Exploratory Analysis of Polygenic Risk Scores for Psychiatric Disorders: Applied to Dual Diagnosis

Abstract

Background:
Concurrence of substance use disorders (SUDs) is high in individuals with psychiatric illnesses; more importantly, individuals with both disorders (dual diagnosis) have more severe symptoms. Psychiatric disorders have been proposed to share a genetic susceptibility with SUDs. To explore this shared genetic susceptibility, we analyzed whether any of the polygenic risk scores (PRSs) for psychiatric disorders could be associated to dual diagnosis in patients with schizophrenia (SCZ) or bipolar disorder (BD).

Methods:
We included 192 individuals of Mexican ancestry: 72 with SCZ, 53 with BD, and 67 unrelated controls without psychiatric disorders. We derived calculations of PRS for autism spectrum disorders, attention-deficit/hyperactive disorder, BD, major depression, and SCZ using summary genome-wide association statistics previously published.

Results:
We found that dual diagnosis had a shared genetic susceptibility with major depressive disorder (MDD) and SCZ; furthermore, in individuals with BD, dual diagnosis could be predicted by PRS for MDD.

Conclusions:
Our results reinforce the notion that individuals with dual diagnosis have a higher genetic susceptibility to develop both disorders. However, analyses of larger sample sizes are required to further clarify how to predict risks through PRS within different populations.

Full text

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ORIGINAL ARTICLERev Invest Clin. 2019;71:321-9
Exploratory Analysis of Polygenic
Risk Scores for Psychiatric Disorders:
Applied to Dual Diagnosis
J J. M-M1,2, T B. G-C1,3, A D. G-M2,4, C A.
T-Z5, I E. J-R1, E S-U6, O R-M7,
N L8, M E9, L M-K10  H N2,8*
1Academic Division of Health Sciences, Universidad Juárez Autónoma de Tabasco, Villahermosa, Tabasco, Mexico;
2Genomics Laboratory of Psychiatric and Neurodegenerative Diseases, Instituto Nacional de Medicina Genómica,
Mexico City, Mexico; 3Multidisciplinary Academic Division of Jalpa de Méndez, Universidad Juárez Autónoma
de Tabasco, Jalpa de Méndez, Mexico; 4Hospital Psiquiátrico Infantil “Juan N. Navarro”, Servicios de Atención
Psiquiátrica, Mexico City, Mexico; 5Multidisciplinary Academic Division of Comalcalco, Universidad Juárez Autónoma
de Tabasco, Comalcalco, Mexico; 6Neurosciences Center, Universidad Autónoma de Nuevo León, Monterrey, Mexico;
7Palliative Care Unit, Instituto Nacional de Cancerología, Mexico City, Mexico; 8Grupo de Estudios Médicos y Familiares
Carracci, Mexico City, Mexico; 9Department of Psychiatry and Center of Excellence in Neurosciences, Texas Tech
University Health Sciences Center, El Paso, Texas, USA; 10Unit of Population Genomics Applied to Health, Faculty
of Chemistry, Universidad Nacional Autónoma de México - Instituto Nacional de Medicina Genómica, Mexico City,
Mexico.
Received for publication: 28-02-2019
Approved for publication: 24-04-2019
DOI: 10.24875/RIC.19003013
ABSTRACT
Background: Concurrence of substance use disorders (SUDs) is high in individuals with psychiatric illnesses; more importantly,
individuals with both disorders (dual diagnosis) have more severe symptoms. Psychiatric disorders have been proposed to share
a genetic susceptibility with SUDs. To explore this shared genetic susceptibility, we analyzed whether any of the polygenic risk
scores (PRSs) for psychiatric disorders could be associated to dual diagnosis in patients with schizophrenia (SCZ) or bipolar
disorder (BD). Methods: We included 192 individuals of Mexican ancestry: 72 with SCZ, 53 with BD, and 67 unrelated controls
without psychiatric disorders. We derived calculations of PRS for autism spectrum disorders, attention-deficit/hyperactive dis-
order, BD, major depression, and SCZ using summary genome-wide association statistics previously published. Results: We found
that dual diagnosis had a shared genetic susceptibility with major depressive disorder (MDD) and SCZ; furthermore, in indi-
viduals with BD, dual diagnosis could be predicted by PRS for MDD. Conclusions: Our results reinforce the notion that individu-
als with dual diagnosis have a higher genetic susceptibility to develop both disorders. However, analyses of larger sample sizes
are required to further clarify how to predict risks through PRS within different populations. REV INVEST CLIN. 2019;71:3219
Key words: Dual diagnosis. Polygenic risk scores. Schizophrenia. Bipolar disorder.
