A family with seizures and minor features of tuberous sclerosis and a novel TSC2 mutation

Drug-Resistant Epilepsy in Tuberous Sclerosis Complex Is Associated With TSC2 Genotype: More Findings From the Preventing Epilepsy Using Vigatrin (PREVeNT) Trial

BACKGROUND

Children with tuberous sclerosis complex (TSC) are at high risk for drug-resistant epilepsy (DRE). The ability to stratify those at highest risk for DRE is important for counseling and prompt, aggressive management, necessary to optimize neurocognitive outcomes. Using the extensively phenotyped PREVeNT cohort, we aimed to characterize whether the TSC genotype was associated with DRE.

METHODS

The study group (N = 70) comprised participants with TSC enrolled at age less than or equal to six months with detailed epilepsy and other phenotypic and genotypic data, prospectively collected as part of the PREVeNT trial. Genotype-phenotype correlations of DRE, time to first abnormal electroencephalography, and time to epilepsy onset were compared using Fisher exact test and regression models.

RESULTS

Presence of a TSC2 pathogenic variant was significantly associated with DRE, compared with TSC1 and participants with no pathogenic mutation identified. In fact, all participants with DRE had a TSC2 pathogenic variant. Furthermore, TSC2 variants expected to result in no protein product were associated with higher risk for DRE. Finally, TSC1 pathogenic variants were associated with later-onset epilepsy, on average 21.2 months later than those with other genotypes.

CONCLUSIONS

Using a comprehensively phenotyped cohort followed from infancy, this study is the first to delineate genotype-phenotype correlations for epilepsy severity and onset in children with TSC. Patients with TSC2 pathogenic variants, especially TSC2 pathogenic variants predicted to result in lack of TSC2 protein, are at highest risk for DRE, and are likely to have earlier epilepsy onset than those with TSC1. Clinically, these insights can inform counseling, surveillance, and management.

A large deletion in TSC2 causes tuberous sclerosis complex by dysregulating PI3K/AKT/mTOR signaling pathway

BACKGROUND/AIM

Tuberous sclerosis complex (TSC) is a multi-system syndrome caused by loss-of-function mutation in TSC1 or TSC2. Most TSC patients present with cardiac rhabdomyoma or cortical tubers during fetal life, and the symptoms are not uniform as their age. The gene products of TSC1/2 are components of the TSC protein complex and are important role in the PI3K/AKT/mTOR (PAM) signaling pathway. Based on three members of a family with variable expressivity, the purpose of this study was to clarify the clinical features of TSC in different age groups and to analyze the genetic characteristics of TSC2 gene.

METHODS

Clinical exome sequencing and co-segregation were used to identify a three-generation family with four affected individuals. HEK-293T cell model was constructed for subsequent experiments. Quantitative RT-PCR, western blotting, and subcellular localization were used to analyze the expression effect of TSC2 mutation. CCK-8 assay, wound healing assay, and cell cycle analysis were used to analyze the function effect of TSC2 mutation.

RESULT

We identified a TSC family with heterozygous deletion of exon 4 in TSC2 by clinical exon sequencing. Sanger sequencing indicated that the affected individuals have 2541-bp deletion that encompassed exon 4 and adjacent introns. Deletion of exon 4 decreased the TSC2 mRNA and protein levels in HEK-293T cells, and activated the PI3K/AKT/mTOR pathway, thereby altering the cell cycle and promoting cell proliferation and migration.

CONCLUSION

We confirmed the pathogenicity of the large deletion in TSC2 in a three- generations family.. Deletion of exon 4 of TSC2 affected cell proliferation, migration, and cell cycle via abnormal activation of the PAM pathway. This study evaluated the pathogenic effect of deletion of exon 4 of TSC2 and investigated the underlying mechanism.

An overview of actionable and potentially actionable TSC1 and TSC2 germline variants in an online Database

Tuberous Sclerosis Complex (TSC) is caused by loss of function germline variants in the TSC1 or TSC2 tumor suppressor genes. Genetic testing for the detection of pathogenic variants in either TSC1 or TSC2 was implemented as a diagnostic criterion for TSC. However, TSC molecular diagnosis can be challenging due to the absence of variant hotspots and the high number of variants described. This review aimed to perform an overview of TSC1/2 variants submitted in the ClinVar database. Variants of uncertain significance (VUS), missense and single nucleotide variants were the most frequent in clinical significance (37-40%), molecular consequence (37%-39%) and variation type (82%-83%) categories in ClinVar in TSC1 and TSC2 variants, respectively. Frameshift and nonsense VUS have potential for pathogenic reclassification if further functional and segregation studies were performed. Indeed, there were few functional assays deposited in the database and literature. In addition, we did not observe hotspots for variation and many variants presented conflicting submissions regarding clinical significance. This study underscored the importance of disseminating molecular diagnostic results in a public database to render the information largely accessible and promote accurate diagnosis. We encourage the performance of functional studies evaluating the pathogenicity of TSC1/2 variants.

