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.”
The article written by
Dr. Kenneth Bovee
is on the following webpage:
http://www.vin.com/proceedings/Proceedings.plx?CID=WSAVA2003&PID=6602&O=Generic
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.
Further information on testing please visit us on the WEB at
http://www.dogenes.com
Use Your Knowledge Wisely
Protect Your Gene Pool and Preserve Genetic Diversity in your breed
Mary H. Whiteley, Ph.D.
DOGenes Inc.
:
+44 (0)1905 830900