Corresponding author:
*Humberto Nicolini
Genomics Laboratory of Psychiatric
and Neurodegenerative Diseases
Instituto Nacional de Medicina Genómica
Grupo de Estudios Médicos y Familiares Carracci
Mexico City, Mexico
E-mail: hnicolini@inmegen.gob.mx
No part of this publication may be reproduced or photocopying without the prior written permission of the publisher. © Permanyer 2019

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INTRODUCTION
Psychiatric disorders are complex diseases caused by
multiple risk factors1,2. During the past decade, the
discovery of genetic factors involved in the suscepti-
bility to these disorders has increased rapidly, due to
novel techniques such as genome-wide association
studies (GWAS)3-6. GWAS have identified multiple loci
among common genomic variants that have been as-
sociated to one or more psychiatric disorders. Meth-
ods to determine polygenic risk scores (PRSs) have
been developed to summarize and use what is known
about these multiple disease-associated loci as risk
prediction tools7,8. PRS can be described as the sum-
mation of disorder-associated alleles across many
loci, weighted by their effect sizes estimated from
GWAS in one individual to predict one person’s likeli-
hood of developing a disease with a genetic compo-
nent. Therefore, a high value of PRS could be trans-
lated into an increased risk of a particular disorder for
having more individual risk variants, each one known
to be associated with the same disorder. The calcula-
tion of how much an individual variant increases a
disease risk, or how groups of variants increase the
risk, derives from former GWAS9. Recent studies of
PRS as risk prediction tools for a disease have exam-
ined how different complex diseases might share
polygenetic backgrounds. They have also evaluated
whether PRS can be used to predict particular traits
or subtypes within groups of individuals who have the
same diagnosis10-12.
Many epidemiological analyses have shown that psy-
chiatric disorders such as bipolar disorder (BD),
schizophrenia (SCZ), and major depressive disorder
(MDD) have high comorbidity with substance use dis-
orders (SUDs)13,14. The comorbidity between SUD
and other psychiatric disorders is so high that the
term “dual diagnosis” was created to specify comor-
bidity of these disorders15-17. Dual diagnosis, when it
occurs, has been associated with multiple negative
physical and psychosocial outcomes such as poorer
quality of life, higher rates of relapse of substance
use, and increased suicide risk18-20. Individuals with a
dual diagnosis show increased severity of symptoms,
which place them at high risk21. Although etiologic
mechanisms underlying dual diagnosis have not been
clearly established, some have suggested that indi-
viduals with dual diagnosis could have a greater ge-
netic susceptibility22,23. Recently, PRSs for psychiatric
disorders have been proposed to be useful for explor-
ing the shared genetic susceptibility between psychi-
atric disorders and SUD, to understand the genetic
basis of dual diagnosis24-26. In addition, PRSs of psy-
chiatric disorders have been studied to find risk pre-
dictors for dual diagnosis27-30. However, one of the
main limitations to this approach is that PRSs of psy-
chiatric disorders were derived from GWAS in Euro-
pean populations, which have a highly homogeneous
genetic background, and they have not been conduct-
ed in populations with a high degree of genetic ad-
mixture. There are nations around the world with a
high degree of admixture, including the Mexican pop-
ulation31, which comes from the combination of sev-
eral indigenous groups and a few European popula-
tions. Therefore, some researchers are concerned
with the generalization of the use of PRS in non-Eu-
ropean populations, and how the use of PRS could be
translated to populations with heterogeneous genet-
ic background32-34. In this sense, our aim was to ex-
plore the performance of PRS calculated from previ-
ous GWAS for psychiatric disorders when applied to
Mexican individuals with a high degree of admixture
who have been diagnosed with SCZ or BD, many of
whom had a dual diagnosis.
