Neuronal network activity and connectivity are impaired in a conditional knockout mouse model with PCDH19 mosaic expression

Mutations in PCDH19 gene, which encodes protocadherin-19 (PCDH19), cause Developmental and Epileptic Encephalopathy 9 (DEE9). Heterogeneous loss of PCDH19 expression in neurons is considered a key determinant of the disorder; however, how PCDH19 mosaic expression affects neuronal network activity and circuits is largely unclear. Here, we show that the hippocampus of Pcdh19 mosaic mice is characterized by structural and functional synaptic defects and by the presence of PCDH19-negative hyperexcitable neurons. Furthermore, global reduction of network firing rate and increased neuronal synchronization have been observed in different limbic system areas. Finally, network activity analysis in freely behaving mice revealed a decrease in excitatory/inhibitory ratio and functional hyperconnectivity within the limbic system of Pcdh19 mosaic mice. Altogether, these results indicate that altered PCDH19 expression profoundly affects circuit wiring and functioning, and provide new key to interpret DEE9 pathogenesis.

PCDH19-clustering epilepsy, pathophysiology and clinical significance

PCDH19 clustering epilepsy (PCDH19-CE) is an X-linked epilepsy disorder associated with intellectual disability (ID) and behavioral disturbances, which is caused by PCDH19 gene variants. PCDH19 pathogenic variant leads to epilepsy in heterozygous females, not in hemizygous males and the inheritance pattern is unusual. The hypothesis of cellular interference was described as a key pathogenic mechanism. According to that, males do not develop the disease because of the uniform expression of PCDH19 (variant or wild type) unless they have a somatic variation. We conducted a literature review on PCDH19-CE pathophysiology and concluded that other significant mechanisms could contribute to pathogenesis including: asymmetric cell division and heterochrony, female-related allopregnanolone deficiency, altered steroid gene expression, decreased Gamma-aminobutyric acid receptor A (GABAA) function, and blood-brain barrier (BBB) dysfunction. Being aware of these mechanisms helps us when we should decide which therapeutic option is more suitable for which patient.

PCDH19-related epilepsy in mosaic males: The phenotypic implication of genotype and variant allele frequency

OBJECTIVE

To analyze the genotypes and phenotypes of mosaic male patients with PCDH19-related epilepsy (PCDH19-RE) and explore the correlation between genotype, variant allele frequency (VAF), and phenotypic severity.

METHODS

Clinical data and peripheral blood samples of 11 male mosaic patients were collected and analyzed in our study. The VAF of the PCDH19 gene from peripheral blood was quantified using amplicon-based deep sequencing. Additional 20 mosaic male patients with PCDH19-RE were collected from the published literature, with 10 patients whose VAFs of the PCDH19 gene were available for analytic purposes.

RESULTS

In our cohort of 11 patients, 10 variants were identified, and four were novel. The VAF of the PCDH19 gene from peripheral blood ranged from 27 to 90%. The median seizure onset age was 6 months (range: 4-9 months). Clinical manifestations included cluster seizures (100%), fever sensitivity (73%), focal seizures (91%), developmental delay/intellectual disability (DD/ID, 82%), and autistic features (45%). Thirty-one mosaic male patients collected from our cohort and the literature developed seizures mostly (87%) within one year of age. Variant types included missense variants (42%), truncating variants (52%), splice variants (3%), and whole PCDH19 deletion (3%). Among 21 patients with a definite VAF from our cohort and the literature, nine had a low VAF ( ≤ 50%) and 12 had a high VAF (> 50%). Seventy-five percent of variants from the high VAF group were missense, whereas 89% of those from the low VAF group were truncations. The median seizure onset age was 6 months in the low VAF group and 9 months in the high VAF group (p = 0.018). Forty-four percent (4/9) of patients from the low VAF group achieved seizure-free for ≥1 year, whereas none of the 12 patients from the high VAF group did (p = 0.021). DD/ID was present in 83% (10/12) of the high VAF group and 56% (5/9) of the low VAF group (p = 0.331).

