As one of the six genes most involved in genetic epilepsy, protocadherin 19 (PCDH19), located at Xq22.1, was highly expressed in multiple organs especially the limbic areas of the brain, which was involved in cell adhesion, neuronal migration, synaptic connections formation (1). With a six-exon structure, PCDH19 encoded protocadherin-19, a transmembrane cell adhesion molecule belonging to the cadherin family participated in the neuronal connections (2). The first exon encoded the extracellular, transmembrane, and a small portion of the intracellular domain. The rest of the C-terminal intracellular domain was encoded by exons 2–6, and cytoplasmic domains (CM1 and CM2) in the C-terminal region were encoded by exons 5 and 6. PCDH19 female limited epilepsy (PCDH19-FE) was an infantile-onset epilepsy syndrome with strong phenotype heterogeneity, and hallmark features of PCDH19-FE included focal seizure, intellectual disability, anti-seizure therapy resistance, etc. (3). PCDH19-FE was also named an epilepsy-intellectual disability in females, which was first reported in 1971 (4). However, its causal gene wasn't identified until 2008 based on seven families (5). In addition to heterozygous females, PCDH19-FE also affected male mosaicism but the male hemizygotes were exempt (6). To date, ~150 variants in PCDH19 have been discovered, which contained non-sense, missense, and frameshift variants (7). Among these variants, missense variants were mostly reported, of which c.1019A > G (p.Asn340Ser) heterozygous variant was one of the common types (8). Besides, while a high variant frequency was found in the extracellular domain (EC) of PCDH19, the variant in the intracellular domain impacting signal transduction was also reported (5, 7). In addition to the genetic variant diversity, the phenotypic spectrum of the disease varied widely including mild epilepsy to epileptic encephalopathy. Seizure types also ranged from focal seizures to generalized tonic-clonic, tonic, absence, and myoclonic seizures (9). With clustered and fever-sensitive attack characteristics, neurodevelopmental disorders such as autism spectrum disorder (ASD), and attention-deficit hyperactivity disorder (ADHD) may also occur (10). Albeit the heterogeneity of variant and phenotype has been summarized, a convincible explanation for this genotype-phenotype heterogeneity still lacked. In this article, we reported a pedigree of PCDH19-FE including five generations, seven patients in the pedigree. The same variant and strong heterogeneity of clinical symptoms were found in this pedigree. A novel variant was identified and potential explanations of strong phenotype heterogeneity compared with symptoms in previous cases and genotype-phenotype heterogeneity were also discussed in detail.

In this research, a five-generation PCDH19-FE pedigree with strong genotype-phenotype heterogeneity was collected and analyzed. In addition to the comprehensive analysis of the pedigree with epilepsy and developmental delay from the clinical perspective, the sequencing of epilepsy-related genes was performed to investigate PCDH19 genotype and phenotype heterogeneity. Two PCDH19 variants in two loci, p.Asn232Ser and p.Asp920Glu, were identified. Besides, AlphaFold2 was adopted for mutated PCDH19 protein structure prediction, which revealed these two variants, and the mutual interaction between these two variants was correlated with PCDH19 structure alterations.

PCDH19 was a member of the delta-cadherin (δ-cadherin) superfamily genes, and the δ-cadherin superfamily included a family of differentially expressed neuro-adhesion molecules involved in a range of neurodevelopmental disorders (21). PCDH19 was predominantly expressed in the nervous system, especially in the developing limbic system, and was essential for calcium-dependent cell-cell interactions and adhesion (22). The increased expression level of PCDH19 was also found in other areas such as the ventromedial telencephalon, thalamus, periventricular zone of the anterior hypothalamus, and numerous sensory and motor nuclei. Multiple variants in PCDH19 have been discovered, which contained missense, non-sense, frameshift variants, etc. Variants can cause changes in protein structure and function in a variety of ways, including interference of homophilic adhesion, impairment of interaction with other calcium adhesion proteins, and a decrease in protein folding function and stability (23). Recent experiments on animal models illustrated that the PCDH19 variant impaired the mossy fiber synapse development in a mouse model, and led to neuronal hyperexcitability in both mosaic and non-mosaic PCDH19 mutation zebrafish models (24, 25). The pathophysiology of PCDH19-related epilepsy has also been reviewed recently with four main theories: gamma-aminobutyric acid type A receptor [GABAA(R)] dysregulation, cellular interference, blood-brain barrier dysfunction, and the aldo-keto reductase 1C family 1-3 (AKR1C1-3) gene product shortage (26). While GABAA(R) mediated the inhibitory postsynaptic potential, the AKR1C1-3 product deficiency led to the decreased allopregnanolone, which was correlated with decreased tonic and phasic inhibitory currents of GABAA(R) (27). As for the blood-brain barrier (BBB) dysfunction, increased substances such as antibodies of N-methyl-D-aspartate (NMDA)-type glutamate receptor (abs-NR) can pass the BBB, which may underlie disease pathogenesis (8). Furthermore, contrary to the loss function hypothesis of PCDH19, the cellular interference theory indicated that clinical manifestations may be related to abnormal communication between normal and mutant cells (28). This hypothesis was also supported by the phenomenon that mutated females and mosaic males were mostly affected while hemizygous males were commonly unaffected. And the correlation of X inactivation and PCDH19-FE phenotype may also be partly explained by this hypothesis.

