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For most cancers, the earlier they are detected, the better the prognosis.
But seeing something deep inside the body isn’t straightforward.
Most of our methods for finding and diagnosing cancer
involve a combination of imaging and invasive tests.
And since many cancers don’t have symptoms early on,
they may go unnoticed until they are at an advanced stage.
But that is changing, thanks to a newer, non-invasive tool.
It’s basically a blood test for cancer.
And some major progress in developing this tool wasn’t related to cancer,
but to genetic testing of fetuses.
The key to this new test lies in what is called cell-free DNA,
which is free-floating DNA in blood plasma, rather than encapsulated within cells.
Now there are some other blood tests being developed,
but we’re going to focus on this one.
Cell-free DNA is thought to come mainly from dying cells throughout the body,
and can be found in healthy people as a result of normal processes.
Though it’s also associated with disease, such as its original discovery in 1948,
when French scientists found it in patients with lupus.
Over the next few decades, scientists observed that people with certain diseases,
including some autoimmune disorders, cancers, and infectious diseases,
have abnormal levels of cell-free DNA.
Like, in 1977, researchers looked at 173 cancer patients and 55 healthy controls
and found that the levels of cell-free DNA were significantly higher
in about half of those with cancer.
Of that 50%, the levels were even higher in those whose cancer had metastasized.
In the 90s, advances in DNA sequencing helped kick
the understanding of cell-free DNA up a few notches.
For instance, in 1994, two groups of researchers found that pieces of cell-free DNA
in cancer patients actually have the same genetic mutations
that tumor cells were known to have.
These studies, and others that followed, suggested that cell-free DNA held the
promise not only to detect cancer, but to pinpoint
the type of cancer and whether treatment is working.
But before that promise could be fulfilled, around the same time,
researchers were also developing ways to learn about fetuses
from cell-free DNA in the pregnant parent’s blood.
In 1989, researchers found that fetal cells are present in the parental bloodstream.
A few years later, the same group hypothesized that maybe cell-free DNA
from fetal tissue would be detectable during pregnancy.
In a 1997 paper in The Lancet, they looked for the presence of DNA
from the Y chromosome in blood samples from the parent.
Now most people who bear children don’t have a Y chromosome.
So detecting it in the bloodstream would mean that it came from the fetus.
The researchers’ hunch panned out, and they found pieces of Y chromosome
in most of those pregnant with fetuses who had Y chromosomes.
That confirmation that cell-free fetal DNA is a thing that shows up
in pregnant people’s blood birthed a new era of prenatal diagnostics.
Eventually, scientists gleaned that, out of all of the cell-free DNA in
a healthy person’s bloodstream, during pregnancy,
about 3 to 15% of it comes from the fetus.
What’s more, since parent and fetus will both typically have 46 chromosomes,
any extras would clearly stick out.
Extra copies of chromosomes, called aneuploidies, are often fatal to the fetus.
In other cases, they can cause conditions that some parents want to prepare for.
Like Down syndrome, in which people have a third copy of chromosome 21.
By 2011, tests for conditions in which people have
a third copy of some chromosomes became available to parents.
So far, these non-invasive prenatal tests, or NIPTs,
are used for screening, not actual diagnosis.
The results still need to be confirmed by other tests.
But they are highly accurate, in that they detect over 90% of fetal trisomies.
They also have fewer false positives than older screening tests,
meaning they’re less likely to give a positive result where none exists.
As doctors increasingly started using NIPTs, one false positive that did happen
helped researchers make the cancer connection.
A 2013 case study described a 37-year-old pregnant patient.
Extra genetic material from chromosomes 13 and 18 suggested fetal trisomies.
But further testing confirmed the fetus had the typical number of chromosomes.
When the patient complained of pelvic pain after delivery,
a fracture in the pubic bone led doctors to find vaginal cancer.
Because here’s the thing: aneuploidies don’t just show up
at the beginning of life, but can develop in cancer too.
That led to the question: could an NIPT predict cancer
before someone feels symptoms?
In a study published in 2015, researchers obtained consent to administer
cell-free DNA tests to thousands of pregnant patients undergoing routine NIPT.
Their test looked not only at chromosomes common in fetal aneuploidies
like 13 and 21, but at DNA fragments from throughout the genome.
Of four thousand samples, they found abnormal results in three patients
and ended up using further tests to diagnose cancer,
before the patients were aware of any symptoms.
These studies and others like them offered tangible evidence to suggest that
cell-free DNA testing could be a powerful, non-invasive predictor of cancer,
and not just in pregnant folks.
It could also be used in the non-pregnant population,
which is most of us, and to monitor for cancer recurrence.
There are still some hurdles to clear before cell-free DNA
is really ready for the big time.
One of the biggest will be to really hone in on separating out
free-floating DNA from everything else in the blood.
Another is to learn more about other conditions
that increase the levels of cell-free DNA.
The hope is that eventually, doctors will even be able to use blood tests to suss out
which therapies will be most effective for an individual’s unique cancer.
Getting information from someone’s blood is a huge step up from having to
take tissue from harder-to-reach body parts,
and from the expensive hassle of imaging.
So all in all, cell-free DNA is a big win against cancer.
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