Question about RNA and expression - (Dec/19/2012 )

We all know that RNA expression is tissue-specific.
So in many studies, researchers analyze the RNA expressions of certain genes in affected tissues and compare them to matched normal tissues.
For example, some breast cancer researchers would investigate the expression of BRCA1 in cancerous and noncancerous breast tissue, so that they can know whether deregulated BRCA1 expression occurs in breast cancer.
These are what I understand from my undergraduate course.

But, recently I came across quite a lot of studies, which use blood or salivary RNA to study gene expression of tissue-specific diseases such as breast cancer.
I fail to figure out the rationale of using blood or salivary RNA instead of RNA from the tissue to study gene expression.
Could someone please enlighten me on this?

Well, they are easiest to collect...

Also just found this via google http://iranpath.org/Papers/s0272271209000055.pdf , a review where they also discuss, starting from p.7, the similarities between breast tissue and the salivary glands and the respective fluids, although it seems to focus on proteins rather than RNA...

Tabaluga on Wed Dec 19 19:31:52 2012 said:

Oh, thanks for the reply.
Anyone else could tell me why RNA from blood/saliva is used instead of RNA from tissue?

Cells from cancer tend to get into blood stream so you can get the specific genes that are normally expressed only in one type of tissue and overexpressed in cancerous tissue from blood. But you need normally expressed gene as well for reference.

bigudukaz on Thu Mar 28 12:24:28 2013 said:

Wow, thanks for the reply after so long.
Then how about RNA from saliva?

jamestoon1 on Thu Apr 25 13:18:54 2013 said:

Cant think up the way how RNA in salive would tell about the cancer other then the one that can be in the "mouth".
Maybe cancer is lifting the levels of some proteins in the blood and some other tissues react to it messing up with gene expression, but this is just jibber jabber