The First Molecular Assay to Help Diagnose Autosomal Dominant Polycystic Kidney Disease

Athena Diagnostics is the only laboratory to offer a direct DNA analysis of the two genes (PKD1 and PKD2) responsible for the majority of autosomal dominant polycystic kidney disease (ADPKD). Unlike haplotype or linkage analysis, the PKDx Molecular Diagnostic Testing Service is a private blood test requiring a blood sample from only the person being tested.

Most ADPKD is caused by alterations in one of two genes, PKD1 and PKD2. These two genes code for two proteins that are important in transporting molecules across cellular membranes. Certain alterations in PKD1 and PKD2 have been shown to alter the structure of the proteins in a way that leads to cyst formation.

While the genetics of ADPKD have been known for some time, the size and complexity of the two genes hindered development of a clinically useful DNA sequencing test. However, recent discoveries ^1,2 have led to the development of the first commercially available clinical assay to test for the presence or absence of alterations in both the PKD1 and PKD2 genes of an individual. The testing service utilizes direct DNA sequencing of both genes, a highly accurate and technologically sensitive method of DNA analysis. Direct sequencing enables the identification of the precise DNA sequence of the two genes in an individual. Unlike linkage or haplotype analysis, multiple family members are not required for this type of test.

Indications for Testing

• Unclear clinical diagnosis
• Typical presentation without family history
• Prenatal diagnosis
• Transplant evaluation
• Differentiate between PKD1 and PKD2

Unclear Clinical Diagnosis
In families with a history of PKD in which the disease appears to be inherited in autosomal dominant fashion, an accurate diagnosis can often be made based on imaging studies. However, there are cases where there is a family history of ADPKD, but where the proband does not fit the customary clinical criteria for the disease. In these cases, sequencing may provide valuable information about the cause of the patient's symptoms.

Typical Presentation Without Family History
Patients may present with customary clinical symptoms, but without a positive family history of ADPKD. This may occur because i) other family members have less symptomatic disease and thus never were evaluated by imaging, ii) the patient may have another disease (such as multiple benign simple cysts, multicystic dysplastic kidney disease, tuberous sclerosis, acquired cystic disease, etc), or iii) the patient may have ADPKD as a result of a de novo mutation. In the absence of a family history of PKD, especially when parents or siblings are not available or are unwilling to undergo imaging studies, it may be useful to conduct DNA sequencing analysis of such a patient to determine if the patient's symptoms are actually due to ADPKD or another disease with similar symptoms.

Prenatal Diagnosis
Sequencing analysis can sometimes be used to provide information about a pregnancy. In a variety of circumstances, it may be useful to determine if a fetus carries an alteration in the PKD1 or PKD2 genes. For example, ultrasound imaging may reveal kidney cysts in a fetus and it can be difficult to determine their cause. Alternatively, a couple, one of whom has ADPKD, may want to learn whether or not their pregnancy will give rise to a child with ADPKD. In such situations it may be useful to analyze the DNA of the fetus. The result may indicate that the fetus has ADPKD, or alternatively, may direct the search for other causes of disease.

It is important to note that PKD1 and PKD2 are large, complex genes, and considerable time is required to sequence them. The time required must be factored into a decision about testing for a prenatal diagnosis. In some cases, directed (single amplicon) testing can be used if a mutation had been previously identified in the parent known to have the disease. Athena's Family Testing Service for ADPKD is available as the PKDx™ familial mutation evaluation (Unit Code 728). It should be noted that the role of prenatal testing in ADPKD is controversial, and its use is a highly personal decision that should be considered only after discussions with knowledgeable physicians or genetic counselors.

Transplant Evaluation
Many patients with PKD will ultimately develop end stage renal failure, and will require a kidney transplant. In most cases, the best organ donor is a living relative. However, ADPKD is inherited as an autosomal dominant trait, meaning that for PKD patients requiring a transplant, family members themselves have a 50% chance of carrying the same DNA alteration that led to the recipient's disease. Many physicians feel that transplantation in such a case should be approached very carefully because of the risk that the living-related potential donor may be presymptomatic for ADPKD. In most cases, imaging studies can effectively exclude or establish a diagnosis in at-risk individuals. There are situations, however, where the diagnosis cannot be determined in an unambiguous manner by imaging (e.g. young adults or younger individuals from families with late onset disease, etc.). The actual risks of using such an individual as a donor are unknown as there have been no careful studies of this topic. There are theoretical risks, however. It is possible that removal of a kidney from an at-risk individual with the same DNA mutation as the recipient could harm the donor since he or she may go on to develop PKD. Given such risks, many transplant centers will not allow individuals with an ambiguous or uncertain diagnosis serve as donors. In these circumstances, sequencing may be able to help resolve the question. In these situations, order the PKDx™ familial mutation evaluation (Unit Code 728) after the familial mutation has been identified.

Desire to Know
For some asymptomatic people, it may be important to know whether or not they carry the same alterations that caused PKD in their relatives. For example, a person with a history of ADPKD in his or her family may want to know their status for family planning reasons. Others may want to know their status in order to take precautionary measures (such as changes in diet, exercise and control of hypertension) that may provide some benefit or even possibly delay the onset of symptoms. Still others may simply want to know their status. This, too, is a very personal decision that should be discussed with one's health care provider prior to testing. While in most cases imaging studies can identify affected individuals before they manifest symptoms, there are clinical situations where DNA testing may be very helpful, such as in individuals with non-diagnostic imaging results (usually of younger age) or individuals that are from later-onset families. It should be noted that the clinical diagnostic criteria commonly used to establish the diagnosis of ADPKD have not been as rigorously established for PKD2PKD1.

Differentiate Between PKD1 and PKD2 The disease caused by alterations in PKD1 and PKD2 can have different clinical courses.³ In general, alterations in PKD2 result in a later onset disease with less severe symptoms. However, there is significant clinical overlap between the two. Thus, it may not be possible to determine, based on clinical signs alone, if the disease in a given family is caused by alterations in PKD1 or PKD2. Once the cause of disease in a family is determined, family members can be inexpensively tested using Athena's PKDx™ familial mutation evaluation (Unit Code 728).

1. Phakdeekitcharoen B, et al.: Thirteen novel mutations of the replicated region of PKD1 in an Asian population 2000 Kidney Int. Oct;58(4):1400-1412.
2. Phakdeekitcharoen B, et al.: Mutation analysis of the entire replicated portion of PKD1 using genomic DNA samples. J Am Soc Nephrol. 2001 May;12(5):955-963.
3. Pei Y, et al.: Bilineal disease and trans-heterozygotes in autosomal dominant polycystic kidney disease, 2001 Am. J. Hum. Genetics 68:355-363.