# Joint modeling of hematology traits yields epistatic signal in stock of mice

    library(mvMAPIT)
    library(ggplot2)
    library(dplyr)

> NOTE: Due to the size of the data, this vignette does not contain the output of the code.

# Preprocessing of the heterogenous stock of mice dataset

This example study makes use of GWA data from the Wellcome Trust Centre for Human Genetics\(^{1,2,3}\) (http://mtweb.cs.ucl.ac.uk/mus/www/mouse/index.shtml). The genotypes from this study were downloaded directly using the `BGLR-R` package. This study contains \(N =\) 1,814 heterogenous stock of mice from 85 families (all descending from eight inbred progenitor strains)\(^{1,2}\), and 131 quantitative traits that are classified into 6 broad categories including behavior, diabetes, asthma, immunology, haematology, and biochemistry. Phenotypic measurements for these mice can be found freely available online to download (details can be found at http://mtweb.cs.ucl.ac.uk/mus/www/mouse/HS/index.shtml). In the main text, we focused on 15 hematological phenotypes including: atypical lymphocytes (ALY; `Haem.ALYabs`), basophils (BAS; `Haem.BASabs`), hematocrit (HCT; `Haem.HCT`), hemoglobin (HGB; `Haem.HGB`), large immature cells (LIC; `Haem.LICabs`), lymphocytes (LYM; `Haem.LYMabs`), mean corpuscular hemoglobin (MCH; `Haem.MCH`), mean corpuscular volume (MCV; `Haem.MCV`), monocytes (MON; `Haem.MONabs`), mean platelet volume (MPV; `Haem.MPV`), neutrophils (NEU; `Haem.NEUabs`), plateletcrit (PCT; `Haem.PCT`), platelets (PLT; `Haem.PLT`), red blood cell count (RBC; `Haem.RBC`), red cell distribution width (RDW; `Haem.RDW`), and white blood cell count (WBC; `Haem.WBC`). All phenotypes were previously corrected for sex, age, body weight, season, year, and cage effects \(^{1,2}\). For individuals with missing genotypes, we imputed values by the mean genotype of that SNP in their corresponding family. Only polymorphic SNPs with minor allele frequency above 5% were kept for the analyses. This left a total of \(J =\) 10,227 autosomal SNPs that were available for all mice.

# Analyze hematology traits in mice

In this section, we apply mvMAPIT to individual-level genotypes and 15 hematology traits in a heterogeneous stock of mice dataset from the Wellcome Trust Centre for Human Genetics\(^{1,2,3}\). This collection of data contains approximately \(N =\) 2,000 individuals depending on the phenotype, and each mouse has been genotyped at \(J =\) 10,346 SNPs. Specifically, this stock of mice are known to be genetically related with population structure and the genetic architectures of these particular traits have been shown to have different levels of broad-sense heritability with varying contributions from non-additive genetic effects.

## Apply mvMAPIT

The number of complete samples in the data varies for different traits and trait pairs. For this study we created separate data sets for each trait and trait pair containing the genotype data in a genotype matrix encoded as `{0, 1, 2}` (minor allele count) and the trait or trait pair in a phenotype matrix. Apply `mvmapit()` to each data set by running the following.

    mvmapit_TRAIT <- mvmapit(
      t(TRAIT$genotype),
      t(TRAIT$phenotype),
      test = "hybrid"
    )

As a result, we get redundant \(P\)-values for some of the univariate variance components. The statistical detection of epistasis is sensitive to sample size. Therefore, we coalesce the redundant data by keeping the analysis results of the largest data set used in the analysis and impute missing data from the next smaller data set that has no missing data.

