March 21, 2013 | 11
Laboratories that sequence an individual’s entire genome should limit the results they report to clinicians and their patients based on certain usability criteria, according to the first set of guidelines on the subject from the American College of Medical Genetics (ACMG). Geneticists simply don’t know what to make of most of the information that they find and it’s much too easy to misinterpret many of the results.
The ACMG’s advice is not as contradictory as it sounds—and could probably apply to a lot of other medical situations. Whenever a doctor asks me or a loved one to undergo any kind of diagnostic test, my first response is usually “What are we going to do with the results?” Mostly, I just want to get an idea of what kind of treatment the physician is likely to recommend based on the test findings. After all, if the recommended course of action is something that I don’t intend to follow, then what is the point of getting the test in the first place?
For example, if a pregnant woman has already decided she could never get an abortion, I don’t think it makes a lot of sense in most cases for her to agree to an amniocentesis, in which a needle is inserted into her womb to withdraw some amniotic fluid for genetic testing. The procedure itself has a small risk of triggering a miscarriage, so why take chances to get information about a possible abnormality? She’ll find out soon enough—after the baby is born—if the child is healthy or needs medical attention.
But there’s another reason to ask “What are we doing with the results?” whenever a clinician wants “just to run a few tests.” Sometimes, in the search for one thing, you may discover something else. Such so-called incidental findings can be quite momentous—such as when an x ray for a cracked rib reveals a shadow on the lung that’s suggestive of cancer. Or they can lead to a lot of anxiety for no good reason.
Years ago, someone in a biology lab at my school unexpectedly discovered that he had an extra X chromosome while completing one of the class assignments. (This was in the early days of do-it-yourself chromosome testing and the news spread pretty quickly among the other students.) The young man in question exhibited no clear signs of his unusual genetic makeup and probably could have spent the rest of his life happily unaware—at least until he tried to have children. (Men with the XXY condition often have fertility problems.) As it was, he was profoundly embarrassed—although no one teased him, as far as I knew. The school later took measures to make sure students were better informed about what they might discover when they took part in this lab project, but the incident made a big impression on me.
Robert C. Green, a genetics researcher and the co-chair of the ACMG working group that came up with the new guidelines, knows first hand about how common incidental genetic findings are. He recently had the protein-coding part of his genetic code (the so-called exome, which represents about 1 percent of any given individual’s entire genetic makeup) sequenced by a commercial gene-typing company. They reported that he had a genetic variant for Treacher-Collins syndrome, an inherited birth defect that often leads to an underdevelopment of the bones in the head, resulting cleft palate or in its most severe manifestations, severe disfiguration of the jaws and cheek bones.
The thing is, Green does not exhibit any of the outward signs of Treacher-Collins syndrome. He apparently has what’s now being called a non-pathogenic variant of the gene. As Green’s own example makes clear, there is no reason to report an incidental finding of a variant for Treacher-Collins syndrome taken from an adult. (The new ACMG guidelines do not apply to genetic screening tests of newborns, which have their own problems.)
Unless you are specifically looking for a Treacher-Collins gene, the vast majority of its genetic variants have no clinical bearing. For whatever reason, the variants that are most likely to show up as result of a search for something else—the definition of an incidental finding—are not going to give you any medically meaningful information, and they could do a lot of harm.
Update: You can read the full text of the ACMG’s guidelines, with my annotations, below. See also what Ricki Lewis has to say about the new recommendations and the tension between “genetic empiricists” like her and “genetic libertarians.”
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