A novel missense mutation in the GTPase activating protein homology region of TSC2 in two large families with tuberous sclerosis complex

Clinical practice recommendations for kidney involvement in tuberous sclerosis complex: a consensus statement by the ERKNet Working Group for Autosomal Dominant Structural Kidney Disorders and the ERA Genes & Kidney Working Group

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by the presence of proliferative lesions throughout the body. Management of TSC is challenging because patients have a multifaceted systemic illness with prominent neurological and developmental impact as well as potentially severe kidney, heart and lung phenotypes; however, every organ system can be involved. Adequate care for patients with TSC requires a coordinated effort involving a multidisciplinary team of clinicians and support staff. This clinical practice recommendation was developed by nephrologists, urologists, paediatric radiologists, interventional radiologists, geneticists, pathologists, and patient and family group representatives, with a focus on TSC-associated kidney manifestations. Careful monitoring of kidney function and assessment of kidney structural lesions by imaging enable early interventions that can preserve kidney function through targeted approaches. Here, we summarize the current evidence and present recommendations for the multidisciplinary management of kidney involvement in TSC.

Genotype and Phenotype Landscape of 283 Japanese Patients with Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by multiple dysplastic organ lesions and neuropsychiatric symptoms, caused by loss of function mutations in either TSC1 or TSC2. Genotype and phenotype analyses are conducted worldwide, but there have been few large-scale studies on Japanese patients, and there are still many unclear points. This study analyzed 283 Japanese patients with TSC (225 definite, 53 possible, and 5 genetic diagnoses). A total of 200 mutations (64 TSC1, 136 TSC2) were identified, of which 17 were mosaic mutations, 11 were large intragenic deletions, and four were splicing abnormalities due to deep intronic mutations. Several lesions and symptoms differed in prevalence and severity between TSC1 and TSC2 patients and were generally more severe in TSC2 patients. Moreover, TSC2 missense and in-frame mutations may attenuate skin and renal symptoms compared to other TSC2 mutations. Genetic testing revealed that approximately 20% of parents of a proband had mild TSC, which could have been missed. The patient demographics presented in this study revealed a high frequency of TSC1 patients and a low prevalence of epilepsy compared to global statistics. More patients with mild neuropsychiatric phenotypes were diagnosed in Japan, seemingly due to a higher utilization of brain imaging, and suggesting the possibility that a significant amount of mild TSC patients may not be correctly diagnosed worldwide.

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.

First five generations Chinese family of tuberous scleroses complex due to a new mutation of the TSC1 gene

Tuberous scleroses complex (TSC) is a rare neurocutaneous syndrome and has autosomal dominant inheritance. However, larger family with TSC is very rare. Here, we report the first five generations family with TSC from China, and localize the pathogenic gene. A boy with TSC and epilepsy underwent preoperative evaluation and epileptic surgery. His TSC family history was gotten, and the clinical data of a Chinese family with TSC were collected in 2016. Complete exons sequencing was performed in the proband and his parents, and whole exons sequence of TSC was performed in the other family members. The family showed autosomal dominant inheritance, and it was the largest reported family with TSC. In this pedigree, there were 14 patients in 5 generations, but only 1 case with epilepsy in them. All of examined patients had TSC 1 gene exon 15 c.1846delG p.A616Pfs^*13 mutation. In conclusion, TSC patients with TSC 1 deletion presented mild neurological symptom and rendered larger family.

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.

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.

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.

Transgenic expression of the N525S-tuberin variant in Tsc2 mutant (Eker) rats causes dominant embryonic lethality

The Tsc2 product, tuberin, negatively regulates the mTOR pathway. We have exploited the Eker (Tsc2-mutant) rat system to analyse various Tsc2 mutations. Here, we focus on the N525S-Tsc2 variant (NSM), which is known to cause distinct symptoms in patients even though normal suppression of mTOR is observed. Unexpectedly, we were repeatedly unable to generate viable rats carrying the NSM transgene. Genotypic analysis revealed that most of the embryos carrying the transgene died around embryonic day after 14.5—similar to the stage of lethality observed for Eker homozygotes. Thus, the NSM transgene appeared to have a dominant lethal effect in our rat model. Further, no significant differences were observed for various signal transduction molecules in transiently expressed NSM cells compared to WT. These results indicate that a non-mTOR pathway, critical for embryogenesis, is being regulated by tuberin, providing a link between tuberin expression and the severity of Tsc2 mutation-related pathogenesis.