## Analysis Data Availability

The results of the paper data are published on Harvard Dataverse. Find the files for Download here\(^{23}\). For running the code snippets in this vignette, download the two files

- mice_HCTHGB_MCVMCH.rds
- mice_SI_paper.rds

and read the files using the following:

    mice_SI_paper <- readRDS("mice_SI_paper.rds")
    mice_HCTHGB_MCVMCH <- readRDS("mice_HCTHGB_MCVMCH.rds")

## All Traits Overview

We also include results corresponding to the univariate MAPIT model and the covariance test for comparison. Overall, the single-trait marginal epistatic test does only identifies significant variants for the large immature cells (LIC) after Bonferroni correction (\(P = 4.83\times 10^{-6}\)). A complete picture of this can be seen in the following figure, which depicts Manhattan plots of our genome-wide interaction study for all combinations of trait pairs. Here, we can see that most of the signal in the combined \(P\)-values from mvMAPIT likely stems from the covariance component portion of the model.

    for_ticks_chr <- aggregate(position ~ chr, mice_data$fisher, function(x) c(first = min(x), last = max(x))) %>%
      mutate(tick = floor((position[,"first"] + position[,"last"]) / 2)) %>%
      mutate(chr2 = case_when(chr %% 5 == 0 ~ as.character(chr),
                              chr == 1 ~ as.character(chr),
                              TRUE ~ ""))
    for_facetgrid_row <- as_labeller(c(`1` = "Trait #1", `2` = "Trait #2", `3` = "Covariance", `4` = "Combined"))
    gg <- mice_SI_paper$fisher %>% ggplot(aes(
          x = position,
          y = -log10(pplot),
          color = factor(color)
        )) +
          geom_point_rast(
            size = 0.7) +
          scale_color_manual(
            values = c("#8b8b8b", "#bfbfbf", "#1b9e77")
          ) +
          scale_y_continuous(breaks = c(0, 5, 10),
                             labels = c("0", "5", ">10")) +
          geom_hline(
            aes(
              yintercept = -log10(5.179737e-06),
              linetype = "Bonferroni"
            ),
            color = "#d95f02",
            size = 0.3
          ) +
          theme_bw() +
          facet_grid(x ~ y) +
          theme(
            panel.grid.major.x = element_blank(),
            legend.position = "bottom",
            text = element_text(family = "Times"),
          ) +
          labs(
            y = bquote(-log[10](p)),
            color = "") +
          scale_x_continuous("Chromosome",
                             breaks = for_ticks_chr$tick,
                             labels = for_ticks_chr$chr2) +
          scale_linetype_manual(name = "", values = c('dashed'))
    show(gg)

## Two trait pairs HCT & HGB as well as MCV & MCH

The hypothesis that most of the signal in the combined \(P\)-values from mvMAPIT likely stems from the covariance component portion of the model holds true for the joint pairwise analysis of hematocrit (HCT) and hemoglobin (HGB) and mean corpuscular hemoglobin (MCH) and mean corpuscular volume (MCV) (e.g., see the third and fourth rows of the following figure).

    gg <- mice_HCTHGB_MCVMCH$fisher %>% ggplot(aes(
          x = position,
          y = -log10(pplot),
          color = factor(color)
        )) +
          geom_point_rast(
            size = 0.7) +
          scale_color_manual(
            values = c("#8b8b8b", "#bfbfbf", "#1b9e77")
          ) +
          scale_y_continuous(breaks = c(0, 5, 10),
                             labels = c("0", "5", ">10")) +
          geom_hline(
            aes(
              yintercept = -log10(5.179737e-06),
              linetype = "Bonferroni"
            ),
            color = "#d95f02",
            size = 0.3
          ) +
          theme_bw() +
          facet_grid(row ~ case, labeller = labeller(row = for_facetgrid_row)) +
          theme(
            panel.grid.major.x = element_blank(),
            legend.position = "bottom",
            text = element_text(family = "Times"),
          ) +
          labs(
            y = bquote(-log[10](p)),
            color = "") +
          scale_x_continuous("Chromosome",
                             breaks = for_ticks_chr$tick,
                             labels = for_ticks_chr$chr2) +
          scale_linetype_manual(name = "", values = c('dashed'))
    show(gg)

