Suggestive linkage of ADHD to chromosome 18q22 in a young genetically isolated Dutch population

Article (PDF Available)inEuropean Journal of Human Genetics 17(7):958-66 · January 2009with24 Reads
DOI: 10.1038/ejhg.2008.260 · Source: PubMed
Abstract
Attention deficit/hyperactivity disorder (ADHD) is a common, highly heritable, neuropsychiatric disorder among children. Linkage studies in isolated populations have proved powerful to detect variants for complex diseases, such as ADHD. We performed a genome-wide linkage scan for ADHD in nine patients from a genetically isolated population in the Netherlands, who were linked to each other within 10 generations through multiple lines of descent. The genome-wide scan was performed with a set of 400 microsatellite markers with an average spacing of +/-10-12 cM. We performed multipoint parametric linkage analyses using both recessive and dominant models. Our genome scan pointed to several chromosomal regions that may harbour ADHD susceptibility genes. None exceeded the empirical genome-wide significance threshold, but the Log of odds (LOD) scores were >1.5 for regions 6p22 (Heterogenetic log of odds (HLOD)=1.67) and 18q21-22 (HLOD=2.13) under a recessive model. We followed up these two regions in a larger sample of ADHD patients (n=21, 9 initial and 12 extra patients). The LOD scores did not increase after increasing the sample size (6p22 (HLOD=1.51), 18q21-22 (HLOD=1.83)). However, the LOD score on 6p22 increased to 2 when a separate analysis was performed for the inattentive type ADHD children. The linkage region on chromosome 18q overlaps with the findings of association of rs2311120 (P=10(-5)) and rs4149601 (P=10(-4)) in the genome-wide association analysis for ADHD performed by the Genetic Association Information Network consortium. Furthermore, there was an excess of regions harbouring serotonin receptors (HTR1B, HTR1E, HTR4, HTR1D, and HTR6) that showed a LOD score >1 in our genome-wide scan.
ARTICLE
Suggestive linkage of ADHD to chromosome 18q22
in a young genetically isolated Dutch population
Najaf Amin
1
, Yuri S Aulchenko
1
, Marieke C Dekker
1,2
, Robert F Ferdinand
3
,
Alwin van Spreeken
4
, Alfons H Temmink
5
, Frank C Verhulst
6
, Ben A Oostra
1
and
Cornelia M van Duijn*
,1
1
Genetic Epidemiology Unit, Department of Epidemiology & Biostatistics and Clinical Genetics, Erasmus MC,
Rotterdam, The Netherlands;
2
Department of Neurology, University Medical Centre, Nijmegen, The Netherlands;
3
De Waag Center for Forensic Psychiatry, Rotterdam, The Netherlands;
4
Department of Neurology, Sint Franciscus
Hospital, Roosendaal, The Netherlands;
5
Department of Neurology, Amphia Hospital, Breda, The Netherlands;
6
Department of Child and Adolescent Psychiatry, Erasmus MC Sophia, Rotterdam, The Netherlands
Attention deficit/hyperactivity disorder (ADHD) is a common, highly heritable, neuropsychiatric disorder
among children. Linkage studies in isolated populations have proved powerful to detect variants for complex
diseases, such as ADHD. We performed a genome-wide linkage scan for ADHD in nine patients from a
genetically isolated population in the Netherlands, who were linked to each other within 10 generations
through multiple lines of descent. The genome-wide scan was performed with a set of 400 microsatellite
markers with an average spacing of
±
1012 cM. We performed multipoint parametric linkage analyses
using both recessive and dominant models. Our genome scan pointed to several chromosomal regions that
may harbour ADHD susceptibility genes. None exceeded the empirical genome-wide significance threshold,
but the Log of odds (LOD) scores were 41.5 for regions 6p22 (Heterogenetic log of odds (HLOD) ¼ 1.67)
and 18q2122 (HLOD ¼ 2.13) under a recessive model. We followed up these two regions in a larger sample
of ADHD patients (n ¼ 21, 9 initial and 12 extra patients). The LOD scores did not increase after increasing
the sample size (6p22 (HLOD ¼ 1.51), 18q2122 (HLOD ¼ 1.83)). However, the LOD score on 6p22 increased
to 2 when a separate analysis was performed for the inattentive type ADHD children. The linkage region on
chromosome 18q overlaps with the findings of association of rs2311120 (P ¼ 10
5
)andrs4149601(P ¼ 10
4
)
in the genome-wide association analysis for ADHD performed by the Genetic Association Information
Network consortium. Furthermore, there was an excess of regions harbouring serotonin receptors
(HTR1B, HTR1E, HTR4, HTR1D, and HTR6) that showed a LOD score 41 in our genome-wide scan.
