Researchers develop cheap, simple SMA screening test
Researchers have developed a new spinal muscular atrophy (SMA) screening method that’s quick, inexpensive and easy to use with standard lab equipment.
The team described their method in a paper, “A rapid and easy-to-use spinal muscular atrophy screening tool based on primers with high specificity and amplification efficiency for SMN1 combined with single-stranded tag hybridization assay,” published in PLOS One,
SMA is a genetic disorder caused by mutations in the SMN1 gene. Most cases of SMA are specifically caused by a mutation called exon 7 deletion, where a portion of this gene is completely absent.
Though SMA was untreatable for most of human history, in recent years several therapies that can slow or even stop SMA progression have become widely available. These treatments generally cannot reverse damage that’s already happened, so starting treatment as early as possible is paramount.
To facilitate early treatment, it’s necessary for patients to be diagnosed as quickly as possible. In the U.S. and other countries, newborn screenings systematically test all babies for SMA to help facilitate diagnoses. “With the advent of new treatments for SMA in recent years, the importance of SMA screening has become unquestionably vital,” the researchers wrote, noting that even a week or two delay in diagnosis can have massive impacts on a patient’s health.
Distinguishing genes
Although screening for SMA is essential, the process of testing for the disease can be complex. Part of the problem is that there’s a second gene called SMN2 that functions as a backup to SMN1, and lab tests often have trouble distinguishing between the two genes.
The researchers devised a method to specifically look for the exon 7 mutation in the SMN1 gene, while avoiding the SMN2 gene.
The method relies on a standard laboratory tool called polymerase chain reaction, or PCR. Very simply, PCR works to copy small amounts of a patient’s DNA into larger quantities that can be more easily detected. To do this, PCR uses tiny fragments of DNA called primers, which match up to specific sequences in the target gene(s) and act as a seed to grow new copies of the DNA.
The test relies on using primers that are specific for the SMN1 gene but not the SMN2 gene. When a healthy copy of SMN1 is present, the primer allows the gene to be replicated — but if the gene carries an exon 7 deletion, the primers are no longer able to function. Clinicians can simply run a PCR using these primers, then carry out standard tests to detect the amplified gene, and if it’s undetectable, the patient’s test is positive for SMA.
“We developed an innovative, inexpensive, and easy-to-use SMA screening assay based on the development of the SMN1 extremely specific primers with high amplification efficiency,” the researchers wrote.
As proof of principle for their test, the researchers tested biobanked DNA samples from several SMA patients, SMA carriers, and people without SMA. They also tested saliva samples from SMA patients and carriers. Results consistently showed that the test was positive for SMA in patients, but not carriers or individuals without SMA.
Though limited to a relatively small number of patients, “the newly developed assay achieved 100% sensitivity, 100% specificity, and 100% positive predictive value in this investigation,” the researchers wrote. They said they are conducting prospective tests using this method as part of SMA screening.
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