Promising New Blood Test to Detect Early Cancers
Researchers at John Hopkins University have developed a new blood test that shows promise for identifying colorectal, breast, lung, and ovarian cancers at their earliest stages. It will need validation in much larger studies but could someday be useful for screening people who have an increased risk of developing cancer.
The blood test detects tumor DNA in the bloodstream, called cell-free circulating tumor DNA (ctDNA). Such tests have received considerable research attention in recent years because ctDNA has potential as a non-invasive test for screening and monitoring cancer.
Detecting ctDNA relies on the fact that dying tumor cells release small DNA fragments into the bloodstream when they break down. These tumor cells have mutations that are acquired during a person's lifetime, called somatic mutations (as opposed to mutations that are present in all of the body's cells, which are referred to as constitutional or inherited mutations). Because these acquired mutations are typically present only in the tumor DNA, it allows researchers to identify whether a DNA fragment is from a healthy cell or a tumor cell.
While past studies have investigated ctDNA's use as a cancer monitoring tool, this study, reported in Science Translational Medicine, looks at how ctDNA could potentially detect cancer in its earliest stages. Early detection and diagnosis are important because many people do not show symptoms during cancer's early and most treatable stages.
In the past, DNA from a biopsied tumor was used to determine which tumor-specific alterations to look for in the ctDNA. However, using ctDNA to screen apparently healthy people means starting without a tumor sample and not knowing what specific genetic alterations to look for. The challenge is to do that in a way that is highly accurate and not prone to false-positive results.
The researchers developed a panel of 58 genes that are all known to be associated with various types of cancer and then screened these genes for mutations. Evaluation of this method of screening involved testing blood samples from 200 people with all different stages (I, II, III or IV) of breast, lung, ovarian, and colorectal cancer, as well as 44 healthy individuals.
The researchers used a type of genomic sequencing called "targeted error correction sequencing." This method of sequencing reads each nucleotide (chemical code) in the DNA sample an average of 30,000 times to help ensure that the test detects hard-to-find mutations.
Using this method, 86 of 138 (68%) individuals with stage I or II cancer screened positive based on their ctDNA analysis. The researchers also found that in 138 people with stage I cancer (cancer that is in a single area and has not begun to spread), the ctDNA blood test detected tumor DNA in 50% of those with colon cancer, 67% with breast cancer, 45% with lung cancer, and 67% with ovarian cancer. While these percentages show that 33-55% of cancers were not detected, there were no false positives among the 44 healthy people in the study.
While ctDNA screening tests need more study before they become part of clinical practice, this study shows their potential to save lives by detecting cancers at their earliest and most treatable stages. It should be considered, however, that the study was among people in early stages of cancer and therefore the results may not be applicable to people at normal risk with no signs or symptoms of cancer.