European Journal of Human Genetics (2009) 17, 958966; doi:10.1038/ejhg.2008.260; published online 21 January 2009
Keywords: children; homozygosity mapping; psychiatric; genome search; genetic isolate
Introduction
Attention deficit/hyperactivity disorder (ADHD) is a dis-
ruptive behaviour disorder characterized by a persistent
pattern of inattention and/or hyperactivity impulsivity
that is more frequent and severe than is typical for
unaffected individuals in the same stage of development.
Some impairment from these symptoms must be present in
at least two settings, for example, at home and at school.
1
ADHD has an onset in childhood but it can persist through
adolescence and into adulthood. It has been estimated that
5 7% of children and 3% of adults are affected with
ADHD.
2,3
Boys are 3 4 times more often diagnosed with
ADHD than girls.
3,4
Received 1 July 2008; revised 27 November 2008; accepted 4 December
2008; published online 21 January 2009
*Correspondence: Professor CM van Duijn, Department of Epidemiology
and Biostatistics, Erasmus MC Rotterdam, Dr Molewaterplein 50,
Rotterdam 3015 GE, The Netherlands.
Tel: þ 31 1 704 3394; Fax: þ 31 1 704 4657;
E-mail: c.vanduijn@erasmusmc.nl
European Journal of Human Genetics (2009) 17, 958 966
&
2009 Macmillan Publishers Limited All rights reserved 1018-4813/09
$32.00
www.nature.com/ejhg
Attention deficit/hyperactivity is a complex disorder
influenced by both genetic and environmental factors.
Heritability estimates from twin and adoption studies show
a strong genetic component ranging from 60 90%,
5–10
and sibling relative risk estimates range from a four- to an
eightfold increase.
11,12
A large number of genetic studies of ADHD followed a
candidate gene approach focusing mainly on the genes
involved in the dopaminergic and serotonergic path-
ways.
13
Genes studied most are the dopamine transporter
gene (DAT1), which maps to 5p15, the dopamine D4 and
D5 receptor genes ((DRD4, 11p15), (DRD5, 4p16)). These
studies have yielded a number of replicated findings, but
meta-analyses show that the associated variants are
of small effect sizes, with odds ratio ranging from
1.13 to1.9.
14,15
Until now, there have been seven independent genome-
wide linkage scans for ADHD. These include affected
sib-pair (ASP) linkage studies
16 24
and studies of extended
multigenerational families.
25,26
These studies suggested
linkage to 1p36, 2q21, 2q35, 4q13.2, 5p13, 5q13.1,
5q33.3, 6q12, 6q22 23, 7p13, 7q21, 9q22, 11q22, 13q12,
14q12, 15q15, 16q23, 17p11, and several other regions
with nominally significant evidence of linkage but no
outstanding replications. The continued failure to replicate
linkage findings for ADHD has led researchers to believe
that genes affecting ADHD have common variants with
very small effects that cannot be detected successfully with
methods relying on linkage and hence advocated the use of
association analysis.
20
Alternatively, rare variants with
strong effects may exist that fail to replicate because of
family specific mutations. A recent genome-wide associa-
tion study of ADHD performed by the International
Multisite ADHD Genetics (IMAGE) group, which was
conducted as a part of Genetic Association Information
Network (GAIN), included 958 parent child trios and
600 000 single-nucleotide polymorphisms (SNPs), but
failed to provide convincing evidence for a number of
common risk variants.
