The Cataract Conundrum

Understanding and using the HSF4 hereditary cataract test 

by C.A. Sharp

First published in the July-August 2008 issue of the Aussie Times. Rev. May 2013

 

Cataracts are one of the Australian Shepherd’s most common hereditary problems and easily the breed’s most common inherited eye disease. In March of 2008, at the world-famous Crufts Dog Show in Birmingham, England, Britain’s Animal Health Trust (AHT,) a century-old charitable foundation dedicated to improving diagnosis and treatment of disease for dogs, cats and horses, announced a new DNA test for hereditary cataract (HC) in the Australian Shepherd.

The international breed community solidly supported the study while it was in progress.  Aussie owners and breeders in the UK, North America and Europe submitted samples from over 400 dogs.  AHT researchers found a mutation in a gene called HSF4 which is significantly associated with cataracts in Aussies.  They subsequently developed a DNA screening test for that mutation.  This test is a tool that can enable breeders to drastically reduce the frequency of cataracts in the breed.

That we have the test at all was due to a stroke of good luck.  Dr. Cathryn Mellersh and her team initially found the mutation that causes HC in Staffordshire Bull Terriers, a recessive mutation of a gene called HSF4.  They decided to screen all the other breeds for which they had DNA samples from dogs with cataracts to see if any shared the Staffy Bull mutation.  One of those breeds was the Australian Shepherd; DNA from seven Aussies with cataracts had been stored several years before.  Although Aussies didn’t carry the Staffy Bull mutation, to Dr. Mellersh’s surprise, they did carry another mutation in the HSF4 gene.  To validate this initial result, she began to collect additional Aussie samples, first from dogs in the United Kingdom and later from North America and Europe.  Ultimately, she determined that her inadvertent discovery was associated with cataracts in the breed and was able to develop a DNA test for the Australian Shepherd.

What are hereditary cataracts?

  The typical inherited cataract in Australian Shepherds is bilateral, involving both eyes, and on the posterior (back) side of the lens. These cataracts usually begin in the cortex, the outer layer of the lens.

Generally, cataracts are first detected during a routine eye exam on a mature adult, but they can arise in young adulthood or in old age. Cataracts in very young Aussie puppies are extremely unusual and generally due to a non-heritable cause.

Disease progression varies. Some dogs’ cataracts progress so slowly the dogs retain functional vision throughout their lives; others become blind in short order. One eye may first show signs of the disease before the other, but with hereditary cataracts the other eye will most likely develop one within a few months.

Cataracts start small and grow. When first noticed they may be referred to as “punctate.” Not every small cataract will advance, which is why the American College of Veterinary Ophthalmologists gives dogs with punctate cataracts a passing exam report. Even though the dog has passed its exam, if it is a breeding animal, the owner should hold off any breeding plans for six months to a year until a subsequent exam can determine whether the punctate is developing into a more advanced cataract.

Corrective surgery is available, but it is costly and not always successful. Fortunately for the affected dog, the disease causes no pain and most dogs adjust well to diminishing eyesight; dogs are much more attuned to sound and scent than vision.

The variable—and sometimes quite late—development of cataracts has been very frustrating for breeders. A dog might have litters on the ground before the disease develops. The advent of the new DNA test for the HSF4 mutation has the potential to reduce that risk considerably if we put it to good use.



Types and Frequency of Cataracts

The American College of Veterinary Ophthalmologists (ACVO) classifies cataracts by where they form in the lens. The lens has three layers of tissue: The nucleus at its core, an outer layer called the cortex, and a skin referred to as the capsule. The front and back are termed anterior and posterior, respectively. The outer rim of the lens is called the equator. Each lens also has anterior and posterior suture lines where the tissue came together during development.

Since not every cataract is inherited, ACVO classifies those that are likely due to environmental causes—generally injury or other diseases—as “significance unknown.” Small cataracts are referred to as “punctate.” These may or may not grow and those that do not aren’t considered significant. Therefore, a dog with punctate cataracts will pass an exam, but if the cataracts advanced to either the intermediate or diffuse type at a subsequent exam it will not.

