Human-Specific Abnormal Alternative Splicing of Wild-Type <i>PKD1</i> Induces Premature Termination of Polycystin-1.

Abstract

The major form of autosomal dominant polycystic kidney disease is caused by heterozygous mutations in <i>PKD1</i>, the gene that encodes polycystin-1 (PC1). Unlike <i>PKD1</i> genes in the mouse and most other mammals, human <i>PKD1</i> is unusual in that it contains two long polypyrimidine tracts in introns 21 and 22 (2.5 kbp and 602 bp, respectively; 97% cytosine and thymine). Although these polypyrimidine tracts have been shown to form thermodynamically stable segments of triplex DNA that can cause DNA polymerase stalling and enhance the local mutation rate, the efficiency of transcription and splicing across these cytosine- and thymine-rich introns has been unexplored. We used RT-PCR and Western blotting (using an mAb to the N terminus) to probe splicing events over exons 20-24 in the mouse and human <i>PKD1</i> genes as well as Nanopore sequencing to confirm the presence of multiple splice forms. Analysis of PC1 indicates that humans, but not mice, have a smaller than expected protein product, which we call Trunc_PC1. The findings show that Trunc_PC1 is the protein product of abnormal differential splicing across introns 21 and 22 and that 28.8%-61.5% of <i>PKD1</i> transcripts terminate early. The presence of polypyrimidine tracts decreases levels of full-length <i>PKD1</i> mRNA from normal alleles. In heterozygous individuals, low levels of full-length PC1 may reduce polycystin signaling below a critical "cystogenic" threshold.