Influence of Epigenetic Differences on the Etiology of Bipolar

Disorder and Schizophrenia

Zhiyu Yan

University College London, London, WC1H 0AJ, U.K.

Keywords: Epigenetic, Schizophrenia, Bipolar Disorder, Psychosis, DNA Methylation.

Abstract: Schizophrenia and bipolar disorder are two mental disorders that have attracted worldwide attention.

However, The role of DNA methylation in epigenetics in schizophrenia and bipolar disorder is unclear. This

paper investigates the role of epigenetics in the pathogenesis of schizophrenia and bipolar disorder.

Genome-wide analyses of monozygotic twins have identified specific genetic loci at which DNA

methylation may be responsible for schizophrenia and bipolar disorder. Certain conclusions were made by

genome-wide DNA methylation analysis of DNA samples from identical twins with inconsistent major

psychiatric disorders. In twins with schizophrenia, the largest differential methylation region associated with

mental illness was significant hypomethylation of the ST6GALNAC1 promoter, which overlaps with

previously reported rare schizophrenia genomes. The average difference in DNA methylation at this locus is

6%, but there is considerable variation between families, with some twins even showing a 20% difference in

methylation. These results suggest that DNA methylation differences play a role in phenotypic differences

in identical twins and may influence the etiology of schizophrenia and bipolar disorder to some extent.

1 INTRODUCTION

Schizophrenia and bipolar disorder are two related

mental disorders that are common across the globe

(Patel et al. 1996). Schizophrenia is a highly

inherited neuropsychiatric disease, which is mainly

manifested in the presence of psychotic symptoms,

but also characterized by dysfunctional emotional

response and cognitive changes. Although people

have succeeded in identifying the gene variants

associated with schizophrenia, they are still

uncertain about the pathogenic genes of the

pathogenesis of the disease and how their functions

are regulated (Hannon et al. 2016). Bipolar disorder

is an extremely debilitating mental illness. It is

characterized by paroxysmal mood swings. Patients

often experience both manic and depressive moods,

and often have cognitive impairment. People with

this disease have serious destructive attacks,

frequent recurrence and serious psychosocial

disorders. The disease usually begins in adolescence

and even in the late childhood of some patients,

much earlier than previously thought (Miklowitz et

al. 2008). Since the two diseases may have the same

etiology, the symptoms of schizophrenia and bipolar

disorder overlap and can be classified as major

psychosis (Cardno et al. 2002). Bipolar disorder and

schizophrenia have strong aggregation in the family.

Quantitative genetic analysis showed that both had

strong genetic components. However, although the

heritability of schizophrenia and bipolar disorder is

estimated to be 70%, the disease consistency of

monozygotic twins with the same DNA sequence is

not 100% (Cardno et al. 2000). This means that

non-genetic and environmental factors are also

important in the etiology of the diseases.

Epigenetics is a rapidly developing field that

includes regulatory mechanisms of gene expression

that do not involve genotype change. Epigenetic

mechanism mainly realizes heritable changes in gene

expression during mitosis through DNA methylation

and chromatin structure changes, but does not change

genomic DNA sequence. And the study of

epigenetics is increasingly relevant to neuroscience.

Epigenetic mechanisms involve brain development

and neuronal differentiation. Epigenetic regulation

involves multiple levels of gene expression, with

direct modifications from DNA and histone tails that

regulate transcription levels to interactions with

messenger RNA that regulate translation levels (Roy

et al. 2015). It is generally believed that epigenetic

dysfunction of human brain can be related to a series

Yan, Z.

Inﬂuence of Epigenetic Differences on the Etiology of Bipolar Disorder and Schizophrenia.

DOI: 10.5220/0011372300003438

In Proceedings of the 1st International Conference on Health Big Data and Intelligent Healthcare (ICHIH 2022), pages 471-477

ISBN: 978-989-758-596-8

471

of mental diseases, including psychosis. In recent

years, researchers have studied the brains of

psychiatric patients and healthy controls, and found

that there are significant epigenetic changes in the

genome related to schizophrenia and bipolar disorder

(Mill et al. 2008). But how the epigenome plays a

role in schizophrenia and bipolar disorder is not well

understood. The role of DNA methylation in

epigenetics in schizophrenia and bipolar disorder is

unclear.