27
Although association analysis is a powerful tool to
detect common variants with small effects, linkage
analysis has proven successful in the detection of rare
variants with large effects. Linkage, for common diseases,
has been very successful in isolated populations,
28,29
as
drift and founder effects lead to the extinction of most rare
variants, while a small number is retained, which, over
subsequent generations, become frequent.
30
This is crucial
for linkage as it implies that genetic heterogeneity is
reduced.
31,32
In this study, we report the results of an independent
genome-wide linkage scan of ADHD children, which was
performed in a genetically isolated population in the
Netherlands. We compare our linkage findings to that of
the genome-wide association study of ADHD performed by
the IMAGE group.
Materials and methods
Study population
This study was conducted within the framework of the
Genetic Research in Isolated populations (GRIP)
programme. Approximately 150 individuals founded this
population, located in the South West of The Netherlands,
in the middle of the eighteenth century. The population
expanded from 700 inhabitants in 1848 to more than
20 000 inhabitants at present. For this population, a
genealogical database including records for more than
100 000 individuals is available.
For this study, two paediatric neurologists, to whom
ADHD patients are referred in GRIP, asked all of their
patients diagnosed with ADHD to participate in this study
(n ¼ 49, 22% females).
33
Thirty-three (67%) patients and
their parents agreed to participate. Of these 33 patients, 2
were excluded from analysis because their genealogy could
not be worked out, and 5 children were excluded because
they did not fulfil the criteria used for the diagnosis of
ADHD in this study. Of the remaining 26 patients, 21 were
inbred, of whom only 9 patients, who could be linked
to each other within no more than 10 generations, were
used in the initial analysis, and all 21 inbred patients
were used in the follow-up analysis.
Psychiatric assessment
The Dutch version of the National Institute of Mental
Health Diagnostic Interview Schedule for Children (NIMH
DISC or DISC)-IV was used to assess DSM-IV diagnoses.
34 36.
Psychologists and psychology students trained by the
authors of the Dutch DISC-IV administered the DISCs.
The training schedule used was similar to that used by
the authors of the original English version, at Columbia
University, New York. To obtain information regarding a
wide range of current DSM-IV axis 1 diagnoses, parent
DISCs (DISC-P) were administered during face-to-face
contacts, at a community general health centre or in a
children’s hospital. Furthermore, lifetime ADHD symptoms
were also assessed with the DISC-P. Teachers were
interviewed with the ADHD section (current, not lifetime)
of the teacher DISC (DISC-T) through telephone. The
child version of the DISC (DISC-C) was not applied, as
most of the children included in our sample were too
young (o11 years of age). Children receiving treatment
were withdrawn from medication for this study before
the interview.
Phenotypic subgroups (inattentive, hyperactive/impul-
sive, and combined) of ADHD were formed based on
application of the DSM-IV criteria that had been assessed
with the DISC. Present ADHD diagnoses were based on
information from both parents and teachers. Two types of
ADHD diagnoses were derived: (1) ‘based on one infor-
mant’, and (2) ‘based on two informants’. A diagnosis of
ADHD based on one informant was applied when either a
parent or a teacher scored six or more criteria for the
A genome-wide scan for ADHD
N Amin et al
959
European Journal of Human Genetics
inattentive, hyperactive, or combined phenotype positive,
whereas the other informant scored less than three criteria
positive. A diagnosis of ADHD based on two informants
was applied when one informant scored six or more criteria
of one of the ADHD subgroups positive and the second
informant scored three or more criteria positive. The
threshold of ‘three criteria positive’ was chosen arbitrarily
for the purpose of this study. The DSM-IV does not provide
explicit rules for the number of criteria that need to be
positive in two settings to obtain an ADHD diagnoses. It
merely states that symptoms have to be present in at least
two settings. If a child did not fulfil the criteria for present
ADHD with the DISC-P, lifetime information from the
DISC-P was used to obtain a lifetime diagnosis of ADHD.
