Next, the researchers transferred each of the variants into a yeast cell, yielding 84 yeast strains that differed at only a single place in the CBS gene. A control strain containing the fully functional major allele of the CBS gene was constructed for comparison.
"Once we put an allele into yeast we can conduct experiments to ask whether it's functional or not by growing the colonies, both in dishes and in liquid culture," the geneticist explains. A mildly mutated allele may allow yeast cells to grow normally, but severely mutated cells fail to grow at all. Many variants function somewhere in between. "When you know where two sequences differ, you can start to identify changes that affect biological function from those that don't," he says.
But "the really interesting and exciting part of this project is what we learned about treating the disease," Mayfield adds. Some homocystinuria patients respond to simple vitamin B6 supplementation, while others require severe dietary restriction to avoid disease symptoms. In a series of experiments with yeast in liquid culture, the researchers added different levels of supplemental vitamin B6 to the 84 disease alleles and found that 37 percent of the alleles were functional or could be rescued by vitamin B6. "In the clinic, then, you can expect the patients with these alleles to respond to B6 treatment," the geneticist explains.
"This method gives us information about allele function and is applicable to other disease genes. Metabolic diseases and proteins that need vitamin co-factors are amenable to similar studies. Our findings indicate clearly that sequences alone can't tell the full story. This is a first step in gaini
|Contact: Janet Lathrop|
University of Massachusetts at Amherst