By David N. Leff
BOSTON - Wednesday morning's BIO Science Symposium crystal-balled "Biomedical Research: A Vision for the Future." A leading academic player in this hands-on futurism is embryologist and molecular biologist Rudolph Jaenisch, of the Massachusetts Institute of Technology in Cambridge, Mass. His topic: "DNA Methylation, Development and Disease."
Methylation is a universal biological process that consists of adding a methyl group - a molecule of CH3 - to a DNA sequence. "Say you have two texts in your word processor," Jaenisch noted. "One is nicely formatted, the other disordered - but their information content is exactly the same. What methylation does is make the genes better readable by the transcription machinery of the cell."
It's a controlling factor in two developmental phenomena - epigenesis and imprinting - that affect the fate of genes before and after birth.
Jaenisch defined epigenetics as "the change in control of gene activity states that do not involve changes in nucleotide sequences. I'm talking about the modification of the DNA by methylation of that gene's chromatin states. These states are stable for mitotic divisions, but can in principle be changed."
First of all, methylation is important for development because it determines that a gene in a given tissue can be expressed, or not be expressed. So during differentiation, some genes are expressed in one tissue but not in another.
"For example, the globin protein is not expressed in neurons, because globin is in an epigenetic state - which means its chromatin is methylated - so it's not available for expression," Jaenisch said. "Whereas erythrocytes [red blood cells] are in a different state of chromatin methylation, so there they can be expressed."
He added that "epigenesis is one mechanism where the environment can directly influence gene expression states. It's very important because, as we know, what you eat - your diet - will influence the incidence of cancer. And this is an environmental stimulus. I suspect we can actually put our hands on some such genes now." Indeed, he said, "diet or other environmental stimuli may influence the extent that a given gene will be active or not active. And this will certainly influence profoundly what, for example, your probability is of developing a certain disease. And I think this is not very appreciated yet. I think it's been neglected so far because we don't have the technology in hand to test this phenomenon."
Jaenisch told his symposium audience that epigenetics "is also very exciting now in nuclear cloning. Remember, Dolly the Scottish sheep was made from a nucleus, which came from a mammary gland. Since then, cows, goats, mice and, recently, pigs have been cloned." The interesting issue here, he said, is that "the epigenetic state of the mammary glands is appropriate for expressing mammary-gland-specific proteins like milk, but is not appropriate for early embryonic development of the early cell nucleus.
"For Dolly to be made," Jaenisch said, "for the cloning experiment to succeed, one has to postulate that the epigenetic state can be reversed through the process of cloning. But also, when you look at all clones achieved so far - those sheep, cows, goats, pigs and mice - it is a very inefficient process for generating long-lived animals. At this moment, it's very low, a few percent and you may well ask why this is so." When you look at the way these clones are lost, he noted, "they are lost very early, during cleavage. Because they probably damage the nucleus or the embryo, so they die. What's interesting is that the fatality occurs very late in gestation. For example, many of Dolly's sisters died late, just before being born."
Gene Imprinting - The Disease Connection
"Imprinting is a mammalian-specific phenomenon," Jaenisch said. "In most normal genes, you have two copies - one from father, one from mother. Both of these genes are expressed. So, in general, when you mutate one allele [parental variant] you don't have the disease, if it's a recessive mutation. To get it, you have to mutate both."
He added, "It's security in a way. In imprinting genes, only one copy is expressed. For a given gene, it depends on whether the copy is inherited from father or mother. Only one allele is expressed. So the basis for monoallelic expression is methylation. That's what maintains the epigenetic state of the two alleles that are different: One is methylated, the other not."
As for future utility or application of these phenomena, Jaenisch said, "I think the biotech industry is interested in cloning, which is potentially important for transplantation medicine. In transplantation, the main problem is to find an immune-typed matched donor. Which brings us to stem cell generation," he told BioWorld Today. As was shown last year, human embryonic stem cells can be generated from expanded blastocysts.
"Let's say you have a patient who has leukemia or bone-marrow disease," Jaenisch said. "He needs a bone marrow transplant from a matched donor. If you could take his skin cells, which are normal, not diseased, and convert those dermal cells to hematopoietic cells, your patient would have an isogenic [self] cell to serve as a donor, because it's his own."
"How would you do that?" he asked rhetorically. "The only way is to clone that skin cell by nuclear injection between oocytes, get blastocysts out of this, and from them obtain an embryonic stem cell. It's well established in the mouse that that can then be differentiated very easily into bone marrow cells, or into heart muscle cells, or into skeletal muscle cells, or into neurons - into all tissues. This would be a perfect source for me, if I could do that. So this is one of the future visions why cloning is of interest to the biotech industry - in addition to making farm animals that have defined properties."
As for the putative effects of epigenesis or imprinting on the highly hypothetical concept of cloning an adult human, Jaenisch made the point that, "You probably wouldn't get an exact carbon copy of the original adult, because a large percentage of one's personality is due to environmental influences - like how you grew up. So it's not just how your face looks, but how your mind works. And that you cannot clone. But the genetic makeup, as in identical twins. would not be affected."
Even identical twins, he said, "have different fingerprint patterns. So how your fingers develop is clearly epigenetically controlled: It's different; it's not genetic only." He observed in a deadpan delivery, "The FBI is quite relieved by this. If different clones were committing crimes, they couldn't do any more forensic DNA fingerprinting, but would have to resort to old-fashioned fingerprints to identify the perpetrator."