Mild TSC Phenotype and Non-Penetrance Associated with a Frameshift Variant in TSC2 Prompts Caution in Evaluating Pathogenicity of Frameshift Variants

INTRODUCTION

Technological advances in genetic testing, particularly the adoption of noninvasive prenatal screening (NIPS) for single gene disorders such as tuberous sclerosis complex (TSC, OMIM# 613254), mean that putative/possible pathogenetic DNA variants can be identified prior to the appearance of a disease phenotype. Without a phenotype, accurate prediction of variant pathogenicity is crucial. Here, we report a TSC2 frameshift variant, NM_000548.5(TSC2):c.4255_4256delCA, predicted to result in nonsense-mediated mRNA decay (NMD) and cessation of TSC2 protein production and thus pathogenic according to ACMG criteria, identified by NIPS and subsequently detected in family members with few or no symptoms of TSC. Due to the lack of TSC-associated features in the family, we hypothesized that the deletion created a non-canonical 5' donor site resulting in cryptic splicing and a transcript encoding active TSC2 protein. Verifying the predicted effect of the variant was key to designating pathogenicity in this case and should be considered for other frameshift variants in other genetic disorders.

METHODS

Phenotypic information on the family members was collected via review of the medical records and patient reports. RNA studies were performed using proband mRNA isolated from blood lymphocytes for RT-PCR and Sanger sequencing. Functional studies were performed by transient expression of the TSC2 variant proteins in cultured cells, followed by immunoblotting.

RESULTS

No family members harboring the variant met any major clinical diagnostic criteria for TSC, though a few minor features non-specific to TSC were present. RNA studies supported the hypothesis that the variant caused cryptic splicing, resulting in an mRNA transcript with an in-frame deletion of 93 base pairs r.[4255_4256del, 4251_4343del], p.[(Gln1419Valfs*104), (Gln1419_Ser1449del)]. Expression studies demonstrated that the canonical function of the resulting truncated TSC2 p.Gln1419_Ser1449del protein product was maintained and similar to wildtype.

CONCLUSION

Although most frameshift variants are likely to result in NMD, the NM_000548.5(TSC2):c.4255_4256delCA variant creates a cryptic 5' splice donor site, resulting in an in-frame deletion that retains TSC2 function, explaining why carriers of the variant do not have typical features of TSC. The information is important for this family and others with the same variant. Equally important is the lesson that predictions can be inaccurate, and that caution should be used when designating frameshift variants as pathogenic, especially when phenotypic information to corroborate testing results is unavailable. Our work demonstrates that functional RNA- and protein-based confirmation of the effects of DNA variants improves molecular genetic diagnostics.

TSC1 and TSC2: Tuberous Sclerosis Complex and Its Related Epilepsy Phenotype

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by a multisystemic involvement. In TSC, reduced function of TSC1 and TSC2 genes products (hamartin and tuberin, respectively) leads to an hyperactivation of the mechanistic target of rapamycin (mTOR) pathway and to a consequent cell growth dysregulation. In TSC patients, neurological and neuropsychiatric manifestations, especially epilepsy and neuropsychiatric comorbidities such as autism or intellectual disability, represent the most disabling features. In particular, epilepsy occurrs up to 80% of patients, is often drug resistant and is frequently associated with neurological impairment. Due to the burden of this morbidity, different treatment strategies have been proposed with the purpose to make patients epilepsy free, such as the use of different antiepileptic drugs like vigabatrin, carbamazepine, valproic acid, and levetiracetam. More recently, a mTOR inhibitor (i.e. everolimus) has showed promising results in terms of seizures reduction.

Epilepsy Risk Prediction Model for Patients With Tuberous Sclerosis Complex

BACKGROUND

Individuals with tuberous sclerosis complex are at increased risk of epilepsy. Early seizure control improves developmental outcomes, making identifying at-risk patients critically important. Despite several identified risk factors, it remains difficult to predict. The purpose of the study was to evaluate the combined risk prediction of previously identified risk factors for epilepsy in individuals with tuberous sclerosis complex.

METHODS

The study group (n = 333) consisted of individuals with tuberous sclerosis complex who were enrolled in the Tuberous Sclerosis Complex Autism Center of Excellence Research Network and UT TSC Biobank. The outcome was defined as having an epilepsy diagnosis. Potential risk factors included sex, TSC genotype, and tuber presence. Logistic regression was used to calculate the odds ratio and P value for the association between each variable and epilepsy. A clinical risk prediction model incorporating all risk factors was built. Area under the curve was calculated to characterize the full model's ability to discriminate individuals with tuberous sclerosis complex with and without epilepsy.

RESULTS

The strongest risk for epilepsy was presence of tubers (95% confidence interval: 2.39 to 10.89). Individuals with pathogenic TSC2 variants were three times more likely (95% confidence interval: 1.55 to 6.36) to develop seizures compared with those with tuberous sclerosis complex from other causes. The combination of risk factors resulted in an area under the curve 0.73.

CONCLUSIONS

Simple characteristics of patients with tuberous sclerosis complex can be combined to successfully predict epilepsy risk. A risk assessment model that incorporates sex, TSC genotype, protective TSC2 missense variant, and tuber presence correctly predicts epilepsy in 73% of patients with tuberous sclerosis complex.

Tuberous Sclerosis Complex Genotypes and Developmental Phenotype

BACKGROUND

Children with tuberous sclerosis complex (TSC), caused by pathogenic variants in TSC1/TSC2, are at risk for intellectual disability. TSC2 pathogenic variants appear to increase the risk, compared with TSC1. However, the effect of TSC2 pathogenic variants on early and specific domains of development hasn't been studied. Using an extensively phenotyped group, we aimed to characterize differences in early intellectual development between genotypes.