Cataracts may be in one eye (unilateral) or both (bilateral) and the later are considered the most likely to be inherited. Cataracts may start in one eye before the other, so a unilateral cataract may be discovered to be bilateral on a subsequent exam. Of the types of bilateral cataracts that will not pass CERF exams, posterior cataracts are anywhere from 4 to 10 times more likely to be found in Aussies than any other bilateral cataract. The HSF4 mutation identified by AHT is significantly associated with bilateral posterior cataracts.

A review of Canine Eye Research Foundation statistics for the breed from 1999 – 2006 indicated that approximately .78%, or roughly one Aussie in 130, has this type of cataract. None of the other types of bilateral cataract (anterior, equatorial cortical, capsular, or nuclear) occur even half as frequently. The second most frequent type is bilateral nuclear cataracts, which occur in only in 1 of 550 dogs. The other types all occur in less than 1 in 1000 dogs.

To further complicate the picture, the Aussie mutation is not a simple dominant; it is dominant with incomplete penetrance.  This means that not every dog that has the mutation will have cataracts.  Instead, having the mutation confers a significantly increased risk of developing cataracts.  A dog with even one copy of this mutation is twelve times more likely to develop cataracts sometime in its life than a dog that has normal versions of HSF4.  The mutation was associated with all types of cataracts, but the majority of dogs had bilatera

The fly in the ointment

Early on some people are questioning the new DNA test’s value and others were confused about what the test results mean for their dogs and their breeding programs.  These misunderstandings have arisen due to several problematic aspects of HC in Aussies:

  • There are multiple genes and gene versions that cause cataracts in dogs
  • The HSF4 gene has more than one cataract-causing mutation
  • The mutation found in Aussies is not a simple recessive
  • Not every cataract is hereditary
  • Not every hereditary cataract in Aussies is due to this gene
  • Some Aussies that have the mutation don’t develop cataracts

All this left more than a few people scratching their heads and wondering how to react to test results or whether to bother with testing at all.  Even today people struggle to understand the way this gene operates and how best to use the test.  A dog might have a cataract even though it does not have a mutation of HSF4, If a dog has just one copy of the Aussie mutation can develop cataracts, and – in what must seem a perverse whim of the DNA gods – not every dog that has the mutation has or will get cataracts, nor every dog with cataracts have the mutation.

Unlike the Staffy Bull mutation, the one found is Aussies is a dominant. While both are mutations of the same gene; they are separate alleles (versions) of HSF4.  The Staffy Bull mutation is not known to occur in Aussies (though it is found in Boston Terriers and French Bulldogs, two much more closely related breeds) nor does the Aussie mutation occur in Staffy Bulls.

l posterior cataracts – the type that has long been recognized typical of HC in our breed.  [See sidebar: “What are Hereditary Cataracts?”]  70% of Aussies with hereditary cataracts have this mutation.

The fact that some dogs who have the mutation do not have cataracts is probably the biggest conundrum for breeders.  Statistics on the several thousand dogs tested indicate that roughly one Aussie in four has this mutation, a much higher frequency than the frequency of cataracts in the breed.  Inheritance with incomplete penetrance means that other factors – other genes, gene regulators, and/or environment – are involved in the process that determines whether cataracts do or do not develop in any particular dog.  As of this writing, we know nothing about those other factors.

Cataracts can also be referred to as having variable expressivity.  This means that not every case is exactly the same.  They develop in different parts of the lens.   Age of onset varies considerably from young adulthood to old age.  Progression, or how fast the cataracts advance, also varies.  This variation in phenotype also indicates the importance of other factors to cataract development.  Other genes almost certainly play a role in this variation, though they are yet to be identified.

As a final complication, not every Aussie with cataracts has the HSF4 mutation.  Some cataracts are not hereditary at all, arising from injury or infectious diseases.  Others may or may not be hereditary, as with nuclear sclerosis – the classic “old age” cataract.  Still others, like those sometimes associated with hyaloid artery remnants, appear to be hereditary but the genetic cause is not yet known.  Some cataracts are secondary to other genetic diseases:  Homozygous merles with severe ocular defects not infrequently develop cataracts and dogs with the most severe form of Collie Eye Anomaly have detached retinas, which can cause secondary cataracts.  Finally, there are other genetic mutations in the breed that cause cataracts, which isn’t surprising given our breed’s comparatively short history and broad founder base.