METHODS
Participants
Target sample
All participants were recruited at the Carracci Medical
Group and evaluated using the Diagnostic Interview
for Genetic Studies (DIGS)35. Diagnoses were as-
signed using the DSM-V criteria for BD, SCZ, and SUD.
A total of 192 individuals of Mexican ancestry were
included in the analysis. Inclusion criteria: The par-
ticipant’s parents and grandparents had to be of
Mexican ancestry, meaning that mother, father, and
four grandparents were born in Mexico; participants
had to be 18 years of age or older. The group of cases
consisted of 125 unrelated outpatients; 72 (49 males
and 23 females) had a lifetime diagnosis of SCZ and
53 (25 males and 28 females), a lifetime diagnosis of
BD. All patients were under psychiatric treatment for
at least 3 weeks after this study began. For the con-
trol group, we used the same inclusion criteria and
excluded individuals with BD, SCZ, MDD, SUD,
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José J. Martínez-Magaña, et al.: PRS FOR PSYCHIATRIC DISORDERS IN DUAL DIAGNOSIS
anxiety disorders, or history of suicidal behavior. The
control group consisted of 67 subjects with no life-
time history of any psychiatric disorder and who had
no relatives with a known history of psychiatric disor-
ders. We defined dual diagnosis as diagnosis of SCZ
or BD with one or more lifetime SUDs (tobacco, alco-
hol, and illegal drugs)36. The prevalence of lifetime
SUD (dual diagnosis) in the group of patients with
SCZ was 40.28% (n = 29) and 57.72% (n = 29) in
patients with BD. An overview of sociodemographic
characteristics of the sample is reported in Table 1.
This protocol was approved by the ethics and investi-
gation committee of the National Institute of Ge-
nomic Medicine under the approval number
23/2015/I. All participants were informed of the
aims of the study and gave their written consent be-
fore the study began. All protocols were performed
the following guidelines of the Helsinki Declaration.
Discovery samples
As discovery samples, we used the publicly available
summary statistics from GWAS to obtain the single-
nucleotide polymorphisms (SNPs) and associated
effect sizes, minor allele frequencies, and effect al-
leles, to be included in the PRSs calculation for each
disorder. Data from these discovery samples came
from the Psychiatric Genomics Consortium (PGC)37
and could be accessed from https://www.med.unc.
edu/pgc/results-and-downloads. Data in the PGC
portal are summary statistics derived from GWAS
previously performed in the following disorders: au-
tism spectrum disorders (ASD)38, attention-deficit/
hyperactivity disorder (ADHD)39, BD3, MDD40, and
SCZ41.
Procedures
Genotyping and imputation
of target sample
DNA was extracted from peripheral leukocytes using
a salting-out commercial protocol, following the
specifications established by the provider (Qiagen,
USA). Genotyping was performed using the whole-
genome genotyping platform PsychArray BeadChip
(Illumina, USA) following the protocol and conditions
established by the provider. PsychArray includes
Table 1. Summary of sociodemographic data
Sociodemographic
characteristic
Bipolar disorder
(n = 53)
Schizophrenia
(n = 72)
Control
(n = 67)
Age (years, ± SD) 37.58 (14.03) 33.53 (9.98) 34.37 (12.51)
Years of education
(years, ± SD)
12.49 (4.03) 10.53 (3.54) 13.16 (5.22)
Gender
Male (n, %) 25 (47.17) 49 (68.06) 26 (38.81)
Female (n, %) 28 (52.83) 23 (31.94) 41 (61.19)
SUDs
No SUD 24 (45.28) 43 (59.72) 67 (100.00)
Dual diagnosis 29 (57.72) 29 (40.28) 0 (0.00)
Alcohol 23 (43.40) 21 (29.17) 0 (0.00)
Tobacco 29 (54.72) 29 (40.28) 0 (0.00)
Cocaine 8 (15.09) 1 (1.39) 0 (0.00)
Cannabis 5 (9.43) 9 (12.50) 0 (0.00)
Inhalants 3 (5.66) 1 (1.39) 0 (0.00)
Stimulants 3 (5.66) 1 (1.39) 0 (0.00)
Dual diagnosis was defined as having a psychiatric disease diagnosis (BD or SCZ) and at least one SUD. SUDs included abuse or dependence of
alcohol, cocaine, cannabis, inhalants, and/or stimulants. There were no other SUDs in this sample. SD: Standard deviation, No SUD: Patients
without dual diagnosis. BD: Bipolar disorder, SUD: Substance use disorder, SCZ: Schizophrenia.
