SUPPLEMENTARY INFORMATION
Sample ascertainment procedure
A list of twins born between 1973 and 1980 where both were registered as living in
Stockholm County was made available from the Swedish Twin Registry1,2 (N=366 pairs). A
postal letter with study information was sent to all twins. After exclusion due to one or both
twins in a pair having migrated, (n=23 pairs), declined participation in the study (n=97 pairs),
was not Swedish speaking (n=1 pair) or investigators failing to establish personal contact
(n=79 pairs), 144 pairs remained for telephone screening by a clinical psychologist and
psychiatrist (J.B. and S.C.). Of these, 82 pairs were excluded due to one or both twins in a
pair reporting present or previous neurological condition including for example migraine and
seizures (n=16), present of previous psychiatric conditions including substance use (n=9),
present or previous treatment with CNS-active drugs (n=11), present or previous major
somatic disorder (n=10), major psychiatric illness in first- and second-degree relatives,
regular nicotine use (n=15), recent use of illegal substances (n=5) and metal implants in the
body.
Zygosity status was determined with a twin similarity interview and later confirmed by means
of genotyping of 80 Single Nucleotide Polymorphisms (SNPs) and 9 variable number tandem
repeats (VNTRs). At the late stage only DZ pairs remained to be included whereas 43 pairs
were excluded due to being MZ according to the interview. In total, this procedure made 34
twin pairs available for further screening. Of these 68 subjects, 65 were subjected to a family
history of psychiatric disorders and a psychiatric interview performed by two experienced
psychiatrists (E.G.J. and P.S.) including the Structured Clinical Interview for DSM-III-R3
1
which includes the global assessment of functioning (GAF) scale.4 In addition, questions
regarding present and previous tobacco smoking/snuffing, present alcohol consumption and
the estimated maximal regular alcohol consumption ever, as well as the Alcohol Use
Disorders Identification Test (AUDIT)5 were administered. If any psychiatric diagnosis was
suspected, symptoms were also evaluated according to DSM-IV criteria6. As part of the
screening procedure, participants were also subjected to a standard somatic investigation
performed by a physician (S.C., E.G.J., P.S.), a standard screening blood test, and brain
magnetic resonance imaging (MRI), the results of which were examined by a neuroradiologist
for gross anomalies.
Exclusion criteria were evidence for present or previous major physical illness or mental
disorder, substance use disorders, brain damage and brain disorders, as well as present
tobacco use, aberrant findings on somatic investigation, in blood analyses and on brain MRI.
In addition, a history of treatment for psychiatric disorder in any first degree relative was a
cause for exclusion. After the total screening procedure 13 of the twin pairs were excluded.
The reasons for exclusion were: only one of the two twins came to the screening
investigations (n=3), one in a pair had metal implant (n=1), one in a pair had signs of previous
bleeding on MRI (n=1), one or two in the pairs were judged to have had a previous
psychiatric diagnosis, i.e. attention deficit hyperactivity disorder (ADHD, n=1), major
depressive episode (n=2) or panic disorder (n=1), one in a pair had been treated with oral
corticosteroids because of acute allergic reaction shortly before the screening and the co-twin
was a regular smoker (n=1), one in a pair stuttered since childhood and the co-twin had
repeated fainting episodes, which were subjects for investigation (n=1), one of the twins died
in an airplane accident three weeks after the screening procedure (n=1), and in one pair a first
degree relative had been treated for depression (n=1).
