This package acts as a wrapper to the penalized R package to add the following functionality to that package:
It also provides a function for simulation of collinear high-dimensional data with survival or binary response.
This package was developed in support of the study by Waldron et al. (2011).
This paper contains greater detail on proper application of the methods provided here. Please cite this paper when using the pensim package in your research, as well as the penalized package (Goeman (2010)).
pensim provides example data from a microarray experiment investigating survival of cancer patients with lung adenocarcinomas (Beer et al. (2002)). Load this data and do an initial pre-filter of genes with low IQR:
library(pensim) data(beer.exprs) data(beer.survival) ##select just 100 genes to speed computation, just for the sake of example: set.seed(1) beer.exprs.sample <- beer.exprs[sample(1:nrow(beer.exprs), 100),] # gene.quant <- apply(beer.exprs.sample, 1, quantile, probs = 0.75) dat.filt <- beer.exprs.sample[gene.quant > log2(100),] gene.iqr <- apply(dat.filt, 1, IQR) dat.filt <- as.matrix(dat.filt[gene.iqr > 0.5,]) dat.filt <- t(dat.filt) dat.filt <- data.frame(dat.filt) # library(survival) surv.obj <- Surv(beer.survival$os, beer.survival$status)
Note that the expression data are in “wide” format, with one column per predictor (gene). It is recommended to put covariate data in a dataframe object, rather than a matrix.
Unbiased estimation of prediction accuracy involves two levels of cross-validation: an outer level for estimating prediction accuracy, and an inner level for model tuning. This process is simplified by the opt.nested.crossval function.
It is recommended first to establish the arguments for the penalized regression by testing on the penalized package functions optL1
(for LASSO), optL2
(for Ridge) or cvl
(for Elastic Net). Here we use LASSO. Setting maxlambda1=5
is not a generally recommended procedure, but is useful in this toy example to avoid converging on the null model.
library(penalized)
## Welcome to penalized. For extended examples, see vignette("penalized").
testfit <- optL1( response = surv.obj, penalized = dat.filt, fold = 5, maxlambda1 = 5, positive = FALSE, standardize = TRUE, trace = FALSE )
Now pass these arguments to opt.nested.crossval()
for cross-validated calculation and assessment of risk scores, with the additional arguments:
outerfold
and nprocessors
: number of folds for the outer level of cross-validation, and the number of processors to use for the outer level of cross-validation (see ?opt.nested.crossval
)opt1D
for LASSO or Ridge, opt2D
for Elastic Net) - see ?opt.splitval
.nsim
defines the number of times to repeat tuning (see ?opt1D
. opt2D
has different required arguments.)set.seed(1) preds <- opt.nested.crossval( outerfold = 5, nprocessors = 1, #opt.nested.crossval arguments optFUN = "opt1D", scaling = FALSE, #opt.splitval arguments setpen = "L1", nsim = 1, #opt1D arguments response = surv.obj, #rest are penalized::optl1 arguments penalized = dat.filt, fold = 5, positive = FALSE, standardize = TRUE, trace = FALSE )
Ideally nsim would be 50, and outerfold and fold would be 10, but the values below speed computation 200x compared to these recommended values. Note that here we are using the standardize=TRUE
argument of optL1
rather than the scaling=TRUE
argument of opt.splitval. These two approaches to scaling are roughly equivalent, but the scaling approaches are not the same (scaling=TRUE
does z-score, standardize=TRUE
scales to unit central L2 norm), and results will not be identical. Also, using standardize=TRUE
scales variables but provides coeffients for the original scale, whereas using scaling=TRUE scales variables in the training set then applies the same scales to the test set.
Cox fit on the continuous risk predictions:
## Call:
## coxph(formula = surv.obj ~ preds)
##
## n= 86, number of events= 24
##
## coef exp(coef) se(coef) z Pr(>|z|)
## preds 0.01639 1.01653 0.07769 0.211 0.833
##
## exp(coef) exp(-coef) lower .95 upper .95
## preds 1.017 0.9837 0.873 1.184
##
## Concordance= 0.543 (se = 0.057 )
## Likelihood ratio test= 0.04 on 1 df, p=0.8
## Wald test = 0.04 on 1 df, p=0.8
## Score (logrank) test = 0.04 on 1 df, p=0.8
Dichotomize the cross-validated risk predictions at the median, for visualization:
preds.dichot <- preds > median(preds)
Plot the ROC curve:
library(survivalROC) nobs <- length(preds) cutoff <- 12 preds.roc <- survivalROC( Stime = beer.survival$os, status = beer.survival$status, marker = preds, predict.time = cutoff, span = 0.01 * nobs ^ (-0.20) ) plot( preds.roc$FP, preds.roc$TP, type = "l", xlim = c(0, 1), ylim = c(0, 1), xlab = paste("FP", "\n", "AUC = ", round(preds.roc$AUC, 3)), lty = 2, ylab = "TP", main = "LASSO predictions\n ROC curve at 12 months" ) abline(0, 1)
Finally, we can get coefficients for the model fit on all the data, for future use. Note that nsim should ideally be greater than 1, to train the model using multiple foldings for cross-validation. The output of opt1D
or opt2D
will be a matrix with one row per simulation. The default behavior in opt.nested.crossval()
is to take the simulation with highest cross-validated partial log likelihood (CVL), which is the recommended way to select a model from the multiple simulations.
