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As everyone is no doubt aware, a worldwide statement
was issued on 20th April 2007 by Dr. Cathryn Mellersh
from the Animal Health Trust (AHT) and Dr. Gary Johnson
from the University of Missouri USA, stating that a
mutation has been identified that is a major risk factor
for the development of PRA in English Springer Spaniels.
They also announced that a DNA test for this mutation
has been developed and is now available in both the
USA and UK.
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| Following that announcement, a number of breeders, who
had donated samples from clinically affected dogs and
their close relatives to the original PRA research project
at the AHT, were invited to request from them the genetic
results for these dogs. |
| These breeders had, naturally, expected that their clinically
affected dogs would be confirmed as genetically affected,
but when some results showed this not to be the case (i.e.
their dogs tested as either genetically clear or carriers),
considerable confusion and alarm ensued concerning the
validity of the DNA test. |
| It is the opinion of both Dr. Mellersh and Dr. Johnson,
that, barring any errors in the testing procedures (possible
but unlikely), these unexpected genetic results can be
explained by either of the following: |
- In some rare instances, a dog can develop PRA as
a result of carrying a single copy of the mutation
(i.e. a genetic carrier), rather than two mutant copies
as would be expected in affected dogs. This would
be unusual, based on what is known, but cannot formally
be excluded at this stage.
- There is another form of PRA in the English Springer
Spaniel caused by a different mutation in a different
gene that, as yet, has not been identified. Multiple
forms of the same eye disease occurring in the same
breed are not unheard of, and it is entirely possible
that some dogs can carry either or both mutations
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| The original announcement made by Dr. Mellersh and Dr.
Johnson, may, unintentionally, have given the impression
that the new test was the whole answer with regard to
PRA in the Breed. Whilst it certainly mentioned that a
second form of PRA was a possibility, the figures quoted
(which were based on statistics from the USA studies)
suggested that this was rare. Unfortunately, with regard
to being able to advise us on how the statistics for a
second form of PRA might apply within the UK population,
Dr. Mellersh was and is unable to give any estimates or
make any predictions, as she only has a very limited number
of 'affected' samples to work with, and these are currently
nowhere near enough for her to make any valid judgement. |
| The DNA test for PRA that has been made available is
for one specific mutation in one specific gene (known
as 'PRA Cord 1 Mutation' or RPGRIP1), and the test cannot,
therefore, detect any other mutation that may be responsible
for any other form of PRA. It does not, however, diminish
its validity for this particular mutation, which is undoubtedly
associated with PRA in English Springer Spaniels. |
| Statistics from the AHT show that approximately 20%
of dogs have been found to be genetic carriers for the
Cord 1 (RPGRIP1) mutation, with approximately 3% being
genetically affected. Irrespective of whether a second
form of PRA exists in the Breed, the prevalence of this
mutation alone in the UK population is very significant.
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| The DNA test allows breeders to identify those dogs
either carrying or affected by this mutation, and, through
careful and sensible breeding over several generations,
to eliminate it from their breeding. The fact that it
is not the whole of the answer to PRA is, of course, disappointing,
but the fact that a DNA test is available for any form
of PRA has to be a step forward. |
| This new test, as with any other, is simply another
tool that breeders now have at their disposal should they
wish to make the most of the information available to
them. When making decisions on breeding, the results of
any test should be taken in the overall context of the
merits or weaknesses of each dog and the severity or consequences
of the disease to which the test relates. |
| With regard to PRA, it is the intention to carry out
further research into both of the following areas: |
- a) The identification of the mutation that causes
a second form of PRA in the Breed. This will require
samples from dogs that are diagnosed as clinically
affected but their DNA test shows them not to be genetically
affected for the Cord 1 (RPGRIP1) mutation.
- b) The identification of dogs that have been tested
as genetically affected for the Cord 1 (RPGRIP1) mutation,
but are not clinically affected. From a breeding point
of view, it is the genetic status that counts, but
Dr. Mellersh and Dr. Johnson have advised that other
genes may influence the age of onset and rate of progression
in dogs genetically affected for the Cord 1 (RPGRIP1)
mutation, and further studies are required to explain
this.
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| Therefore, ESS owners and breeders, who wish to do so,
should be encouraged to use the new DNA test for PRA in
addition to continuing with clinical eye testing for all
breeding stock. |
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UK Breed Clubs Health Co-ordinators
Lesley Bloomfield
Louise Scott
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