knitr::opts_chunk$set( collapse = TRUE, comment = "#", fig.width=7, fig.height=5 ) library(knitr)

If you use multivariate MAPIT in published research, please cite:

Crawford L, Zeng P, Mukherjee S, & Zhou X (2017). Detecting epistasis with the marginal epistasis test in genetic mapping studies of quantitative traits. PLoS genetics, 13(7), e1006869. https://journals.plos.org/plosgenetics/article?id=10.1371/journal.pgen.1006869

Stamp J, DenAdel A, Weinreich D, Crawford, L (2023). Leveraging the Genetic Correlation between Traits Improves the Detection of Epistasis in Genome-wide Association Studies. G3 Genes|Genomes|Genetics 13(8), jkad118; doi: https://doi.org/10.1093/g3journal/jkad118

Stamp J, Crawford L (2022). mvMAPIT: Multivariate Genome Wide Marginal Epistasis Test. https://github.com/lcrawlab/mvMAPIT, https://lcrawlab.github.io/mvMAPIT/

Load necessary libraries. For the sake of getting started, `mvMAPIT`

comes with
a small set of simulated data. This data contains random genotype-like data and two
simulated quantitative traits with epistatic interactions. To make use of this data,
call the genotype data as `simulated_data$genotype`

, and the simulated trait data
as `simulated_data$trait`

.
The vignette traces the analysis of simulated data. The simulations are described in `vignette("simulations")`

.

For a working installation of mvMAPIT please look at the`README.md`

or `vignette("docker-mvmapit")`

library(mvMAPIT) library(dplyr) library(ggplot2) library(ggrepel) library(tidyr) library(kableExtra) library(RcppAlgos)

The R routine `mvmapit`

can be run in multiple modes. By default it runs in a hybrid mode, performing tests both wtih a normal Z-test as well as the Davies method. The resulting `p`

-values can be combined using functions provided by `mvMAPIT`

, e.g. `fishers_combined()`

, that work on the `pvalues`

tibble that `mvmapit`

returns.

NOTE:mvMAPIT takes the X matrix as $p \times n$;notas $n \times p$.

mvmapit_hybrid <- mvmapit( t(data$genotype), t(data$trait), test = "hybrid" ) fisher <- fishers_combined(mvmapit_hybrid$pvalues)

fisher <- mvmapit_data$fisher pairs <- mvmapit_data$exhaustive_search thresh <- 0.05 / (nrow(fisher) / 2)

To visualize the genome wide `p`

-values, we use a Manhattan plot.
The `p`

-values are plotted after combining the results from the multivariate analysis using Fisher's method.

manhplot <- ggplot(fisher, aes(x = 1:nrow(fisher), y = -log10(p))) + geom_hline(yintercept = -log10(thresh), color = "grey40", linetype = "dashed") + geom_point(alpha = 0.75, color = "grey50") + geom_text_repel(aes(label=ifelse(p < thresh, as.character(id), ''))) plot(manhplot)

To control the type I error despite multiple testing, we recommend the conservative Bonferroni correction.
The significant SNPs returned by the `mvMAPIT`

analysis are shown in the output below. There are in total 6 significant SNPs after multiple test correction. Of the significant SNPs, 4 are true positives.

thresh <- 0.05 / (nrow(fisher) / 2) significant_snps <- fisher %>% filter(p < thresh) # Call only marginally significant SNPs truth <- simulated_data$epistatic %>% ungroup() %>% mutate(discovered = (name %in% significant_snps$id)) %>% select(name, discovered) %>% unique() significant_snps %>% mutate_if(is.numeric, ~(as.character(signif(., 3)))) %>% mutate(true_pos = id %in% truth$name) %>% kable(., linesep = "") %>% kable_material(c("striped"))

To compare this list to the full list of causal epistatic SNPs of the simulations, refer to the following list. There are 5 causal SNPs. Of these 5 causal SNPs, 4 were succesfully discovered by mvMAPIT.

truth %>% kable(., linesep = "") %>% kable_material(c("striped"))

Now we may take only the significant SNPs according to their marginal epistatic effects and run a simple exhaustive search between them.

