High rate of mosaicism in tuberous sclerosis complex

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.

Unusual cardiac "masses" in a newborn with infantile pompe disease

Glycogen storage disease type II (OMIM #232300), or Pompe disease, may present in the newborn period with moderate-to-severe biventricular hypertrophy with or without left ventricular outflow tract obstruction that typically leads to death from cardiorespiratory failure in the first year of life. Glycogen deposition tends to be uniform, and is only occasionally accompanied by patchy areas of fibrosis. Here, we present an infant identified with biventricular hypertrophy and cardiac masses by prenatal ultrasound. Postnatal molecular studies did not support the diagnosis of tuberous sclerosis in this case. Additional evaluation for infantile hypertrophic cardiomyopathy confirmed the diagnosis of Pompe disease. We discuss whether the "cardiac masses," which brought this infant to medical attention and facilitated an early diagnosis of Pompe disease, may represent an unusual manifestation of GSD type II or the coincidental occurrence of an unrelated disease process.

Numerous cortical tubers and rhabdomyomas in a case of sudden unexpected infant death

Sudden infant death syndrome is the leading cause of death in infants between the ages of 1 month to 1 year. Sudden infant death syndrome, a diagnosis of exclusion, can only be made after other explanations for unexpected death have been ruled out. Tuberous sclerosis complex is occasionally the findings in these patients with unexpected infant death. Here, we present a case of an unexpected infant death during sleep with multiple factors that confound the cause of death. We discuss these factors and attempt to delineate their contributions to arrive at a cause and mechanism of death.

Multiplex ligation-depending probe amplification is not suitable for detection of low-grade mosaicism

'Apparent non-penetrance' occurs in several genetic disorders, including tuberous sclerosis complex and neurofibromatosis type 1: clinically unaffected parents may have multiple affected offspring. Germ line or somatic mosaicism in one of the parents of the index patient is the probable cause and results in an enhanced recurrence risk. Therefore, it is of great importance to use the most sensitive technology for testing DNA of the parents of the index patient for the presence/absence of the familial mutation. To detect large rearrangements multiplex ligation-depending probe amplification (MLPA) is often used. Here we show that MLPA is less sensitive in detecting low-grade somatic mosaicism than fluorescence in situ hybridization (FISH) or a mutation-specific PCR test. Therefore, we recommend FISH (if possible) or PCR analysis for the analysis of parental DNA.

Spectrum of CREBBP mutations in Indian patients with Rubinstein-Taybi syndrome

Rubinstein-Taybi syndrome (RSTS), a developmental disorder comprising abnormalities that include mental retardation, an unusual facial appearance, broad thumbs and big toes is frequently associated with molecular lesions in the CREB-binding protein gene, CREBBP. The objective of the present study was to identify and analyse CREBBP mutations in Indian RSTS patients on which there are no data. Direct sequencing of CREBBP performed in 13 RSTS patients identified the three zinc fingers (CH1, CH2, CH3) and HAT domain as mutational hotspots in which ten novel pathogenic mutations were localized. Functional analysis revealed that three of these mutations affecting amino acids Glu1459, Leu1668 and Glu1724 were critical for histone acetyltransferase activity. Twenty-eight novel CREBBP single-nucleotide polymorphisms (SNPs) were also identified in the Indian population. Linkage disequilibrium studies revealed associations between (i) SNP (rs129974/c.3836-206G greater than C) and mutation (p.Asp1340Ala); (ii) (rs130002) with mutation (p.Asn435Lys) and (iii) SNPs rs129974, rs130002 and SNP (c.3836-206G greater than C) signifying a disease affection status. In conclusion, the present study reports the highest detection rate of CREBBP mutations (76.9%) in RSTS patients to date, of which ten are predicted to be pathogenic and three critical for histone acetyltransferase activity. Moreover, identification of the association of CREBBP polymorphisms with disease susceptibility could be an important risk factor for the pathogenesis of RSTS.

First systematic experience of preimplantation genetic diagnosis for de-novo mutations

Standard preimplantation genetic diagnosis (PGD) cannot be applied for de-novo mutations (DNM), because neither origin nor relevant haplotypes are available for testing in single cells. PGD strategies were developed for 80 families with 38 genetic disorders, determined by 33 dominant, three recessive and two X-linked DNM. All three recessive mutations were of paternal origin, while of 93 dominant mutations, 40 were paternal, 46 maternal and seven detected in affected children. The development of specific PGD strategy for each couple involved DNA analysis of the parents and affected children prior to PGD, including a mutation verification, polymorphic marker evaluation, whole and single sperm testing to establish the normal and mutant haplotypes and PGD by polar body analysis and/or embryo biopsy. Overall, 151 PGD cycles were performed for 80 families, for which a specific PGD design has been established. The application of these protocols resulted in pre-selection and transfer of 219 (1.72 per cycle) DNM-free embryos in 127 (84.1%) PGD cycles, yielding 63 (49.6%) unaffected pregnancies and birth of 59 (46.5%) healthy children, confirmed to be free of DNM. The data show feasibility of PGD for DNM, which may routinely be performed with accuracy of over 99%, using the established PGD strategy.

Somatic mosaicism in a case of apparently sporadic Creutzfeldt-Jakob disease carrying a de novo D178N mutation in the PRNP gene

Transmissible spongiform encephalopathies (TSEs) are a group of rare fatal neurodegenerative disorders. Creutzfeldt-Jakob disease (CJD) represents the most common form of TSE and can be classified into sporadic, genetic, iatrogenic and variant forms. Genetic cases are related to prion protein gene mutations but they only account for 10-20% of cases. Here we report an apparently sporadic CJD case with negative family history carrying a mutation at codon 178 of prion protein gene. This mutation is a de novo mutation as the parents of the case do not show it. Furthermore the presence of three different alleles (wild type 129M-178D and 129V-178D and mutated 129V-178N), confirmed by different methods, indicates that this de novo mutation is a post-zygotic mutation that produces somatic mosaicism. The proportion of mutated cells in peripheral blood cells and in brain tissue was similar and was estimated at approximately 97%, suggesting that the mutation occurred at an early stage of embryogenesis. Neuropathological examination disclosed spongiform change mainly involving the caudate and putamen, and the cerebral cortex, together with proteinase K-resistant PrP globular deposits in the cerebrum and cerebellum. PrP typing was characterized by a lower band of 21 kDa. This is the first case of mosaicism described in prion diseases and illustrates a potential etiology for apparently sporadic neurodegenerative diseases. In light of this case, genetic counseling for inherited and sporadic forms of transmissible encephalopathies should take into account this possibility for genetic screening procedures.