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approximately 560,000 polymorphisms distributed
across the whole genome, as well as some variants
previously associated with diverse mental psychiatric
disorders including BD and SCZ. As quality control
(QC), we removed all the SNPs with a minor allele
frequency (MAF) of 5%, a Hardy–Weinberg equilib-
rium (HWE) p < 0.00001 for a Chi-square test; ad-
ditionally, polymorphisms with a genotyping rate
lower than 95% were removed. The genotyped data-
base is available as supplementary information 1.
After the genotyping process, we performed an impu-
tation using Beagle software; the 1000 Genomes da-
tabase was utilized as reference42-44. For the following
analyses, we included only SNP with a Chi-square test
p-value for an HWE lower than 0.00001, a MAF high-
er than 0.05, and an allelic R2 higher than 0.445. After
imputation and QC filter, we obtained a total of
4,835,917 SNPs.
Analyses
Polygenic risk score calculation
PRSs are measures developed to reduce the calcula-
tion of risk profiles due to polygenicity in complex
diseases to a simpler and more manageable, single
score for everyone. The PRSs for one individual are
the summation of GWAS associated alleles to a dis-
order/trait, weighted by their effect sizes46. To calcu-
late the individual PRS, we used summary statistics
based on published GWAS from the Psychiatric Ge-
nomic Consortium (data free to download from:
https://www.med.unc.edu/pgc/results-and-down-
loads) utilizing the algorithm implemented in PRSice9.
PRSice calculated PRS using two sets of data, one
discovery and one target sample. The discovery sam-
ple is the summary statistics from a GWAS for a
specific trait or disease, with enough power to detect
an association at genome-wide significance (i.e., data
downloaded from GWAS and meta-analysis reported
by the PGC). The target sample is the sample where
PRSs are going to be calculated (our target sample
was the genotype data obtained after genotyping,
genotyping QC, imputation, and imputation QC).
Once we had the discovery and the target samples,
PRSice performed two steps: first, the clumping pro-
cess, where polymorphisms in linkage disequilibrium
(LD) between the associated loci in the target sample
and the discovery sample were unified47. In this analy-
sis, we used the following clumping criteria: 250 kb
and pairwise LD R2 < 0.1. Second, PRSice calculated
the individuals’ PRS using different p-values thresholds
for the associated variants in the discovery sample
(with a lower bound of p = 0.0001 and an upper bound
of p = 0.5; increments of 0.00005, which generated
9999 different thresholds). After the different thresh-
olds were performed, the best-fitted model estimates
were reported. This high-resolution approach allowed
us to calculate the best-fitted PRS for the target sam-
ple. In this study, we report the PRS for the best esti-
mates of models. All the models were adjusted by age,
gender, and the first five principal components of
global ancestry; for p-value multiple testing correc-
tions, we performed 1000 permutations tests. The
global ancestry estimation was performed with princi-
pal components analysis implemented in the GCTA
software48, using a panel of 200 ancestry informative
markers previously reported to reach at ancestry es-
timations in American populations49. For ancestry cal-
culation, the following populations were used as refer-
ences: Utah’s residents with northern and western
European ancestry (CEU), Yoruba residents in Ibadan
from a Nigeria population (YRI) reported in the 1000
Genomes Project44, and 25 individuals of Mexican
Amerindian (MA) ancestry genotyped with the multi-
ethnic genotyping array (Illumina, San Diego, CA,
USA). Once we calculated the best-fitted PRS, we
compared the mean of these PRSs for psychiatric dis-
orders between cases (subjects with BD and SCZ) and
controls using a Welch t-test and considered a sig-
nificant association when p < 0.05.