METHODS

The study group (n = 92) included participants with TSC enrolled in a multicenter study involving genetic testing and detailed prospective phenotyping including the Mullen Scales of Early Learning, a validated measure of cognition, language, and motor development in babies and preschool children. Mean T-scores at 24 months for each Mullen Scales of Early Learning domain were calculated for children with, versus without, a TSC2 pathogenic variant. Multivariable linear regression models were used to compare the groups, adjusting for seizures.

RESULTS

T-scores on every Mullen Scales of Early Learning domain were significantly worse in the TSC2 group. Below average composite scores were present in three-fourths of the TSC2 group, compared with one-fourth of those without TSC2. Having a TSC2 pathogenic variant was associated with lower composite Mullen Scales of Early Learning scores, even when corrected for seizures.

CONCLUSIONS

In a well-characterized patient population with standardized assessment of multiple aspects of development, we found that having a TSC2 pathogenic variant was associated with significantly lower Mullen Scales of Early Learning scores at age 24 months, independent of seizures. These data suggest that a baby with a TSC2 pathogenic variant is at high risk for significant developmental delays by 24 months.

Whole exome sequencing identifies a novel intron heterozygous mutation in TSC2 responsible for tuberous sclerosis complex

This study was aimed to identify the potentially pathogenic gene variants that contribute to the etiology of the tuberous sclerosis complex. A Chinese pedigree with tuberous sclerosis complex was collected and the exomes of two affected individuals were sequenced using the whole exome sequencing technology. The resulting variants from whole exome sequencing were filtered by basic and advanced biological information analysis and the candidate mutation was verified as heterozygous by sanger sequencing. After basic and advanced biological information analysis, a total of 9 single nucleotide variants were identified, which were all follow the dominant inheritance pattern. Among which, the intron heterozygous mutation c.600-145 C > T transition in TSC2 was identified and validated in the two affected individuals. In silico analysis with human splicing finder (HSF) predicted the effect of the c.600-145 C > T mutations on TSC2 mRNA splicing, and detected the creation of a new exonic cryptic donor site, which would result in a frame-shift, and finally premature termination codon. Our results reported the novel intron heterozygous mutation c.600-145 C > T in TSC2 may contribute to TSC, expanding our understanding of the causally relevant genes for this disorder.

Genetics, genomics, and genotype-phenotype correlations of TSC: Insights for clinical practice

Tuberous Sclerosis Complex (TSC) is a multisystem autosomal dominant condition caused by inactivating pathogenic variants in either the TSC1 or the TSC2 gene, leading to hyperactivation of the mTOR pathway. Here, we present an update on the genetic and genomic aspects of TSC, with a focus on clinical and laboratory practice. We briefly summarize the structure of TSC1 and TSC2 as well as their protein products, and discuss current diagnostic testing, addressing mosaicism. We consider genotype-phenotype correlations as an example of precision medicine, and discuss genetic counseling in TSC, with the aim of providing geneticists and health care practitioners involved in the care of TSC individuals with useful tools for their practice.

A novel TSC2 missense variant associated with a variable phenotype of tuberous sclerosis complex: case report of a Chinese family

Background

Tuberous sclerosis complex (TSC) is an autosomal dominant genetic disorder characterized by the development of hamartomas in multiple organs, including the brain, heart, skin, kidney, lung and retina. A diagnosis of TSC is established with a recently revised clinical/radiological set of criteria and/or a causative mutation in TSC1 or TSC2 gene.

Case presentation

We report a Chinese TSC family with two siblings presenting with multiple hypomelanotic macules, cardiac rhabdomyomas and cortical tubers associated with a small subependymal nodule. The older child had seizures. A novel heterozygous missense variant in the TSC2 gene (c.899G > T, p.G300 V) was identified and shown to be inherited from their father as well as paternal grandfather, both of whom presented with variable TSC-associated signs and symptoms.

Conclusion

We identified a novel heterozygous TSC2 variant c.899G > T as the causative mutation in a Chinese family with TSC, resulting in wide intrafamilial phenotypic variability. Our study illustrates the importance of clinical evaluation and genetic testing for family members of the patient affected with TSC.

TSC2 c.1864C>T variant associated with mild cases of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominantly inherited disorder with variable expressivity associated with hamartomatous tumors, abnormalities of the skin, and neurologic problems including seizures, intellectual disability, and autism. TSC is caused by pathogenic variants in either TSC1 or TSC2. In general, TSC2 pathogenic variants are associated with a more severe phenotype than TSC1 pathogenic variants. Here, we report a pathogenic TSC2 variant, c.1864C>T, p.(Arg622Trp), associated with a mild phenotype, with most carriers meeting fewer than two major clinical diagnostic criteria for TSC. This finding has significant implications for counseling patients regarding prognosis. More patient data are required before changing the surveillance recommendations for patients with the reported variant. However, consideration should be given to tailoring surveillance recommendations for all pathogenic TSC1 and TSC2 variants with documented milder clinical sequelae. © 2017 Wiley Periodicals, Inc.