Ultra deep sequencing detects a low rate of mosaic mutations in tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal dominant neurocutaneous syndrome caused by mutations in TSC1 and TSC2. However, 10-15% TSC patients have no mutation identified with conventional molecular diagnostic studies. We used the ultra-deep pyrosequencing technique of 454 Sequencing to search for mosaicism in 38 TSC patients who had no TSC1 or TSC2 mutation identified by conventional methods. Two TSC2 mutations were identified, each at 5.3% read frequency in different patients, consistent with mosaicism. Both mosaic mutations were confirmed by several methods. Five of 38 samples were found to have heterozygous non-mosaic mutations, which had been missed in earlier analyses. Several other possible low-frequency mosaic mutations were identified by deep sequencing, but were discarded as artifacts by secondary studies. The low frequency of detection of mosaic mutations, two (6%) of 33, suggests that the majority of TSC patients who have no mutation identified are not due to mosaicism, but rather other causes, which remain to be determined. These findings indicate the ability of deep sequencing, coupled with secondary confirmatory analyses, to detect low-frequency mosaic mutations.

Parallel sequencing used in detection of mosaic mutations: comparison with four diagnostic DNA screening techniques

We have made an evaluation of mutation detection techniques for their abilities to detect mosaic mutations. In this study, Sanger sequencing, single-strand conformation polymorphism (SSCP)/heteroduplex analysis (HD), protein truncation test (PTT), and denaturating high-performance liquid chromatography (DHPLC) were compared with parallel sequencing. In total DNA samples from nine patients were included in this study. Mosaic mutations were artificially constructed from seven of these samples, which were from heterozygote mutation carriers with the mutant allele present at 50%. The mutations analyzed were as follows: c.646C>T, c.2626C>T, c.2828C>A, c.1817_1818insA, c.2788dupA, c.416_419delAAGA, and c.607delC in the APC gene. The lowest degree of mutant alleles detected with SSCP/HD and DHPLC varied between 5% and 25%, and between 15% and 50% for Sanger sequencing. Three of the mutations were analyzed with PTT with considerable variations in detection levels (from 10 to 100%). Using parallel sequencing a detection frequency down to 1% was reached, but to achieve this high sensitivity sufficient coverage was required. Two patients with natural mosaic mutations were also included in this study. These two mutations had previously been identified with Sanger sequencing (NF2 c.1026_1027delGA) and SSCP/HD (APC c.2700_2701delTC). In conclusion, all the evaluated methods are applicable for mosaic mutation screening even though combinations of the conventional methods should be used to reach an adequate sensitivity. Sanger sequencing alone is not sensitive enough to detect low mosaic levels. Parallel sequencing seems to be the ultimate choice but the possibilities to use this technique is today limited by its complexity, economics, and availability of instruments.

Semi-automated unidirectional sequence analysis for mutation detection in a clinical diagnostic setting

BACKGROUND

The past 10 years have seen an improvement in sequence data quality due to the introduction of capillary sequencers and new sequencing chemistries. In parallel, new software programs for automated mutation detection have been developed. We evaluated the sensitivity of semiautomated unidirectional sequence analysis for the detection of heterozygous base substitutions using the Mutation Surveyor software package.

METHODS

Detection rates for heterozygous base substitutions in 29 genes by automated and visual inspection were compared. Examples of heterozygous bases not detected in one direction during bidirectional analysis were also sought through a national survey of United Kingdom (UK) genetics laboratories. Sequence quality was assessed in a consecutive cohort of 50 patients for whom the 39 exons of the ABCC8 gene had been sequenced in one direction.

RESULTS

A total of 701 different heterozygous base substitutions were detected by the software with no false negatives (sensitivity >or=99.57%). Four examples of heterozygous bases missed in one direction during bidirectional analysis were reported. Two were detected using unidirectional analysis settings, and the other two bases had low-quality scores. Of the 1950 amplicons examined, 97.2% had a quality score >or=30 and an average PHRED-like score >or=50 for the defined region of interest, and 98.1% of the 323,650 bases had a PHRED score >40.

CONCLUSIONS

We found no evidence to support a requirement for bidirectional sequencing. Semiautomated analysis of good quality unidirectional sequence data has high sensitivity and is suitable for heterozygote mutation scanning in clinical diagnostic laboratories. Further work is required to determine minimum quality parameters for semiautomated analysis.

Epilepsy surgery and tuberous sclerosis complex: special considerations

Epilepsy surgery for medically refractory seizures among patients with tuberous sclerosis complex (TSC) is a well-accepted treatment option. Many epilepsy centers around the world have published their experience over the past several years, supporting the idea that the best seizure control is obtained when a single tuber and associated epileptogenic zone is documented and targeted surgically. Recent advances in imaging and physiological techniques that reveal the epileptogenic zone have been used successfully in children with TSC who are being evaluated for surgery. As a result, a number of different surgical strategies have emerged, each reflecting the experience, strengths, and referral biases of the individual treating teams. Experience suggests that some patients with TSC who present with seizures that are difficult to localize and do not meet the classic selection criteria for epilepsy surgery may, nevertheless, benefit from surgery. Tuberectomy alone is often not sufficient for obtaining seizure control. Intracranial electrode recordings performed in a large number of children with TSC undergoing epilepsy surgery have raised new questions about the relationship of the cortical tuber to the epileptogenic zone in TSC. A careful assessment of the risks and benefits of any surgical strategy, compared with those associated with continued refractory epilepsy, should be considered by the treating team in conjunction with the patient's family. Epilepsy surgery has not only benefited many children with TSC, but it also facilitates the understanding of epileptogenesis in TSC.

Focal cortical dysplasia: a genotype-phenotype analysis of polymorphisms and mutations in the TSC genes

PURPOSE

Focal cortical dysplasia (FCD) is a common cause of pharmacoresistant human epilepsy. FCD has frequently been discussed as a "forme fruste" of tuberous sclerosis complex (TSC) because of the radiologic and histologic resemblance of dysplastic areas to tubers in TSC. Mutations or a germ-line predisposition in terms of increased polymorphisms in the TSC genes have been presumed to influence the pathogenesis of FCD. A detailed genotype-phenotype analysis of these patients has not been performed so far.

