Molecular Diagnostics in Autosomal Dominant Polycystic Kidney Disease: Utility and Limitations

Background and objectives: Gene-based mutation screening is now available and has the potential to provide diagnostic confirmation or exclusion of autosomal dominant polycystic kidney disease. This study illustrates its utility and limitations in the clinical setting.Design, setting, participants, & measurements: Using a molecular diagnostic service, genomic DNA of one affected individual from each study family was screened for pathologic PKD1 and PKD2 mutations. Bidirectional sequencing was performed to identify sequence variants in all exons and splice junctions of both genes and to confirm the specific mutations in other family members. In two multiplex families, microsatellite markers were genotyped at both PDK1 and PKD2 loci, and pair-wise and multipoint linkage analysis was performed.Results: Three of five probands studied were referred for assessment of renal cystic disease without a family history of autosomal dominant polycystic kidney disease, and two others were younger at-risk members of families with autosomal dominant polycystic kidney disease being evaluated as living-related kidney donors. Gene-based mutation screening identified pathogenic mutations that provided confirmation or exclusion of disease in three probands, but in the other two, only unclassified variants were identified. In one proband in which mutation screening was indeterminate, DNA linkage studies provided strong evidence for disease exclusion.Conclusions: Gene-based mutation screening or DNA linkage analysis should be considered in individuals in whom the diagnosis of autosomal dominant polycystic kidney disease is uncertain because of a lack of family history or equivocal imaging results and in younger at-risk individuals who are being evaluated as living-related kidney donors.Autosomal dominant polycystic kidney disease (ADPKD) is the most common hereditary kidney disease worldwide affecting one in 500 to 1000 live births (1,2). It is characterized by focal and sporadic development and progressive enlargement of renal cysts, leading to ESRD in late middle age. Typically, only a few renal cysts are detected in most affected individuals before 30 yr of age; however, by the fifth decade of life, hundreds to thousands of renal cysts will be found in the majority of patients. Overall, it accounts for 5 to 8% of ESRD in developed countries (1,2). ADPKD is a systemic disorder associated with multiple extrarenal complications, such as cysts in nonrenal organs, valvular heart disease, colonic diverticula, inguinal hernias, and intracranial arterial aneurysms. It is genetically heterogeneous, with most cases arising from mutations in PKD1 (MIM 601313) and PKD2 (MIM 173910), located on chromosome 16p13.3 and 4q21–23, respectively (1–4). In a linkage-characterized European sample, PKD1 accounts for approximately 85% of the cases, whereas PKD2 accounts for most of the remainder (3,4). Although the clinical manifestations of the two gene types overlap completely, a strong locus effect is evident with more severe renal disease in PKD1 than PKD2 (median age at ESRD 54 versus 74, respectively) (5). In addition, both environmental and genetic modifiers have been implicated to account for the significant intrafamilial renal disease variability observed (6–8), and a mild allelic effect has been suggested for PKD1 but not PKD2 (7,8).PKD1 is a large gene consisting of 46 exons with an open reading frame of approximately 13 kb and is predicted to encode a protein of 4302 amino acids. Its entire 5′ region up to exon 33 has been duplicated six times more proximally on chromosome 16p, and the presence of these highly homologous pseudogenes has made genetic analysis of PKD1 difficult (1,2). Recent availability of protocols for long-range and locus-specific amplification of PKD1 has enabled the complete mutation screening of this complex gene (9–11). By contrast, PKD2 is a single-copy gene that consists of 15 exons with an open reading frame of approximately 3 kb and is predicted to encode a protein of 968 amino acids (1,2). Marked allelic heterogeneity is evident for ADPKD, with more than 200 different PKD1 and more than 50 different PKD2 mutations reported to date (2,9–11). The majority of these mutations are unique and scattered throughout both genes. Most of them are also predicted to be protein truncating (as a result of frame-shift deletion/insertion, nonsense changes, or splice defects), although a significant number of unclassified variants (UCV; e.g., in-frame deletions, missense changes) have been reported (9–11). Despite sequencing of all of the coding regions and exon-intron splice junctions in both genes only 45 to 63% of pathogenic mutations could be identified in three large clinical series (9–11).The diagnosis of ADPKD is generally straightforward when affected individuals present with a positive family history and enlarged kidneys with multiple cysts (12). Renal ultrasound is a sensitive method for this purpose, and age-dependant criteria based on cyst number have been derived for individuals who are born with 50% risk for PKD1 (13); however, because cyst formation is an age-dependent process, the false-negative rate of ultrasound-based diagnosis is higher in younger at-risk individuals or in those who are affected by PKD2, which is associated with later onset disease (14). Equivocal imaging results can be a source of diagnostic uncertainty in the clinic because the underlying gene type for most patients is unknown. In addition, renal cystic disease without a family history of ADPKD and evaluation of younger at-risk individuals as living-related kidney donors are clinical scenarios that often pose diagnostic challenges (12). Using a case series, we illustrate the utility and limitations of molecular diagnostics for ADPKD in the clinical setting.