The baseline characteristics of the patients are given in
Table 1.
Genotyping
Blood was drawn for all patients and their parents. DNA
was extracted from peripheral leucocytes using standard
procedures.
37
We performed the genome-wide linkage scan
on nine patients and their parents with a set of 400
fluorescently labelled, highly polymorphic microsatellite
markers (distance between markers
±
10 12 cM) covering
the whole genome. The remaining, distantly related, 17
patients and their parents were only typed for the markers
in the regions of interest on chromosomes 6 (n ¼ 12) and
18 (n ¼ 7). The genotyping experiments were carried out
following the manufacturer’s instructions (Applied Bio-
systems, Foster City, CA, USA).
Statistical analyses
For the nine patients used in the initial genome-wide scan,
a pedigree was extracted from the database. As the size of
the pedigree was too large (n ¼ 2206) to be analysed with
any of the available software packages for linkage analysis,
we divided the pedigree into two smaller, analysable
subpedigrees using PEDCUT software. This program allows
identification of sub-pedigrees that come under within a
pre-specified pedigree bit-size limit that can be analysed,
maximizing the size of the subgroups.
38
One of subpedi-
grees (Figure 1a) had bit-size of 46 and contained 146
individuals (six patients) and the other subpedigree
(Figure 1b) had bit-size of 19 and contained 41 individuals
(three patients).
We checked all the markers for Mendelian inconsis-
tencies using PEDCHECK, and in the case of inconsisten-
cies, a second round of laboratory quality control was
performed. In case the reasons for the problem could not
be identified, the genotypes of the parents and child(ren)
were set to missing. Marker allele frequencies were
estimated by pooling the data from a sample of 447 people
from the same population, using the maximum likelihood
method as implemented in the PoolSTR software.
39
Data
handling and preparation of input files was done with
MEGA2.
40
For the genome-wide linkage analysis, we performed
affected-only analyses using both dominant and the
recessive models. Multipoint parametric linkage analysis
under the dominant model was performed assuming a
disease allele frequency of 0.001, complete penetrance, and
a phenocopy rate of 0.01 using SIMWALK2.
The recessive analysis was performed using homo-
zygosity mapping.
41
We adjusted for inbreeding using the
shortest loop and a hypothetical loop capturing all cryptic
inbreeding.
42
The disease allele frequency was set to 0.01.
A model with complete penetrance and no phenocopies
was used to perform heterogeneity log of odds (LOD)
score computations with MERLIN.
43
Haplotypes were
constructed using SIMWALK2.
The genome-wide significance thresholds were deter-
mined empirically by performing 1000 genome-wide
simulations of our data under the null hypothesis of no
linkage. We used the complete pedigree, including all 2206
members, for marker simulation. Unlinked markers were
dropped in the complete pedigree. Number of markers and
intermarker distances were simulated according to the
typed marker set. We performed linkage analysis using the
subpedigrees. Disease-allele frequency, genetic models,
pedigrees, and penetrances were the same as those we used
in the actual linkage analysis. Genotypes of untyped
individuals were set to ‘missing’. For each genome screen,
the highest heterogenetic log of odds (HLOD) score was
recorded. The cumulative density function of the obtained
1000 maximum HLOD scores approximates the distribu-
tion of the genome-wide type I error rates. Our simulations
showed that an HLOD score of 2.65 corresponds to a
genome-wide type I error rate of 5% and that an HLOD of
1.78 corresponds to a genome-wide type I error of 50%.