One explanation for observing more signal in the covariance components over the univariate test could be derived from the traits having low heritability but high correlation between epistatic interaction effects. In our simulation studies (see publication) we showed that the sensitivity of the covariance statistic increased for these cases. Notably, the non-additive signal identified by the covariance test is not totally dependent on the empirical correlation between traits. Instead, as previously shown in our simulation study, the power of mvMAPIT over the univariate approach occurs when there is correlation between the effects of epistatic interactions shared between two traits. Importantly, many of the candidate SNPs selected by the mvMAPIT framework have been previously discovered by past publications as having some functional nonlinear relationship with the traits of interest. For example, the multivariate analysis with traits MCH and MCV show a significant SNP rs4173870 (\(P = 4.89\times 10^{-10}\)) in the gene hematopoietic cell-specific Lyn substrate 1 (Hcls1) on chromosome 16 which has been shown to play a role in differentiation of erythrocytes\(^7\). Similarly, the joint analysis of HGB and HCT shows hits in multiple coding regions. One example here are the SNPs rs3692165 (\(P = 1.82\times 10^{-6}\)) and rs13482117 (\(P = 8.94\times 10^{-7}\)) in the gene calcium voltage-gated channel auxiliary subunit alpha2delta 3 (Cacna2d3) on chromosome 14, which has been associated with decreased circulating glucose levels\(^8\), and SNP rs3724260 (\(P = 4.58\times 10^{-6}\)) in the gene Dicer1 on chromosome 12 which has been annotated for anemia both in humans and mice\(^9\).

## Notable SNPs with marginal epistatic effects after applying the mvMAPIT framework to 15 hematology traits

For full analysis, we provide a summary table which lists the combined \(P\)-values after running mvMAPIT with Fisher’s method. The following table lists a select subset of SNPs in coding regions of genes that have been associated with phenotypes related to the hematopoietic system, immune system, or homeostasis and metabolism. Each of these are significant (after correction for multiple hypothesis testing) in the mvMAPIT analysis of related hematology traits. Some of these phenotypes have been reported as having large broad-sense heritability, which improves the ability of mvMAPIT to detect the signal. For example, the genes Arf2 and Cacna2d3 are associated with phenotypes related to glucose homeostasis, which has been reported to have a large heritable component (estimated \(H^2 = 0.3\) for insulin sensitivity\(^{10}\)). Similarly, the genes App and Pex1 are associated with thrombosis where (an estimated) more than half of phenotypic variation has been attributed to genetic effects (estimated \(H^2 \ge 0.6\) for susceptibility to common thrombosis\(^{11}\)).

