Developmental and epileptic encephalopathy-9 (DEE9) is characterized by seizure onset in infancy, mild to severe intellectual impairment, and psychiatric features and is caused by a mutation in the PCDH19 gene on chromosome Xq22. The rare, unusual X-linked type of disorder affects heterozygous females and mosaic males; transmitting males are unaffected. In our study, 165 patients with epilepsy were tested by Next Generation Sequencing (NGS)-based panel and exome sequencing using Illumina technology. PCDH19 screening identified three point mutations, one indel, and one 29 bp-long deletion in five unrelated female probands. Two novel mutations, c.1152_1180del (p.Gln385Serfs*6) and c.830_831delinsAA (p.Phe277*), were identified and found to be de novo pathogenic. Moreover, among the three inherited mutations, two originated from asymptomatic mothers and one from an affected father. The PCDH19 c.1682C>T and c.1711G>T mutations were present in the DNA samples of asymptomatic mothers. After targeted parental testing, X chromosome inactivation tests and Sanger sequencing were carried out for mosaicism examination on maternal saliva samples in the two asymptomatic PCDH19 mutation carrier subjects. Tissue mosaicism and X-inactivation tests were negative. Our results support the opportunity for reduced penetrance in DEE9 and contribute to expanding the genotype-phenotype spectrum of PCDH19-related epilepsy.

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OBJECTIVE: PCDH19 mutations cause epilepsy and mental retardation limited to females (EFMR) or Dravet-like syndromes. Especially in the first years of life, epilepsy is known to be highly pharmacoresistant. The aim of our study was to evaluate the effectiveness of antiepileptic therapy in patients with PCDH19 mutations.
METHODS: We report a retrospective multicenter study of antiepileptic therapy in 58 female patients with PCDH19 mutations and epilepsy aged 2-27 years (mean age 10.6 years).
RESULTS: The most effective drugs after 3 months were clobazam and bromide, with a responder rate of 68% and 67%, respectively, where response was defined as seizure reduction of at least 50%. Defining long-term response as the proportion of responders after 12 months of treatment with a given drug in relation to the number of patients treated for at least 3 months, the most effective drugs after 12 months were again bromide and clobazam, with a long-term response of 50% and 43%, respectively. Seventy-four percent of the patients became seizure-free for at least 3 months, 47% for at least one year.
CONCLUSIONS: The most effective drugs in patients with PCDH19 mutations were bromide and clobazam. Although epilepsy in PCDH19 mutations is often pharmacoresistant, three quarters of the patients became seizure-free for at least for 3 months and half of them for at least one year. However, assessing the effectiveness of the drugs is difficult because a possible age-dependent spontaneous seizure remission must be considered.