METHODS

In this study, 33 patients with FCD (among them 23 with FCD type 2 and 4 patients with multifocal FCD) were investigated (1) clinically as to dermatologic manifestations, retinal hamartoma, cardial rhabdomyoma, and renal angiomyolipoma, and (2) genetically by considering lesional brain tissue and blood using single strand conformation polymorphism (SSCP) electrophoresis and sequencing of the TSC1 and TSC2 genes.

RESULTS

In the clinical examinations, no subtle features of TSC could be detected in this large group of patients with FCD, pointing to the fact that this is a different patient group without clinical overlap. Several sequence alterations were found in the TSC1 and TSC2 genes in both lesional brain tissue and blood of FCD patients, however, in similar frequencies to that of the normal population. Moreover, most of these sequence alterations were silent.

DISCUSSION

This study shows that FCD-even multifocal FCD-is not caused by mutations in the TSC genes and seems not to be promoted by polymorphisms in the TSC genes. Therefore, FCD cannot be regarded as a "forme fruste" of TSC.

The pathogenesis and imaging of the tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is an autosomal-dominant disorder characterized by the formation of hamartomatous lesions in multiple organ systems. It is the second most common neurocutaneous syndrome after neurofibromatosis type 1 and has been recognized since the late 1800s. Although the disease has complete penetrance, there is also high phenotypic variability: some patients have obvious signs at birth, while others remain undiagnosed for many years. In addition to skin lesions, TSC patients develop numerous brain lesions, angiomyolipoma (AMLs), lymphangiomyomatosis (LAM) in the lungs, cardiac rhabdomyomas, skeletal lesions, and vascular anomalies, all of which are well seen with medical imaging. Our knowledge of TSC genetics and pathophysiology has expanded dramatically in recent years: two genetic loci were discovered in the 1990s and recent elucidation of TSC's interaction with the mTOR pathway has changed how we manage the disease. Meanwhile, medical imaging is playing an increasingly important role in the diagnosis, management, and treatment of TSC. We provide an update on the genetics and pathophysiology of TSC, review its clinical manifestations, and explore the breadth of imaging features in each organ system, from prenatal detection of cardiac rhabdomyomas to monitoring rapamycin therapy to treatment of AMLs by interventional radiology.

Novel CCM1, CCM2, and CCM3 mutations in patients with cerebral cavernous malformations: in-frame deletion in CCM2 prevents formation of a CCM1/CCM2/CCM3 protein complex

Cerebral cavernous malformations (CCM) are prevalent cerebrovascular lesions predisposing to chronic headaches, epilepsy, and hemorrhagic stroke. Using a combination of direct sequencing and MLPA analyses, we identified 15 novel and eight previously published CCM1 (KRIT1), CCM2, and CCM3 (PDCD10) mutations. The mutation detection rate was >90% for familial cases and >60% for isolated cases with multiple malformations. Splice site mutations constituted almost 20% of all CCM mutations identified. One of these proved to be a de novo mutation of the most 3' acceptor splice site of the CCM1 gene resulting in retention of intron 19. A further mutation affected the 3' splice site of CCM2 intron 2 leading to cryptic splice site utilization in both CCM2 and its transcript variant lacking exon 2. With the exception of one in-frame deletion of CCM2 exon 2, which corresponds to the naturally occurring splice variant of CCM2 on the RNA level and is predicted to result in the omission of 58 amino acids (CCM2:p.P11_K68del), all mutations lead to the introduction of premature stop codons. To gain insight into the likely mechanisms underlying the only known CCM2 in-frame deletion, we analyzed the functional consequences of loss of CCM2 exon 2. The CCM2:p.P11_K68del protein could be expressed in cell culture and complexed with CCM3. However, its ability to interact with CCM1 and to form a CCM1/CCM2/CCM3 complex was lost. These data are in agreement with a loss-of-function mechanism for CCM mutations, uncover an N-terminal CCM2 domain required for CCM1 binding, and demonstrate full-length CCM2 as the essential core protein in the CCM1/CCM2/CCM3 complex.

Mosaicism in autosomal dominant polycystic kidney disease revealed by genetic testing to enable living related renal transplantation

Patients with end-stage renal disease (ESRD) secondary to autosomal dominant polycystic kidney disease (ADPKD) receive fewer living-related kidney (LRK) transplants than other groups with ESRD. This relates to the difficulties in excluding the disease in potential donors. We report a case which highlights these difficulties and, by discovery of mosaicism for a new mutation, illustrates the role of clinical and molecular genetic resources in assessing young related kidney donors for patients with ADPKD.

Importance of investigating somatic and germline mutations in hemophilia A: a preliminary study from All India Institute of Medical Sciences, India

BACKGROUND

Hemophilia A is a common hereditary bleeding disorder caused mainly by mutations in the Factor VIII (FVIII) gene, which results in defective or absent FVIII protein. Most of the causative mutations arise from the germ cells, which leads to either heterozygous or hemizygous state for the mutation in the next generation. Germline or somatic mosaic may result due to a de novo mutation during early embryogenesis.

METHOD

We analyzed 14 families of Indian origin with Hemophilia A [sporadic and severe] for the presence of mosaic individuals by employing Allele Specific PCR, mutation enrichment experiment and sequencing.

RESULT

Nine families had point mutations, 3 families had small deletions or insertions, 2 families had splice site mutations. The origin of the de novo mutation was assigned to the patients' mother in 8 families. For 4 families it was assigned to the maternal grandmother and to the maternal grandfather in 2 families. In a single family somatic mosaic was detected.

CONCLUSION

The presence of somatic mosaic in families with sporadic Hemophilia A in India may confound risk estimation during genetic counseling.

Somatic APC mosaicism: a frequent cause of familial adenomatous polyposis (FAP)

Somatic mutational mosaicism presents a challenge for both molecular and clinical diagnostics and may contribute to deviations from predicted genotype-phenotype correlations. During APC mutation screening in 1,248 unrelated patients with familial adenomatous polyposis (FAP), we identified 75 cases with an assumed or confirmed de novo mutation. Prescreening methods (protein truncation test [PTT], DHPLC) indicated the presence of somatic mosaicism in eight cases (11%). Sequencing of the corresponding fragments revealed very weak mutation signals, pointing to the presence of either nonsense or frameshift mutations at low level. All mutations were confirmed and quantified by SNaPshot analysis: in leukocyte DNA from the eight patients, the percentage of mosaicism varied between 5.5% and 77%, while the proportion of the mutation in DNA extracted from adenomas of the respective patient was consistently higher. The eight mutations identified as mosaic are localized within codons 216-1464 of the APC gene. According to the known genotype-phenotype correlation, patients with mutations in this region exhibit typical or severe FAP. However, six of the eight patients presented with an attenuated or atypical polyposis phenotype. Our data demonstrate that in a fraction of FAP patients the causative APC mutation may not be detected due to weak signals or somatic mosaicism that is restricted to tissues other than blood. SNaPshot analysis was proven to be an easy, rapid, and reliable method of confirming low-level mutations and evaluating the degree of mosaicism. Some of the deviations from the expected phenotype in FAP can be explained by the presence of somatic mosaicism.

