Aims:

To assess the theory of the Gateway Hypothesis, whereby the use of substances such as tobacco and alcohol ('Gateway Drugs') are a risk factor for the use of harder drugs such as cannabis, cocaine and opiods.

Hypotheses:

The hypotheses of this project can be subdivided into three phases:

1. Assessment of the relationship between patterns of smoking and patterns of alcohol consumption in adults. These hypotheses will be tested using the MR technique, where patterns of smoking can relate to either daily cigarettes smoked or short-term smoking cessation.

2. Assessment of the relationship between patterns of smoking and patterns of alcohol consumption in adolescents. This hypotheses will be tested used either standard epidemiological techniques or Mendelian randomisation.

3. Assessment of the relationship between cannabis and other illegal substances. These hypotheses will be tested using hair sample data taken from adolescents.

There is also scope for further analysis linking hypotheses 1 and 2 with that of 3; however this will be assessed at a later date when preliminary results have been obtained.

The following exerpt it taken from the proposal written in application of the PhD studentship for which this research is a part of and will explain all variables being used in the study:

Phase 1 - Relationship between smoking and alcohol in adults

SNP rs1051730 has been shown to be causally related to both heaviness of smoking and smoking cessation in adults. With this in mind, I propose to use the data collected from the mothers in the ALSPAC cohort in order to use MR to assess both the relationship between heaviness of smoking and alcohol consumption and smoking cessation and alcohol consumption. With regards to heaviness of smoking, the exposure variable used will cigarettes smoked per day in the sample of those who smoke, and for smoking cessation, smoking status before and during pregnancy. The outcome variable used for alcohol consumption will focus on heaviness of use.

Once observational epidemiology has been used to initially assess the relationship between tobacco and alcohol, the relationship between genotype and smoking status will be tested using a model with and without interaction terms between the two variables, using a likelihood ratio test. The relationship between genotype and alcohol consumption will also be tested in this manner in order to show whether the genotype is associated with alcohol consumption independently of smoking status. The effect of both heaviness of smoking and smoking cessation on alcohol consumption will be assessed using instrumental variable estimation by performing a two stage least squares under an additive model. This method will first fit the regression of the exposure (smoking) on the instrument (genotype) before the outcome (alcohol) is regressed on the predicted values of continuing smoking, where the estimate of the causal effect will be the coefficient produced. I will also test whether there is a direct effect of genotype on alcohol consumption in non-smokers.

Phase 2 - Relationship between smoking and alcohol in adolescents

To further the work undertaken in phase1 of this research, the relationship between heaviness of smoking and alcohol consumption and adolescents will be assessed using the ALSPAC child based questionnaires and interviews. As a current genetic signal for smoking in adolescence is not known, this work will explore the availability of a polygene risk score for use with MR, or use a standard epidemiological technique of this is unavailable. The exposure and outcome variables used for this section of analysis will be smoking initiation and heaviness of alcohol use.

The first stage of this analysis will be to gather cross sectional studies that have included the rs1051730 across a range of ages in order to assemble a longitudinal map of the effect of this variant, as has been previously done with the FTO gene.

If a genotypic signal is available for use in this analysis then statistical methods for the rest of the phase will be as for phase 1, however if a standard epidemiological method is applied then multinomial logistic regression will be used with smoking as the exposure and alcohol as the outcome with confounding from environmental factors taken into account.

Phase 3 - Relationship between cannabis and illicit drugs in adolescents

Finally, the relationship between cannabis use and the use of other substances as described in the Gateway Hypothesis will be assessed in ALSPAC adolescents using drug information obtained from hair samples. The hair samples can be used in one of two ways: (1) to validate the self report measures taken from the ALSPAC adolescents and (2) to use the information provided by the hair samples itself. With regards to the first option, this method will be used in assessment of cannabis, as hair samples are only able to provide information of regular or heavy use in relation to this drug. The hair sample analysis for other drugs is more sensitive than for cannabis, meaning that this data can both be used on its own and to validate the self report measures, therefore, for this section of the analysis we will use the biological marker as the outcome. As hair samples were not taken from all members of ALSPAC, this section of the study will have a smaller sample size than others; however, the loss of power from the biological marker will be made up for by increased precision and lack of bias. In order to link this with previous phases, cotinine levels of each of the ALSPAC adolescents will also be used to assess the relationship between tobacco and cannabis, ecstasy, cocaine, amphetamines and heroin.

Standard epidemiological statistical methods will be used here, with cannabis use from self reported questions (validated by hair sample data) as the exposure and the use of other illicit substances as shown by hair samples as the outcome.

Analysis:

All analysis will be undertaken by University of Bristol staff (Michelle Taylor) with Glyn Lewis, Matthew Hickman and Marcus Munafo acting as PhD supervisors.