is here suggested that the mdd rs35936514 t - carriers may function as one of several factors that can be used to diagnose mdd .
it is possible that variation in lhpp may contribute to the different alff values in temporal lobe in individuals with or at risk for mdd . in this way ,
the subgroup with mdd that carries the t allele may be a risk factor to abnormal brain activity .
however , exploratory analyses did not reveal significant correlations between alff values in regions of significant group differences and clinical factors within each group .
we speculate that the relative small sample size or other clinical factors may limit our ability to detect the relation .
one lhpp snp , rs11245316 , was found to confer risk of mdd by qtl - specific association analysis .
however , these data do not provide preliminary indications of how genetic variation in high - risk regions influences susceptibility to neural circuits abnormalities in patients with mdd .
first , many snps and genes implicated in mdd were not included in the study .
gene - gene interaction studies and even pathway studies are needed to search for valid mdd - associated genetic and neuroimaging biomarkers .
second , owing to the small sample size , the study placed individuals with ct and tt genotype in a single group . as a result
, no analysis of the alff values among 3 genotype groups could be performed , and the effect of the risk t allele could not be established as dominant or codominant . a larger - sample study should be performed to further investigate the effect of genes .
in conclusion , the present study demonstrated for the first time that lhpp rs35936514 ct / tt genotype may affect regional brain activity in mdd patients .
this suggests that the influence of lhpp variation may be one mechanism that contributes to the neural circuits of mdd .
 a single - nucleotide polymorphism at the lhpp gene ( rs35936514 ) has been reported in genome - wide association studies to be associated with major depressive disorder ( mdd ) . however , the neural system effects of rs35936514 that mediate the association are unknown . 
 the present work explores whether the lhpp rs35936514 polymorphism moderates brain regional activity in mdd . 
 a total of 160 subjects were studied : a cc group homozygous for the c allele ( 23 individuals with mdd and 57 controls ) and a t - carrier group carrying the high risk t allele ( ct / tt genotypes ; 22 mdd and 58 controls ) . 
 all participants underwent resting - state functional magnetic resonance imaging ( rs - fmri ) scanning . 
 brain activity was assessed using the amplitudes of low - frequency fluctuations ( alff ) . 
 mdd patients showed a significant increased alff in the left middle temporal gyrus and occipital cortex . 
 the t - carrier group showed increased alff in the left superior temporal gyrus . 
 significant diagnosis  genotype interaction was noted in the bilateral lingual gyri , bilateral dorsal lateral prefrontal cortex ( dlpfc ) , and left medial prefrontal cortex ( mpfc ) ( p < 0.05 , corrected ) . 
 results demonstrated that mdd patients with lhpp rs35936514 ct / tt genotype may influence the regional brain activity . 
 these findings implicate the effects of the rs35936514 variation on the neural system in mdd . 