Tuberous sclerosis

Tuberous sclerosis complex (TSC) is a genetic multisystem disorder characterized by widespread hamartomas in several organs, including the brain, heart, skin, eyes, kidney, lung, and liver. The affected genes are TSC1 and TSC2, encoding hamartin and tuberin respectively. The hamartin-tuberin complex inhibits the mammalian-target-of-Rapamycin (mTOR) pathway, which controls cell growth and proliferation. Variations in the distribution, number, size, and location of lesions cause the clinical syndrome to vary even between relatives. About 85% of children and adolescents with TSC have CNS complications, including epilepsy, cognitive impairment, challenging behavioral problems, and autism-like symptoms. Epilepsy generally begins during the first year of life, with focal seizures and spasms. The discovery of the mTOR pathway upregulation in TSC-associated lesions presents new possibilities for treatment strategy. Increasing understanding of the molecular abnormalities caused by TSC may enable improved management of the disease.

Mutational analysis of TSC1 and TSC2 in Japanese patients with tuberous sclerosis complex revealed higher incidence of TSC1 patients than previously reported

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterized by multiple hamartias and hamartomas involving throughout the body. To date, many TSC1 and TSC2 mutations have been reported all over the world, however, few TSC mutation studies have been performed in the Japanese population, and genetic characteristics of Japanese TSC patients are not yet clear. In this study, we analyzed TSC1 and TSC2 in 57 Japanese patients with TSC (8 familial and 49 sporadic; 46 definite and 11 suspect TSC) and identified 31 mutations including 11 TSC1 mutations (two familial and nine sporadic; all definite TSC) and 20 TSC2 mutations (2 familial and 18 sporadic; 19 definite and 1 suspect TSC). We also reviewed all Japanese TSC mutations previously reported. Our study demonstrates significantly higher incidence (P=0.007) of TSC1 mutations among sporadic TSC patients in the Japanese population compared with US and European studies. No differences emerged in mutation distributions and types in precedent studies, excepting low frequency of the TSC2 nonsense mutation. Comparing clinical manifestations, developmental delay and/or mental retardation were milder in TSC1 patients than TSC2 patients for its frequency (P=0.032) and severity (P=0.015); however, no other symptoms were clearly different.

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.

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.

Hereditary genodermatoses with cancer predisposition

In this article hereditary genodermatoses with cancer predisposition are reviewed, including nevoid basal cell carcinoma syndrome, neurofibromatosis types 1 and 2, tuberous sclerosis complex, xeroderma pigmentosum, and dyskeratosis congenita. Hereditary melanoma is also included, though it differs from the others in several respects. The underlying genetic aberrations causing these syndromes are largely known, allowing novel treatments to be developed for some of these disorders. Early recognition and diagnosis allows for close follow-up and surveillance for associated malignancies.

Autism spectrum disorders in tuberous sclerosis: pathogenetic pathways and implications for treatment

Autism spectrum disorders have been reported as being much more frequent in individuals with tuberous sclerosis than in the general population. Previous studies have implicated early seizure onset and the localization of cortical tubers in the temporal lobes as risk factors for autism. However, the underlying reasons for this association remain largely unclear. The dysregulation of intracellular signaling through the activation of mTOR pathway could play a direct role in determining susceptibility to autism. Early control of seizures and an early intensive behavioral intervention of autism during the period of brain plasticity can mitigate, but not reverse the final outcome. A greater understanding of the pathogenetic mechanisms underlying autism in tuberous sclerosis could help in devising targeted and potentially more effective treatment strategies.

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.

