"Wild-type dog-1" does not describe a canine that forsook domesticity to go feral. Rather, dog-1 is the acronym of a gene that stands for "deletions of guanine-rich DNA." It was recently discovered in wild-type nematodes - Caenorhabditis elegans - a mini-organism much studied by molecular biologists.
C. elegans has a lot going for it research-wise. The transparent, millimeter-long roundworm is the most abundant of all multicellular organisms on earth. Its 959 cells comprise 19,099 protein-coding genes in its 97-million-nucleotide genome. An ever-increasing number of those worm genes have homologues in humans (Homo sapiens), which makes C. elegans an elegant laboratory model for studying people in sickness and in health. Like folks, the worm has a nervous system, digests food and pursues a busy sex life. In its 3.5-day life span, C. elegans spawns some 300 progeny daily, partly by self-fertilization.
The discoverers of dog-1 were a research group led by cell molecular biologist Peter Lansdorp at the B.C. Cancer Agency in Vancouver, also a professor of medicine at the University of British Columbia. "We've come up with a new gene that seems to be important to maintaining genomic integrity," he told BioWorld Today. "It's interesting that this molecule in the worm has a homologue in humans, which interacts with the BRCA gene's protein, involved in breast cancer. Genetic instability," he explained, "is key to cancer. The evolution of cells in the normal genome into cancer cells is highly abnormal. One of the critical elements there is the loss of genetic integrity, so cancer researchers are interested in the mechanism the molecules use to prevent genetic instability throughout the genome.
"What is unstable," Lansdorp went on, "is the actual sequence. Every time a cell divides, its DNA needs to be duplicated, and one hopes that the two daughter cells retain exactly the same genetic copy, the same DNA, as their original parent cell. But if that copy is altered in any way - as in the case of a mutated dog-1 gene in the worm - if there's a piece missing or deleted, then you're talking about genetic instability."
Guanine Nucleotide Forms Complex Quartet
Lansdorp is senior author of a paper in Nature Genetics, released online July 8, 2002. Its title: "Disruption of dog-1 in Caenorhabditis elegans triggers deletions upstream of guanine-rich DNA."
"There's more than one finding here," Lansdorp said. "On the one hand we found a new type of genetic instability. It has a characteristic signature in that throughout the worm's genome there are deletions that start as runs - stretches - of guanines. These are one of the four bases in nucleic acid, the others being adenine, cytosine and thymine. That type of signature in terms of genome abnormalities hasn't been seen before," Lansdorp continued, "so this is the first time that this type of genetic instability has been published. Also, we came up with a model that implied the formation of G quartets - four guanine bases interacting with each other, like small kinky knots. That type of DNA structure has been seen in the laboratory in vitro. But I think it's the first example supporting the existence of these configurations in vivo. In the G quartet you get four G's exchanging electrons, and a very stable complex that we see in the lab but not in actual animal cells.
"Dog-1 genes and the proteins they express have homologues in humans," Lansdorp pointed out. "As to what their function there might be, we're speculating because we have no data. We looked under the microscope at mutant worms and observed that they gave rise to various mutations, which we characterized by culturing them and observing phenotypes. Then we reintroduced the normal, nonmutant dog-1 gene into the worm to show it was the gene we thought it was. However, I think a reasonable speculation is that the human homologue will have a similar function to what we postulate for the worm. Namely, to maintain stretches of guanines or of structures that can form G quartets during cell replication. When that protein is missing or absent in the worm, we see instability. The net result always is that the daughter cells are not identical to the parental cells, and you have a genetic instability in the phenotype."
Cancer Problems Seek Therapeutic Solutions
"The possible relevance of dog-1 to human breast cancer," Lansdorp observed, "might be one of its most exciting aspects because it's been speculated - and there are some data - that BRCA1 is involved in DNA repair. But what is the DNA that needs to be repaired? By studying the worm, we may get a handle on that. And certainly we need to look at it further. We would now say that looking at the DNA of patients with breast cancer that has mutations in the homologue of the dog-1 gene - and those have been described - we would like to examine the stability of these polyguanine genomic stretches.
"What we found in the worm so far is that it takes at least 20 G's next to each other before you have a pathological problem. And if the sequences are longer, say a stretch of 30 G's, you have more problems than with 20. So there seems to be a length dependency in frequency of deletions, which is compatible with this G-quartet model. These are similarities to CAG triplet codon repeats in Huntington's disease, which are also assumed to result from the replication of a particular DNA sequence," he said. (See BioWorld Today, July 11, 2002.)
"The human gene equivalent to nematode dog-1 resides on chromosome 17. This is a genomic locus neighborhood in which this gene has its problems: It is mutated in a proportion of breast cancer patients. In the long term this may imply or suggest a possible therapeutic application at some time in the future. For therapy in most cases one wants to know what the problem is. This Nature Genetics study suggests that we still need more data before we have a handle on tumors where BRCA1 or dog-1 homologue is missing or defective. Then maybe," Lansdorp concluded, "we can think about ways to come up with specific interventions, which might be beneficial to people who have acquired this problem."