Short communicationIllicit drug use and abuse/dependence: modeling of two-stage variables using the CCC approach
Introduction
Illicit drug use is moderately heritable (26–40% due to genetic factors or A) with evidence for the role of shared environmental (environment shared by members of a twin pair or C) and individual-specific environmental factors (E). Abuse/dependence of illicit drugs is highly heritable (28–79% due to A) with the remainder of the variance due to individual-specific environmental influences (Kendler et al., 2000, Kendler et al., 1999a, Tsuang et al., 2001, Van den Bree et al., 1998). However, drug involvement is a two-stage process where drug use (the so-called “upstream” variable), in certain cases, is followed by the clinically diagnosed stage of abuse/dependence (the “downstream” variable). This contingent nature (use must precede abuse/dependence) was not considered in previous univariate models. The causal-common-contingent (CCC) model is an improvement over the standard independent assessments of drug use and abuse/dependence (Kendler, Neale, & Sullivan et al., 1999). The model allows for partitioning of genetic and environmental factors common to drug use and abuse/dependence from the factors that are unique to abuse/dependence. Using the CCC model, in the present study we sought to examine the role of common and specific additive genetic (A), shared environmental (C) and unique environmental (E) factors on the two stages (use and abuse/dependence) of cannabis, cocaine, sedatives, stimulants and any illicit drug involvement and test for the presence of quantitative gender differences.
Section snippets
Sample
This study utilized drug use and abuse/dependence information from 1191 (702 MZ and 489 DZ pairs) male and 934 (556 MZ and 378 DZ pairs) female same-sex monozygotic (MZ) and dizygotic (DZ) Caucasian twin pairs from the Virginia Adult Twin Study of Psychiatric and Substance Use Disorders (VATSPSUD), a longitudinal study based on participants from the Virginia Twin Registry (now a part of the Mid-Atlantic Twin Registry). The most recent and complete waves of interview were used for these
Results
Mean age of the female twins was 35.8 years (range 21–62 years) at the fourth wave of interviews with a mean education level of 14.3 years. The male twin pairs had a mean age of 35.5 years (range 20–58 years) and a mean education level of 13.6 years at the second wave of interviews. Standardized parameter estimates for each illicit drug are presented in Table 1, separately for males and females. For all the illicit drugs, except sedatives, ai2 ranges between 0.48 and 0.73, ci2 ranges between
Discussion
Drug use and abuse/dependence should be modeled as a two-stage process. Previous studies have typically modeled use and abuse/dependence, individually, without considering the contingent relationship between them. Two previous studies used data from the same data set to assess use and abuse/dependence with the CCC approach. In this study, we sought to obtain the best estimates of additive genetic, shared environmental and unique environmental factors on illicit drug use and abuse/dependence
Acknowledgements
This work was supported by NIH grants MH-40828, MH/AA/DA-49492, AA-09095 and DA-11287. We acknowledge the contribution of the Virginia Twin Registry, now part of the Mid-Atlantic Twin Registry (MATR), for ascertainment of subjects for this study. The MATR, directed by Dr. J Silberg, L Corey and L. Eaves, has received support from the National Institutes of Health, the Carman Trust and the WM Keck, John Templeton and Robert Wood Johnson Foundations.
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