Monozygotic twins carrying the same disease

mutation can be clinically quite different, and

investigating inconsistent monozygotic twins pairs is

a useful method for discovering disease-related

epigenetic mechanisms, as It can detect the

epigenome independently of potential variation of

genome sequence (Bell et al. 20011. A study found

substantial differences in DNA methylation variation

between monozygotic twins, suggesting that

epigenetic variation can lead to phenotypic

inconsistencies between humans with the same gene

(Kaminsky et al. 2009).

Because the role of DNA methylation in

schizophrenia and bipolar disorder is unclear, it is not

known exactly which DNA sites are involved. The

new study proposes genome-wide analysis to identify

the genetic loci most affected by the two diseases.

This paper investigates the role of epigenetics in

the pathogenesis of schizophrenia and bipolar

disorder. Genome-wide analyses of monozygotic

twins have identified specific genetic loci at which

DNA methylation may be responsible for

schizophrenia and bipolar disorder. Genome-wide

analysis of DNA methylation variations in identical

twins caused by schizophrenia and bipolar disorder,

with genetically damaged DNA extracted from a

unique twin. Many DNA methylation differences

associated with disease were found, many of which

were located near genes previously associated with

psychosis. The results agree with the hypothesis that

epigenetic changes can influence the causes of

schizophrenia and bipolar disorder (Emma et al.

2011).

Table 1: Group of monozygotic twin pairs utilised in the study, values shown are average plus standard deviation.

Schizophrenia-discordant twin

airs

Bipolar Disorder-discordant twin

airs

Psychosis-discordant twin

airs

Sex

(

males:females

)

8:3 2:9 10:12

Ethnicity 10 Caucasian, 1 unknown 10 Caucasian, 1 Afro-Caribbean

20 Caucasian, 1 unknown, 1

Afro-Caribbean

Time discordant

(years)

10.4 ± 10.6 14.6 ± 10.7 12.6 ± 10.6

Age of onset (years) 20.0 ± 4.6 21.7 ± 12.3 20.9 ± 9.3

2 METHODOLOGY

Since monozygotic twins share the same genetic

sequence, studying epigenetic changes in diseases in

inconsistent identical twins is a powerful approach

because it allows independent epigenetic assessment

of any potential genome sequence variation.

Inconsistent DNA methylation in schizophrenia

or bipolar disorder was measured in 22 pairs of

twins (44 individuals) using the Illumina Infinium

HumanMethylation27 BeadChip. Standard protocols

is used to extract genomic DNA from whole blood

of 22 pairs of inconsistent monozygotic twins

recorded in the Maudsley Bipolar disorder and

Schizophrenia Twin Study. In the experiment, the

twins were clinically diagnosed by at least two

psychiatrists and two psychologists to ensure mental

inconsistencies. On average, the twins had been ill

for 12.6 (+ 10.6) years when they were taken blood

(Table 1). The EZ 96-DNA methylation Kit (Zymo

Research, CA, USA) is used to replicate 500 nm of

genomic DNA per person with sodium bisulfite. In

order to study genome-wide DNA methylation,

Illumina Infinium human methylation 27 beadchip

was mainly used. The chip investigated 27578 CpG

sites related to about 14000 genes. Also, Illumina

GenomeStudio software play a important role in

extracting the signal strength of each probe and

perform an quality control inspection when all data

sets are available. The probes with p value of 0.05 (n

= 733) detected in all samples were removed, and

the probes with poor quality were strictly controlled.

This experiment mainly uses microarray data

analysis. In the experimental analysis procedures, all

calculations and statistical analyses are performed in

the R statistical analysis environment. The ratio of

the normalized signals of methylated probes to the

sum of the normalized signals of methylated and

unmethylated probes calculates the relative

ICHIH 2022 - International Conference on Health Big Data and Intelligent Healthcare

472

methylation levels of each detected CpG site. This

gives an mean β value for each CpG loci, which is

from 0 to 1, where 0 means unmethylated and 1

means fully methylated.