CONCLUSION

The predominant variant types were truncating and missense variants. Missense variants tended to have higher VAFs. Patients with a high VAF were more likely to have a more severe epileptic phenotype. Our findings shed light on the phenotypic implications of VAF in mosaic males with PCDH19-RE.

PCDH19-Related Epilepsies

Protocadherin-19 (PCDH19) is considered one of the most relevant genes related to epilepsy. To date, more than 150 mutations have been identified as causative for PCDH19-female epilepsy (also known as early infantile epileptic encephalopathy-9, EIEE9), which is characterized by early onset epilepsy, intellectual disabilities, and behavioral disturbances. More recently, mosaic-males (i.e., exhibiting the variants in less than 25% of their cells) have been described as affected by infant-onset epilepsy associated with intellectual disability, as well as compulsive or aggressive behavior and autistic features. Although little is known about the physiological role of PCDH19 protein and the pathogenic mechanisms that lead to EIEE9, many reports and clinical observation seem to suggest a relevant role of this protein in the development of cellular hyperexcitability. However, a genotype–phenotype correlation is difficult to establish. The main feature of EIEE9 consists in early onset of seizures, which generally occur in clusters lasting 1 to 5 minutes and repeating up to 10 times a day for several days. Seizures tend to present during febrile episodes, similarly to the first phases of Dravet syndrome and PCDH19 variants have been found in ∼25% of females who present with features of Dravet syndrome and testing negative for SCN1A variants. There is no “standardized” treatment for PCDH19-related epilepsy and most of the patients receiving a combination of several drugs. In this review, we focus on the latest researches on these aspects, with regard to protein expression, its known functions, and the mechanisms by which the protein acts. The clinical phenotypes related to PCDH19 mutations are also discussed.

Pathogenic convergence of CNVs in genes functionally associated to a severe neuromotor developmental delay syndrome

BACKGROUND

Complex developmental encephalopathy syndromes might be the consequence of unknown genetic alterations that are likely to contribute to the full neurological phenotype as a consequence of pathogenic gene combinations.

METHODS

To identify the additional genetic contribution to the neurological phenotype, we studied as a test case a boy, with a KCNQ2 exon-7 partial duplication, by single-nucleotide polymorphism (SNP) microarray to detect copy-number variations (CNVs).

RESULTS

The proband presented a cerebral palsy like syndrome with a severe motor and developmental encephalopathy. The SNP array analysis detected in the proband several de novo CNVs, nine partial gene losses (LRRC55, PCDH9, NALCN, RYR3, ELAVL2, CDH13, ATP1A2, SLC17A5, ANO3), and two partial gene duplications (PCDH19, EFNA5). The biological functions of these genes are associated with ion channels such as calcium, chloride, sodium, and potassium with several membrane proteins implicated in neural cell-cell interactions, synaptic transmission, and axon guidance. Pathogenically, these functions can be associated to cerebral palsy, seizures, dystonia, epileptic crisis, and motor neuron dysfunction, all present in the patient.

CONCLUSIONS

Severe motor and developmental encephalopathy syndromes of unknown origin can be the result of a phenotypic convergence by combination of several genetic alterations in genes whose physiological function contributes to the neurological pathogenic mechanism.

Right Place at the Right Time: How Changes in Protocadherins Affect Synaptic Connections Contributing to the Etiology of Neurodevelopmental Disorders

During brain development, neurons need to form the correct connections with one another in order to give rise to a functional neuronal circuitry. Mistakes during this process, leading to the formation of improper neuronal connectivity, can result in a number of brain abnormalities and impairments collectively referred to as neurodevelopmental disorders. Cell adhesion molecules (CAMs), present on the cell surface, take part in the neurodevelopmental process regulating migration and recognition of specific cells to form functional neuronal assemblies. Among CAMs, the members of the protocadherin (PCDH) group stand out because they are involved in cell adhesion, neurite initiation and outgrowth, axon pathfinding and fasciculation, and synapse formation and stabilization. Given the critical role of these macromolecules in the major neurodevelopmental processes, it is not surprising that clinical and basic research in the past two decades has identified several PCDH genes as responsible for a large fraction of neurodevelopmental disorders. In the present article, we review these findings with a focus on the non-clustered PCDH sub-group, discussing the proteins implicated in the main neurodevelopmental disorders.

