SEQUENCE AND STRUCTURAL DETERMINANTS OF POLYCYSTIC KIDNEY DISEASE
Autosomal Dominant Polycystic Kidney Disease (ADPKD) is a severe but relatively common (1/500 incidence) renal disease. It results from an inherited mutation in one PKD1 gene, followed by a “second hit” mutation in the other normal gene leading to a cellular recessive mechanism. The PKD1 gene appears to be prone to these somatic inactivation events, but the mutagenic mechanisms are unknown. Moreover, suitable molecular markers for diagnosis and prevention remain elusive. We have discovered that the PKD1 gene in humans contains repeats of guanine capable of forming four-stranded structures known as G-quadruplex (G4) DNA. G4 DNA is a structure that leads to genetic instability, and it is found at oncogenic translocation hot spots, cancer genes, and at sites of recombination. Interestingly, the murine Pkd1 homolog is not prone to mutation like the human gene, so we compared the genome sequences to look for patterns that may explain the differences in mutation rate. Compared to human, murine Pkd1 is not repetitive. The human homolog contains G4 sequence motifs throughout the gene, and extensive repeats concentrated in several introns. Circular Dichroism spectroscopy confirms that those sequences indeed adopt G4 structures in solution. Our results suggest a mechanism for PKD1 inactivation and the emergence of ADPKD. G4 structure formation during replication or transcription blocks polymerase activities, leading to DNA breaks and recombination. This elevates the risk of sequence changes that inactivate the gene. Our results not only provide a molecular rationale for ADPKD, but also identify a DNA structure within PKD1 that could be targeted for diagnosis or disease prevention.