Calculation of SUD correlation
with psychiatric diseases PRSs
One common application of PRS is to test for common
genetic variation shared by two different traits or
disorders46. To determine whether any of the psychi-
atric PRS (out of those for ADHD, ASD, MDD, SCZ,
and BD) calculated with the previous algorithm, had
a shared genetic etiology with a dual diagnosis phe-
notype within our sample, we performed a Nagelker-
ke test implemented in PRSice50,51. PRSice reports the
variance explained by PRS in the analyzed phenotype,
calculated as the difference in the Nagelkerke’s pseu-
do-R2 from a model including the score and covariates
versus a model including only the covariates. In these
correlation tests, we recorded the phenotype of dual
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José J. Martínez-Magaña, et al.: PRS FOR PSYCHIATRIC DISORDERS IN DUAL DIAGNOSIS
diagnosis as cases (29 individuals with both SUD and
BD and 29 individuals with both SUD and SCZ) and
defined a non-SUD phenotype consisting of all indi-
viduals without SUD diagnoses (67 individuals with
either BD or SCZ who did not have a comorbid SUD
and the 67 controls). Furthermore, for these analy-
ses, we used the best-fitted models as described
above. After finding which PRS explained a higher
variance with SUD, we performed a Welch t-test
comparison of this PRS between patients diagnosed
with one psychiatric disease only (BD or SCZ without
SUD) and patients with dual diagnosis (BD or SCZ
with SUD) and considered a significant association
when p < 0.05.
Effect of global ancestry estimation
on the best-fitted PRS
To establish whether ancestry influences the PRS that
shared a genetic effect with dual diagnosis, we per-
formed a Spearman correlation test implemented in
R52. We compared the global ancestry principal com-
ponent 1 (PC1) and principal component 2 (PC2) with
the PRS (best-fitted PRS and covariants adjusted). We
only included PC1 and PC2 because these two global
ancestry principal components divide individuals into
two main populations31. We considered a correlation
with PRS and global ancestry component if p < 0.05.
RESULTS
In the analysis of PRS for psychiatric disorders, we
found that the only PRS that showed statistical dif-
ferences between psychiatric cases (72 subjects
diagnosed with SCZ and 53 subjects diagnosed with
BD) and controls (67 subjects) was the PRS for MDD.
A summary of the means of each PRS for ADHD, ASD,
BD, MDD, and SCZ is reported in Table 2.
SCZ and major depression shared
genetic etiology with dual diagnosis
PRSs for ADHD, ASD, and BD did not share a genetic
etiology at a significant level with dual diagnosis in
our population: ADHD p = 0.1570, ASD p = 0.0538,
and BD p = 0.1585. In contrast, SCZ and MDD:PRS
each showed a significant shared genetic etiology
with the dual diagnosis phenotype: SCZ (Nagelkerke
Pseudo-R2 = 0.0283, corrected p = 0.0423, n = 8058
SNPs) and MDD (Nagelkerke Pseudo-R2 = 0.0451,
corrected p = 0.0118, n = 334 SNPs). As can be ob-
served, the MDD:PRS explained a higher amount of
variance (4.51%) predicting placement in the dual
diagnosis group than did the SCZ:PRS (2.83%).
Once we identified that MDD and SCZ PRS had a
shared genetic etiology with a dual diagnosis in our
sample, we performed a pair-wise comparison of the
individual MDD and SCZ PRS in patients with a dual
diagnosis and patients without a dual diagnosis, to
determine whether these PRSs (for MDD or SCZ)
could be used to detect a subgroup of patients who
had SUD within each diagnostic category (BD or SCZ
patients). In this analysis, patients with a dual diag-
nosis in the BD group had a higher MDD:PRS when
compared to patients with BD without a dual diagno-
sis, and this difference reached statistical significance
(p < 0.05) (Fig. 1). In contrast, when the MDD:PRS
was applied only to patients diagnosed with SCZ, it
Table 2. Comparisons of psychiatric disorders polygenic risk scores between patients diagnosed with psychiatric disorders and
controls
Polygenic risk score Cases
(n = 125)
Controls
(n = 67)
p-value
ADHD:PRS −0.0013 (0.0002) −0.0013 (0.0002) 0.1817
ASD:PRS −0.0035 (0.0024) −0.0032 (0.0021) 0.3738
BD:PRS 0.0213 (0.0196) 0.0211 (0.0189) 0.9404
SCZ:PRS 0.0011 (0.0004) 0.0011 (0.0005) 0.7602
MDD:PRS −0.0041 (0.0018) −0.0033 (0.0025) 0.04754
ADHD:PRS: Attention-deficit hyperactivity disorder:polygenic risk score, ASD:PRS: Autism spectrum disorders:polygenic risk score,
BD:PRS: Bipolar disorder:polygenic risk score, SCZ:PRS: Schizophrenia:polygenic risk score, MDD-PRS: Major depression disorder-polygenic
risk score. The reported p-value is result of a Welch t-test.
