Phenotype Health Dictionary

n. 1. the genetic constitution of an individual or group, as determined by the particular set of genes it possesses. 2. the genetic information carried by a pair of alleles, which determines a particular characteristic. 3. a gene or pattern of genes the precise details of which are defined. Compare phenotype.... genotype

n. a situation in which a single gene is responsible for more than one effect in the *phenotype. The mutation of such a gene will therefore have multiple effects. —pleiotropic adj.... pleiotropy

either of two inherited disorders in which renal cysts are a common feature. Autosomal recessive polycystic kidney disease (ARPKD) occurs in about 1 in 20,000 live births. It is due to a single mutation on chromosome 6 for the gene encoding the protein fibrocystin. The majority of cases are diagnosed before or at birth. The most severely affected fetuses have enlarged kidneys and *oligohydramnios due to poor fetal renal output. These fetuses develop the ‘Potter’ phenotype with characteristic facies, pulmonary hypoplasia, and deformities of the spine and limbs. Those surviving the neonatal period (50–70%) develop varying degrees of renal impairment but this may not proceed to end-stage until early adulthood.

Autosomal dominant polycystic kidney disease (ADPKD) affects between 1 in 400 and 1 in 1000 individuals and is one of the most common hereditary diseases. Two types have been defined. ADPKD 1 is the commonest and responsible for about 85% of cases. It is due to a mutation in the PKD1 gene on chromosome 16, which encodes polycystin 1, an *ion-channel-regulating protein. ADPKD2 is due to a mutation in the PKD2 gene on chromosome 4, which encodes the protein polycystin 2, a calcium-release channel. ADPKD2 tends to be a milder disease with later presentation.

ADPKD is a multisystem disorder that is also associated with cyst formation in other organs (particularly the liver), cardiovascular disorders, and colonic diverticular disease. Renal disease presents in early adult life with haematuria, loin pain, urinary tract infection, hypertension, renal stone disease, or the finding of a mass in the abdomen. Other cases are identified by family contact tracing; the findings of a few cysts on renal ultrasonography in a young adult with a family history of ADPKD is highly suggestive of the disease. Renal disease is progressive and about 50% of patients will have reached end-stage by the time they enter their seventh decade. The progress of the renal failure can be slowed by good blood pressure control. In the UK, patients with ADPKD are responsible for 5–10% of the total on renal replacement therapy.

There are a number of separate rare autosomal dominant conditions other than ADPKD1 and ADPKD 2 that can present with polycystic kidneys. These include *von Hippel-Lindau disease and *tuberous sclerosis.... polycystic disease of the kidneys