Two novel PCDH19 mutations in Russian patients with epilepsy with intellectual disability limited to females: a case report

Background

Epilepsy with intellectual disability limited to females (Epileptic encephalopathy, early infantile, 9; EIEE9) is a rare early infantile epileptic encephalopathy characterized by an unusual X-linked inheritance: females with heterozygous mutations are affected, while hemizygous males are not.

Case presentation

We describe the clinical and molecular characteristics of 2 Russian patients with EIEE9 (females, ages 3 years and 7 years). In these patients seizures developed at the age of 3 years. Additionally, for our patients and for cases described in the literature we searched for a possible relationship between the type and localization of the mutation and the EIEE9 clinical phenotype.

Conclusions

We identified two novel PCDH19 mutations in EIEE9 patients: a missense mutation in exon 1 (c.1236C > A, p.Asp412Glu) and a frameshift in exon 3 (c.2386_2387insGTCT, p.Thr796fs). We conclude that the age of seizure onset and the presence of intellectual disability may depend not on the type and localization of PCDH19 mutations, but on the X-inactivation status. The study also highlights the need to screen for EIEE9 among young female epilepsy patients.

Characterization of seizure susceptibility in Pcdh19 mice

OBJECTIVE

PCDH19-related epilepsy is characterized by a distinctive pattern of X-linked inheritance, where heterozygous females exhibit seizures and hemizygous males are asymptomatic. A cellular interference mechanism resulting from the presence of both wild-type and mutant PCDH19 neurons in heterozygous patients or mosaic carriers of PCDH19 variants has been hypothesized. We aim to investigate seizure susceptibility and progression in the Pchd19 mouse model.

METHODS

We assessed seizure susceptibility and progression in the Pcdh19 mouse model using three acute seizure induction paradigms. We first induced focal, clonic seizures using the 6-Hz psychomotor test. Mice were stimulated with increasing current intensities and graded according to a modified Racine scale. We next induced generalized seizures using flurothyl or pentylenetetrazol (PTZ), both γ-aminobutyric acid type A receptor function inhibitors, and recorded latencies to myoclonic and generalized tonic-clonic seizures.

RESULTS

Pcdh19 knockout and heterozygous females displayed increased seizure susceptibility across all current intensities in the 6-Hz psychomotor test, and increased severity overall. They also exhibited shorter latencies to generalized seizures following flurothyl, but not PTZ, seizure induction. Hemizygous males showed comparable seizure incidence and severity to their wild-type male littermates across all paradigms tested.

SIGNIFICANCE

The heightened susceptibility observed in Pcdh19 knockout females suggests additional mechanisms other than cellular interference are at play in PCDH19-related epilepsy. Further experiments are needed to understand the variability in seizure susceptibility so that this model can be best utilized toward development of future therapeutic strategies for PCDH19-related epilepsy.

The role of somatic mosaicism in brain disease

In this review we discuss the importance of genetic somatic mosaicism and its impact on brain diseases. We start from introducing the different types of somatic mutations, their frequencies and abundances across development and lifespan. We then describe how weakness in DNA repair mechanisms influences their prevalence. Finally, we address their functional consequences in the brain and review recent research showing their unsuspected importance in several neurodevelopmental, psychiatric, and neurodegenerative diseases.