In genome-wide correlation analysis, variable

probes are recorded by calculating the standard

deviation of the entire data set, and then those

probes whose standard deviation is less than the

estimated standard deviation are filtered out. Two

independent ranking tests were analyzed. The first is

the standard paired T-test, which looks at the

meaning of differences in DNA methylation

between the members of each twin pair. The another

test was used to measure the size of the methylation

difference, with a calculated Δβ-value describing the

average difference in methylation between the

members of each twin pair (Δβ refers to the

unaffected minus the affected twin). The results of

the two tests were sorted by P value and size. Next,

the two ranking lists were added together to produce

the final CpG site ranking list. The table shows the

CpG sites with the greatest differences in DNA

methylation and the most consistent in all twins. For

statistical analysis of the magnitude of change

observed at each locus for affected and unaffected

twins, the custom weighted T-test was used.

3 RESULTS

DNA methylation at a single CpG site illustrated

important difference in monozygotic twins.

Analytical methods was used to determine the

biggest differences in DNA methylation at specific

CpG sites. Many sites in the whole genome is

identified which showed differences in

disease-related DNA methylation (Figures 1 & 2).

Figure 1: Characteristic map of each CpG locus in all 44 individuals of psychotic discordant twins.

Figure 2: DNA methylation differences (Δβ-value) for the top-ranked probes from (A) the combined psychosis-discordant

analysis group: ST6GALNAC1 (cg13015534), (B) Schizophrenia-discordant analysis group: PUS3 (cg02659232) and (C)

the bipolar disorder-discordant analysis group: GPR24 (cg21342728).

Inﬂuence of Epigenetic Differences on the Etiology of Bipolar Disorder and Schizophrenia

473

Table 2 shows the genomes closest to the

differentially methylated CpG sites associated with

the eight top diseases in the three experimental

groups (Schizophrenia, bipolar disorder and

combined psychosis). By analyzing the location of

100 CpG sites with the highest methylation

difference associated with psychosis, the

representation of CpG sites in CpG island was

significantly insufficient.

Table 2: The first eight of the three test groups had differential methylation CpG sites.

Analysis

rou

Rank Gene name Chromosome Paired t-test P-value

Mean Δβ (minimum–

maximum

)

Weighted Δβ

P-value

Weighted q-

value

Psychosis

ST6GALNA

17q25.1 4.03E − 04 0.06 (−0.11–0.20) 1.19E − 07 7.97E − 04

2 ACADL 2

34 2.49E − 04 0.05

(

−0.05

–

0.17

)

1.59E − 06 3.19E − 03

3 TBC1D10A 22

12.2 8.56E − 04 0.06

(

−0.05

–

0.23

)

9.40E − 08 7.97E − 04

4 PUS3 11q24.2 7.66E − 04 −0.05 (−0.16

–

0.06) 1.71E − 06 3.19E − 03

5 FXR2 17p13.1 1.74E − 03 0.06 (−0.09

–

0.26) 9.03E − 08 7.97E − 04

6 TSP50 3p21.31 4.92E − 04 0.04 (−0.05

–

0.12) 2.26E − 05 1.12E − 02

7 PCOLN3 16

24.3 1.27E − 03 0.04

(

−0.06

–

0.21

)

1.79E − 05 1.03E − 02

8 SOX1 13

34 1.04E − 03 0.04

(

−0.04

–

0.13

)

2.64E − 05 1.18E − 02

Schizophr

enia

1 PUS3 11

24.2 7.66E − 04 −0.07

(

−0.16

–

0.03

)

5.16E − 05 0.10

2 SYNGR2 17q25.3 8.29E − 04 0.07 (0.01

–

0.13) 9.82E − 05 0.14

3 KDELR1 19q13.3 1.25E − 03 −0.06 (−0.14

–

0.01) 3.07E − 04 0.18

4 PDK3 Xp22.11 7.54E − 04 0.06 (0.00

–

0.14) 3.67E − 04 0.18

5 PPARGC1A 4

15.1 1.85E − 03 0.06

(

−0.02

–

0.12

)

2.89E − 04 0.18

6 ACADL 2

34 3.74E − 03 0.07

(

0.00

–

0.17

)

