DNA test for late onset ataxia in the Parson Russell terrier_October 2012

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New DNA test for Late onset ataxiia iin the Parson Russellll terriier

Background

Late onset ataxia (LOA) in the Parson Russell terrier (PRT) is disease of incoordination of gait and lack of balance. The onset age for the disease is usually between 6 months and 1 year of age, when owners may start to notice that their dog is showing changes in gait pattern (often weaving of the hind limbs) and some difficulty balancing. The disease is progressive and affected dogs become increasingly uncoordinated with difficultly balancing, which makes moving around and everyday tasks such as going up and down stairs difficult. There is no treatment or cure for LOA and affected dogs are often euthanized, typically around two years after onset, on humane grounds as their quality of life diminishes.

Genetic investigations

At the Kennel Club Genetics Centre at the Animal Health Trust we have been investigating the genetics of LOA in the PRT. In our initial experiments we performed a genome-wide screen of markers using DNA from 16 LOA affected dogs (cases) and 16 unaffected (control) PRTs. This approach enabled us to locate a small region of the genome (approximately 0.1 %) which was present in 15 of the 16 cases, but not present in any of the controls, and almost certainly contained the mutation causing the majority of LOA cases.

In our second round of experiments we used a technique known as target-enrichment to separate the LOA associated region from the remaining 99.9 % of the genome in 2 cases and 3 controls. The LOA associated region, which still consisted of 2.5 million ‘letters’ of code, was then sequenced almost entirely using an advanced sequencing technique.

Results in the PRT

Although many sequence variants were identified between the five dogs whose DNA we sequenced, one interesting mutation within the protein coding part of a gene was identified, which is predicted to have a damaging effect. The mutation was investigated in additional cases and controls and found to be very highly associated with LOA in the Parson Russell terrier – in other words, we could use the mutation to predict very accurately whether a dog had LOA or not. Results are summarised in the table below:

DNA test for late onset ataxia in the Parson Russell terrier_October 2012

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Clinically normal PRT Clinically affected PRT 0 copies of the mutation 136 3 1 copy of the mutation 68 1 2 copies of the mutation 1 22 Total 205 26

Out of the 26 LOA affected dogs in our sample collection 22 had two copies of the mutation (85 %). Of the remaining four dogs, three had no copies of the mutation (clear) and one had a single copy of the mutation (carrier). We believe there is a different hereditary (another mutation) or environmental cause (such as head trauma) of ataxia in these four dogs.

We also tested 205 healthy PRTs. 204 of these dogs (over 99 %) were clear or carriers of the mutation. One 4 year old dog that is reported by his owner to be clinically normal has two copies of the mutation, and there are two possible reasons for this:

(a) the mutation that we have identified may work in conjunction with another ‘modifier’ mutation in the PRT. The modifier mutation would be harmless on its own, but disease-causing when inherited along with the LOA mutation. In this case we would speculate that the modifier mutation is very common in the PRT, and it is therefore rare for dogs to have two copies of the LOA mutation but be free of the modifier mutation and as a result be clinically free from LOA

(b) the mutation that we have identified is not the causal mutation, but is a variant that lies very close to the causal mutation with very high diagnostic value (the LOA mutation correctly predicts the LOA status for 227 of the 232 (97.8%) dogs we genotyped).

Results in the Jack Russell terrier

We have collected DNA from five Jack Russell terriers (JRT) with ataxia. Only one dog had two copies of the mutation, and the remaining four dogs were clear of the associated mutation. The size of the sample collection is too small to draw any firm conclusions, but the results suggest that there may be at least two genetically distinct forms of ataxia in the JRT.

DNA test for late onset ataxia in the Parson Russell terrier_October 2012

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DNA test

Because of the diagnostic value of the identified mutation, a DNA test for LOA in the PRT will be launched on the 1st November 2012 and will be priced £48. The test will be based on the LOA associated mutation we have identified and will accurately predict the risk of developing LOA for over 97% of PRTs. Results will be explained as follows:

CLEAR: These dogs have two normal copies of DNA and are highly likely to be clear of LOA. The results of our research suggest that there may be other causes of ataxia in the breed so we cannot exclude the formal possibility that clear dogs could develop a genetically different form of ataxia due to other mutations that are not detected by this test.

CARRIER: These dogs have one copy of the LOA associated mutation and one normal copy of DNA. These dogs will not develop LOA themselves as a result of the LOA mutation but they will pass the mutation on to approximately 50% of their offspring. The results of our research suggest that there may be other causes of ataxia in the breed so we cannot exclude the formal possibility that carriers could develop a genetically different form of ataxia due to other mutations that are not detected by this test.

