identifying a cancer type genetically

A friend had unsuccessful chemotherapy. The tumour was not shrinking.
Then, he had a genetic test done through the Mayo clinic to identify the source of the tumour. Once directed chemotherapy was administered, he is now responding to treatment.
If all the cells in our body have the same DNA, how can a genetic test identify the source of a tumour.

I’m not a doctor, but it’s my understanding that: 1. The cancer cells themselves have changed and are no longer genetically the same as the non-cancerous cells. 2. The testing determines what specific type of cancer it is. Different cancers respond differently to different therapies.
3. Some genetics can make a person more susceptible to certain cancers. Once this is determined, the specific type of treatment can be better focused.

I hope a real oncologist comes by who can explain better. It is important to note that there is not just one disease called “cancer” the same way there is one disease called “diptheria” or “whooping cough.” There are many different kinds.

Just to add to what** MLS** said, cancers also spread. Things like skin cancer and testicular cancer are notorious for breaking off pieces that lodge in other tissues and start to grow. One of the more common ways that people get diagnosed with such cancers is when they get tested for a persistent cough and are told they have *lung *cancer. Because the blood vessels int he lung are so fine and so numerous that’s one of the first places that cancers spread to.

The problem is that these people don’t *actually *have lung cancer or brain cancer or whatever.They have skin cancer that that has spread to the lungs or brain. Many of the treatments that work really well for one type of cancer are ineffective against other types cancer, so if a treatment fails it is routine to do a “genetic” test to find out where the cancer originated.

Even though the original cells have the same DNA, every tissue type has different genes “turned on”. That’s how your liver cells keep dividing into more liver cells, your heart cells keep dividing into heart cells and so forth. By culturing the cancer cells and testing for which genes are active and which are inactive you can get a good idea of what organ they originated in.

Then you can start treating your lung cancer using testicular cancer therapy, or treating your liver cancer using bowel cancer therapy or whatever. Might seem odd, but the organ where the cancer is found is often not the place where it originated.

The short answer to this is that they probably don’t look at the DNA; they look at the RNA profile. This tells you which genes are being expressed and at what levels. Certain patterns of gene expression correspond to different tissue types, different cancer types, different prognoses, and suggest different treatments. A lot of work has gone into figuring out these patterns.

I used to do genetic testing of cancer tissues, and without more details, it’s hard to say exactly what was done. There are some useful things to be learned from the DNA, but the tissue source is not generally one of them. We used to test for a couple of specific mutations that are associated with certain types of cancer. There was a mutation in a gene called JAK2 that, if your cancer had it, meant that your cancer was more likely to respond to a specific treatment. Other DNA tests would include karyotyping, to figure out how many chromosomes the tumor has. Chromosomal instability is a hallmark of many cancers, so tumor cells often have lots of extra and missing chromosomes.

There are also translocations, like the 9;22 BCR-abl “Philadelphia” chromosome, but at least in our lab, it was considered more informative to look at the RNA levels from that translocation rather than simply test if it was present in the DNA.

And so forth.

The type of test you’re referring to is apparently the one done by specialized labs to try to narrow down the identification of “unknown primary” sites.

Routine tissue testing in pathology labs can often identify where a tumor originates with reasonable certainty (there are pretty good immunohistochemical markers for lung, colon, kidney and so on). In some cases the markers are less specific and the pathologist’s diagnosis can only give a range of likely primary sites (for instance, mentioning that the tumor in the liver could be a primary cholangiocarcinoma, a met from the pancreas or extrahepatic bile ducts, or possibly originating from the upper gastrointestinal tract). The “tumor of unknown primary test” is supposed to genetically determine the primary source; however in my experience it is less than definite and gives a percentage likelihood that the tumor comes from source A as opposed to B.

It’s also very expensive. But if the surgery/treatment is going to radically differ based on the primary site, it may be worth going for it.

Jackmannii, M.D.

This is very true. However, with the advances in Illumina technology, some tumors are now also being directly sequenced at the DNA level - resequencing is actually pretty cheap these days for organisms like humans that have a ton of sequence data already available.

Makes me nostalgic for undergrad/grad school when cloning and sequencing a gene was cause enough for beer celebration.

But it seems that this test is to find the tissue/cell type origin, not the cancerous genotype. A cancer genotype (or full genome!) can tell you a lot, but it won’t distinguish between the tissue of origin. Expression profiling can.

Edit: whoops, I read too quickly and thought mozchron was arguing about this technology. So… nevermind really. But Illumina is some crazy hot shit…

Just a random comment here, as there is no known human version of this, but there is such a thing as a parasitic cancer.