Rare familial TSC2 gene mutation associated with atypical phenotype presentation of Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is a neurocutaneous disorder that results from mutations within either the TSC1 gene or the TSC2 gene. Diagnosis is based on well-established clinical criteria or genetic criteria. We describe an 18-month-old boy who presented with seizures and a single hypopigmented macule. He did not meet consensus criteria for the clinical diagnosis of TSC. Exome sequencing revealed a heterozygous TSC2 mutation (c.5138G>A (p.Arg1713His)) in the patient. This heterozygous alteration was detected in his mother as well as several other maternal family members. The mother and other family members with the mutation were asymptomatic except for the presence of hypopigmented macules. The phenotypic characteristics of the individuals in this family were not suggestive of a TSC2 mutation as none satisfied the clinical criteria for even a diagnosis of possible TSC. This case provides evidence for a unique TSC2 mutation that resulted in an atypical clinical presentation and indicates potential shortcomings of the current diagnostic criteria for TSC. These findings may have implications for genetic counseling and screening. © 2017 Wiley Periodicals, Inc.

Tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that affects multiple organ systems and is caused by loss-of-function mutations in one of two genes: TSC1 or TSC2. The disorder can affect both adults and children. First described in depth by Bourneville in 1880, it is now estimated that nearly 2 million people are affected by the disease worldwide. The clinical features of TSC are distinctive and can vary widely between individuals, even within one family. Major features of the disease include tumours of the brain, skin, heart, lungs and kidneys, seizures and TSC-associated neuropsychiatric disorders, which can include autism spectrum disorder and cognitive disability. TSC1 (also known as hamartin) and TSC2 (also known as tuberin) form the TSC protein complex that acts as an inhibitor of the mechanistic target of rapamycin (mTOR) signalling pathway, which in turn plays a pivotal part in regulating cell growth, proliferation, autophagy and protein and lipid synthesis. Remarkable progress in basic and translational research, in addition to several randomized controlled trials worldwide, has led to regulatory approval of the use of mTOR inhibitors for the treatment of renal angiomyolipomas, brain subependymal giant cell astrocytomas and pulmonary lymphangioleiomyomatosis, but further research is needed to establish full indications of therapeutic treatment. In this Primer, we review the state-of-the-art knowledge in the TSC field, including the molecular and cellular basis of the disease, medical management, major knowledge gaps and ongoing research towards a cure.

Variants Within TSC2 Exons 25 and 31 Are Very Unlikely to Cause Clinically Diagnosable Tuberous Sclerosis

Inactivating mutations in TSC1 and TSC2 cause tuberous sclerosis complex (TSC). The 2012 international consensus meeting on TSC diagnosis and management agreed that the identification of a pathogenic TSC1 or TSC2 variant establishes a diagnosis of TSC, even in the absence of clinical signs. However, exons 25 and 31 of TSC2 are subject to alternative splicing. No variants causing clinically diagnosed TSC have been reported in these exons, raising the possibility that such variants would not cause TSC. We present truncating and in‐frame variants in exons 25 and 31 in three individuals unlikely to fulfil TSC diagnostic criteria and examine the importance of these exons in TSC using different approaches. Amino acid conservation analysis suggests significantly less conservation in these exons compared with the majority of TSC2 exons, and TSC2 expression data demonstrates that the majority of TSC2 transcripts lack exons 25 and/or 31 in many human adult tissues. In vitro assay of both exons shows that neither exon is essential for TSC complex function. Our evidence suggests that variants in TSC2 exons 25 or 31 are very unlikely to cause classical TSC, although a role for these exons in tissue/stage specific development cannot be excluded.

Genotype/Phenotype Correlations in Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by the development of widespread hamartomatous lesions in various organs, including brain, skin, kidneys, heart, and eyes. Central nervous system is almost invariably involved, with up to 85% of patients presenting with epilepsy, and at least half of patients having intellectual disability or other neuropsychiatric disorders including autism spectrum disorder. TSC is caused by the mutation in one of the 2 genes TSC1, at 9q34, and TSC2, at 16p13.3. They respectively encode for hamartin and tuberin, which form an intracellular complex inhibiting the mammalian target of rapamycin. Mammalian target of rapamycin overactivation following the genetic defect determines the cell growth and proliferation responsible for TSC-related lesions, as well as the alterations in neuronal excitability and synaptogenesis leading to epilepsy and neuropsychiatric disorders. A causative mutation for the disorder is identified in about 85% of patients with a clinical diagnosis of TSC. Mosaicism and technology limits likely explain most of the no mutation identified cases. This review confirms that patients with TSC2 mutations considered as a group usually present a more severe phenotype, characterized by higher number of tubers, earlier age at seizure onset and higher prevalence of intellectual disability. However, the clinical phenotype of the disease presents a high variability, thus making the prediction of the phenotype on an individual basis still challenging. The increasing application of new molecular techniques to subjects with TSC has the potential to significantly reduce the rate of patients with no mutation demonstrated and to identify an increasing higher number of mutations. This would hopefully allow a better characterization of higher risk mutations, which might help clinicians to plan individualized surveillance plans. Furthermore, the increasing availability of disease registries to collect clinical and genetics data of patients help to define more valid and clinically oriented genotype or phenotype correlations.

