By David N. Leff
A milky-white "mustache" on the upper lip of a celebrity is the U. S. dairy industry's way of saying milk is good for you.
This marketing message may be true in general, but in particular it's lost on the large majority of African-Americans and Native Americans who are born with intolerance for lactose, or milk sugar. Their bodies lack the intestinal enzyme lactase, which splits lactose into two more gut-friendly sugars, glucose and galactose.
Statistics are shaky, but across Asia, 90 percent or more of the population — notably in China and Japan — are lactose-intolerant. So are people of Mediterranean origin. But, among those of northern European ancestry, only 5 percent are lactose-intolerant.
When such individuals ingest milk, cheese, ice-cream or other dairy foods, they suffer swift abdominal retribution: gas, bloating, stomach rumbling, cramps, nausea and diarrhea.
Just as people with the now-newsy allergy to peanuts shun goobers, those with lactose intolerance tend to avoid all dairy products. But this abstinence triggers another side effect, namely calcium deficiency. Besides the obvious loss of bone mass and strength, lack of dietary calcium degrades a wide range of essential metabolic functions.
One way around this serious effect — not to mention the craving for ice-cream — is oral lactase, widely sold in pill or soft-capsule form, to be taken with dairy foods. But, on a lifetime basis, this regimen doesn't come cheaply, or easily. Nor does supplemental calcium.
Speculations differ as to the reason for this global dichotomy between milk-accepting and milk-resisting peoples. How did evolution make half the world's population averse to dairy products?
"Humankind has evolved in this respect," recounted neuroendocrinologist Matthew During, "because the domestication of cattle 10,000 years ago conferred a survival advantage — the ability to digest dairy products beyond gestation." People who could eat or drink the bovine milk products were bigger, better, biologically stronger, more productive. And their genes would get passed down — or the mutation in the gene, whatever that is.
"This lactose-tolerating trait is inherited as an autosomal recessive," During said, "but, in fact, the underlying genetic change is not well defined. We humans really have a gain-of-function mutation that enables us to metabolize lactose. We really are genetic mutants."
He explained: "We're all born with genes that are developmentally regulated to some extent. So you can have a gene that's very active during a certain stage of fetal life, and gets switched off in adulthood."
During is director of the Central Nervous System Gene Therapy Center at Jefferson Medical College, in Philadelphia. He is senior corresponding author of a paper in the current issue of Nature Medicine, dated October 1998, titled: "Peroral gene therapy of lactose intolerance using an adeno-associated virus [AAV] vector."
AAV: Gene-Delivery Vehicle Of Choice
"Adenoviruses," During told BioWorld Today, "naturally infect the digestive and respiratory systems, and the adeno-associated virus normally has a very similar tropism. AAV is a very small and hardy virus," he went on, "resistant to all sorts of things, such as solvents and extremes of acidity.
"So, I thought immediately," he recalled, "that it might serve as a vector in our effort to treat lactose intolerance by gene therapy. It's the sort of virus you can put into drinking water or food, or some sort of pill, and swallow it. It would survive the stomach acids, and probably transduce the intestinal cells."
He started with a breed of rats low in lactase, and screened out a subset colony that fully mimicked human lactose intolerance. Into the stomachs of these rodents, the co-authors tube-fed orally a gene-therapy construct, consisting of an enzyme-complex gene centered on beta-galactosidase hooked to an eviscerated, recombinant adeno-associated virus vector.
One week later, and again at four months, they challenged these animals with an oral lactose load, and later fed them a lactose-only diet for several weeks. As late as six months out, 17 to 19 percent of the gut's target lamina propria cells were transduced with the gene, for a total ranging from 8 to 13 million cells.
Control animals sustained weight loss, a marker of lactose intolerance.
"We used just a standard, off-the-shelf vector," During recalled, "and were amazed to see how efficiently it transduced the cells in the gut. It just got in there and worked like gangbusters.
"Potentially, 20 percent of the cells and part of the gut were transduced," he went on. "And given just a 40-microliter amount of the virus, we were getting something like 10 million cells transduced — pretty phenomenal. Those cells in the gut seem to be prone to getting infected with this virus, and the gene seems to get into a very transcriptionally stable state."
New Disorders To Conquer
Fortified by this "proof-of-principle" result, During is now extending his oral gene therapy strategy to other disease entities.
"We're trying to develop a product for diabetics now," he said. "I believe we can use this same technology for the insulin gene, if we could potentially get some regulation of insulin secretion from this. So, that's a big one to go for."
At this point, he and his co-authors "have made the virus and constructs, and are just starting animal experiments in diabetic rats.
"I'm really a neuroscientist," During said, "and was the first gene therapist to use AAV in the brain, while at Yale University. Right now, we have a pending patent, and are pushing toward clinical trials in Parkinson's disease [PD]." This would not be oral, but rather "direct injection of the tyrosine hydroxylase gene into the brain." That enzyme makes L-dopa, precursor to dopamine, and the principal present PD drug." (See BioWorld Today, Dec. 21, 1994, p. 1.)
During foresees applying his oral gene therapy approach to clotting-factor genes for treating hemophilia, and eventually to vaccines.
As for the lactose-intolerance aspect, During — himself a New Zealander — is now sounding out the "huge New Zealand dairy industry, to see if it is sufficiently interested in developing this approach toward a product and pushing it toward clinical trials." *