Question: What is (are) Polycystic Kidney Disease ?
Answer: Polycystic kidney disease is a genetic disorder that causes numerous cysts to grow in the kidneys. A kidney cyst is an abnormal sac filled with fluid. PKD cysts can greatly enlarge the kidneys while replacing much of their normal structure, resulting in chronic kidney disease (CKD), which causes reduced kidney function over time. CKD may lead to kidney failure, described as end-stage kidney disease or ESRD when treated with a kidney transplant or blood-filtering treatments called dialysis. The two main types of PKD are autosomal dominant PKD and autosomal recessive PKD. PKD cysts are different from the usually harmless simple cysts that often form in the kidneys later in life. PKD cysts are more numerous and cause complications, such as high blood pressure, cysts in the liver, and problems with blood vessels in the brain and heart.
Source: NIDDK

Question: What causes Polycystic Kidney Disease ?
Answer: A gene mutation, or defect, causes polycystic kidney disease. Genes provide instructions for making proteins in the body. A gene mutation is a permanent change in the deoxyribonucleic acid (DNA) sequence that makes up a gene. In most cases of PKD, a person inherits the gene mutation, meaning a parent passes it on in his or her genes. In the remaining cases, the gene mutation develops spontaneously. In spontaneous cases, neither parent carries a copy of the mutated gene. Researchers have found three different gene mutations associated with PKD. Two of the genes are associated with autosomal dominant PKD. The third gene is associated with autosomal recessive PKD. Gene mutations that cause PKD affect proteins that play a role in kidney development. Genetic Disorders Each cell contains thousands of genes that provide the instructions for making proteins for growth and repair of the body. If a gene has a mutation, the protein made by that gene may not function properly, which sometimes creates a genetic disorder. Not all gene mutations cause a disorder. People inherit two copies of most genes; one copy from each parent. A genetic disorder occurs when one or both parents pass a mutated gene to a child at conception. A genetic disorder can also occur through a spontaneous gene mutation, meaning neither parent carries a copy of the mutated gene. Once a spontaneous gene mutation has occurred, a person can pass it to his or her children. Read more about genes and genetic conditions in the U.S. National Library of Medicines (NLMs) Genetics Home Reference.
Source: NIDDK