2
Subject description
The remaining 42 participants were all working or studying full-time. Their mean (SD) GAF
scores were 88.5 (4.96). Mean (SD), minimum, maximum and median years of education
were 14.6 (2.53), 11, 19, and 14.0, respectively. Their mean (SD), minimum, maximum and
median of alcohol consumption during the last month was 222 (159), 0, 647, and 181 g/week,
respectively. Their mean (SD), minimum, maximum and median of maximal regular alcohol
consumption during life was 786 (717), 25, 2880, and 493 g/week, respectively. Average
(SD), minimum, maximum and median AUDIT scores were 5.1 (2.27), 1, 12, and 5,
respectively. None of the subjects fulfilled criteria for alcohol abuse or alcohol dependence
according to DSM-III-R (6) or DSM-IV. Twenty-nine had never used illegal drugs, whereas
13 had used illegal drugs. Of these 11 had smoked cannabis one to ten (median 1.5) times,
two had used stimulants (amphetamine or ephedrine) one and two times, two had used
cocaine one to two times, one had used ecstasy once, and one had used anabolic steroids once.
The person who had used illegal drugs most frequently had taken cannabis five to ten times,
cocaine twice and ecstasy once. The average (SD) duration at PET since the subjects used
illegal drugs was 6.9 (2.18) years (range 2.6 – 10.2, median 7.0). None of the subjects fulfilled
criteria for any substance abuse or dependence diagnosis according to DSM-III-R or DSM-IV.
Of the remaining subjects 26 had never smoked tobacco, or only “tried”, whereas 15 had to
various degrees been former regular smokers. These 15 smokers had used on average (SD)
390 (404) packages of cigarettes (range 3 - 1004, median 193). Twenty-six had never used
oral tobacco (“snus”, snuff), whereas 16 were former snuff users, who had on average (SD)
used 1327 (852) packages (range 15 – 3120, median 1222).
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At the screening interview four subjects used legal drugs: cutaneous emulsion hydrocortisone
butyrate 2% once daily (n=1), nasal spray fluticasone propionate 100 microgram once daily
(n=1), tablet cetirizine 10 mg daily, inhalation of combination fluticasone propionate 50
microgram/salmeterol 250 microgram once to twice daily, ointment hydrocortisone 1% once
daily (n=1) and oral ferrous succinate, anhydrous 120 mg once daily (n=1). However, at the
PET investigation, none of the subjects used any drugs.
Parametric PET image preprocessing and analysis
Parametric PET images were obtained with wavelet-aided parametric imaging (WAPI) using
the non-invasive Logan plot7 with start times (t*) of 18 and 30 minutes for [11C]raclopride and
[11C]WAY100635 respectively. These images were resliced into the space of MR images
using SPM coregistration parameters. Preprocessing steps for the parametric analyses were
performed using FSL 5.0.78 within NeuroDebian9. T1 MR images were pre-masked and skullstripped using BET10. MR images were normalised to the MNI template (MNI152, 2mm) by
first applying the affine registration tool FLIRT11, and then performing non-linear registration
using FNIRT12. The FNIRT non-linear transformation matrix was then applied to the resliced
PET images in MR-space to generate parametric PET images in MNI space. The PET images
were brain-masked and then smoothed using a FWHM of 8 mm.
To perform a confirmatory heritability analysis on parametric PET images, the parametric
images were imported into R software (version 3.1.3, “Smooth Sidewalk”) using the oro.nifti
package13 (http://cran.r-project.org/web/packages/oro.nifti/oro.nifti.pdf), and masked for the
structures analysed in the ROI analysis using masks generated by FSL. ICC coefficients were
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calculated for each voxel within the specified masks using the “psych” package14
(http://personality-project.org/r/psych/).
References
1
Lichtenstein P, De Faire U, Floderus B, Svartengren M, Svedberg P, Pedersen NL. The
Swedish Twin Registry: a unique resource for clinical, epidemiological and genetic
studies. J Intern Med 2002; 252: 184–205.
2
Pedersen NL, Lichtenstein P, Svedberg P. The Swedish Twin Registry in the third
millennium. Twin Res 2002; 5: 427–32.
3
Spitzer RL, Williams JBW, Gibbon M. Structured Clinical Interview for DSM-III-R Non Patient Version (SCID-NP). Biometrics Research Department, New York State
Psychiatric Institute: New York, 1986.