PCDH19-Related Epilepsy in Early Onset of Chinese Male Patient: Case Report and Literature Review

Mutations in PCDH19 are associated with epilepsy, intellectual disability and behavioral disturbances, mostly related to females. The unique X-linked pattern of inheritance affects females predominantly, while usually is transmitted through asymptomatic males. Recently, new research has demonstrated that males with a mosaic pattern of inheritance could also be affected. As yet, PCDH19 mutations have been reported in hundreds of females; however, only 15 mosaic males were reported to exhibit epileptic seizures with the onset ranges between 6 and 31 months. These patients were usually reported to carry various mutations in the PCDH19. Here we describe a non-sense variant at the PCDH19 (c.498C>G; p.Y166^*) in the Chinese male that exhibited early developmental delay and frequent seizures starting from the age of 5 months. We aim that this case report, focusing on studying clinical seizures, therapeutic approaches, and the patient's prognosis, will contribute to the cumulative knowledge of this rare and complex genetic disorder.

A 2020 View on the Genetics of Developmental and Epileptic Encephalopathies

Developmental and epileptic encephalopathies (DEEs) can be primarily attributed to genetic causes. The genetic landscape of DEEs has been largely shaped by the rise of high-throughput sequencing, which led to the discovery of new DEE-associated genes and helped identify de novo pathogenic variants. We discuss briefly the contribution of de novo variants to DEE and also focus on alternative inheritance models that contribute to DEE. First, autosomal recessive inheritance in outbred populations may have a larger contribution than previously appreciated, accounting for up to 13% of DEEs. A small subset of genes that typically harbor de novo variants have been associated with recessive inheritance, and often these individuals have more severe clinical presentations. Additionally, pathogenic variants in X-linked genes have been identified in both affected males and females, possibly due to a lack of X-chromosome inactivation skewing. Collectively, exome sequencing has resulted in a molecular diagnosis for many individuals with DEE, but this still leaves many cases unsolved. Multiple factors contribute to the missing etiology, including nonexonic variants, mosaicism, epigenetics, and oligogenic inheritance. Here, we focus on the first 2 factors. We discuss the promises and challenges of genome sequencing, which allows for a more comprehensive analysis of the genome, including interpretation of structural and noncoding variants and also yields a high number of de novo variants for interpretation. We also consider the contribution of genetic mosaicism, both what it means for a molecular diagnosis in mosaic individuals and the important implications for genetic counseling.

PCDH19 Pathogenic Variants in Males: Expanding the Phenotypic Spectrum

Protocadherin-19 (PCDH19) pathogenic variants cause an infantile onset epilepsy syndrome called Girls Clustering Epilepsy due to the vast majority of affected individuals being female. This syndromic name was developed to foster early recognition and diagnosis in infancy. It has, however, sparked debate, as, there are rare males with postzygotic somatic, and therefore, mosaic, PCDH19 pathogenic variants with similar clinical features to females. Conversely, "transmitting" males with germline inherited PCDH19 variants are considered asymptomatic. To date, there has been no standardized neuropsychiatric assessment of males with PCDH19 pathogenic variants. Here, we studied 15 males with PCDH19 pathogenic variants (nine mosaic and six transmitting) aged 2 to 70 years. Our families completed a survey including standardized clinical assessments: Social Responsiveness Scale, Strengths and Difficulties Questionnaire, Behavior Rating Inventory of Executive Function, and Dimensional Obsessive-Compulsive Scale. We identified neuropsychiatric abnormalities in two males with germline PCDH19 possibly pathogenic variants. One had a prior history of a severe encephalopathic illness, which may have been unrelated. We also describe a non-penetrant somatic mosaic male with mosaicism confirmed in blood, but not identified in skin fibroblasts. Our data suggest that transmitting hemizygous males are generally unaffected, in contrast to males with postzygotic somatic mosaic variants who show a similar neuropsychiatric profile to females who are naturally mosaic, due to X-chromosome inactivation. The penetrance of PCDH19 pathogenic variants has been estimated to be 80%. Like females, not all mosaic males are affected. From our small sample, we estimate that males with mosaic PCHD19 pathogenic variants have a penetrance of 85%. With these insights into the male phenotypic spectrum of PCDH19 epilepsy, we propose the new term Clustering Epilepsy (CE). Both affected females and males typically present with infantile onset of clusters of seizures.