beer.coefs <- opt1D( setpen = "L1", nsim = 1, response = surv.obj, penalized = dat.filt, fold = 5, maxlambda1 = 5, positive = FALSE, standardize = TRUE, trace = FALSE )
We can also include unpenalized covariates, if desired. Note that when keeping only one variable for a penalized or unpenalized covariate, indexing a dataframe like [1]
instead of doing [, 1]
preserves the variable name. With [, 1]
the variable name gets converted to "".
beer.coefs.unpen <- opt1D( setpen = "L1", nsim = 1, response = surv.obj, penalized = dat.filt[-1], # This is equivalent to dat.filt[,-1] unpenalized = dat.filt[1], fold = 5, maxlambda1 = 5, positive = FALSE, standardize = TRUE, trace = FALSE )
Note the non-zero first coefficient this time, due to it being unpenalized:
beer.coefs[1, 1:5] #example output with no unpenalized covariates
## L1 cvl J04130_s_at U20758_rna1_at L11672_r_at
## 4.99896033 -115.19258054 0.00000000 0.09998177 -0.10312924
beer.coefs.unpen[1, 1:5] #example output with first covariate unpenalized
## L1 cvl J04130_s_at U20758_rna1_at L11672_r_at
## 4.99906823 -117.76317249 -0.15193598 0.10888431 -0.03394335
The pensim also provides a convenient means to simulation high-dimensional expression data with (potentially censored) survival outcome or binary outcome which is dependent on specified covariates.
First, generate the data. Here we simulate 20 variables. The first 15 (group “a”) are uncorrelated, and have no association with outcome. The final five (group “b”) have covariance of 0.8 to each other variable in that group. The response variable is associated with the first variable group “b” (firstonly=TRUE
) with a coefficient of 2.
Binary outcomes for \(n_s= 50\) samples are simulated as a Bernoulli distribution with probability for patient s:
\[\begin{equation} p_{s} =\frac{1}{1 + exp(-\beta X_{s})} \end{equation}\]
with \(\beta_{s,16} = 0.5\) and all other \(\beta_{s,i}\) equal to zero.
The code for this simulation is as follows:
set.seed(9) x <- create.data( nvars = c(15, 5), cors = c(0, 0.8), associations = c(0, 2), firstonly = c(TRUE, TRUE), nsamples = 50, response = "binary", logisticintercept = 0.5 )
Take a look at the simulated data:
summary(x)
## Length Class Mode
## summary 6 data.frame list
## associations 20 -none- numeric
## covariance 400 -none- numeric
## data 21 data.frame list
x$summary
## start end cors associations num firstonly
## a 1 15 0.0 0 15 TRUE
## b 16 20 0.8 2 5 TRUE
A simple logistic model fails at variable selection in this case:
simplemodel <- glm(outcome ~ ., data = x$data, family = binomial)
## Warning: glm.fit: algorithm did not converge
## Warning: glm.fit: fitted probabilities numerically 0 or 1 occurred
summary(simplemodel)
##
## Call:
## glm(formula = outcome ~ ., family = binomial, data = x$data)
##
## Deviance Residuals:
## Min 1Q Median 3Q Max
## -2.156e-04 -2.100e-08 2.100e-08 4.490e-06 2.110e-04
##
## Coefficients:
## Estimate Std. Error z value Pr(>|z|)
## (Intercept) 394.817 40723.092 0.010 0.992
## a.1 -2.017 8340.192 0.000 1.000
## a.2 1054.116 107650.687 0.010 0.992
## a.3 432.929 41344.208 0.010 0.992
## a.4 187.507 21631.886 0.009 0.993
## a.5 -403.302 39003.798 -0.010 0.992
## a.6 -236.102 20862.924 -0.011 0.991
## a.7 -258.201 23085.549 -0.011 0.991
## a.8 7.364 18988.262 0.000 1.000
## a.9 229.596 21020.703 0.011 0.991
## a.10 -519.764 55465.620 -0.009 0.993
## a.11 -718.759 73461.234 -0.010 0.992
## a.12 94.096 16745.928 0.006 0.996
## a.13 -619.513 65350.574 -0.009 0.992
## a.14 585.887 61785.544 0.009 0.992
## a.15 -271.199 30368.663 -0.009 0.993
## b.1 