4
Endicott J, Spitzer RL, Fleiss JL, Cohen J. The global assessment scale. A procedure
for measuring overall severity of psychiatric disturbance. Arch Gen Psychiatry 1976;
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5
Saunders JB, Aasland OG, Babor TF, de la Fuente JR, Grant M. Development of the
Alcohol Use Disorders Identification Test (AUDIT): WHO Collaborative Project on
Early Detection of Persons with Harmful Alcohol Consumption--II. Addiction 1993;
88: 791–804.
6
American Psychiatric Association. Diagnostic and statistical manual of mental
disorders, fourth edition, international version. 1995.
7
Cselényi Z, Olsson H, Halldin C, Gulyás B, Farde L. A comparison of recent
parametric neuroreceptor mapping approaches based on measurements with the high
affinity PET radioligands [11C]FLB 457 and [11C]WAY 100635. Neuroimage 2006;
32: 1690–708.
8
Jenkinson M, Beckmann CF, Behrens TEJ, Woolrich MW, Smith SM. FSL.
Neuroimage 2012; 62: 782–90.
9
Halchenko YO, Hanke M. Open is Not Enough. Let’s Take the Next Step: An
Integrated, Community-Driven Computing Platform for Neuroscience. Front
Neuroinform 2012; 6: 22.
10
Smith SM. Fast robust automated brain extraction. Hum Brain Mapp 2002; 17: 143–55.
11
Jenkinson M, Smith S. A global optimisation method for robust affine registration of
brain images. Med Image Anal 2001; 5: 143–56.
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12
Andersson JLR, Jenkinson M, Smith S. Non-linear registration, aka Spatial
normalisation. FMRIB Analysis Group Technical Reports TR07JA2. FMRIB Centre,
Oxford, United Kingdom: Oxford, 2007
http://www.fmrib.ox.ac.uk/analysis/techrep/tr07ja2/tr07ja2.pdf.
13
Whitcher B, Schmid V, Thornton A. Package ‘oro.nifti’. 2011. http://cran.rproject.org/web/packages/oro.nifti/oro.nifti.pdf.
14
Revelle W. The Psych Package. 2014. http://personality-project.org/r/psych.
Supplementary Figure legends
Supplementary Figure S1. D2/3-dopamine receptor binding in the limbic striatum plotted pair
wise for MZ and DZ pairs. Each twin pair is represented by one dot with the BP value for
twin 1 in the pair on the x-axis and the BP value for twin 2 (the co-twin) on the y-axis.
Supplementary Figure S2. D2/3-dopamine receptor binding in the associative striatum plotted
pair wise for MZ and DZ pairs. Each twin pair is represented by one dot with the BP value for
twin 1 in the pair on the x-axis and the BP value for twin 2 (the co-twin) on the y-axis.
Supplementary Figure S3. D2/3-dopamine receptor binding in the sensorimotor striatum
plotted pair wise for MZ and DZ pairs. Each twin pair is represented by one dot with the BP
value for twin 1 in the pair on the x-axis and the BP value for twin 2 (the co-twin) on the yaxis.
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Supplementary Figure S4. 5-HT1A receptor binding in the temporal cortex plotted pair wise
for MZ and DZ pairs. Each twin pair is represented by one dot with the BP value for twin 1 in
the pair on the x-axis and the BP value for twin 2 (the co-twin) on the y-axis.
Supplementary Figure S5. 5-HT1A receptor binding in the hippocampus plotted pair wise for
MZ and DZ pairs. Each twin pair is represented by one dot with the BP value for twin 1 in the
pair on the x-axis and the BP value for twin 2 (the co-twin) on the y-axis.
Supplementary Figure S6. 5-HT1A receptor binding in the amygdala plotted pair wise for MZ
and DZ pairs. Each twin pair is represented by one dot with the BP value for twin 1 in the pair
on the x-axis and the BP value for twin 2 (the co-twin) on the y-axis.
Supplementary Figure S7. Box plots showing the distribution and individual values of
regional D2/3 receptor BP (top) and regional 5-HT1A receptor BP (bottom).
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