Tag Archives: carrier screening panels

Screening Everyone For Everything: A Changing Model of Screening For Carrier Status of Genetic Diseases?

The heterozygote carrier screening paradigm  is  starting to shift from ancestry based screening  for carriers of a single or a few genetic diseases to pan-ethnic screening for carriers of a wide range of genetic diseases. New techniques of DNA sequencing make it possible to test a single sample to determine carrier status for dozens of genetic conditions at prices that make carrier screening panels (CSPs) very tempting to healthcare providers and patients. Since carrier screening for one or another genetic disease – cystic fibrosis, Tay-Sachs, hemoglobinopathies – is already offered to essentially all women who are pregnant or planning a pregnancy, why not “screen everyone for everything” at no greater up front cost? And it doesn’t even require a blood sample; a less intimidating buccal sample works just fine.

Part of the understandable justification to move beyond targeted carrier screening programs is the futility of trying to classify people into distinct ancestry/racial/ethnic categories. Gene mutations and genetic diseases have a pesky habit of flowing fluidly between populations, and cultural heritages can be lost through assimilation (See Interesting Digression* below).

People

So why object to CSPs? After all, don’t people have a right to “know their DNA” and to understand what health and reproductive risks they face?

I am not suggesting that we should stand in the way of anyone’s wishes to “know their DNA.” If someone chooses to acquire that knowledge without the benefit of meeting with a genetic counselor, well, I may disagree with that decision but I respect it. But whether people decide to undergo carrier testing through a genetic counselor or through an online testing company, they need information that is forthright, complete, and transparent; they do not need subtly biased sales pitches. Private companies do not have a vested interest in talking people out of genetic testing.

Before we examine how some labs “objectively” describe carrier screening to patients, we must acknowledge an ethically uncomfortable truth. Carrier screening does not consistently lead to better treatments,  encourage greater tolerance of disabilities,  stimulate research into cures, or improve psychosocial adaptation to genetic disease. The only compelling reason to devote economic and medical resources to carrier screening is to reduce disease incidence. For better or worse, that is the measure of success of Tay-Sachs screening in Ashkenazi Jews and thalassemia screening on Cyprus.

Three strategies can reduce the incidence of genetic disease. One is mate selection based on carrier status, which is rare except in select populations such as Ultra-orthodox Jews (Hey Single Women out there, when was the last time you met potential Mr. Right and said “Er, you can buy me a drink but do you mind if I take a pedigree and a cheek swab from you before I give you my phone number?”). A second approach is preimplantation genetic diagnosis, but it is available to only a miniscule percentage of the population. The third and only realistic option for most patients is the elephant in the room – prenatal diagnosis with termination of affected fetuses.

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Take a look at the  web sites of companies such as Counsyl or 23andMe and you get a different narrative. The word “abortion” does not appear. Instead, you read about sperm/ovum donation, preimplantation diagnosis, mental preparation, watchful waiting, and early treatment. No mention is made that early treatment requires testing the baby anyway and that some treatment is available for only a handful of the screened conditions. The websites do not bring up the point that there are no large-scale studies that have shown better familial adaptation to genetic disease when parents have prenatal awareness of their carrier status, so couples really cannot know if testing really will result in mental preparedness. And I am still not sure what watchful waiting is, and how it differs from mental preparedness.

waiting

A second concern is that screening for very rare conditions plays on the emotionally vulnerable state of many pregnant women and the difficulty almost anyone has in understanding very, very small numbers in a psychologically meaningful way. Take for example, a condition that has an incidence of 1/100,000 births with a 75% carrier detection rate. Before carrier testing, a couple would have an ~99.9999% of NOT having an affected child; after carrier testing that probability would increase to ~99.99999%. Really, who can tell the difference between those two statistics? It’s difficult just trying to count the number of nines in those numbers. But read about the condition’s severe intellectual disabilities and physical birth defects, and, damn the statistics, give me that test.

A third concern is the lack of complete information about test sensitivity on the information portion of the website. For example, a patient with normal carrier test results might understandably think they would not have to be concerned about having a child with Bardet Biedl syndrome. What the site does not indicate however is that BBS1 and BBS10, the two loci included in the  panel, account for less than half of patients with Bardet Biedl syndrome, and that the dozen or so other genes that can cause Bardet Biedl syndrome are not included in the test panel.

A fourth, and maybe the greatest, concern is the ethical difficulty of deciding which conditions to include on a CSP. Tay-Sachs screening among Ashkenazi Jews and thalassemia screening in Cyprus developed with significant input from families, medical professionals, and community and religious leaders. There was widespread agreement in those communities that these were serious diseases and that carrier screening, mate selection, or prenatal diagnosis were ethically acceptable ways of reducing disease incidence. Very little community dialogue has taken place over CSPs. Do we really believe that the world is a better place if we screen for carriers of a common form of hereditary deafness or, God help us, red hair color?

Redhead

And ruminate on this: a study of 3 million cystic fibrosis carrier tests performed at a single US lab found that 25,000 CF carrier screens needed to be performed to detect one affected fetus. And this is for a relatively common genetic condition with a frequency of about 1/4000 US newborns and  a screening program whose success remains debatable. How many carrier screens will need to be performed to detect a fetus or newborn with a rare disorder like isovaleric academia, with a frequency of 1/250,000 births?

It could be that I am just the last of the old wave of genetic counselors who are out of touch with new technologies and changing ethical values, the proverbial last leaf on the tree. Maybe I am a 20th century genetic counselor in a  21st century world in which private industry will become the primary mode for the delivery of medical genetic services. Perhaps when I retire in a decade or so the genetic counseling community will issue a collective sigh of relief. But sometimes Old School cranks have a point.

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* – Interesting Digression: I recently learned about the Jews of Acadiana, Jewish merchants who settled among, and who were often culturally and reproductively assimilated into, Louisiana’s Cajuns (although the Cajun Tay-Sachs mutations stem from their French Canadian origins and predates the Ashkenazi admixture). Also, an exploration of why Tay-Sachs screening caught on among Ashkenazi Jews but not among Cajuns would make for  an interesting socio-medical-historical study. If a  large scale  Tay-Sachs screening program were to be introduced among Cajuns, perhaps its motto would be Laissez les bon genes roulez.

Accordion

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