7.81E − 05 0.12

7 FLJ90650 5q23.1 4.19E − 04 0.05 (−0.01

–

0.09) 6.98E − 04 0.19

8 TUBB6 18p11.21 3.54E − 03 0.06 (−0.01

–

0.18) 1.78E − 04 0.16

Bipolar

disorder

1 GPR24 22q13.2 1.30E − 03 0.07 (0.00

–

0.16) 7.59E − 05 0.17

2 TLE6 19

13.3 1.97E − 03 −0.09

(

−0.21

–

0.01

)

1.54E − 05 0.12

3 STAB1 3

21.1 1.63E − 03 −0.07

(

−0.18

–

0.02

)

8.11E − 05 0.17

4 PPYR1 10

11.2 5.13E − 04 −0.06

(

−0.12

–

0.01

)

3.44E − 04 0.25

5 CTNNA2 2p12 3.59E − 03 0.09 (0.00

–

0.21) 1.56E − 05 0.12

ST6GALNA

17q25.1 3.06E − 03 0.06 (−0.01–0.15) 2.82E − 04 0.23

7 C1orf35 1

42.13 5.30E − 03 0.06

(

−0.01

–

0.18

)

1.88E − 04 0.23

8 IQCH 15q23 3.94E − 03 0.05 (−0.06

–

0.10) 7.81E − 04 0.30

In all 22 pairs of uncoordinated monozygotic

twins, the methylation difference in the promoter site

of the gene sequence ST6GALNAC1 was the

highest, and the methylation degree of the affected

individuals was lower than that of the unaffected

monozygotic twins. The highest differentially

methylated CpG locus in discordant schizophrenic

twins is located upstream of the gene encoding

PUS3, which is highly methylated in affected twins.

The higher ranked CpG locus in inconsistent

comorbid psychiatric pairs is located upwards of the

gene site GPR24, which is methylated in diseased

twins.

Although the influence of the top loci in each

diagnostic group was in the same direction, the

significance of methylation difference between the

diseased and non-diseased twins at some loci was

different, suggesting familial heterogeneity even at

the top loci. The first psychotic differential

methylation site in the ST6GALNAC1 promoter

showed binomial Δβ values as high as 0.20 (Table

2). Psychosis has significant clinical heterogeneity,

so it is generally accepted that there are some rare

etiologies that are highly influential in some cases.

Individual twins may also vary greatly in

methylation at sites associated with particular

diseases (Merikangas et al. 2009). Therefore, DNA

methylation differences in individual twins to find

the greatest specific changes involved In families.

Many new and known psychiatric candidate genes

show significant differences in DNA methylation

between patients of one or more pairs of twins, and

some sites can be calculated with Δβ values greater

than 0.60 between diseased and non-diseased twins.

Some CpG loci ranked higher in schizophrenic

discordant and bipolar disorder discordant twin

analyses, but DNA methylation changes were

reversed between diseases, suggesting that

epigenetic mechanisms can be disease identified. In

monozygotic twins with dissonant schizophrenia and

ICHIH 2022 - International Conference on Health Big Data and Intelligent Healthcare

474

bipolar disorder, the CpG loci in the ZNF659

promoter was the highest of 100 sites and was

hypomethylated in bipolar disorder twins and

hypermethylated in schizophrenia twins (Figure 3).

Among the 100 highest-ranked schizophrenia and

bipolar disorder loci, several other studies have

shown commonly large diversities in the alternative

diagnostic group in the opposite direction.

Figure 3: DNA methylation differences (Δβ-value) for a CpG loci located at ZNF659 (cg18267381), where Schizophrenia

group is blue bars, and Bipolar disorder group is red bars.

Investigating that the validation and reproduction

of disease-associated unmethylated at the

ST6GALNAC1 gene loci. The Sequenom

EpiTYPER data, which accurately repeated arrays of

specific CpG sites on the Illumina platform, showed

hypommethylation in diseased psychiatric twins

(mean ¼ 35% methylation in psychotic twins and ¼

41% methylation in unaffected twins). Notably,

validation data for schizophrenic twins showed

greater differences in DNA methylation (an average

of 15% hypomethylation) between the diseased and

undiseased twins than was spotted at this locus. The

overall level of DNA methylation in the brain (85%)

was higher than that in the blood (40%). Although

no overall significant difference was found between

psychiatric patients and the control group at the

dominant site of DNA methylation array (CpG4

sequenom analysis), thirty (13.3%) psychiatric

patients tested illustrated clear (27%) methylated

and large difference at this CpG and many similar

CpG loci (Fig. 4).

