Introduction Mutations in the PCDH19 gene have mainly been reported in female patients with epilepsy. To date, PCDH19 mutations have been reported in hundreds of females and only in ten male epileptic patients with mosaicism.

Objective We aimed to investigate the occurrence of mosaic PCDH19 mutations in 42 families comprising at least one patient with PCDH19-related epilepsy.

Methods Two male patients with mosaic PCDH19 variants were identified using targeted next-generation sequencing. Forty female patients were identified by Sanger sequencing/Multiplex Ligation Probe Amplification (MLPA). Micro-droplet digital PCR (mDDPCR) was used to quantify the mutant allelic fractions (MAFs) in 20 families with PCDH19 variants.

Results Five mosaic individuals were identified in total. Mosaic variant was confirmed in multiple somatic tissues from one male patient, and in blood from the other male patient. Among 22 female patients harboring a newly occurred PCDH19 variant identified by Sanger sequencing and MLPA, Sanger sequencing revealed two mosaic fathers (9%, 2/22), one with two affected daughters and one with an affected child. Two asymptomatic mosaic fathers were confirmed as gonosomal mosaicism, with MAFs ranging from 4.16% to 37.38% and from 1.27% to 19.13%, respectively. In 11 families with apparent de novo variants, one female patient was identified as a mosaic with a blood MAF of 26.72%. Three females with incomplete penetrance was unrelated to mosaicism.

Conclusion Our study provides new insights into phenotype-genotype correlations in PCDH19 related epilepsy and the finding of high-frequency mosaicism has important implications for genetic counselling.