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Which unwanted factors influence laboratory mice?

By Leticia Wüthrich and Martin Quintas.

Supervisor: Frédéric Schütz.

Link to presentation: https://docs.google.com/presentation/d/1EEJKlP73pZFCd6OF_fqTT9LVO_TrEVl3JDRN23b1BYE/edit?usp=sharing

Introduction

Mice play a crucial role in a wide range of research contexts, from genetic studies to drug development. While it is often assumed that genetically identical mice are clones of one another, this is not always true. Environmental differences, such as cage effects, can influence the mice. These cage effects can stem from various factors, including different environmental conditions between cages, social interactions, varying microbial exposures, and even how the cages are handled or maintained. Cage effects contribute to variation among mice, often resulting in greater similarity among mice within the same cage and greater differences between mice from different cages.

Initial Simulations

To illustrate the impact of cage effects on experimental data analysis, we perform simulations for the weights of mice in two different groups, distributed in a hierarchical design, based on the following formula:

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With:

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Thus, ϵij represents the inter-individual variance for the i-th mouse within the j-th cage, and γj represents the variance specific to the j-th cage that affects all mice within that cage.

Essentially, each data point is created based on a certain mean 𝑢, to which we add random inter-individual variance. Additionally, based on the cage where the mice are housed, another random source of variance is added, which is the same for all mice within the same cage. This is modeled using a normal distribution. The treatment effect represents the mean difference between the two groups of mice, so it impacts the 𝑢 parameter.

Simulations conducted with and without a cage effect and with and without a treatment effect reveal that the presence of both a treatment effect and a cage effect increases the likelihood of false negatives. Conversely, the presence of a cage effect without a treatment effect increases the likelihood of false positives.

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Note: in these simulations, when there was a treatment or cage effect, these were set to 5 grams.

Data Description

Results

Final Simulations

To determine how to account for the cage effect, we considered three different types of statistical models:

1. When the cage effect is not considered
2. Fixed effects models
3. Mixed effects models

We used the same simulations as before but adapted them to our statistical models. Initially, we applied parameters that reflect our data, setting the cage effect to 8.5 (calculated from ANOVA), with 4 cages and 4 mice per cage. The false positive rates and power we obtained were as follows:

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None of the statistical models performed particularly well, likely due to the large cage effect.

This led us to question how the experimental design could be improved to reduce the impact of the cage effect on model performance. We conducted numerous simulations (around 50 or 60) to explore this. Generally, we found that power and false positive rates improve if:

1. The cage effect is smaller
2. The treatment effect is larger
3. The number of cages is increased
4. The number of mice per cage is decreased

The only parameters we can realistically control in the experimental design are the number of cages and the number of mice per cage.

When we increased the number of cages and decreased the number of mice per cage, the performance of the models improved:

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For comparison, with the same total number of mice and the same cage effect, but with fewer cages and more mice per cage, the performance of the models was much worse:

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Thus, with improved experimental design, only the mixed models became truly appropriate for data with a cage effect as large as ours.