Response to everolimus is seen in TSC-associated SEGAs and angiomyolipomas independent of mutation type and site in TSC1 and TSC2

Tuberous sclerosis complex is an autosomal dominant disorder that occurs owing to inactivating mutations in either TSC1 or TSC2. Tuberous sclerosis complex-related tumors in the brain, such as subependymal giant cell astrocytoma, and in the kidney, such as angiomyolipoma, can cause significant morbidity and mortality. Recently, randomized clinical trials (EXIST-1 and EXIST-2) of everolimus for each of these tuberous sclerosis complex-associated tumors demonstrated the benefit of this drug, which blocks activated mammalian target of rapamycin complex 1. Here we report on the spectrum of mutations seen in patients treated during these trials and the association between mutation and response. TSC2 mutations were predominant among patients in both trials and were present in nearly all subjects with angiomyolipoma in whom a mutation was identified (97%), whereas TSC1 mutations were rare in those subjects (3%). The spectrum of mutations seen in each gene was similar to those previously reported. In both trials, there was no apparent association between mutation type or location within each gene and response to everolimus. Everolimus responses were also seen at a similar frequency for the 16–18% of patients in each trial in whom no mutation in either gene was identified. These observations confirm the strong association between TSC2 mutation and angiomyolipoma burden seen in previous studies, and they indicate that everolimus response occurs regardless of mutation type or location or when no mutation in TSC1 or TSC2 has been identified.

The neural crest lineage as a driver of disease heterogeneity in Tuberous Sclerosis Complex and Lymphangioleiomyomatosis

Lymphangioleiomyomatosis (LAM) is a rare neoplastic disease, best characterized by the formation of proliferative nodules that express smooth muscle and melanocytic antigens within the lung parenchyma, leading to progressive destruction of lung tissue and function. The pathological basis of LAM is associated with Tuberous Sclerosis Complex (TSC), a multi-system disorder marked by low-grade tumors in the brain, kidneys, heart, eyes, lung and skin, arising from inherited or spontaneous germ-line mutations in either of the TSC1 or TSC2 genes. LAM can develop either in a patient with TSC (TSC-LAM) or spontaneously (S-LAM), and it is clear that the majority of LAM lesions of both forms are characterized by an inactivating mutation in either TSC1 or TSC2, as in TSC. Despite this genetic commonality, there is considerable heterogeneity in the tumor spectrum of TSC and LAM patients, the basis for which is currently unknown. There is extensive clinical evidence to suggest that the cell of origin for LAM, as well as many of the TSC-associated tumors, is a neural crest cell, a highly migratory cell type with extensive multi-lineage potential. Here we explore the hypothesis that the types of tumors that develop and the tissues that are affected in TSC and LAM are dictated by the developmental timing of TSC gene mutations, which determines the identities of the affected cell types and the size of downstream populations that acquire a mutation. We further discuss the evidence to support a neural crest origin for LAM and TSC tumors, and propose approaches for generating humanized models of TSC and LAM that will allow cell of origin theories to be experimentally tested. Identifying the cell of origin and developing appropriate humanized models is necessary to truly understand LAM and TSC pathology and to establish effective and long-lasting therapeutic approaches for these patients.

Graded loss of tuberin in an allelic series of brain models of TSC correlates with survival, and biochemical, histological and behavioral features

Tuberous sclerosis complex (TSC) is a neurodevelopmental disorder with prominent brain manifestations due to mutations in either TSC1 or TSC2. Here, we describe novel mouse brain models of TSC generated using conditional hypomorphic and null alleles of Tsc2 combined with the neuron-specific synapsin I cre (SynIcre) allele. This allelic series of homozygous conditional hypomorphic alleles (Tsc2(c-del3/c-del3)SynICre(+)) and heterozygote null/conditional hypomorphic alleles (Tsc2(k/c-del3)SynICre(+)) achieves a graded reduction in expression of Tsc2 in neurons in vivo. The mice demonstrate a progressive neurologic phenotype including hunchback, hind limb clasp, reduced survival and brain and cortical neuron enlargement that correlates with a graded reduction in expression of Tsc2 in the two sets of mice. Both models also showed behavioral abnormalities in anxiety, social interaction and learning assays, which correlated with Tsc2 protein levels as well. The observations demonstrate that there are graded biochemical, cellular and clinical/behavioral effects that are proportional to the extent of reduction in Tsc2 expression in neurons. Further, they suggest that some patients with milder manifestations of TSC may be due to persistent low-level expression of functional protein from their mutant allele. In addition, they point to the potential clinical benefit of strategies to raise TSC2 protein expression from the wild-type allele by even modest amounts.

Epilepsy in Tuberous Sclerosis Complex

Tuberous Sclerosis Complex (TSC) is an autosomal dominant multisystem disorder, characterized by the presence of hamartomatous lesions involving different organ systems, including the brain. Epilepsy is the most common presenting symptom, representing a major source of morbidity and mortality. Despite multiple antiepileptic drug combinations, in about two thirds of cases the patients present high-frequency drug-resistant epilepsy, and nonpharmacologic options may be considered. The aim of this work was to point out the current knowledge on epileptogenesis in TSC, the available medical therapies and diagnostic tools, and possible surgical strategies, with the intent to better understand the actual difficulties in controlling seizures and the results reported in the literature. There is also a section dedicated to the common association with cognitive impairment and the role of epilepsy control on its outcome.