Of the dogs that have the mutation do not have cataracts at the time of testing, some almost certainly will develop them later in life, though at this point we aren’t sure how many.  The study was essentially a snapshot; it used information on dogs’ eye phenotype and HSF4 genotype as of a particular point in time.  While the genotype won’t, change, we need to wait for a longitudinal study, the “movie version” as it were, before we’ll know how many of those normal-eye dogs go on to develop cataracts. When that information is known we’ll have a better idea what percentage of the dogs with the mutation will ultimately develop HC.

Since HSF4 is not connected to every cataract that occurs in Aussies, breeders need to accept the fact that use of this test will not necessarily prevent them from producing dogs that develop cataracts.  However, effective use of the test will result in a significant decrease in the frequency of the disease across the breed as a whole.  [See “Types and Frequency of Cataracts”]

In the Trenches

With the other genetic tests currently available for Australian Shepherds, most notably MDR1, CEA and PRA, the answer to the “what to do” question is straightforward:  Having a single copy of the CEA-CH or prcd/PRA mutations or even two of the MDR1 are not reasons to remove a dog from your breeding program.  HSF4 has not provided us with such a nice tidy little package.  It also presents some tough ethical questions that can prove emotionally difficult to the breeder who discovers that one or more of her dogs have the HSF4 mutation.

People wonder why HSF4 is so important when there are other types of hereditary cataracts.  But the correlation between the mutation and those dogs that have cataracts is so high that we cannot afford to ignore it.

More than a few people have questioned the wisdom of removing all HSF4 positive dogs from the breeding population, fearing that it could have a negative effect on the overall gene pool.  Given that so many Aussies have at least one copy of this mutation coupled with the fact that many of them do not go on to develop cataracts, removing them all from breeding is unwise.  However, when cataracts occur this gene is by far the most likely to be the culprit so ignoring it isn’t an option either.  No dog with cataracts and the mutation should be bred.  Ideally, those with two copies should be passed over in favor of relatives that are clear or have only one copy.  Dogs with the mutation and at least four years of age (to have some assurance that they won’t have cataracts) should be bred only to clear tested dogs and preference should be given to clear tested offspring to carry on with.  Over time and generations, the frequency of the mutation will reduce without sacrificing valuable traits those dogs may have or severely restricting the breed gene pool.  However, males with two copies ought not to be offered at public stud due the potential for increasing the frequency of the mutation if that dog or any of his sons should become popular sires.

In North America, home to the bulk of the world breed population the frequency will be very close to that seen in the overall test results.  However, depending on the breed population structure in countries outside North America, the frequency of the mutation might be higher or lower.  If it is lower a more aggressive approach might be employed.  But in a country with a tiny breed population or where the population has a limited founder base and a high frequency of the HSF4 mutation, the national clubs in those places may need to develop a multi-generational plan for slowly reducing the frequency of the mutation.  Breeding of dogs with the mutation could be conducted as described above.  In the meantime, efforts could be made to import new breeding stock, preferably from more varied bloodlines, that has been tested clear.

Another aspect of the HSF4 test’s potential effect on the breed population relates to the breed’s other significant health issues.  Epilepsy is at least as common as cataract and far more devastating.  The Australian Shepherd Health & Genetics Institute (ASHGI) Cancer Survey, completed in 2007, indicated that hemangiosarcoma and lymphoma are very common in the breed.  We also have hips, elbows and a variety of other health issues to deal with.  Overzealous application of test results in an effort to eradicate the HSF4 mutation has the potential to cause ourselves and our dogs’ greater grief through neglect of health issues with a greater quality-of-life impact.