Tuberous sclerosis complex: advances in diagnosis, genetics, and management

Tuberous sclerosis complex (TSC) is an autosomal dominant multisystem neurocutaneous syndrome characterized by the development of multiple hamartomas distributed throughout the body, skin, brain, heart, kidneys, liver, and lungs. Two-thirds of patients represent sporadic mutations. The classic triad is seizures, mental retardation, and cutaneous angiofibromas. However, the full triad occurs in only 29% of patients; 6% of them lack all three of them. Two tumor suppressor genes responsible for TSC have been identified: TSC1 gene on chromosome 9 and TSC2 on chromosome 16. This article highlights the most recent significant advances in the diagnosis and genetics of TSC, along with a discussion on the limitations and the usefulness of the revised 1998 clinical criteria for the tuberous sclerosis complex. The "ash leaf" macule often comes in other shapes, such as round; most are polygonal, usually 0.5 cm to 2.0 cm in diameter, resembling a thumbprint. Since the death of its describer, Thomas Fitzpatrick, we call each a "Fitzpatrick patch." Special attention is paid in this work to TSC treatment options, including therapeutic trials with rapamycin, also known as sirolimus.

LEARNING OBJECTIVE

After completing this learning activity, participants should familiar with tuberous sclerosis complex, its cutaneous signs and systemic findings stratified by patient age, its genetics, and the potential for meaningful therapeutic intervention.

[Pulmonary lymphangioleiomyomatosis with or without tuberous sclerosis]

INTRODUCTION

Pulmonary lymphangioleiomyomatosis (LAM) is a rare disease affecting young women and presenting with recurrent pneumothorax.

BACKGROUND

Other lesions such as chylothorax or renal angiomyolipoma may suggest the diagnosis. The condition is related to a proliferation of abnormal smooth muscle cells staining for the monoclonal antibody HMB45. LAM can appear sporadically or be associated with tuberous sclerosis with abnormalities of the TSC2 suppressor gene. High resolution thoracic CT scanning shows bilateral, thin walled pulmonary cysts. Pulmonary function tests reveal bronchial obstruction and over-inflation with a reduced DLCO being the earliest abnormality.

VIEWPOINT

Although there are non-progressive forms, LAM usually leads to chronic respiratory insufficiency within a few, or ten or so years. In the absence of a controlled clinical trial hormone therapy has not been shown to be effective. Lung transplantation is the last therapeutic resort; recurrences in the transplanted lung have been occasionally reported.

CONCLUSIONS

Analysis of the molecular mechanisms induced by mutations of the TSC2 suppressor gene and the demonstration of the migratory properties of smooth muscle cells, whose origin may be extra-thoracic, reveal new specific antiproliferative therapeutic options.

The presence of germ line mosaicism in cleidocranial dysplasia

Cleidocranial dysplasia (CCD) is typically an autosomal dominant condition. The possibility of alternative causes, such as an autosomal recessive form or germ line mosaicism, have been suggested in some families with CCD, but not proven. We present a family consisting of a mother having three sons affected with CCD. One of the affected boys is a half brother to the other two affected children. The diagnosis of CCD was confirmed by DNA analysis of the RUNX2 gene in all three of the boys in blood; however, initial DNA testing in the mother's blood did not detect the presence of a RUNX2 mutation in the mother. Further testing through heteroduplex analysis applying high-resolution melting analysis followed by subcloning detected low-level mosaicism in DNA isolated from maternal blood and buccal swab, confirming low-level mosaicism in somatic cells. We present the first case of confirmed germ line mosaicism in CCD.

Giant cells: contradiction to two-hit model of tuber formation?

Tuberous sclerosis (TSC) is an autosomal dominant disease characterized by the formation of hamartomatous lesions in many organs, including brain, heart or kidneys. It has been found that TSC is caused by the mutation in one of the two tumor suppressor genes: TSC1 or TSC2, encoding hamartin and tuberin, respectively. According to Knudson's two-hit model of tumorigenesis, second-hit mutation and resulting loss of heterozygosity (LOH) of a tumor suppressor gene is necessary for tumor formation. In fact, LOH is commonly found in several types of hamartomas formed in the process of tuberous sclerosis, but, interestingly, not in brain lesions, containing characteristic giant cells. In this paper, we review literature covering origination of giant cells and present several hypotheses explaining why in spite of the presence of hamartin and tuberin, brain lesions form in TSC patients.

Genotype/phenotype correlation in 325 individuals referred for a diagnosis of tuberous sclerosis complex in the United States

Tuberous sclerosis complex is an autosomal dominant neurocutaneous disorder marked by hamartoma growth in multiple organ systems. We performed mutational analyses on 325 individuals with definite tuberous sclerosis complex diagnostic status. We identified mutations in 72% (199/257) of de novo and 77% (53/68) of familial cases, with 17% of mutations in the TSC1 gene and 50% in the TSC2 gene. There were 4% unclassified variants and 29% with no mutation identified. Genotype/phenotype analyses of all observed tuberous sclerosis complex findings in probands were performed, including several clinical features not analyzed in two previous large studies. We showed that patients with TSC2 mutations have significantly more hypomelanotic macules and learning disability in contrast to those with TSC1 mutations, findings not noted in previous studies. We also observed results consistent with two similar studies suggesting that individuals with mutations in TSC2 have more severe symptoms. On performing meta-analyses of our data and the other two largest studies in the literature, we found significant correlations for several features that individual studies did not have sufficient power to conclude. Male patients showed more frequent neurologic and eye symptoms, renal cysts, and ungual fibromas. Correlating genotypes with phenotypes should facilitate the disease management of tuberous sclerosis complex.

Molecular and clinical analyses of 84 patients with tuberous sclerosis complex

BACKGROUND

Tuberous sclerosis complex (TSC) is an autosomal dominant disease characterized by the development of multiple hamartomas in many internal organs. Mutations in either one of 2 genes, TSC1 and TSC2, have been attributed to the development of TSC. More than two-thirds of TSC patients are sporadic cases, and a wide variety of mutations in the coding region of the TSC1 and TSC2 genes have been reported.

