A researcher's valued elbow-jogger is often serendipity.
So it was that while studying mice with a mutant gene whose human counterpart causes inherited glaucoma, researchers discovered a second gene mutation that worsens the structural eye defect that causes this type of eye disease.
Their unexpected findings and their ramifications are reported in today's Science, dated March 7, 2003, under the title: "Modification of ocular defects in mouse developmental glaucoma models by tyrosinase." The paper's senior author is geneticist/molecular biologist Simon John. Its co-lead authors are Richard Libby, a post-doctoral fellow, and Richard Smith, a research scientist - all at the Jackson Laboratory in Bar Harbor, Maine.
"I'm here at the Jackson," Smith told BioWorld Today, "because I get frustrated seeing patients with nasty diseases like congenital glaucoma and not knowing what the Dickens is going on. One reason we eye pathologists didn't know what's going on in humans is that we never saw eyes under the microscope until the patients had many years of the disease, plus surgery, plus drugs, and it's impossible to reconstruct that history.
"Now we know a lot more about the molecular mechanisms that lead to congenital glaucoma. - thanks to these experimental mice that have the human gene. No one had an inkling of how the normal eye structures are formed," he went on "before we made this finding in Science.
"The finding is that in this particular form of glaucoma at least, the amount of pigmentation in the eye affects severity. In the congenital glaucoma, the less pigment the eye has, the more severe the disease. That's almost a strong black-and-white correlation," Smith observed. "Albino animals lacking pigment," he explained, "have very severe widespread malformations. So pigmentation plays a big role in prenatal eye development.
"There was no suspicion that pigment played any role in formation of these normal ocular structures that carry fluid out of the eye on a regular basis. The importance is that what has been used drug-wise to treat this primary congenital glaucoma has a very limited success. The ophthalmologists tried out the gamut of available medications. Our thought that affecting the production of pigment in the eye might alter the disease opens up a whole range of pharmacologic medications that we could consider worth trying. We've already shown that L-dopa, a precursor pigment molecule, seems to prevent the disease."
Consanguinity Fosters Recessive Genes
To determine the link between pigmentless albinism and glaucoma, the team assembled a mixed bag of black and albino mice.
"Where congenital glaucoma and albinism have co-existed in human populations," Smith pointed out, "professionals brushed this off as a coincidence. We considered that it's probably not pure chance.
"When we first got these mice, made by one of the Science co-authors - Frank Gonzales at the National Cancer Institute - he sent us batches of the animals with mixed offspring, pigmented and albino.
"The tyrosinase enzyme," he digressed, "is part of the pathway that produces pigmentation, melanin production. The glaucoma gene itself, CYP1B1, is short for cytochrome P 450 b1 form, which is an oxidative enzyme. It's expressed in the eye but no one knows what it's doing there. We transferred this gene onto an inbred mouse pigmented background. It lacked the tyrosinase gene, which makes pigment, so it was albino. This seems to carry over to the human condition from the few reports that we've been able to scare up," Smith added. "We'd gotten two not rare but uncommon diseases. In order to have a person with albinism and congenital glaucoma, you've got to have - because they're both recessive genes - parents who are both carriers. And the equation reads: uncommon times uncommon = rare.
"So the chance of that happening is small, but it has happened," Smith noted. "I think there's an exact carryover, because the cytochrome P 450 gene is the human gene. It's as common as one in 2,500 live births with congenital glaucoma in the Mediterranean basin area, particularly Saudi Arabia and the Gypsy population of Eastern Europe. There's a high incidence of the disease in these areas because there's a high degree of consanguinity.
"The key question we asked our mice," Smith recounted, "was the pigment issue. We had suspicions about this from the start, when we looked at those assorted mice from NIH, and saw a disparity between pigmented and albino. So we thought: What if we changed the pigment production pathway products?' And that's why we did the L-dopa experiment last year. Mice have a 20-day pregnancy duration. So starting on the 7th day, about the time eye formation begins, we put L-dopa into the drinking water of the pregnant mothers. The embryos were exposed to that drug until birth, 13 or 14 days later.
"Under the light and electronic microscopes, I looked at these eyes and said, We've got a winner.' Our treatment cohort was about 50 mice, and 100 percent of the treated ones looked entirely normal. But 100 percent of the untreated animals looked entirely abnormal - with glaucoma."
What's Parkinson's L-Dopa Doing In The Eye?
"The L-dopa stimulated pigmentation. The mice got more pigment in their eyes, but less reaction to coat color. To turn them completely black would have called for a much higher L-dopa dose. We think we're on to some interesting stuff, that could have an effect in how we treat people with glaucoma.
"It probably would not be with L-dopa," Smith said. "First of all, you wouldn't give it to anyone who did not have an at-risk genotype for this rare disease. And second, the FDA and physicians are going to want to take steps before pregnant women dose their offspring with L-dopa - even though it's approved, for Parkinson's disease.
"L-dopa has its warts," Smith allowed. "We look at it not as an end in itself but a new way of looking at something in terms of glaucoma that may be particularly useful. We're looking very hard at adult glaucoma, because that of course is a pretty common disease. Some 70 million people worldwide have adult-onset glaucoma. It's a major economic and social issue.
"We're also working hard to adapt some of the human techniques to the eye examinations of mice, and have been pretty successful at that. We can do just about anything," he concluded, "except getting them to read the eye charts. We're working on that one!" he quipped.