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They were mystery diseases that had stumped doctors for years: adults with strange symptoms and children with neurological problems, mental slowness or muscles too weak to let them stand. Scientists say they were able to crack one-quarter of these cases by decoding the patients’ genes.

The study is the first large-scale effort to move gene sequencing out of the lab and into ordinary medical care, and it shows that hopes for the technology are finally paying off, the researchers said.

“This is a direct benefit of the Human Genome Project,” the big effort to decode our DNA, said Dr. Christine Eng of Baylor College of Medicine in Houston. She led the study, published online Wednesday by The New England Journal of Medicine. It gives results on the first 250 patients referred to Baylor for a newer type of sequencing: just the DNA segments that hold the recipes for all the proteins the body needs. That’s only about 1 percent of the whole genome.

Baylor has sequenced more patients beyond those in the study — 1,700 — and found gene flaws in 1 out of 4, Eng said.

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That rate will improve as more genes are linked to diseases, but it’s already much higher than the gene tests done now, said Rebecca Nagy, a scientist at Ohio State University and president of the National Society of Genetic Counselors. “For some of these conditions there could be treatments that are lifesaving,” she said.

Three people tested at Baylor were found to have a muscle disorder that can cause respiratory problems and even death. The condition is aggravated by infections and stress, and there are drugs to treat those and prevent serious episodes, Eng said.

In other cases, having a diagnosis helped parents such as Lindsey and Brandon Collier decide whether to have more children. The Colliers, who live in Georgetown, Texas, about 30 miles north of Austin, searched for years for an answer to what was plaguing their son, Cannon, now 4.

“He was a pretty floppy baby” with poor muscle tone and problems eating, Lindsey Collier said. “It is a huge weight lifted off our shoulders” she said of testing at Baylor that found a rare muscle disorder.

Genetic counselors said the problem was not likely to occur in other offspring, so the Colliers had a second child. Their 6-week-old daughter, Smith, is fine, and Cannon is being helped by intensive physical therapy and other treatments.

Having a diagnosis is valuable because it ends the expensive and emotionally exhausting testing that parents go through, said Dr. Robert Green, a geneticist at Brigham and Women’s Hospital and Harvard Medical School. “Many of these are children or adults that have had a mystery illness for many years,” and it’s not possible to find a treatment until you find the cause, he said.

In the study of the first 250 patients at Baylor, 62 were found to have gene flaws. In 33 cases, only one faulty copy of a gene was responsible. In 16 other cases, both copies of a gene were bad. Four patients had problems in two different genes. Nine patients had faulty genes on the X chromosome.

Baylor gets revenue from gene testing, and two study leaders are consultants or paid speakers for gene-testing companies not involved in the study. The government’s National Human Genome Research Institute helped pay for the study, and insurers covered much of the testing. It cost $7,000 per case, which usually included sequencing the parents’ genes.

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