Polycystic kidney disease

Question 1

What is polycystic kidney disease

Answer 1

PKD is an inherited condition that causes multiple cysts to form within the kidneys
- it is considered to be caused by a basement membrane defect that affects the renal tubules and allows cysts to form and grow

Question 2

Explain how PKD can lead to CKD

Answer 2

The cysts are present from birth, but they start out microscopic, grow larger with time, and can vary from less than a millimeter to more than a centimeter in diameter

Eventually the enlarging cysts deplace normal functional renal tissue, severely disrupting the kidney and ultimately causing kidney failure to develop

The cysts usually grow quite slowly, so most affected cats will not show any signs of kidney disease until mid to late in life, typically at around five to eight years of age

Question 3

Is it possible to predict the evolution of the disease

Answer 3

At present there is no way of predicting how rapidly the disease will progress in any particular cat

The reason for the marked phenotypic variation in the disease presentation is not entirely clear

Question 4

In which breed of cats PKD is considered a major problem

Answer 4

PKD is recognized as a major problem in Persian cats because in this breed it is inherited as an autosomal dominant disease
- in some countries 30-50% of Persian cats are affected

Other breeds related to Persians anf/or that have used Persian cats in breeding programs also have a recognized predisposition for PKD (e.g., British shorthair, Burmillas, Exotics)

The disease is seen occasionally in a wide variety of different breeds and also non–pedigree cats

Question 5

Could you define the genetic defect responsible for PKD in cats

Answer 5

Studies have demonstrated that cats affected with PKD have an underlying defect in a gene corresponding to the most common cause of autosomal dominant PKD (ADPKD) in humans
- it is not yet known whether the defect of this gene accounts for all cases of PKD in cats, or if some individuals have different genetic defects (as in humans)

Question 6

Has genetic testing for PKD definitely surpassed ultrasonographic examination

Answer 6

Gene testing for a point mutation in exon 29 of the PKD1 gene is now easily available
- PCR on blood sample

Nucleotide analysis of exon 29 indicated that male cats had a higher point mutation rate than female cats

Some cases of PKD confirmed on ultrasound were gene test negative suggesting there are other mutations capable of causing PKD

Question 7

How is PKD inherited

Answer 7

PKD is the result of a single, autosomal, dominant gene abnormality
- every cat with the abnormal gene will have PKD (no unaffected carriers)
- Every cat with PKD will have the abnormal gene
- Only one affected parent is needed to pass on the defect to some kittens
- Every breeding cat with PKD will pass the disease on to a proportion of its kittens

All affected cats appear to be heterozygotes for the PKD gene
- breeding studies suggest that homozygosity for PKD is a lethal defect with early death occuring in utero

Question 8

How is a diagnosis of PKD made

Answer 8

Ultrasound examination
- used to be the primary way of diagnosis
- often a late diagnosis and diseased cats have already been used for breeding

Use of genetic screening
- has now largely superseded the use of ultrasonography in the screening of potential breeding cats
- the test can be performed on either peripheral blood cells (EDTA blood sample) or on a buccal mucosal swab

Question 9

What is the ultrasonographic appearance of PKD

Answer 9

Several black areas of different sizes corresponding to fluid filled cysts within the renal parenchyma