Genotype and cognitive phenotype of patients with tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant, multisystem disorder, which affects 1 in 6000 people. About half of these patients are affected by mental retardation, which has been associated with TSC2 mutations, epilepsy severity and tuber burden. The bimodal intelligence distribution in TSC populations suggests the existence of subgroups with distinct pathophysiologies, which remain to be identified. Furthermore, it is unknown if heterozygous germline mutations in TSC2 can produce the neurocognitive phenotype of TSC independent of epilepsy and tubers. Genotype-phenotype correlations may help to determine risk profiles and select patients for targeted treatments. A retrospective chart review was performed, including a large cohort of 137 TSC patients who received intelligence assessment and genetic mutation analysis. The distribution of intellectual outcomes was investigated for selected genotypes. Genotype-neurocognitive phenotype correlations were performed and associations between specific germline mutations and intellectual outcomes were compared. Results showed that TSC1 mutations in the tuberin interaction domain were significantly associated with lower intellectual outcomes (P<0.03), which was also the case for TSC2 protein-truncating and hamartin interaction domain mutations (both P<0.05). TSC2 missense mutations and small in-frame deletions were significantly associated with higher IQ/DQs (P<0.05). Effects related to the mutation location within the TSC2 gene were found. These findings suggest that TSC2 protein-truncating mutations and small in-frame mutations are associated with distinctly different intelligence profiles, providing further evidence that different types and locations of TSC germline mutations may be associated with distinct neurocognitive phenotypes.

Evidence for population variation in TSC1 and TSC2 gene expression

BACKGROUND

Tuberous sclerosis complex (TSC) is an autosomal dominant neurogenetic disorder caused by mutations in one of two genes, TSC1 or TSC2, which encode the proteins hamartin and tuberin, respectively 123. Common features of TSC include intractable epilepsy, mental retardation, and autistic features. TSC is associated with specific brain lesions, including cortical tubers, subependymal nodules and subependymal giant cell astrocytomas. In addition, this disease frequently produces characteristic tumors, termed hamartomas, in the kidneys, heart, skin, retina, and lungs. Disease severity in TSC can be quite variable and is not determined by the primary mutation alone. In fact, there is often considerable variability in phenotype within single families, where all affected individuals carry the same mutation. Factors suspected to influence phenotype in TSC include the specific primary mutation, random occurrence of second-hit somatic mutations, mosaicism, "modifying genes", and environmental factors. In addition to these factors, we hypothesize that differences in mRNA expression from the non-mutated TSC allele, or possibly from the mutated allele, play a part in modifying disease severity. Common genetic variants that regulate mRNA expression have previously been shown to play important roles in human phenotypic variability, including disease susceptibility. A prediction based on this idea is that common regulatory variants that influence disease severity in TSC should be detectable in non-affected individuals.

METHODS

A PCR/primer extension assay was used to measure allele specific expression of TSC1 and TSC2 mRNAs in leukocytes isolated from normal volunteers. This assay can be used to measure "allelic expression imbalance" (AEI) in individuals by making use of heterozygous "marker" single nucleotide polymorphisms (SNPs) located within their mRNA.

RESULTS

In this study we show for the first time that TSC1 and TSC2 genes exhibit allele-specific differences in mRNA expression in blood leukocytes isolated from normal individuals.

CONCLUSIONS

These results support the possibility that allele-specific variation in TSC mRNA expression contributes to the variable severity of symptoms in TSC patients.

Epilepsy secondary to tuberous sclerosis: lessons learned and current challenges

BACKGROUND

In tuberous sclerosis complex (TSC), a substantially increased risk of developing epilepsy is present as a result of a disruption of a TSC gene expression in the brain and secondary abnormal cellular differentiation, migration, and proliferation. Dysregulated excitation probably has its roots in the disruption of GABAergic interneuron development. There is an age-dependent electroclinical expression of seizures, and epilepsy is often quite severe and unremitting.

DISCUSSION

The majority of patients (>60%) who are candidates for surgery remain seizure-free after tuberectomy. During the recent years technical advances in the localization of the epileptogenic zone during the recent years have lead to a 63% of Engel class I status after surgery compared with a previous 52%. In medically refractory patients not suitable for surgery, vagus nerve stimulation has proved efficacy in significantly reducing seizure frequency in more than 50% of cases. New evidence suggests that mTOR inhibitors may be helpful in the management of intractable epilepsy for individuals with TSC.

A hypomorphic allele of Tsc2 highlights the role of TSC1/TSC2 in signaling to AKT and models mild human TSC2 alleles

Tuberous sclerosis complex (TSC) is a tumor suppressor gene syndrome in which hamartomas develop in multiple organ systems. Knockout and conditional alleles of Tsc1 and Tsc2 have been previously reported. Here, we describe the generation of a novel hypomorphic allele of Tsc2 (del3), in which exon 3, encoding 37 amino acids near the N terminus of tuberin, is deleted. Embryos homozygous for the del3 allele survive until E13.5, 2 days longer than Tsc2 null embryos. Embryos die from underdevelopment of the liver, deficient hematopoiesis, aberrant vascular development and hemorrhage. Mice that are heterozygous for the del3 allele have a markedly reduced kidney tumor burden in comparison with conventional Tsc2(+/-) mice. Murine embryo fibroblast (MEF) cultures that are homozygous for the del3 allele express mutant tuberin at low levels, and show enhanced activation of mTORC1, similar to Tsc2 null MEFs. Furthermore, the mutant cells show prominent reduction in the activation of AKT. Similar findings were made in the analysis of homozygous del3 embryo lysates. Tsc2-del3 demonstrates GTPase activating protein activity comparable to that of wild-type Tsc2 in a functional assay. These findings indicate that the del3 allele is a hypomorphic allele of Tsc2 with partial function due to reduced expression, and highlight the consistency of AKT downregulation when Tsc1/Tsc2 function is reduced. Tsc2-del3 mice also serve as a model for hypomorphic TSC2 missense mutations reported in TSC patients.