From the clinical perspective, the PCDH19 variant caused a spectrum of clinical symptoms of PCDH19-FE (23). While some shared clinical features such as seizures characterized by fever sensitivity, and early-onset seizures in infancy or early childhood, the phenotype heterogeneity was strong (3, 29, 30). Multiple seizure types have been observed such as GTCS, myoclonic, and absence seizures (31). The concomitant psychiatric symptoms were also diverse including hyperactive, autistic, and obsessive-compulsive features (7). In addition, epileptic encephalopathy was also reported in previous PCDH19-FE pedigree (32, 33).

In our pedigree, a strong genotype-phenotype heterogeneity was exhibited in the result part and Table 1. The clinical characteristic that symptoms of PCDH19-FE were observed in heterozygous females but not in hemizygous males was consistent with previous reports. In addition, the clinical manifestations of five generations in our pedigree seemed to become more severe in the newer generation. And patients with early onset tended to suffer from more severe symptoms. The clinical symptoms of V:1 with epileptic encephalopathy manifestations were more severe than those of IV:11. The intellectual disability was observed on IV:3, IV:4, and IV:5 but was not identified on II:6. Although there have been some reports of genotype-phenotype heterogeneity, mechanisms were still vague. Age was one of the influencing factors of PCDH19-FE as seizures commonly became less severe with growth (29). And onset age and seizure frequency were prognostic factors of intellectual disability (7). Besides, while most patients appeared cognitive delay after the epilepsy symptoms, 15% of patients exhibited cognitive delay before the epilepsy onset. The intellectual impairment was probably related to genetic factors, environmental factors, etc. In addition, the X chromosome inactivation was also a potential reason for phenotype heterogeneity. Previous publications revealed the association of X-inactivation and PCDH19-related epileptic encephalopathy with skewed X-chromosome inactivation as a potential protective factor (32). Epigenetic modifications participated in the skewed X-inactivation, which preferentially inhibited the expression of wild-type or mutant proteins returning the heterozygous to the homozygous state. Based on the cellular interference hypothesis, the comparable wild-type cells and mutant cells interacted with each other probably impacting the neurodevelopment or synaptogenesis (34, 35). And the skewed X-inactivation may reverse this pathogenic status reducing seizures and disorders.

Besides, PCDH19 variants in two loci were identified in our research, and the p.Asn232Ser has been reported in previous cases of the clinical diagnosis of PCDH19-related epilepsy or Dravet syndrome with phenotype heterogeneity (3, 18–20, 33, 36). In addition to the common symptoms including cluster and febrile seizures, generalized seizures such as absence seizure and GTCS and focal seizure have been reported in multiple patients with the p.Asn232Ser variant (3, 18, 36). Intellectual disability, language, and motor delay were also reported (18, 20). Another female patient with the p.Asn232Ser variant typically began with hypomotor-apneic semiology mostly followed by tonic-clonic activity with posterior discharge on ictal EEG in infancy. Later, after an aura of lower extremity or abdominal pain, and myoclonia, tonic-clonic clusters and hypermotor seizures were observed (19). Intriguingly, the p.Asn232Ser variant has been detected in a European (non-Finnish) female sample but was not detected in the east Asian population based on the record on the gnomAD database (v3.1.1). The allele frequency of this variant was 8.84e-6 in the total population indicating that it can be detected in the asymptomatic person but was not a common variant in the population. Numerous cases of asymptomatic carrier mothers were also reported (37–39). As for the mechanism of the existence of asymptomatic carriers, the penetrance rate and X-inactivation are potential influencing factors. The penetrance of PCDH19-FE was 80% approximately but was still uncertain because of the indefinite definition of unaffected individuals and the estimation based only on reported cases with tested mothers (7). Previous parts have also discussed that the X-inactivation may involve in the phenotype heterogeneity, and the skewed X-inactivation was probably related to unaffected heterozygous female carriers. However, further researches were still necessary to obtain more accurate explanations.