The role of PCDH19 in refractory status epilepticus

PCDH19-Girls Clustering Epilepsy (GCE) is an epileptic syndrome with infantile onset, characterized by clustered and fever-induced seizures, often associated with intellectual disability (ID) and autistic features. Seizures clusters could progress into status epilepticus (SE) with different semiology, both convulsive and nonconvulsive SE (NCSE), and often refractory to conventional antiepileptic drugs. We reviewed literature on PCDH19-GCE, in order to define prevalence, semiology, treatments, and outcome of SE. We conducted a comprehensive review of the PCDH19-GCE literature on the public databases PubMed and EMBASE from January 2008 to July 2019. An overall number of 59 full-text articles were selected, retrieved, and assessed for eligibility. We collected 269 cases with PCDH19-GCE, in 85 of them, a history of SE was reported. Prevalence of SE in all selected series of PCDH19-GCE series is 31.5%. Data on SE were fully exhaustive in 21 cases. There was no gender difference in SE occurrence. Median age at first SE occurrence was 12 months (6 months-11 years). Semiology of SE was reported in 17 cases: it was convulsive in 15 and nonconvulsive in 2. Status epilepticus was refractory in 15 out of 21 cases (71.4%). Benzodiazepine was the most commonly used drug for SE. Alternative treatments with steroids and ketogenic diet were reported as well. We found a high prevalence of ID and autism (19 out of 21 patients, 90%). Despite the relatively high frequency of SE in those patients, there are few specific descriptions of the semiology, EEG pattern, and treatment approach. We strongly believe that a multicenter study looking specifically at SE characteristics might improve the knowledge and consequently the overall outcome. This article is part of the Special Issue "Proceedings of the 7th London-Innsbruck Colloquium on Status Epilepticus and Acute Seizures".

A mutation update for the PCDH19 gene causing early-onset epilepsy in females with an unusual expression pattern

The PCDH19 gene consists of six exons encoding a 1,148 amino acid transmembrane protein, Protocadherin 19, which is involved in brain development. Heterozygous pathogenic variants in this gene are inherited in an unusual X-linked dominant pattern in which heterozygous females are affected, while hemizygous males are typically unaffected, although they pass on the pathogenic variant to each affected daughter. PCDH19-related disorder is known to cause early-onset epilepsy in females characterized by seizure clusters exacerbated by fever and in most cases, onset is within the first year of life. This condition was initially described in 1971 and in 2008 PCDH19 was identified as the underlying genetic etiology. This condition is the result of pathogenic loss-of-function variants that may be de novo or inherited from an affected mother or unaffected father and cellular interference has been hypothesized to be the culprit. Heterozygous females are symptomatic because of the presence of both wild-type and mutant cells that interfere with one another due to the production of different surface proteins, whereas nonmosaic hemizygous males produce a homogenous population of cells. Here, we review novel pathogenic variants in the PCDH19 gene since 2012 to date, and summarize any genotype-phenotype correlations.

A systematic review and meta-analysis of 271 PCDH19-variant individuals identifies psychiatric comorbidities, and association of seizure onset and disease severity

Epilepsy and Mental Retardation Limited to Females (EFMR) is an infantile onset disorder characterized by clusters of seizures. EFMR is due to mutations in the X-chromosome gene PCDH19, and is underpinned by cellular mosaicism due to X-chromosome inactivation in females or somatic mutation in males. This review characterizes the neuropsychiatric profile of this disorder and examines the association of clinical and molecular factors with neuropsychiatric outcomes. Data were extracted from 38 peer-reviewed original articles including 271 individual cases. We found that seizure onset ≤12 months was significantly associated (p = 4.127 × 10^-7) with more severe intellectual disability, compared with onset >12 months. We identified two recurrent variants p.Asn340Ser and p.Tyr366Leufs*10 occurring in 25 (20 unrelated) and 30 (11 unrelated) cases, respectively. PCDH19 mutations were associated with psychiatric comorbidities in approximately 60% of females, 80% of affected mosaic males, and reported in nine hemizygous males. Hyperactive, autistic, and obsessive-compulsive features were most frequently reported. There were no genotype-phenotype associations in the individuals with recurrent variants or the group overall. Age at seizure onset can be used to provide more informative prognostic counseling.