Table 1 Baseline characteristics of the study population
Characteristics All
Included in genome-
wide linkage analysis
Number of patients 26 9
Number of inbred
patients
21 9
Number of patients who
received diagnosis from
two informants
16 7
Mean age at examination
(range)
10 (6 16) 10 (6 15)
Females (%) 23.1 33.3
Mean kinship 0.002 0.014
Mean inbreeding 0.002 0.006
ADHD subtype
Inattentive 12 5
Hyperactive impulsive 3 F
Combined 11 4
A genome-wide scan for ADHD
N Amin et al
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European Journal of Human Genetics
Results
As expected, due to the selection on relationship within 10
generations, those included in the initial genome-wide
linkage analysis had an almost 10-fold higher average
kinship compared with that of all patients. These patients
also showed an increased inbreeding coefficient, which
may point to a recessive form of disease.
Results of the complete genome-wide scan from both the
dominant model and recessive model with shortest and
hypothetical loops are illustrated in Figure 2a c.
There was no genome-wide significant evidence for
linkage under either model. The highest LOD score under
the dominant model was observed at 6q16 (HLOD ¼ 0.91).
Other regions that showed weak evidence of linkage
include 2q23 24 (HLOD ¼ 0.81), 3q24 (HLOD ¼ 0.75),
and 12p13 (HLOD ¼ 0.71). Homozygosity mapping yielded
five genomic regions with HLOD Z1. The strongest
evidence of linkage was observed at 18q21 22 (D18S64,
HLOD ¼ 2.13). Other regions with HLOD Z 1 include 6p23
(D6S470, HLOD ¼ 1.68), 6p12 (D6S257, HLOD ¼ 1.07),
1p36 (D1S214, HLOD ¼ 1.09), 18p11 (D18S59,
HLOD ¼ 1.15), and 15q25 (D15S205, HLOD ¼ 1.19). Details
are provided in Table 2. Adjusting for multiple inbreeding
loops (Figure 2c) did not alter our findings, decreasing LOD
scores only marginally.
The patients’ haplotypes at chromosome 18 (presented
in Figure 1) show excess of homozygosity but not at a
single marker. Four out of nine patients are homozygous
for allele 1 of the marker D18S464. This is, however, the
most common allele, with a frequency of homozygosity of
0.42. Also, at marker D18S64, five patients are homo-
zygous: two are homozygous for allele 1, which has a
frequency of homozygosity of 0.05; two are homozygous
for allele 2, which has a frequency of homozygosity of
about 0.002; and one patient is homozygous for allele 5,
which has a frequency of homozygosity of 10
9
.
The regions of interest on chromosomes 18 and 6 were
additionally typed for the remaining distantly related 17
patients, and the data from all 21 inbred patients (Figure 3)
were reanalysed. For this analysis, we used only the
recessive model of inheritance as it yielded the evidence
for linkage in the initial analyses.
The LOD scores did not increase by increasing the
sample size, but rather decreased due to adding non-
informative individuals (Figure 4). On chromosome 6p, the
highest LOD score we observed was HLOD ¼ 1.51, at
marker D6S470, using homozygosity mapping. This HLOD
score increased to 2 at marker D6S1574 when a recessive
model with a disease allele frequency of 0.01 and complete
penetrance was run separately for the 10 patients with only
inattentive type ADHD (Figure 4). For chromosome 18, the
highest LOD score observed was 1.83 at marker D18S1161.
The haplotype analysis, however, showed that 11 out of 21
inbred patients were homozygous for the same allele at
marker D18S464 (Figure 3), and 10 patients were homo-
zygous at marker D18S64.
Figure 1 Subpedigrees after breaking the complete pedigree. Inbreeding coefficients and haplotypes for chromosome 18 for all nine patients are
shown at the bottom.
A genome-wide scan for ADHD
N Amin et al
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European Journal of Human Genetics
Finally, we compared our findings with those from
the genome-wide association analysis of the GAIN
consortium. The chromosome 18 region identified in
our linkage analysis also showed evidence for association
in the GAIN analysis; rs2311120 (P ¼ 10
5
), rs4149601
(P ¼ 10
4
), rs9973180 (P ¼ 10
4
), and rs2006776
(P ¼ 10
4
) are located in the linkage region we identified
(Figure 4). These SNPs, particularly rs2311120, which was
the third most significant SNP, were among the top
100 most strongly associated SNPs in the GAIN
consortium. Also for chromosome 6p in the region we
identified in our genome-wide scan, there was also some
evidence of association in the GAIN study (rs2772387,
P ¼ 3 10
4
).