The possible cause of the clinical phenotype heterogeneity between our cases and previous reports was probably associated with the novel p.Asp920Glu variant found in our pedigree. This p.Asp920Glu variant was highly conserved in biological species conservation and was not a polymorphic change in the population with a low frequency of occurrence. The pathogenicity of the p.Asp920Glu variant was testified by multiple in silico analyses in Table 2 including Polyphen2 and Mutation Taster. And the influence of p.Asp920Glu on PCDH19 protein structure has also been predicted by AlphaFold2. The hydrogen bond formed between Asp at position 920 and His at position 919 disappeared when the amino acid at position 920 was mutated to Glu. As a membrane protein, PCDH19 consisted of six extracellular cadherin repeats, a transmembrane domain (TM), and a C-terminal intracellular tail with two motifs, CM1 and CM2 (40). More than one hundred variants and a variety of variant types led to structural alterations in different domains of the PCDH19 protein based on previous publications (7). And novel variants were also increasingly reported, which were mostly located at exon 1 and missense variations (7). Various variant types were recorded including missense/non-sense variants, small deletions, splicing variants, etc. However, the association of different missense variant sites and phenotype heterogeneity has not been summarized before but several publications have reported the correlation among gene variant sites, protein structure, and phenotype. For instance, a milder intellectual disability and later onset seizures were found in the patients with truncating variants from EC domain 5 to the cytoplasmic domain compared with truncating variants from EC domain 1 to EC domain 4 (22). As c.2760T>A was a novel variant located at the exon 5 of the PCDH19 gene influencing the structure of the CM1 domain of PCDH19 protein, the phenotype differences between our pedigree and previous cases were probably related to this variant requiring further experiments to verify this hypothesis. From the function perspective, PCDH19 functions were also diverse such as its adhesive or signal transduction properties with potential influence on structural layering on hippocampus layering (41, 42). A complete PCDH19 ectodomain model was put forward recently, which can assist in the evaluation of epilepsy-related variants and protein interaction sites (43). Diverse variants probably affected different functions of the PCDH19 protein to varying degrees leading to heterogeneity. Besides, the mutual interactions between different mutated amino acids may also exert an influence on the structure and function of PCDH19. Our research revealed the amino acid alteration at position 230 led to the disappearance of the hydrogen bond between His919 and Asp920. This indicated that the interplay among simultaneous variants at different sites in PCDH19 also probably impacted the intermolecular interaction of PCDH19 protein.

In addition to our findings, there were a few limitations that existed in our research. First, the mechanism of genotype-phenotype heterogeneity, which has been observed in our pedigree and previous reports, requires further research to explore such as the X chromosome inactivation mode. Second, the functional verification on the animal model should be subsequently conducted to ascertain whether the p.Asp920Glu variant influences the PCDH19 protein function and PCDH19-FE phenotype.

Based on this PCDH19-FE pedigree with five generations, two variants, p.Asn232Ser and p.Asp920Glu in the PCDH19, were identified, of which p.Asp920Glu was a novel variant site without being reported before. And a strong genotype-phenotype heterogeneity has been observed among female patients with the same genotype in our pedigree. As for the potential mechanism of this genotype-phenotype heterogeneity, multiple factors including age, and X-chromosome inactivation mode were possible related factors but still needed further investigation. Other factors related to phenotype heterogeneity such as variant sites and the mutual interaction between concurrent variants in PCDH19 were also discussed based on the genotype and clinical manifestations of patients in our pedigree and previous cases.

Two variants in PCDH19 were identified in seven female patients with different clinical phenotypes in a five-generation pedigree with PCDH19-FE. And the c.2760T>A (p.Asp920Glu) variant is a novel variant site probably related to PCDH19-FE.

The datasets presented in this study can be found in online repositories. Our sequencing data have been deposited to the NCBI Sequence Read Archive (SRA) database of the national center for biotechnology information, and the accession number of it is PRJNA859639.

The studies involving human participants were reviewed and approved by Third Xiangya Hospital. Written informed consent to participate in this study was provided by the participants' legal guardian/next of kin. Written informed consent was obtained from the individual(s), and minor(s)' legal guardian/next of kin, for the publication of any potentially identifiable images or data included in this article.

WZ and YO were involved in the conception and design of the study and the drafting of the manuscript. YJ participated in the drafting of the manuscript. WZ, QX, and LZ reviewed the manuscript for its intellectual content and revised the entire work. All authors approved the final manuscript as submitted and agree to be accountable for all aspects of the work.