Discovery of the Mutation and development of a
direct DNA test for JRD
by Dr. Mary H. Whiteley, Ph.D. - DOGenes Inc.
Background
Juvenile renal dysplasia (JRD) is an important category of kidney diseases
in canines. Dysplasia is defined as abnormal growth or development of cells
or organs. In the case of JRD the kidney fails to develop properly during
embryogenesis in the womb. At birth immature structures consisting of undifferentiated
fetal cells or tissue types are found in the kidney, and are persistent throughout
the life of the animal.
Many breeds of dogs are afflicted with JRD, and this has been documented
in veterinary text books, as well as case reports and articles in the scientific
literature. Of note is that JRD in these breeds share a common phenotype,
characterized by immature glomeruli, and/or tubules and persistent esenchyme.
The mode of inheritance of JRD has been widely debated, as this disease
can present itself with a wide range of symptoms and pathological findings.
Definitive diagnosis of JRD is done by a wedge biopsy which reveals dysplastic
lesions, including abnormal ducts, and glomeruli. Individuals with an abnormal
biopsy can be asymptomatic, showing no signs of the disease, On the other
hand, they may present with classic signs of chronic end stage renal failure,
or somewhere between these two extremes. Given this broad spectrum of symptoms
affected individuals often go unnoticed, and remain in the breeding population.
This is why development of a genetic test is critical to the management and
elimination of this disease. Further a genetic test will shed light on the
mode inheritance.
The gene for JRD is discovered and a direct genetic test is developed.
During the development of the fetus, certain genes act in a pathway or cascade
to direct the development of the various organs and structures that make up
the body (think of it as dominos: if one domino is missing the other ones
won’t perform their task in the array). Since every cell contains two copies
of every gene, one non-mutated copy may be sufficient to complete to cascade
(recessive inheritance). The human genome contains about 20,000 to 25,000
genes, and the canine genome is likely to contain about the same number. Of
these several hundred or more may be dedicated to the maturation of specific
organs. Not all of these “tissue specific” genes are absolutely essential
to the development of specific cell types, as has been shown in “knockout
mice”, where genes have been completely eliminated with no observable effect
on the animal. In the case of renal dysplasia in model organisms such as the
mouse, mutated transcription factors (genes that code for proteins that turn
on other genes) or growth factors (genes that code for proteins that promote
the growth of cells), have been implicated in causing the disease.
One approach to discover genes that are cause genetic diseases is to use
candidate genes known to cause specific diseases in model organisms, like
the mouse, rats, zebra fish or even the lowly fruit fly.
This was the approach used in the quest to find the mutation for JRD in
dogs. After DNA sequencing six candidate genes, the causative mutation was
finally uncovered in a gene in Lhasa apsos, and Shih Tzus. This mutation
was then discovered in other breeds with JRD, including Cairn terriers, and
a direct genetic test is now available for many breeds afflicted with this
disorder.
Through pedigree studies, the mode of inheritance was finally revealed as
Dominant with Incomplete Penetrance.
What does Dominant with Incomplete Penetrance Mean?
All chromosomes exist in pairs in the nucleus of cells. Each pair is comprised
of one chromosome from the sire and one from the dam. Therefore, every animal
has two copies of every gene. In dogs there are 78 chromosomes, or 39 pairs.
The traits that we see in an individual are collectively known as the "phenotype",
while the "genotype" refers to genetic constitution or makeup of an individual.
A mutation is a permanent change in the DNA sequence of a gene, whether
it is good, bad or neutral. Mutations that cause a genetic disease can be
inherited as dominant where one bad copy of the gene is sufficient to cause
disease or a phenotypic trait to be observed, or recessive where two bad
or mutated copies of the gene are needed to cause disease or phenotypic trait
to be observed.
Dominant with incomplete penetrance refers to a situation where an inherited
mutation may or may not be expressed in an individual.
Penetrance refers to the frequency that the phenotype (or some characteristics
of the disease) is observed. If for example, the penetrance is 75%, then the
chances of offspring to develop a disease are 3 out of 4. In the case of
JRD, the penetrance is low, with a penetrance estimated to be about 5%. Therefore
only a small number of individuals with the mutation will show signs of the
disease. However, they can pass the disease on to their offspring. This is
why a genetic test is critical to manage JRD; this is the only way to eliminate
this disorder. There may be risk factors or triggers that are yet undiscovered
that may increase the chances of an individual to develop JRD.
How can the new test be used to eliminate this disorder from a breed
without compromising the gene pool?
Genetic tests are designed to manage and eventually eliminate disorders
without compromising the diversity in a gene pool. If you have just found
out that your dog carries the mutation for juvenile renal dysplasia, do not
panic. Now you have the opportunity to manage and eliminate this disease.
The frequency of this mutation is extremely high in many breeds. This mutation
has been elusive and impossible to eliminate prior to the development of
a genetic test, as the disease appears sporadically because it is inherited
with incomplete penetrance.
Meaning that an animal that carries this mutation may or may not show clinical
signs of the disease, but can still pass it on to the next generation.
All dogs (and living organisms) are carriers of multiple mutations.
If a genetic disease is produced in an animal, it is not necessarily the
result of poor breeding practices, but is the nature of inheritance as a random
event. Although the exact mutation rate for caninies is difficult to determine,
by extrapolation from other species, there is a good chance that every individual
produced has a new mutation in some gene. Therefore, with every generation
of breeding, new mutations arise, but since they are present at a low frequency,
they are generally lost in subsequent breeding. There is no such thing as
a perfect animal!
