The Molecule of Life

It's the greatest scientific puzzle.

Deoxyribonucleic acid (DNA), a.k.a. the molecule of life, harbors an estimated 20,000 to 200,000 canine genes hidden in the DNA across from 38 pairs of chromosomes, all contributing to health and disease. Sleuthing out defects along genes nucleotides, or base pairs, seemingly turns into a numbers game that can add up into the billions.

Dr. Elaine Ostrander, director of Cancer Genetics Branch of the National Human Genome Research Institute of the National Institutes of Health, reports the canine model represents true scientific potential.

The unveiled sequenced canine genome announced last December was heralded as a critically important scientific marker, and it sets the stage for major exploration during the next 10 years. To date, about 2.5 million single nucleotide polymorphisms have been identified. These variants in the DNA code contribute to disease.

So, why the dog? With only 350 varieties (families) and 15,000 years since man began selective breeding, it represents a mere tick on the genetic clock. Each breed offers a unique panel of polymorphisms never encountered in other mammals. About 70 percent of dog DNA is identical. Now consider canine cancer. The presentation of disease in humans is quite similar. This combination and the crossover potential to human medicine makes the purebred dog a genetics researcher's best friend.

Ostrander remains hopeful that the next 10 years will be rife with discovery of genetic variants that lead to new screening tests and new improved therapies for treating disease from cancers to heart disease.

Cracking the code

This crossover potential to human medicine also opens up the coffers to major research cash. In other words, dog stands to benefit a great deal.

"Understanding the genes in dogs is going to be important for anyone interested in pursuing cancer genetics and cancer biology," Ostrander says.

"Dogs offer an opportunity for us to tackle some of the cancers that have been very difficult to understand the genetic makeup nature in humans because canine families are much bigger and because of the breed barrier. In humans, there might be a dozen genes for osteosarcoma just to pull a number out of thin air, but for any one breed, the Rottweiler, Irish Wolfhound or Scottish Deerhound, that number is going to be considerably less because the breed barrier has restricted the number of deleterious genes for any disease," she explains.

"So, there has been a real sense of excitement of people in the field to look at lymphomas, osteosarcomas, melanomas and a variety of different types of cancers that are of interest for human health as well as some of the cancers that aren't such a big deal for human health, like hemangiosarcoma, malignant histiocytosis."

Under investigation

Ostrander's Bethesda, Md. laboratory is focused on finding the susceptibility gene for three important canine cancers: malignant histiocytosis in Bernese Mountain Dogs and Flat-coated Retrievers; bladder cancer working in conjunction with Purdue University and University of Minnesota and lymphoma, especially T-cell lymphomas.

She adds that other laboratories around the country are focused on melanoma, osteosarcoma, B-cell lymphomas, hemangiosarcomas and CNS tumors.

"There have been groups coming forward expressing a strong interest for all of the canine cancers," she says. "I think it is an area that is really going to explode in the next few years.

"As we start to unravel the complex genetics of cancer, it is really going to set the stage for how to tackle many of these other diseases."

Hip dysplasia is one such disease that affects an estimated 50 percent of all dogs, especially large breed dogs. "I think the availability of the genome sequence has dramatically changed the rate at which we do our work," she adds.

The worldwide publicity spawned by the decoded canine genome helped generate more collaboration among scientists and sped the ability to pinpoint genes of interest.

"The rate at which we can scurry through the genome to find the regions of the genome we think are important is much faster. And once we have regions of the genome we think are responsible for disease, the rate of which we can interrogate those regions very fully and come up with a set of candidate genes to interrogate further has changed dramatically."

Recruiting breed clubs

Oddly, one limiting factor to progress is access to quality samples, she says.

"Breed clubs have more control than they think they do. If we can't get samples, we can't work on it. Those that are motivated are the ones that get our attention."

The Portugese Water Dog Club is case in point.

"The club recognizes their breed suffers from a number of ills ranging from Addison's disease to hip dysplasia to cancer," she says. The club began to collaborate with Gordon Lark at the University of Utah to actively gather samples to study. With the help of their veterinarians, more than 1,000 blood samples along with medical information, contact information and X-ray data were obtained through a point person for this AKC-registered breed.

"As a result, that breed club has gotten an enormous amount of scientific attention. That is the breed in which the locus for osteoarthritis has been described for hip dysplasia, which is under review."

In comparison, Ostrander says many studies have trouble getting off the ground simply due to lack of quality samples, not funding.

Having a rich variety of samples also allowed researchers to comb through the genome using a set of markers, or variation in DNA. "They looked at a set of 500-600 sets of markers, interrogating each of the chromosomes at every 5-10 million base pairs. And for each of these diseases, they looked at what each of these dogs affected share, and dog's unaffected lack. It is very powerful information," she says.

"Addison's disease is a hard disease to study. One way to reduce the complexity of the problem is to look within one breed. It is often hard to get enough samples within one breed, but when you have 1,000 people responding to a health study with X-rays, DNA samples, blood samples with medical information, pedigree information totals, you have enough power to start tackling and investigating hip dysplasia, osteoarthritis, bladder cancer and Addison's disease."

When work commences, Ostrander's group dives in to scrutinize the genetic map in that region.

"We have ways we can pull out all of the genes very efficiently, and then look the sequence of genes in affected or unaffected dogs and see if we can find differences. We interrogate those difference very closely to see if there are things that are likely responsible for the disease or not. Are there things that change the structure of the encoded protein or amino acids? What sets are critical to the function of the protein? What do we know about those proteins and other animal systems? Are they intact in affected dogs or unaffected dogs?"

Busting myths

Ostrander adds that "she's dazzled" by the educational process veterinarians have undertaken in regard to genetics. But she does have a few myths to bust about the science.

Myth No. 1: "Everything that doesn't show a clear pattern of genetics is some sort of complex multigenic effect. That is not going to turn out to be true. In some of those cases, it is going to turn out that there is one major gene, it is just that it has variable penetrants. So, it is not the RCND gene that we identified in the German Shepherd kidney cancer case, where the penetrant seems to be nearly 100 percent. Everyone who has the mutation gets the disease. That is not the case; that is not the norm in genetics. And it is going to be much more gray."

To take the analogy further, 10 dogs of the same breed could have the same genetic mutation, yet two of dogs will get the cancer early in life, two may get it later in life, and six might not get it at all.

And who can forget the role of diet, exercise, and in-utero effects, other minor genetic factors and just plain chance.

"I think veterinarians shouldn't be so quick to pass everything off as multigenic or complex genetics. I think actually some of this is going to be a lot more solvable than they think, but then the ball is going to bounce back into the veterinarian's court, because all those things I mentioned like diet, exercise, environmental factors, are all things that we are going to tease out the effects on genetic expression. That is going to go through a logarithmic educational process," she says.

In other words, exploration of the genome likely will unearth a tremendous amount of knowledge on breed predisposition, lifestyle and dietary recommendations based on genetics. With it, Ostrander says, comes "a wealth of medical knowledge over the next 10 years."