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Phenotype: Cataracts may start forming after 2 years of age and show variable rate of progression and vision impairment. The HC mutation in Australian Shepherds is mostly associated with bilateral posterior cataracts. Both sexes are equally affected. In general, dogs that have 2 copies of the HC mutation tend to have cataracts of the nuclear type, which progresses more rapidly and results in blindness at an earlier adult age. Dogs that have 1 copy of the HC mutation tend to have posterior polar subcapsular type of cataract that is not progressive and does not interfere with vision. Not all dogs that have the mutation will develop cataracts.
Mode of Inheritance: Autosomal dominant (incomplete penetrance)
Alleles: N = Normal, HC = Hereditary cataract
Breeds appropriate for testing: Australian Shepherd
Explanation of Results:
- Dogs with N/N genotype do not have this hereditary cataract variant and cannot transmit this variant to their offspring. However, cataracts may develop because of other genetic and environmental factors.
- Dogs with N/HC genotype may develop or are at risk to develop hereditary cataracts. They will transmit this hereditary cataract variant to 50% of their offspring, which will also be at risk to develop hereditary cataracts.
- Dogs with HC/HC genotype may develop or are at risk to develop the more severe form of hereditary cataracts that leads to blindness. If bred, dog will transmit this hereditary cataract variant to all of their offspring.
Results of this test can be submitted to the OFA (Orthopedic Foundation for Animals)
Cataracts are a common cause of blindness in dogs. Cataracts are defined as the clouding (opacity) of the lens of the eye. The lens focuses light on the retina to enable vision. Cataracts can therefore impair vision and, if progressive, they can lead to total blindness. Cataracts can develop in one eye (unilateral) or both (bilateral) as a result of the normal aging process, underlying diseases, injury, or be caused by a genetic defect (primary hereditary cataracts). Primary hereditary cataracts tend to be of the bilateral type. In dogs, mutations in the HSF4 gene are associated with two different forms of cataracts, Juvenile Hereditary Cataract (JHC) and Hereditary Cataract (HC).
Hereditary Cataract (HC) in the Australian Shepherd is associated with a mutation in HSF4 different from the mutation associated with JHC. Cataracts may start forming after 2 years of age and show variable rate of progression and vision impairment. The inheritance of HC in this breed is complex and this mutation is not the sole determinant for development of cataracts. Other unknown genetic and non-genetic factors contribute to cataract development and progression. The HC mutation in Australian Shepherds is mostly associated with bilateral posterior cataracts. Both sexes are equally affected. HC is inherited as a dominant trait with incomplete penetrance, which means that not all dogs that have the mutation will develop cataracts. Dogs that have the HC mutation are 17 times more likely to develop bilateral cataracts compared to dogs that do not have any copies of the mutation. In general, dogs that have 2 copies of the HC mutation tend to have cataracts of the nuclear type, which progresses more rapidly and results in blindness at an earlier adult age. Dogs that have 1 copy of the HC mutation tend to have posterior polar subcapsular type of cataract that is not progressive and does not interfere with vision.
Owners and breeders can benefit from this test by identifying at an early age which dogs have this mutation and are at significant risk for developing HC. Selection of breeding stock that are free (N/N) of this mutation will help reduce incidence of cataract in the breed. Veterinarians can benefit from this test by determining if a clinical cataract case in this breed has a known genetic etiology.
Species
Breed
| Test Result | Hereditary Cataract |
|---|---|
|
N/N |
No copies of HC mutation. Cataracts may however develop because of other genetic and environmental factors. |
|
N/HC |
1 copy of the HC mutation. Dog may be affected or is at risk to develop hereditary cataracts. If bred to an N/N dog, dog will pass on a copy of HC to 50% of offspring. |
|
HC/HC |
2 copies of HC mutation. Dog is affected or at risk to develop the more severe form of HC that leads to blindness. If bred, dog will pass on a copy of HC to all offspring. |
Mellersh, C.S., Pettitt, L., Forman, O.P., Vaudin, M., & Barnett, K.C. (2006). Identification of mutations in HSF4 in dogs of three different breeds with hereditary cataracts. Veterinary Ophthalmology, 9(5), 369-378. doi: 10.1111/j.1463-5224.2006.00496.x
Mellersh, C.S., Graves, K.T., McLaughlin, B., Ennis, R.B., Pettitt, L., Vaudin, M., Barnett, K.B. (2007). Mutation in HSF4 associated with early but not late-onset hereditary cataract in the Boston Terrier. Journal of Heredity, 98(5), 531-533. doi: 10.1093/jhered/esm043
Mellersh, C.S., McLaughlin, B., Ahonen, S., Pettitt, L., Lohi, H., & Barnett, K.C. (2009). Mutation in HSF4 is associated with hereditary cataract in the Australian Shepherd. Veterinary Ophthalmology, 12(6), 372–378. doi: 10.1111/j.1463-5224.2009.00735.x