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did not distinguish between SCZ patients with and
without a dual diagnosis of SUD. The SCZ:PRS did not
show statistically significant differences in detecting
a dual diagnosis when it was applied only to patients
with SCZ (SCZ patients with and without SUD) or to
patients with BD (BD patients with and without SUD).
The pair-wise comparisons are shown in Table 3.
Possible global ancestry deviation
of MDD and SCZ-PRS
To explore whether genetic admixture could influ-
ence PRS, we analyzed how global ancestry within
the sample could affect the best-fitted PRS (also,
adjusted after covariants). As PRSs of MDD and SCZ
were the only ones that shared a genetic etiology
with a dual diagnosis, we performed correlation tests
with all the participants (SCZ, BD, and healthy con-
trols) of each individual PRS and the global ancestry
principal components (PC1 and PC2), which are the
two components that separated main populations31, to
search whether PRS calculations could have an ances-
try-dependent deviation. In this analysis, both MDD
(PC1: rho = −0.20, p = 0.01 and PC2: rho = −0.19,
p = 0.01) and SCZ (PC1: rho = −0.61, p = 2.2e-16
and PC2: rho = −0.61, p = 2.2e-16) PRSs were cor-
related with PC1 and PC2. Of the two, SCZ:PRS had
a stronger correlation with the two global ancestry
components (Fig. 2).
Figure 1. Comparisons of schizophrenia (SCZ) and major depressive disorder (MDD) polygenic risk score (PRS) in patients with
and without dual diagnosis (substance use disorder). (A) Comparison of MDD:PRS in patients with bipolar disorder (BD) (dual
diagnosis patients had significantly higher scores than BD only subjects, p = 0.0005) and patients with SCZ (non-significant
difference). (B) Comparison of SCZ-PRS in BD and SCZ patients. SCZ:PRS differences between dual diagnosis patients and pa-
tients with only the primary disorder were not significantly different in either BD or SCZ subjects.
Table 3. Comparisons of MDD and SCZ polygenic risk scores
Bipolar disorder Schizophrenia
Polygenic risk
score
No substance
use
Dual
diagnosis
p-value No substance
use
Dual
diagnosis
p-value
SCZ:PRS 0.0010
(0.0006)
0.0011
(0.0004)
0.6392 0.0011
(0.0005)
0.0012
(0.0004)
0.7409
MDD:PRS −0.0048
(0.0017)
−0.0028
(0.0020)
0.0007 −0.0041
(0.0018)
−0.0040
(0.0016)
0.6099
Dual diagnosis was considered if a patient had a psychiatric disease diagnosis (BD or SCZ) and substance use. SCZ:PRS: Schizophrenia:polygenic
risk score, MDD:PRS: Major depression disorder:polygenic risk score. The reported p-value resulted from Welch t-test. BD: Bipolar disorder,
SUD: Substance use disorder.
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José J. Martínez-Magaña, et al.: PRS FOR PSYCHIATRIC DISORDERS IN DUAL DIAGNOSIS
DISCUSSION
PRSs are of potential value for determining subphe-
notypes within a larger phenotype or main diagnosis
in psychiatric disorders53,54. In the present study, we
evaluated whether the current PRS (available for phe-
notypes of ADHD, ASD, BD, SCZ, and MDD) could also
correlate with a lifetime history of dual diagnosis, in
individuals diagnosed with SCZ or BD. Our results
showed that both MDD and SCZ-PRSs had an impact
on a dual diagnosis in the total sample. Nevertheless,
when applied only to patients with one diagnosis, only
MDD:PRS was found to differentiate patients diag-
nosed with BD and dual diagnosis from patients diag-
nosed with BD without a dual diagnosis.
