How to calculate chromosomal enrichment of differentially expressed genes

Question: How to calculate chromosomal enrichment of differentially expressed genes

5.6 years ago by

Sakti • 420

United States

Sakti • 420 wrote:

Dear all,

I have a list of differentially expressed genes between 2 experimental conditions. I observed that one of the chromosomes (3 in mouse) has a lot more genes coming up as differential expressed, and I want to know whether this number is statistically significant.

I was thinking about using a permutation test in R or perl to randomly select differentially expressed genes for the whole pool of genes in the genome, and then check how many of these are located in chromosome 3, and compare these values against my obtained experimental values.

However, I'm a little lost whether this is a valid approach to solve my question, and also I don't know how to whether there are already algorithms out there doing this exact same thing.

Thanks!!

differential expression rna-seq permutation • 2.5k views

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modified 2.3 years ago by Biostar ♦♦ 20 • written 5.6 years ago by Sakti • 420

5.6 years ago by

Devon Ryan ♦ 96k

Freiburg, Germany

Devon Ryan ♦ 96k wrote:

While you certainly could use permutation to answer this, it would seem much simpler to just Fisher's exact test. The four number you'd need are (1) The number of differentially expressed genes on chromosome 3, (2) the number of non-DE genes on chromosome 3, (3) the number of DE genes not on chromosome 3, and (4) the number of non-DE genes not on chromosome 3. Then it's simply fisher.test() in R and you're done.

Edit: BTW, you can use a one-sided test.

ADD COMMENT • link modified 5.6 years ago • written 5.6 years ago by Devon Ryan ♦ 96k

Thank you so much Devon for your reply. I think I need to re-take some stats courses it seems. Thanks!!

ADD REPLY • link written 5.6 years ago by Sakti • 420

So for those who want to do a similar thing, I ended up doing what Devon said in this piece of code:

mat = matrix(c(#DEchr3 #non-DEchr3,#DEgenesnotonchr3,#non-DEgenesnotonchr3),2,2)
fisher.test(mat, alternative = "greater")

I think it's "greater" in the alternative, as suggested by Devon, unless he corrects me about it.

ADD REPLY • link written 5.6 years ago by Sakti • 420

And not noticing this post I ended up wrapping this in a function to test over all chromosomes:

ADD REPLY • link written 5.4 years ago by A. Domingues • 2.3k

Wonderful post, very informative. Thanks for sharing the function with us!

ADD REPLY • link written 5.3 years ago by Sakti • 420

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