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Multi-SKAT R-package

This package produces kernel Regression based rare-variant association tests for Multiple phenotypes. The functions aggregate variant-phenotype score statistic in a particular region/gene and computes corresponding p-values efficiently.

Overview

In this vignette we display an elementary workflow to obtain the association test results corresponding to different Multi-SKAT tests (omnibus and with pre-specified kernel)

Unrelated individuals

Multi-SKAT with pre-specified kernels

An example dataset MultiSKAT.example.data has a genotype matrix Genotypes of 5000 individuals and 56 SNPs, a phenotype matrix Phenotypes of 5 continuous phenotypes on those individuals, and a covariates vector Cov denoting intercept.

The first step is to create a null model using MultiSKAT_NULL function (for unrelated individuals) with the phenotype matrix and covariate matrix. Subsequently, MultiSKAT function is used with appropriate kernel to obtain the association p-value.

library(MultiSKAT)
data(MultiSKAT.example.data)
attach(MultiSKAT.example.data)

### Create null model
obj.null <- MultiSKAT_NULL(Phenotypes,Cov)

### Phenotype-Kernel: PhC; Genotype-Kernel: SKAT
out1 <- MultiSKAT(obj.null,Genotypes,Sigma_p = cov(Phenotypes),verbose = FALSE)

### Phenotype-Kernel: Het; Genotype-Kernel: SKAT
out2 <- MultiSKAT(obj.null,Genotypes,Sigma_p = diag(5),verbose = FALSE)

### Phenotype-Kernel: Hom; Genotype-Kernel: SKAT
out3 <- MultiSKAT(obj.null,Genotypes,Sigma_p = matrix(1,ncol = 5,nrow = 5),verbose = FALSE)

### Phenotype-Kernel: PC-Sel; Genotype-Kernel: SKAT
### Select top 4 principal components
sel <- 4
L <- obj.null$L
V_y <- cov(Phenotypes)
V_p <- cov(Phenotypes%*%L)
L_sel <- cbind(L[,1:sel],0)
pc_sel_kernel <- V_y%*%L_sel%*%solve(V_p)%*%solve(V_p)%*%t(L_sel)%*%V_y
out4 <- MultiSKAT(obj.null,Genotypes,Sigma_p = pc_sel_kernel,verbose = FALSE)

### Phenotype-Kernel: PhC; Genotype-Kernel: Burden
out5 <- MultiSKAT(obj.null,Genotypes,Sigma_p = cov(Phenotypes),verbose = FALSE,r.corr = 1)

### Phenotype-Kernel: Het; Genotype-Kernel: Burden
out6 <- MultiSKAT(obj.null,Genotypes,Sigma_p = diag(5),verbose = FALSE,r.corr = 1)

Assign weights to variants

It is assumed that rarer variants are more likely to be causal variants with large effect sizes. The default version of MultiSKAT uses Beta(1, 25) weights as per Wu et al(2011). Other weighting schemes can also be incorporated in the MultiSKAT function through the weights and weights.beta option.

### No weights
MultiSKAT(obj.null,Genotypes,Sigma_p = cov(Phenotypes),
                   weights.beta = c(1,1),verbose = FALSE)$p.value

Omnibus Tests

minP

To combine MultiSKAT tests with pre-specified phenotype kernels minP function can be used given the genotype kernels remain the same.

### Combining PhC, Het and Hom with genotype kernel being SKAT
obj.list = list(out1,out2,out3)
obj.minP = minP(obj.null,obj.list,Genotypes)

### Combining PhC and Het with genotype kernel being Burden
obj.list = list(out5,out6)
obj.minP2 = minP(obj.null,obj.list,Genotypes,r.corr = 1)

minPcom

To combine MultiSKAT tests with pre-specified phenotype kernels with simultaneously varying genotype kernels minPcom function can be used.

### Getting minPcom p-value combining PhC, Het and Hom kernels
Sigma_Ps = list(cov(Phenotypes),diag(5),matrix(1,ncol = 5,nrow = 5))
obj.com = minPcom(obj.null,Sigma_Ps,Genotypes,verbose = FALSE)

Related individuals

Multi-SKAT functions can analyse related individuals by incorporating kinship correction. An example dataset with related individuals MultiSKAT.Kinship.example.data includes a kinship matrix Kinship of 500 individuals and 20 SNPs, a co-heritability matrix V_g, residual covariance matrix V_e in addition to 5 phenotypes, genotype matrix and covariates. Additionally, if the the kinship matrix K has a spectral decomposition as K = UDU' with D being a diagonal matrix of eigen values, the dataset contains U and D. The workflow for the related individuals remains the same. First the construction of the null model through MultiSKAT_NULL_Kins followed by obtaining the p-value through MultiSKAT

detach(MultiSKAT.example.data)
data(MultiSKAT.Kinship.example.data)
attach(MultiSKAT.Kinship.example.data)

### Create null model
obj.null = MultiSKAT_NULL_Kins(Phenotypes,Cov,U,D,V_g,V_e)

### Phenotype-Kernel: PhC; Genotype-Kernel: SKAT
out1 <- MultiSKAT(obj.null,Genotypes,Sigma_p = cov(Phenotypes),verbose = FALSE)

All the omnibus tests (minP and minPcom) can be carried out in a similar fashion with related individuals.

Ref: Diptavo Dutta, Laura Scott, Michael Boehnke, Seunggeun Lee. Multi-SKAT: General framework to test multiple phenotype associations of rare variants

Please direct any questions to diptavo@umich.edu