AFFECTED: These dogs have two copies of the LOA associated mutation and have a very high chance of developing LOA (96 % of dogs studied in our research programme with two copies of the LOA associated mutation were clinically affected with ataxia).

The test will effectively reduce the number of LOA cases in the PRT. We encourage owners to keep us updated on the health of dogs they have tested. We are particularly interested to hear from owners of dogs that:

i) are clinically affected with ataxia but that do not have two copies of the mutation, or

ii) are over 4 years old and remain free from ataxia despite having two copies of the mutation.

This information will help us to monitor the effectiveness of the test, to possibly refine the test in the future if necessary and to start to investigate additional cause(s) of ataxia in PRTs and JRTs.

DNA tests can be ordered online from November 1st 2012. Full details available from our website: http://www.aht.org.uk/cms-display/genetics_tests.html

Acknowledgements

We would like to thank all the Breed Clubs and individuals who have contributed financially to this research and to all the Parson Russell terriers and their owners who have contributed DNA and information to this research.

A Layman’s Guide to Primary Lens Luxation (PLL) in dogs.

 

Description:

This is  a condition where the lens is partially or fully dislocated from the threads that hold it in position within the eye.  These tiny threads, sometimes called ligaments are known as zonules and it is they that weaken and break causing this condition.  Using normal ophthalmic instruments (such as your own vet may possess) it is difficult to even see these small threads.  What the experienced ophthamist (not your own vet) should also be looking for is a wobble in the lens as the dog moves its head around.  This indicates that the zonules are becoming weak.  Weak zonules are called subluxated, completely detached (luxated). 

 

Unfortunately, each eye can behave differently, so a rapid onset on one eye, may or may not lead to a gradual (and predictable) onset with the other.  As this appears to be a genetic defect in dogs, then it is almost certain that eventually both eyes will be affected.

 

There are two directions in which the lens can go; forward and backward.  Forward is called anterior luxation and backwards is call posterior luxation.  The lens can also detach at an angle which can cause bruising of the cornea (edema of the cornea).  This causes the cornea to loose its transparency in the area of contact.  This may or may not be permanent.

 

What to look for:

Forward is the usual way.  The iris, the coloured part of the eye, surrounding the pupil (the normally dark spot in the centre of the eye) can be almost invisible, being fully open and unable to contract even in strong light.  The pupil is consequently very large revealing the greenish hue (in fact, a reflection) from the back of the eye, the sort of colour exhibited by animal eyes when illuminated at night.

 

In addition, the eye might appear slightly swollen and the dog may have difficulty holding his eye open.  These are VERY BAD signs.  They indicated that pressure has already built up in the eye, possibly to damaging levels.

 

Other symptoms can be shivering – due to pain.  Some dogs have a habit of shivering for no reason in any case but it is worth checking the eyes if he/she is shivering for no apparent reason. 

 

If the lens goes backwards, there may not be any symptoms and what the prognosis may be.  However, there is the possibility that the lens could go forward at sometime and cause glaucoma and the other conditions mentioned.

 

What happens:

When the lens goes forward in the eye, it usually blocks the flow of fluid (aqueous humour) which is continually produced in the normal eye by a ring of tissue (the cilary body) between the iris and the lens.  The majority of this fluid normally flows out of the eye.  Because the fluid cannot drain away, a condition arises which can cause irreparable damage to the eye due to the build up of pressure.  This condition is known as glaucoma.

 

Nowadays, pressure (tonometry, the measurement of eye pressure) is easily measured with a small hand held digital (no moving parts!) pressure gauge (a tonopen) held against the front of the eye.  Normal pressure should be in the range of 15 to 25 torr (mm of mercury) but can reach as high as 60 (with glaucoma).  That is over one pound per square inch.

 

At the back of the eye is the light receptive area.  These cells (rods and cones) are mounted in a structure called the retina, which is held on to the interior wall of the eye.  The blood flow and information from the light sensitive cells pass out of a central area in the back of the eyeball, which contains the optic nerve (which transmits the information from the cells).  Pressure build-up in the eyeball can cause permanent damage to the optic nerve itself and constrict the blood flow, which can lead to the death of the light sensitive cells.  A consequence of this is that the retina detaches from the eyeball.  This can be seen eventually by examination of the eye, thought it might take some months to happen.  Partial detachment can also occur.  Detachment can be progressive.  The amount of detachment relates to the level of vision available to that eye. 

