By David N. Leff
At first glance, an automobile looks perfectly symmetrical, left to right, along both sides of its long axis. So does a human body.
But the car's driver sits behind a steering wheel, floor pedals, gearshifts and dashboard dials, all grouped to the car's left.
Similarly, in the normal human body, the heart nests to the left, along with stomach and spleen, while the liver lies to the right, and intestines coil counterclockwise. Even the paired lungs are asymmetrical: the right-hand one has three lobes; the left, only two.
To be sure, in many countries, cars drive on the left, so their controls are grouped on the right. And a certain number (actually, uncertain) of humans are born with their internal organs flipped mirror-image-wise. They wear their hearts on the right (dextrocardia), liver on the left, and so on.
This reversal-of-organ condition — called situs inversus, in contradistinction to the normal situs solitus — causes no medical problems, as a rule. In fact, situs inversus usually comes to light only when its subject is examined by X-ray.
But there's a third form of jumbled visceral organs, situs ambiguus, a.k.a. heterotaxia, that is a menace. Pushing the automotive analogy, it recalls the warning never to buy a car assembled on a Monday morning — following a relaxed weekend by the workers on the assembly line.
In situs ambiguus, the gestational assembly line — which dictates left-right asymmetry in the growing embryo during its first six to eight weeks of development — has lost it. It positions the internal organs randomly in no order, marked by major congenital heart malformations.
"Typically," molecular geneticist Brett Casey told BioWorld Today, "the children born with heterotaxia carry quite severe heart defects. And this manifests itself straight up front, at birth, as some altered circulation. Often, the babies will be a bit blue, because they are not pumping oxygenated blood to their systemic circulation."
When Heart Is Not In Right Place
"Such complex cardiac defects," Casey continued, "are quite common, and often fatal without surgical intervention. The usual situation is that the precise anatomic defects are defined by imaging techniques, and then a surgical plan made. This often means multiple surgeries over a period of years."
Casey is senior author of an article in the November issue of Nature Genetics titled: "X-linked situs abnormalities result from mutations in ZIC3."
"ZIC3," he told BioWorld Today, "is the first human gene that has been shown to be altered or mutated in this particular birth defect." He went on to list other particularly common, clinically significant abnormalities of situs ambiguus: asplenia, intestinal malrotation, defects of the lumbosacral spine and hind-gut.
Casey directs a laboratory focused on the molecular genetics of human left-right axis malformations, at the Baylor College of Medicine, in Houston.
He observed that "this type of left-right mix-up has been recognized for centuries, reportedly even by Aristotle. Heterotaxia now afflicts about one in 10,000 male and female victims."
Casey and his co-authors set out to find, by positional cloning, the gene or genes that account for situs ambiguus.
"At the beginning," he recounted, "we identified a very large American family, back through four generations, that looked to have sex-linked inheritance, because 11 of the females were carriers, but at least 12 of the males were affected with heterotaxia. In addition, the inheritance of whatever genes had gone bad seemed to always come through a female carrier."
This immediately suggested to the team that the hunted gene was on the X chromosome. The researchers used linkage mapping to narrow the region on that chromosome where they thought the gene must lie, because it was always that region that showed up in the affected and carrier individuals.
In a second, unrelated family, they confirmed that that narrow region on X was a hot spot for the disease gene.
"Then," Casey went on, "we simply began to isolate genes that lay in that region. One of them had a very strong homology to a fruit fly gene called odd-paired, which is involved in the insect's very early embryonic development. So we thought: 'Okay, this might be the gene we're looking for.'"
They found it in their original mega-family, as well as in a number of other kindreds with sporadic (not familial) male cases, "and asked the simple question: 'Do we find mutations in this gene, in these families and in these individuals?' And indeed we did. That closed the circle."
Multi-Genes Govern Scrambled-Organ Disorder
Having thus isolated the gene for familial heterotaxia, Casey made the point that this disease — unlike X-linked hemophilia — is transmitted to females as well as males by numerous other sporadic autosomal genes.
The Nature Genetics paper's first author is molecular geneticist Marinella Gebbia, a visiting scientist in Casey's lab.
"The ZIC3 gene," she pointed out, "encodes a zinc-finger transcription factor. And zinc fingers are the active domain of the gene that encodes the protein mutated in situs ambiguus. Probably," she added, "though we don't know yet, the mutated protein cannot fold properly. Several of the mutations truncate the gene, so it can make no protein. But some of them can make the protein change its structure, so it can't attach, for example, to DNA."
The lab is now engaged, Gebbia observed, "in finding the functional evidence of what this gene is doing. We don't know where exactly, and on what other genes, ZIC3 is acting."
To which Casey added: "Certainly, one future focus of our work will be to try to place this molecule into the larger picture of the molecular aspect of left-right axis development."
As for practical applications, he suggested, "There's a proximal genetic-counseling aspect to this research." But his long-range goal is "trying to modify the rate at which this birth defect will occur. The idea is: Can we do something to increase the percentage of those not affected? Something simple, like modifying the diet, or whatever? Like prevention of neural tube defects by folic acid." *