Tuberous sclerosis

Tuberous sclerosis is a genetic multisystem disorder characterised by widespread hamartomas in several organs, including the brain, heart, skin, eyes, kidney, lung, and liver. The affected genes are TSC1 and TSC2, encoding hamartin and tuberin respectively. The hamartin-tuberin complex inhibits the mammalian-target-of-rapamycin pathway, which controls cell growth and proliferation. Variations in the distribution, number, size, and location of lesions cause the clinical syndrome to vary, even between relatives. Most features of tuberous sclerosis become evident only in childhood after 3 years of age, limiting their usefulness for early diagnosis. Identification of patients at risk for severe manifestations is crucial. Increasing understanding of the molecular abnormalities caused by tuberous sclerosis may enable improved management of this disease.

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.

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: 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.

Tumour suppressors hamartin and tuberin: intracellular signalling

Tumour suppressors hamartin and tuberin, encoded by tuberous sclerosis complex 1(TSC1) and TSC2 genes, respectively, are critical regulators of cell growth and proliferation. Mutations in TSC1 and TSC2 genes are the cause of an autosomal dominant disorder known as tuberous sclerosis complex (TSC). Another genetic disorder, lymphangioleiomyomatosis (LAM), is also associated with mutations in the TSC2 gene. Hamartin and tuberin control cell growth by negatively regulating S6 kinase 1 (S6K1) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1), potentially through their upstream modulator mammalian target of rapamycin (mTOR). Growth factors and insulin promote Akt/PKB-dependent phosphorylation of tuberin, which in turn, releases S6K1 from negative regulation by tuberin and results in the activation of S6K1. Although much has been written regarding the molecular genetics of TSC and LAM, which is associated with either the loss of or mutation in the TSC1 and TSC2 genes, few reviews have addressed the intracellular signalling pathways regulated by hamartin and tuberin. The current review will fill the gap in our understanding of their role in cellular signalling networks, and by improving this understanding, an integrated picture regarding the normal function of tuberin and hamartin is beginning to emerge.

GAPs galore! A survey of putative Ras superfamily GTPase activating proteins in man and Drosophila

Typical members of the Ras superfamily of small monomeric GTP-binding proteins function as regulators of diverse processes by cycling between biologically active GTP- and inactive GDP-bound conformations. Proteins that control this cycling include guanine nucleotide exchange factors or GEFs, which activate Ras superfamily members by catalyzing GTP for GDP exchange, and GTPase activating proteins or GAPs, which accelerate the low intrinsic GTP hydrolysis rate of typical Ras superfamily members, thus causing their inactivation. Two among the latter class of proteins have been implicated in common genetic disorders associated with an increased cancer risk, neurofibromatosis-1, and tuberous sclerosis. To facilitate genetic analysis, I surveyed Drosophila and human sequence databases for genes predicting proteins related to GAPs for Ras superfamily members. Remarkably, close to 0.5% of genes in both species (173 human and 64 Drosophila genes) predict proteins related to GAPs for Arf, Rab, Ran, Rap, Ras, Rho, and Sar family GTPases. Information on these genes has been entered into a pair of relational databases, which can be used to identify evolutionary conserved proteins that are likely to serve basic biological functions, and which can be updated when definitive information on the coding potential of both genomes becomes available.

Genetic linkage of attention-deficit/hyperactivity disorder on chromosome 16p13, in a region implicated in autism

Attention-deficit/hyperactivity disorder (ADHD) is the most commonly diagnosed behavioral disorder in childhood and likely represents an extreme of normal behavior. ADHD significantly impacts learning in school-age children and leads to impaired functioning throughout the life span. There is strong evidence for a genetic etiology of the disorder, although putative alleles, principally in dopamine-related pathways suggested by candidate-gene studies, have very small effect sizes. We use affected-sib-pair analysis in 203 families to localize the first major susceptibility locus for ADHD to a 12-cM region on chromosome 16p13 (maximum LOD score 4.2; P=.000005), building upon an earlier genomewide scan of this disorder. The region overlaps that highlighted in three genome scans for autism, a disorder in which inattention and hyperactivity are common, and physically maps to a 7-Mb region on 16p13. These findings suggest that variations in a gene on 16p13 may contribute to common deficits found in both ADHD and autism.