Nuclear Translocation of Activation-Induced Cytidine Deaminase (AID) in ADPKD Tissue Reveals a Mechanism for Second Hit Mutations Leading to ADPKD

Background: ADPKD results from inheritance of a pathogenic PKD1 allele, however, a somatic second hit inactivation of the normal allele is thought to be necessary for cyst development. Despite similarities in gene sequence and function with human PKD1, Pkd1-/+ heterozygous mice do not naturally develop ADPKD. We have recently identified abundant tandem guanine repeats in the human PKD1 gene that are capable of adopting four-stranded DNA conformations called G4 DNA. In contrast, these sequences are sparse in the mouse or rat Pkd1 genes. Activation-induced cytidine deaminase (AID) contains both nuclear import and export signals and is primarily expressed in B lymphocytes during differentiation where it regulates class switch recombination (CSR) and somatic hypermutation in antibody diversification. AID binds to G4 DNA and deaminates deoxycytidines into deoxyuracils resulting in mismatches that are processed into double strand breaks in CSR. AID is also capable of targeting non-immunoglobulin genes where it binds G4 DNA and introduces mutations and chromosomal translocations. Methods: To begin to determine whether AID plays a role in inducing second hit mutations in polycystic kidney disease we evaluated AID expression in human ADPKD kidneys and in kidneys from Pkd1 mutant mice. Results: In human ADPKD tissue, AID protein was translocated to the nuclei of cyst lining cells, but was restricted to the cytoplasm in normal human kidneys. In addition, AID co-localized with gamma-H2AX, a marker for DNA breaks in human ADPKD kidneys. Moreover, chromatin immunoprecipitation of G4 DNA was associated with DNA breaks in human PKD1 but not in mouse Pkd1. In contrast, in both normal and Pkd1 mutant mouse kidney tissue, AID protein was excluded from the nucleus and restricted to the cytoplasm. Conclusion: Taken together, our results suggest that the abundant G4 sequences in the human PKD1 gene are targeted by AID where deoxycytidine deamination leads to double strand breaks and second hit mutations. Funding: NIDDK Support