METHODS

Mutational analysis of TSC1 and TSC2 genes was performed in 84 Taiwanese TSC families using denaturing high-performance liquid chromatography (DHPLC) and direct sequencing.

RESULTS

Mutations were identified in a total of 64 (76 %) cases, including 9 TSC1 mutations (7 sporadic and 2 familial cases) and 55 TSC2 mutations (47 sporadic and 8 familial cases). Thirty-one of the 64 mutations found have not been described previously. The phenotype association is consistent with findings from other large studies, showing that disease resulting from mutations to TSC1 is less severe than disease due to TSC2 mutation.

CONCLUSION

This study provides a representative picture of the distribution of mutations of the TSC1 and TSC2 genes in clinically ascertained TSC cases in the Taiwanese population. Although nearly half of the mutations identified were novel, the kinds and distribution of mutation were not different in this population compared to that seen in larger European and American studies.

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.

The origin of EFNB1 mutations in craniofrontonasal syndrome: frequent somatic mosaicism and explanation of the paucity of carrier males

Craniofrontonasal syndrome (CFNS) is an X-linked disorder that exhibits a paradoxical sex reversal in phenotypic severity: females characteristically have frontonasal dysplasia, craniosynostosis, and additional minor malformations, but males are usually mildly affected with hypertelorism only. Despite this, males appear underrepresented in CFNS pedigrees, with carrier males encountered infrequently compared with affected females. To investigate these unusual genetic features of CFNS, we exploited the recent discovery of causative mutations in the EFNB1 gene, which encodes ephrin-B1, to survey the molecular alterations in 59 families (39 newly investigated and 20 published elsewhere). We identified the first complete deletions of EFNB1, catalogued 27 novel intragenic mutations, and used Pyrosequencing and analysis of nearby polymorphic alleles to quantify mosaic cases and to determine the parental origin of verified germline mutations. Somatic mosaicism was demonstrated in 6 of 53 informative families, and, of 17 germline mutations in individuals for whom the parental origin of mutation could be demonstrated, 15 arose from the father. We conclude that the major factor accounting for the relative scarcity of carrier males is the bias toward mutations in the paternal germline (which present as affected female offspring) combined with reduced reproductive fitness in affected females. Postzygotic mutations also contribute to the female preponderance, whereas true nonpenetrance in males who are hemizygous for an EFNB1 mutation appears unusual. These results highlight the importance of considering possible origins of mutation in the counseling of families with CFNS and provide a generally applicable approach to the combined analysis of mosaic and germline mutations.

A case of solitary subependymal giant cell astrocytoma: two somatic hits of TSC2 in the tumor, without evidence of somatic mosaicism

Subependymal giant cell astrocytoma (SEGA) is a unique brain tumor arising in tuberous sclerosis complex (TSC), an autosomal dominant inherited phacomatosis. There are several case reports of solitary SEGA without any other manifestations of TSC. Usually these cases are thought to be forme fruste of TSC due to somatic mosaicism. However, no previous reports have used molecular methodology to fully investigate mutations in TSC genes or the possibility of somatic mosaicism. Here, we report a 20-year-old woman with a brain tumor. Pathological diagnosis was consistent with SEGA, but comprehensive clinical screening found no other lesions indicative of TSC. Molecular analysis of the tumor revealed loss of heterozygosity and allelic mutation (5228G>A, R1743Q) of TSC2. To detect the small fraction of mosaic mutation in somatic cells, we developed a highly sensitive new method: triple-nested polymerase chain reaction-restriction fragment length polymorphism. The identical TSC2 missense mutation was not detected in any other tissues from the same patient, including peripheral blood, buccal mucosa, urinary sediment, nail, and hair. According to these results, this patient should be considered as having SEGA that developed from two somatic hit mutations in TSC2, rather than being a TSC2 patient with a very small fraction of somatic mosaicism.

Giant cells: contradiction to two-hit model of tuber formation?

1. Tuberous sclerosis (TSC) is an autosomal dominant disease characterized by the formation of hamartomatous lesions in many organs, including brain, heart or kidneys. It has been found that TSC is caused by the mutation in one of two tumor suppressor genes: TSC1 or TSC2, encoding hamartin and tuberin, respectively. 2. According to Knudson's two-hit model of tumorigenesis, second-hit mutation and resulting loss of heterozygosity (LOH) of a tumor suppressor gene is necessary for tumor formation. In fact, LOH is commonly found in several types of hamartomas formed in the process of tuberous sclerosis, but, interestingly, not in brain lesions, containing characteristic giant cells. 3. In the present paper we review literature covering origination of giant cells and present several hypotheses explaining why in spite of the presence of hamartin and tuberin, brain lesions form in TSC patients.

Mutational analysis of the TSC1 and TSC2 genes in a diagnostic setting: genotype – phenotype correlations and comparison of diagnostic DNA techniques in Tuberous Sclerosis Complex

Tuberous sclerosis complex (TSC) is an autosomal dominant disorder characterised by the development of hamartomas in multiple organs and tissues. TSC is caused by mutations in either the TSC1 or TSC2 gene. We searched for mutations in both genes in a cohort of 490 patients diagnosed with or suspected of having TSC using a combination of denaturing gradient gel electrophoresis, single-strand conformational polymorphism, direct sequencing, fluorescent in situ hybridisation and Southern blotting. We identified pathogenic mutations in 362 patients, a mutation detection rate of 74%. Of these 362 patients, 276 had a definite clinical diagnosis of TSC and in these patients 235 mutations were identified, a mutation detection rate of 85%. The ratio of TSC2:TSC1 mutations was 3.4:1. In our cohort, both TSC1 mutations and mutations in familial TSC2 cases were associated with phenotypes less severe than de novo TSC2 mutations. Interestingly, consistent with other studies, the phenotypes of the patients in which no mutation was identified were, overall, less severe than those of patients with either a known TSC1 or TSC2 mutation.