Somatic mosaicism and neurodevelopmental disease

Traditionally, we have considered genetic mutations that cause neurodevelopmental diseases to be inherited or de novo germline mutations. Recently, we have come to appreciate the importance of de novo somatic mutations, which occur postzygotically and are thus present in only a subset of the cells of an affected individual. The advent of next-generation sequencing and single-cell sequencing technologies has shown that somatic mutations contribute to normal and abnormal human brain development. Somatic mutations are one important cause of neuronal migration and brain overgrowth disorders, as suggested by visible focal lesions. In addition, somatic mutations contribute to neurodevelopmental diseases without visible lesions, including epileptic encephalopathies, intellectual disability, and autism spectrum disorder, and may contribute to a broad range of neuropsychiatric diseases. Studying somatic mutations provides insight into the mechanisms underlying human brain development and neurodevelopmental diseases and has important implications for diagnosis and treatment.

Mosaicism and incomplete penetrance of PCDH19 mutations

BACKGROUND

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 10 mosaic 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 with PCDH19 variants were identified by Sanger sequencing and Multiple Ligation Probe Amplification (MLPA). Microdroplet digital PCR was used to quantify the mutant allelic fractions (MAFs) in 20 families with PCDH19 variants.

RESULTS

Five mosaic individuals, four males and one female, 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 harbouring 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 the other 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, 1 female patient was identified as a mosaic with a blood MAF of 26.72%.

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.

Chinese cases of early infantile epileptic encephalopathy: a novel mutation in the PCDH19 gene was proved in a mosaic male- case report

Background

The link between the protocadherin-19 (PCDH19) gene and epilepsy suggests that an unusual form of X-linked inheritance affects females but is transmitted through asymptomatic males. Individuals with epilepsy associated with mutations in the PCDH19 gene display generalized or focal seizures with or without fever sensitivity. The clinical manifestation of the condition ranges from mild to severe, resulting in intellectual disability and behavioural disturbance. In the present study, we assessed mutations in the PCDH19 gene and the clinical features of a group of Chinese patients with early infantile epileptic encephalopathy and aimed to provide further insight into the understanding of epilepsy and mental retardation limited to females (EFMR; MIM 300088).

Case Presentation

We described three variations in the PCDH19 gene in Chinese patients with epilepsy who developed generalized seizures occurring in clusters with or without triggering by fever. Candidate genes were screened for mutations that cause epilepsy and related paroxysmal or nervous system diseases in the coding exons and intron–exon boundaries using polymerase chain reaction (PCR) of genomic deoxyribonucleic acid (DNA) followed by sequencing. The variations were sequenced using next-generation sequencing technology and verified with first-generation sequencing. Exome sequencing of a multigene epilepsy panel revealed three mutations in the PCDH19 gene in a mosaic male and two unrelated females. These included a frameshift mutation c.1508_1509insT (p.Thr504HisfsTer19), a missense mutation c.1681C > T (p.Pro561Ser) and a nonsense mutation c.918C > G (p.Tyr306Ter). Of the three mutations in the PCDH19 gene associated with early infantile epileptic encephalopathy, the frameshift variation in a mosaic male is novel and de novo, the missense variation is de novo and is the second ever reported in females, and the nonsense variation was inherited from the paternal line and is the first example discovered in a female.

Conclusions

The results from our current study provide new insight into and perspectives for the molecular genetic link between epilepsy and PCDH19 alterations. Moreover, our new findings of the male mosaic variant broaden the spectrum of PCDH19-related epilepsy and provide a new understanding of this complex genetic disorder.

Electronic supplementary material

The online version of this article (10.1186/s12881-018-0621-x) contains supplementary material, which is available to authorized users.