Science Editor

Of all the mice (Mus musculus) that populate the world's research labs, the commonest one is "Black six" (B6 for short). It outnumbers all the other murine models that keep bioscientists in business. One of the popular animal's steadiest costumers is mammalian molecular geneticist Miriam Meisler, professor of human genetics at the University of Michigan in Ann Arbor.

"The human and mouse genome sequences came along 10 years ago during the Human and Mouse genome projects," she recalls. "And they were essential tools. It's really an example of why the investment in this genome sequencing was so valuable. Because once we localized the gene in our cross-hairs to a million-base-pair region by genetic mapping, we were able to go directly to the sequence databases. The rest of it was in silico - chip analysis.

"We could look at all the genes that were known and protected in our region of genomic interest. There were 37 genes. We could get the mouse sequence and the human sequence for several different susceptible mice. It wasn't present in normal B6 mice and not in humans.

"Ten years ago," Meisler continued, "we would have had to isolate all these genes. You could imagine what an effort that was. So all we had to do at that point was to verify the gene identity by confirming the defect that its mutation causes, then putting in the wild-type genes."

Meisler's research focuses on sodium channel genes, which control the flow of electrical signals between brain neurons and muscle cells. Mutations in sodium channel genes generate a variety of neurological disorders - including several types of epilepsy, ataxia, poor muscle coordination, paralysis and cardiac arrhythmias.

She and her co-authors identified the mouse mutation in the Scnm1 modifier gene by comparing the DNA sequence in B6 mice with data from the Humane Genome Project. The B6 mouse gene carries a mutation called a premature stop codon, which blocks some of the genetic instructions needed to make normal Acnm1 modifier protein. As a result, the B6 gene is 20 percent shorter than the mouse or human gene. "That mutation doesn't by itself cause disease," she explained, "but produces a genetic susceptibility in other genes."

Science Article Carries Two Interesting Things'

Meisler is senior author of a paper in Science dated Aug. 15, 2003. Its title is "SCNM1, a putative RNA splicing factor that modifies disease severity in mice."

"This article consists of two interesting things," Meisler observed, "the findings we report and the methods we used to get there. Our basic finding indicates that variation in mutant splice factors may exist in a population undetected until they're combined with a secondary mutation in another disease gene.

"And most likely," she continued, "there's a variation in the human population, too; what you'd call disease susceptibility.' In fact, we've already found some that may be similar. It only becomes evident if a person inherits another defect, and the combination can then be very severe. For example, there are mutations in the cystic fibrosis gene, and individuals in the same family - or different families - that have the same mutation may suffer a severe disease or a mild disease. This has been a big puzzle. And the same is true in another disorder, familial dysautonomia. It involves abnormal functioning of the autonomic nervous system.

"Most affected individuals," she pointed out, "have precisely the same splice-site mutation. But the clinical severity, marked in part by demyelination, is very variable. You have what are called nodes of Ranvier,'" she explained. "They're little regions along the motor neuronal axon that lacks myelin. Most of the axon is insulated with myelin. That axonal coating facilitates transmission of the electrical signal down those long axons, down the whole leg, for instance. These nodes of Ranvier that stud the axons are where the sodium channels cluster. So the transmission runs from node to node. Obviously, if your axons don't have enough sodium channel, the mice become paralyzed because they're not getting enough signal to the muscle."

The disease Meisler is studying is caused by the interaction of two genes. The primary gene, Scn8a, encodes a sodium channel. If at least 50 percent of the Scn8a protein is functional, the mice appear perfectly normal.

The secondary gene, Scnm1 (for sodium channel modifier 1), carries the B6 mouse built-in sodium channel mutation. It contributes to expression of the sodium channel modifier gene as well as other genes.

If the amount of functional protein falls to 10 percent, the animals have some degree of neurological deficit, but can still live a normal life span. But if protein levels dip below 5 percent, these mice are paralyzed, and don't survive more than one month after birth.

To rescue B6 mice that had the lethal combination of mutations in Scn81 and Scnm1, the team injected a normal copy of the Scnm1 gene into fertilized mouse eggs. In two different experiments, they were able to prevent paralysis and juvenile lethality.

Do Human Versions Of Murine Diseases Exist?

"We're now looking for the human counterparts of these mouse diseases," Meisler recounted. "We haven't found them yet, but I don't have any doubt they exist. Ten different mutations in the mouse have arisen. We're actually screening them in human DNA," but have not yet identified any neurological diseases - nor has anyone else.

"Now that we have identified the modifier gene and the mechanism by which it works," Meisler summed up, "as well as the precise chromosome location of the human genes, we can begin looking for interactions with other mutations linked to human neurological disorders, such as epilepsy. This modifier is likely to interact with other types of genes, in addition to human sodium channels. If we can find a way to change the secondary effects of modifier genes," Meisler concluded, "we may be able to minimize the impact of the original genetic defect."

In a related "Perspective", molecular geneticist Joseph Nadeau commented that identifying modifier genes, as described in Science, "will provide insights into organismal resilience to genetic and environmental perturbations, and perhaps novel ways to treat disease."