Primary Lens Luxation (PLL)

Primary Lens Luxation (PLL) is a well-recognized, painful and blinding inherited eye condition that affects many breeds of dog, particularly terrier and terrier-type breeds including (but not restricted to) Miniature bull terriers, Tibetan terriers, Jack and Parson Russell terriers, Lancashire Heelers and Chinese Crested dogs.

The lens is held in place in the eye by fibers known as zonules. If these zonules stretch or break, the lens can fall out of place, or luxate. When this happens it often requires immediate veterinary attention to remove the displaced lens and prevent painful secondary glaucoma, and sometimes loss of vision.

PLL is recently the most known and redoubtable hereditary disease for Miniature Bull Terriers. The assumption that the luxation is now more common is due to the breeding of minis which has increased during the last years.

According to the Dortmunder Kreis – DOK (Diagnostic Organization for Genetic Related Animal Eye Diseases) about 5% of the examined minis have a lens luxation.

Recent genetic research show more sad statistics. According to OFA (Orthopedic Foundation for Animals) by DNA tests

We have to difference Primary Lens Luxation (PLL) from Secondary Lens Luxation (not hereditary) that causes trauma, inflammation and other reasons..

DNA testing

DNA test development

At the beginning of September 2009 practically simultaneously were made public reports of two independent teams about identified the mutation that causes PLL.
The discovery was made by two teams of researchers:

- team of researchers led by Drs Gary Johnson & Elizabeth Giuliano at the University of Missouri College of Veterinary Medicine..

- members of the AHT canine genetics team, in collaboration with David Sargan's research group ( Cambridge University ) and David Gould (Davies Veterinary Specialists).

Shortly after the announcements about the discovery Dr Catherine Mellersh and Dr David Sargen contacted Dr Johnson, and both research teams have agreed to share data and co-publish this discovery.

 Working independently and using different breeds, the researchers from the University of Missouri and from the Animal Health Trust found the same mutation. This independent confirmation of the finding makes both labs confident that the correct mutation has been identified, and that the test is valid for many breeds.

Our kennels support these investigations by providing samples of our dogs. AHT letters about it is here.

DNA test

Breeders and owners are able to test their dogs using a simple test kit.  It is possible to collect DNA from dogs to be tested via a simple cheek swab which is included in the test kit, along with all information, necessary forms and full instructions.

In OFA the testing kits  can be ordered online through the OFA website (www.OFFA.org).

In AHT the testing kits can be requested by e-mail swab.request@aht.org.uk. If you do not have access to email please telephone 01638 555621 or fax 01638 555666.

Testing results and inheritance of PLL

From pedigree studies done previously, there has been general agreement that PLL is inherited as a simple recessive trait. This means that a dog needs 2 mutated, or “bad” copies of the gene to show the disease. With the PLL mutation identified, and the research groups able to compare notes on the dogs used in the study, it has become apparent that there are some exceptions. While the vast majority of dogs with PLL have tested AFFECTED, as small percentage of the dogs that test CARRIER are also at risk of developing PLL.

Dogs that test AFFECTED have 2 mutated copies of the gene. The vast majority of these dogs will luxate at 4-8yrs of age, the typical age of onset for PLL. There were a few dogs in the study group that tested as AFFECTED but did not luxate until after 8 years of age, and some dogs testing AFFECTED have died from other causes without luxating. A search of published veterinary literature revealed that about 10% of the dogs reported to be clinically affected with PLL had onset of symptoms after 8 yrs of age. Because of this, the test results will say “AFFECTED/HIGH RISK”. 

All genetically affected dogs should have their eyes examined by a veterinary ophthalmologist every 6 months, from the age of 18 months, so the clinical signs of PLL are detected as early as possible.

 As stated earlier, dogs testing CARRIER are at a slight risk of developing PLL. Carriers have one normal and one mutated copy of the gene. They could pass either the normal copy or the mutated copy on to their offspring. Because there were a very few cases of dogs in the research groups testing CARRIER who did appear to have PLL, the test results will say “CARRIER/LOW RISK”.

 Researchers from AHT say that they do not currently know why some carriers develop the condition whereas the majority does not, and they advise that all carriers have their eyes examined by a veterinary ophthalmologist every 6- 12 months, from the age of 2 years, throughout their entire lives.

A dogs testing CLEAR have two normal copies of DNA. Research has demonstrated clear dogs will not develop PLL as a result of the mutation we are testing for, although we cannot exclude the possibility they might develop PLL due to other causes, such as trauma or the effects of other, unidentified mutations.

Breeding Advice

Now breeders are given an estimate of each dog’s risk of developing PLL depending on their genotype and are able to make sensible breeding decisions that minimize the risk of producing dogs that will become affected by this serious and debilitating condition.

Breeding advice of Animal Health Trust:

“Our research has also demonstrated that the frequency of the PLL mutation is extremely high in the PLL-affected breeds that we have studied in depth.  This means that allowing only CLEAR dogs to breed could have a devastating effect on breed diversity and substantially increase the likelihood of new inherited diseases emerging. Therefore, we strongly advise breeders to consider all their dogs for breeding, regardless of their PLL genotype. GENETICALLY AFFECTED and CARRIER dogs can be bred with, but should only be bred to DNA tested, CLEAR dogs. All puppies from any litter that has at least one CARRIER parent should be DNA tested, so that the CARRIERS can be identified and followed clinically throughout their lives. This practice should be followed for at least one or two generations, to allow the PLL mutation to be slowly eliminated from the population without severely reducing the genetic diversity of breeds at risk.”

Advice for breeders about DNA tests use

(Vestnik RKF ¹2 2007)

References

1.      http://www.aht.org.uk/

2.      http://www.caninegeneticdiseases.net/GLX/mainGLX.htm

3.      http://www.offa.org/

4.      Vestnik RKF ¹2 (68), M., 2007, p. 28 

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