Autosomal Dominant vs Autosomal Recessive Polycystic Kidney Disease (PKD)

Polycystic kidney disease (PKD) exists in two variants, which are inherited in different ways, named autosomal dominant PKD (ADPKD) as opposed to autosomal recessive (ARPKD). Apart from this obvious difference in the pattern of inheritance, these behave like quite different diseases.

Autosomal Dominant PKD

• ADPKD usually starts to present with signs and symptoms from the age of 30 years, despite the cysts starting to develop from childhood, or even being present at birth. It exists in two forms, type 1 and type 2, which is carried by gene mutations called PKD1 and PKD2 on chromosome 16 and chromosome 4 respectively. Type 1 accounts for 85% of ADPKD.
• The overall incidence of the gene may be up to one in 400 people.
• It is transmitted to 50 percent of the offspring and one copy of the defective gene is sufficient to pass on the clinical disease.
• Thus every generation is likely to have at least one person affected by the disease.

The common identifying features and manifestations of ADPKD include:

• Bilateral renal cysts
• Liver and pancreatic cysts
• Brain aneurysms
• Dilatation of the root of the aorta
• Aortic dissection
• Aortic valve stenosis
• Abdominal hernia
• Pain in the loins and the back
• Renal dysfunction
• Complications such as hypertension, urinary infections, urinary stones, and hyperlipidemia

Autosomal Recessive PKD

On the other hand, ARPKD is much rarer.

• It occurs in one person out of 20 000 to 40 000 and is carried via two copies of the gene PKHD1 on chromosome 6.
• Individuals with the recessive form typically present at birth or during infancy and die early. Since the cysts are often apparent in utero or in early infancy, it is also called infantile PKD.
• It affects the liver in addition to the kidneys. Both parents must have a copy of the mutated gene which they pass on, for the child must be born with a copy of the mutated PKHD1 gene from both parents to have this condition.
• The parents have a 25 percent chance of passing on the defective gene to each of their offspring. If only one parent has a copy of this gene mutation, no child will suffer from the clinical disease but may become a carrier if the copy of the gene is inherited. Carriers are important because they can transmit the gene to their children.
• The nature of the disease may also be hinted at from the family history, apart from the timing of signs and symptoms, because autosomal recessive disorders rarely occur in every generation.

Signs and symptoms are apparent in early childhood or even in utero and may include:

• The earliest sign of ARPKD is apparent upon an ultrasound and consists of bilaterally enlarged echogenic kidneys, often with hepatic cystic development.
• Individuals with classical ARPKD are typically smaller-than-normal, which is called growth failure. This is because kidney function is essential for physiological development in early life but the abnormal kidney fails to function properly.
• The liver in these children is scarred even at birth and liver cirrhosis sets in as they grow older, leading to hepatic dysfunction and portal hypertension, which may be complicated by variceal development and serious internal bleeding.
• They also develop hypertension and urinary infections are common.

Diagnosis

Both AD and AR types of PKD are diagnosed by imaging, usually with ultrasound. In both cases, liver involvement is also looked for. However, a clinical history of PKD with no family history for the disease is diagnostic of ARPKD, whereas the ADPKD shows a positive family history from generation to generation.

Treatment

Treatment is symptomatic in the case of ADPKD and also includes monitoring for complications such as heart valve disease and brain aneurysms. In children with ARPKD, treatment is more urgent and intensive. It may involve the treatment of respiratory failure, and ventilator support. This is sometimes with nephrectomy, followed by peritoneal dialysis, the management of growth failure, the emergency treatment of respiratory arrest, and the management of liver disease.

Prognosis

In ARPKD, the prognosis depends on how early the cysts start to develop. Thus if a developing fetus shows kidney cysts, the disease is more severe than if the cysts first occur after birth.

Severity varies even within families and one in three babies die soon after birth, or within the first month, with respiratory failure. Death is due to pulmonary hypoplasia, as a result of oligohydramnios, in addition to pressure on the chest cavity from the large kidneys, and lung infections. In a few individuals, the signs are delayed for years or even decades.

80-90% of children with ARPKD who live beyond one month survive for at least five years. Renal failure occurs in over 50% of children within the first decade, while in ADPKD, its onset is usually in the 60s. The prognosis is worse in individuals with ADPKD if symptoms of renal enlargement and hematuria start before the age of 30 or if hypertension is diagnosed before age 35.

References

• http://www.nhs.uk/conditions/autosomal-recessive-polycystic-kidney-disease/Pages/Introduction.aspx
• http://www.niddk.nih.gov/health-information/health-topics/kidney-disease/polycystic-kidney-disease-pkd/Pages/facts.aspx
• https://ghr.nlm.nih.gov/condition/polycystic-kidney-disease#inheritance
• http://www.ncbi.nlm.nih.gov/books/NBK1326/