Separating human DNA mixtures using denaturing high-performance liquid chromatography

DNA mixtures represent challenging samples that are rarely amenable to direct DNA sequence analysis and many of the strategies available to separate mixtures are both labor and time intensive. Denaturing high-performance liquid chromatography is an accurate and rapid approach for the detection and scoring of mutations. It can also be used to separate DNA mixtures. The technique relies on the chromatographic separation of crosshybridization products to isolate the individual components of a mixture. By eliminating secondary amplification and excessive manipulation prior to sequencing, denaturing high-performance liquid chromatography can streamline the analysis of conditions ranging from somatic mosaicism, microchimerism and mitochondrial heteroplasmy to evidentiary material containing mixtures of DNA encountered in forensic investigations.

The phakomatoses

The "phakomatosis" concept was formulated early in the twentieth century by the ophthalmologist van der Hoeve. He included 3 disorders in the group-neurofibromatosis, tuberous sclerosis complex, and von Hippel-Lindau syndrome--on the basis of the occurrence of patchy ophthalmologic manifestations in each disorder. Since the name was coined, much has been learned about the pathogenesis of these 3 disorders. It is clear that 2 of them--neurofibromatosis and tuberous sclerosis--are collective terms for multiple disorders. Each of the conditions is caused by distinct genetic defects, with little commonality in terms of protein function. Yet, in some respects, the disorders share a pathogenetic mechanism, that of the tumor suppressor gene. This review will briefly describe these disorders in light of what has been learned about underlying molecular pathogenesis. In each case, genetic testing is beginning to be available; principles of the use of genetic tests will be described.

Mutations within the programmed cell death 10 gene cause cerebral cavernous malformations

Cerebral cavernous malformations (CCMs) are hamartomatous vascular malformations characterized by abnormally enlarged capillary cavities without intervening brain parenchyma. They cause seizures and cerebral hemorrhages, which can result in focal neurological deficits. Three CCM loci have been mapped, and loss-of-function mutations were identified in the KRIT1 (CCM1) and MGC4607 (CCM2) genes. We report herein the identification of PDCD10 (programmed cell death 10) as the CCM3 gene. The CCM3 locus has been previously mapped to 3q26-27 within a 22-cM interval that is bracketed by D3S1763 and D3S1262. We hypothesized that genomic deletions might occur at the CCM3 locus, as reported previously to occur at the CCM2 locus. Through high-density microsatellite genotyping of 20 families, we identified, in one family, null alleles that resulted from a deletion within a 4-Mb interval flanked by markers D3S3668 and D3S1614. This de novo deletion encompassed D3S1763, which strongly suggests that the CCM3 gene lies within a 970-kb region bracketed by D3S1763 and D3S1614. Six additional distinct deleterious mutations within PDCD10, one of the five known genes mapped within this interval, were identified in seven families. Three of these mutations were nonsense mutations, and two led to an aberrant splicing of exon 9, with a frameshift and a longer open reading frame within exon 10. The last of the six mutations led to an aberrant splicing of exon 5, without frameshift. Three of these mutations occurred de novo. All of them cosegregated with the disease in the families and were not observed in 200 control chromosomes. PDCD10, also called "TFAR15," had been initially identified through a screening for genes differentially expressed during the induction of apoptosis in the TF-1 premyeloid cell line. It is highly conserved in both vertebrates and invertebrates. Its implication in cerebral cavernous malformations strongly suggests that it is a new player in vascular morphogenesis and/or remodeling.

Genodermatoses 2003-2004

PURPOSE OF REVIEW

Many genodermatoses have been linked in recent years to their respective genes. The underlying biology and integrative nature of these genes with other genes and organ systems is beginning to be understood. This paper reviews recent advances in neurocutaneous disorders, ectodermal dysplasias, and the phenomenon of revertant gene mosaicism.

RECENT FINDINGS

In neurofibromatosis type 1, molecular assays are being developed to distinguish malignant from benign and premalignant lesions. Clinical mutation analysis for the NF1 gene has been problematic; a sensitive new assay using automated comparative sequence analysis may be helpful. Revision of clinical care guidelines is ongoing. New data for the prospective management of optic pathway gliomas is reviewed. The two genes that underlie tuberous sclerosis complex, tuberin and hamartin, lie at the center of an important signal transduction pathway with significant implications for pharmacologic treatment. Issues in genetic counseling for this highly variable disease are updated. Extensive progress has been made in understanding the basis of several forms of ectodermal dysplasia. Disorders caused by mutations in p63 and the connexin and NF-kappaB gene families will be reviewed. Finally, phenotypic in vivo amelioration of genodermatoses via revertant gene mosaicism will be discussed as a possible mechanism to be exploited in directed therapeutic approaches.

SUMMARY

This paper reviews recent developments in the molecular and biologic bases of neurofibromatosis type 1, tuberous sclerosis, and ectodermal disorders related to p63 and the connexin and NF-kappaB gene families. The concept of revertant gene mosaicism is also discussed as a potential model for gene therapy.

The phakomatoses

The phakomatoses are a diverse set of disorders related principally by a similar tendency to produce patchy manifestations that affect the nervous system and various other tissues. All of the disorders involve the occurrence of benign neoplasms or hamartomatous growths, and all involve the action of a distinct gene that functions as a tumor suppressor. At present, none of the phakomatoses is amenable to effective medical treatment, so management is limited to surveillance and treatment of progressive lesions. The radiologist plays a key role in this management, participating in diagnosis and follow-up of affected patients.

The Tuberous Sclerosis Complex Genes in Tumor Development

The study of hereditary tumor syndromes has laid a solid foundation toward understanding the genetic basis of cancer. One of the latest examples comes from the study of tuberous sclerosis complex (TSC). As a member of the phakomatoses, TSC is characterized by the appearance of benign tumors, most notably in the central nervous system, kidney, heart, lung, and skin. While classically described as "hamartomas," the pathology of the lesions has features suggestive of abnormal cellular proliferation, size, differentiation, and migration. Occasionally, tumors progress to become malignant (i.e., renal cell carcinoma). The genetic basis of this disease has been attributed to mutations in one of two unlinked genes, TSC1 and TSC2. Cells undergo bi-allelic inactivation of either gene to give rise to tumors in a classic tumor suppressor "two-hit" paradigm. The functions of the TSC1 and TSC2 gene products, hamartin and tuberin, respectively, have remained ill defined until recently. Genetic, biochemical, and biologic analyses have highlighted their role as negative regulators of the mTOR signaling pathway. Tuberin, serving as a substrate of AKT and AMPK, mediates mTOR activity by coordinating inputs from growth factors and energy availability in the control of cell growth, proliferation, and survival. Emerging evidence also suggests that the TSC 1/2 complex may play a role in modulating the activity of beta-catenin and TGFbeta. These findings provide novel functional links between the TSC genes and other tumor suppressors responsible for Cowden's disease (PTEN), Peutz-Jeghers syndrome (LKB1), and familial polyposis (APC). Common sporadic cancers such as prostate, lung, colon, endometrium, and breast have ties to these genes, highlighting the potential role of the TSC proteins in human cancers. Rapamycin, a specific mTOR inhibitor, has potent antitumoral activities in preclinical models of TSC and is currently undergoing phase I/II clinical studies.

