By David N. Leff
Diabetes mellitus and diabetes insipidus are two entirely different diseases — but stay tuned.
In the U.S. alone, some 16 million people have the mellitus version, and one-third of them don't even know it. Insipidus affects a far smaller number, less than 0.2 percent of the U.S. population, and they know it well.
Mellitus is the Latin word for "honey-sweet"; it describes the urine of diabetics, marked by the glucose their inadequate insulin hasn't metabolized. Insipidus means insipid, the pale, watery urine that its sufferers excrete in gargantuan amounts, reflecting the intense thirst that drives them to drink inordinate quantities of water. It's caused by a defect in the brain's pituitary hormone, vasopressin, which governs urinary output.
But there's a far rarer form of diabetes that afflicts its childhood victims with both mellitus and insipidus, along with blindness, deafness and early death. In fact, this syndrome carries the name DIDMOAD, standing for "diabetes insipidus, diabetes mellitus, optic atrophy and deafness." It's also known as Wolfram syndrome (WFS).
Don J. Wolfram, an American physician, first described the syndrome in 1938 as a combination of familial juvenile-onset diabetes mellitus and optic atrophy. Since then, researchers in many countries have been trying to track down the gene or genes responsible for Wolfram syndrome's protean, horrendous hallmarks.
At Washington University, in St. Louis, molecular diabetologist Alan Permutt took up the challenge in 1992. "Because the most common forms of diabetes are polygenic," he told BioWorld Today, "I decided to attack monogenic disorders. We knew from its inheritance pattern that WFS was an autosomal recessive disease. That is, the fact that neither parent of WFS children is affected, but multiple siblings are. And often those parents are inbred, consanguineous cousins. It looked like a recessive, monogenic disease."
WFS strikes an estimated one in 100,000 North Americans; one in 770,000 Britons. And it accounts for one of every 150 children with juvenile-onset diabetes.
Permutt is senior author of a paper in this month's Nature Genetics, titled, "A gene encoding a transmembrane protein is mutated in patients with diabetes mellitus and optic atrophy (Wolfram syndrome)."
Six Families On Three Continents Gave DNA
By genetic linkage studies of blood samples from six WFS families — three Japanese, one each from Europe, Australia and Saudi Arabia — the co-authors could confirm that the single WSF1 gene resides on human chromosome 4's short arm, and expresses numerous mutated proteins.
For the first six or eight years of a child's life, WFS looks like ordinary, garden-variety juvenile-onset diabetes, now better known as insulin-dependent diabetes mellitus. Then, by age 10 or so, visual problems arise, and the child goes slowly blind from atrophy of the optic nerve.
This loss of vision is distinct from the blinding diabetic retinopathy that marks the commoner forms of diabetes. "It's not a retinal disease," Permutt pointed out, "but rather [occurs] in the visual centers in the brain, so it's more of a neurodegeneration." The affected areas include, presumably, the brain's pituitary gland, which accounts for diabetes insipidus.
Permutt's research technologist, Jon Wasson, a co-author, recalled that, in mapping and sequencing the DNA from the six affected families, researchers benefited from the new genetic markers being made available in the 1990s by the international Human Genome Project. "Most helpful," he told BioWorld Today, "was direct sequencing of regions and the database searches.
"We were able to identify a host of genomic markers," Permutt recounted, "that span the suspect chromosome 4 region. And we confirmed linkage in our WFS families. By recombinant mapping in a monogenic disease," he explained, "we could exclude regions.
"In other words, if two affected siblings show the same chromosomal pattern, and the third child has the disease like the other two, but doesn't have the same pattern, then you know that the gene, or the marker, lies outside that area," he said.
"So, by doing that in multiple families, we were able to narrow the critical region of suspicion," Permutt went on. "Then we began to do physical mapping, isolating clones that span the region. We identified a number of genes in that region, and then did mutation screening in the patients."
Wasson described those mutations he found: "All are in exon 8 of the gene, and all different — all private. That is, each family had a different type of mutation. One had a 15-base-pair deletion; another, a 2-base-pair deletion. One family had a 7-base-pair insertion."
Saudi Arabian Child's DNA Revealed Truncated Gene
The significance of all these private genes, Permutt pointed out, "is that WFS ain't sickle cell anemia [SCA]. In SCA, there's one base that's mutated. This alters amino acids, and causes the globin molecule to sickle. The Wolfram syndrome gene has a bunch of different mutations. The ones that we have identified include missense, nonsense, and we think a microdeletion," he said.
As to how these mutations actually act on the disease, investigators "haven't the slightest clue," Permutt allowed. "It's just guilt by association. We know that this gene is in the critical region of the chromosome, and we found pretty devastating mutations. The Saudi Arabian family that I described in Nature Genetics is the most intriguing. The affected child is homozygous for a 7-bp insertion, which causes a frame shift in a premature truncation.
"The ribosome reads along," he continued, "and reads every three bases. Now, all of a sudden, there's an extra base in there. Then it goes a little while longer, and hits a stop codon in the middle of the protein. So, we know that this child, for whatever reason, has half a protein. We think of that as pretty much an effective gene knockout."
Given that some 80,000 American children are diagnosed each year with insulin-dependent diabetes mellitus, Permutt does not see his genotypic mutations as a practical test to detect WFS before optic atrophy makes it manifest.
Rather, he suggested, it "could be enormously helpful for genetic counseling. For instance, say you had a child who developed diabetes and then optic atrophy, and this child had two younger sib[ling]s. If you found a mutation in the Wolfram gene in this child, then you could confirm a WFS diagnosis, and do some useful prediction about the younger children." *