Discussion
We performed a genome-wide scan using two extended
families from a genetically isolated Dutch population. As
these pedigrees were selected based on consanguinity, we
Figure 2 LOD score plots from multipoint analyses of the whole autosomal genome in nine ADHD patients. (a) Shows the LOD scores for
dominant model, b and c for recessive model under homozygosity mapping with the shortest loop and hypothetical loop, respectively. The horizontal
axis depicts the whole genome divided into 22 autosomes, and the vertical axis depicts the LOD scores in each panel.
Table 2 Loci with MLS Z1 under homozygosity mapping
Most likely
Multipoint HLOD based on
homozygosity mapping
Chromosome Position (cM) cytogenetic location Nearest marker Liberal
a,b
Conservative
a,c
1 17 1p36 D1S450 1.1 0.258
5 174 5q33 D5S422 0.73 1.03
6 20 6p22 D6S289 1.67 0.85
6 69 6p12 D6S257 1.07 0.47
6 102 6q15 D6S462 1.125 0.58
15 78.5 15q25 D15S205 1.19 0.54
18 117 18q21 D18S64 2.13 1.27
18 140 18q22 D18S1161 1.2 1.81
18 0 18p11 D18S59 1.15 0.22
a
Estimated using MERLIN.
b
Results from the linkage analysis of pedigrees based on shortest inbreeding loop.
c
Results from the linkage analysis of pedigrees constructed hypothetically using the inbreeding coefficient.
A genome-wide scan for ADHD
N Amin et al
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European Journal of Human Genetics
selected families with recessive form of disease. There was
an increased inbreeding so we did not expect a dominant
form of disease. We, however, tested a dominant model
because in isolated populations the disease may appear in
pseudo-dominant forms. We did not observe evidence of
significant or even suggestive linkage under a dominant
model. Using a recessive model, we identified six genomic
regions with HLOD score 41. Although none of these
regions passed the genome-wide significance threshold,
there was suggestive evidence of linkage at 18q21 22
(HLOD ¼ 2.13, marker D18S64). The LOD score did not
increase with the increase in the sample size. This may be
explained by the fact that the patients selected for the
genome-wide linkage analysis were the ones showing
the strongest evidence for a recessive form of the disease,
as the inbreeding coefficient was the highest for these
patients (Table 1). The region 18q2122 has been im-
plicated earlier as a major susceptibility locus for bipolar
disorder.
44
None of the children in our sample received a
clinical or DISC-P diagnosis of bipolar disorder, and none
of the children received a clinical diagnosis of schizo-
phrenia or were scored positive on the psychosis screen of
the DISC-P. As none of the patients included in our study
showed evidence of bipolar disorder or schizophrenia,
there is no evidence of misdiagnosis explaining our
finding. Considering the fact that some of the symptoms
of ADHD and bipolar disorder coincide, and that ADHD in
childhood increases the risk for later developing bipolar
disorder, this finding suggests that this locus might
harbour pleiotropic genes that increase the risk of both
ADHD and bipolar disorder. Our findings are further
supported by the fact that this region showed evidence of
association in the GAIN study; rs2311120 was the third
most significant SNP in the GAIN analysis. There were also
other, less significant, SNPs in the same region (rs9973180,
P ¼ 10
4
, rs2006776, P ¼ 10
4
) supporting the hypothesis
that this region is implicated in ADHD. This region,
however, did not show evidence of linkage when an ASP
linkage scan
19
was performed by the IMAGE group, which
used a sample largely from the GAIN families.
