Candidate Gene Studies in Substance-Dependent Adolescents, their Siblings, and Controls

1
Candidate Gene Studies in Substance-Dependent
Adolescents, their Siblings, and Controls S. E.
Young, A. Smolen, M. C. Stallings, R. P. Corley,
T. J. Crowley and J. K. Hewitt
University of Colorado School of
Medicine Division of Substance Dependence
STUDY III Are dopamine risk alleles associated
with severity of substance dependence or conduct
problems? Patient Sample means(s.d.)

• BACKGROUND
• Why study the dopamine system?
• Dopamine is known to mediate motor activity and
  reward mechanisms.
• Dopamine imbalance is implicated in
  psychopathology
• Schizophrenia
• Tourette Syndrome
• Depression
• Substance Abuse
• ADHD
• WHICH CANDIDATE GENES?
• Dopamine Transporter (DAT1)
• Mapped to 5p15.316
• Non-coding region at 3 end
• 40 base-pair VNTR (3-11 repeats)
• DAT1 polymorphisms have been associated with
• Substance Dependence8,12

• ASSESSMENTS
• Substance Use and Dependence
• Composite International Diagnostic Interview
• Substance Abuse Module4
• Average Dependence Symptoms
• Sum of symptoms across substances
• substances used gt 5 times
• Conduct Disorder Symptoms
• Diagnostic Interview Schedule for Children 13
  
• SAMPLE CHARACTERISTICS

• SUMMARY I
• We found no significant differences in genotypic
  frequencies for DAT1 among the three groups.
• Genotypic frequencies for DRD2 were significantly
  different among the three groups (?212.7
  p.01).
• There were significant differences in the
  genotypic frequencies among the three groups for
  DRD4 (?214.1 p.01).
• However, these differences were driven by the
  sibling sample, and do not remain significant
  when comparing the patient sample to the control
  sample.

Genotype Nic Sx Alc Sx Avg Sx
Cond Sx __________________________________________
DAT1 (N) 9/9 (9) 1.7 (1.9) 2.3 (2.5)
1.6 (1.0) 8.2 (4.2) 9/10 (65) 2.9 (2.3)
2.1 (2.0) 2.5 (1.4) 5.4 (3.0) 10/10 (100) 2.8
(2.2) 1.9 (2.0) 2.5 (1.3) 5.8 (2.9) DRD2
A1/A1 (16) 2.6 (2.6) 1.8 (2.3) 2.3 (1.6)
4.9 (2.9) A1/A2 (73) 2.9 (2.2) 2.2 (2.1)
2.4 (1.3) 5.9 (2.5) A2/A2 (88) 2.7 (2.1)
1.9 (1.9) 2.5 (1.2) 5.9 (3.2) DRD4
L/L ( 7) 3.6 (2.1) 1.6 (2.4) 2.9 (1.0)
6.5 (3.3) L/S (54) 2.6 (2.2) 1.7 (1.5) 2.2
(1.1) 5.9 (3.1) S/S (110) 2.8 (2.2) 2.2
(2.1) 2.5 (1.4) 5.7 (3.0)

• STUDY II
• Are frequency differences for DRD2 confirmed in
  family-based analyses?
• The Transmission/Disequilibrium Test15
• The transmission/disequilibrium test (TDT)
  assesses deviations from equal transmission of
  two alleles from heterozygous parents to affected
  children.
• TDT (t21 t12)2
• --------------------
• t21 t12
• -- which is asymptotically distributed as
  chi-square with one degree of freedom.
• The TDT requires genotypic data on both parents
  and an affected child only heterozygous parents
  are informative.
• One advantage of the TDT is that it provides a
  test of association while avoiding Type I error
  due to population stratification.
• Example (A1 risk allele)

• SUMMARY III
• Analysis of variance revealed no significant
  differences in the mean symptom counts for
  nicotine dependence, alcohol dependence, average
  dependence or conduct disorder by genotype for
  DAT1, DRD2 or DRD4.
• There was a trend towards higher conduct disorder
  symptoms for the 9/9 genotype for DAT1, which
  will be followed up as the sample size increases.
• DISCUSSION
• We found preliminary evidence for an association
  between the DRD2 A1 risk allele and substance
  dependence in an analysis of adolescent patients,
  their siblings, and controls (Study I).
• This association was not confirmed when a
  family-based TDT was applied (Study II). The
  sample size was modest (n107 informative trios)
  however, the pattern of transmission was not
  consistent with the predicted association.
• Risk alleles for DAT1, DRD2 and DRD4 were not
  associated with symptom severity in the patient
  (affected) group (Study III).
• These results obtained in an adolescent sample do
  not confirm findings previously reported on adult
  alcohol or nicotine dependent samples.
• Differences in these findings may also be due, in
  part, to the fact that our patient sample is a
  polysubstance abusing sample with extensive
  comorbid psychopathology.

STUDY I Are there increased frequencies of
dopamine risk alleles in the clinical samples
compared to the control samples?
Allele Frequencies
Genotype Frequencies
DRD2 DRD2
Non-transmitted Transmitted A1
A2 A1 -- 48
A2 56 -- TDT (?2)
(56 48)2 0.62 p .43
(56 48)
Allele Frequencies
Genotype Frequencies

• SUMMARY II
• Of the 104 informative family trios, 32 of the
  parental genotypes were unambiguously inferred
  from sibling genotypes.
• The TDT was non-significant (?2 .62 p .43),
  suggesting that the allelic association shown in
  Study I (greater frequency of the A1 allele in
  clinical vs. control samples) may have been due
  to population stratification.
• Previous analysis of trios without inferred
  parental genotypes produced similar results (?2
  1.03 p .75).

Allele Frequencies
Genotype Frequencies