UC Davis School of Veterinary Medicine Veterinary Genetics Laboratory

Hereditary Cataracts

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).

Juvenile Hereditary Cataract (JHC) found in Boston Terriers, Staffordshire Bull Terriers, and French Bulldogs is a recessive disorder (2 copies of the mutation are necessary for cataracts to develop). JHC affects both eyes symmetrically and has early onset, with cataract formation starting before 1 year of age and progressing to maturity and blindness by 2-3 years of age. JHC affects both sexes equally. Mating of two carriers (N/JHC x N/JHC) is predicted to produce 25% of affected pups. JHC is distinct from the late-onset hereditary cataract also found in Boston Terriers and other breeds, the cause of which has not yet been identified.

Hereditary Cataract (HC) in the Australian Shepherd is associated with another mutation in HSF4. 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 more 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 Aussies 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.

Testing recommendation:

Juvenile Hereditary Cataracts (JHC): Boston Terriers, French Bulldogs, Staffordshire Bull Terriers. Owners and breeders can benefit from this test by identifying at an early age which puppies will have the disease and which will not. Littermates that are either carriers (N/JHC) or clear of the mutation (N/N) are better candidates to retain for breeding purposes. Breeders should avoid mating 2 carriers that can produce affected dogs. Veterinarians can benefit from this test by determining if a clinical cataract case in these breeds has a known genetic etiology.

Hereditary Cataracts (HC): Australian Shepherds. 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.

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Juvenile Hereditary Cataract reported as:

N/N

No copies of JHC mutation. Cataracts may however develop because of other genetic and environmental factors.

N/JHC

1 copy of the JHC mutation. Dog is a carrier. If bred to an N/N dog, 50% of offspring are predicted to be JHC carriers.

JHC/JHC

2 copies of JHC mutation. Dog is affected. If bred, dog will pass on a copy of JHC to all offspring.

Hereditary Cataract reported as:

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.


References:

Mellersh, CS, B McLaughlin, S Ahonen, L Pettitt, H Lohi, KC Barnett. (2009) Mutation in HSF4 is associated with hereditary cataract in the Australian Shepherd. Vet. Ophthalmol 12(6):372–378.

Mellersh CS, L Pettitt, OP Forman, M Vaudin, KC Barnett. (2006) Identification of mutations in HSF4 in dogs of three different breeds with hereditary cataracts. Vet Ophthalmol 9(5):369-378.

Mellersh CS, KT Graves, B McLaughlin, RB Ennis, L Pettitt, M Vaudin, KB Barnett. (2007) Mutation in HSF4 associated with early but not late-onset hereditary cataract in the Boston Terrier. J Hered 98(5):531-533.

 
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