This detachment is not in anyway painful to the dog.  However, if glaucoma is allowed to remain untreated for as little as six hours, then you dog will be fully blind in that eye.  It is important that the dog is immediately seen by a vet who is fully knowledgeable in this subject (See above list).  Most vets aren’t, or don’t appreciate the seriousness of the situation.  This is an ACUTE (requires urgent attention)condition not CHRONIC (slow).

 

The treatment:

It was originally understood that the only treatment was an immediate operation to remove the lens.  However, now there are drugs, which, if given soon enough, will reduce the pressure in the eye.  By careful monitoring, a dog can be maintained (at the veterinary centre) until an operation is undertaken.  This can be a period of some days or even weeks.  Alternatively, some surgeons allow dogs to go home with the owner administering the pressure reducing treatment, until the lens has become almost completely detached and moved to a suitable position so that it can be easily removed.

 

The lens is also attached to the vitreous humour (the jelly like substance) in the eyeball itself.  In order to remove the lens, it has to be severed from the vitreous humour.  This is a delicate operation, as it is possible that any outward pressure on the lens can move the whole of the vitreous humour forward which can detach the retina with consequent loss of vision.  Some vitreous humour is inevitably removed with the lens, though in a successful operation the surgeon takes as little as possible because the space left by the lens and removed vitreous humour causes an imbalance within the eye.  In this situation, there is a tendency for the vitreous humour to move forward causing partial or complete retinal detachment.  Some surgeons remove the lens prematurely by forcible suction, the majority hold that this is not advisable that because of the danger of retinal detachment by this method.  In addition, there is usually more chance of haemorrhage (internal bleeding), more scar tissue, again increasing the likelihood of retinal detachment. 

 

Because stability of the eye has been upset, surgeons recommend that the dog does nothing that might cause the movement of the retina.  For instance ‘rat killing’ (violent toy shaking).  Even straining against a collar is considered by some to be inadvisable, so they recommend a body harness (which does not bear on the neck).

 

After successful treatment, the eye has no focusing ability and the light image which would (at least in humans) be inverted by the lens now falls directly on the retina.  However, in dogs, apparently, the cornea (the structure in front of the lens) provides an appreciable focusing (though fixed) element to the eye and although focusing is absent, some resolution of an image should be possible.  Presumably, in time, the brain will eventually learn to interpret what falls on the retina, which should give the dog the so called ‘guidance vision’.

 

Sometimes, the short term results are good, particularly if both eyes have retained their retinas.  However, as mentioned, prolonged glaucoma may have already caused irreparable damage to the retina.  In addition, this major alteration to the structure of the eye can lead to glaucoma again!  The eventual consequences of glaucoma require removal of the eye.

 

Aftercare:

Surgeons recommend that the dog should generally be kept quite for a number of days and stress the importance of the regular and continued use of the eye drops supplied.

 

At least one specialist recommends that the dog be seen at intervals of several months to check that the IOP has not risen to dangerous levels. 

 

One ophthalmic surgeon states that further surgery is essential to maintain vision.  He recommends laser treatment to reduce to capability of the eye to produce aqueous humour.  In human eye conditions where the retina is detaching from the eye, lasers can be used to pin the retina back against the eye.

 

Advice:

Be prepared.  A normal vet will NOT be able to carry out this treatment, so he will refer you to a specialist.

 

Is there one within a reasonable distance of you, which has suitably qualified staff for this operation?

 

Do these staff maintain a 24 hour rota and will they see a luxated dog immediately and at ANY time/day?

 

Do you have their telephone number?

 

Does the phone number you have been given actually work at any time?  Why not try it some bank holiday?

 

If you think your dog is at risk, find a centre that is prepared to take action, when necessary, quickly.

 

Keep looking at your dog and notice any particular change in behaviour that might be eye related.

 

Other Stuff:

In America other aftercare treatments have been tried, for instance cyclocryothermy: freezing of some of the ciliary body to prevent the production of fluid, however this can apparently cause complications and may not be successful.

 

It is possible to insert (permanently) which is called a shunt,  a small valve, into the eye which can relieve excess pressure, though this tube can sometimes become blocked.

 

If glaucoma necessitates removal of the eye (enucleation), this is usually a straightforward operation with no complications.  It is possible to insert a silicone implant within the eye after removing the contents (called evisceration).   This apparently results in a normal looking pain free eye, but with no vision of course. 

 

Maybe you think all vets should have a tonopen?  They could then be in a position to administer the pressure reducing drugs, however, the cost of one of these instruments approaches £2,000.