Analysis of an ADTKD family with a novel frameshift mutation in MUC1 reveals characteristic features of mutant MUC1 protein

Medullary cystic kidney disease Type 1 is an autosomal dominant tubulointerstitial kidney disease (ADTKD). Recently, mucin 1 (MUC1) was identified as a causal gene of medullary cystic kidney disease (ADTKD-MUC1). However, the MUC1 mutation was found to be a single cytosine insertion in a single copy of the GC-rich variable number of tandem repeats (VNTRs), which are very difficult to analyze by next-generation sequencing. To date, other mutations have not been detected in ADTKD-MUC1, and the mutant MUC1 protein has not been analyzed because of the difficulty of genetically modifying the VNTR sequence. We conducted whole-exome analyses of an ADTKD family by next-generation sequencing. We also performed histopathological analyses of a renal biopsy from a pedigree family member. We constructed a mutant protein expression vector based on the patient genome sequence and characterized the nature of the mutant protein. We found a novel frameshift mutation before the VNTR in the MUC1 gene. The resulting mutant MUC1 protein had a very similar amino acid sequence and predicted 3D structure to the previously reported mutant protein. Notably, the recombinant mutant MUC1 protein was trapped in the cytoplasm and appeared to self-aggregate. The patient native mutant protein was also found in urine exosomes. This novel frameshift mutation in the MUC1 gene and consequent mutant protein may contribute to the future discovery of the pathophysiology of ADTKD-MUC1. The mutant MUC1 protein in urine exosomes may be used for non-DNA-related diagnosis.