Three independent mutations in the TSC2 gene in a family with tuberous sclerosis

Tuberous sclerosis complex (TSC) is a rare autosomal dominant disorder characterized by hamartomas and hamartias in multiple organs. TSC is caused by a wide spectrum of mutations within the TSC1 and TSC2 genes. Here, we report a unique family with three independent pathological mutations in TSC2. A c.1322G>A mutation in exon 12 created a stop codon, whereas a second mutation in exon 23 (c.2713C>T) was a missense change. The third mutation was a 4 base pair deletion in intron 20 of TSC2. We showed that this mutation was responsible for abnormal splicing. The three mutations were most likely de novo, as parents of affected patients did not present any features of TSC. In addition, we showed gonadal mosaicism in a branch of the family. To our knowledge, several independent mutations in TSC2 have never been observed in a single family. The probability of finding a family with three different pathological TSC2 mutations is extremely low. We discuss two main hypotheses that may be raised to explain this recurrence: (i) the TSC2 mutation rate is underestimated. In such a case, the likelihood of finding a family with three independent mutations in TSC2 may not be dramatically low; (ii) a heritable defect in a DNA repair gene (eg, mismatch repair gene) segregating in the family that is unlinked to the TSC2 gene might predispose to the occurrence of multiple TSC2 gene mutations, used as a specific target during embryogenesis.

Recent advances in neurobiology of Tuberous Sclerosis Complex

Tuberous Sclerosis Complex (TSC) is a multisystem genetic disorder with variable phenotypic expression, due to a mutation in one of the two genes, TSC1 and TSC2, and a subsequent hyperactivation of the downstream mTOR pathway, resulting in increased cell growth and proliferation. The central nervous system is consistently involved in TSC, with 90% of individuals affected showing structural abnormalities, and almost all having some degree of CNS clinical manifestations, including seizures, cognitive impairment and behavioural problems. TSC is proving to be a particularly informative model for studying contemporary issues in developmental neurosciences. Recent advances in the neurobiology of TSC from molecular biology, molecular genetics, and animal model studies provide a better understanding of the pathogenesis of TSC-related neurological symptoms. Rapamycin normalizes the dysregulated mTOR pathway, and recent clinical trials have demonstrated its efficacy in various TSC manifestations, suggesting the possibility that rapamycin may have benefit in the treatment of TSC brain disease.

Distinct clinical characteristics of tuberous sclerosis complex patients with no mutation identified

Tuberous Sclerosis Complex (TSC) is a multi-system disorder that is highly variable in its clinical presentation. Current molecular diagnostic methods permit identification of mutations in either TSC1 or TSC2 in 75-85% of TSC patients. Here we examine the clinical characteristics of those TSC patients who have no mutation identified (NMI). A retrospective review of our patient population that had comprehensive testing for mutations in TSC1/TSC2 identified 23/157 (15%) that were NMI. NMI patients had a lower incidence of brain findings on imaging studies, neurological features, and renal findings than those with TSC2 mutations. In contrast, NMI patients had a lower incidence of seizures than TSC patients with TSC1 mutations, but had a higher incidence of both renal angiomyolipomas and pulmonary lymphangioleiomyomatosis. This distinct constellation of findings suggest that NMI patients may have a unique molecular pathogenesis, different from that seen in TSC patients with the usual mutations in TSC1 and TSC2. We suggest that the mechanisms of disease in these patients include both mosaicism for a TSC2 mutation, and unusual non-coding region mutations in TSC2.

A reliable cell-based assay for testing unclassified TSC2 gene variants

Tuberous sclerosis complex (TSC) is characterised by seizures, mental retardation and the development of hamartomas in a variety of organs and tissues. The disease is caused by mutations in either the TSC1 gene or the TSC2 gene. The TSC1 and TSC2 gene products, TSC1 and TSC2, form a protein complex that inhibits signal transduction to the downstream effectors of the mammalian target of rapamycin (mTOR). We have developed a straightforward, semiautomated in-cell western (ICW) assay to investigate the effects of amino acid changes on the TSC1-TSC2-dependent inhibition of mTOR activity. Using this assay, we have characterised 20 TSC2 variants identified in individuals with TSC or suspected of having the disease. In 12 cases, we concluded that the identified variant was pathogenic. The ICW is a rapid, reproducible assay, which can be applied to the characterisation of the effects of novel TSC2 variants on the activity of the TSC1-TSC2 complex.

Intrafamilial phenotypic variability in tuberous sclerosis complex

Clinical manifestations were retrospectively assessed in 5 families with tuberous sclerosis complex, including 1 pair of monozygotic twins. Interfamilial variation in tuber count was significantly larger than intrafamilial variation. Severity of epilepsy and cognitive profiles varied both between and within families, particularly between the monozygotic twins, and IQ was inversely related to tuber count. Cutaneous, renal, and cardiac findings did not appear to cluster within families. Although the monozygotic twins displayed similar physical manifestations of tuberous sclerosis complex (renal and cardiac hamartomas), they differed markedly in neurocognitive profiles. Phenotypic variation within these families may be explained largely as a function of the randomness of second-hit events that cause hamartomas in tuberous sclerosis complex or by as-yet-unidentified genetic modifiers. Familial variation in tuberous sclerosis complex phenotype has important implications for genetic counseling.

