For the first time, researchers have developed tests that could improve the diagnosis of two rare childhood diseases known as congenital disorders of glycosylation (CDGs) and metachromatic leukodystrophy, and that could even lead to new treatments for CDGs. The findings were published in the Mass Spectrometry issue of Clinical Chemistry, the journal of AACC.
Altogether, rare diseases affect nearly 25 million Americans and there are more than 6,000 of these diseases worldwide, 50% of which impact children. However, because each individual rare disease affects a very small percentage of the population, this has hampered scientists' ability to study them for reasons ranging from inadequate sample sizes to lack of funding. It has only been in the last 30 years that researchers have begun to make a concerted effort to develop tests and treatments for these conditions, according to the National Organization for Rare Disorders. CDGs and metachromatic leukodystrophy are two such rare diseases for which optimal testing and treatment still do not exist.
A Test to Distinguish Between Types of CDG
A team of researchers led by Miao He, PhD, of the Children's Hospital of Philadelphia, set out to change this by developing a more efficient test for the three most common CDG subtypes: ALG1-CDG, PMM2-CDG, and MPI-CDG. All CDGs are characterized by defects in the body's ability to form glycoproteins or glycolipids (proteins and fats with sugar attachments that serve critical functions in all tissues and organs) and the current test for CDGs detects alterations in the glycoprotein transferrin. However, this fails to distinguish between CDG subtypes.
He's team has discovered that the presence of a novel N-tetrasaccharide along with levels of N-linked Man3GlcNAc2 (two types of sugar found in the body) can be used to diagnose ALG1-CDG, PMM2-CDG, and MPI-CDG, and to differentiate patients with ALG1-CDG from those with PMM2-CDG or MPI-CDG. To determine this, the researchers first compared the sugars found in CDG patients with those in healthy people to identify potential indicators of CDG. Once they zeroed in on the N-tetrasaccharide and Man3GlcNAc2, they used a lab technique known as liquid chromatography tandem-mass spectrometry (LC-MS/MS) to test for these sugars in 31 patients with one of the three CDG subtypes and in 20 unaffected individuals. This confirmed that the N-tetrasaccharide only appeared in the CDG patients; additionally, Man3GlcNAc2 levels were elevated in PMM2-CDG and MPI-CDG patients, but found at normal or low levels in ALG1-CDG patients.
Significantly, He's group also observed that the N-tetrasaccharide's levels went down after treating an MPI-CDG patient with the sugar mannose as well as after incubating ALG1-CDG patient cells in a solution of mannose. Currently, MPI-CDG is the only CDG subtype with a known treatment. This result indicates that future studies could use the N-tetrasaccharide test to investigate whether mannose is an effective treatment for ALG1-CDG as well, and to aid in the development of much-needed treatments for the other CDGs.
Making Newborn Screening Possible for Metachromatic Leukodystrophy
A second team of researchers led by Michael H. Gelb, PhD, of the University of Washington, Seattle, has created a test that could enable all infants in the U.S. to get screened for metachromatic leukodystrophy and receive prompt treatment to halt the cognitive deterioration it causes. In patients with this condition, a fatty substance known as sulfatide accumulates in the brain and other areas of the body. Previous studies had shown, however, that concentrations of sulfatides in dried blood—the sample type used by newborn screening programs—do not differ enough between metachromatic leukodystrophy patients and healthy individuals to serve as a diagnostic marker.
To solve this problem, Gelb's team developed a method that optimizes the extraction of sulfatides from dried blood samples and improves the signal-to-noise ratio for sulfatide detection using MS/MS and another lab technique known as ultra high performance liquid chromatography. They then tested this method by analyzing 19 sulfatide species in dried blood samples from 14 metachromatic leukodystrophy patients and 50 unaffected newborns. Their method successfully detected sulfatide concentrations in patients that were either 3.3-fold or 9.5-fold higher than in healthy infants. They also discovered that sulfatide concentration in dried blood seems to correlate with disease severity. Patients with lower sulfatide concentrations developed a milder form of metachromatic leukodystrophy, with symptoms appearing when they were 8-22 years old. The group with higher concentrations, particularly of the sulfatide species C18:0, developed an aggressive form of the disease with symptom onset at 11-26 months.
"The difference in sulfatide concentration was sufficient to unambiguously detect metachromatic leukodystrophy patients and stratify them based on severity," Gelb concluded. "This suggests the feasibility of using the mass spectrometry method for newborn screening of metachromatic leukodystrophy and sets the stage for a larger-scale newborn screening pilot study. The interest in screening for metachromatic leukodystrophy seems to be timely, due to the new treatment options that are being investigated in the clinic."
American Association for Clinical Chemistry