SNP | Location | Trait 1 | Trait 2 | Trait 1 \(P\) | Trait 2 \(P\) | Cov. \(P\) | Comb. \(P\) | Gene | Genomic Annotation | Reference
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
rs3699393 | 2:5887012 | MCV | PLT | 0.21 | 0.23 | 5.75e-7 | 4.9e-06 | Upf2 | anemia and abnormal bone marrow cell development | \(^{12}\)
rs13478092 | 5:3601413 | LIC | PLT | 0.034 | 0.58 | 1.67e-10 | 1.26e-9 | Pex1 | abnormal venous thrombosis | \(^{13}\)
rs3694887 | 5:102770070 | ALY | LIC | 1.26e-4 | 0.013 | 2.54e-6 | 1.55e-9 | Aff1 | abnormal B and T cell number and morphology | \(^{14}\)
rs3694887 | 5:102770070 | LIC | PLT | 0.013 | 0.28 | 5.47e-27 | 4.49e-26 | Aff1 | abnormal B and T cell number and morphology | \(^{14}\)
rs13478923 | 6:99475169 | ALY | LIC | 2.8e-4 | 0.12 | 1.79e-6 | 1.81e-8 | Foxp1 | abnormal B cell differentiation, physiology, count | \(^{15,16}\)
rs13478924 | 6:99571626 | ALY | LIC | 3.11e-4 | 0.12 | 2.70e-6 | 2.86e-8 | Foxp1 | abnormal B cell differentiation, physiology, count | \(^{15,16}\)
rs13478985 | 6:115245823 | MCV | WBC | 0.16 | 0.40 | 1.14e-81 | 1.34e-78 | Atg7 | decreased bone marrow cell count | \(^{17}\), \(^{18}\)
rs3723163 | 11:103800737 | HCT | LYM | 0.072 | 0.30 | 3.99e-107 | 2.66e-104 | Arf2 | decreased fasting circulating glucose level | \(^8\)
rs3723163 | 11:103800737 | HGB | WBC | 0.069 | 0.25 | 1.85e-7 | 6.76e-7 | Arf2 | decreased fasting circulating glucose level | \(^8\)
rs3724260 | 12:100163212 | HGB | HCT | 0.030 | 0.062 | 1.44e-5 | 4.58e-6 | Dicer1 | anemia | \(^9\)
rs3692165 | 14:27756640 | HCT | HGB | 0.026 | 0.037 | 9.9e-6 | 1.8e-06 | Cacna2d3 | decreased circulating glucose level | \(^8\)
rs3697466 | 14:27485228 | HCT | HGB | 0.026 | 0.037 | 9.9e-6 | 1.8e-06 | Cacna2d3 | decreased circulating glucose level | \(^8\)
rs13482117 | 14:27614362 | HCT | HGB | 0.023 | 0.03 | 5.9e-6 | 9.0e-07 | Cacna2d3 | decreased circulating glucose level | \(^8\)
rs6159786 | 14:27820736 | HCT | HGB | 0.026 | 0.037 | 9.9e-6 | 1.8e-06 | Cacna2d3 | decreased circulating glucose level | \(^8\)
rs6244569 | 14:27044891 | HCT | HGB | 0.026 | 0.037 | 9.9e-6 | 1.8e-06 | Cacna2d3 | decreased circulating glucose level | \(^8\)
rs13482288 | 14:81840412 | ALY | BAS | 0.036 | 0.65 | 1.78e-8 | 1.1e-07 | Tdrd3 | abnormal B cell differentiation and physiology | \(^{19}\)
rs3680448 | 14:81934085 | ALY | BAS | 0.036 | 0.65 | 1.78e-8 | 1.1e-07 | Tdrd3 | abnormal B cell differentiation and physiology | \(^{19}\)
rs4173870 | 16:35764290 | MCH | MCV | 0.14 | 0.71 | 1.20e-11 | 4.89e-10 | Hcls1 | differentiation of erythrocytes | \(^7\)
rs4212102 | 16:84204704 | PLT | WBC | 0.17 | 0.35 | 1.16e-10 | 2.44e-9 | App | increased susceptibility to induced thrombosis | \(^{20,11}\)
rs4212186 | 16:84273330 | PLT | WBC | 0.17 | 0.36 | 5.88e-11 | 1.31e-9 | App | increased susceptibility to induced thrombosis | \(^{20,11}\)
rs3711994 | 19:45078018 | ALY | LYM | 3.71e-4 | 0.10 | 1.04e-12 | 2.80e-14 | Btrc | abnormal lymphocyte morphology | \(^{21}\)

In the first two columns, we list SNPs and their genetic location according to the mouse assembly NCBI build 34 (accessed from \(^{21}\)) in the format `Chromosome:Basepair`. Next, we give the results stemming from univariate analyses on traits 1 and 2, respectively, the covariance (cov) test, and the overall \(P\)-value derived by mvMAPIT using Fisher’s method. The last columns detail the closest neighboring genes found using the Mouse Genome Informatics database\(^4\) \(^5\) \(^6\), a short summary of the suggested annotated function for those genes, and the reference to the source of the annotation.

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