First of all, the existence of the HSF4 test is not a reason to ignore other health issues.  Exams for hips, elbows and eye diseases of all types need to continue, as does use of the other DNA-based screening tests currently available.  (For reference, ASHGI provides a suggested testing and screening protocol on the website.)  It is important to know what the relative risks are for all these health issues in every individual breeding dog, then set priorities for these health traits.  This can be done by careful and extensive pedigree health research by individuals or cooperative groups of breeders, or through utilization of the ASHGI/IDASH Pedigree Analysis Service.

To breed or not to breed

Based on the author’s correspondence and conversations with Aussie breeders since the release of the test, their one biggest conundrum is the fact that not all dogs with the mutation actually have cataracts.  They want to know, for a certainty, whether or not the dog will get cataracts.  And if it does not, why?  Some have even asked whether there might not be some entire families that that have the mutation but never get cataracts – in which case why not breed them?

It is very possible, given the disparity between the numbers of dogs with the mutation and the frequency of cataracts that HSF4 positive/no cataracts families might be possible, but without further study we cannot be sure.  Most breeders cannot track every single puppy they produce throughout its lifetime no will they necessarily get information on the offspring and descendants of dogs they have sold to others.  Gathering comprehensive data on a dog’s progenitors and their other descendants can also be difficult when they belonged to someone else.  There are clearly individuals who have reached old age without cataracts with one and even two copies of the mutation.  There are also groups of related dogs with the mutation that don’t have cataracts, but some of those dogs are still young and may develop them later.  The median age for cataract onset in Aussies is around 4½ years old and the oldest diagnosed was over 10.  Until a longitudinal study is done that follows carriers of the HSF4 mutation into old age, we can’t be sure how many of them will go on to develop cataracts or what factors might prevent a dog from doing so.

The complex inheritance of HSF4-related cataracts leaves some people wondering why HSF4 alone is so important.  HSF4 is a gene of major effect for this trait.  70% of Aussies with cataracts of any type have at least one copy of this mutation.  Other factors may well affect the final phenotype, but HSF4 provides a key indicator of dogs that are at significant risk for developing cataracts at some time in their lives.

Testing

With all the “ifs” and “buts” connected to the HSF4 mutation, some may wonder whether they might not be better off relying on eye exams as they have always done.  The problem is an eye exam only tells you what the examining veterinarian saw at the time the exam was done.  For diseases present in young puppies it can be very effective at identifying affected dogs early in life, but HC frequently arises much later—often after a dog may have been bred.   The advantage of a DNA test is that you know the dog’s genotype whether or not it exhibits signs of the disease.  You can hold off breeding and get a few annual eye exams done first, then select mates that don’t carry the mutation in hopes of producing clear offspring of good quality to replace the dog with the mutation.

Cataracts, and therefore the HSF4 mutation, are sufficiently common that every breeding dog not out of two tested clear individuals ought to be tested before being bred.  If your dog is clear of the mutation, it has a low risk for developing cataracts from some other cause.  Dogs with normal HSF4 test results can be advertised as such, though such ads should specifically mention HSF4 and not simply say “cataract free” since the test will not identify other cataract-causing mutations.

Test results, like other genetic health information, should be shared.  If your dog has the mutation, let the owners of its parents and other near relatives know and advise them to test their dogs.  If you are going to breed it, make sure the owner of the prospective mate knows its status and you know that their dog is test clear.  The Orthopedic Foundation for Animals and the ASHGI/IDASH Open Health Database will register HSF4 test results.  This is an excellent way to establish a permanent record that will be accessible to breeders world-wide and for generations to come.

While it is true we do not yet know what other mutations might also cause cataracts in Australian Shepherds or why some HSF4 Aussies remain normal, cataract research is still underway at AHT.  We should continue to support their effort by submitting samples from dogs affected with any type of cataract.  Eventually they may be able to provide us with an even more detailed understanding of the genetics of cataracts in the Aussies.

In the meantime, we must make effective use of the HSF4 test.  It’s the best tool we have for reducing the frequency of our breed’s most common blinding eye disease.

 

The author would like to thank Dr. Cathryn Mellersh, of the Animal Health Trust, for reviewing this article prior to publication.