The other interesting region in our genome-wide scan
comprises of two adjacent regions on chromosome 6:
6p22 24 (HLOD 1.67, marker D6S257) and 6p12 (HLOD
1.07, marker D6S257). 6p22 is one of the most frequently
replicated susceptibility regions for reading disability (RD)
or dyslexia,
45 48
and has also been implicated as a
susceptibility locus for ADHD
49
in a study of ADHD within
sibpairs identified for RD . In a subanalysis of this region,
the LOD score increased to HLOD ¼ 2 (marker D6S309)
when the data were analysed separately for the patients
Figure 3 Subpedigrees after breaking the complete pedigree for all 26 patients and haplotypes for chromosome 18 for only the inbred patients
(n ¼ 21) are shown at the bottom.
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N Amin et al
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European Journal of Human Genetics
having predominantly inattentive type ADHD. However,
subgroup analyses may cause false-positive findings and
remain to be confirmed. When considering other regions
that were suggested by others, the region 6q15 that
had a HLOD of 1.13 in our study and also harbours
serotonin receptor genes HTR1B and HTR1E is adjacent
(distance ¼ 10 cM) to the region 6q14, which was identified
by Ogdie et al
23,24
as a nominally significant susceptibility
locus for ADHD. Our genome scan also showed some
evidence of linkage to 5q33 (conservative homozygosity
mapping HLOD 1.03, marker D5S422) (Figure 2c). This
region harbours serotonin receptor 4 (HTR4) and was first
identified with significant evidence of linkage in a genome-
wide scan for ADHD in an isolated population from
Colombia.
25
The region 1p36 (HLOD ¼ 1.09, marker
D1S450) has recently been identified as a susceptibility
locus for ADHD with significant evidence of linkage in a
linkage study of quantitative ADHD traits performed by the
IMAGE group.
22
This region harbours serotonin receptor 6
(HTR6) and serotonin receptor 1D (HTR1D) genes. It is of
interest that each of these marginally linked regions
includes serotonin receptors. HTR1B, HTR1E, HTR4,
HTR1D, and HTR6 genes are known candidate genes for
ADHD, although, except for HTR1B,
50 52
the association of
these genes with ADHD has not been established.
Although our study sample was small, the strength of our
population lies in the fact that we can select patients based
on genealogy. In that way, we can specifically target
patients with dominant, or, in this case, recessive forms
of disease based on their consanguinity. Taken together
with the GAIN results, our study yields evidence that
18q21 22 may be relevant for ADHD. Our findings, in
conjunction with those of GAIN, ask for further follow-up
of the region. Furthermore, our study suggests that the
serotonin receptors HTR1B, HTR1E, HTR4, HTR1D, and
HTR6 might be implicated in ADHD.
Although isolated populations may facilitate the detec-
tion of linkage, caution is required in generalizing the
results to other populations. It is, therefore, necessary that
these regions be followed up in other populations.
Although our findings are compatible with those of GAIN,
their credibility will increase if confirmed elsewhere.
Acknowledgements
We acknowledge the collaboration of all patients, their parents, and
their teachers. We thank Petra Veraart for the genealogy. We also
thank Aaron Isaacs for verifying the manuscript. This study was
supported by a grant from the Sophia Foundation for Scientific
Research (SSWO, no. 318) and the Center of Medical Systems Biology
(CMSB).