Our study suggests that both the MDD:PRS and the
SCZ:PRS might be of use in detecting risk for a dual
diagnosis; however, when PRSs are applied only to a
specific diagnosis, we suggest that MDD:PRS, used in
patients with BD, is the only specific PRS which cor-
relates with a dual diagnosis within the specific diag-
noses of SCZ or BD, respectively. The shared genetic
susceptibility between MDD and alcohol dependence
(AD) might be what drove this result within our BD
patients, especially when noting that the main sub-
stance of SUD in our samples was (apart from nico-
tine) alcohol abuse/dependence. These findings are
consistent with the study of Andersen et al.55, where
they suggested that shared genetic susceptibility con-
tributed to MDD and AD comorbidity.
Although other studies have applied PRS to explore
the shared genetic background between psychiatric
disorders and SUDs24,25, all the approximations have
been made in populations of European ancestry and
not in admixed populations. Our study is one of the
first approximations on how to apply psychiatric PRS
in admixed populations. Our results suggest that PRS
must be applied with caution in admixed populations
such as the Mexican population, which has individuals
with varying levels of admixture31,49,56. In relation to
this, we found that SCZ:PRS showed a correlation with
global ancestry components. The difference in PRS
based on demographic history has been previously
explored32,57. Martin et al. evaluated eight complex
traits PRS in the 1000 Genomes Project panel and
found similar results to ours; they observed that
SCZ:PRS could be deviated based on the main popula-
tion’s ancestry. In their analysis, they also reported
Figure 2. Analysis of the correlation of schizophrenia (SCZ) and major depressive disorder (MDD) polygenic risk scores (PRSs)
with global ancestry components. Triangles represent patients with bipolar disorder, circles represent patients with SCZ, and
squares represent non-psychiatric controls. Each triangle, circle, or square represents any of the 195 individuals included in the
analysis. The individuals’ graph in both plots is the same in the same position, the only change is the gradient of SCZ:PRS or
MDD:PRS value. (A) Correlation of SCZ:PRS with global ancestry components. The gradient represents the SCZ-PRS, red color
means a higher SCZ:PRS, and blue means a lower SCZ:PRS. Individuals with a higher degree of global Mexican Amerindian an-
cestry are grouped in values lower than zero in both principal components’ axes. The high correlation between the ancestry
components and the SCZ:PRS is shown. (B) Correlation of MDD:PRS with global ancestry components. The gradient represents
the MDD:PRS, red color means a higher MDD:PRS, and blue means a lower MDD:PRS. Individuals with a higher degree of global
Mexican Amerindian ancestry are grouped in values lower than zero in both principal components axes. The low correlation
between the ancestry components and the MDD:PRS is shown.
No part of this publication may be reproduced or photocopying without the prior written permission of the publisher. © Permanyer 2019

328
REV INVEST CLIN. 2019;71:321-9
that it was not possible to predict how PRS could
change according to ancestry. In this sense, we think
that the application of PRS in different populations,
with distinct admixtures and diverse phenotypes,
could give us more information on the use of PRS for
psychiatric disorders as a translational risk prediction
tool.
The results obtained from this study should be con-
sidered as preliminary due to the small sample size; it
will be necessary to increase the sample size to have
a better understanding of both the utility of PRS to
determine dual diagnosis risk in BD and SCZ, and to
assess how to correct for genetic population factors
that influence PRS. This study is among the first ones
looking at how PRSs for psychiatric disorders perform
as markers of dual diagnosis in admixed populations.
Nevertheless, we found that dual diagnosis had a
shared etiology with MDD and SCZ. The present
study can help reduce disparities in what is known
about the PRSs in different populations.
ACKNOWLEDGMENTS
This work was supported by the Instituto Nacional de
Medicina Genómica with a grant to Humberto Nicolini
(Grant number 23/2015/I). We want to thank all the
participants in the present study. We are grateful to
Samuel Canizales-Quinteros for the critical revision of
the manuscript. José Jaime Martínez-Magaña is a
doctoral student at the Doctorado en Ciencias Bio-
médicas, Universidad Juárez Autonóma de Tabasco
(UJAT) and was supported by the National Council of
Science and Technology (CONACYT) of Mexico.
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