Epilepsy surgery in tuberous sclerosis: a systematic review

PURPOSE

Tuberous sclerosis complex (TSC) is often associated with intractable epilepsy. Although epilepsy surgery has gained interest in recent years uncertainties exist about which patients are good surgical candidates. A systematic review of the available literature has been undertaken to assess the overall outcome of epilepsy surgery and identify risk factors of seizure recurrence.

METHODS

We searched MEDLINE, Embase, and bibliographies of reviews and book chapters to identify articles published in English since 1960. Twenty-five articles, describing postoperative seizure outcome and type of surgery in 177 TSC patients, were included in this study. Seizure outcome was analyzed both as seizure freedom and good outcome, including patients with >90% seizure reduction.

RESULTS

Seizure freedom was achieved in 101 patients (57%). Seizure frequency was improved by > 90% in 32 patients (18%). Moderate or severe intellectual disability (IQ < 70) (RR 1.8; 95% CI 1.2-2.8) and the presence of tonic seizures (RR 1.7; 95 % CI 1.2-2.4) were related to seizure recurrence.

CONCLUSIONS

A relation between multiple seizures types with early onset, multiple cortical tubers and multifocal epileptogenicity, and poor outcome is not supported by this systematic analysis. Although there is considerable variation among studies reviewed here, the literature suggests that resective surgery may offer benefit in a selected population of TSC patients with drug-resistant epilepsy.

Unusually mild tuberous sclerosis phenotype is associated with TSC2 R905Q mutation

OBJECTIVE

To report the clinical manifestations and functional aspects of Tuberous Sclerosis Complex (TSC), resulting from Codon 905 mutations in TSC2 gene.

METHODS

We performed a detailed study of the TSC phenotype and genotype in a large French-Canadian kindred (Family A). Subsequently, clinical and molecular data on 18 additional TSC families with missense mutations at the same codon of TSC2 were collected. Functional studies were performed on the different missense changes and related to the phenotype.

RESULTS

A 2714G>A (R905Q) mutation was identified in Family A. The TSC phenotype in this family was unusually mild and characterized by hypomelanotic macules or focal seizures that remitted spontaneously or were easily controlled with medication. Diagnostic criteria were met in only a minority of mutation carriers. Other families with the R905Q mutation were found to have a similar mild phenotype. In contrast, patients with a 2713C>T (R905W) or a 2713C>G (R905G) mutation had more severe phenotypes. Although all three amino acid substitutions were pathogenic, the R905W and R905G substitutions affected tuberin function more severely than R905Q.

INTERPRETATION

Codon 905 missense mutations in TSC2 are relatively common. The TSC2 R905Q mutation is associated with unusually mild disease, consistent with functional studies. Combined with previous reports, it is apparent that certain TSC2 missense mutations are associated with a mild form of tuberous sclerosis, which in many patients does not meet standard diagnostic criteria. These findings have implications for the large number of patients with limited clinical features of TSC and for genetic counseling in these families.

Tuberous sclerosis

Tuberous sclerosis is a serious inherited disease which poses major challenges for affected families and those caring for them. Identification of the genes causing the condition and study of their protein products has shed light on the pathogenesis of the disease and provided valuable new information about signalling pathways regulating protein synthesis and cell growth. There is now the exciting possibility of drug therapy for some of the manifestations of the disease.

Tuberous sclerosis: a GAP at the crossroads of multiple signaling pathways

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder that is characterized by benign tumors (hamartomas and hamartias) involving multiple organ systems, due to inactivating mutations in TSC1 or TSC2. Here, we review recent advances in our understanding of the growth and signaling functions of the TSC1 and TSC2 proteins. Led by seminal studies in Drosophila, the TSC1/TSC2 complex has been positioned in an ancestrally conserved signaling pathway that regulates cell growth. TSC1/TSC2 receives inputs from at least three major signaling pathways in the form of kinase-mediated phosphorylation events that regulate its function as a GTPase activating protein (GAP): the PI3K-Akt pathway, the ERK1/2-RSK1 pathway and the LKB1-AMPK pathway. TSC1/TSC2 functions as a GAP towards Rheb, which is a major regulator of the mammalian target of rapamycin (mTOR). In the absence of either TSC1 or TSC2, high levels of Rheb-GTP lead to constitutive activation of mTOR-raptor signaling, thereby leading to enhanced and deregulated protein synthesis and cell growth. As a specific inhibitor of mTOR, rapamycin has therapeutic potential for the treatment of TSC hamartomas.

Imagerie cérébrale de la sclérose tubéreuse de bourneville chez l’enfant

Tuberous sclerosis (TS) is a frequent phakomatosis, with autosomal dominant transmission. The diagnosis relies on a list of criteria (2 major criteria or 1 major criterion and 2 minor criteria). Brain MRI is very important for diagnosis, already showing the major signs during fetal life: subependymal nodules, tubers and giant cell astrocytomas. In childhood, FLAIR sequences are the more interesting while in fetuses, neonates and infants T1 sequences are required because of the myelination process.