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    • "Our significant QTL for inattention on chromosome 18q21.1– 18q22.3 has previously shown a suggestive linkage in the young genetically isolated population from the Netherlands using the Dutch version of the NIMH Diagnostic Interview Schedule for children that utilizes ADHD DSM-IV diagnostic criteria for the phenotypic definition [Amin et al., 2009]. An intergenic rs2311120 SNP on 18q21.2 was recently reported to be the third most associated SNP after the TDT-bias correction (P ¼ 1.22EÀ05). "
    [Show abstract] [Hide abstract] ABSTRACT: Attention deficit/hyperactivity disorder (ADHD) is a neurodevelopmental phenotype that persists into adulthood. This study investigated the heritability of inattentive and hyperactive symptoms and of total ADHD symptomatology load (ADHD index) in adults and performed linkage scans for these dimensions. Data on sibling pairs and their family members from the Netherlands Twin Register with genotype and phenotype data for inattention, hyperactivity and ADHD index (∼750 sib-pairs) were analyzed. Phenotypes were assessed with the short self-report form of the Conners' Adult ADHD Rating Scales (CAARS). Heritabilities were estimated in SOLAR under polygenic models. Genome-wide linkage scans were performed using variance components (VC) in MERLIN and MINX and model-based linkage analysis was carried out in MENDEL with empirical evaluation of the results via simulations. Heritability estimates for inattention, hyperactivity and ADHD index were 35%, 23%, and 31%, respectively. Chromosomes 18q21.31-18q21.32 (VC LOD = 4.58, p(emp)  = 0.0026) and 2p25.1 (LOD = 3.58, p(emp)  = 0.0372) provided significant evidence for linkage for inattention and the ADHD index, respectively. The QTL on chromosome 2p25.1 also showed suggestive linkage for hyperactivity. Two additional suggestive QTLs for hyperactivity and the ADHD index shared the same location on chromosome 3p24.3-3p24.1. Finally, a suggestive QTL on 8p23.3-8p23.2 for hyperactivity was also found. Heritability of inattention, hyperactivity and total ADHD symptoms is lower in adults than in children. Chromosomes 18q and 2p are likely to harbor genes that influence several aspects of adult ADHD.
    Full-text · Article · Apr 2011
  • [Show abstract] [Hide abstract] ABSTRACT: Attention deficit hyperactivity disorder (ADHD) is a common neuropsychiatric disorder. Genetics has an important role in the aetiology of this disease. In this study, we describe the clinical findings in a Dutch family with eight patients suffering from ADHD, in whom five had at least one other psychiatric disorder. We performed a genome-wide (parametric and nonparametric) affected-only linkage analysis. Two genomic regions on chromosomes 7 and 14 showed an excess of allele sharing among the definitely affected members of the family with suggestive LOD scores (2.1 and 2.08). Nonparametric linkage analyses (NPL) yielded a maxNPL of 2.92 (P=0.001) for marker D7S502 and a maxNPL score of 2.56 (P=0.003) for marker D14S275. We confirmed that all patients share the same haplotype in each region of 7p15.1-q31.33 and 14q11.2-q22.3. Interestingly, both loci have been reported before in Dutch (affected sib pairs) and German (extended families) ADHD linkage studies. Hopefully, the genome-wide association studies in ADHD will help to highlight specific polymorphisms and genes within the broad areas detected by our, as well as other, linkage studies.
    Full-text · Article · Sep 2009
  • [Show abstract] [Hide abstract] ABSTRACT: Abnormalities in electrophysiological measures of stimulus-evoked brain activity (including the P3 event-related potential (ERP) and its associated delta and theta time-frequency (TF) components), and intrinsic, resting state brain activity (including EEG in the beta frequency band) have each been associated with biological vulnerability to a variety of externalizing (EXT) spectrum disorders, such as substance use disorders, conduct disorder, and antisocial behavior. While each of these individual measures has shown promise as an endophenotype for one or more aspects of EXT, we proposed that the power to identify EXT-related genes may be enhanced by using these measures collectively. Thus, we sought to explore a multivariate approach to identifying electrophysiological endophenotypes related to EXT, using measures identified in the literature as promising individual endophenotypes for EXT. Using data from our large twin sample (634 MZ and 335 DZ, male and female same-sex pairs), and fitting multivariate biometric Cholesky models, we found that these measures (1) were heritable, (2) showed significant phenotypic and genetic correlation with a general vulnerability to EXT (which is itself highly heritable), (3) showed modest phenotypic and genetic correlation with each other, and (4) were sensitive to genetic effects that differed as a function of gender. These relationships suggest that these endophenotypes are likely tapping into neurophysiological processes and genes that are both common across them and unique to each-all of which are relevant to a biological vulnerability to EXT psychopathology.
    Full-text · Article · Feb 2010
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