Diagnosis of tuberous sclerosis complex

Tuberous sclerosis complex is a dominantly inherited disorder affecting multiple organs; because of its phenotypic variability, the diagnosis of tuberous sclerosis complex can be difficult in the young or in individuals with subtle findings. Recently revised consensus diagnostic criteria for tuberous sclerosis complex reflect an improved understanding of its clinical manifestations and its genetic and molecular mechanisms. The diagnostic criteria are based on the premise that there are probably no truly pathognomonic clinical signs for tuberous sclerosis complex; signs that were once regarded as specific occur as isolated findings in individuals with no other clinical or genetic evidence of tuberous sclerosis complex. Consequently, the revised criteria require tuberous sclerosis complex-associated lesions of two or more organ systems or at least two dissimilar lesions of the same organ to confirm the diagnosis. The addition of DNA testing complements clinical diagnosis and allows more precise genetic counseling and, in some individuals, prenatal diagnosis. Nevertheless, the 15% false-negative rate for DNA testing and the occurrence of germline mosaicism in about 2% of individuals with tuberous sclerosis complex make it difficult to exclude the diagnosis of tuberous sclerosis complex in family members.

Mosaic mutations of the FLN1 gene cause a mild phenotype in patients with periventricular heterotopia

X-linked periventricular nodular heterotopia (PNH) (OMIM 300049) is a neuronal migration disorder, associated with mutations of the FLN1 gene (Xq28), accompanied by severe epilepsy and normal to mildly impaired cognitive function in affected women. The recurrence risk has been estimated to be 50% in daughters of affected women, with early post-natal lethality in boys. Mutation analysis [denaturing high-performance liquid chromatography (DHPLC) and sequencing], performed in a woman and a man with PNH, was suggestive of somatic mosaicism. Both patients were investigated using single nucleotide primer extension (SNuPE) and DHPLC. To better characterize mosaicism in the affected man, SNuPE-DHPLC analysis was also performed on a pool of hair roots and single hair roots. The affected woman had features of PNH on magnetic resonance imaging. She had well-controlled epilepsy and normal cognitive function. She was mosaic for a nucleotide insertion (c.568_569ingG). SNuPE-DHPLC findings showed 17% of mutant allele. The affected man had classical PNH and was mosaic for an A>G substitution (intron 11 acceptor splice site). SNuPE-DHPLC on both leukocyte and hair root DNA revealed 42% and 69% of mutant allele. Single hair root analysis confirmed that this patient did not harbor the mutation in all ectodermal derivative cells. His daughter had not inherited the mutation. Phenotypic heterogeneity associated with X-linked PNH may depend on the type of mutation, its location on the protein, as well as on somatic mosaicism. Mosaicism can influence the recurrence risk rates in affected women. Mosaic mutations in men may not be transmitted to their daughters, masking the X-linked nature of the disorder.

Recurrent familial hypocalcemia due to germline mosaicism for an activating mutation of the calcium-sensing receptor gene

De novo activating mutations in the calcium-sensing receptor (CASR) gene are a common cause of sporadic isolated hypoparathyroidism. Here, we describe a family in which two affected siblings were found to be heterozygous for a novel F788L mutation in the fifth transmembrane domain encoded by exon 7 of the CASR. Both parents and the third sibling were clinically unaffected and genotypically normal by direct sequencing of their leukocyte exon 7 PCR amplicons. However, the mother was revealed to be a mosaic for the mutation by sequence analysis of multiple subclones as well as denaturing HPLC of the CASR exon 7 leukocyte PCR product. A functional analysis of the mutation was performed by transiently transfecting wild-type and mutant CASRs tagged with a c-Myc epitope in human embryonic kidney (HEK293) cells. The mutant CASR was expressed at a similar level as the wild type. The F788L mutant produced a significant shift to the left relative to the wild-type CASR in the MAPK response to increasing extracellular calcium concentrations. This is the first report of mosaicism for an activating CASR mutation and suggests that care should be exercised in counseling for risks of recurrence in a situation where a de novo mutation appears likely.

New approaches to investigating heterogeneity in complex traits

Great advances in the field of genetics have been made in the last few years. However, resolving the complexity that underlies the susceptibility to many polygenic human diseases remains a major challenge to researchers. The fast increase in availability of genetic data and the better understanding of the clinical and pathological heterogeneity of many autoimmune diseases such as multiple sclerosis, but also Parkinson's disease, Alzheimer's disease, and many more, have changed our views on their pathogenesis and diagnosis, and begins to influence clinical management. At the same time, more powerful methods that allow the analysis of large numbers of genes and proteins simultaneously open opportunities to examine their complex interactions. Using multiple sclerosis as a prototype, we review here how new methodologies such as gene expression profiling can be exploited to gain insight into complex trait diseases.

Characterizing mutations in samples with low-level mosaicism by collection and analysis of DHPLC fractionated heteroduplexes

Somatic mosaicism is a frequent phenomenon in mendelian disorders that exhibit a high proportion of new mutations; however, mutant alleles present at low frequency are difficult to detect and characterize. We have previously shown that denaturing high-performance liquid chromatography (DHPLC) can detect TSC1 and TSC2 mutations in tuberous sclerosis patients with low-level somatic mosaicism, even when direct sequencing cannot identify the causative lesion. Characterization of these mutations traditionally involves extensive sequencing of cloned products. To overcome this limitation, we have utilized DHPLC with an in-line fraction collector to isolate low-level heteroduplex peaks that can be directly sequenced to reveal the mutation. We have successfully applied this technique to resolve the mutations 2724-1G>C in TSC1and 1462-28del42bp, 1774del4bp, and N1643K (4947C>G) in TSC2, which were present in only 6.5-17% of the patients' alleles. We have also applied this technique to successfully resolve seven somatic APC mutations in colorectal tumor samples that were previously undetectable by direct PCR product sequencing. This method may simplify many of the currently challenging goals in mutation detection.