Figure 4: DNA methylation at several CpG sites in the ST6GALNAC1 promoter of psychotic patients was compared with

postmortem brain tissue of controls.

Inﬂuence of Epigenetic Differences on the Etiology of Bipolar Disorder and Schizophrenia

475

This hypomethylated ST6GALNAC1 promoter

was not found in any of the control groups in this

study, indicating consistent and consistent

methylation in the area. These brain data flesh back

large heterogeneity in blood data between twins

(Figure 1), with some affected siblings showing

hypomethylation of up to 20%, but other twins

showing smaller differences. These data suggest that

major changes associated with disease in this site

may affect some patients.

4 DISCUSSION

A genome-wide approach was used to

comprehensively analyze disease-related DNA

methylation differences in monozygotic twins with

discorrelating schizophrenia and bipolar disorder. It

is preliminarily found that no significant changes in

total DNA methylation between the diseased and

undisased twins, but there were important

disease-related changes between the twins at

identified sites on the genome. Some methylation

differences persisted in the comorbidities, while

other differences may be specific to schizophrenia or

bipolar disorder. Although large differences were

found in nearly all pairs of discordant twins in each

diagnostic category, other differences related to

specificity for schizophrenia or bipolar disorder

were found for only one or a few pairs of twins.

Although these sites in the experiment had not

previously been linked to mental illness, evidence of

DNA methylation differences in genes involved in

mental illness could still be found. This illustrates

the data of this study and provides more

demonstrations to support the usage of DNA

methylation in the etiology of schizophrenia and

bipolar disorder.

The first total difference in methylation

combined with mental illness found in the study was

a CpG loci (17q25.1) in the ST6GALNAC1

promoter, and it is hypomethylation at this site in the

ill twins. They also found that 0.13% of autopsy

brain samples from patients with schizophrenia and

bipolar disorder showed significant hypomethylation

in extended regions containing the designated CpG

locus, which could indicate that epigenetic

mechanisms in this area can exist in a subset of

patients with psychosis. CpG sites associated with

mental illness do not exist in the same CpG island,

and differential methylation sites are significantly

deficient in classical CpG-rich promoters,

suggesting that phenotypic related variations in

DNA methylation generally occur outside these

regions, which is consistent with data from another

epigenomic analysis study (Weber et al. 2007).

However, the study did have some limitations.

For example, it was limited to 22 monozygotic

twins, calculations are based on standard deviation

estimates of the entire dataset from Illumina's array

data showing that at a strict Bonferroni-corrected

assumed value cutoff. The 22 monozygotic twins in

the study provided greater than 80% accuracy in

obtaining Δβ=0.06, although the ability to find small

differences at this level of significance was limited.

5 CONCLUSIONS

The role of epigenetics in the etiology of

schizophrenia and bipolar disorder has been

demonstrated experimentally. Genome-wide

analysis of monozygotic twins found that specific

sites of DNA methylation also contribute to the

cause of schizophrenia and bipolar disorder. In

patients with schizophrenia, the largest differential

methylation region associated with psychiatric

illness was hypomethylation of the ST6GALNAC1

promoter. The average difference in DNA

methylation at this site is 6%. However, there is

significant heterogeneity between families, since

some twin pairs having as much as 20% difference

in methylation. These results suggest that DNA

methylation differences play a role in phenotypic

differences in identical twins and may influence the

etiology of schizophrenia and bipolar disorder to

some extent.

This study provides insight into the etiology of

mental illness, particularly schizophrenia and

bipolar disorder. However, it is not particularly clear

how DNA methylation at specific sites affects

patients with both diseases. Epigenetics with

neuroscience is a hot research direction in recent

years, and great progress has been made in the

genetic etiology and treatment of mental diseases. In

the future, human diseases can be better understood

at the epigenome level by studying different

epigenetic modifications of DNA.

ACKNOWLEDGEMENTS

I would like to thank Prof. Wang and the supervisor

Ms. Wang for their guidance and academic

explanation of my paper.

ICHIH 2022 - International Conference on Health Big Data and Intelligent Healthcare

476

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