The search itself is a simple regression on the interaction terms between all significant interactions.

# exhaustive search for p-values pairs <- NULL if (nrow(significant_snps) > 1) { pairnames <- comboGeneral(significant_snps$id, 2) # Generate unique pairs of SNP names; # for length(names) = n, the result is a (n * (n-1)) x 2 matrix with one row corresponding to a pair for (k in seq_len(nrow(pairnames))) { fit <- lm(y ~ X[, pairnames[k, 1]]:X[, pairnames[k, 2]]) p_value1 <- coefficients(summary(fit))[[1]][2, 4] p_value2 <- coefficients(summary(fit))[[2]][2, 4] tib <- dplyr::tibble( x = p_value1, y = p_value2, u = pairnames[k, 1], v = pairnames[k, 2] ) pairs <- bind_rows(pairs, tib) } } colnames(pairs) <- c(colnames(y), "var1", "var2")

We plot the $-\mathrm{log}_{10}(p)$ of the `p`

-values for the regression coefficients as tile plot to highlight the identified interaction structure.

plotable <- pairs %>% pivot_longer( cols = starts_with("p_"), names_to = "trait", names_prefix = "trait ", values_to = "p", values_drop_na = TRUE ) %>% mutate(trait = case_when( trait == "p_01" ~ "Trait 1", trait == "p_02" ~ "Trait 2")) tiles <- ggplot(data = plotable, aes(x=var1, y=var2, fill=-log10(p)))+ geom_tile() + facet_wrap(~trait) + theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust=1)) + scale_fill_viridis_c() plot(tiles)

The only significant interactions after multiple testing correction are the interaction
between `snp00068`

and `snp_00665`

as well as `snp_00465`

and `snp_00917`

.

pairs %>% filter(p_01 < 0.05/15 | p_02 < 0.05/15) %>% kable(., linesep = "", digits = 14) %>% kable_material(c("striped"))

Compare the results of the exhaustive search to the true interaction structure. Notice that the only significant interactions in the exhaustive search are the two with the largest true effects.

true_interactions <- simulated_data$interactions %>% mutate(var1 = sprintf(group1, fmt = "snp_%05d")) %>% mutate(var2 = sprintf(group2, fmt = "snp_%05d")) %>% mutate(trait = case_when( trait == 1 ~ "Trait 1", trait == 2 ~ "Trait 2")) %>% select(-c("group1", "group2")) X <- true_interactions[, c("var1", "var2")] X <- t(apply(X, 1, sort)) true_interactions[,c("var1", "var2")] <- X epistatic_pairnames <- comboGeneral(simulated_data$epistatic$name %>% unique(), 2) true_pairs <- NULL for (k in seq_len(nrow(epistatic_pairnames))) { tib <- dplyr::tibble(var1 = epistatic_pairnames[k, 1], var2 = epistatic_pairnames[k, 2]) true_pairs <- bind_rows(true_pairs, tib) } anti <- anti_join(true_pairs, true_interactions) %>% mutate(effect_size = 0) true_int_plot <- true_interactions %>% bind_rows(anti %>% mutate(trait = "Trait 1")) %>% bind_rows(anti %>% mutate(trait = "Trait 2")) true_tiles <- ggplot(data = true_int_plot, aes(x=var1, y=var2, fill=effect_size)) + geom_tile() + facet_wrap(~trait) + theme(axis.text.x = element_text(angle = 90, vjust = 0.5, hjust=1)) + scale_fill_gradient2(low = "blue", high = "red", mid = "white") plot(true_tiles)

While mvMAPIT does not identify the explicit partner, it still implicates more correct SNPs in this example. All true epistatic SNPs are listed in the following table.

true_interactions %>% kable(., linesep = "") %>% kable_material(c("striped"))

**Any scripts or data that you put into this service are public.**

Embedding an R snippet on your website

Add the following code to your website.

For more information on customizing the embed code, read Embedding Snippets.