Mechanisms and consequences of somatic mosaicism in humans

Somatic mosaicism -- the presence of genetically distinct populations of somatic cells in a given organism -- is frequently masked, but it can also result in major phenotypic changes and reveal the expression of otherwise lethal genetic mutations. Mosaicism can be caused by DNA mutations, epigenetic alterations of DNA, chromosomal abnormalities and the spontaneous reversion of inherited mutations. In this review, we discuss the human disorders that result from somatic mosaicism, as well as the molecular genetic mechanisms by which they arise. Specifically, we emphasize the role of selection in the phenotypic manifestations of mosaicism.

Temperature modulation of DHPLC analysis for detection of coexisting constitutional and mosaic sequence variants in TSC2

Somatic mosaicism is a frequent phenomenon in Mendelian disorders that exhibit a high proportion of new mutations. However, mutant alleles present at low frequency may escape detection. We have previously shown that denaturing high-performance liquid chromatography (DHPLC) at the recommended melt temperature can detect TSC1 and TSC2 mutations in tuberous sclerosis patients with low-level somatic mosaicism, even when direct sequencing cannot identify the causative lesion. Here, we report the use of temperature modulation in DHPLC analysis to facilitate the robust detection of a mosaic mutation, N1643K, in the presence of a coexisting constitutional polymorphism.

Neurologic manifestations of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a multiorgan disorder that primarily affects the brain, skin, and kidneys. Recent advances have elucidated the genetics of this complex, which has helped lead to an increased understanding of the basic neurobiology of this disorder. There is both phenotypic and geneotypic heterogeneity. The treatment of epilepsy remains a major challenge in these patients, and there is an increasing role for epileptic surgery. Many patients with TSC continue to have intractable seizures. Early identification to ensure proper monitoring and genetic counseling continue to be important clinically. The neurologist must be aware of other organ involvement, particularly the kidneys, and the lungs in female patients, to ensure appropriate monitoring for complications. It is also important to be aware of the marked variability of expression in all the clinical features of TSC.

Human germline mutation in the factor IX gene

The molecular epidemiology of factor IX germline mutations in patients with hemophilia B has been studied in detail because it is an advantageous model for analyzing recent germline mutations in humans. It is estimated that mutations have been defined in the majority of nucleotides that are the target for mutation. The likelihood that a factor IX missense mutation will cause disease correlates with the degree of evolutionary conservation of the amino acid. Mutation rates per base-pair have been estimated after careful consideration and correction for biases, predicting about 76 de novo mutations per generation per individual resulting in 0.3 deleterious changes. The male-to-female sex ratio of mutation varies with the type of mutation. There is evidence for a maternal age effect and an excess of non-CpG G:C to A:T transitions. The factor IX mutation pattern is similar among geographically, racially and ethnically diverse human populations. The data support primarily endogenous mechanisms of germline mutation in the factor IX gene. Mutations at splice junctions are compatible with simple rules for predicting disease causing mutations.

Somatic mosaicism in hemophilia A: a fairly common event

Mutations in the large gene of clotting factor VIII (FVIII) are the most common events leading to severe human bleeding disorder. The high proportion of de novo mutations observed in this gene raises the possibility that a significant proportion of such mutations does not derive from a single germ cell but instead should be attributed to a germline or somatic mosaic originating from a mutation during early embryogenesis. The present study explores this hypothesis by using allele-specific PCR to analyze 61 families that included members who had sporadic severe hemophilia A and known FVIII gene defects. The presence of somatic mosaicisms of varying degrees (0.2%-25%) could be shown in 8 (13%) of the 61 families and has been confirmed by a mutation-enrichment procedure. All mosaics were found in families with point mutations (8 [25%] of 32 families). In the subgroup of 8 families with CpG transitions, the percentage with mosaicism increased to 50% (4 of 8 families). In contrast, no mosaics were observed in 13 families with small deletions/insertions or in 16 families with intron 22 inversions. Our data suggest that mosaicism may represent a fairly common event in hemophilia A. As a consequence, risk assessment in genetic counseling should include consideration of the possibility of somatic mosaicism in families with apparently de novo mutations, especially families with the subtype of point mutations.

Molecular genetic aspects of the phakomatoses: tuberous sclerosis complex and neurofibromatosis 1

The phakomatoses are a diverse group of diseases characterized by skin lesions in early childhood followed by the development of tumors in many other organs. Tuberous sclerosis complex and neurofibromatosis 1 are of special interest to the neurologist because of their prominent neuro-oncological and neuro-developmental consequences. The cloning of genes responsible for these two diseases has led to the identification of causative mutations, an understanding of basic cellular pathophysiology and the development of animal models. Current laboratory investigations are focused on bringing clinical relevance to these findings, including the prospects of molecular diagnostics and rational therapeutics.

Somatic mosaicism and variable expressivity

For more than 50 years geneticists have assumed that variations in phenotypic expression are caused by alterations in genotype. Recent evidence shows that 'simple' mendelian disorders or monogenic traits are often far from simple, exhibiting phenotypic variation (variable expressivity) that cannot be explained entirely by a gene or allelic alteration. In certain cases of androgen insensitivity syndrome caused by identical mutations in the androgen receptor gene, phenotypic variability is caused by somatic mosaicism, that is, somatic mutations that occur only in certain androgen-sensitive cells. Recently, more than 30 other genetic conditions that exhibit variable expressivity have been linked to somatic mosaicism. Somatic mutations have also been identified in diseases such as prostate and colorectal cancer. Therefore, the concept of somatic mutations and mosaicism is likely to have far reaching consequences for genetics, in particular in areas such as genetic counseling.

Denaturing high-performance liquid chromatography (DHPLC) is a highly sensitive, semi-automated method for identifying mutations in the TSC1 gene

Sensitive and automated methods for the detection of DNA sequence variation are required for a wide variety of genetic studies. Diagnostic testing in human genetic disorders is one application of such methods. Tuberous sclerosis complex (TSC) is an autosomal dominant familial tumor syndrome characterized by the development of benign tumors (hamartomas) in multiple organs (OMIM # 19110, #191092). There is a high frequency of sporadic cases and significant demand from patients and families for genetic testing information. Two TSC genes have been identified (TSC1 and TSC2) and together account for all cases [1,2]. Here we report our methods for DHPLC analysis of the TSC1 gene and demonstrate the high sensitivity of this method in a blinded analysis of 21 TSC patients with known TSC1 mutations. In this series, DHPLC detected 27/28 (96%) known TSC1 sequence variations. The only sequence variation not identified by DHPLC in this study is a mosaic case.