Hands-on with cancer mutational signatures, part 2

In this second part of the “Hands On” series, I want to address how to create the input for the MatLab mutational signature framework from the output of my python code to prepare the SNP data for analysis.

First, creating a Matlab .mat file for input to the program.   The code is expecting an input file that contains a set of mutational catalogues and metadata information about the cancer type and the mutational types and subtypes represented in the data.

Fig 1. The required data types within one .mat file to run the framework.
Fig 1. The required data types within one .mat file to run the framework.

As you can see from Fig 1., you need to provide a 96 by X matrix, where X is the number of samples in your mutational catalogue.  You also need an X by 1 cell array specifying sample names, a 96 by 1 cell array specifying the subtypes (ACA, ACC, ACG, etc…) and a 96 by 1 cell array specifying the types (C>A, C>A, C>A, etc…).  These must correspond to the same order as specified in the “originalGenomes” matrix or the results won’t make sense.

My code outputs .csv files for all of these needed inputs.  For example, when you run my python code on your SNP list, you will get a “subtypes.csv”, “types.csv”, “samples.csv”, and “samples_by_counts.csv” matrix (i.e., originalGenomes) corresponding to the above cell arrays in Fig 1.

Now, the challenge is to get those CSV files into MatLab.  You should have downloaded and installed MatLab on your PC.  Open MatLab and select “Import Data.”

Fig 2. Select the "Import Data" button.
Fig 2. Select the “Import Data” button.

Browse to one of the output CSV files and select it.  It will open in a new window like in Fig 3 below:

Fig 3. The data import window from MatLab.
Fig 3. The data import window from MatLab.

Be sure to select the correct data type in the “imported data” section.  Also, select only row 1 for import (row 2 is empty).  Once you’re finished, click Import Selection.  It will create a 1×96 cell called “types.”  It looks like this:

Fig 4. The new imported cell data "types."
Fig 4. The new imported cell data “types.”

We’re almost done, but we have to switch the cell to be 96×1 rather than 1×96.  To do this, just double-click it and select “transpose” in the variable editor.   Now you should repeat this process for the other CSV input files, being sure to select “matrix” as the data type for the “samples_by_counts” file.   Pay special attention to make sure the dimensions and data types are correct.

Once you have everything in place you should be ready do run the mutational analysis framework from the paper.   To do this, open the “example2.m” Matlab script included with the download.  In the “Define parameters” section, change the file paths to the .mat file you just created:

Fig 5. Define your parameters for the signature analysis.
Fig 5. Define your parameters for the signature analysis.

 

Here you can see in Fig 5, I’ve specified 100 iterations per core, a number of possible signatures from 1-8, and the relative path to the input and output files.  The authors say that ~1000 iterations is necessary for accuracy, but I’ve found little difference in the predictions between 50-500 iterations.   I would perform as many iterations as possible, given constraints from time and computational power.

Note also that you may need to change the part of the code that deals with setting up a parallel computing pool.  Since MatLab 2014, I believe, the “matlabpool” processing command has been deprecated.   Substituting the “parpool” command seems to work fine for me (Mac OS X 10.10, 8 core Macbook Pro Retina) as follows:

if ( parpool('local') == 0 )

parpool open; % opens the default matlabpool, if it is not already opened

end

This post is getting lengthy, so I will stop here and post one more part later about how to compare the signatures you calculate with the COSMIC database using the cosine similarity metric.

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