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Because you watch SciShow, Brilliant is offering you a 30-day free trial and 20% off an annual premium subscription at Brilliant.org/SciShow. There's currently no simple way to diagnose cancer.

Cancer is a lot of different things, and there’s just no way to develop one single, straightforward test where doctors prick your finger and a machine goes yep, that’s cancer. Except one day there might be, thanks to a gene that doesn’t do anything. See, cancer is prone to turning on this one weird genetic sequence that’s widely considered junk.

It doesn’t do much of anything for our bodies, even though it does make cancer worse. But one team of researchers is on the trail of this genetic interloper, and by finding it, they think they can find cancer. As in, in general.

Here’s how. [♪ INTRO] Meet the genetic snippet known as long interspersed nuclear element-1, or LINE-1 for short. It doesn’t do much. “Junk DNA” is thought to be like stuff that takes up space but has no useful function, and far less of our DNA is “junk” than scientists used to believe. But LINE-1 takes up space and has no useful function.

It’s grade-A certified spam. LINE-1 is a retrotransposon – a 6,000-ish-letter sequence of genetic material that replicates by making RNA copies of itself and wandering all over the genome, inserting itself inside strands of DNA. This sequence makes a couple proteins called ORF1p and ORF2p, which have the extremely important jobs of copying the LINE-1 sequence and pasting it elsewhere.

And that is all LINE-1 does. Control C, control V. I do want to point out that a molecular biologist calling a protein “Orf1p” is like calling it Protein McProteinface.

As generic as it can possibly get. This cut-and-paste tomfoolery accounts for 17% of our genome, and yet it contributes exactly diddly-squat in terms of gene function. Every once in a while an optimistic paper comes out assigning some helpful function to LINE-1, but it’s optimistic.

Some researchers have gone as far as to characterize retroposons like LINE-1 as genomic parasites -- the body needs a pretty enthusiastic watchdog to keep these things in line. Transponsons happen in a lot of organisms. Scientists think that at some point there must have been a really good reason for a genetic sequence whose only function was to cut and paste itself.

But if there used to be a good reason, it's not super evident now. Under most circumstances these traveling sequences do nothing but clog up your chromosomes. That’s a big source of stress for cells, and can even cause disease.

For instance, if LINE-1 cuts into the middle of a gene, it can break that gene entirely. If a gene is a set of instructions, and you put a totally different set of instructions in the middle of that set of instructions, it’s gonna get pretty garbled. It’s actually been shown that LINE-1 butting in on genes that make blood clotting proteins can break the blood clotting process and cause hemophilia.

As a result, healthy cells don't usually give LINE-1 much opportunity to do its thing, and they silence it in a few different ways, including by attaching chemical compounds called methyl groups to LINE-1 elements. This is a common strategy cells use to turn down genetic material they’re not using – a brain cell doesn’t need to make insulin, right? Leave it to the pancreas.

And since nobody needs to make LINE-1, everybody shuts it up. Here’s the fun thing, though. LINE-1 is allowed to turn back on in about 50% of all human cancers.

Researchers don’t really know why, but cancer cells do like to throw away all those little methyl group flags that keep genes silenced. Maybe they just benefit from the chaos. Whatever the reason, LINE-1 in particular nearly always seems to lose its methyl groups.

Which gives it the ability to turn back on, and the absolutely nothing it does can lead to genetic chaos. Like, in colon cancer, activated LINE-1 is sometimes shown to be stuck in the middle of a tumor-suppressing gene called APC, breaking it, just like with hemophilia. Other studies show that by inserting itself into DNA, LINE-1 causes a bunch of breaks in that twisty backbone.

That compromises the genome’s overall stability, and that damage might aid tumor growth. Research also suggests that LINE-1 expression is related to damage to the tumor-suppressing p53 protein that regulates cell division and suppresses tumor growth in the body. In other words, LINE-1 can cause an awful lot of mischief for some supposedly harmless genetic junk mail.

Just by doing its cut-and-paste thing, it’s tilting the odds in cancer’s favor. Even if a cancer wasn’t looking to turn on LINE-1, so to speak, a cancer that does might have a good time. So it goes that LINE-1 expression seems to be associated with basically half of the cancers.

Including ones like ovarian and gastroesophageal cancer that are often hard to diagnose because they keep a low profile, symptom-wise, until they're too advanced to easily treat. Cancers of the esophagus, colon, lung, breast, prostate, ovaries, uterus, pancreas, head and neck all also seem to fire up in concert with the LINE-1 garbage machine. But here's the amazing thing: it just so happens that if LINE-1 is making ORF1p, one of the proteins that carry it around, it's pretty easy to detect it in the blood.

In a 2023 paper, researchers presented a super-sensitive machine that can look for ORF1p in our blood plasma. In fact, the method works on as little as half a drop of blood! The researchers found the test was extremely accurate in detecting a wide range of cancers.

They claim the test itself has a turnaround of just a couple hours and costs less than $3 to produce. Imagine a procedure no more complicated than the finger sticks used to monitor blood sugar in diabetes. That’s what we might be looking at in the future.

Of course, this test can't tell your doctor where in your body the cancer is hiding – only that ORF1p is present, so you've probably got cancer somewhere and more tests are needed. That means it’s a simple, first line screen that’s easy to get to patients, not something to replace more sophisticated tests like PET scans. However, this test could also be used to monitor whether treatments are working.

If a certain treatment is effective, doctors should be able to see blood levels of ORF1p dropping. LINE-1 may be a couch surfing, freeloading genetic parasite, but its one redeeming quality seems to be that it's not crafty enough to hide its antics. In fact, it may end up snitching on the very cancers it’s aiding.

Finally, it’s doing something worthwhile. This new tool is a great example of what’s at the core of a lot of scientific thinking. Take something useless, or even something that has a purpose, and put it to work solving a totally different problem.

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And thanks to all the people who worked on this episode. It was fascinating. [♪ OUTRO]