447.844 49587.944 0.009 0.993
## b.2 -100.723 21001.800 -0.005 0.996
## b.3 -197.254 35209.129 -0.006 0.996
## b.4 1540.346 146783.886 0.010 0.992
## b.5 -1080.818 105901.366 -0.010 0.992
##
## (Dispersion parameter for binomial family taken to be 1)
##
## Null deviance: 6.6406e+01 on 49 degrees of freedom
## Residual deviance: 3.7489e-07 on 29 degrees of freedom
## AIC: 42
##
## Number of Fisher Scoring iterations: 25
But LASSO does a better job, selecting several of the collinear variables in the “b” group of variables which are associated with outcome:
lassofit <- opt1D( nsim = 3, nprocessors = 1, setpen = "L1", penalized = x$data[1:20], response = x$data[, "outcome"], trace = FALSE, fold = 10 ) print(lassofit)
## L1 cvl (Intercept) a.1 a.2 a.3 a.4 a.5 a.6 a.7
## [1,] 4.404100 -32.62007 0.5318400 0 0.04073869 0 0 0 -0.08727491 0
## [2,] 4.992938 -34.51760 0.5204535 0 0.01012467 0 0 0 -0.02152966 0
## [3,] 4.133468 -33.02222 0.5429307 0 0.06228851 0 0 0 -0.11206135 0
## a.8 a.9 a.10 a.11 a.12 a.13 a.14 a.15 b.1 b.2 b.3
## [1,] 0 0 -0.00379697 -0.3295077 0 0 0 0 0.00000000 0 0
## [2,] 0 0 0.00000000 -0.2793558 0 0 0 0 0.00000000 0 0
## [3,] 0 0 -0.02862559 -0.3483136 0 0 0 0 0.05135596 0 0
## b.4 b.5
## [1,] 0.2525967 0.1969338
## [2,] 0.2313165 0.1668551
## [3,] 0.2559668 0.1831810
And visualize the data as a heatmap:
dat <- t(as.matrix(x$data[,-match("outcome", colnames(x$data))])) heatmap(dat, ColSideColors = ifelse(x$data$outcome == 0, "black", "white"))
We simulate these data in the same way, but with response="timetoevent"
. Here censoring is uniform random between times 2 and 10, generating approximately 34% censoring:
set.seed(1) x <- create.data( nvars = c(15, 5), cors = c(0, 0.8), associations = c(0, 0.5), firstonly = c(TRUE, TRUE), nsamples = 50, censoring = c(2, 10), response = "timetoevent" )
How many events are censored?
## [1] 0.42
Kaplan-Meier plot of this simulated cohort:
library(survival) surv.obj <- Surv(x$data$time, x$data$cens) plot(survfit(surv.obj ~ 1), ylab = "Survival probability", xlab = "time")
## R Under development (unstable) (2020-01-28 r77738)
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##
## attached base packages:
## [1] stats graphics grDevices utils datasets methods base
##
## other attached packages:
## [1] survivalROC_1.0.3 penalized_0.9-51 survival_3.1-8 pensim_1.2.9
##
## loaded via a namespace (and not attached):
## [1] Rcpp_1.0.3 knitr_1.28 magrittr_1.5 splines_4.0.0
## [5] MASS_7.3-51.5 lattice_0.20-40 R6_2.4.1 rlang_0.4.5
## [9] highr_0.8 stringr_1.4.0 tools_4.0.0 grid_4.0.0
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## [17] assertthat_0.2.1 digest_0.6.25 rprojroot_1.3-2 pkgdown_1.5.0
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## [25] evaluate_0.14 rmarkdown_2.1 stringi_1.4.6 compiler_4.0.0
## [29] desc_1.2.0 backports_1.1.5
Beer, David G., Sharon L.R. Kardia, Chiang-Ching Huang, Thomas J. Giordano, Albert M. Levin, David E. Misek, Lin Lin, et al. 2002. “Gene-Expression Profiles Predict Survival of Patients with Lung Adenocarcinoma.” Nat Med 8 (8): 816–24. https://doi.org/10.1038/nm733.
Goeman, Jelle J. 2010. “L1 Penalized Estimation in the Cox Proportional Hazards Model.” Biometrical Journal. Biometrische Zeitschrift 52 (1): 70–84. https://doi.org/10.1002/bimj.200900028.
Waldron, Levi, Melania Pintilie, Ming-Sound Tsao, Frances A Shepherd, Curtis Huttenhower, and Igor Jurisica. 2011. “Optimized Application of Penalized Regression Methods to Diverse Genomic Data.” Bioinformatics 27 (24): 3399–3406. https://doi.org/10.1093/bioinformatics/btr591.