Chromosomes exist in cells in pairs, one from the sire and one from the
dam. Dogs have 39 sets of chromosomes. Each set or pair is composed of two
chromosomes, one from the sire, and one from the dam. In the case of a simple
recessive mutation, one of the chromosomes, either from the sire or the dam,
makes enough protein from for the animal to survive. Therefore, the “wild
type” chromosome of the pair provides enough protein (gene product) to compensate
for the chromosome that carries a mutation. In the case of a dominant mutation,
only one copy of the chromosome carrying the mutation is necessary to produce
disease.
With the identification of one of the many mutations that your animal carries,
you can now proceed to at least eliminate this identified mutation, and not
inadvertently select for another deleterious mutation that your animal carries.
Wholesale elimination of carriers is the worst decision that you can make
as this would deplete the gene pool.
As in any breeding you must consider the positive and negative traits of
each partner, and how the parent’s traits can best balance and compliment
each other.
A brief History of JRD in the GSD Breed
Over the last 15 or so years, the GSD breed in the UK has experienced about
10-20 litters where most of the litter died “apparently” of this JRD disease.
Although it seems to be the general impression that these cases can be traced
to one or two popular sires, this mutation is probably more widespread than
is currently appreciated.
Throughout the course of my work with the various breeds, breeders who denied
ever having a case in their kennel were surprised to find that the mutation
was present, or suddenly after years of breeding a litter is born with this
disease. This disease appears to be sporadic in nature, and is probably
influenced by genetic background.
To those who deny that this mutation is in their lines, I always ask “have
you looked, and how many biopsies have you done on your dogs. “ The
answer in generally that they have not been at all vigilant about this disease
in their lines, and when an entire litter goes down at an early age, breeders,
except in the breeds that are well known to have this disorder, may not consider
the possibility. Many veterinarians have likewise pigeonholed this disease
as one of Lhasa apsos, and Shih tzus.
Here is an excerpt form an article written by Dr. Kenneth Bovee regarding
JRD in Shih Tzus:
“The prevalence of renal dysplasia in this breed is very high in North America.
In a study of 74 random dogs evaluated with wedge renal biopsy, only 16% were
free of any histologic evidence of the disease. Among the remaining, about
52% had 1-5% fetal glomeruli, while 20% were moderately affected with 6-15%
fetal glomeruli. The remaining 12% had more than 15% fetal glomeruli. These
findings suggest that the genetic character of the disease is very high and
variable in this breed. Because many dogs are mildly affected and escape
detection in the absence of renal biopsy, the question arises as to the genetic
transmission of the defect in normal appearing dogs.”
In the case of JRD in your breed, ideally animals identified as carriers
of this mutation should be bred to a clear. In this case, approximately
50% of the offpsring will be clear of the mutation. Individuals with two copies
of the gene (homozygous mutant allele = homozygote) by default breeding a
homozygote to a clear will produce all carriers. This is the only way to eventually
eliminate the mutation in these circumstances, but such a breeding is only
recommended if this is the only breeding option. If the frequency of the
mutation in a breed is high, breeders have been doing this mating all along
without even knowing it. And the carriers can then be bred to a clear, in
subsequent generations. Obviously and where possible, it is ideal to breed
clear to clear. However it is important for your breed to maintain genetic
diversity.
What will happen to the resultant litter if breeders mate 2 of the (c) ‘clear’
animals together? Dr. Bovee states in his article this mating still produced
carrier’s 1-3% fetal glomeruli.
Dr. Bovee is referring to two animals that are clear by biopsy (phenotype),
not by DNA test (genotype). Animals that have the mutation may have a completely
normal biopsy. This is why people who have tried to eliminate this disorder
by biopsy (phenotype) have failed. Only a genetic test can tell you what you
are dealing with.
What does this DNA test actually tell the breeder?
The DNA test results are reported as follows:
a) Carrier (one copy of the JRD mutation)
b) Homozygous mutant allele = Homozygote (two copies of the JRD mutation)
c) Clear.
With a & b results above – the animal is also potentially affected by
JRD.
This DNA test information is clearly important for breeders, and important
for the future of the breed.
Before the development of this DNA test, JRD could only be positively diagnosed
by a biopsy. A biopsy will reveal the phenotype. A wedge biopsy may reveal
dysplastic lesions, including abnormal ducts, and immature glomeruli. Individuals
with an abnormal biopsy can be asymptomatic, showing no signs of the disease,
On the other hand, they may present with classic signs of chronic end stage
renal failure or somewhere between these two extremes. Given this broad spectrum
of symptoms affected individuals often go unnoticed, and remain in the breeding
population.
At the moment, in German Shepherd Dogs (GSD), and many other breeds, we
have no idea what the frequency is of this mutation. We will only be testing
20 GSD in the initial Test Study, and much more testing needs to be done
to determine the extent of the occurrence of this mutation in this breed.
Only with this knowledge, will you be able to proceed and eliminate this
disorder effectively.
Organizations representing other breeds should contact me at Dogenes Inc.
http://www.dogenes.com to discuss a JRD
DNA Test Study for their breed.
DOGenes uses cheek swabs for DNA testing.
We ask for three samples per dog. We also require certain documentation
to be completed, and 3 to 5 generation pedigree information.
Protect Your Gene Pool and Preserve Genetic Diversity in your breed
reprinted with kind permission from Dogenes.com
The
above information is simply informational. It's intent is not to replace the
advice of a veterinarian nor to assist you in making a diagnosis of your pet.
Please consult with your own veterinarian for confirmation of any diagnosis.
Your pets life may depend on it.
